20050666 Ver 1_Stormwater Info_20050418

Mint Hill Water Quality
Design Manual
December 31, 2003
Prepared by:
Mecklenburg County Water Quality Program
Prepared For:
Town of Mint Hill

Section 1 Purpose...
Section 2 Definitions
Table of Contents
...........................................
...........................................
Section 3 Performance Criteria
......................1
......................1
......................................................5
Section 4 Low Impact Development (LID) Site Planning ...........................................6
4.1 Introduction ..................................................................................................6
4.2 LID Goals .....................................................................................................7
4.3 Key Considerations for LID Site Planning ..................................................7
4.4 Minimizing Impacts to the Curve Number (CN) .........................................8
4.4.1 Introduction .................................................................................................. 8
4.4.2 Land Cover Type .........................................................................................9
4.4.3 Reduction and Disconnection of Impervious Areas ..................................13
4.5 Providing Retention Storage to Maintain Existing CN ..............................14
4.5.1 Residential Retention Storage ....................................................................14
4.5.2 Commercial/Industrial Retention Storage ..................................................15
4.6 Methods for Additional Detention Storage ............................................... 15
4.7 Maintaining Existing Time of Concentration (Tc) ................................... 17
Section 5 LID Hydrologic Analysis .......................................................................... 18
5.1 Introduction ............................................................................................... 18
5.2 Hydrologic Comparison Between Conventional and LID Approaches.... 18
5.3 Key LID Hydrologic Definitions .............................................................. 22
5.4 Hydrologic Evaluation .............................................................................. 23
5.4.1 LID Runoff Curve Number ........................................................................24
5.4.2 Maintaining the Pre-Development Time of Concentration (Tc) ............... 27
5.4.3 Maintaining the Pre-Development Curve Number and Runoff Volume ...29
5.4.4 Potential Requirement for Additional Detention Storage ..........................30
5.5 Process and Computational Procedure for LID Hydrologic Analysis .......31
5.5.1 Introduction ................................................................................................31
5.5.2 Data Collection ......................................................................................... 32
5.5.3 Determining the LID Runoff Curve Number (CN) ...................................32
5.5.4 Development of the Time of Concentration (Tc) .................................... 33
5.5.5 LID Storm Water Management Requirements ........................................ 33
5.5.6 Determination of Design Storm Event ...................................................... 46
Section 6 LID Site Design Best Management Practices (BMPs) ..............................46
6.1 Introduction ................................................................................................46
6.2 LID BMPs and Their Functions .................................................................46
6.3 Runoff Reduction BMPs ............................................................................50
6.4 Retention BMPs ........................................................................................ 51
6.5 Detention BMPs ........................................................................................ 55

6.6 Pollution Prevention Practices
.............. 57
Section 7 Erosion and Sediment Control Considerations for LID ............................58
7.1 Introduction ................................................................................................ 5 8
7.2 Erosion and Sediment Control Principles ..................................................58
7.3 Principle One: Planning .............................................................................59
7.4 Principle Two: Scheduling of Operations ..................................................60
7.5 Principle Three: Soil Erosion Control Practices ........................................61
7.6 Principle Four: Sediment Control Practices ...............................................62
7.7 Principles Five and Six: Maintenance and Inspection .............................. 63
Section 8 Construction and Maintenance ..................................................................64
8.1 Introduction .............................................................................................. ..64
8.2 Permitting and Processing ........................................................................ ..64
8.3 Erosion and Sediment Control Principles for Bioretention Applications ...64
Principle: Planning and Phasing .............................................................. ..64
Principle 2: Schedule of Operations ........................................................ ..64
Principle 3: Soil Erosion Control ............................................................. ..65
Principle 4: Sediment Control .................................................................. ..65
Principle 5: Inspection and Maintenance ..................................... ..65
8.4 Construction Technique and Sequencing for Bioretention ..................65
8.4.1 Site Preparation and Planning ...................................................65
Method One ...................................................................... ..66
Method Two ...................................................................... ..66
8.4.2 Minimize Lot Grading/Clearing ............................................... ...66
8.4.3 Install Sediment Control Devices ............................................. ...66
8.4.4 Excavation Preparation ......................................................... ...68
8.4.5 Underdrain Specification ....................................................... ..68
8.4.6 Observation/Cleanout Standpipe .............................................. ..69
8.4.7 Gravel Bed ....................................................................... ...69
8.4.8 Pea Gravel Diaphragm ............................................................70
8.4.9 Filter Fabric ...................................................................... ...70
8.4.10 Soil Preparation and Installation .............................................. ...71
8.4.11 Plant Preparaton and Planting Methodology ................................ ...73
8.4.12 Installation of Mulching Materials ............................................ ...75
8.5 Maintenance and Operation .................................................... ..76
8.5.1 General Care ..................................................................... ...76
8.5.2 Maintenance Responsibilities ....................................................78
8.5.3 Warrantees ....................................................................... ...79
8.6 Typical Sequence of Construction for Bioretention ........................ ...79
8.6.1 Inspectors Checklist for Bioretention ...........................................80
Sequence of Construction for Bioretention .................................. ..81
Section 9 Plan Submittal/Review ............................................................................ ...82
9.1 Introduction ............................................................................................. ...82
9.2 Site Evaluation Tool ............................................................................... ...82
9.3 Plan Submittal Requirements .................................................................. ...83

Section 10 Inspection and Enforcement .....................................................................85
10.1 Introduction .............................................................................................. .. 8 5
10.2 Authority .................................................................................................. ..85
10.3 Inspection Responsibilities ...................................................................... ..86
10.4 Inspection Requirements During Construction ........................................ ..86
10.4.1 Drywell or Infiltration Trench ................................................................. ..86
10.4.2 Bioretention BMPs ................................................................................... ..87
10.5 Maintenance Inspections .......................................................................... ..88
10.5.1 Introduction .............................................................................................. .. 8 8
10.5.2 Responsibilities and Procedures .............................................................. ..88
10.6 Approvals and Reports ............................................................................. ..88
10.6.1 Field Inspection Report ............................................................................ ..88
10.7 Administration and Enforcement ............................................................. ..89
10.7.1 Right to Appeal ........................................................................................ ..89
10.7.2 Penalties ................................................................................................... ..89
List of Tables:
Table 5.1. LID Techniques and Hydrologic Design and Analysis Components ........... 23
Table 5.2. Comparison of Conventional and LID Land Covers .................................... 25
Table 5.3. LID Planning Techniques to Reduce the Post-Development LID CN ......... 25
Table 5.4. LID Techniques to Maintain the Pre-development Time of Concentration . 29
Table 5.5 Representative LID Curve Numbers ............................................................. 32
Table 5.6. Representative Percentages of Site Required for Volume and Peak
Control .......................................................................................................... 42
Table 6.1. Approved LID BMPs for Use in Mint Hill ................................................... 48
Table 6.2. Conventional Storm Water BMPs ................................................................ 49
Table 6.3. Grassed Swale Design Considerations .......................................................... 56
List of Figures:
Figure 4.1. Schematic of Residential Single-Family Low Impact Lot Layout ............... 10
Figure 4.2. Grass Swale and Infiltration Trench ............................................................. 12
Figure 4.3. Providing Bioretention Within Landscaped Buffer Areas ............................ 16
Figure 5.1. LID Analysis Procedure ................................................................................ 19
Figure 5.2. Hydrologic Response of Conventional BMPs .............................................. 20
Figure 5.3. Comparison of the Hydrologic Response of Conventional and LID
BMPs ............................................................................................................. 21
Figure 5.4. Comparison of Land Covers Between Conventional and LID CNs ............. 24
Figure 5.5. Effect of LID CN on the Post-Development Hydrograph Without
Storm Water BMP ......................................................................................... 26
Figure 5.6. LID Hydrograph that has a Reduced CN and Maintains Tc Without
Storm Water BMPs ....................................................................................... 28

Figure 5.7. Retention Storage Required to Maintain Peak Development Runoff Rate...30
Figure 5.8. Effect of Additional Detention Storage on LID Retention Practices............ 31
Figure 5.9. Procedure to Determine Storage Volume Required for BMPs to
Maintain Pre-Development Volume and Peak Flow Rate ............................ 37
Figure 5.10. Comparison of Retention of Storage Volumes Required to Maintain
Peak Runoff Rate Using Retention and Detention ....................................... 39
Figure 5.11. Storage Volume Required to Maintain Peak Runoff Rate ............................ 40
Figure 5.12. Comparison of Storage Volumes for Various Tcs ........................................ 40
Figure 6.1 Typical Swale Design ................................................................................... 49
Figure 6.2. Typical Bioretention BMP ............................................................................ 52
Figure 6.3. Typical Dry Well .......................................................................................... 53
Figure 6.4. Typical Rain Barrel ....................................................................................... 54
Figure 6.5. Typical Rock Trench Level Spreader ........................................................... 57
Figure 8.1. Rain Garden Detail .................................................................. 67
Figure 8.2. Installation of Underdrain .......................................................... .69
Figure 8.3. Filter Fabric Over Underdrain ...................................................... 70
Figure 8.4 Installing Amended Soil Mix ...................................................... .72
Figure 8.5. Soil in place, V-Notch weir, Forebay and Spillway Built ......................73
Figure 8.6. Hal Marshall Rain Garden Planting Design ...................................... .74
Figure 8.7 Planted and Mulched Rain Garden ................................................ 76
Figure 8.8. Sequence of Construction for Bioretention ...................................... .81
List of Examples:
Example 5.1: Detailed CN Calculation ............................................................................34
Example 5.2: Determine Site Area Required to Maintain Volume (CN) ........................38
Example 5.3: Calculation of Volume for Water Quality Control ....................................38
Example 5.4: Calculation of Additional Storage Above Volume Required to
Maintain CN and Maintain Pre-development Peak Runoff Rate
Using Hybrid Approach .............................................................................43
Example 5.5: Calculation of Percentage of Site Area Required to Maintain the Peak
Runoff Rate Using the Hybrid Approach of Retention and Detention......45
Example 7.1: Calculation of Slope Steepness and Slope Length .....................................59
List of Appendices:
Appendix A -Mint Hill Water Quality Ordinance
Appendix B -Chart Series A: Storage Volume Required to Maintain the Pre-development
Runoff Volume Using Retention Storage
Appendix C -Chart Series B: Storage Volume Required to Maintain the Pre-development
Peak Runoff Rate Using 100% Retention Storage
Appendix D -Chart Series C: Storage Volume Required to Maintain the Pre-development
Peak Runoff Rate Using 100% Detention Storage

Appendix E - Design Examples & Cost Analyses for Use of LID in Medium and High
Density Residential, Commercial and Institutional Developments
Appendix F - Plant List
Appendix G -Sample Maintenance Covenant

Mint Hill Water Quality Design Manual Date of Adoption
Section 1. Pur ose
The purpose of this document is to provide the information necessary for compliance
with the Mint Hill Water Quality Ordinance adopted by the Mint Hill Town Board on
??????????? and incorporated into the Mint Hill Zoning Ordinance. Appendix A
contains a copy of this Ordinance. The goal of the Ordinance and this Manual is to
protect stream water quality from surface water degradation in Goose, Duck and Stevens
Creeks and to protect habitats for the Carolina heelsplitter, a federally endangered species
of freshwater mussel, through the application of land use requirements for the control of
non-point source pollutants. The Goose, Duck and Stevens Creek Watershed Overlay
District is that area within the Town of Mint Hill and its Extra Territorial Jurisdiction
(ETJ) that contributes surface drainage into Goose Creek and its tributaries as specifically
defined on the Town of Mint Hill Zoning Maps. Low Impact Development (LID)
techniques combined with conventional storm water retention and detention structures are
the primary mechanisms discussed in this document for meeting this goal and complying
with the Ordinance. The goal of LID is to develop site design techniques, strategies,
BMPs, and criteria to store, infiltrate, evaporate, retain, and detain runoff on the site to
replicate pre-development runoff characteristics and mimic the natural and unique
hydrology of the site thereby preventing an increase in pollutant loads above pre-
development conditions. The selection of these strategies and techniques for compliance
with Water Quality Performance Criteria is discretionary and shall be detailed in a Water
Quality Management Plan submitted during the Preliminary Plan review process. This
manual provides the information necessary to select and design these strategies and
techniques.
Section 2. Definitions
Best Management Practices (BMPs) - A structural or non-structural management-based
practice used singularly or in combination to reduce non-point source input to receiving
waters in order to achieve water quality protection goals.
Non-structural BMPs -Non-engineering methods to control the amount of non-
point source pollution. These may include land-use controls and vegetated
buffers.
Structural BMPs -Engineered structures that are designed to reduce the delivery
of pollutants from their source or to divert contaminants away from the
waterbody.
Bioretention -The use of vegetation in retention areas designed to allow infiltration of
runoff into the ground and transpiration by plants as well as evaporation. The plants
provide additional pollutant removal and filtering functions while infiltration allows the
temperature of the runoff to be cooled. Also referred to as a Biofilter or Rain Garden.
Cistern - A receptacle for holding water or other liquid (i.e. tank for catching and storing
rainwater).
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Conveyance -The process of moving water from one place to another.
Curbs -Concrete barriers on the edges of streets used to direct storm water runoff to an
inlet or storm drain and to protect lawns and sidewalks from vehicles.
Curve Number - An index that represents the amount of runoff from the combined
hydrologic effect of soil, land use, agricultural land treatment class, hydrologic condition,
and antecedent soil moisture. Curve numbers have a range of 0 to 100.
Design Storm - A rainfall event of specific depth or intensity (i.e. 3.12 inches or 4.80
inches/hour) and return frequency (e.g., 1-year storm) that is used to calculate runoff
volume and peak discharge rate.
Detain - To store and slowly release storm water runoff following precipitation by means
of a surface depression or tank and an outlet structure. Detention structures are
commonly used for pollutant removal, water storage, and peak flow reduction.
Dry Well -Small excavated trenches filled with stone to control and infiltrate runoff,
usually from rooftops.
Evaporation -The change by which any substance is converted from a liquid state and
carried off in vapor.
Evapotranspiration -The combination of evaporation and transpiration of water into the
atmosphere from living plants and soil.
Filter Strip -Grass strips along roads or parking lots that remove pollutants from runoff
as it passes through, allowing infiltration and velocity reduction.
Floodplain - An area adjacent to a stream or river where water overflows its banks during
high flow events.
Ground Water -The supply of fresh water found beneath the earth's surface (usually in
aquifers) that provides base flow to streams and rivers and is often used for supplying
wells and springs. The inflow to a ground water reservoir is called ground water
recharge.
Gutter -The edge of a street (below the curb) designed to drain water runoff from streets,
driveways, parking lots, etc. into catch basins and storm drains.
Hydrograph - A graphic plot of changes in the flow of water or in the elevation of water
level plotted against time.
Hydrologic Cycle (Water Cycle) -The cycle of water movement from the atmosphere to
the earth and back to the atmosphere through various processes.
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Hydrologic Soil Group (HSG) -See Soil Group definition.
Hydrology -The science dealing with the properties, distribution and circulation of
water.
Impervious Surface - A surface that cannot be penetrated by water such as pavement or
rock and prevents infiltration, thus generating runoff.
Integrated Management Practices (IMP) - A LID practice or combination of practices
that are the most effective and practicable (including technology, economic, and
institutional considerations) means of controlling the pre-development site hydrology.
Level Spreader - An outlet designed to convert concentrated runoff to sheet flow and
disperse it uniformly across a slope to prevent erosion.
Low Impact Development (LID) -The integration of site ecology and environmental
goals and requirements into all phases of urban planning and design from the individual
residential, commercial and industrial lot level to the entire watershed.
Mecklenburg County Land Use & Environmental Services Agency (LUESA) -The
department or division of Mecklenburg County government (regardless of the title given
to it by Mecklenburg County) which has responsibility for storm water and water quality
matters, acting as the agent of the Town of Mint Hill for various purposes in connection
with the enforcement of this regulation.
National Pollution Discharge Elimination System (NPDES) Permit - A permit issued
pursuant to the federal Clean Water Act for the purpose of controlling discharges of
pollutants to surface waters and protecting water quality. In North Carolina, NPDES
Permits are issued by the N.C. Department of Environment and Natural Resources.
Nonpoint Source (NPS) Pollution -Forms of pollution caused by sediment, nutrients,
organic and toxic substances originating from land use activities, which are carried to
lakes and streams by surface runoff. Nonpoint source pollution occurs when the rate of
materials entering these waterbodies exceeds natural levels.
Permeability -The property of a soil to transmit water under a gradient. It is measured by
the quantity of water passing through a unit cross section, in a unit time, under a
hydraulic gradient.
Pollutant Load - A calculated quantity that is the result of a flow rate and pollutant
concentration applied over a given amount of time.
Rain Barrels -Barrels designed to collect and store rooftop runoff.
Receiving Waters - A river, ocean, stream, or other watercourse into which runoff from
precipitation is discharged.
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Recharge -The addition of water into the ground water via the surface of the ground.
Retain - To capture and hold storm water runoff following precipitation by means of a
surface depression allowing the water to infiltrate into the soil, evaporate and possibly
transporate thus reducing the hydrologic and pollution impacts downstream. Retention
structures are commonly used for pollutant removal, water storage, and peak flow
reduction.
Riprap - A facing layer (protective cover) of stones placed to prevent erosion or the
sloughing off of a structure or embankment of a waterbody.
Runoff -Water from a precipitation event that flows across the ground surface. Run-off
carries nonpoint source pollutants to receiving streams.
Sediment -The layer of soil, sand and minerals at the bottom of surface water, such as
streams, lakes, and rivers that may absorb contaminants.
Sedimentation -The removal, transport, and deposition of detached soil particles by
flowing water or wind.
Siltation -The deposition of finely divided soil and rock particles upon the bottom of a
waterbody.
Site Evaluation Tool (SET) - A spreadsheet-based model that assesses and compares pre-
development and post-development runoff, infiltration, and pollutant loading rates, which
provides a methodology to aid in better site design and evaluation of BMP effectiveness.
Site Fingerprinting - A development approach that places land disturbing activities away
from environmentally sensitive areas (wetlands, steep slopes, etc.), future open spaces,
tree save areas, future restoration areas, and temporary and permanent vegetative forest
buffer zones. Ground disturbance is confined to areas where structures, roads, and rights-
of-ways will exist after construction is completed.
Soil Group -Hydrologic Soil Groups (HSG) are used to estimate runoff from
precipitation. Soils not protected by vegetation are placed in one of four (4) groups on
the basis of the intake of water (infiltration) after the soils have been wetted and have
received precipitation from long duration storms. Group A soils have a high infiltration
rate and usually include course sand and gravel. Group B soils have a moderate
infiltration rate and include soils with moderately fine to coarse texture. Group C soils
have a slow infiltration rate and include soils that have a moderately fine to fine texture.
Group D soils have a very slow infiltration rate and include fine textured clays (Soil
Conservation Service, 1986).
Soil Moisture -Water diffused in the soil. It is found in the upper part of the zone of
aeration where water is discharged by transpiration from plants or by soil evaporation.
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Storm Water -Water that runs across the surface of the ground during and after
precipitation events.
Swale - An open depression or wide, shallow ditch that intermittently contains or conveys
runoff. Can be used as a BMP to detain and filter runoff.
Time of Concentration (Tc) -The time required for runoff to travel from the
hydraulically most distant point (in time) in the watershed, to the point of interest in the
watershed.
Total Suspended Solids (TSS) -Total suspended matter in water, which is commonly
expressed as a concentration in terms of milligrams per liter or parts per million.
Travel Time (Tt) -The time it takes water to travel from one location to another in a
watershed. It is a component of the time of concentration (Tc).
Treatment Train - A series of BMPs or natural features, each designated to treat runoff
that are implemented together to maximize pollutant removal effectiveness.
Water Table -The upper surface of a zone of saturation or ground water.
Watershed -The area of land that contributes surface runoff to a waterbody.
Wet Pond - A BMP designed to detain urban runoff and always contain water.
Wetlands - An area (including swamp, marsh, bog, prairie pothole, or similar area) that is
typically inundated or saturated by surface or groundwater at a frequency and duration
sufficient to support the growth and regeneration of vegetation requiring an abundant
water source.
Section 3. Performance Criteria
Land development activities shall be performed in such a manner as to minimize the
degradation of water quality conditions through compliance with the Performance
Criteria listed below. Tables 6.1 and 6.2 in Section 6 of this Manual contain a description
of approved BMPs for meeting each of these Criteria.
a) All storm water treatment systems used to meet these Performance Criteria shall be
designed to achieve average annual 85% Total Suspended Solids (TSS) removal for
the developed area of a site. Areas designated as open space that are not developed
do not require storm water treatment. All sites must employ LID practices to control
and treat runoff from the first 1-inch of rainfall.
b) LID practices or a combination of LID and conventional storm water management
practices shall be used to control and treat the increase in storm water runoff volume
associated with post-construction conditions as compared with pre-construction
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Mint Hill Water Quality Design Manual Date of Adoption
(existing) conditions for the 2-year frequency, 24-hour duration storm event (3.12
inches). This may be achieved by hydrologic abstraction, recycling and/or reuse, or
other accepted management practice as described in Section 6 of this Manual.
c) Where any storm water BMP employs the use of a temporary water quality storage
pool as a part of its treatment system, the drawdown time shall be a minimum of 48
hours and a maximum of 120 hours.
d) Peak storm water runoff rates shall be controlled for all development above 12%
imperviousness. The peak storm water runoff release rates leaving the site during
post-construction conditions shall be equal to or less than the pre-development peak
storm water runoff release rates for the 2-year frequency, 24-hour duration storm
event and 10-year frequency, 24-hour duration storm event. The emergency overflow
and outlet works for any pond or wetland constructed as a storm water BMP shall be
capable of safely passing a discharge with a minimum recurrence frequency of 50
years. For detention basins, the temporary storage capacity shall be restored within
72 hours. Requirements of the Dam Safety Act shall be met when applicable.
e) No one BMP shall receive runoff from an area greater than five (5) acres. However,
the total drainage area from BMPs used in series (i.e., integrated) can exceed this five
(5) acre maximum.
Section 4. Low Im act Develo ment Site Plannin
4.1 Introduction
The Water Quality Performance Criteria discussed in Section 3 shall be achieved using
LID site planning and techniques (BMPs) or a combination of LID and conventional
storm water structures. The selection of these strategies and techniques is discretionary
and shall be detailed in a Water Quality Management Plan submitted during the plan
review process (see Section 9, Plan Submittal/Review). This Section focuses on the LID
planning strategies and techniques that can be used in the development of the Water
Quality Management Plan for achieving the Performance Criteria. The goal of LID site
planning is to allow for the maximum reasonable utilization of the property while
maintaining the pre-development hydrologic regime (volume, peak runoff rate for a given
frquency). The LID approach combines a hydrologically functional site design with
pollution prevention measures (BMPs) to reduce development impacts on hydrology and
water quality. The goal is to maintain the pre-development storm water runoff volume,
peak runoff rates, and to mimic pre-development runoff conditions. Storm water is
managed in small, source control landscape features rather than in large, end-of--pipe
pond structures located at the downstream extent of drainage areas. However, ponds may
be required in addition to LID BMPs to create a "treatment train" affect designed to meet
the Performance Criteria described in Section 3. Through LID, hydrologic functions such
as infiltration, peak and volume of discharges, and ground water recharge can be
maintained with the use of reduced impervious surfaces, functional grading, open channel
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Mint Hill Water Quality Design Manual Date of Adoption
sections, disconnection and utilization of runoff, and the use of bioretention/filtration
landscape areas.
4.2 LID Goals
The goal of LID is to develop site design techniques, strategies, BMPs, and criteria to
store, infiltrate, evaporate, transpire, retain, and detain runoff on the site to replicate pre-
development runoff characteristics and mimic the natural and unique hydrology of the
site. Since every aspect of site development affects the hydrologic response of the site,
LID runoff control techniques also can address every aspect of site development. There
is a wide array of impact reduction and site design techniques that allow the site designer
to create storm water control mechanisms that function in a similar manner to natural
control mechanisms. The net result will be to mimic the watershed's natural hydrologic
functions or water balance between runoff, infiltration, storage, ground water recharge,
and evapotranspiration. With the LID approach, receiving waters experience little
change in the volume, frequency, or quality of runoff or in the base flows fed by ground
water.
The goals of LID are discussed and demonstrated throughout this Manual. The list below
highlights and reinforces some of the main goals and principles of LID:
• Provide enhanced technology for environmental protection of receiving
waters.
• Provide economic incentives to provide environmentally sensitive
development.
• Develop the full potential of environmentally sensitive site planning and
design.
• Encourage public education and participation in environmental protection.
• Reduce maintenance costs of the storm water infrastructure.
• Utilize concepts, technologies, and objectives for storm water
management such as micromanagement and multi-functional landscape
features (rain gardens, swales, and conservation areas) to mimic or
replicate hydrologic functions, and maintain the ecological biological
integrity of receiving streams.
LID planning and techniques will reduce impervious areas, reduce the need for
conventional pipe and pond technology, and reduce or limit clearing and grading. LID
techniques integrate storm water controls throughout the site in small, discrete units.
BMPs are distributed across each site, reducing the need for a centralized BMP.
4.3 Key Considerations for LID Site Planning
Examples of the LID site planning techniques include, but are not limited to:
Maintaining natural drainage ways and patterns and directing runoff to
depression areas.
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Mint Hill Water Quality Design Manual Date of Adoption
• Preserving as many trees as possible, especially those located on soils with
the highest permeability rates.
• Reducing the percentage of impervious area.
• Locating BMPs in soils with the highest permeability rates.
• Disconnecting impervious areas.
• Limiting clearing and grading in areas containing soils with the highest
permeability rates.
• Locate impervious areas on less permeable soils.
• Maintaining existing natural topography and terrain. Avoid disturbance
of, and construction in, steep slope areas (>15%).
• Limiting clear-cutting and mass-grading through "site fingerprinting"
techniques. Selectively clear wooded lots to preserve the tree canopy,
understory vegetation, and natural vegetative buffers. See Article 7 of the
Mint Hill Zoning Ordinance.
• Flattening slopes only within existing cleared and graded areas, where
feasible, to facilitate on-lot storage and infiltration.
• Revegetating areas that have been cleared and graded.
• Dispersing storm water flow to the natural drainage ways rather than
concentrating it in swales, pipes, or channels.
The appropriate combination of these techniques to maintain the curve number (CN) and
time of concentration (Tc) will result in a design that maintains the pre-development
runoff volume, peak rate, and frequency. The remainder of this Section offers guidance
on how these and other LID site planning techniques can be used to achieve the LID
hydrology and Performance Criteria (Section 3). The Section is divided into the four
hydrologic components that are discussed in the hydrologic analysis:
• Minimizing development impacts on the existing CN (Section 4.4).
• Providing retention storage to maintain the CN (Section 4.5).
• Providing additional detention storage to maintain the CN (Section 4.6).
• Maintaining the Tc (Section 4.7).
4.4 Minimizing Impacts to the Curve Number (CN)
4.4.1 Introduction
The CN is used to determine the runoff volume from both the pre- and post-development
condition. The aggregate CNs assigned to each area are used to arrive at a composite, or
weighted, CN for the site watershed. As the CN from the pre-development condition to
the post-development condition increases, storage is required to mimic the pre-
development CN. Site development factors most responsible for the determination of the
CN fall into four categories:
Land cover type.
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Mint Hill Water Quality Design Manual Date of Adoption
Percentage and connectivity of impervious areas.
Hydrologic conditions (good, fair, poor).
Soil infiltration rate (HSG).
The evaluation and management of land cover type, and percentage and connectivity of
impervious areas are different for LID and discussed in detail below.
4.4.2 Land Cover Type
The most important factor for the change in CN from the pre- to post-development
condition is land cover. The land cover type is the distribution of the physical
components of the site. It is typically a broad definition, such as residential '/4 acre lots,
but can be defined in detail, such as trees in good, fair, or poor condition. For example,
the land use might be low-density residential on an HSG B, which carries an average CN
of 65 (see TR55 Table 2-2a, Soil Conservation Service, 1986); however, if examined
more closely, each lot might incorporate LID site planning and retention practices that
would effectively reduce the CN. Figure 4.1 illustrates slow-density residential lot that
includes tree preservation, minimized impervious areas, and low-impact BMP retention
and detention practices. As shown in Figure 4.1, the land cover of a site is directly
affected by the extent of the limits of clearing and grading. To minimize these
hydrologic impacts, LID site planning strategically reduces clearing and grading,
preserves more trees wherever possible, and minimizes the percent and connectivity of
impervious areas within a given drainage area. The designer should keep in mind that the
change in the CN from pre- to post-development will directly affect the need for
compensatory storm water storage volume; hence, reducing this change can potentially
reduce the storage volume requirement, thereby increasing the reasonable use of the
property.
Determining the existing land cover requires delineating the surface coverage of existing
land cover features and sensitive areas such as woodlands, permeable soils, streams,
floodplains, critical areas, wetlands, and steep slopes. Delineation of these areas is
important for understanding existing site conditions and site hydrology. It should be
noted that this process is exactly the same as the conventional site planning process. The
difference is the increased amount of detail and effort required to calculate the CN.
Reduction in Land Cover Changes.
Reduction in land cover changes is the first step in maintaining existing CN. There are
several ways to reduce land cover changes including:
• Reduce the size of cleared area (i.e., preserve as much woodland as
possible) and increase reforestation areas.
• Locate cleared/graded areas outside permeable soils and vegetated areas.
• Design roads, sidewalks, and parking areas to minimize land cover
impacts.
• Reduce or disconnect site imperviousness.
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Mint Hill Water Quality Design Manual Date of Adoption
I ~ ~ ~`'~
i • s zY ~ Ex[stng
1 ~~ ~ ~ '~~ Tree Line
,°r~ ~ 1Q0»F©o# Maximum f~verland Flaw at Minimum i~o Slope
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~._1` ~~~_~.~ ~ I' I~-r~-~!~~„~._~_~~i~'.~11-=-ffc ~=L 1~==~_---'~! ;==_~ i1 _~_~Y 13 =L!-=1~~;
;,f...tl I -I; II if 1~ .I tt~ It ~t ii [ i r. iz _~L. 3~r ~i. '.. i, ~.~.-~ '.- ,: _,... r.
rigure 4.1. schematic of Kesidential Single-Family Low Impact Lot Layout
10

Mint Hill Water Quality Design Manual Date of Adoption
Reduce Limits of Clearing and Grading.
The limits of clearing and grading refer to the site area to which development is directed.
This development area will include all impervious areas such as roads, sidewalks,
rooftops, and graded lawn areas and open drainage BMPs. To have minimal hydrologic
impacts on existing site land cover (i.e., to reduce the percent change in the CN), the area
of development should be located where the impact on the pre-development CN is less
sensitive (e.g., developing barren C and D type soils will have a lesser hydrologic impact
than developing forested A and B type soils). At a minimum, current requirements place
the areas of development outside stream and lake buffer areas. Increasing these buffer
areas, thereby reducing the limits of clearing and grading, will lead to a reduction in the
percent change in the CN.
Site Fingerprinting.
Site fingerprinting (minimal disturbance techniques) can be used to further reduce the
limits of clearing and grading, thereby minimizing the percent change in the CN. Site
fingerprinting includes restricting ground disturbance by identifying the smallest possible
area and clearly delineating it on the site. Land cover impacts can be minimized through
minimal disturbance techniques that include the following:
• Minimizing the size of construction easements, materials storage areas,
and siting stockpiles within the development envelope.
• Careful siting of lots and home layout, clearing and grading to avoid steep
slopes, removing existing trees and excessive grading.
• Minimizing imperviousness by reducing paved surfaces on private areas.
• Homes on crawl spaces or basements that conform to natural grades
without creating a flattened pad area for slab construction; thus saving
clearing and grading costs.
• Flagging the smallest site disturbance area possible to minimize soil
compaction on the site. Install orange construction fencing and tree
protection areas where necessary to protect root structure along the limit
of encroachment during the construction phase.
• Disconnecting all impervious areas to increase the Tc and flow paths.
• Re-vegetating cleared and graded areas to provide an effective way to
decrease the post-development runoff CN. Sometimes it is impractical or
impossible to develop a lot or group of lots using the minimal disturbance
techniques. Re-vegetation can be used to connect bioretention facilities to
natural drainage ways or to increase the size of riparian buffer areas. Re-
vegetated areas also increase the permeability of the soil. In addition,
these BMPs can add aesthetic value to a site.
Maintaining existing topography and drainage patterns to encourage
dispersed flow.
Locate Cleared and Graded Areas Outside Permeable Soils and Vegetated Areas.
Locating roadways, houses, and graded lawn areas within the following areas should be
minimized as much as possible:
11

Mint Hill Water Quality Design Manual Date of Adoption
Within pervious soils. Sensitive soils for LID sites include soils with good infiltration
characteristics. Site planning that locates impervious surfaces or directs compaction
within areas of highly pervious soils creates the greatest possible change in infiltration
between pre-development and post-development conditions. Areas that contain well-
drained soils should be preserved. Preserving permeable soils should be promoted in all
unpaved areas throughout the site, when considering optional ways to reduce the
difference between the pre- and post-development CNs. Maintaining permeable soils
also helps maintain recharge areas for ground water and base stream flows.
Within existing vegetated areas. Protection of woodland areas can help to reduce
impacts on existing land cover and associated CN. Expansion of protected vegetated
areas adds to the benefits of reducing CN changes. Saving existing trees on a
development site is acost-effective and quality-enhancing practice. If the trees are
appropriate for preservation and adequate protective measures are taken during
construction, the preservation of existing trees on a site has many rewards. When these
protected vegetated areas are combined with riparian buffers, they can provide added
benefits of reduced storm water velocities, increased storm water infiltration, filtration of
pollutants, protection of existing wildlife habitat, and stream bank and bed stability.
Use Alternative Roadway Designs.
Roadways, sidewalks, driveways, and parking areas are the greatest contributors to
increasing the CN due to associated imperviousness and land clearing for cutting and
filling. The primary considerations of road design are safety and balanced earthwork for
the site. For LID sites, minimizing the effective imperviousness contributed by road and
parking area pavement is also an important site planning and design consideration. In a
LID layout, the roadway is designed to minimize hydrologic impacts by using minimal
grading and clearing techniques and open drainage sections. LID roadway designs
emphasize the need to keep paved areas to an absolute minimum. Reducing both
pavement width and road length can help achieve this goal. The following features can
be incorporated into the roadway to minimize impacts:
Narrow road sections. Small sections can be used to minimize site imperviousness and
clearing and grading impacts.
Grassed swales and infiltration trenches. In Mint Hill's "R" Zone, grassed swales and
infiltration trenches can be used in place of curb and gutter in some circumstances (see
h 1~ure 4
i t i w; f t r t a t i~ m T t e ~ t e r~ P
r.
L
a<
P
f
t
9
S . , . I .. _. ...-~~
Figure 4.2. Grass Swale and Infiltration Trench
12

Mint Hill Water Quality Design Manual Date of Adoption
Curvilinear road layouts. Local and collector streets with curves and changes in
alignment allow the flexibility to fit the road into the existing site topography. Following
existing contours will minimize grading and make earthwork operations easier. It should
be noted that curvilinear road layouts must meet current AASHTO design criteria.
Location of roads on ridges. Strategically locate the roadway entrance to minimize
disturbance by positioning the entrance on the high point of the ingress, following mild
contour slopes to the greatest degree possible. As an added benefit, following mild
contour slopes when laying out roads allows reduction in cut and fill requirements and
associated grading costs. It is also more aesthetically pleasing than rigid rectangular
blocks.
Reduced application of sidewalks to one side of primary roads. Decrease site
imperviousness by reducing sidewalk widths or by using alternate materials such as
pervious pavers. The reduction of sidewalks to one of side of the street is allowed only in
Mint Hill's "R" Zone.
4.4.3 Reduction and Disconnection of Impervious Areas
By reducing and disconnecting site impervious areas, the amount of direct runoff can be
significantly decreased. This requires careful planning. There must be adequate
circulation so that pedestrians and vehicles do not use grassed areas and cause erosion. In
turn, reduced widths and lengths result in reduction of associated infrastructure such as
drainage pipes, etc.
Minimize rooftop imperviousness. Rooftops contribute to site imperviousness, and the
number of lots per acre or lot coverage generally determines the site's rooftop impervious
area. House type, shape, and size can affect rooftop imperviousness. As an example,
more rooftop coverage is generally required for ranch-type homes that spread out square
footage over one level. With this in mind, vertical construction is favored over horizontal
layouts to achieve less impact.
Disconnect impervious areas. Reductions in post-development CNs can be gained by
redirecting and dissipating concentrated flows from impervious areas onto vegetated
surfaces. Strategies for accomplishing this include:
• disconnecting roof drains and directing flows to vegetated areas;
directing flows from small swales to stabilized vegetated areas;
• breaking up flow directions from large paved surfaces; and
• encouraging sheet flow through vegetated areas.
Carefully locate impervious areas. To the extent possible, place impervious areas so that
they drain to natural drainage features, vegetated buffers, natural resource areas, or
permeable zones/soils. Additional LID site planning techniques aimed at reducing site
imperviousness include:
Using shared driveways in "R" Zone. (Using shared driveways in other
areas is recommended but may require a waiver. Shared driveways also
need the approval of the Town of Mint Hill.)
Limiting residential driveway width to 9 feet (for both single and shared
driveways).
Minimizing building setback to reduce driveway lengths.
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Mint Hill Water Quality Design Manual Date of Adoption
Efficient parking lot layout. Avoid single loaded aisles, angled parking,
and excessive spaces.
Using private roads where possible to allow flexibility in reducing road
widths (see Figure 4.2).
Planning efficient lot layout to minimize the road length, single loaded
streets and double frontage lots.
4.5 Providing Retention Storage to Maintain Existing CN
Once all of the appropriate strategies have been applied to the site to take advantage of
infiltration and storage opportunities in the watershed, there still may be a need for
additional retention storage to maintain the CN. The objective is to provide this
additional storage as source control BMPs. These are small BMPs distributed
strategically throughout the site and close to the origin of storm water runoff. When the
need arises, additional detention storage basins may be required to maintain the pre-
development peak runoff rate.
4.5.1 Residential Retention Storage
For residential LID sites, lot layouts must be planned to distribute retention storage
volume as much as possible throughout the site. At the site planning stage, it is important
to allocate enough area to provide for needed storm water retention storage. This
involves developing a preliminary layout where LID techniques can be incorporated. At
a minimum, this should include designating (1) road layout with potential LID geometric
modifications, (2) sidewalk areas, (3) building footprint, driveway and accessory
structures, and (4) potential BMP areas. When laying out the site, the designer should
always keep in mind the four important factors that affect the change in pre- to post-
development storm water runoff volume and peak rate including: land cover type,
percentage and connectivity of impervious areas, hydrologic conditions, and soil type
(HSG). The site should be planned to maximize lot yield while minimizing impacts on
the hydrologic regime. Residential retention storage can be incorporated onto individual
lots or common areas. Due to concerns with the future maintenance of these BMPs,
locating them in common areas dispersed throughout the site is recommended.
Information and calculations provided in Section 5 can be used to determine the required
retention storage area. Zoning requirements exist regarding maximum lot coverage in
residential zones for various lot sizes in accordance with the Mint Hill Zoning Ordinance.
Lot coverage for residential zones relates to maximum impervious surface allowed per lot
and can be used to determine available green space for on-lot retention storage area.
Keep in mind the need for allowing "reasonable use" of a property and restrictions on
locating storage areas within the building restriction line. Estimated requirements for
retention storage areas (evenly dispersed throughout the development) can be compared
with estimated available green space. In most cases, adequate space will be available to
provide retention storage in zones. However, situations may arise for %-acre lots and
14

Mint Hill Water Quality Design Manual Date of Adoption
smaller where the magnitude of change in pre-development and post-development curve
numbers results in storage requirements that cannot be fully accommodated while
allowing reasonable use of lawn and/or open space area. Refer to Section 4.6 for more
detailed information on this situation (i.e., incorporating more detention storage). This is
important to realize when identifying the limits of clearing and grading on each lot. It
may be necessary to minimize the clearing and grading to reduce changes in the CN to
accommodate storage retention requirements. Terraces and enlarged drainage swales can
also be used to provide additional detention storage as necessary. When locating on-lot
retention storage areas on residential LID sites, follow these recommended guidelines:
• Locate swales and bioretention BMPs (rain gardens) where they can
provide a green space connection between existing wooded or natural
areas.
• Bioretention practices must be located outside a public road right-of--way
to avoid conflict with underground utilities.
• Infiltration or enhanced swales may be used in the public right-of--way.
• Locate bioretention and infiltration in areas containing permeable soils.
• Keep all LID storm water management BMPs outside all sensitive areas
and respective buffers.
• Insure that the contributing drainage area to the site is stabilized prior to
bioretention installation.
4.5.2 CommerciaVIndustrial Retention Storage
Planning for retention storage volume for commercial and industrial LID sites is focused
on two areas: perimeter buffer areas and green areas located within parking lots. On-site
retention storage can be provided as interior bioretention, preferably located within the
required landscape islands, or as cistern or rainbarrel facilities. In the event that the
available green space within the parking area is insufficient to provide for required
storage volume (as computed in Section 5), additional space can be obtained by providing
bioretention within the landscaped buffer area located on the perimeter of the commercial
or industrial site (see Figure 4.3). For example, retention storage volume could be
located within required commercial and industrial site setback areas or landscape buffers.
Existing minimum green space requirements plus the size of perimeter buffers and
parking space requirements will dictate the feasibility of providing all required storage
within surface swales, terraces, or bioretention facilities (refer to the Mint Hill Zoning
ordinance and the landscape details for specific requirements).
4.6 Methods for Additional Detention Storage
In some cases, it may be necessary to provide for additional detention storage to augment
retention in maintaining the pre-development storm water runoff peak discharge rate.
The following LID practices can be used for detention within residential or commercial/
industrial sites:
15

Mint Hill Water Quality Design Manual Date of Adoption
• swales with check dams;
• restricted drainage pipe and inlet entrances;
• wider swales and terraces;
• rain barrels;
• parking lot storage;
• rooftop storage; and
• diversion structures.
Additional planning considerations for swales include:
• Open drainage conveyance (swales or natural drainage ways) should be
used to the extent allowed by existing ordinances. Terraces also can be
designed for and used as detention. Drainage along primary roads must be
contained in piped storm drains.
• Use 4:1 slopes for roadway swales out of the public right-of--way. These
slopes, once graded, are to be stabilized with fiber mats or netting and then
planted with perennials or wild flowers or dense ground cover or woody
shrubs. The use of 4:1 slopes will minimize disturbance and preserve
more existing trees. The slope of the graded swale to the building pad
must be 1 percent minimum.
• Locate on-lot swale facilities where they can provide green space
connection between existing wooded or natural areas.
Woodlands
it-----f
~- _~
t .-~~~``~ "i
15' Wide Bioretention Skrip
- -;~
_,
~ ,
i
,_~ ~~~ ~ `t , i
' ~~,1_j ~
,~.
Building A
~,
S ; '~~ 1
n l . `/
r
~ S .. -
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.!~ f
1 .`~L% 1
;.
;i; Inkerior i3ioretention Area
.~
~~E
.~_-
Toil Area of Parking Lot
rigure 4.3. Providing Bioretention Within Landscaped Buffer Areas
4.7 Maintaining Existing Time of Concentration (Tc)
16

Mint Hill Water Quality Design Manual Date of Adoption
The time of concentration (Tc), in conjunction with the CN, determines the peak
discharge rate for a storm event. From theoretical considerations, site and infrastructure
components that affect time of concentration and travel time include:
• travel distance (flow path);
• slope of the ground surface and/or water surface;
• ground surface roughness; and
• channel shape and pattern.
These concepts are applied to LID by using techniques to control the Tc by modifying the
following aspects of flow and conveyance within the development:
• maximize sheet flow;
• modify/lengthen flow path;
• site and lot slopes;
• open swale BMP; and
• site and lot vegetation.
Overland Sheet Flow. The site should be graded to maximize the overland sheet flow
distance and to minimize disturbance of woodland along the Tc flow path. This practice
will increase travel times thereby increasing the time of concentration and ultimately the
peak discharge rate will be decreased. To provide sufficient contact time and allow for
settlement of suspended solids, flow velocity in areas graded to natural drainage ways
should not exceed 1 fps to the extent practicable. Install a stable, level spreader (timber
tie, edging, etc.) along the upland edge of the natural drainage way buffer, or create a flat
grassy area about 30 feet wide on the upland side of the buffer where runoff can spread
out. This grassy area can be incorporated into the Upland Zone of the S.W.I.M. buffer;
however, level spreaders must be located outside the buffer.
Flow Path. Increase flow path or travel distance of surface runoff to increase infiltration
and travel time. One of the challenges of LID is to provide as much overland or sheet
flow as possible to increase the time it takes for rooftop and driveway runoff to reach
open Swale drainage features or in some cases piped storm drains. Typically, the
designer can perform one or both of two potential techniques for accomplishing this task.
First, rooftop and driveway runoff can be permanently infiltrated or stored within
infiltration trenches, dry wells, or cisterns strategically located to capture the runoff prior
to it reaching the lawn. Second, strategic lot grading can increase both the surface
roughness and the travel length of the runoff.
Site and lot slopes. Flatten lot slopes to approach a minimum 1 percent to increase
infiltration and travel time. The building pad area is a 10-foot perimeter around the
building with a positive drainage in accordance with the building code, or a minimum of
five percent (5%) slope. Lot areas outside the building pad perimeter should contain a
positive slope of at least 1 percent. As described in the Charlotte-Mecklenburg Storm
Water Design Manual, the designer is responsible for ensuring that the slope of the lot
does not cause flooding during the 100-year storm event. Soil compaction of original
soils (not fill) in the lot area should be avoided to maximize the infiltration capacity of
the soil. These infiltration areas can receive runoff from impervious surfaces such as
rooftops and driveways to decrease travel times for these areas.
17

Mint Hill Water Quality Design Manual Date of Adoption
Open Swales. Wherever possible, LID aims to use open swales in lieu of more
conventional storm drain structures. To alleviate flooding problems and reduce the need
for conventional storm drain systems wherever possible vegetated or grassed open
drainage BMPs should be provided as the primary means of conveying surface runoff
between lots and along roadways. Lots should be graded so as to minimize the quantity
and velocity of surface runoff within the open drainage BMPs. On-line infiltration BMPs
and terraces can be used to reduce the quantity and travel time of the surface runoff as the
need arises. At no time shall 5 cfs be exceeded.
Site and Lot Vegetation. Re-vegetate and/or plant graded areas to promote natural
retention and increase travel time. Re-vegetating graded areas, planting, or better yet,
preserving existing vegetation can reduce the peak discharge rate by creating added
surface roughness as well as providing for additional retention and reducing the surface
water runoff volume. Designers can connect vegetated buffer areas with existing
vegetation or forest to gain retention/detention credit for runoff volume and peak rate
reduction and to avoid "paved area" as the Tc flow path for the "shallow concentrated
flow" part of the Tc calculation. The benefit of such practices will be to minimize the
need for on-lot bioretention facilities.
Section 5. LID H drolo is Anal sis
5.1 Introduction
The purpose of this Section is to provide LID hydrologic analysis and computational
procedures for use in determining LID storm water management requirements. Design
concepts are illustrated by the use of runoff hydrographs that represent responses to both
conventional and LID approaches. LID site planning and BMPs are defined and
categorized into components of LID objectives. Computational procedures for
determining BMP requirements are demonstrated through design examples located in
Appendix E. A strategy for using these techniques is provided in Section 4. The process
for developing the LID hydrology is illustrated in Figure 5.1. This figure lists the
sequential steps and the sections in the manual where the methods to calculate or
determine the specific requirements are provided.
5.2 Hydrologic Comparison Between Conventional and LID Approaches
Conventional storm water conveyance systems are designed to collect, convey, and
discharge runoff as efficiently as possible. Conventional storm water management BMPs
are typically sited at the most downstream point of the entire site (end-of--pipe control).
The storm water management requirement is usually to maintain the peak runoff rates at
pre-development levels for a particular design storm event. Figure 5.2 illustrates the
hydrologic response of the runoff hydrograph to conventional BMPs.
18

Mint Hill Water Quality Design Manual Date of Adoption
Figure 5.1. LID Analysis Procedure
19

Mint Hill Water Quality Design Manual Date of Adoption
Developed Condition, without BMPs
-Developed Condition, with Conventional CN
Q and BMPs
(Peak discharge ~~~"~
;I
cubic feet per =''
~;
second) ' ~ 2 '
Additional Runoff Volume
~/
i3
;' ~Predevelopment ~' `"'
T (Time)
Figure 5.2. Hydrologic Response of Conventional BMPs
Hydrograph 1 represents the response to a given storm of a site that is in a
pre-development condition (e.g., woods, meadow). The hydrograph is
defined by a gradual rise and fall of the peak discharge and volume.
Hydrograph 2 represents the response of apost-development condition
with no storm water management BMPs. This hydrograph definition
reflects a shorter time of concentration (Tc) and higher runoff curve
number (CN) than that of the pre-development condition, a rapid decrease
in the time to reach the peak runoff rate, a significant increase in the peak
runoff discharge rate and volume, and increased duration of the discharge
volume.
Hydrograph 3represents apost-development condition with conventional
storm water BMPs, such as a detention pond. Although the peak runoff
rate is the same, the hydrograph exhibits significant increases in the runoff
volume and duration of runoff from the pre-development condition.
In comparison with conventional storm water management, the objective of LID
hydrologic design is to retain the post-development excess runoff volume in discrete units
throughout the site to emulate the pre-development hydrologic regime. Management of
both runoff volume and peak runoff rate is included in the design. The approach is to
manage runoff at the source rather than at the end of pipe. Preserving the hydrologic
regime of the pre-development condition requires both structural and nonstructural
techniques to compensate for the hydrologic alterations of development. Typically
alterations to the hydrologic regime as a result of development include, but are not
limited to, the following:
Increased runoff volume and velocity;
Increased flow frequency, duration, and peak runoff rate;
20

Mint Hill Water Quality Design Manual Date of Adoption
• Reduced infiltration (ground water recharge);
• Modification of the flow pattern;
• Faster time to peak, due to shorter Tc through storm drain features; and
• Loss of storage.
In LID, the design approach is to leave as many undisturbed areas as practicable to
reduce runoff volume and runoff rates by maximizing infiltration capacity. Storm water
management BMPs are then integrated throughout the site to compensate for the
hydrologic alterations of development. The approach of maintaining areas of high
infiltration and low runoff potential in combination with small, source control storm
water management BMPs creates a "hydrologically functional landscape." This
functional landscape not only helps maintain the pre-development hydrologic regime but
also enhances the aesthetic and habitat value of the site. Figure 5.3 illustrates a
comparison of LID and conventional BMPs:
For hydrograph 1, refer to Figure 5.2 for description.
For hydrograph 3, refer to Figure 5.2 for description.
Hydrograph 4 represents the response of apost-development condition
that incorporates LID storm water management. LID uses undisturbed
areas and smaller retention storage areas distributed throughout the site
(on-lot or in common areas) to reduce runoff volume. The peak runoff
rate and volume remain the same as the pre-development condition
through the use of common area or on-lot retention and/or detention. The
frequency and duration of the runoff are also much closer to the existing
condition than those typical of conventional BMPs.
Predevelopment peak discharge
Q -Conventional BMP Controls
;` ~`, LID Concepts
i
• r ~=
~•.
~ • ~:
e . ~'•
•i ~~'
i,
is ~ ~'•
O 3
~~ ~ .
4
•,
T
Figure 5.3. Comparison of the Hydrologic Response of Conventional and LID BMPs
21

Mint Hill Water Quality Design Manual Date of Adoption
5.3 Key LID Hydrologic Definitions
The LID "functional landscape" emulates the pre-development temporary storage
(detention) and infiltration (retention) functions of the site. This functional landscape is
designed to mimic the pre-development hydrologic conditions through runoff volume
control, peak runoff rate control, flow frequency/duration control, and water quality
control.
Runoff Volume Control: The pre-development volume is maintained by
a combination of minimizing the site disturbance from the pre-
development to the post-development condition and then providing
distributed retention BMPs. Retention BMPs are structures that retain the
runoff for the design storm event. A "customized" or detailed CN
evaluation is required to determine the required runoff volume. The
storage required to maintain the pre-development volume may also be
sufficient to maintain the pre-development peak rate. This storage volume
determination is based on developed design charts and nomographs, which
are included in the Appendices B, C, and D.
Peak Runoff Rate Control: LID is designed to maintain the pre-
developmentpeak runoff discharge rate for the selected design storm
events. This is done by maintaining the pre-development Tc and then
using retention and/or detention BMPs (e.g., rain gardens, open drainage
BMPs, etc.) that are distributed throughout the site. The goal is to first use
retention practices to control runoff volume and, if these retention
practices are not sufficient to control the peak runoff rate, to then use
additional detention practices to control the peak runoff rate. Detention is
temporary storage that releases excess runoff at a controlled rate. The use
of a combination of retention and detention to control the peak runoff rate
is defined as the hybrid approach.
Flow Frequency/Duration Control: Since LID is designed to emulate the
pre-development hydrologic regime through both volume and peak runoff
rate controls, the flow frequency and duration for the post-development
conditions will be almost identical to those for the pre-development
conditions (see Figure 5.3). Thus, the impacts on the sediment and
erosion and stream habitat potential at downstream reaches can then be
minimized.
Water Quality Control: LID is designed to provide water quality
treatment of runoff from the first 1 inch of rainfall using retention
practices. Storm water treatment shall be designed to achieve average
annual 85% Total Suspended Solids (TSS) removal and must apply to the
volume of post-construction runoff. Drawdown time for this treated
volume of runoff shall be a minimum of 2 days. The storage required for
water quality control is compared to the storage required to control the
increased runoff volume. The greater of the two volumes is the required
retention storage. LID also provides pollution prevention by modifying
human activities to reduce the introduction of pollutants into the
22

Mint Hill Water Quality Design Manual Date of Adoption
environment. LID practices also aid in cooling runoff from developed
sites thus lessening thermal peaks in receiving streams.
The low-impact analysis and design approach focuses on the following hydrologic
analysis and design components:
• CN: Minimizing change in the post-development CN by reducing
impervious areas and preserving more trees and meadows to reduce the
storage requirements to maintain the pre-development runoff volume.
• Tc: Maintaining the pre-development Tc by minimizing the increase of
the peak runoff rate after development by lengthening and flattening flow
paths and reducing the length of the piped runoff conveyance systems.
• Retention: Providing retention storage for volume and peak control, as
well as water quality control, to maintain the same storage volume as the
pre-development condition.
• Detention: Providing additional detention storage, if required, to maintain
the same peak runoff rate and/or prevent flooding and erosion
downstream.
Table 5.1 provides a summary of LID techniques that affect these components.
Table 5.1. LID Techniques and Hydrologic
Design and Analysis Components
LID Techni ue

~ g?
c~ ~
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o
Low-Impact Hydrologic ~ ~ ~ ~ '~ ~ ~ ~ '~ ~ ~ ~ ~ ~ °~ o,o
Design and Analysis ~ ~ ~ ~ ~ c+; °~ o ~ ~' ~ ~~ o o
Com onents w .~ S S ~ w .~ > U ,
Q .~ a: r.~ ci: as r~ >
Lower Post-Develo ment CN
Increase Tc
Retention
Detention
5.4 Hydrologic Evaluation
The purpose of the hydrologic evaluation is to determine storm water management
requirements for LID sites. The evaluation method is used to determine the amount of
retention and/or detention to control the runoff volume and peak runoff rate. Appropriate
detention and/or retention techniques are then selected to meet these requirements.
23

Mint Hill Water Quality Design Manual Date of Adoption
5.4.1 LID Runoff Curve Number (CN)
Calculation of the LID CN is based on a detailed evaluation of the existing and proposed
land cover so that an accurate representation of the potential for runoff can be obtained.
This calculation requires the designer to investigate key parameters associated with a
LID:
• Land cover type;
• Percentage of and connectivity of impervious areas;
• Hydrologic soils group (HSG); and
• Hydrologic condition (average moisture or runoff condition).
Comparing conventional and LID CN calculations, the conventional CNs are based on
the land cover assumptions whereas the LID CN is based on a detailed evaluation of the
land cover and parameters listed above. For example, as illustrated in Figure 5.4,
customizing the CN for a LID site allows the developer/engineer to take advantage of and
get credit for such LID site planning practices as the following:
• Narrower driveways and roads (minimizing impervious areas);
• Maximized tree preservation or re-forestation (tree planting);
• Site fingerprinting (minimal disturbance);
• Open drainage swales, sheet flow, maintain natural drainage patterns;
• Preservation of soils with high infiltration rates to reduce CN; and
• Location of BMPs on high infiltration soils.
Ex. woods
Lawn Area
Lawn Area
Hose- _
_ l-louse
~
> ~
~ ~ Biorefention
~ Detention Area
~ If Required
0 ~. _ Drainage
-

Curb & Gutter '.
~` ---._-..~ Swale
~-. _- --- -
Reduced Road Widih
Conventional CN Typical Low-Impact
for 1-Acre Lot Development Lot
N.T.S. N,T,S.
Figure 5.4. Comparison of Land Covers Between Conventional and LID CNs
Table 5.2 illustrates a comparison of LID CN land covers with those of a conventional
CN for a typical 1-acre lot. Figure 5.4 illustrates a comparison of conventional CN land
covers with a LID customized CN fora 1-acre lot.
24

Mint Hill Water Quality Design Manual Date of Adoption
Table 5.2. Comparison of Conventional and LID Land Covers
Conventional Land Covers
TR-55 Assum tions LID Land Covers
20% impervious 15% imperviousness
80% grass 25% woods
60% rass
Table 5.3 provides a list of LID site planning practices and their relationship to the
components of the LID CN. Key LID techniques that will reduce the post development
CN, and corresponding runoff volumes, are as follows:
• Preservation of Permeable Soils: This approach includes site planning
techniques such as minimizing disturbance of soils, particularly vegetated
areas with high infiltration rates. Additional planning should limit the
placement of infrastructure and impervious areas such as houses, roads,
and buildings on more permeable soils. These areas of permeable soils
should be reserved for infiltration practices. Care must be taken when
determining the suitability of soils for proposed construction practices.
Adequate geotechnical information (in addition to County Soils Maps) is
required for planning practices.
Table 5.3. LID Planning Techniques to Reduce
the Post-Development LID CN
~ = ~
x ~ ~ U O ~ •O

N ~ N
~
'~ ~ ~ sue., m ~ ~ cd ~ N ~ O

4.
^O
N
~ ~
~
~ ~.
~ ~
O
~
~
O
N O
~'"' • ~
'L3 ~ C Ri
N ~ ~ Q„ ~ ~
O ~
• ~ ~
N O
~+'
Suggested Options i
v
y ~ ~ ~ ^°~
3 ~ ~ ~
.~ N
~ N
~ ~ ~ -?
Affectin Curve
g ~ ~ -~
~ '~' ~ -o
~ a o ~ ~ i ss, ~ ~ o
Number a _
~ 3 ~ ~ U ~? ~ a a -b ~ ~ a,
Land Cover Type
Percent of
Im erviousness
Hydrologic Soils
Grou
Hydrologic Condition
Disconnectivity of
Im ervious Area
Stora e & Infiltration
25

Mint Hill Water Quality Design Manual Date of Adoption
Preservation of Existing Natural Vegetation: Woods and other vegetated
areas provide many opportunities for storage and infiltration of runoff. By
maintaining the surface coverage to the greatest extent possible, the
amount of compensatory storage for BMPs is minimized. Naturally
vegetated areas also can be used to provide surface roughness, thereby
increasing the Tc. In addition, plant life functions to filter out and uptake
pollutants, particularly nitrogen, phosphorus and heavy metals.
Minimization of Site Imperviousness: Reducing the amount of
imperviousness on the site will have a significant impact on the amount of
compensatory BMP storage required since there is almost aone-to-one
corresponding relationship between rainfall and runoff for impervious
areas.
Disconnection of Site Imperviousness: Impervious areas are considered
disconnected if they do not connect to a storm drain structure or other
impervious areas through direct or shallow concentrated flow.
Disconnecting and directing impervious areas to sheet flow onto vegetated
or bioretention areas to allow for infiltration results in a direct reduction in
runoff and corresponding storage volume requirements.
Creation of Transition Zones and Bioretention: Transition zones are
vegetated areas that can be used to store and infiltrate runoff from
impervious areas before they discharge from the site. These areas are
located at the sheet or discharge points from graded and impervious areas.
These areas affect the land cover type calculations of the LID CN.
The use of these techniques can provide cost savings to the overall site development and
infrastructure. Figure 5.5 illustrates the hydrologic response using LID to reduce
impervious area and increase the storage volume.
Developed Condition without BMPs
Reduced Q peak
~ Developed Condition, with LID- CN
~ No Controls
~ ~ -Reduced Runoff Volume
Q
' ..
~ ~~
,.
l
i
~ ,
~.
~ ~-Existing
Figure 5.5. Effect of LID CN on the Post-Development Hydrograph Without Storm Water BMP
26

Mint Hill Water Quality Design Manual Date of Adoption
For hydrograph I, refer to Figure 5.2 for description.
For hydrograph 2, refer to Figure 5.2 for description.
Hydrograph 5 represents the resulting post-development hydrograph using
the LID CN only. There is a reduction in both the post-development peak
rate and volume. Section 5.5.3 describes the process and computational
procedure for determining the LID runoff CN.
5.4.2 Maintaining the Pre-development Time of Concentration (Tc)
The LID hydrologic evaluation requires that the post-development time of concentration
(Tc) be greater than or equal to the pre-development Tc. The travel time (Tt) throughout
individual lots or areas should be approximately the same so that the Tc is representative
of the drainage. To maintain the Tc, LID uses the following site planning techniques:
• Maintaining pre-development flow path length by dispersing and
redirecting flows, generally, through open swales and natural drainage
patterns.
• Increasing surface roughness (e.g., preserving woodlands, using vegetated
swales).
• Detaining flows (e.g., open swales, check dams, rain gardens).
• Minimizing disturbance (minimizing compaction and changes to existing
vegetation).
• Flattening grades in impacted areas.
• Disconnecting and dispersing runoff from impervious areas (e.g.,
eliminating curb/gutter and redirecting downspouts).
• Connecting pervious and vegetated areas (e.g., reforestation, tree
planting).
To maintain the pre-development Tc, an iterative process that analyzes different
combinations of the above appropriate techniques may be required. These site planning
techniques are incorporated into the hydrologic analysis computations for post-
development Tc to demonstrate an increase in post-development Tc above conventional
techniques and a corresponding reduction in peak discharge rates. Figure 5.6 illustrates
the hydrologic response to maintaining equal pre-development and post-development Tc.
For hydrograph I, refer to Figure 5.2.
For hydrograph 5, refer to Figure 5.5.
Hydrograph 6 represents the effects of the LID techniques to maintain the
Tc. This effectively shifts the postpeak runoff time to that of the pre-
developmentcondition and lowers the peak runoff rate.
The greatest gains for increasing the Tc in a small watershed can be accomplished by
increasing the Manning's roughness "n" for the initial surface flow at the top of the
watershed and increasing the flow path length for the most hydraulically distant point in
27

Mint Hill Water Quality Design Manual Date of Adoption
Develo ed,
LID-CN -
no control '~ ~ Reduced Qpeak
1 ~
~ ~ ~.
} ~ Developped, LID- CN, No control
~ ~ t ~~ ~ Same Tc as existing condition
Q ~ ~ ,~L ~.
~.' ~~~ ~
~.' ~ ~~ ~~
i .-
~ ~; ~ '. ~ More Runoff Volume than the
~ ~ ; 5 , ~~ predevelopment condition
0~ ~
~ i ~ `'.
ice' ~ '~,
- - -- ~ ~
T
rigure ~.b. 1.,11) Hydrograph that has a Keduced CN and Maintains Tc Without Storm
Water BMPs
the drainage area. After the transition to shallow concentrated flow, additional gains in
Tc can be accomplished by:
• Decreasing the slope;
• Increasing the flow length; and
• Directing flow over pervious areas.
In LID sites, the amount of flow in closed channels (pipes) should be minimized to the
greatest extent possible. Swales and open channels should be designed with the
following features:
• Increase surface roughness to retard velocity.
• Use a network of wider and flatter swales and channels to avoid fast-
moving flow (maximum Scfs for swales during a 10-year, 24-hour storm
event).
• Increase channel flow path.
• Maximize Swale and channel width to increase storage capacity.
• Reduce channel gradients to decrease velocity .
• The Swale and channel should flow over pervious soils wherever possible
to increase infiltration and reduce runoff to maximize infiltration capacity.
Table 5.4 identifies LID techniques and volume objectives to maintain pre-development
Tc.
28

Mint Hill Water Quality Design Manual Date of Adoption
Table 5.4. LID Techniques to Maintain the Pre-development Time of Concentration
Low Im act Deve lo ment Techni ue

a' o
v,
~ ~ ~ 0 0

O
fl ~ ~ p ~ yam,, N
~ `n V ~ N ,~ ~ ~
.
. ~ ~ .^. O ~ O y N af '~
~ ue ~ ~ y > O Q

LID Ob'ective ~
O
3 c~
~ ~ s
.
U ~ O
a ~ ¢,
~ 'a, N
Ca ~
v~ N
a ° 4-~
a .~
Minimize disturbance
Flatten rades
Reduce hei ht of slo es
Increase flow path (divert
and redirect
Increase rou hness "n"
5.4.3 Maintaining the Pre-development Curve Number and Runoff Volume
Once the post-development Tc is maintained at the pre-development conditions and the
impact of CN is minimized, any additional reductions in runoff volume must be
accomplished through distributed on-site storm water management techniques. The goal
is to select the appropriate combination of management techniques (see Section 6) that
emulate the hydrologic functions of the pre-development condition to mimic the existing
CN and runoff volume. LID uses retention to accomplish this goal. Placing these
facilities strategically located in common areas or on individual lots will provide volume
controls at the source.
Retention storage allows for a reduction in the post-development volume and the peak
runoff rate. The increased storage and infiltration capacity of retention BMPs allows the
pre-development volume to be maintained. Retention BMPs to maintain the pre-
development CN include, but are not limited to the following:
• Infiltration trenches;
• Retention ponds;
• Rain barrels;
• Bioretention (Rain Garden);
• Irrigation ponds; and
• Rooftop storage.
As the retention storage volume of the LID BMPs is increased, there is a corresponding
decrease in the peak runoff rate in addition to runoff volume reduction. If a sufficient
29

Mint Hill Water Quality Design Manual Date of Adoption
amount of runoff is stored, the peak runoff rate may be reduced to a level at or below the
pre-development runoff rate. This concept is illustrated in Figure 5.7. This storage may
be all that is necessary to control the peak runoff rate when there is a small change in CN.
However, when there is a large change in CN, it may be less practical to achieve flow
control using volume control only.
Hydrograph 7 represents the BMP inflow hydrograph for the post-
development condition for a site using LID. Because of BMP retention
storage, runoff is not released until the maximum retention storage volume
is exceeded. Line A represents the limit of retention storage.
Hydrograph 8 is the outflow hydrograph from the LID retention BMP.
The release begins at the limit of retention storage, represented by line A.
The storage maintains the pre-development volume and controls the peak
runoff rate. For this situation, the falling limb of the hydrograph
represents a condition where the inflow (hydrograph 7) equals the outflow
(hydrograph 8).
Provide storage - ' •O
using retention i~
BMPs so that the ~' `
Predevelopment ~~ ~~
peak runoff _ . _ _ _ _
rate is i~
maintained ~~
i
i'
.'
' ..
Predevelopment
_ _~PeakQ
a
g
J ~
`0
Figure 5.7. Retention Storage Required to Maintain Peak Development Runoff Rate
5.4.4 Potential Requirement for Additional Detention Storage
Even though the post-development Tc and CN are maintained at the pre-development
level, in some cases additional detention storage is needed to maintain the pre-
development peak runoff rate due to the spatial distribution of the retention storage
provided. The amount of storage that maintains the pre-development runoff volume
might not be sufficient to also maintain the pre-development peak runoff rate. Therefore,
additional common areas or on-lot storage is required in detention storage. LID storm
water management techniques for providing detention storage include, but are not limited
to the following:
• Swales with check dams, restricted drainage pipe, and inlet entrances;
30

Mint Hill Water Quality Design Manual Date of Adoption
• Wider swales;
• Rain barrels;
• Rooftop storage; and
• Diversion structures.
The effect of this additional detention storage is illustrated in Figure 5.8.
For hydrograph 1, refer to Figure 5.2.
Hydrograph 9 represents the response of apost-development condition
that incorporates LID retention practices. The amount of retention storage
provided is not large enough to maintain the pre-development peak runoff
discharge rate. Additional detention storage is required.
Hydrograph 10 illustrates the effect of providing additional detention
storage (hybrid design, see page 44) to reduce the post-development peak
discharge rate to pre-development conditions.

Q

:. -
:,
:~ ~ ;'
:, ;
:,
:; 10
Existing
~-.': ,;
.:
. .
Provide additional detention storage
to reduce peak discharges to be equal
- to that of the existing condition,
.~
`~
`~
.;
. °:,.
~~\
•~~.
0,'~:.~
-~ •~.
Figure 5.8. Effect of Additional Detention Storage on LID Retention Practices
5.5 Process and Computational Procedure for LID Hydrologic Analysis
5.5.1 Introduction
The hydrologic analysis of LID is a sequential decision making process that can be
illustrated by the flow chart shown in Figure 5.1. Several iterations may occur within
each step until the appropriate approach to reduce storm water impacts is determined. The
procedures for each step are given in the following Section. Design charts have been
developed to determine the amount of storage required to maintain the existing volume
and peak runoff rates to satisfy storm water management requirements (see Appendices
B, C, and D).
31

Mint Hill Water Quality Design Manual Date of Adoption
5.5.2 Data Collection
The basic information used to develop the LID site plan and used to determine the CN
and Tc for the pre- and post-development condition is the same as conventional site plan
and storm water management approaches.
5.5.3 Determining the LID Runoff Curve Number (CN)
The determination of the LID CN requires a detailed evaluation of each land cover within
the development site. This will allow the designer to take full advantage of the storage
and infiltration characteristics of LID site planning to maintain the CN. This approach
encourages the conservation of more woodlands and the reduction of impervious area to
minimize the need of BMPs. The steps for determining the LID CN are as follows:
Step 1: Determine percentage of each land use/cover.
Because LID design emphasizes minimal site disturbance (tree preservation and site
fingerprinting), it is possible to retain much of the pre-development land cover and CN.
Therefore, it is appropriate to analyze the site as discrete units to determine the CN.
Table 5.5 lists representative land cover CNs used to calculate the composite "custom"
LID CN.
Table 5.5 Representative LID Curve Numbers
Land Use/Cover Curve Number for H drolo is Soil s Grou s'
A B C D
Impervious Area 98 98 98 98
Grass (good condition, >75%) 39 61 74 80
Woods (fair condition) 36 60 73 79
1 Table 2.2, TR-55 (Soil Conservation Service, 1986).
Step 2: Calculate composite custom CN.
The initial composite CN is calculated using a weighted approach based on individual
land covers without considering disconnectivity of the site imperviousness. This is done
using Equation 5.1. This weighted approach is illustrated in Example 5.1.
Equation 5.1.
CNl A~ + CN2 AZ ... + CND A~
CND = A~ + A2 ... + A~
Where:
CNc =composite curve number;
A~ =area of each land cover; and
CND =curve number for each land cover
Overlays of Soil Conservation Service (HSG) boundaries onto homogeneous land cover
32

Mint Hill Water Quality Design Manual Date of Adoption
areas are used to develop the LID CN. What is unique about the LID custom made CN
technique is the way this overlaid information is analyzed as small discrete units that
represent the hydrologic condition, rather than a conventional TR-55 approach that is
based on a representative national average. This is appropriate because of the emphasis
on minimal disturbance and retaining site areas that have potential for high storage and
infiltration. This approach provides an incentive to save more trees and maximize the use
of more permeable soils for recharge. Careful planning can result in significant
reductions in post-development runoff volume and corresponding BMP costs.
Step 3: Calculate LID CN based on the connectivity of site impervious area. When the
impervious areas are less than 30 percent of the site, the percentage of the unconnected
impervious areas within the watershed influences the calculation of the CN (Figure 2-4,
Soil Conservation Service, 1986). Disconnected impervious areas are impervious areas
that do not connect to a drainage feature or impervious surface through direct or shallow
concentrated flow. For example, roof drains from houses could be directed onto lawn
areas where sheet flow occurs, instead of to a Swale or driveway where shallow
concentrated flow occurs. By increasing the ratio of disconnected impervious areas to
pervious areas on the site, the CN and resultant runoff volume can be reduced. Equation
5.2 is used to calculate the CN for sites with less than 30% impervious area.
Equation 5.2.
CND =CNp+ oo x(98-CNP)x(1-O.SR)
where:
R =ratio of unconnected impervious area to total impervious area;
CND =composite CN;
CNP =composite pervious CN; and
P;,r,p =percent of impervious site area
Example 5.1 uses steps 1 through 3 to compare the calculation of the curve number using
conventional and LID techniques using the percentages of land cover for a typical 1-acre
residential lot from Figure 5.4.
5.5.4 Development of the Time of Concentration (Tc)
The pre- and post-development calculation of the Tc for LID is exactly the same as that
described in the TR-55 (Soil Conservation Service, 1986) and NEH-4 (Soil Conservation
Service, 1985) manuals. Tc can be maintained through techniques discussed in 5.4.2.
5.5.5 LID Storm Water Management Requirements
Once the CN and Tc are determined for the pre- and post-development conditions, the
storm water management storage volume requirements can be calculated. The LID
objective is to maintain both the pre-development volume, pre-development peak runoff
33

Mint Hill Water Quality Design Manual Date of Adoption
Example 5.1. Detailed CN Calculation
(~' (~`

98 x 4,356 + 98 x 2,178 + 61 x 26,136 + 55 x 10,890
CND _ ~~ ~~,.
10
R=-
15
CND =CNp +( 0~ ~x~98-CNp~x~l-0.5xR)

CND=59.2+(100)x 98-59.2 x 1-0.5x0.67
34

Mint Hill Water Quality Design Manual Date of Adoption
rate, for each storm frequency. The required storage volume is calculated using the
design charts in Appendices B, C, and D. The remaining LID hydrologic analysis
techniques are based on the premise that the post-development Tc is the same as the pre-
development condition. If the post-development Tc does not equal the pre-development
Tc, additional LID site design techniques must be implemented to maintain the Tc. Three
series of design charts are needed to determine the storage volume required to control the
increase in runoff volume and peak runoff rate using retention and detention practices.
The required storages shown in the design charts are presented as a depth in hundredths
of an inch (over the development site). Recommended design depths for BMPs are
provided in Section 6. Equation 5.3 is used to determine the volume required for BMPs.
However, a 6-inch depth is recommended as the maximum depth for bioretention basins
used in LID.
Equation 5.3.
Volume forBMP -_ depth from chart, inches)x (developmentsize, acres
100
The amount, or depth, of runoff lost by infiltration or by the process of evapotranspiration
is not included in the design charts. Reducing surface area requirements through the
consideration of these factors can be determined by using Equation 5.4.
Equation 5.4.
Volume of Site Area for BMP -_ initial volume of site for BMP) x (100 - x
100
where:
x = % of the storage volume infiltrated and/or reduced by evaporation or transpiration.
x% should be minimal (less than 10% is considered).
Storm water management is accomplished by selecting the appropriate BMP, or
combination of BMPs, to satisfy the calculations for surface area and volume
requirements. The design charts to be used to evaluate these requirements are:
Appendix B: Storage Volume Required to Maintain the Pre-development
Runoff Volume Using Retention Storage.
Appendix C: Storage Volume Required to Maintain the Pre-development
Peak Runoff Rate Using 100% Retention.
Appendix D: Storage Volume Required to Maintain the Pre-development
Peak Runoff Rate Using 100% Detention.
These charts are based on the following general conditions:
The land uses for the development are relatively homogeneous.
The storm water management measures are to be distributed evenly across
the development, to the greatest extent possible.
The design storm is based on 1-inch increments. Use linear interpolation
for determining intermediate values.
The procedure to determine the BMP requirements is outlined in Figure 5.9 and described
35

Mint Hill Water Quality Design Manual Date of Adoption
in the following Sections.
Step 1: Determine storage volume required to maintain pre-development volume or
CN using retention storage. The post-development runoff volume generated as a result
of the post-development custom made CN is compared to the pre-development runoff
volume to determine the storage volume required for volume control. Use Chart Series in
Appendix B: Storage Volume Required to Maintain the Pre-development Runoff Volume
Using Retention Storage. The procedure for calculating the depth required for
maintaining runoff volume is provided in Example 5.2. The practical and reasonable use
of the site must be considered. The BMPs must not restrict the use of the site. The
storage volume, expressed as hundredths of an inch, is for volume control only;
additional storage may be required for water quality control. The procedure to account for
the first 1-inch of rainfall, which is the current Performance Criteria, is found in Step 2.
Step 2: Determine storage volume required for Performance Criteria. The runoff
volume, expressed as hundredths of an inch, is then compared to the volume required for
water quality control. The volume requirement for storm water management quality
control is based on the requirement to treat the first 1-inch of rainfall. This volume can
be translated to a percent of the site area by assuming a storage depth of 6 inches. The
procedure for calculating the site area required for quality control is provided in Example
5.3. The greater number is used as the required storage volume to maintain the CN.
From the results of Example 5.3, 0.0011 inches of storage is required for water quality
using retention; from Example 5.2, 0.19 inches of storage is required to maintain the
runoff volume using retention. Since the storage required to maintain the volume is
larger, in this case 0.19 inches over the site should be reserved for retention BMPs using
LID measures.
Step 3: Determine storage volume required to maintain peak storm water runoff
rate using 100 percent retention. The amount of storage required to maintain the pre-
developmentpeak runoff rate is based on the chart in Appendix C. This chart is based on
the relationship between storage volume, 'dS ~ ,and discharge, Q~ , to maintain the
r ~
pre-development peak runoff rate.
Where:
b'S =volume of storage to maintain the predevelopment peak runoff rate using
100% retention;
b'r =post development peak runoffvolume;
Qo =peak outflow discharge rate; and
Q; =peak inflow discharge rate.
The relationship for retention storage to control the peak runoff rate is similar to the
relationship for detention storage. Figure 5.10 is an illustration of the comparison of the
storage volume/discharge relationship for retention and detention. Curve A is the
relationship of storage volume to discharge to maintain the pre-development peak runoff
36

Mint Hill Water Quality Design Manual Date of Adoption
Determine storage volume required to maintain runoff volume or CN.
Use Chart Series in Appendix B: Storage Volume Required to Maintain the Pre-
development Runoff Volume Using Retention Storage (Example 5.2).
Step 2:
Step 3:
Step 4:
5 (use if additional detention storage is
Step 6 (use if additional detention storage is required):
Step 7 (use if additional detention storage is required):
37

38
Mint Hill Water Quality Design Manual Date of Adoption

Mint Hill Water Quality Design Manual
Date of Adoption
~8
d
~ 0.7
0
e
0
~
d 0.6
E
a
°
d
-,
, E
v
d
`
> 0.5
a c
e
e
•a e
f E 0.4
o
,9 a
0
u
~~ ~

0
3
.
0

~M
ro a
v
>
0
h
O 0.2
W
E
Z
0.1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Figure 5.10. Comparison of Retention of Storage Volumes Required to Maintain Peak
Runoff Rate Using Retention and Detention
rate using the detention relationship from Figure 6.1 of the TR-55 Manual (SCS, 1986)
for a Type II 24-hour storm event. Curve B is the ratio of storage volume to discharge to
maintain the pre-development peak runoff rate using 100 percent retention. Note that the
volume required to maintain the peak runoff rate using detention is less than the
requirement for retention. This is graphically demonstrated in Figure 5.11.
• For hydrograph 2, refer to Figure 5.1 for description.
• For hydrograph 10, refer to Figure 5.8 for description.
b'i is the storage volume required to maintain the pre-development peak ratio using
100% detention storage. The combination of ~/1 and d2 is the storage volume required to
maintain the pre-development peak discharge rate using 100% retention storage. The
following calculations apply to Chart Series in Appendix C:
The Tc for the post-development condition is equal to the Tc for the pre-
development condition. This equality can be achieved by techniques such
as maintaining sheet flow lengths, increasing surface roughness,
decreasing the amount and size of storm drain pipes, and decreasing open
channel slopes. Section 5.4.2 provides more details on these techniques.
The depth of storage for the retention structure is 6 inches. For other
depths, see Example 5.2.
If the Tc is equal for the pre-development and post-development conditions, the peak
runoff rate is independent of Tc for retention and detention practices. The difference in
volume required to maintain the pre-development peak runoff rate is practically the same

Ipp~R"reryr/o ~u'yNB
y ' R,
arm Even ~/n ent
~~~%
Ty CU~eA
e
pe ~~ tPn .

Fg r
~E hF~enf
l

39

Mint Hill Water Quality Design Manual Date of Adoption
d1
Q
T
Q peak for existing condition
Postdevelopment condition
with detention pond
Postdevelopment condition
/~ with combination of detention
~....~.!~'~••~..._ storage and retention storage
Figure 5.11. Storage Volume Required to Maintain Peak Runoff Rate
if the Tc's for the pre-development and post-development conditions are the same. These
concepts are illustrated in Figure 5.12. In Figure 5.12 note that the difference in the
required BMP area between a Tc of 0.5 and a Tc of 2.0 is minimal if the pre-development
and post-development Tc's are maintained.
70
~, 60
a
f
m
50
.~
c
~ 40
ro
u
Q
v 30
0
v
eo
20
v
u
u
a
10
0
55 60 65 70 75 80 85
Postdevelopment Curve Number
Figure 5.12. Comparison of Storage Volumes for Various Tcs
90 95
40

Mint Hill Water Quality Design Manual Date of Adoption
Step 4: Determine whether additional detention storage is required to maintain the
pre-development peak runoff rate. The storage volume required to maintain the pre-
development volume using retention, as calculated in Step 1, might or might not be adequate to
maintain both the pre-development volume and peak runoff rate. As the CNs diverge, the storage
requirement to maintain the volume is much greater than the storage volume required to maintain
the peak runoff rate. As the CNs converge, however, the storage required to maintain the peak
runoff rate is greater than that required to maintain the volume. Additional detention storage will
be required if the storage volume required to maintain the runoff volume (determined in Step 1) is
less than the storage volume required to maintain the pre-development peak runoff rate using 100
percent retention (determined in Step 3). The combination of retention and detention practices is
defined as a hybrid BMP. The procedure for determining the storage volume required for the
hybrid approach is described in Step 5. Table 5.6 illustrates the percentage of site area required
for volume and peak control for representative curve numbers. For example, using a 5-inch Type
II 24-hour storm event, with apre-development CN of 60, the following relationships exist:
For apost-development CN of 65, 5.9 percent of the site area (column 4)
is required for retention practices to maintain the pre-development
volume. To maintain the pre-development peak runoff rate (column 5),
9.5 percent of the site is required. Therefore, additional detention storage
or a hybrid approach (calculated in column 7) is required.
For apost-development CN of 90, 42.9 percent of the site area (column 4)
is required for retention practices to maintain the pre-development
volume. To maintain the pre-development peak runoff rate (column 5)
37.2 percent of the site is required. Therefore, the storage required to
maintain the runoff volume is also adequate to maintain the peak runoff
rate. However, 42.9 percent of the site for BMPs is not a practical and
reasonable use of the site. Refer to Step 7, hybrid approach, for a more
reasonable combination of retention and detention storage.
Step 5: Determine storage required to maintain pre-development peak runoff rate
using 100 percent detention. (This step is required if additional detention storage is
needed.) Charts contained in Appendix D: Storage Volume Required to Maintain the
Pre-development Peak Runoff Rate Using 100% Detention are used to determine the
amount of storage to maintain the peak runoff rate only. This information is needed to
determine the amount of detention storage required for hybrid design, or where site
limitations prevent the use of retention storage to maintain runoff volume. This includes
sites that have severely limited soils for infiltration or retention practices. The procedure
to determine the site area is the same as that of Step 3. Using chart contained in
Appendix D, the following assumptions apply:
The Tc for the post-development condition is equal to the Tc for the pre-
development condition.
The depth of storage for the detention structure is 6 inches. For other
depths, see Example 5.2.
The storage volume, expressed as a depth in hundredths of an inch, is for
peak flow control.
41

Mint Hill Water Quality Design Manual
Date of Adoption
Table 5.6. Representative Percentages of Site Required for Volume and Peak Control
Runoff C urve No. % of Area Needed for B MP Hybrid ~ Percent of
Design Volume
Type of Volume Control Peak Control Peak Control (Eq. 5.6) Retention
24-Hour Using 100% Using 100% Using 100% (7) for Hybrid
Storm Retention Retention Detention ~ Design
Event Existing Proposed Appendix B Appendix C Appendix D i (Eq. 5.5)
(1) 2 3 4 5 6 ~ 8
55 1.7 1.6 0.9 1.7 ! 100
60 4.0 3.4 2.4 4.0 100
50 65 6.9 6.2 4.5 6.9 100
70 10.4 9.3 7.3 10.4 100
80 19.3 18.0 15.8 19.3 100
65 2.9 3.9 2.3 3.6 ~ 80
70 6.3 6.7 4.4 6.6 96
60 75 10.5 10.0 7.1 10.5 100
3„ 90 27.5 24.9 18.7 27.5 100
75 4.1 5.9 3.4 5.3 '~ 77
80 8.9 9.7 5.8 9.5 94
70 85 14.6 13.9 8.8 14.6 100
90 21.2 18.7 12.6 21.2 100
80 4.8 7.5 4.2 6.6 73
75 85 10.5 11.8 7.0 11.4 91
90 17.1 16.6 10.2 17.1 j 100
55 4.8 6.9 4.0 6.3 77
60 10.1 11.1 6.9 10.9 I 93
50 65 16.0 15.6 10.4 16.0 ~ 100
70 22.4 20.6 14.5 22.4 100
80 36.7 32.8 23.9 36.7 100
65 5.9 ___
9.5 5.3 8.3 71
70 12.3 14.6 8.4 13.9 88
60 75 19.1 19.8 12.0 19.6 I
97
5„ 90 42.9 37.2 25.3 42.9 100
75 6.9 13.2 7.2 10.9 63
80 14.3 18.9 10.7 17.4 82
70 85 22.2 24.5 14.3 23.8 93
90 30.7 30.5 18.2 30.7 100
80 7.4 15.0 8.1 12.3 60
75 85 15.3 20.6 11.6 18.9 81
90 23.8 26.7 15.2 25.7 92
55 7.6 12.3 6.8 10.7 I 71
60 15.6 18.6 10.7 17.7 88
50 65 23.9 25.0 15.1 24.7 97
70 32.5 31.4 19.6 32.5 100
80 50.5 44.5 30.0 50.5 i 100
65 8.3 16.6 9.0 13.6 ~ 61
70 16.9 23.2 13.2 21.2 80
60 75 25.8 29.9 17.3 28.7 90
7„ 90 53.7 49.7 30.7 53.7 100
75 8.9 20.4 10.9 16.1 55
80 17.9 26.8 14.7 23.8 ~, 75
70 85 27.2 33.4 18.9 31.5 87
90 36.7 42.3 23.0 39.2 94
80 9.1 22.1 11.5 17.1 , 53
75 85 18.4 28.6 15.6 25.1 ' 73
90 27.9 35.3 19.8 32.9 ~ 85
42

Mint Hill Water Quality Design Manual Date of Adoption
These charts are based on the relationship and calculations from Figure 5.1 (Approximate
Detention Basin Routing for Rainfall Types I, IA, II and III) in TR-55 (SCS, 1986).
Step 6: Use hybrid facility design (required for additional detention storage).
When the storage volume for peak control exceeds that for volume control as
determined in Step 3, a hybrid approach must be used. For example, a dry Swale
(infiltration and retention) may incorporate additional detention storage. Equation 5.5 is
used to determine the ratio of retention to total storage. Equation 5.6 is then used to
determine the additional storage volume, above the storage volume required for runoff
volume control, needed to maintain the pre-development peak runoff rate.
Equation 5.5.
x - 50 x (-dDloo + d2Dloo + 4 x (dRloo - dDloo) x f/R )
(dR100 -bD100)
where
d R =Storage volume required to maintain pre-development runoff
volume (see Chart Series in Appendix B)
d x~oo =Storage volume required to maintain pre-development peak runoff
rate using 100% retention (see Chart Series in Appendix C)
d D~oo =Storage volume required to maintain pre-development peak runoff
rate using 100% detention (see Chart Series in Appendix D)
x =Area ratio of retention storage to total storage
Equation 5.6.
H= b'Rx (100=x)
where:
H =Hybrid storage volume
Equations 5.5 and 5.6 are based on the following assumptions:
x% of the total storage volume is the retention storage required to maintain
the pre-development CN calculated from the Chart Series in Appendix B:
Storage Volume Required to Maintain Pre-development Volume using
Retention Storage.
There is a linear relationship between the volume of storage required to
maintain the peak pre-development runoff rate using 100% retention and
100% detention (Chart Series in Appendices C and D)
The procedure for calculating hybrid facilities size is shown in Example 5.4.
43

Mint Hill Water Quality Design Manual Date of Adoption
• Existing CN = 60
Step 7: Determine hybrid amount of BMP site area required to maintain peak
runoff rate with partial volume attenuation using hybrid design (required when
retention area is limited).
Site conditions, such as high percentage of site needed for retention storage, poor soil
infiltration rates, or physical constraints, can limit the amount of site area that can be used
for retention practices. For poor soil infiltration rates, bioretention is still an acceptable
alternative, but an underdrain BMP must be installed. In this case, the bioretention basin
is considered detention storage.
When this occurs, the amount available for retention BMPs is less than that required to
maintain the volume, or CN. A variation of the hybrid approach is used to maintain the
peak runoff rate while attenuating as much of the increased runoff volume as possible.
First, the appropriate storage volume that is available for volume control ~b'R'~ is
determined by the designer by analyzing the site constraints. Equation 5.7 is used to
determine the ratio of retention to total storage. Equation 5.8 is then used to determine
the total site BMP area in which the storage volume available for retention practices
(VR') is substituted for the storage volume required to maintain the runoff volume.
44

Mint Hill Water Quality Design Manual Date of Adoption
Equation 5.7
X- 50 x (-d + Ib~ZDloo + 4 x (d d ) x d R~ )
( D100 / R100 D100
1~R100 -~DI00)
where:
`dR =storage volume acceptable for retention BMPs. The total storage with limited
retention storage is:
Equation 5.8
H'='dR'x(100=X')
H'=Hybrid area, with a limited storage volume available for retention BMPs.
Example 5.5 illustrates this approach.
H' = 0.0951001
32.2
H' = 0.30 inches
not runoff volume. Of the 0.30" of ctnraoa n noc» :...._ __-•
45

Mint Hill Water Quality Design Manual Date of Adoption
5.5.6 Determination of Design Storm Event
Conventional storm water management runoff quantity control is generally based on not
exceeding the pre-development peak runoff rate for the 2-year and 10-year 24-hour Type
II storm events. The amount of rainfall used to determine the runoff for the site is
derived from Technical Paper 40 (Department of Commerce, 1963). For Mecklenburg
County, these rainfall amounts are 3.12 and 4.80 inches, respectively. The 2-year storm
event was selected to protect receiving channels from sedimentation and erosion. The
10-year event was selected for adequate flow conveyance considerations. In situations
where there is potential for flooding of structures, the 100-year event is used. For LID
the design storm event shall be the 2-year, 24-hour storm. Post-developed peak and
volume shall not exceed the pre-developed peak and volume.
Section 6. LID Site Desi n Best Mana ement Practices BMPs
6.1 Introduction
In Section 5, the required runoff storage volume, expressed in hundredths of an inch, and
the type of best management practice (BMP) required for its control (i.e., retention,
detention, or hybrid) were determined through a systematic hydrologically-based process.
Section 5 explored site planning considerations for maintaining the curve number (CN)
and time of concentration (Tc), which can be used to achieve the objectives of Section 5.
This Section builds upon the planning strategies presented in Sections 4 and 5 by
providing considerations key to the proper design of the BMPs. The Section begins with
a definition and description of the functions provided by various types of LID BMPs that
are available to the designer and concludes by outlining some consideration for their
design.
6.2 LID BMPs and Their Functions
LID BMPs are sited and designed to provide the following:
• Site hydrologic source controls for Tc, CN, and ground water recharge.
• Small common area or on-lot BMPs that are more easily maintained.
• An added aesthetic value.
• Multiple use of landscaped areas. In some cases, the on-lot or commercial
hydrologic control also can satisfy the Town of Mint Hill's requirements
for green or vegetated buffer space.
LID site design techniques and BMPs can be organized into four major categories, as
follows:
Pollution prevention practices that contribute to the overall improvement
of storm water quality.
46

Mint Hill Water Quality Design Manual Date of Adoption
Runoff prevention measures designed to minimize impacts that would
otherwise change the pre-development curve number (CN) and time of
concentration (Tc).
Retention facilities that store runoff and have no positive outlet, or
release, for the design storm event. Excess runoff is infiltrated, exfiltrated,
or evaporated.
Detention facilities that temporarily store a portion of the increased runoff
volume and release it through a measured outlet.
LID BMPs shall be applied in the following order of preference:
Step One: Incorporate runoff reduction BMPs into the design to minimize changes in the
site hydrology, which result from changes in the CN and Tc .
Step Two: Meet site storm water quality treatment objectives through the use of
retention/detention practices.
Step Three: Use retention BMPs, to the greatest extent possible, for storm water
management volume and peak flow rate control, using the procedures from Section 5.
Step Four: Use detention and hybrid BMPs if required.
Step Five: Use pollution prevention through source reduction and elimination.
Tables 6.1 and 6.2 provide a list of BMPs approved for use to satisfy the Mint Hill
Ordinance as well as the applicable zones for use of the BMPs in Mint Hill, the section of
the Performance Criteria (see Section 3) they are best suited to satisfy and whether they
serve a water quality or volume control function (WQ or VC). It should be noted,
however, that these lists are not all inclusive and that as new innovative BMPs become
available, they will be reviewed and adapted on a case-by-case basis by the Mecklenburg
County Water Quality Program of Land Use and Environmental Services. In some cases,
conventional BMPs (Table 6.2) may be used in conjunction with LID BMPs provided in
Table 6.1. In addition, BMPs placed in series or modifications to BMP designs can be
developed to combine both retention and detention storage. Placing BMPs in series will
provide for the maximum on-lot storm water runoff control (i.e., maximum mitigation of
site development impacts on CN and Tc). This type of design BMP is known as a
"hybrid." Hybrid BMPs can be used to increase the post-development Tc. Table 6.1
gives each LID BMP type and its primary function. The design source for all the BMPs
in Tables 6.1 and 6.2 is the N. C. Department of Environment and Natural Resources,
Storm Water Best Management Practices, April 1999.
The functions listed in Table 6.1 are subdivided into two major categories as follows:
Water Quality: All the BMPs listed in Tables 6.1 and 6.2 provide some
water quality function; however, Section 3(a) of this Manual requires that
only LID BMPs be used to control and treat runoff from the first inch of
rainfall and that they must be designed to achieve average annual 85%
Total Suspended Solids (TSS) removal.
Volume Control: The LID BMPs listed in Table 6.1 or a combination of
LID and conventional storm water management practices as listed in Table
6.2 can be used to satisfy the volume and peak runoff rate control
Performance Criteria described in Sections 3(b) and 3(d) of this Manual.
47

Mint Hill Water Quality Design Manual Date of Adoption
Section 3(c) requires that whenever any storm water BMP employs the use
of a temporary water quality storage pool as a part of its treatment system,
the drawdown time must be a minimum of 48 hours and a maximum of
120 hours. Section 3(e) requires that no one BMP can receive runoff from
an area greater than five (5) acres. However, the total drainage area from
BMPs in series (i.e., integrated) can exceed this five (5) acre maximum.
Some approaches are used to convey storm water while minimizing or eliminating
erosion potential. For example, a swale can be designed to detain, retain, and control
runoff velocities simply by altering its geometry and slope and adding an infiltration
trench beneath the Swale and a weir control structure at the inlet of the culvert beneath the
driveway (see Figure 6.1). The implication of this is that velocities are reduced and
particulate matter is infiltrated or filtered from the runoff prior to its delivery to a
particular BMP or downstream receiving water body. Some conveyance techniques also
can be designed specifically for runoff water quality treatment if that objective is
considered during the concept plan design.
Table 6.1. Approved LID BMPs for Use in Mint Hill
Funci~°
Strategic Clearing & Grading All 3(a) WQ, VC, PC
Reduce Impervious Surfaces All 3(a) WQ, VC, PC
Bioretention (Rain Garden) All 3(a), 3(b) Section 4.0 WQ, VC, PC
Infiltration Trench All 3(a), 3(b) Section 8.0 WQ, VC, PC
Infiltration Swale All 3(a), 3(b) Section 8.0 WQ, VC, PC
Swales All 3(a) Section 5.0 WQ, VC
Swales with Outlet Control R 3(a), 3(a) Section 5.0 WQ, VC, PC
Vegetative Filter Strips & Buffers All 3(a) Section 7.0 WQ, PC
Dry Well, Cistern & Rainbarrel All 3(b) WQ, VC, PC
Porous Paving All 3(b) WQ, VC
Curb & Gutter Elimination R 3(b) WQ, PC
Rooftop Storage C, I 3(b) VC, PC
(1) Applicable Zoning Districts: These are the Zoning Districts where the BMP can be used including
C = Commercial; I = Industrial; R =Residential.
(2) Applicable Performance Criteria: These are the Performance Criteria Section numbers (see Section
3) that the BMP can be used to satisfy.
(3) Design Function: All BMP designs are contained in the N.C. Department of Environment &
Natural Resources, Storm Water Best Management Pratices, April 1999
(4) Functions: These are the dominate functions that the BMPs perform including WQ =Water
Quality; VC =Volume Control, PC =Peak Control.
Table 6.2. Conventional Storm Water BMPs
48

Mint Hill Water Quality Design Manual Date of Adoption
BMP
Funcr:,
Wet Pond All 3(b), 3(d) Section 1.0 WQ, VC, PC
Extended Dry Pond All 3(b), 3(d) Section 6.0 VC, PC
Storm Water Wetlands All 3(b) Section 2.0 WQ, VC, PC
Sand Filter C, I 3(a) Section 3.0 WQ, VC, PC
(1) Applicable Zoning Districts: These are the Zoning Districts where the BMP can be used including
C = Commercial; I = Industrial; R =Residential.
(2) Applicable Performance Criteria: These are the Performance Criteria Section numbers (see Section
3) that the BMP can be used to satisfy.
(3) Design Function: All BMP designs are contained in the N.C. Department of Environment &
Natural Resources, Storm Water Best Management Pratices, April 1999
(4) Functions: These are the dominate functions that the BMPs perform including WQ =Water
Quality; VC =Volume Control, PC =Peak Control.
~. _e._ .. Singte Famlly
._ - ~ ~ "~~ Residential (2 1/3 Aore+)
Lot _
4
j Driveway
``,.; \
Cleanout
-... 1r ... ~ .,,.a , _
..._~.
~-'~ Oriveway'-'+ ~rln
culvert /'^
. _ -~` ~:
-_
To BMP System ~~ ., , " _~~
or Receiving Stream
Pea Gravel
Inlet, Trench
F
A
Qttorn w~~"~,
~ s _ .'
'~^ ~a~ou ar ,,:.... ~ __ ~ Initial IM
r' / / ~ t Point
~ Read Surface ~ a h ravel
-a- p P_LAIy
NITS
Prrvate
Driveway ~
„~ ..............._._._y 1/2 Raund Garr. Metal Pipe Weir,
! ~ Bolted t0 CUivert
%~ Swale
196 to 296 Bottom
Siape A~rerage ~~
Dnveway ,/ --- "•_ ~ ~ '- Depth 1~' `,
Culvert t~ P . -..... ~.;_ T ~ -~'~- ~~____-.~.~~~
~,~
'~~,.,._~~,, _ _. _._. it
~~
Pea ,}, 6 Gravel ~ -.. Fabricated
outran to ~ .~ -~ --~> ( Gravel 1_ _ underarair, P$rmeabia soli RROFILE
Storm Drain No Gravel ar Inlet Syystem with Mixture
Sys#em Pertnratlnns .~n~ F9arfora#ed ~~~-
Under PVC Pipe
Driveway
Bottom YVidth
i0 Year ,~ _-~28 feet) ,~ $houlcler
Oep#h * Paving
r. -~_.. ~ _., m
. __-
Flatter ~-- 2' 6' 6' Freeboard
~ ~i---' ~ ~ 1NQY or
t. _ ,~: . - ~'. z Year depth SECTIQN AA
at SaiirGravei ~ ~' ~ (3ravelf ipe ~~~
ir~r R~to-f ill Underdrain Syst m
gn
49

Mint Hill Water Quality Design Manual Date of Adoption
6.3 Runoff Reduction BMPs
Runoff reduction BMPs are site development practices that are implemented to reduce
changes in storm water runoff volume and peak flow rate. This is accomplished by
minimizing changes to pre-development land cover, reducing impervious surfaces, and
preserving trees, especially those found on pervious soils. Runoff reduction BMPs are
the first line of defense to reduce changes in existing land cover and to disconnect
"effective" impervious surfaces. These runoff reduction BMPs may be the least
expensive way to minimize CN impacts and maintain existing Tc. By including these
techniques, the need for retention and detention BMPs to mitigate development
hydrologic impacts is reduced. A brief description of these techniques/practices is
provided below.
Strategic clearing and grading practices are the most effective method of reducing storm
water quantity and erosion impacts on downstream receiving waters and aquatic habitat.
Minimizing clearing and grading within forested and densely vegetated areas is the most
efficient method of reducing changes to the CN. This is due to the natural dispersion and
infiltration function of forests and densely vegetated areas. It is possible to increase the
post-development Tc by flattening the slopes, increasing the flow path, maximizing sheet
flow, and increasing surface roughness on cleared areas.
Vegetated buffers are strips of vegetation, either natural or planted, around sensitive
areas, such as water bodies, wetlands, or highly erodible soils. In addition to protecting
sensitive areas, vegetated strips help to reduce storm water runoff impacts by trapping
sediment and sediment-bound pollutants, encouraging infiltration, and slowing and
dispersing storm water flows over a wide area.
Proper landscaping is one method of mitigating the hydrologic impacts of clearing and
grading. In some cases, a large portion of the site will have to be cleared and graded,
resulting in the loss of much woody vegetation and debris. A carefully designed
landscaping plan can be used to reestablish some of the vegetative functions lost during
this process. Heavily revegetated areas can improve sediment removal, infiltration, and
community aesthetics.
Curb elimination or flat curbs addresses both quantity as well as water quality functions.
When curbs are removed flattened or depressed, site imperviousness is disconnected by
allowing storm water runoff, normally conveyed along the gutter and discharged directly
into a storm drain BMP or receiving water body, to be dispersed to vegetated buffer areas
or roadside swales. This process helps to minimize CN impacts, increase or maintain the
TC, and filter pollutants leaving a given site.
Changes in CN due to clearing and grading and the addition of impervious surfaces result
in changes in storm water runoff volume and peak flow rate. Both the pre- and post-
development CNs must be determined to evaluate the magnitude of change and potential
requirement for retention and detention storage volume. By reducing the change in CN,
the requirement for onsite BMP storage area is reduced. Calculation of the post-
development CN should be based on prototypical lot layouts that represent the LID
design techniques that will be incorporated into the development.
50

Mint Hill Water Quality Design Manual Date of Adoption
6.4 Retention BMPs
LID BMPs are used to satisfy the water quality and storage volume requirements
(Sections 3(a) and 3(b)) calculated in Section 5. They are the preferred method because
they maintain the pre-development runoff volume. Therefore, they are the most effective
method for managing the hydrologic regime. In the hydrologic analysis presented in
Section 5, storage volume for retention is first calculated to maintain pre-development
runoff volume and peak flow rate. These BMPs are sized to retain 100% of the excess
post-development volume; however, under some circumstances additional detention
storage will be required. Ideally, the design goal is to locate retention BMPs at the source
on level ground within individual lots of the development. LID retention BMPs include
standard infiltration-type facilities. They typically are designed and located to provide
retention controls for very small drainage areas serving up to 2 to 5 acres. Retention
BMPs that are applicable to LID include multifunctional landscape areas, bioretention
facilities, dry wells, and roof runoff controls such as rain barrels and cisterns.
In most cases, LID on-lot or common open space retention storage will be feasible for
storm water management in zones which specify '/2-acre lots and larger. However,
situations may arise where the magnitude of change in pre-development and post-
development CNs results in on-lot storage requirements that cannot be accommodated
within available lawn or open space area. Therefore, in these cases, the use of runoff
reduction BMPs is important. In the case of commercial and industrial zoned properties,
the minimum internal landscaping requirements plus the size of perimeter buffers and
perimeter parking landscape will dictate the feasibility of providing all required storage
within surface swales or bioretention facilities. (Refer to the Town of Mint Hill's Zoning
Ordinance and landscape manual for specific requirements). The following provides a
brief description of retention BMPs.
Bioretention: Bioretention BMPs (sometimes referred to as Rain
Gardens) are applicable as retention facilities for treatment of up to 5
acres. Typically they are designed to mimic forested BMPs that naturally
control hydrology through infiltration and evapotranspiration. They are
especially suited to residential and commercial areas where additional
landscaping can provide aesthetic benefits. Figure 6.2 shows a typical
plan and detail for a bioretention cell.
Design Considerations: In addition to design criteria currently listed in
various state and local manuals, the following design considerations
should be followed for LID sites. Bioretention areas may or may not
include an upstream pretreatment area. The decision to use pretreatment
depends on the type of land use in the upstream service area. It is
recommended that bioretention areas incorporate vegetated filter strips as
pretreatment devices. In commercial areas where space is limited, parking
area sweeping is recommended as a pretreatment practice.
Dry Wells: Dry wells are small excavated trenches backfilled with stone.
Dry wells function as infiltration BMPs used to control runoff from
building rooftops. Figure 6.3 shows a typical detail for a dry well.
51

Mint Hill Water Quality Design Manual Date of Adoption
CURB STOPS ~ '} '~' '~ ,~
E ~~ ~. n
''t-~:t. `a;.'s,~.'g.. :{ `t;.'a, .Fpst. `o ,,~_z,x„a'x~,
w~r+r ~rww+rwww+r~w~+t~x+~~-u~~r +rw ~+av~r~r~+rav~r~wwvvuv ~~'
w+w `iWW+YWwwWWw$~Y~Y~Fik+r~Yi~ Y W+YV+f+6Vi~iY+i+iv+Y+YViiwvw ~t>
~r +r~w~rw~~sv~ww~ru~~rv~mu~~ru~+r +r~~v~~~s~W~t~~c~+~wwu+r GRASSFiITER
v ~rv~+~w+~~+~+rwm+r~+i•ww~vwwu~ ~r+~~~rW~r+~WVw:-wwwwv~wv STRIP
+r~s~~~w~r+~+r~r+r+l•~r+~~ruw+rv ~rw?~~~~r~v+~vwL~~sw+rvw+~
~wv?rWV+~wv~wwv~u+rw~rv+~+r m~w+~w~+-+s~ww+~+r~u~~uwu
`L'w'3!'~F `... ~ ...:..:. ... .. ... .. .. .. .; ... .. .. `. 'tau ~.v.us
•xf+~ ~' u
BERM
UNDERDRAtN COLLECTION SYSTEM
PLAN V1E1N
CURB STOPS
6' PONDING _ ~'
2"-3" MUL.G .. _
4' PLANTfI~G SOIL
~~, e,~ y ~ tea. ~,~ *-FILTER FABRIC
LL a~,'~,~ *rp ~ ~.
tgure 6.l. Typical Bioretention BMP
Design Considerations: Dry wells should infiltrate the design storm
within three (3) days from the beginning of the storm. Dry wells should
be located close to runoff sources. However, they must be located a
52

Mint Hill Water Quality Design Manual Date of Adoption
sufficient distance away on the down slope side of the structure and from
building foundations to prevent seepage into basements. The designer
must make sure to site these facilities away from slopes >20%, particularly
when the slope consists of fill materials over native ground. The designer
must also ensure that the overflow is directed to the downslope
bioretention BMPs, swales, or other management areas in a nonerosive
fashion. Conveyance areas should be well vegetated and have slopes of
<5%, or appropriately sized riprap should be used.
j ROOF LEADER

1 SURCHARGE PI PE
~
~__~ SPLASH BLOCK
~' ~""~ CAP WITH SCREW TOP LtD

_ _ ~:
~ _ = _
- ;`
~ _

~ -- ..
, „ ~.1.=

1
I
~
~
-' FILTER I I
I = FABRIC ~ :~;.
'; .
~ 11=
GRAVEL
~
~ 3' - 12'

I 1 ~
-
-'`~
OUNDATON '.. ~--
I ~
' FOOfi ~: ,
• . OBSERVATION WEL
_ ~0
PLATE
- MINIMUMII;
_~ , -
„ _ ' ,

,,
~ ~:=
~ ~~-
..
MINIMUM 2'
BEDROCK OR
HIGH WATER TABLE
Figure 6.3. Typical Dry Well
• Cisterns: Storm water runoff cisterns are roof water management devices
that provide retention storage volume in underground storage tanks. On
lot storage and later reuse of storm water also provides an opportunity for
water conservation and the possibility of reducing water utility costs.
Design Considerations: Cisterns are applicable to residential,
commercial, and industrial LID sites. Due to the size of rooftops and the
amount of imperviousness of the drainage area, increased runoff volume
and peak discharge rates for commercial or industrial sites may require
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larger capacity cisterns. Individual cisterns can be located beneath each
downspout, or storage volume can be provided in one large, common
cistern. Cisterns should be located where readily accessible in case
maintenance or replacement becomes necessary.
Rain Barrels: Rain barrels are low-cost, effective, and easily
maintainable retention devices applicable to both residential and
commercial/industrial LID sites. Rain barrels operate by retaining a
predetermined volume of rooftop runoff (i.e., they provide permanent
storage for a design volume); an overflow pipe provides some detention
beyond the retention capacity of the rain barrel. Figure 6.4 shows a
typical rain barrel. Rain barrels also can be used to store runoff for later
reuse in lawn and garden watering. However, water from rain barrels is
not to be used for potable use.
WN SPOUT
25~BUILDING PAD
®47.
LID wrtH MOLDED ~~
DEPRESSION OVERFLOW '
PIPE o
HOSE
ADAPTS 4'-O" HICK MAX I
EVERGREEN
SHRVBS
y OPE 2~
PLASTIC RAIN BARRELS
NOTE:
T. RAIN BARREL TO BE KEPT AT HALF-FILLED
DURING WINTER MONTHS TO PREVENT BARREL
FROM BREAKING IF WATER IS FROZEN
HOUSE WITH RAIN BARRELS
Scale: Not to Scale
Figure 6.4. Typical Rain Barrel
Design Considerations: Rain water from any type of roofing material
may be directed to rain barrels. To be aesthetically acceptable, rain barrels
can be incorporated into the lot's landscaping plans, or patio or decking
designs. Rain barrels placed at each corner of the front side of the house
should be landscaped for visual screening. Rain barrels are not allowed
within a public right-of--way nor within the required setback, required side
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yard, or within five (5) feet of an established rear yard as regulated by the
Mint Hill Zoning Ordinance. Since all rain barrels will be located on
private property, they must be included in any private storm water
maintenance agreement. Gutters and downspouts are used to convey
water from rooftops to rain barrels. Filtration screens should be used on
gutters to prevent clogging of debris. Rain barrels should be designed so
that complete draining of the BMP is possible. Rain barrels should also be
equipped with a drain spigot that has garden hose threading, suitable for
connection to a drip irrigation BMP. An overflow adapter could be used
to connect two or more barrels to divert surplus water away from
foundations. An overflow outlet must be provided to bypass runoff from
large storm events. Rain barrels must be designed with removable, child-
resistant covers and mosquito screening on water entry holes. The size of
the rain barrel is a function of the rooftop surface area that drains to the
barrel, as well as the inches of rainfall required for retention storage. For
example, one 42-gallon barrel provides 0.5 inch of runoff storage for a
rooftop area of approximately 133 square feet.
6.5 Detention BMPs
LID detention BMPs are used to satisfy the detention storage volume requirement
calculated in Section 5. As noted in Section 5, the storage required to maintain the pre-
development volume using retention might not be adequate to maintain both the pre-
development volume and peak flow rate. Therefore, additional detention storage is
required in the form of LID detention BMPs. For LID sites, detention BMPs are
preferably used in combination with retention BMPs. The ratio of retention storage to
detention storage is calculated in Section 5.5.5, Step 6. Detention structures store the
volume and then release the discharge through a control structure at the pre-development
rate. Some water quality benefit may be realized as pollutants settle out during detention.
The SCS Rational Method is used to determine the discharge in order to size the outlet.
LID detention BMPs include filter strips, grassed swales, level spreaders, and rooftop
storage. (Refer to Table 6.1 for a complete listing.) The following are brief descriptions
of some LID detention BMPs:
Filter Strips: Filter strips are typically bands of close-growing vegetation,
usually grass, planted between pollutant source areas and a downstream
receiving waterbody. They also can be used as outlet or pretreatment
devices for other storm water control practices. For LID sites, a filter strip
should be viewed as only one component in a storm water management
BMP. Filter strips remove pollutants and help mitigate concentrated peak
flows.
Design Considerations: LID filter strips should be planted in
combination with existing natural vegetation to meet the needed filter
width. Usually the minimum width for grassed filter areas is 15 feet,
while that of wooded areas is 35 feet. Depending on specific site
conditions, filter areas may be as wide as 150 feet. The width is based on
a required detention. Filter strips function best when they are level in the
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direction of storm water flow toward the receiving water. This orientation
creates proper sheetflow through the strip, increasing infiltration and
filtering of sediments and other organic solids. To prevent erosion or
channel formation, a level spreader should be situated along the top edge
of the strip.
• Grassed Swales: Grassed swales are earthen channels covered with a
dense growth of a hardy grass, such as tall fescue. Grassed swales are
typically located at the outlets of road culverts, as conveyance between
homes, and as highway medians. Swales provide detention storage. They
cab also provide a water quality benefit if designed for pollutant removal.
Design Considerations: Maximum Swale depth is to be 2.9 feet, with a
maximum head of flow over a private driveway of 1.5 feet. Culvert
structures should have a weir located no closer than 1 foot from the inlet to
control the peak flow rate. LID swales may be designed with a curvilinear
width to maximize storage volume for detention. For LID sites, side
slopes outside aright-of--way may be 2:1. Table 6.3 provided design
considerations for grassed swales.
Table 6.3. Grassed Swale Desien Considerations
Design Storm 10-year storm event
Channel Capacity Swale must be sized to convey the peak discharge of the
10-year storm event.
Soils The permeability (infiltration rate) of the soils will
determine whether a dry or wet Swale can be used. It is
recommended that soils used for dry swales have
infiltration rates of 0.27 - 0.50 inches per hour.
Channel Shape Trapezoidal or parabolic shape recommended.
Bottom Width 2 foot minimum, 6 foot maximum.
Side Slopes 3:1 or flatter.
Channel Longitudinal Slope 1.0% minimum, 6.0% maximum.
Flow Depth 4.0 inches for water quality treatment (maximum).
Manning's n Value 0.15 for water quality treatment (depth < 4 inches) 0.15 -
0.03 for depths between 4 inches and 12 inches, 0.03
minimum for depth 12 inches.
Flow Velocity 1.0 fps for water quality treatment - 5.0 fps for 2-year and
10-year storms.
Length of Channel Length necessary for 10 minute residence time.
Maintenance Routine landscape maintenance required.
• Level Spreaders: A level spreader typically is an outlet designed to
convert concentrated runoff to sheet flow and disperse it uniformly across
a slope to prevent erosion. One type of level spreader is a shallow trench
filled with crushed stone. The lower edge of the level spreader must be
exactly level if the spreader is to work properly. Figure 6.5 shows a
typical rock-filled trench level spreader detail. Timber ties or landscape
edging can also be used.
Design Considerations: Sheet flow, or overland flow, is the movement of
runoff in a thin (usually less than 1 inch in depth) layer over a wide
surface, which begins when water ponded on the surface of the land
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becomes deep enough to overcome surface retention forces. Level
spreaders can be used to convey sheet flow runoff from lawn areas within
graded areas to bioretention facilities and transition areas. They can also
be used to deliver runoff from parking lots and other impervious areas to
infiltration areas. The receiving area of the outlet must be uniformly
sloped and not susceptible to erosion. Particular care must be taken to
construct the outlet lip completely level in a stable, undisturbed soil to
avoid formation of rilling and channeling. Erosion-resistant matting might
be necessary across the outlet lip, depending on expected flows.
Alternative designs to minimize erosion potential include hardened
structures (landscape edging, timber ties), stiff grass edges, and
segmenting of discharge flows into a number of smaller, adjacent
spreaders. Sheet flow should be used over well-vegetated areas,
particularly lawns to achieve additional opportunity for retention and
increase the Tc.
Figure 6.5. Typical Rock Trench Level Spreader
Detention Ponds: In some unique circumstances, LID structures will
need to be augmented with conventional storm water structures such as
dry detention ponds to achieve the Water Quality Target described in
Section 4 (see the Charlotte-Mecklenburg Storm Water Design Manual for
design specifications). Both dry and wet ponds provide detention storage;
however, only wet ponds are effective for water quality purposes.
6.6 Pollution Prevention Practices
Pollution prevention, through public education, implementation of site maintenance and
management plans, and industrial process changes, is an integral part of low-impact site
development. By emphasizing public education and outreach programs, the site
developer can prevent the entrance of pollutants into storm water runoff, thereby
reducing the burden on retention and detention BMPs and enhancing their pollutant
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removal effectiveness. The nature of pollution prevention techniques will complement
existing structural controls and will help promote citizen stewardship and local
participation in environmental restoration and enhancement efforts.
Section 7. Erosion and Sediment Control Considerations for LID
7.1 Introduction
Erosion and sediment control and storm water management are interrelated. In
conventional practice, the design of erosion and sediment controls tend to follow the
design of storm water management control BMPs. It is not uncommon to find all of the
site area directed to a large sediment control pond, located at the low point of the site.
After the site is stabilized, the sediment pond is cleaned out and converted to a storm
water management pond. Mass grading operations often disturb more area than is
necessary and negative water quality impacts can result from increased soil erosion.
In the application of LID technology the designer must be careful and conscious to carry
the LID concepts through to the erosion and sediment control elements of resource
protection. If this aspect is overlooked erosion and sediment control problems will be
encountered. The application of LID concepts and the associated emphasis on
minimizing the areas disturbed, as well as breaking up drainage areas into small
manageable subcatchment areas, is consistent with the basic principles of erosion and
sediment control outlined below.
7.2 Erosion and Sediment Control Principles
The following six (6) basic commonsense principles govern the development and
implementation of a sound erosion and sediment control plan for any land development
activity.
1. Planning. Plan the operation to fit the existing site features including;
topography, soils, drainage ways, and natural vegetation.
2. Scheduling of Operations. Schedule grading and earthmoving operations to
expose the smallest practical area of land for the shortest possible time.
3. Soil Erosion Control. Apply soil erosion control practices as a first line of
defense against offsite damage.
4. Sediment Control. Apply sediment control practices as a second line of defense
against offsite damage.
5. Maintenance. Implement a thorough maintenance program before, during and
after development is completed.
6. Inspection. All erosion and sediment control structures must be inspected at least
once a week and within 24 hours (weekends and holidays included) after any
storm event >.5 inches of rain per 24 hour period (see Mecklenburg County Soil
Erosion and Sedimentation Ordinance).
The following sections describe key elements of the above principles in detail.
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7.3 Principle One: Planning
The first principle of erosion and sediment control is to plan the development to fit the
site features, including; topography, soils, drainage ways, and natural vegetation. It
should be observed that this principle is very similar to the planning guidelines provided
for LID in Section 4 of this design manual. Listed below are key considerations of the
planning element.
Topography. The primary considerations are slope steepness and slope length (see
Example 7.1). Due to the runoff effects, longer and steeper slopes provide for the
greatest erosion potential. The percent of slope can be determined from site topography.
Areas of similar slope can be grouped together to produce a slope area map, which
identifies areas of similar steepness. Slope steepness can be grouped into three or more
general ranges of erosion potential as listed below:
0 - 7 % -Low erosion hazard (buildable, no constraints)
7 - 15 % -Moderate erosion hazard (moderate building constraints, selective clearing)
15 % or over -High erosion hazard (do not disturb, preserve)
Within these slope ranges the greater the slope length, the greater the erosion hazard.
Therefore in determining potential critical areas the site planner should be aware of
excessively long slopes. As a general rule, the erosion hazard will become critical if slope
lengths exceed the following values:
0 - 7 % - 300 feet
7 - 15 % - 150 feet
15 % or over - 75 feet
Example 7.1: Calculation of Slope Steepness and Slope Length
780 ft msl
650 ft msl
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Drainage ways. Natural drainage patterns existing on the site should be identified in
order to plan around these critical areas where water will concentrate. Where it is
possible natural drainage ways should be used to convey runoff over and off the site to
avoid the expense and problems of constructing an artificial drainage BMP. These
natural drainage ways should be protected with vegetative buffers whenever possible or
required.
Man made ditches, diversions, and waterways will erode if they are not properly
stabilized. Care should also be taken to be sure that increased runoff from the site will
not erode or flood the existing natural drainage feature.
Soils. Major soil considerations from an erosion and sediment control standpoint
include; erodibility, permeability, depth to water table and bedrock, and soils with special
hazards including shrink/swell potential or slippage tendencies. The Mecklenburg
County Soil Survey provides data on these properties for the soils found in the county.
Erodibility is a term that describes the vulnerability of a soil to erosion. Soil erodibility is
influenced by the average particle size and gradation (texture), percentage of organic
matter, and soil structure. The most erodible soils generally contain high proportions of
silt and very fine sand. The presence of clay or organic matter tends to decrease soil
erodibility. Clays are sticky and tend to bind soil particles together, which along with
organic matter helps to maintain stable soil structure (aggregates). By combining the
soils information with the topography, drainage, and vegetation on the site, the planner
can determine the critically erodible and sensitive areas that should be avoided if possible
during construction.
Natural Vegetation. Ground cover is the most important factor in terms of preventing
erosion. Any existing vegetation that can be saved will help prevent erosion. Vegetative
cover shields the soil surface from raindrop impact and the root mass holds soil particles
in place. Vegetation can "filter" sediment from runoff. Thus grass "buffer strips" can be
used to remove sediment from surface runoff. Vegetation also slows the velocity of
runoff and helps maintain the infiltration capacity of a soil. Trees and unique vegetation
protect the soil as well as beautifying the site after construction. Where existing
vegetation cannot be saved, the planner should consider staging construction, temporary
seeding, or temporary mulching.
7.4 Principle Two: Scheduling of Operations
The second erosion and sediment control principle is to expose the smallest practical area
of land for the shortest possible time. The reason for this principle is that 1 acre of
exposed land will yield less sediment than 2 acres of exposed land, and an area exposed
for 3 months will yield less sediment than an area exposed for 6 months.
The clearing, grubbing and scalping (mass clearing or grading) of excessively large areas
of land at one time promotes erosion and sedimentation problems. As previously
described in Section 4, Planning for LID, these initial earth disturbing activities should be
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kept to a bare minimum. On the areas where disturbance takes place the site designer
should consider staging construction, temporary seeding and /or temporary mulching as a
technique to reduce erosion. Staging construction involves stabilizing one part of the site
before disturbing another. In this way the entire site is not disturbed at once and the
duration of soil exposure is minimized. Temporary seeding and mulching involves
seeding or mulching areas that would otherwise lie open for long periods of time. The
time of exposure is limited and therefore the erosion hazard is reduced.
7.5 Principle Three: Soil Erosion Control Practices
The third important principle is to apply soil erosion control practices as a first line of
defense against offsite damage. This principle relates to using practices that control
erosion on a disturbed area to prevent excessive sediment from being produced. Control
does not begin with the perimeter sediment trap or basin, rather it begins at the source of
the sediment and extends down to the control structure.
Soil particles become sediment when they are detached and moved from their initial
resting place. This process, which is called erosion, is accomplished for the most part by
the impact of falling raindrops and the energy exerted by moving water and wind. A
reduction in the rate of soil erosion (Soil Loss) is achieved by controlling the
vulnerability of the soil to erosion processes or the vulnerability of the soil to erosion
processes, or the capability of moving water to detach soil particles. In humid regions,
such as Mecklenburg County, this is accomplished through the use of "soil stabilization"
and "runoff control practices".
Soil stabilization practices include a variety of vegetative, chemical, and structural
measures used to shield the soil from the impact of raindrops, or to bind the soil in place,
thus preventing it from being detached by surface runoff or wind erosion. Approved soil
stabilization practices in the North Carolina Erosion and Sediment Control Design
Manual under Section 6.10, Surface Stabilization, and includes the following practices:
• Temporary Seeding
• Permanent Seeding
• Sodding
• Trees, Shrubs, Vines, and Ground Covers
• Mulching
• Riprap
Additional guidance can be found in the Charlotte Mecklenburg Land Development
Standards Manual, Section 30.17.
The use of mulch to achieve temporary stabilization before permanent vegetation is
established is gaining increased attention and recognition. Ongoing research efforts are
confirming the fact that mulching is a very effective method of reducing runoff as well as
removing pollutants from runoff.
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Runoff control practices, in contrast, include a number of measures designed to reduce
the amount of runoff that is generated on a construction site, prevent offsite runoff from
entering the disturbed area, or slow the runoff moving through and exiting the disturbed
area. Drainage system outlet velocities shall not exceed 5 feet per second (fps).
Approved runoff control practices are provided in the following Sections of the North
Carolina Erosion and Sediment Control Planning and Design Manual:
Section 6.2 -Runoff Control Measures, Provides standards for the following practices: 1)
temporary diversions; 2) permanent diversions; 3) diversion dikes (perimeter protection);
and 4) right-of--way diversions (water bars).
Section 6.3 -Runoff Conveyance Measures, provides standards for the following
practices: 1) grass lined channels; 2) riprap channels; and 3) temporary slope drains.
Section 6.4 -Outlet Protection, provides standards for the following practices: 1) level
spreader; and 2) outlet stabilization structure.
Additional guidance is available in the Charlotte-Mecklenburg Land Development
Standards Manual.
7.6 Principle Four: Sediment Control Practices
The fourth principle is to apply sediment control practices as a second line of defense
against offsite damage. Even with the best erosion control plan, some sediment will be
generated and controlling it is the objective of this principle. Whereas soil erosion control
practices are designed to prevent soil particles from being detached, sediment control
involves using practices that prevent the detached particles from leaving the disturbed
area and getting to receiving waterways. This is accomplished by reducing the ability of
surface runoff to transport sediment and by containing the sediment onsite.
Sediment control practices are designed to slow the flow of water by spreading, ponding,
or filtering. By so doing, the ability of the water to transport sediment is reduced, and
sediment settles out of suspension. Commonly used control practices include: 1) the
preservation or installation of vegetated buffer areas downslope of the disturbed area to
slow and filter the runoff, 2) the construction of small depressions or dikes to catch
sediment (particularly coarse-textured material) as close to its point of origin as possible;
and 3) the construction of sediment traps or basins at the perimeter of the disturbed area
to capture additional sediment from the runoff.
The amount of sediment removed from the runoff is mostly dependent upon (1) the speed
at which the water flows through the filter, trap or basin; (2) the length of time the water
is detained; and (3) the size, shape and weight of the sediment particles.
Sediment control practices which have been approved for use in North Carolina are
presented in the North Carolina Erosion and Sediment Control Planning and Design
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Manual under Section 6.60 -Sediment Traps and Barriers, which provides information on
the following control devices:
• Temporary Sediment Trap
• Sediment Basin
• Sediment Fence
• Rock Dam
Section 6.5 -Inlet Protection provides standards for the following practices; 1)
Excavated Drop Inlet Protection, 2) Fabric Drop Inlet Protection, 3) Block and Gravel
Inlet Protection, and 4) Sod Drop Inlet Protection.
Currently, the most frequently used approach to sediment control is simply to direct all
surface runoff into a large sediment basin, which is later cleaned out and converted to a
storm water management pond. While this approach is arguably the simplest and lowest
cost method to control sediment, it often fails to address the other principles described
above and thus may not represent the best way to prevent and control sediment.
One of the underlying concepts of LID technology involves breaking up the drainage
areas of a given site into very small catchment areas and to provide opportunities to
increase the time of concentration and thus reduce peak discharges. Accordingly this
approach will benefit sediment control efforts by diffusing surface flow into many
directions and providing more flexibility in the use of a variety of sediment control
practices.
This approach will provide more opportunity to use silt fences and small traps to control
small catchment areas generally in the range of one to three acres in size. It will also
allow more opportunity to integrate the use of vegetative buffers in sediment control.
When bioretention practices are planned for storm water management, they can first be
used as a small temporary trap, by excavating the top two feet of soil. After the site is
stabilized, the trap and accumulated silt can be removed and the bioretention cell can be
completed. See Section 8.2.1 of this manual for additional information.
7.7 Principles Five and Six: Maintenance and Inspection
The final important control principle is to implement a thorough maintenance and follow
up operation. This principle is vital to the success of an erosion and sediment control
program. A site cannot be controlled effectively without thorough, periodic checks of all
erosion and sediment control practices. When inspections reveal problems, modifications,
repairs, cleaning or other maintenance operations must be performed expeditiously.
Particular attention must be paid to water-handling structures such as; 1) diversions,
2) sediment traps, 3) grade control structures, 4) sediment basins, and 5) areas being
revegetated. Breaches in the structures or areas being revegetated must be repaired
before the next rainfall.
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Section 8. Construction and Maintenance
8.1 Introduction
The effectiveness of an LID system is a function of the design and the construction
techniques employed. Of these two parameters, construction is far more critical at
achieving quality results. Poor construction techniques will cause the best designed
BMP to fail prematurely, usually from sedimentation and/or clogging. To prevent this
from occurring, adequate and proper inspection is required. This Section covers the basic
concepts associated with LID construction and inspection. Refer to the North Carolina
Department of Environment and Natural Resources (NCDENR) Best Management
Practices Manual, April 1999, Section 4.0 for specific bioretention design criteria.
8.2 Permitting and Processing
Typically, the installation of bioretention is a component of the grading and storm water
management permit associated with the development or individual lot. Conceptual storm
water management approval and construction permits have already been obtained by this
point and are not covered here. For specific permitting information, contact Mecklenburg
County Land Use and Environmental Services Agency (LUESA).
8.3 Erosion and Sediment Control Principles for Bioretention Applications
During the construction phase, sedimentation and erosion problems can be greatest due to
exposed earth, clearing and grubbing operations, and equipment soil compaction. For
this reason, erosion and sediment controls are required to contain sediment onsite. For
conventional storm water management design, this meant that the designer simply had to
place a sediment control pond at the lowest point of the property under development.
The sediment basin would then be used for storm water management control after
construction was completed. Sites that incorporate bioretention for stormwater control
require closer attention to detail because drainage areas are reduced and massive site
grading to one low point is discouraged. As a result, grading and sediment control
practices are typically applied on a lot-by-lot basis to minimize the opportunity for soil
transport. The following principles are identified and briefly explained for the user of
this manual.
Principle 1: Planning and phasing. PRIOR to construction and even design, proper
planning for sediment control is needed for each lot. Bioretention is a source BMP that
requires placement within the lot area or common open space area. Therefore, when
laying out the development, the designer must analyze the topography, existing tree cover
to be preserved, the building location and associated setbacks, slope steepness and length,
drainage ways, and soil types.
Principle 2: Schedule of Operations. Expose the smallest area of land for the shortest
possible time. All sediment control devices must be in place prior to the start of the main
construction. At the end of each workday, inspect the devices to be sure of their
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adequacy and safeguard any trenches or excavations. Provide temporary stabilization for
disturbed areas as quickly as possible or as directed by the inspector. Areas that have
been disturbed and are not actively being worked, as well as areas that are on final grade,
must be stabilized within 14 days.
Principle 3: Soil Erosion Control. First line of defense against contamination of the
bioretention area. This would include the installation of on-lot silt fences, diversion
swales, stabilization and runoff control techniques. Make sure that silt fencing is
properly keyed into the ground to prevent undermining. See the Charlotte Mecklenburg
Land Development Standards for guidance.
Principle 4: Sediment Control. Even with the best erosion control techniques, sediment
transport will occur. For this reason, on-lot sediment traps and/or super silt fence control
practices are recommended.
Principle 5: Inspection and Maintenance. Erosion and sediment control practices must
be inspected and maintained on a routine schedule. Accumulated sediment must be
removed on a periodic basis, and inspected for excessive accumulation after every major
storm. Particular attention should be paid to the stabilization of disturbed areas and
integrity of the sediment control devices. All sediment traps must have room for
additional sediment loading capacity. Proper disposal of removed sediment is imperative
to reduce the probability of downstream contamination.
Sedimentation Rates for Bioretention. Although no specific sedimentation studies have
been done on bioretention to determine the rate of accumulated sediment, estimated
vertical settling rates may be derived from Stokes Law. Sedimentation rates will vary
significantly and are a function of the following factors:
• Soil Particle size distribution and load of the influent (affected by land use
activities)
• Retention time in the ponded area (24 hour drawdown time)
• Physical features of the bioretention facility surface and resulting flowpath length
• Surface area
• Water temperature affecting fluid viscosity
• Wind resuspension (minimal effect)
8.4 Construction Technique and Sequencing for Bioretention
8.4.1 Site Preparation and Planning
Most importantly, the erosion and sediment control principles discussed in Section 7
must be followed to insure sediment will not affect the BMP. In the planning and lot
layout phase, the potential bioretention locations are identified. Bioretention facilities
should be located within the development envelope, minimizing the need to clear areas
unnecessarily. Bioretention areas may make use of existing wooded areas, with minimal
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Mint Hill Water Quality Design Manual Date of Adoption
clearing required. It is important to insure that the inflow velocity into the bioretention
facility is dissipated, and dispersed. Level spreaders, sodded swales, forebays, or other
such stilling methods should be employed so as not to displace the mulch or cause
erosion in the facility. Two methods of sediment control techniques are typically applied
to bioretention facilities.
Method One
The first method (most typical) is to avoid disturbing the proposed bioretention area after
the initial rough grading and temporary stabilization has been performed. During the
construction phase, all drainage must be directed away from the BMP location to avoid
excessive sedimentation. Flow can be directed away from the bioretention BMP by
utilizing silt fencing materials and temporary diversion swales that direct flows to small
on-lot silt traps.
Method Two
The second method of erosion and sediment control design allows the area proposed for
the bioretention BMP to be used for the installation of a sediment control structure. If a
sediment control structure is to become a bioretention BMP, the following conditions
must be met:
• The proposed invert of the bioretention BMP must be greater than 1 foot
below the invert for the sediment control structure.
• All remnant sediment must be removed.
• If geotechnical tests show that the in-situ soils meet or exceed the soil
medium guidelines for infiltration rates (see section 8.4.10), no underdrain
will be required, although it is still highly recommended.
• The in-situ soils and ponded sediment materials shall be removed and the
remaining surface scarified to increase the likelihood of adequate
infiltration potential.
8.4.2 Minimize Lot Grading/Clearing
Bioretention facilities should be located within the development envelope, minimizing
the need to clear areas unnecessarily. Bioretention areas may make use of existing
woded areas, without grading the wooded area to install the facility. Grading of any
catchment area draining to the facility should be done sparingly and stabilized
immediately (within 14 days).
8.4.3 Install Sediment Control Devices
Utilizing the approved sediment and erosion control plans, install necessary sediment
control devices to protect the facility from contamination by sediment. Essentially, the
placement of silt fence material around the perimeter should be sufficient to prevent flow
entering the area during construction.
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PERF aTED P UM R a N tD A. VARIES. iH I €7
NOTES;
1. AVOID DISTURBING THE PROPOSED BIORETENTION tRAIN GARDEN) AREA AFTER THE INITIAL ROUGH GRADING
AND TEMPORARY $TABILI2ATlON IBS BEEN PERFORMED
2. EXCAVATE THE RAIN GARDEN TO THE DESIGN DIMENSIONS.
3, UNDER DRAIN SYSTEMS WAY BE Ci~OSED OF A VARIETY OF MATERIALS vttw PVC CPERFGRATED t
NON-PERFORATED) PIPE MATERIAL BEING THE MIST COHM{INLY USED. OTHER PiP£ MATERIALS MAY B£
SUBSTITUTED ONLY WITH THE APPROVAL FROM TF~ DESIGN ENGINEER AND INSPECTOR,
4, SOIL AND GRAVEL COVER OVER THE UNDER DRAIN SHALL BE AT LEAST 2 FEET IN DEPTH.
5. GRAVEL HED MATERIALS (STONE) S12£ SHALL NOT BE GREATER THEN t!2 TO i i!2 IN DIAMETER (BLUE STONE,
DOUBLE VASHED, M57>. DEPTH Of GRAVEL SHALL NDT EXCEED 12 INCHES.
b. FILTER FABRIC MUST I~ET A MIN. PERMEAB1LiTY RATE OF 7S GALIMINJsq. ft, AND SMALL NOT IMPEDE
THE INFILTRATI{R1 RATE OF THE SOIL MEDIUM, NON-V~OVEN FABRIC IS PREFERRED LIVER uDVE~1 AND
INSTALLATION RE4l1IRES AT LEAST I FOIIT OVERLAP AT THE ENDS AND STAKING IN PLACE DURING
CONSTRUCTION AT THE TURNED UP SURFACES..
7, T~tE SOIL MIXTURE FOR USE IN A BIORET£NTIDN FACILITY SHALL CT;#lTAIN BDk SAND AND 20k EIRGANIC
MATERIAL CCOMPOST, TOPSOIL) AND HAVE A PERMEABILIIY Ck5 RANGING FROM I inlhr TO 6 inrhr, THE SOIL
SHQU~D HAVE A off BETVEEN 5.5 TO 6.5. A LABDRATQRY TEST MUST BE PERFt~MED TO VERIFY Tt~
PERMEAHIUTY RATE OF MATERIAL_ SOIL MUST NDT B£ INSTALLED UNT1L ALL ~ THE CONTRIBUtING DRAINAGE
AREA HAS BEEN STABILIZED AWD APPROVED BY THE INSPECTOR. SOIL MIXTURES TO BE INSTALLED IM $ Tp
12 INCN LIFTS TO ENSURE ADEQUATE FILTRATION.
B, MULCH IS TO BE PLACED ON THE SURFACE PpNDiNG AREA DF THE RAIN GARDEN, THE LAYER DF FRESH
MULCH SHOULD NOT EXCEED 3 iNCI~S IN DEPTH.
9. AN OBSERVATIONtCL£ANOUi STANDPIPE MUST BE 1NSTALL£D IN EVERY BIORETENTION FACILITY THAT HAS
A DEPTH GREATER THAM R FEET ANDfpR AN UNDER DRAIN SYSTEM. tHE STANIWIP£ MUST CONSIST OF A
R1G1D NCIN-PERFORATED PVC PIPE, 4 TO 6 INCHES IN DIAMETER. IT SHOULD HE LOCATED IN THE CENTER OF
THE STRUCTURE AND BE CAPPED FLUSHED VITH THE GRROUND ELEVATION OF THE FACILITY. THE TOP OF TW£
WELL SHALL EITHER 9E CAPPED vITN A SCREv OR FLANGE TYPE.
67

Mint Hill Water Quality Design Manual Date of Adoption
8.4.4 Excavation Preparation
Excavate the facility to the design dimensions (see figure 8.1). Excavated materials must
be placed away from the facility sides to avoid contamination and possible side wall
instability. Large tree roots (>2 inches) must be trimmed flush with the side walls in
order to prevent fabric puncturing or tearing during subsequent installation procedures.
The sidewalls of the trench must be roughened where sheared and sealed by heavy
equipment.
8.4.5 Underdrain Specification
Where Underdrains are specified, the following information provides guidance for
underdrain installation. Figure 8.2 shows the installation of a typical underdrain.
Location -Underdrains are typically located at the invert of the bioretention BMP to
capture and remove any filtered water that does not infiltrate into the surrounding soils.
Soil and gravel cover over the underdrain shall be at least 2 feet in depth. Placement of
2 to 3 inches of gravel bedding is recommended beneath the discharge points.
Underdrains must "daylight" or connect to an existing drainage system to achieve
positive flow. Prior to covering the underdrain system, the inspector must observe the
underdrain itself, the connections and the gravel bedding.
Suitable discharge points include:
• Grass swale areas, flush cut with sideslope areas
• Storm drain pipe conveyance system
Underdrain Material Types -Underdrain systems may be composed of a variety of
materials, with PVC pipe material being the most commonly used. PVC pipe comes in 8
to 12 foot sections. Alternative pipe material may include flexible HDPE pipe. Other pipe
materials may be substituted at the designer's prerogative and with the Inspector's
approval.
Connections -Pipe joints and storm drain structure connections must be adequately
sealed to avoid leaks. Pipe sections must be coupled using suitable connection rings and
flanges. Field connections to storm drain structures and pipes must be sealed with
polymer grout material that is capable of adhering to surfaces. The underdrain pipe must
be capped (at structure) until site completion.
Perforations -Perforated PVC pipe sections are available from local hardware and
building supply stores. The perforation locations are not critical for proper operation, as
long as the total opening area exceeds the expected flow capacity of the underdrain itself.
Commonly marketed perforated PVC pipe has'/4 or'/z inch perforations, 6 inches from
center to center, along two or three longitudinal rows. Whether or not the perforations are
placed at the invert of pipe or elsewhere, depend upon the design of the facility.
Typically, the perforations are placed closest to the invert of the pipe to achieve
maximum potential for draining the facility. The perforations can be placed near the top
of the pipe if an anaerobic zone is intended. Water below the perforated portion of the
68

Mint 1-sill Watcr Quality I~esi~w,n Manual Date of Adoption
underdrain will have a tendency to accumulate during periods of saturation. Otherwise,
ils.
An observation/cleanout standpipe must be installed in every bioretention BMP that has
a depth greater than 2 feet and/or an underdrain system. The standpipe will serve two
primary functions: 1) it will indicate how quickly the bioretention BMP dewaters
following a storm; and 2) it provides a maintenance cleanout port.
The observation well must consist of a rigid non-perforated PVC pipe, 4 to 6 inches in
diameter. It should be located in the center of the structure and be capped flush with the
ground elevation of the facility. The top of the well shall be capped with a screw, or
flange type cover to discourage vandalism and tampering. Locking is not necessary.
8.4.7 Gravel Bed
Gravel bed materials are used to protect an underdrain pipe to reduce clogging
potential. Placement of the gravel over the underdrain must be done with care. Avoid
dropping the gravel from a backhoe at elevated heights. Spill gravel gently over the
underdrain and spread manually. The following gravel specifications are used to protect
bioretention underdrains:
• Gravel stone size shall be no greater than '/z to 1'h inches in diameter.
(Blue stone, double washed, #57 stone)
• The use of "pea gravel" in place of geotextile fabric is optional, but
preferred
• Depth of the gravel shall not exceed 12 inches
• River-run, washed gravel is preferred.
69
8.4.6 Observation/Cleanout Standpipe

Mint Hill Water Quality Design Manual Date of Adoption
8.4.8 Pea Gravel Diaphragm
Some specifications for bioretention utilize a geotextile fabric to filter water and soil
before passing through to the underdrain gravel blanket. The use of a pea gravel
diaphragm in place of fabric has gained acceptance because of the reduced likelihood of
blockage. If a pea gravel diaphragm is used in this manner, it should have a minimum
thickness of 3 to 4 inches and a maximum thickness of 8 inches. Where situations permit,
a greater depth may be applied.
8.4.9 Filter Fabric
Filter fabric is needed for two purposes in bioretention facilities: 1) controlling transport
of silt, and 2) controlling the direction of flow. In some designs, the filter fabric
placed on top of the gravel bed is used to control sediment transport into the gravel bed,
which otherwise may become clogged. Figure 8.3 shows filter fabric placement over
underdrain. Filter fabric installations must meet the following specifications:
• Must meet a minimum permeability rate of 75 gal/min/ftZ and shall not
impede the infiltration rate of the soil medium.
• Filter fabric may be placed along the "walls" of the facility to help direct
the water flow downward and to reduce lateral flows.
Non-woven fabric is preferred over the woven variety. Filter fabric
installation requires at least 1-foot overlap at the ends and staking in-place
during construction at the turned up surfaces. After soil is placed over the
filter fabric, excess fabric may be removed by cutting along desired
elevations.
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Mint Hill Water Quality Design Manual Date of Adoption
8.4.10 Soil Preparation and Installation
Soil preparation for bioretention facilities can be performed offsite and transported to the
facility location when ready for installation. Soil preparation can be accomplished by
thoroughly mixing soil components, amendments and additives, as needed utilizing a
backhoe or front-end loader. The soil mixture for use in a bioretention facility should
contain 80% sand (coarse, washed sand with no clay content; preferably creek sand if
available), 20% organic material (such as compost and topsoil ). The mixture should
have a permeability (K) ranging from 1.0 in./hour to 6 in./hour as measured in the field.
The soil mixture should have a pH between 5.5 and 6.5. The clay content of the soil
mixture cannot exceed 6%. Soil must not be installed until all of the contributing
drainage area has been stabilized and approved by the Inspector. Provisions for sediment
control must be installed and maintained as specified in the sediment and erosion control
plans. Soil should not be delivered to the site until excavation has taken place and the
fabric and underdrain system has been installed. Scarification of in-situ soil surfaces by
manually raking to aerate and reduce soil compaction is recommended. Soil
specifications for proposed bioretention sites are required and must be submitted as part
of the Preliminary Plans. Figure 8.4 shows initial placement of amended soil mix.
The County inspector must inspect the soil on-site prior to installation. Installation of
soils must be done in a manner that will ensure adequate filtration. After scarifying the
invert area of the proposed facility, place soil mixture at 8 to12 inch lifts. Lifts are not to
be compacted but are performed in order to reduce the possibility of excessive settlement.
Lifts may be lightly watered to encourage natural compaction. Minimal compaction may
be performed using mechanical equipment (such as a backhoe bucket) to reduce the
possibility of excessive settlement. Removing overfill is easier than adding soil when
attempting to bring the facility to the correct elevation.
Avoid over compaction by allowing time for natural compaction and settlement. No
additional manual compaction of soil is necessary. Rake soil material as needed to level
out. Overfill above the proposed surface invert to accommodate natural settlement to
proper grade. Depending upon the soil material, up to 20% natural compaction may
occur. In order to speedup the natural compaction process, presoaking the placed soil
may be performed. Significant settlement can occur after the first presoak, and additional
settlement may occur subsequent to the initial wetting. If time and construction
scheduling permits, it is preferable to allow natural settlement to occur with the help of
rain events to presoak the soil medium. The surface of the facility does not necessarily
have to be uniform. A slight variation due to settling or mulching application is
acceptable as long as the possible ponding depth does not exceed 6". For areas where
excessive settlement occurs, apply sand to fill spots and cover with mulch as needed.
In-situ (or in-place) soil used for bioretention must also be prepared. Scarification of the
soil surfaces by manually raking to aerate and reduce soil compaction is recommended.
When in-situ soils are being used without an underdrain system, soils investigation/
geotechnical reports are required. A copy of the geotechnical report shall be supplied to
the inspector at the preconstruction meeetng. The report shall include the boring
71

Mint Hill Water Quality Design Manual Uate of /1clu~~tion
location at the bioretention facility and include USDA soil classification, boring log with
penetration depths at least 2' below the proposed facility invert, depth to groundwater or
impervious layer (if present), and infiltration rate of the in-situ soil.The bioretention
facility shall not be placed in service until all of the contributing drainage area has been
stabilized and approved by the inspector. Provisions for sediment control shall be in-
place as specified within the sediment and erosion control plans.
Delivery of materials such as soil medium, plants, gravel, geotextile, and underdrains will
need to be coordinated to avoid stockpiling and contamination problems. Soil materials
should not be delivered until the bioretention facility has been excavated or graded to the
design elevations and geotextile fabrics and underdrain systems are in place. Planting
materials should not be delivered until after the soil medium has had time to settle and
trimmed to the proper grade elevation. Weather and seasonal conditions will also affect
planting requirements.
Figure 8.5 shows V-notch weir, forebay and concrete spillway. These were installed to
provide the ability to monitor flows, dissipate entrance velocity, and provide controlled
release of excess runoff.
72

Mint Ilill Watcr Quality l~csign I~1anual Uarc c~f~ ~iclt~~~ti~~n
w.
~~~ r...
~__N
8.4.11 Plant Preparation and Planting Methodology
When ordering plants to be installed in a bioretention BMP, adequate preparation of the
bedding soils must occur prior to delivery. Timing in relation to season and readiness of
the BMP is very important. The recommended ordering times for plants are early spring
or fall, depending upon the species selected.
Often times, plant materials need to be stockpiled while the BMP is being prepared.
Keeping root balls wet during this period, and providing a shaded storage location will
improve the plants survivability.
The initial density of the planting arrangement will be thick. This is to ensure that
adequate vegetative cover will quickly take hold. Once the plants continue to grow and
spread out, some plants may be removed or divided by the property owner and
transplanted elsewhere in the yard. Refer to Appendix F for specific species
requirements. Selected plants must be able to survive in the 80% sand mixture.
A minimum of three (3) species of trees and three (3) species of shrubs should be
selected to insure diversity. In addition to reducing the potential for monoculture
mortality concerns, a diversity of trees and shrubs with differing rates of transpiration
73

Mint Hill Water Quality Design Manual Date of Adoption
may ensure a more constant rate of evapotranspiration and nutrient and pollutan~ uptake
throughout the growing season. To add interest to the rain garden a variety of punt
material should be selected to provide blooming and color at different times of tie year.
Figure 8.6 illustrates the planting design used at a rain garden installed at Mecklenburg
County Hal Marshall Building, 700 N. Tryon Street in Charlotte in November 2003. The
rain garden is located at the eastern side of the parking lot located at 11`h and College
treets and is available for inspection.
~r~es
~#~. 5pie~tFflc #~art~r~ Cornrnon ~I~rne
t As;errubruur Red hABpte
2 GQ~us car~aols~nsr's '~'or~sf P~r~sy Fart pansy Fted~ud
3 M~~jrrctfira virgdnlaraa Sw~l ~Y hlagnalda
tirut>w~
4 ~deld~r~ ~drarlorda ~~urrirrrdrrgbirri
v ~c~r»us seraecle
B dt$x w9rikrdJ6la - mater
7 drax verticrdrala 'Nirrd~ Red"
t3 dr~x ti+mr~rforda ~aendule`
4 tte'a virgdntce 'Nenr}+'s Ger~gf'
HerbaCet~uS platt#~
tl~ Andr~pogtxa glom~a~tus
ti A~f~rrrova-a»~fdea
i 2 Aster aLrdara~!{vtdUS 'C~ctQ6er 5k~s'
13 Aster t9t8riCrJS
id GdaA~merrtdtre~rrrdatrldtdrxar~
1~ d-fgdi~rnfAkes angustdfotta
16 d-fibtscti~5 cacclrtarrs
1 T drys vr~vrriC~'S
1~ 4erro~fd~9-8 frrrtfcosa
1~3 d~$rrtcr+m virRaf[rrrr 7~eau~ d-feta~'
2tI i~tcrdbecdcda fuJ~idB vsr, t~dgdda
21 Sct~izaclrpri+~rrt scquarir~raa
22 S+dt.triurrr perfotfatrrrn
23 Snted6~r3 rt,+~vss ° Fdrawortcs.
Flurnrnh~gbkd Ct-ethra
Fled 'T`wig Dogwood
t]eclduous FttaMly
~Vtidirrt9r Deciduous HollX
YVeeplr~~ ~`aupcn h~IQUIr
Henryrs {3amal Sweetspire
Bushy Bluesiern
New England drier
I~IY~2r ~db5
Srrarnp Sunllvwer
R±~el Filbiscus
glue Flag Iris
Sur7draps
Nea,ry Mehl Switcl~grass
Mack-eyed Swan
Little E~lue~stem
Cup Plant
Firew~rka Raugh-eared ~~Id~rrrod
74
Hal Marshall Rain Garden Planting Design
Designed By: George Morris, HARP
Frgure 8.6

Mint Hill Water Quality Design Manual Date of Adoption
Shipping of the plant materials is typically the responsibility of the nursery or
landscaping contractor. It is preferable to have the plants shipped directly to the facility
site ready for planting.
Tags. All plant materials shall be tagged for easy identification with the American
Standard for Nursery Stock. Tags shall be checked by the inspector for compliance with
the landscaping planting list shown on the stormwater design plans, or water quality
management plans. Variations of plant type, quantity or quality, requires Mecklenburg
County L.U.E.S.A. approval and may necessitate a plan revision submittal.
Herbaceous ground covers are important to prevent erosion of the mulch and soil layers.
Suitable herbaceous ground covers are identified in Appendix F.
The number of tree and shrub plantings may vary, especially in areas where aesthetics
and visibility are vital to the site development, and should be determined on an individual
site basis. On average, 1000 trees and shrubs should be planted per acre. For example, a
bioretention area measuring 15' x 40' would contain a combination of trees and shrubs
totaling 14 individuals. At installation, trees should be 2-1/2 inches in caliper, and shrubs
3 to 4 feet in height or 18 to 24 inches in spread. Ground cover may be as seed, or
preferably, plugs. The relatively mature size requirements for trees and shrubs are
important to ensure that the installation of plants are readily contributing to the
bioretention process (i.e., evapotranspiration, pollutant uptake).
8.2.12 Installation of Mulching Materials
Mulch should be placed on the surface ponding area of the BMP. The mulch material
should be fresh, shredded hardwood to help retain soil moisture and maximize nutrient
uptake. This type of mulch material also helps resist flotation when BMP is fully
ponded. If "aged" mulch is used, select the shredded type over the chip variety to
minimize floatation and wash-outs. Select your mulch carefully. Mulch cannot contain
fine organics, which have a tendency to create a barrier to infiltration. Do not use pine
straw mulch.
The layer of mulch should not exceed 3" in depth. Greater depths keep plant roots
from making good contact with the soil. Mulch materials should not be mounded around
the base of tree trunks as this practice encourages pests and diseases. The mulch layer
should be placed after the plants and groundcover have been installed. Protect and lift
groundcover vegetation to place mulch material underneath and between plantings. The
mulch layer surface should approximate the final elevation as shown on the design plans.
Figure 8.7 shows the rain garden constructed at the Hal Marshall Building parking lot
after installation of hardwood mulch.
75

illint I lilt ~ti`allcr (~w.tu~~litmy 1)c~i~~n h9~ii~ual Date of Adoption
o-. ~. D.. .d3awa`'.~`%N ..e 3mJhYa,: .o i e~. ~~IRf ~,'~°.ra.'....9W"^~aN,i~+^9Lffi ,....

8.5 Maintenance and Operation
8.5.1 General Care
In traditional, intensively cropped landscapes, soil fertility (and especially the level of
available nitrogen) is considered the limiting factor to plant growth. By design,
Bioretention facilities are located in areas where nutrients (especially nitrogen) are
significantly elevated above natural levels. Therefore, it is unlikely that soil fertility will
be the limiting factor in plant growth, and thus fertilization would be unnecessary. Excess
fertilization, (besides compromising the BMP's pollutant reduction effectiveness) leads
to weak plant growth, promotes disease and pest outbreaks, and inhibits soil life. If soil
fertility is in doubt, a simple soil test can resolve the question. Persons responsible for
maintenance may consult with their local nursery or contact the Mecklenburg County
Cooperative Extension Office to determine fertility needs. If fertilization should become
necessary, an organic fertilizer will provide nutrients as needed without disrupting soil
life.
76
Figure 8.7 Planted and Mulched Rain Garden

Mint Hill Water Quality Design Manual Date of Adoption
Like any garden area that includes grasses or woody plant materials, harvesting and
pruning of excess growth will need to be done occasionally. Trimmed materials may be
recycled back in with replenished mulch material. Mulch should be inspected after every
significant precipitation event and replaced as necessary. In most cases, shredded mulch
should only need replacement once per year.
Typically, watering of the BMP will not be necessary once plants have become
established, except during drought conditions. Plant species for bioretention have been
selected based on their hardiness and ability to survive extreme conditions. However,
watering will be needed during the plant establishment stage. As with any landscaping
feature, the designer should consider affects on moisture condition and the ability of the
owner to apply watering as needed. Facilities susceptible to drying conditions include:
• Landscape parking lot islands
• Median areas
• Windy, exposed areas
Weeding of the facilities is not absolutely necessary for the proper functioning of the
bioretention BMP. However, unwanted plants can be invasive, consuming the intended
planting and destroying the aesthetic appeal. Therefore, weeding is encouraged to control
growth of unwanted plants, especially where facilities are placed in prominent settings.
Seasonal Care
Spring
Prune deciduous trees and shrubs before leaves appear (usually early to mid-
March).
Prune flowering trees and shrubs after blossoming (usually early June).
Divide ornamental grasses and perennials as soon as the soil becomes soft.
Summer
• During extended drought, water deeply in the morning every seven to ten days.
• Check trees and shrubs for signs of disease or insect pests. Plant diseases usually
can be easily treated when detected early.
• Weed regularly, preferably by hand.
Fall
Cut perennials back to the ground after the first frost and remove annuals.
Plant new trees and shrubs as long as the soil temperature remains above 32
degrees.
Mulch trees and shrubs to help conditions the soil for spring and to protect roots.
Winter
• Cut back ornamental grasses and remove clippings.
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Mint Hill Water Quality Design Manual Date of Adoption
Troubleshooting Problems
• Look for signs that plants are too wet including wilting, yellowing, ringed spots
on leaves, and a soft or rotting base.
• If erosion is occurring at drainage paths, stabilize the erosion.
• If plants are dying, it may be necessary to choose plants more tolerant of
drier/wetter conditions.
• If water is not dissipating within a couple of days, the BMP is not functioning
properly. Water will pond longer in winter and early spring.
• Do not walk or mow in ponding areas.
8.5.2 Maintenance Responsibilities
BMPs that are constructed on privately-owned land and that are not within a public
easement shall be maintained by a Homeowners Association or the owner of the
subject property. BMPs that are constructed on public land within public rights-of-
way, and/or within public easements shall be maintained by the public body with
ownership/jurisdiction of the subject property. The following requirements shall be
met for all BMPs that have been constructed on privately-owned property and not
within a public easement.
a) Maintenance Covenants. Prior to the issuance of an Occupancy Permit for
any building within a permitted development served by a BMP, the applicant
or owner of the BMP shall establish a formal Maintenance Covenant approved
by the Mecklenburg County Land Use and Environmental Services Agency
and recorded in the Office of the Register of Deeds in which the owner
acknowledges the duty of the owner and all subsequent owners of the property
to maintain the BMP in accordance with the terms of the Covenant. A
maintenance plan and schedule shall be included as part of the covenant as
well as a mechanism for funding maintenance and repairs. This Maintenance
Covenant shall also specify the Homeowners Association or other party
responsible for maintenance of the BMP. A Homeowners Association or
similar legal entity has the power to compel contributions from residents of a
development to cover their proportionate shares of the costs associated with
BMP maintenance. Appendix G contains an example of a typical
Maintenance Covenant.
b) Requirements for the Maintenance Covenants. BMPs shall be periodically
inspected as described in the Maintenance Covenant to identify maintenance
and repair needs and to ensure compliance with the requirements of these
regulations. Any identified maintenance and/or repair needs shall be
addressed in a timely manner. The inspection and maintenance requirement
may be increased as deemed necessary by the Mecklenburg County Land Use
and Environmental Services Agency to ensure proper functioning of the BMP.
c) Records of Installation and Maintenance Activities. Parties responsible for
the inspection, operation, and maintenance of a BMP shall make records of
the installation of all the maintenance and repairs and shall retain the records
for at least five years. Those records shall be made available to the
78

Mint Hill Water Quality Design Manual Date of Adoption
Mecklenburg County Land Use and Environmental Services Agency upon
request and/or as specifically outlined in the Maintenance Covenant.
Watering and maintenance responsibilities during different phases of a project shall
generally be defined as follows, unless contractual obligations require otherwise:
• Construction Phase: Developer/Builder
• Project Acceptance: Builder
• Property Ownership Transfer: Builder/Property Owner
• Warrantee Phase: Property Owner
• Operation Phase: Property Owner/Homeowners Association
8.5.3 Warrantees
The landscaping work and materials shall be guaranteed by the developer/builder for one
year from the date of the final inspection approval, although watering, mulching and
general care during this period is the responsibility of the property owner. After one year
the property owner will assume full responsibility for all landscaping work and materials
and shall replace plants if they die.
8.6 Typical Sequence of Construction for Bioretention
The sequence of construction for Bioretention areas is closely tied to the grading plans for
the development. Because Bioretention is a source control BMP, drainage area
catchments are kept relatively small and therefore, manageable during the construction
phase for control of sediment. Basic sediment control practices are employed for each
lot.
A typical sequence of construction with typical construction schedule is provided in
Figure 8.8 of this document. The sequence of construction will vary for every project but
the designer may utilize this sequence of construction as a general guide. Variations to
the sequence must be noted and conveyed to the County inspector. The sequence of
construction shall be placed on the plans.
8.6.1 Inspectors Checklist for Bioretention
The following checklist has been derived and modified from a checklist developed by the
Prince George's County Maryland, Community Standards Division, Site Development
Inspection Section for use when evaluating a Bioretention facility during different phases:
79

Mint Hill Water Quality Design Manual Date of Adoption
Bioretention Inspection Checklist
1. Pre-construction Meeting
- Two copies of approved Stormwater Management Plan with Latitude & Longitude of
BMPs
- Submit one copy to Joyce Brown for BMP mapping
- Disseminate inspection requirements; what needs inspection
- Ticket and tag requirements & a copy of the geotechnical report (if available)
2. Excavation of Bioretention Area
- Soil Permeability
- Suitable sub-grade materials
- Presence of moisture or water
- Dimensions and placement of excavation conforms with plans
- Sediment and erosion control devices in place
3. Installation Phase (Only after all contributing drainage area has been stabilized)
- Proper Placement of gravel layer
- Correct placement of underdrains, cleanout (size, schedule, location) where required
- Correct placement of filter fabric (non-woven)
- Backfill soil conforms with specifications and placed per details and specifications
- Proper grade establishment
- Proper placement of plant materials (type, size, quantity, tags)
- Correct placement of ground cover or mulch cover
4. Final Inspection and As-Built
- Changes in grading, facility depth, size, soil medium, plant materials, etc., shall
require an As-built Plan whether private or public to reflect the changes.
- Maintenance Agreement/Covenant for Bioretention facilities located on private
property
- All landscaping installed/landscape warrantee documentation received
- Bioretention configuration, size and depth are in accordance with approved plans
- Landscaping certification documentation for Bioretention facility(ies)
- Drainage area conforms to approved plan
- Drainage area completely stabilized
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Mint Hill Water Quality Design Manual Date of Adoption
Install sediment control devices as shown on the plans.
-Construction time: '/Z Day
2. Grade site to elevations shown on plan. If applicable, construct curb openings, and/or
remove and replace existing concrete as specified on the plan. Curb openings shall be
blocked or other measures taken to prohibit drainage from entering construction area. At
the end of each workday, all excavations shall be protected by construction safety fencing
or temporary backfill as needed.
-Construction time: 1 Day
Stabilize grading within Limit of Disturbance except for Bioretention Area. Bioretention
areas may be utilized as sediment traps if the proposed invert of the bioretention facility
is 1' lower then the sediment trap and if approved on the plans.
-Construction time: '/2 Day
4. Excavate bioretention area to proposed invert depth and scarify the existing soil surfaces,
taking care not to compact the in-situ materials.
-Construction time: 'h Day
4a. Install gravel filter, underdrain system, observation wells, and outlet device, if specified.
-Construction time: '/Z Day
4b. Inspection required of underdrain and amended soil mix.
5. Backfill bioretention area with washed stone and planting soil as shown in the plans and
detailed in the specifications. Overfilling is recommended to account for settlement.
-Construction time: 'h Day
Presoak the planting soil prior to planting vegetation to allow for settlement. This can be
done by water truck or allowing water to enter the pit from a rain event.
-Construction time: '/4 Day
7. Excavate or fill to achieve proper design grade, leaving space for the upper layer of
mulch and/or topsoil that will bring the surface to final grade and ready for planting.
Create sheet flow into bioretention area to avoid concentrated flow and subsequent
erosion.
-Construction time: '/4 Day
8. Plant vegetation specified in the planting plan for Bioretention Area.
-Construction time: '/~ Day
9. Mulch and install erosion protection at entrance points; remove sediment control
practices or entrance blocks with inspector authorization.
-Construction time: 1/~ Day
10. Final inspection required.
Total Estimated Construction Time - 5.0 Days
Figure 8.8 Sequence of Construction for Bioretention
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Section 9. Plan Submittal/Review
9.1 Introduction
Plan submittal requirements will remain the same for all commercial and residential
projects. Water Quality Management Plans demonstrating compliance with Section 3,
Performance Criteria, are required for all projects, unless exempted under Applicability
of the Mint Hill Water Quality Ordinance.
9.2 Site Evaluation Tool
The Site Evaluation Tool (SET) is a water quality model that assesses pre-development
runoff and pollutant loading rates and provides a methodology for implementing LID best
management practices (BMPs) into a development for achieving the established
Performance Criteria (Section 3). The SET is intended to be an integral part of the
development of a conceptual site design and SET output must be submitted along with
preliminary sketch plans. The SET requires the following input describing pre- and post-
construction conditions:
1. The size of the project.
2. For residential development, the number of homes within the project to be served
by septic systems.
3. For commercial development using on-site wastewater disposal BMPs, the
estimated waste volume in gallons/year.
4. The fraction of the project area that is distributed within each of the hydrologic
soil groups A, B, C, or D.
5. The land areas (in square feet) shown on the detailed site plan for the proposed
development as occupied by the following pervious areas:
a. Forest/Wetland
b. Meadow (open space maintained in a natural condition)
c. Lawn
6. The areas (in square feet) of the project that will be covered by the following
impervious areas:
a. Rooftops (all buildings)
b. Driveways and/or Parking Lots (including gravel surfaces)
c. Roads
d. Sidewalks
e. Other Impervious Areas (e.g. tennis courts, patios)
7. The surface areas (in square feet) of all storm water management facilities or
BMPs planned for the project including structural features (ponds, wetlands) and
design features such as swales, channels and infiltration galleries).
8. The same division of land uses, impervious areas, and storm water management
facilities as items 5-7 for the condition of the site prior to development. For most
sites the existing land use will be a combination of forest/wetland and meadow.
9. A division of the project area into distinct drainage areas that are served by
specific storm water management facilities and/or BMPs.
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10. Additional inputs are required for some BMPs; ponds and wetlands require entry
of detention storage, and stream buffers require average buffer width and the
proportion of the drainage area within 150 feet + the buffer width of the stream.
Also, pollutant removal efficiencies and hydraulic properties must be specified on
the site plan that are not included in the menu of BMP choices.
11. The average slope and the longest flow length for each drainage area.
A copy of the SET and detailed documentation can be obtained from the following
sources:
Via USMaiI: Mecklenburg County Water & Land Resources,
700 North Tryon Street
Charlotte, NC 28202
Attn: SET,
Via Telephone: [phone]
Via the Internet: http:f'1www.co.charmeck.or~ (search SET)
9.3 Plan Submittal Requirements
Plan submittal procedures will remain as described in the Town of Mint Hill Zoning
Ordinance and the Mecklenburg County Land Use and Environmental Services Agency
Plan Submittal Guidelines, with the following additions:
A sketch plan meeting is required with Town staff and the Mecklenbur County
Water Quality Program (700 N. Tryon Street, Charlotte, NC 28202) prior to a
formal plan submittal. The plan shall be on a topographical map showing original
contours at intervals of not less than two feet and existing tree lines. It should
show in sketch form the proposed layout of streets, lots, and other features in
relation to existing conditions. This plan shall include the following:
• The boundary/property lines of the property being developed as well as
the location of property lines that intersect the property being developed;
• Water courses on the land to be subdivided or developed;
• Impervious area calculations;
• The location, names, and rights-of--way of any existing streets on or within
300 feet of the land to be subdivided or developed;
• The location of all property lines which intersect the boundaries of the
property being subdivided or developed;
• Limits of all wooded areas (locate all trees 6-inches in diameter or larger
for special or conditional uses);
• Soils type (HSG) and limits;
• Contour map at two (2) foot intervals extending 100 feet beyond the
property boundary.
• Slope Analysis showing (0% - 10%, 10% - 15%, 15% - 25%, >25% );
• Natural drainageways (woodland swales, concentrated flows), ponds;
• Wetlands limits (copy of appropriate Federal and State
permits/verification to be submitted with preliminary plans);
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• Rough finished grades, the location of proposed streets, lots, parks or other
open spaces, building lines, street cross-sections, number and type of
buildings, and the location of any building restriction flood lines;
• Zoning information for the proposed project site and adjacent properties;
• Proposed front, rear, and side yard dimensions for each building type
along each street type;
• The location and width of required S.W.I.M. buffers;
• SET Calculations;
• Proposed LID BMPs and locations;
• The location and width of any S.W.I.M. stream buffers;
• The location of general buffers or screens required for the project area, as
a whole;
• The scale of the plan, which shall not be smaller than 100 feet to the inch;
north point; date; and
• A small scale vicinity map.
2. Preliminary Plans shall include the above listed information in addition to the
Preliminary Plan submittal requirements outlined in the Mint Hill Zoning
Ordinance and the Mecklenburg County Land Use and Environmental Services
Agency Plan Submittal Guidelines. A Water Quality Management Plan shall be
submitted demonstrating compliance with the Performance Criteria (see Section
3) and should contain the following information:
• BMP summary table, which lists all BMPs on the site and corresponding
NAD 83 (feet);
• Calculations and design drawings for each BMP and overall site
hydrology calculations (see Section 5) illustrating compliance with
Section 3, Performance Criteria.
• Planting plan/schedule for each BMP illustrating plant location, species,
and quantities; and
• A maintenance plan for all BMPs. This plan shall include the responsible
party for each BMP and a schedule of routine maintenance activities to
ensure proper performance.
Prior to approval of the Water Quality Management Plan and the issuance of any
land development permits, a posting of financial security is required in
compliance with the Mint Hill Water Quality Ordinance.
3. In addition to typical Final Plat requirements in the Mint Hill Zoning Ordinance
and the Mecklenburg County Land Use and Environmental Services Agency Plan
Submittal Guidelines, the following additional requirements shall apply:
• All BMPs shall be recorded on the final plat and applicable deeds, with
their corresponding NAD 83 (feet); and
• Any vegetation, tree save areas, open space or site conditions that
contribute to the project's compliance with Section 3, Performance
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Mint Hill Water Quality Design Manual Date of Adoption
Criteria, shall be called out, protected, and recorded through the plat and
applicable deeds.
4. Prior to the issuance of an Occupancy Permit for any building within a permitted
development served by a BMP, the applicant or owner of the BMP shall establish
a formal Maintenance Covenant, approved by the Mecklenburg County Land Use
and Environmental Services Agency, and recorded in the Office of the Register of
Deeds in which the owner acknowledges the duty of the owner and all subsequent
owners of the property to maintain the BMP in accordance with the terms of the
Covenant. This Maintenance Covenant is required for all BMPs constructed on
privately-owned property and not within a public easement. Appendix G contains
an example of a typical Maintenance Covenant. See Section 8.3.2 of this manual
for additional information on maintenance responsibilities.
Section 10. Ins ection ar~d Enforcement
10.1 Introduction
This Section establishes inspection and enforcement guidelines to be followed for this
Manual and corresponding Ordinance. The guidelines are to be applied uniformly for
inspection of private and public water quality measures.
10.2 Authority
The provisions of this Ordinance shall be enforced by the Mecklenburg County Land Use
and Environmental Services Agency (LUESA). LUESA staff shall be authorized by the
Mint Hill Town Manager to perform the following functions:
1. Conduct inspections and file reports as necessary during construction of water
quality BMPs to ensure compliance with the approved plans and permits.
2. Furnish the permitting agent or owner of the property the results of the inspection
in a timely manner (See Section 10.6).
3. Issue a Field Inspection Report (FIR) to the permitting agent or owner when any
portion of the work does not comply with the approved plans and/or permits (see
Section 10.6.1).
4. Issue a Notice of Violation in accordance with these guidelines as the result of
unsatisfactory work or progress, failure to comply with approved plans and
permits, and any non-compliance of the requirements of this Ordinance.
5. Issue a Stop Work Order or Revocation of Permit as the result of unsafe
conditions, working without a permit, unsatisfactory work or progress, or other
non-compliance.
6. Issue Civil Citations(s), Civil Actions(s), or Criminal Action(s) listed in Section
10.7 due to unsafe conditions, non-compliance with a Stop Work Order,
unsatisfactory work or progress, or other non-compliance (see appropriate Section
of the Mint Hill Zoning Ordinance).
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7. Perform a final inspection upon the completion of water quality structures to
determine if the completed work is constructed in accordance with the approved
Water Quality Management Plan and associated plans/documents.
10.3 Inspection Responsibilities
LUESA staff shall be responsible for performing all inspection activities described in this
Section under authorization of the Mint Hill Town Manager.
10.4 Inspection Requirements During Construction
Inspections will be conducted at the request of the owner/developer and at specified
stages of construction for each water quality BMP and/or technique described below.
Final inspections/approvals are required for all BMPs, but additional inspections
/approvals are required during the construction phase of ~rywell/Infiltration Trenches
and Bioretention BMPs, as specified in Section 10.4.1 and 10.4.2. The Inspectors will
also make unscheduled inspections to ensure compliance with the requirements of this
Manual and corresponding Ordinance as they deem necessary. The Inspector will be
authorized to accept minor field changes proposed by the owner/contractor. Inspections
shall be requested at least 24 hours in advance. When requesting an inspection, the
permit number, type of inspection, contact name and phone number must be provided.
10.4.1 Drywell or Infiltration Trench
The following inspections are required for construction of a Drywell or Infiltration
Trench. The Inspector may require additional inspections.
1. Excavation Inspection - An inspection must be conducted after excavation and
immediately prior to the final topsoil installation. During this inspection, the
Inspector shall verify the following:
a. Trench dimensions comply with approved plans.
b. Adequate sediment control protection has been installed.
c. Approved filter fabric has been cut and installed to conform to the trench
perimeter. A six-inch minimum overlap is required between strips of
cloth. Extruding tree roots or other obstacles must be removed from the
trench walls and base to prevent the fabric from tearing.
d. Observation well or inlet has be installed as specified on plans prior to
stone placement. Perforations shall not extend beyond gravel trench.
e. Installation of 1 1/2 inch to 3 inch washed stone. Care must be used when
dumping the stone to ensure the filter cloth does not tear.
f. Installation of the inlet pipe for Drywells.
g. Placement of topsoil on-site (but not installed).
2. Final Inspection- During this inspection, the Inspector will verify the following:
a. Gravel surface must be completely covered with cloth and backfilled with
topsoil for Drywells and Infiltration Trenches. For Dry wells. surface
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inlets must be installed as per approved plans and the observation well
must be capped.
Upon stabilization of the area, if the above items have been completed
satisfactorily, afinal inspection report will be issued and approved by the
Inspector. The final inspection is required before requesting a Final Certificate of
Occupancy. For privately owned and maintained BMPs, a signed and sealed
letter from a North Carolina Registered Professional Engineer must be submitted
prior to the final inspection, which states that the BMP has been constructed as
per the approved plan.
10.4.2 Bioretention BMPs
The following inspections are required for construction of a Bioretention BMP. The
Inspector may require additional inspections.
1. Excavation and Underdrain Inspection - An inspection shall be conducted after
excavation and Underdrain installation and immediately prior to the final stone
and topsoil installation. During this inspection, the Inspector shall verify the
following:
a. Trench dimensions comply with approved plans.
b. Adequate sediment control protection has been installed.
b. Stone, Underdrain and filter fabric have been installed as per the approved
plan.
c. Proper grade has been establishment.
d. Placement of topsoil on-site (but not installed).
2. Final Inspection -During this inspection, the Inspector shall verify the following:
a. Proper topsoil installation.
b. Proper plant installation (native plants, size) and plant health as per the
approved plan.
c. Maintenance agreement/covenant complete.
d. Outlet properly installed.
e. Proper placement of ground cover/ mulch.
f. Drainage area completely stabilized.
g. Drainage area conforms to approved plan.
Upon stabilization of the area, if the above items have been completed satisfactorily, a
final inspection report will be issued and approved by the Inspector. The final inspection
is required before requesting a Final Certificate of Occupancy. For privately owned and
maintained BMPs, a signed and sealed letter from a North Carolina Registered
Professional Engineer must be submitted prior to the final inspection, which states that
the BMP has been constructed as per the approved plan.
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10.5 Maintenance Inspections
10.5.1 Introduction
Maintenance Inspections will be done on a routine basis to ensure that water quality
BMPs remain functional. Preventive maintenance inspections will be performed on
water quality BMPs during the first year of operation and at least once every three years
thereafter or as stipulated in the maintenance agreement.
10.5.2 Responsibilities and Procedures
The owner of the property will be contacted prior to the maintenance inspection. If the
owner cannot be contacted, the inspection will be performed and a report will be sent to
the owner.
1. A Warning Citation and/or Notice of Violation will be issued for any maintenance
and/or repairs required.
2. The owner shall begin to make corrections to the violations within 10 days of the
date of the Warning Citation and/or Notice of Violation. After 10 days, Civil
Citations may be issued as governed in Section 10.7.2.3.
3. If an imminent hazard exists, the Inspector may post the Water Quality BMP
unsafe as per the North Carolina State Building Code and require immediate
repair.
4. Upon the owner's failure to comply with the Warning Citation, Notice of
Violation, Civil Citation(s), the Inspector may issue a Civil or Criminal Action
against the property owner and/or contractor.
10.6 Approvals and Reports
Approvals and inspection reports shall be maintained by the Inspector to insure proper
notification of construction approvals and failure to comply with the approved plans to
the owner, contractor or developer.
10.6.1 Field Inspection Report
The purpose of the Field Inspection Report (FIR) is to notify the owner and or
contractor/developer of construction deficiencies noted by the Inspector, and to direct
repairs and corrections.
FIRs will be issued when the permittee or agent is directed to make changes to their work
to satisfy this Manual and corresponding Ordinance, approved storm water design plans,
or specifications. The notice shall set forth the nature of the corrections required and the
time allotted to make the necessary corrections. FIRS shall be issued when the following
has occurred:
1. Failure to comply with the design plan. Incorrect measurements, using improper
materials or failing to follow proper procedures can prompt the issuance of a FIR.
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FIRS shall be issued in writing except when a verbal notice would result in
immediate compliance as the work is being completed. Verbal notices shall be
noted in the project file.
2. Failure to provide certification for water quality structures. The Inspector shall
issue a FIR to the owner/developer requesting certifications and/or as-builts. A
compliance date and a mailing address for sending needed information shall be
supplied.
10.7 Administration and Enforcement
If an application for a Building Permit, Grading Permit, Storm Water Permit, Zoning
Land Use Permit or Certificate of Occupancy is denied because ofnon-compliance with
this Ordinance, the Inspector shall provide notification of the denial and of the reasons
therefore.
If the owner fails to comply with the FIR during construction, the Inspector shall provide
notification of the denial and of the reasons therefore.
10.7.1 Right to Appeal
If a request for a permit is disapproved or if a ruling of the Zoning Administrator is
questioned, any aggrieved party may appeal such ruling to the Mint Hill Board of
Adjustment as provided in Section 14.2 of the Mint Hill Zoning Ordinance. An appeal or
variance to the Board of Adjustment, lawfully and completely filed within 30 days of the
date of the decision, shall stay enforcement action and penalties until a hearing has been
held and a decision rendered by the Board of Adjustment.
10.7.2 Penalties
In case any water quality BMP is installed, constructed, reconstructed, altered, repaired,
converted or maintained in violation of these regulations, an action for injunction,
mandamus, or other appropriate action or proceeding to prevent such violation may be
instituted by the Zoning Administrator or other authority designated by the Board of
Commissioners as enforcement agent(s) for this ordinance. Penalties and remedies are
stated in Section 16.4 and 16.5 of the Mint Hill Zoning Ordinance and as listed below:
Notice of Violation. The Inspector will issue a Notice of Violation to the owner
upon non-compliance of this Ordinance. In most cases a FIR will be used for the
first offense. Subsequent noncompliance with these regulations or failure to
complete the items on the FIR within the specified period of time will result in
Notice of Violation.
2. Stop Work Order Issuance and Revocation of Permits. Whenever a water
quality BMP or part thereof is being installed, constructed, renovated, altered, or
repaired in violation of any provisions of this ordinance, the Inspector may order
the specific part of the work that is in violation, or would be when the work is
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completed, to be immediately stopped. The Stop Work Order shall be in writing,
directed to the person doing the work and/or owner, and shall state the specific
work to be stopped, the specific reasons for cessation and the action(s) necessary
to lawfully resume work.
The Zoning Administrator may revoke any permit (e.g. building, grading, storm
water, zoning use, certificate of occupancy) by written notification to the permit
holder and/or owner when violations of this Manual or corresponding Ordinance
have occurred. Permits may be revoked when false statements or
misrepresentations were made in securing the permit, work is being or has been
done in substantial departure from the approved application or plan, there has
been a failure to comply with the requirements of this Manual or corresponding
Ordinance, or a permit has been mistakenly issued in violation of this Manual or
corresponding Ordinance.
Failure to comply with the proper construction sequence as outlined in this
Ordinance may cause a Notice of Violations/ Stop Work Orders to be issued in
such cases as described below:
a. Failure to notify the Department before beginning any work to implement
the water quality BMP (including not requesting apre-construction
meeting):
Any work that has been placed without a required inspection approval
shall be certified in writing by a registered professional engineer before
the next phase of construction begins. The Inspector reserves the right to
require investigative materials testing on all un-inspected facilities or
devices at the sole expense of the permittee/owner. Any deficiencies that
need to be corrected for work already started shall be listed and given a
compliance date. The permittee shall be notified to call for future
inspections as required, as well as any additional inspections required by
the Inspector.
b. Failure to have work inspected and approved before continuing work:
It is required that inspection point(s) not approved be certified in writing
by a registered professional engineer. The Inspector reserves the right to
require investigative or materials testing on all un-inspected facilities or
devices at the sole expense of the permittee/owner.
c. Failure to call for a final inspection:
The Inspector shall list all deficiencies that need to be corrected, give a
compliance date, and request a letter of certification and/or as-builts (if
required) be submitted. The owner or contractor/developer shall request a
re-inspection after completing the corrections so another final inspection
can be made.
d. Failure to provide certification for completed water quality BMPs:
If an engineer's certification and/or as-built is not received by the
compliance date as required by a previously issued FIR, a Notice of
Violation to the owner and/or developer shall be sent requesting
certification and/or as-builts.
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3. Civil Penalty. Pursuant to NC General Statute 160A-175, the regulations and
standards of the Manual or corresponding Ordinance may be enforced through the
issuance of civil penalties by the Zoning Administrator for the following
situations:
a. When a FIR, Notice of Violation and/or Stop Work Order has not been
complied with or there has not been substantial progress in complying
with the Notice of Violation and/or Stop Work Order.
b. On abandoned sites where no work has been on going, and continued non-
compliance with a Notice of Violation may result in the issuance of
repeated citations.
c. When a Stop Work Order is in effect and work continues in defiance of
the order.
d. When repeated recurring violations of the same section of this Manual or
corresponding Ordinance. Each day that a violation remains uncorrected
constitutes a separate violation of applicable code or ordinance.
Subsequent citations for the same violation may be issued by the Zoning
Administrator if the offender does not pay the citation (except as otherwise
provided in a warning situation) after it has been issued unless the offender has
sought an Appeal to the Decision of the Zoning Administrator through the Board
of Adjustment. Once the ten-day warning period has expired, each day which the
violation continues shall subject the violator to additional citations to be issued by
the Zoning Administrator.
The following penalties are hereby established:
Warning Citation and/or Notice of Violation -Correct Violation within 10
days.
First Citation - $50
Second Citation for Same Offense - up to $200
Third and Subsequent Citations for Same Offense - up to $500, then $500 per day
A citation may be issued every day the violation continues after the 10 day
warning period. Hazardous violations or violations that cause risk to the public's
health or welfare may be fined immediately. If the offender fails to pay the civil
penalties within 10 days after having been cited, the Town of Mint Hill may
recover the penalties in a civil action in the nature of debt.
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APPENDIX A
Mint Hill Water Quality Ordinance

4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
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Section 7.4 . Goose, Duck and Stevens Creek Watershed Overlay (GDS-O).
Section 7.401. Purpose.
The purpose of the Goose, Duck and Stevens Creek Watershed Overlay District is to
protect stream water quality from surface water degradation and to protect habitats for the
Carolina heelsplitter, a federally endangered species of freshwater mussel, through the
application of land use requirements for the control of non-point source pollutants.
The Goose, Duck and Stevens Creek Watershed Overlay District is that area within the
Town of Mint Hill and its Extra Territorial Jurisdiction (ETJ) that contributes surface
drainage into Goose Creek and its tributaries as specifically defined on the Town of Mint
Hill Zoning Maps.
Section 7.402. Applicability.
7.402.1 All properties shall be subject to the buffer requirements of this Section
except those properties which, as of the effective date of ,fit into one of the
following categories:
a. Have been issued a Certificate of Building Code Compliance.
b. Have a valid building permit.
c. Are a small subdivision less than five lots.
d. Have been subdivided by a recorded subdivision plat.
e. Are included on a valid preliminary subdivision plan.
£ Are located within the Downtown Boundaries.
g. Have otherwise secured a vested property right under State law or local
ordinance.
7.402.2 Redevelopment or expansions to uses included in the above categories are
not subject to the requirements of this Part unless it would result in an increase in the
total impervious area within the buffer and/or result in the creation or addition of
more than 5,000 square feet of new impervious area.
7.402.3 In the event that stream buffers are required by another Section of this
Ordinance, the more stringent stream buffer requirements apply.
Section 7.403. Definitions.
Best Management Practices (BMPs). A structural or nonstructural management based
practice used singularly or in combination to reduce non-point source input to receiving
waters in order to achieve water quality protection goals.
- Non-structural BMPs -Non-engineering methods to control the amount of non-point
source pollution. These may include land-use controls and vegetated buffers.

4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
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- Structural BMPs -Engineered structures that are designed to reduce the delivery of
pollutants from their source or to divert contaminants away from a waterbody.
Carolina heelsplitter (Lasmigona decorata). A rare species of freshwater mussel with
only six (6) known populations in existence, one of which is found in Goose Creek. To
help secure its continued existence, the U.S. Fish and Wildlife Service designated this
mussel as an endangered species under the jurisdiction of the Federal Endangered Species
Act on June 30, 1993. This Act provides for the establishment of special requirements to
help preserve the species and its habitat.
Detain. To store and slowly release storm water runoff following precipitation by means
of a surface depression or tank and an outlet structure. Detention structures are
commonly used for pollutant removal, water storage, and peak flow reduction.
Hydrologic Abstractions. Physical processes of interception of rainfall or overland storm
water flow by vegetation, evaporation from land surfaces and upper soil layers,
transpiration by plants, infiltration of water into soil surfaces, and storage of water in
surface depressions.
Intermittent Stream. A well defined channel that contains water for only part of the year,
typically during the winter and spring when the aquatic bed is below the water table. The
flow may be heavily supplemented by storm water runoff. An intermittent stream often
lacks the biological and hydrological characteristics commonly associated with the
conveyance of water.
Low Impact Development (LID). The integration of site ecology and
environmental goals and requirements into all phases of urban planning and design
from the individual residential lot level to the entire watershed.
Mecklenburg County Land Use and Environmental Services Agency. The department or
division of Mecklenburg County government (regardless of the title given to it by
Mecklenburg County) which has responsibility for storm water and water quality matters,
acting as the agent of the Town of Mint Hill for various purposes in connection with the
enforcement of this regulation.
National Pollution Discharge Elimination System (NPDES) Permit. A permit issued
pursuant to the federal Clean Water Act for the purpose of controlling discharges of
pollutants to surface waters and protecting water quality. In North Carolina, NPDES
Permits are issued by the N.C. Department of Environment and Natural Resources.
Non-Point Source (NPS) Pollution. Forms of pollution caused by sediment, nutrients,
organic and toxic substances originating from land use activities and carried to lakes and
streams by surface runoff.
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4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
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Perennial Stream. A well defined channel that contains water year round during a year of
normal rainfall with the aquatic bed located well below the water table for most of the
year. Groundwater is the primary source of water for a perennial stream, but it also
carries storm water runoff. A perennial stream exhibits the typical biological,
hydrological, and physical characteristics commonly associated with the continuous flow
of water.
Retain. To capture and hold storm water runoff following precipitation by means of
surface depression allowing the water to infiltrate into the soil, thus reducing the
hydrologic and pollution impacts downstream. Retention structures are commonly used
for pollutant removal, water storage, and peak flow reduction.
Site Evaluation Tool (SET). A spreadsheet-based model that assesses and compares pre-
development and post-development runoff, infiltration, and pollutant loading rates, which
provides a methodology to aid in better site design and evaluation of BMP effectiveness.
Total Suspended Solids (TSS). Total suspended matter in water, which is commonly
expressed as a concentration in terms of milligrams per liter (mg/1) or parts per million
(ppm)•
Section 7.404. Development Standards for the Goose, Duck and Stevens Creek
Watershed Overlay.
The purpose of this Section is to establish storm water management requirements and
controls to prevent surface water quality degradation and to protect habitats for the
Carolina heelsplitter to the extent practicable in the Goose, Duck and Stevens Creek
Watershed Overlay. This Section seeks to meet this purpose by fulfilling the following
objectives:
a. Minimize increases in storm water runoff from development or redevelopment in
order to reduce flooding, siltation and streambank erosion, and maintain the integrity
of stream channels;
b. Minimize increases in non-point source pollution caused by storm water runoff from
development or redevelopment that would otherwise degrade local water quality;
c. Minimize the total volume of surface water runoff that flows from any specific site
during and following development in order to replicate pre-development hydrology to
the maximum extent practicable;
d. Reduce storm water runoff rates and volumes, soil erosion and non-point source
pollution, to the extent practicable, through storm water management controls
(BMPs) and to ensure that these management controls are properly maintained and
pose no threat to public health or safety; and
e. Meet the requirements of the National Pollution Discharge Elimination System
(NPDES) Storm Water Permit and other requirements as established by the Clean
Water Act.
This Section and the Mint Hill Water Quality Design Manual encourages the use of Low
Impact Development (LID) practices, which more closely replicate a site's
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4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
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predevelopment characteristics compared to conventional storm water management
techniques.
7.404.1. Mint Hill Water Quality Design Manual.
The Mecklenburg County Land Use and Environmental Services Agency shall
furnish, maintain and update a Water Quality Design Manual for the Town of Mint
Hill containing additional policy, criteria and information that shall be followed for
the proper implementation of the requirements of this Section. BMPs that are
designed and constructed in accordance with the criteria contained in this Manual
shall be accepted as meeting the minimum Performance Criteria specified in Section
7.404.2.
7.404.2. Performance Criteria.
Land development activities shall be performed in such a manner as to minimize the
degradation of water quality conditions and protect habitats for the Carolina
heelsplitter through compliance with the Performance Criteria listed below. Section 6
of the Mint Hill Water Quality Design Manual contains a description of approved
BMPs for meeting each of these Criteria.
a. All storm water treatment systems used to meet these Performance Criteria shall
be designed to achieve average annual 85% Total Suspended Solids (TSS)
removal for the developed area of a site. Areas designated as open space that are
not developed do not require storm water treatment. All sites must employ Llll
practices to control and treat runoff from the first inch of rainfall.
b. LID practices or a combination of LID and conventional storm water management
practices shall be used to control and treat the increase in storm water runoff
volume associated with post-construction conditions as compared with pre-
construction (existing) conditions for the 2-year frequency, 24-hour duration
storm event. This may be achieved by hydrologic abstraction, recycling and/or
reuse, or other accepted management practice(s) as described in Section 6 of the
Mint Hill Water Quality Design Manual.
c. Where any storm water BMP employs the use of a temporary water quality
storage pool as a part of its treatment system, the drawdown time shall be a
minimum of 48 hours and a maximum of 120 hours.
d. Peak storm water runoff rates shall be controlled for all development above 12%
imperviousness. The peak storm water runoff release rates leaving the site during
post-construction conditions shall be equal to or less than the pre-development
peak storm water runoff release rates for the 2-year frequency, 24-hour duration
storm event and 10-year frequency, 24-hour duration storm event. The
emergency overflow and outlet works for any pond or wetland constructed as a
storm water BMP shall be capable of safely passing a discharge with a minimum
recurrence frequency of 50 years. For detention basins, the temporary storage
capacity shall be restored within 72 hours. Requirements of the Dam Safety Act
shall be met when applicable.
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4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
1/14/04
e. No one BMP shall receive runoff from an area greater than five (5) acres.
However, the total drainage area from BMPs used in series (i.e., integrated) can
exceed this five (5) acre maximum.
7.404.3. Low Impact Development (LID).
The Performance Criteria discussed in Section 7.404.2. shall be achieved using LID
site planning and techniques or a combination of LID and conventional storm water
management practices. The goal of LID is to develop site design techniques,
strategies, and BMPs to store, infiltrate, evaporate, retain, and detain runoff on the
site to more closely replicate pre-development runoff characteristics and to better
mimic the natural and unique hydrology of the site thereby limiting the increase in
pollutant loads caused by development. The selection of these strategies and
techniques for compliance with the Performance Criteria in Section 7.404.2. is
discretionary and shall be detailed in a Water Quality Management Plan submitted
during the preliminary plan review process. Specific requirements regarding the
design, installation and maintenance of LID structures and a discussion of LID site
planning is contained in the Mint Hill Water Quality Design Manual.
7.404.4. Plan Submittal and Review.
a. Sketch Plans. Preliminary sketch plans shall contain the information necessary to
evaluate the proposed development site for compliance with Performance Criteria
as detailed in Section 9 of the Mint Hill Water Quality Design Manual. Stream
and wetland delineations must be included with all sketch plans.
b. Site Evaluation Tool. Site Evaluation Tool (SET) output shall be submitted along
with sketch plans for each proposed development site. Section 9 of the Mint Hill
Water Quality Design Manual contains detailed information concerning the
submission requirements for SET.
c. Water Ouality Management Plan. A Water Quality Management Plan shall be
submitted along with preliminary plans for each proposed development. The Plan
shall demonstrate compliance with Section 7.404.2. Performance Criteria, unless
otherwise exempted. The Plan shall contain SET outputs, supporting
computations, drawings, soil analyses, calculations for each BMP, and overall site
hydrology calculations as well as other information sufficient to describe the
manner, location, and type of measures for managing storm water from the
development in compliance with Section 7.404.2 In addition, the Plan shall
specify those parties responsible for long-term maintenance of all BMPs. Section
9 of the Mint Hill Water Quality Design Manual contains specific submission
requirements. The Mecklenburg County Land Use and Environmental Services
Agency shall review the Plan to determine compliance with the Performance
Criteria listed in Section 7.404.2. Approval of the Water Quality Management
Plan by the Mecklenburg County Land Use and Environmental Services Agency
is required prior to the initiation of land disturbing activities and said Plan shall
serve as the basis for all subsequent construction.
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4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
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d. Adjustments of Information Requirements. Information requirements may be
adjusted by the Zoning Administrator or designee for a particular development
application upon written request of the applicant, provided at least one of the
following circumstances can be demonstrated:
(i) Alternative measures for on-site and/or offsite management of storm water
have been proposed and these measures are approved for
compliance with the Performance Criteria in Section 7.404.2.
(ii) It is otherwise demonstrated that the proposed development will not produce
any significant change to the existing, pre-development stream or site
hydrology and pollutant loading.
7.404. S. Incentives.
a. Purpose. The purpose of this Section is to set forth incentives to offset
restrictions that LID may have on the development of certain sites.
b. Sidewalks. To reduce impervious cover and promote LID, sidewalks are required
on one side of the street with the exception of the following:
1. Arterial and Secondary Streets and extensions thereof.
2. Streets with such continuity through a subdivision or with such potential
continuity through a subdivision and adjacent areas that they may serve as
general traffic access streets for the neighborhood.
3. Streets providing access to existing elementary schools, junior high
schools, high schools, colleges, and official sites for such schools and
streets that provide access to existing places of public assembly.
c. Encroachments. Water quality BMPs may encroach into a required buffer yard as
long as the encroachment does not disturb existing vegetation. Minor understory
may be disturbed in order to accommodate water quality structures. Trees and
shrubs shall be placed to maximize screening where the encroachment takes
place. If the encroachment runs parallel to the buffer, the width of the buffer shall
be increased by the amount of the encroachment.
7.404.6. Posting of Financial Security.
Approval of a Water Quality Management Plan shall be subject to the owner filing a
surety bond or letter of credit or making other financial arrangements in favor of
Mecklenburg County as agent for the Town of Mint Hill acceptable to the
Mecklenburg County Land Use and Environmental Services Agency guaranteeing the
installation and maintenance of required BMPs until the issuance of certificates of
occupancy for seventy-five percent (75%) of all construction which might reasonably
be anticipated to be built within the area which drains into the BMP, allowing credit
for improvements completed prior to the submission of the final plat. At such time
that this level of occupancy is achieved, written notice thereof shall be submitted by
the owner to the Mecklenburg County Land Use and Envirorunental Services Agency.
The owner shall verify the adequacy of the Maintenance Covenant for the BMPs
6

4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
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including the necessary financing to support the proposed maintenance practices. The
owner shall also provide professional engineer certification that the BMP is designed
and constructed in accordance with approved plans and specifications. The
Mecklenburg County Land Use and Environmental Services Agency will inspect the
structural BMP and verify the effectiveness of the Maintenance Covenant and, if both
are found to be satisfactory, the Mecklenburg County Land Use and Environmental
Services Agency will notify the owner in writing within 30 days of the date of notice
regarding approval of the BMP. Following the issuance of this written approval, the
owner can request the release of the surety bond, letter of credit or other financial
arrangements at which time the maintenance responsibilities for the BMPs shall
revert to the Homeowners Association, property owner or other party responsible for
long term maintenance as specified in the Maintenance Covenant. It shall be
expressly stated within the restrictive covenants or homeowners' association
documents that it will be the responsibility of the owner or assigns to maintain BMPs
until such time as maintenance responsibilities have been transferred to the
Homeowners Association Board of Directors, property owner or other party
responsible for long term maintenance of the BMPs. It shall be the sole responsibility
of the owner or assigns to correct any deficiencies prior to said transfer of
maintenance responsibilities.
7.404.7. Final Plat Requirements.
The exact boundary of all water quality best management practices shall be shown on
final plats prepared by a registered surveyor. Also, these plats shall contain the
following statement: "This lot contains a water quality feature that must be
maintained in accordance with the recorded Maintenance Covenant as specified in
Section 7.404.8. (a) of the Mint Hill Zoning Ordinance."
7.404.8. Maintenance.
BMPs that are constructed on privately-owned land and that are not within a public
easement shall be maintained by a Homeowners Association or the owner of the
subject property. BMPs that are constructed on public land within public rights-of-
way, and/or within public easements shall be maintained by the public body with
ownership/jurisdiction of the subject property. The following requirements shall be
met for all BMPs that have been constructed on privately-owned property and not
within a public easement.
a. Maintenance Covenants. Prior to the issuance of an Occupancy Permit for any
building within a permitted development served by a BMP, the applicant or owner
of the BMP shall establish a formal Maintenance Covenant approved by the
Mecklenburg County Land Use and Environmental Services Agency and recorded
in the Office of the Register of Deeds in which the owner acknowledges the duty
of the owner and all subsequent owners of the property to maintain the BMP in
accordance with the terms of the covenant. A maintenance plan and schedule
shall be included as part of the covenant as well as a mechanism for funding
maintenance and repairs. This Maintenance Covenant shall also specify the
7

4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
1/14/04
Homeowners Association or other party responsible for maintenance of the BMP.
A Homeowners Association or similar legal entity has the power to compel
contributions from residents of a development to cover their proportionate shares
of the costs associated with BMP maintenance.
b. Requirements for the Maintenance Covenants. BMPs shall be periodically
inspected as described in the Maintenance Covenant to identify maintenance and
repair needs and to ensure compliance with the requirements of these regulations.
Any identified maintenance and/or repair needs shall be addressed in a timely
manner. The inspection and maintenance requirement may be increased as
deemed necessary by the Mecklenburg County Land Use and Environmental
Services Agency to ensure proper functioning of the BMP.
c. Records of Installation and Maintenance Activities. Parties responsible for the
inspection, operation, and maintenance of a BMP shall make records of the
installation of all the maintenance and repairs and shall retain the records for at
least five years. Those records shall be made available to the Mecklenburg
County Land Use and Environmental Services Agency upon request and/or as
specifically outlined in the Maintenance Covenant.
d. Failure to Maintain Practices. It is unlawful for a property owner to fail to meet
the requirements of the Maintenance Covenant. Any person or association that
fails to meet the requirements of the Maintenance Covenant shall be subject to a
civil penalty payable to the Town of Mint Hill of not more than $500. Each day
that the violation continues shall constitute a separate violation. No penalties
shall be assessed until the person alleged to be in violation has been notified in
writing of the violation by registered or certified mail, return receipt requested, or
by other means which are reasonably calculated to give actual notice. The notice
shall describe the nature of the violation with reasonable particularity, specify a
reasonable time period within which the violation must be corrected, and warn
that failure to correct the violation within the time period will result in assessment
of a civil penalty or other enforcement action. In the event that a BMP becomes a
danger to public safety or public health, the Mecklenburg County Land Use and
Environmental Services Agency, after reasonable notice as described above, may
seek a mandatory or prohibitory injunction and order of abatement commanding
the property owner to correct a violation of the design criteria standards or
maintenance needs contained in the Maintenance Covenants by performing all
necessary work to place the BMP in proper working condition. If the property
owner fails to take corrective action as ordered by the Court, the Town of Mint
Hill may seek an order holding the property owner in contempt of court and for
authority to execute the order of abatement by taking the corrective action. If the
Town of Mint Hill executes the order of abatement it shall have a lien upon the
property for the cost of executing the order of abatement.
7.404.9. Inspection of BMPs.
a. Inspections. Inspections shall be conducted as prescribed by the Maintenance
Covenant. Additional inspections may be conducted by the Mecklenburg County

4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
1/14/04
Land Use and Environmental Services Agency on any reasonable basis, including
but not limited to: routine inspections; random inspections; inspections based
upon complaints or other notice of possible violations; inspections of drainage
basins or areas identified as higher than typical sources of sediment or other
contaminants or pollutants; inspections of businesses or industries of a type
associated with higher than usual dischargers of contaminants or pollutants or
with discharges of a type which are more likely than the typical discharge to cause
violations of State or Federal water quality standards or the NPDES Storm Water
Permit; and joint inspections with other agencies inspecting under environmental
and safety laws. Inspections may include, but are not limited to: reviewing
maintenance and repair records; sampling discharges, surface water, groundwater,
and material or water in BMPs; evaluating the condition of BMPs and storm
water management practices.
b. Right-of--Entry for Inspection. When any new BMP is installed on private
property, the property owner shall grant to the Mecklenburg County Land Use
and Environmental Services Agency the right to enter the property at reasonable
times and in a reasonable manner for the purpose of inspection. This includes the
right to enter a property when the Mecklenburg County Land Use and
Environmental Services Agency has a reasonable basis to believe that a violation
of this regulation is occurring or has occurred, and to enter when necessary for
abatement of a public nuisance or correction of a violation of this regulation.
7.405. Stream Buffer Requirements.
The purpose of this Section is to establish stream buffer requirements to prevent surface
water quality degradation and to protect habitats for the Carolina heelsplitter in the
Goose, Duck and Stevens Creek Watershed Overlay. All buffers are measured
horizontally on a line perpendicular to the surface water, landward from the top of the
bank on each side of the stream. Required stream buffer widths vary based on the size of
the upstream drainage basin. Mecklenburg County's Geographic Information System
(GIS) will serve as a tool to delineate the size of drainage basins and specify the
corresponding buffer widths for all streams draining 50 acres or greater. For streams
draining less than 50 acres, a stream delineation must be performed by a registered
professional using U.S. Army Corps of Engineers and N.C. Division of Water Quality
methodology to delineate both perennial and intermittent stream sections, both of which
will require buffers. Stream buffer requirements specified in this Section begin at the
point where intermittent streams have been delineated according to the above
methodology.
7.405.1. Buffer widths for intermittent and perennial streams.
Buffers are required for intermittent and perennial streams as specified below.
9

4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
1/14/04
Upstream Stream Side 'Lone Upland Zone "Total Width
Drainage Area (Gone 1) (Zone 2} of Buffer on each side
__ ___ of Stream
> 300 acres __
100 feet Remainder of the Entire FEMA floodplain
FEMA floodplain but no less than 100 feet
be and 100 feet~~~
50 to 300 acres 50 feet None 50 feet
<50 acres 30 feet 20 feet 50 feet
Footnotes:
(1) Applies to those streams draining 640 acres or greater in which case the first 100
feet will include the Stream Side Zone and the rest of the buffer to the edge of the
FEMA floodplain will comprise the upland zone. If the floodplain is less than 100
feet wide, the total width of the buffer on that side of the stream will be 100 feet.
(2) Intermittent and perennial streams draining less than 50 acres must be delineated
by a registered professional using U.S. Army Corps of Engineers and N.C.
Division of Water Quality methodology. Streams equal to and greater than 50
acres and corresponding buffer widths will be delineated by Mecklenburg County
GIS.
7.405.2. Buffer description.
Buffer function, vegetation and use vary according to the different buffer zones as
described in the following table.
Characteristics StreamSide°Zone= ~ Upland;Zone=-~_ --
Function Protect the integrity of the Prevent encroachment into the
ecosystems Stream Side Zone and filter
runoff
Vegetative Undisturbed (no cutting or Grass or other herbaceous
Targets ~l~ clearing allowed) - If existing ground cover allowed -Forest
tree density is inadequate, is encouraged
reforestation is encouraged
Uses ~ ~ Very restricted -Permitted uses Restricted -Permitted uses
limited to: bank stabilization as limited to: structural BMP's
well as utility and road crossings and all uses allowed in the
as allowed by state and federal Stream Side Zone as well as
permits only grading for lawns
Footnotes:
(1) Re-vegetation of disturbed buffers is required as specified in the Charlotte-
Mecklenburg Land Development Standards Manual when such disturbances result
in the failure of the buffer system to comply with the vegetative targets specified
10

4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
1/14/04
above. The manual also contains recommended tree densities for each zone for
voluntary reforestation efforts.
(2) Fill material cannot be brought into the buffer. Grading is allowed only in the
Upland Zone. Commercial buildings or occupied structures are not allowed in the
buffer.
(3) Utilities remain outside the buffer when practicable and stream crossings
minimized. Utilities are allowed in certain circumstances by approval from state
and federal agencies. Water and utility crossings shall be near perpendicular to
stream when practicable.
(4) Pesticides shall not be used within 200' of streams, wetlands or floodplain except
when needed to protect native flora.
7.405.3. Diffuse flow requirement.
Direct discharges of runoff to streams are not allowed. Diffuse flow of runoff shall be
maintained in the buffer by dispersing concentrated flow and reestablishing
vegetation. Techniques for providing diffuse flow are specified in the Charlotte-
Mecklenburg Land Development Standards Manual.
a. Concentrated runoff from ditches or other manmade conveyances shall be
converted to diffuse flow before the runoff enters the buffer.
b. Periodic corrective action to restore diffuse flow shall be taken by the property
owner as necessary to prevent the formation of erosion gullies.
7.405.4. Ponds,
Ponds which intersect the stream channel shall have the same buffers as the original
stream measured from the top of the bank of the pond.
7.405.5. Wetlands.
Sewer lines and structures must be a minimum of 50' from jurisdictional wetlands
associated with the floodplain.
7.405.6. Buffer delineation.
The following buffer delineations are required:
a. Streams and buffer boundaries including all buffer zones must be clearly
delineated on all construction plans, including grading and clearing plans, erosion,
drainage and sediment control plans and site plans.
b. Outside buffer boundaries must be clearly marked on-site prior to any land
disturbing activities.
c. The outside boundary of the buffer must be permanently marked at highway
stream crossings.
d. Streams and buffer boundaries including the delineation of each buffer zone must
be specified on all surveys and record plats.
e. Buffer boundaries including the delineation of each buffer zone as well as all
buffer requirements must be specified on all surveys and record plats, on
individual deeds and in property association documents for lands held in common.
11

4th DRAFT Mint Hill Goose, Duck and Stevens Creek Watershed Overlay District
1/14/04
7.406. Appeals and Variances.
Appeals and variances from this Part shall be subject to Article 14 of these regulations.
The Mecklenburg County Land Use and Environmental Services shall review and submit
findings to the Town of Mint Hill Board of Adjustment prior to any decision being
rendered for appeal or variance.
12

APPENDIX B
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DEVELOPMENT RUNOFF VOLUME USING RETENTION
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APPENDIX C
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APPENDIX D
STORAGE VOLUME REQUIRED TO MAINTAIN THE PRE-
DEVELOPMENT PEAK RUNOFF RATE USING 100% DETENTION
STORAGE
TYPE 2 24-HOUR STORM CHART SERIES

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APPENDIX E
DESIGN EXAMPLES & COST ANALYSES FOR USE OF LID
Includes Single Family Residential, Multifamily Residential,
Commercial and Institutional Developments

TECHNICAL MEMORANDUM:
Cost Comparison of Conventional and Low Impact
Development Designs for a High Density
Residential Site in Mecklenburg County, North
Carolina
January 2003
Prepared for
Mecklenburg County Land Use and
Environmental Services Agency
Prepared by
Tetra Tech, Inc.
P.O. Box 14409
3200 Chapel Hill -Nelson Highway, Suite 105
Research Triangle Park, NC 27709
[phone]

Cost Estimation for High Density Residential Development Example January, 2003
BACKGROUND AND INTRODUCTION
Tetra Tech, Inc. compared the costs of conventional and Low Impact Development (LID) designs
for a proposed high density residential development in Huntersville, North Carolina. This memo
presents the preliminary design and cost comparison.
For purposes of conducting this cost comparison, the Town of Huntersville provided conceptual site
plans and information about the subject development. Tetra Tech, with design support from Soil
and Environmental Consultants (S&EC), then provided conceptual designs of conventional and LID
stormwater systems for the site (Figures 1 and 2). It should be noted that the LID design was
constrained by the Town and County to work with existing building footprints, road configurations,
parking areas, and use of curb and gutter. The conceptual nature of the designs also limited the
precise sizing of stormwater systems.
The LID design achieves more stringent water quantity and quality guidelines than the conventional
design. The wet detention pond would meet the existing Mecklenburg County requirement to
remove 85% of Total Suspended Solids (TSS) from the runoff of the first inch of rainfall. Due to
spatial constraints, the conventional design would not meet the existing Mecklenburg County
requirements to approximate pre-development peak discharge runoff rates for the 2-year and 10-
year storms. If additional detention capacity were added to meet the peak discharge runoff rate
requirements, the infrastructure cost of the conventional design would substantially increase. In
addition, the site may need to be redesigned to accommodate the added detention capacity,
potentially reducing parking or landscaping area. These additional infrastructure and redesign costs
have not been included in this analysis.
The LID design complies with Huntersville's proposed water quality regulations for the Transit-
Oriented Development, Town Center, and Highway Commercial Zoning Districts. The regulations
for these high-density districts are to meet the existing requirements described above and to retain
the post-development minus pre-development runoff volume for the 1-year, 24-hour storm. The
LID design achieves these requirements with bioretention areas. By complying with more stringent
guidelines, the LID design retains far more runoff onsite and removes considerably more pollutants
than the conventional design.'
This analysis shows that the current LID design costs about $25,000 less than the conventional
design. The conventional design would cost more if it were designed to meet the peak flow
discharge requirements. Therefore, the net LID savings would be higher than reported here.
However, the conventional design cost maybe reduced if a shared, offsite pond is used. The
accuracy of this study is limited due to the conceptual nature of the designs.
1 Tetra Tech will provide a detailed comparison of pollutant removal and runoff retention.
TETRATECN INC.

Cost Estimation for High Density Residential Development Example January, 2003
Figure 1. Conventional Site Plan for the Subject Development
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~~
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Wa Demnuon Pond 0.12 AC ` ' " ~ '"'

Figure 2. Low Impact Development Plan for Subject Development
TETRA TECH.INC.

Cost Estimation for High Density Residential Development Example January, 2003
ESTIMATION SOURCES AND METHODS
This study was designed to use a development site as an example for potential, future development
of Huntersville. All prices, both national averages and non-local prices, were converted to
Charlotte prices for the year 2003. Unit prices were obtained from S&EC, RS Means Manuals, and
Yarbrough-Williams and Houle, Inc., a development engineering firm local to Huntersville. The
sites did not differ in pavement or curb and gutter.
The designs differed, in part, by storm drain length and inlet type (Table 1). The storm drain price
was estimated from trends in the Yarbrough-Williams and Houle cost data and agreed with prices in
RS Means (2001). It was assumed that both designs would employ poured-in-place storm drain
inlets. Due to the use of bioretention, the LID design would require fewer inlets than the
conventional design.
Table 1. Unit Prices and Quantities for Conventional and LID Storm Systems
Component Price Quantity
Conventional LID Source
Storm Drain, 12" $12 per LF 775 LF 540 LF Yarbrough-Williams and Houle, Inc.'
Reinforced Concrete
Catch Basins, Curb Inlets $1,100 per unit 15 units 5 units Yarbrough-Williams and Houle, Inc.
and Yard Inlets
~ Price was extrapolated from Yarbrough-Williams and Houle, Inc. cost data and confirmed with RS Means (2001).
The costs of the wet detention pond included excavation, hauling, fill, and outlet structures (Table
2). Since this analysis was conducted at a conceptual level, S&EC based the price of the pond
outlet structure2 on similar project experience and not site-specific routing data. Therefore, the
price of the outlet may vary considerably depending on the stormwater routing of the final site
design.
The costs of the bioretention cells included excavation, hauling, soil fill, and subdrain piping (Table
3). The LID design would contain five bioretention cells of varying sizes, and catch basins would
function as outlet structures for the bioretention cells. It was assumed that pond and bioretention
landscaping would fulfill some of the landscaping requirements of the development, and, therefore,
planting costs were not considered. For both the LID and conventional stormwater controls, design
and engineering costs were estimated as 25% of the total construction costs (US EPA, 1999).
z The pond outlet structure would include the following components: riser, barrel, anchor slab, flared end section (or
head wall), riprap outlet protection apron, erosion protection blanket, and seepage control device.
3
TETRA TECH INC.

Cost Estimation for High Density Residential Development Example January 2003
Table 2. Cost Estimation of Wet Detention Pond (Conventional Design)
--
Component 1
Quantity
Unit Price
Total
Source
Excavation (CY) 1,275 $3.74 $4,769 RS Means, 2001
Hauling' (CY) 765 $3.56 $2,723 RS Means, 2001
Outlet (Unit) 1.00 $35,000 $35,000 S&EC
Backfill (CY)2 382.5 $0.90 $345 RS Means, 2001
Compaction (CY)2 382.5 $2.00 $765 RS Means, 2001
Design and Engineering 25% $10,900 USEPA, 1999
Total $54,502
' Quantity assumes 60% of excavated material is dumped offsite; price refers to hauling distance of 2 miles.
z Quantity assumes 30% of excavated material is used in earthworks, and 10% is spread elsewhere onfite.
Table 3. Cost Estimation for Bioretention Areas (LID Design)
~- Components Quantity Price Total Source
Excavation (CY) 1,650 $3.74 $6,171 RS Means, 2001
Hauling' (CY) 1,485 $3.56 $5,287 RS Means, 2001
Soil (CY) 1,460 $15.00 $21,900 S&EC
Subdrain (LF) 350 $3.50 $1,225 S&EC
Design and Engineering 25% $8,646 USEPA, 1999
Total $43,228
Quantity assumes 90% of excavated material is dumped offsite; price refers to hauling distance of 2 miles.
O TETRATECN~INC. r

Cost Estimation for High Density Residential Development Example January, 2003
RESULTS
The net savings for choosing LID over conventional design was calculated by subtracting the LID
costs from the infrastructure costs (Table 4). The net savings was estimated at about $25,000.
About 45% of the net savings was due to the difference in cost between the bioretention and a
conventional stormwater pond. Differences in storm drain piping and inlets (including catch
basins) represented 11 % and 44% respectively of the net savings for LID.
Table 4. Net Savings to the Developer for Choosing LID over Conventional Design.
Component Conventional LID Net ~
Storm Drain Piping $ 9,300 $6,480 $2,820
Storm Drain Inlets $16,501 $5,500 $11,000
Wet Detention Pond $54,502 - $54,502
Bioretention - $43,228 $ 43,228
Net Savings $25,094
S&EC considered this site as a separate development, as requested by the County, and estimated the
dimensions of an on-site wet detention pond that would remove 85% of TSS from the runoff
generated during the first inch of rainfall. An on-site pond was not designed to meet the peak
discharge runoff rate requirements due to spatial constraints. If on-site ponds were to be employed,
the site would need to be redesigned to accommodate more detention capacity. The additional
detention capacity and site redesign would increase the costs of the conventional design, and the net
savings for LID would be greater than $25,000.
It should be noted that, in the proposed design, the subject development is part of a larger
development, and the entire site will drain to a larger, more economical pond than the one
considered in this analysis. Tetra Tech was unable to compare the off-site pond to the LID design
because the data for the off-site pond was not available. The conventional design cost may be
reduced if the shared, off-site pond is used as a part of the larger development.
REFERENCES
RS Means. 2001. Facilities Construction Cost Data. Robert Snow Means Company, Inc.,
Kingston, Massachusetts.
US EPA. 1999. Preliminary Data Summary of Urban Stormwater Best Management Practices.
Report EPA-821-R-99-012. United States Environmental Protection Agency, Office of Water.
Washington, DC. [Online] Available: http://www.epa.gov/OST/stormwater/
O TETRATECH,IIVG.

TECHNICAL MEMORANDUM:
Cost Comparison of Conventional and Low
Impact Development Designs for a Medium
Density Residential Subdivision Site in
Mecklenburg County, North Carolina
October 2002
Prepared for
Mecklenburg County Land Use and
Environmental Services Agency
Prepared by
Tetra Tech, Inc.
P.O. Box 14409
3200 Chapel Hill -Nelson Highway, Suite 105
Research Triangle Park, NC 27709
[phone]

Cost Estimation for Medium Density Residential Development Example October, 2002
BACKGROUND AND INTRODUCTION
Tetra Tech, Inc. compared the costs of conventional and Low Impact Development (LID)
designs for a medium density residential site in Huntersville, North Carolina. Results of
this cost analysis were presented to the Huntersville Planning Board and Town Council
members on September 4, 2002. This memo provides the detailed documentation for
how the analysis was performed and the results.
For purposes of conducting this cost comparison, the Town of Huntersville in
cooperation with the DMC Land Development Company and Alliance Engineering
provided site plans and information for the subject development. Tetra Tech then
provided redesigns of the property using conventional and LID approaches based on the
Town's proposed density zoning changes with new open space provisions. The overall
lot and street layout were the same for the conventional and LID designs (Figure 1).
However, the designs differed in several aspects. The conventional design had two 5-
foot wide sidewalks, and the streets were drained with curb and gutter and storm drain
systems. The LID plan had one 4-foot sidewalk, and instead of curb and gutter, the LID
plan used bioretention swales to drain the streets and rain gardens to treat stormwater
from each roof and lawn. This analysis showed that a developer would save construction
costs by choosing the LID development instead of the conventional development.
Figure 1. Low Impact Design Site Plan for Subject Development
O TETIIA TE[H~ING.

Cost Estimation for Medium Density Residential Development Example October, 2002
The most significant difference between the two designs is the chosen goal for
stormwater management. The goal of the conventional design was to effectively move
stormwater off the development site, through stormwater conveyance systems in the
development and nearby streams (i.e. efficient drainage). In contrast, the goal of the LID
design was to retain stormwater onsite (specifically to retain the runoff generated by the
development during the 2-year storm or 3.1-inch storm event).
The previous cost analysis was expanded in this memo to consider a wet detention pond
in the conventional design. This pond would remove a similar amount of sediment when
compared to LID but was not as effective at managing nutrients and water volume. In
addition, the conventional design that included the pond was more expensive than the
LID and initial conventional design.
Please note that a few corrections were made to the original analysis. Catch basins were
added to provide more realistic costs of the conventional development. A range of costs
was estimated for the infrastructure as well as the LID to account for variation in design.
Curb and gutter and storm drain frontage improvements were added to the conventional
development to equal the length of the LID frontage improvements. Driveway culverts
were added to the Swale costs so that the swales would have the potential to convey large
volumes of runoff.
ESTIMATION SOURCES AND METHODS
The rain gardens and swales used in the LID plan are among several Best Management
Practices (BMPs) used as residential stormwater controls. Sources exist to estimate the
costs of BMPs, but the estimates vary widely in both form and value. Two frequently
used methods are the cubic-foot and unit price methods.
In the cubic-foot method, the estimator uses an equation that relates cubic-foot capacity
to cost for each BMP. Wiegand et al. (1986) and Brown and Schueler (1997) estimate
equations that predict the cost of BMPs per cubic-foot capacity. Cubic-foot estimates
return about a 15% margin of error and are most appropriate for preliminary estimates
(RS Means, 1990). For example, CH2M-Hill used cubic-foot estimates to report general
costs within a watershed, and The Center for Watershed Protection used cubic-foot
estimates to compute the difference in costs between development types. EPA (1999)
and Rouge River (1997, 2001) also report cubic-foot estimates as guidelines for planning.
Unlike the cubic-foot method, the unit price method is accurate to about a 5% margin of
error and is most appropriate when site plans are available (RS Means, 1990). To obtain
unit prices, an estimator uses a cost data manual that lists average unit prices of
construction materials, labor, and equipment. The Robert Snow Means Company (RS
Means) publishes construction cost data manuals with BMP components. The cost
estimator can take the design of a BMP, separate it into components, and estimate the
total cost using RS Means data. The cost estimator also can convert national averages to
local prices with RS Means city indexes. Southeastern Wisconsin Regional Planning
O TETRATECH,INC.
2

Cost Estimation for Medium Density Residential Development Example October, 2002
Commission (SEWRPC, 1991) estimated BMP costs with the unit price method; their
work is cited in Baker (2001), Center for Watershed Protection (1998), EPA (1999), and
Rouge River (1997, 2000).
The unit price method was chosen for the majority of this estimation. Unit prices were
obtained from RS Means Manuals, North Carolina Agricultural Extension, US EPA, and
the Low Impact Development Center. The cubic-foot method was used for estimating the
cost of the wet detention pond.
COST ESTIMATION PROCEDURE
This study was designed to use a single development site as an example for potential,
future development of Huntersville. Because of this, it was important to estimate the
costs as conservatively as possible. Minimum and maximum prices were chosen for each
component, and a minimum, midpoint, and maximum were reported for the total costs.
All prices, both national averages and non-local prices, were converted to Charlotte
prices for the year 2002.
Conventional Infrastructure
Price ranges for conventional infrastructure were chosen to capture the variation in
choice of design (Table 1). Sidewalk prices varied depending on the extent of the gravel
base. The curb and gutter costs varied by contractor and method of installation (e.g. pre-
cast or cast in place). The street drainage was chosen as a typical system but not
designed for a specific storm event; this drainage included storm drain piping and catch
basins. The storm drain prices varied by width of pipe; the minimum price refers to the
minimum diameter required by NCDOT, and the maximum price refers to the pipe that
would carry a similar flow to the LID Swale during a bank full event. The average
number of catch basins was assumed to be 20, and catch basin depth was varied from 4 to
8 feet. Catch basin inspection and cleaning was not included because it would cost the
developer less than $60 for the first year (Rouge River, 2001).
O TETRA TECN.INC

Cost Estimation for Medium Density Residential Development Example October, 2002
Table 1. Prices for Infrastructure
Component Minimum Maximum Source Notes
Sidewalk $2.41 $3.06 RS Means, 2001 4" thick, 3000 psi, CIP,
(per SF) 6x6-W1.4xW1.4
mesh, broomed finish
Curb and Gutter $12.88 $15.91 Barrus Construction Straight with 6-inch
(per LF) and RS Means, 2001 high curb and 6-inch
thick gutter, wood
forms; 30-inch wide,
0.066 CY per LF
Storm Drain $13.36 $38.94 RS Means, 2001 Porous wall concrete
(per LF) underdrain, standard
strength, 15" to 30"
diameters
Catch Basins $586.21 $1045.35 RS Means, 2001 Concrete, pre-cast, 4
(per unit) feet I.D., 4 to 8-feet
deep
Conventional Stormwater Control
A wet detention pond was added to the conventional development as a conventional
stormwater control. This pond would detain the first inch of storm runoff and is designed
to remove 85% Total Suspended Solids (TSS) from runoff during along-term average
storm (NCDENR, 1999). The cost of the pond was varied by choice of depth, from 6 to 8
feet, and the pond volume ranged from about 200,000 to 230,000 cubic feet as calculated
from North Carolina state guidelines (NCDENR, 1999). The pond cost was estimated
with cubic foot equations and ranged from about $150,000 to $190,000 (Table 2). ~ Pond
maintenance, ranged from $5,000 to $11,000 per year and included lawn care, sediment
and litter removal, inspection, and administration (EPA, 1999 and SEWRPC, 1991).
1 -If this pond was sized to retain the runoff from the 2-year storm, it would cost significantly more and
require significantly more space within the development.
O TETRATECH,INC.
4

Cost Estimation for Medium Density Residential Development Example October, 2002
Table 2. Cost Estimation of Wet Detention Pond
Type of Pond Equation" Design and
Engineering Cost Equation Source
Ponds greater C - 40.6V0~64 25% $154,361 SEWRPC, 1991
than 100,000 CF
All Ponds C = 30.6V0~70 32% $186,805 Brown and
Schueler, 1997
*C = cost; V =cubic foot volume, including permanent and temporary pool.
Low Impact Design
The two stormwater controls employed in the subject development LID plan were
bioretention swales and rain gardens. Construction, maintenance, design, and
engineering costs were considered. Design and engineering costs were estimated as 25%
of the total BMP construction costs (EPA, 1999).
Bioretention Swales
The cost estimate of the bioretention swales was separated into grass Swale and trench
components (Table 3). The grass swales had a width of 10 feet, and their price included
grading, seeding, topsoil stockpiling and spreading, straw spreading, and watering. A
breakdown of component costs was not necessary for the grass swales because the
description for the linear-foot price matched the subject development Swale design. No
variation was assumed for the grass swales cost. Culverts were included for each 9-foot
wide driveway.
The cost for the trench included excavation, hauling, gravel, drainage pipes, and
conditioned soil (Table 4). The cost per cubic yard of excavation and hauling varied by
type of machinery used and hauling distance. Prices for all other components were
assumed fixed.
Table 3. Total Prices for Bioretention Swale ($ per linear foot)
BMP Minimum Maximum... Source
Trench $3.64 $6.23 See Table 3
Grass Swale* $3.07 $3.07 RS Means, 1997
Total for Swale $6.71 $9.30
Driveway
Culvert*'' $13.83 $40.10 RS Means, 2001
* Includes 10' wide, grading, seeding, topsoil stockpiling and spreading, straw, and watering.
**Price of conventional storm drain piping plus excavation.
O TETRATECH,INC.

Cost Estimation jor Medium Density Residential Development Ezample October, 2002
Table 4. Unit Prices for Trench Components
Component Price Source
#57 Stone $8.00 per CY Weinstein, 2001
4" HDPE Underdrain $1.00 per LF Weinstein, 2001
Excavation and Hauling Minimum: $2.00 per CY
Maximum: $5.38 per CY Weinstein, 2001
Rouge River, 1997
Conditioned Soil $15.00 per CY Weinstein, 2001
Rain Gardens
The cost estimate for rain gardens was separated into 9 components (Table 5).
Vegetation accounted for 60% of the variation in cost for rain gardens. The minimum
vegetation price represents vegetation transplanted on-site, and the maximum price is for
gardens planted with mature nursery plants. Excavation and hauling varied by type of
soil and hauling distance, and this variation accounted for about 20% of the total
variation. Soil, mulch, and stone varied slightly by either brand or price source. Stone,
pipes, weirs, and end sections were assumed to not vary significantly.
Table 5. Unit Prices for Rain Garden Components
Components Minimum Maximum
Excavation (per CY) $3.74 $4.70
Hauling (per CY)* $0.00 $5.17
Pipes -- 4" HDPE Underdrain $1.00 $1.00
-- 15" outlet $10.00 $10.00
Stone (per CY) $8.00 $8.00
Soil (per CY) $10.80 $15.00
Stone Weirs (per unit) $125.00 $125.00
End Section (per unit) $125.00 $125.00
Vegetation (per SF)`* $0.30 $2.00
Mulch: 2" of wood chips (per SF) $0.16 $0.28
*Ranges from dumping on-site to 5 miles round trip.
**Ranges from on-site transplanting to mature nursery plants.
Maintenance
For the bioretention swales, the cost of cutting grass with a tractor was used as the
minimum cost of maintenance (Table 6). The maximum cost included grass cutting,
sediment removal, drainpipe inspection, and reseeding. Grass cutting was assumed to
O TETRA TE[M,INC.
6

Cost Estimation for Medium Density Residential Development Example October, 2002
occur biweekly. This was a conservative estimate because grass cutting should occur less
frequently, given that swales require a grass height of about 4 to 6 inches (Rouge River,
1997).
Mulching comprised the majority of rain garden maintenance costs (Table 6). Mulching
was assumed to occur twice per year as a conservative estimate, but it is likely to occur
less frequently. Mulch prices varied depending on how the labor is performed. The cost
of machine spreading was used for the minimum price, and the cost of hand spreading
was used for the maximum price. Pipe unclogging and plant replacement were not
considered because the developer is unlikely to face these costs in the first year.
Table 6. Yearly prices for Maintenance of Low Impact Design
Component Minimum Maximum Source
Grass swales $0.22 / LF $0.51 / LF RS Means, 2001 and Rouge River, 2001
Rain Gardens $72 per unit $135 per unit RS Means, 2001 and Hunt, 2002
Discounting
The developer's maintenance costs were calculated for one year after construction,
assuming that homeowners and the government would maintain the BMPs after the first
year. These maintenance costs were discounted because they occur one year after
construction. During that one year's time, the money allocated to maintenance can earn
an average return of 6% in another investment (Helfert, 1997). The developer's
maintenance costs were decreased by 6% and compounded at the end of each year.
Purpose of Spreadsheets
Spreadsheets were used to organize the price and quantity data (Table 7). Each
development design had a separate set of spreadsheets. This series of spreadsheets
allows for ease of movement between stages of estimation, and the spreadsheets have
room for expansion by adding components, increasing maintenance years, or considering
property value and growth rates.
O TETRA TECN.INC.
7

Cost Estimation for Medium Density Residential Development Example October, 2002
Table 7. Description of Spreadsheets
~
Sheet Title - -
Description
P_Mnt Lists minimum and maximum maintenance prices, with specifications and
sources.
P_Cnst Lists minimum and maximum construction prices, with specifications and
sources.
Dim Lists dimensions of all components in the same units as the prices.
P_Min Organizes minimum prices (from P_mnt and P_Const) in a table with room for
price changes over 10 years.
P_Max. Organizes maximum prices (from P_mnt and P_Const) in a table with room for
price changes over 10 years
O Organizes dimensions from Dim in a table by timing of construction and
maintenance over 10 years.
CashFlow_Min Displays product of Q and P_Min tables: the cost of each component, subtotals
for each category, total cost, and the present value of total cost (maintenance is
discounted). This sheet also has a separate cash flow table used for calculating
the distribution of costs (all costs are discounted separately in this table).
CashFlow_Max Same as CashFlow_Min but uses P_Max instead of P_Min.
Results Summarizes the total costs for each category; shows incremental cost between
LID and conventional; shows distribution of costs, both for the design in question
and incrementally.
RESULTS
Net Cost savings
The LID development provides a net cost savings for the developer over conventional
design. The total cost of LID stormwater controls includes the cost of 94 rain gardens,
10,740 feet of bioretention swales, and 1 year of discounted maintenance by the
developer (Table 8). The net savings for choosing LID over conventional design was
calculated by subtracting the LID costs from the infrastructure savings (Table 9). This
net savings ranged from about $190,000 to $410,000 with a midpoint of $300,000. When
a wet detention pond is included in the conventional site, the net cost savings for LID
ranged from about $350,000 to $610,000 with a midpoint of $480,000 (Table 10). Since
the cost range for LID was less than the conventional cost range, LID is likely to be the
efficient choice for developments similar to the subject development (Figure 2).
O TETRATECN,INC.
8

Cost Estimation for Medium Density Residential Development Example October, 2002
Table 8. Total Costs of Low Impact Design for Subject Development
-- _
Component __
Minimum
Maximum
Midpoint
Rain Gardens $78,678 $137,146 $107,912
Bioretention Swales $83,727 $133,793 $108,760
Design and Engineering $40,601 $67,735 $54,168
Maintenance (2002 dollars) $8,268 $16,549 $12,409
Total $211,274 $355,223 $283,249
Table 9. Cost Savings to the Developer for Choosing LID over Conventional Design.
Component Minimum Maximum
Midpoint
Curb and Gutter
(11,586 minus 1,100 ft)
$149,265
$184,283
$166,774
Sidewalks: Conv. minus LID
(57,930 minus 23,172 ftZ)
$83,782
$106,295
$95,039
Storm Drain Piping
(12,686 minus 1,100 ft)
$154,836
$451,209
$303,023
Catch Basins
(20 units)
$11,724
$20,907
$16,316
Subtotal $399,608 $762,694 $581,151
LID costs ($211,274) ($355,223) ($283,249)
Net Savings $188,334 $407,471 $297,902
Table 10. Cost Savings to the Developer for Choosing LID over Conventional Design
with a Stormwater Pond.
Component Minimum Maximum Midpoint
Subtotal from Table 9
$399,608 $762,694 $581,151
Wet Detention Pond
(cost plus 1-year maint.) $158,993 $198,013 $178,503
Subtotal
$558,601
$960,707
$759,654
LID costs
($211,274)
($355,223)
($283,249)
Net Savings
$347,327
$605,484
$476,406
O TETRA TECH.INC.
9

Cost Estimation for Medium Density Residential Development Example October, 2002
$1,200,000
$1,000,000
$800,000
$600,000
$400,000
$200,000
$0
Conventional Conventional with Pond LID
Figure 2. Total Cost Ranges for the Three Development Designs
Limitations
The net cost savings reported here does not include maintenance costs for Huntersville
town government or the homeowner's association. Once the developer sells a majority of
the homes, the homeowners would be responsible for the rain garden maintenance. The
town government most likely would maintain the swales, and Swale maintenance is
typically more expensive than maintenance of storm drain systems (SEWRPC, 1991).
However, LID maintenance would be similar in cost to the maintenance of the wet
detention pond.
The conservative estimates in this study provide a probable range of cost savings for
developments similar to the subject development. Each site will differ in the feasibility
and cost of LID, and the developers need to evaluate which BMPs are most economic for
their specific sites.
O TETRATECH,INC.
10

Cost Estimation for Medium Density Residential Development Example October, 2002
REFERENCES
Brown, W., and T. Schueler. 1997. The Economics of Stormwater BMPs in the Mid-
Atlantic Region, Final Report. Center for Watershed Protection. Chesapeake Research
Consortium.
Center for Watershed Protection. 2000. The Economics of Stormwater Treatment: An
Update. Technical Note #90. Watershed Protection Techniques, 2(4): 395-499. [Online]
Available: http://www.stormwatercenter.net/
Baker, S. Paige. 2001. Management Options Preliminary Cost Estimate, Upper Neuse
River Watershed Management Plan. CH2M-Hill.
Helfert, Erich A. 1997. Techniques of Financial Analysis, 9th edition. McGraw-Hill,
New York.
Hunt, William F. 2002. Urban Waterways: Designing Rain Gardens (Bio-retention
Areas). North Carolina Cooperative Extension, AG-588-3. [Online] Available:
http://www.bae.ncsu.edu/people/faculty/hunt/index.html
NCDENR. 1999. Stormwater Best Management Practices. North Carolina Department
of Environment and Natural Resources Division of Water Quality Water Quality Section.
[Online] Available:
http://h2o.enr. state.nc.us/su/PDF_Files/S W_Documents/BMP_Manual.PDF
NCDOT. 2001. Subdivision Roads Minimum Construction Standards. North Carolina
Department of Transportation Division of Highways. [Online] Available:
http://www. doh. dot. state.nc.us/preconstruct/highway/dsn_srvc/value/manuals/newsubdiv
3-23-OO.pdf
RS Means. 1997. Environmental Restoration Unit Cost Book. Robert Snow Means
Company, Inc., Kingston, Massachusetts.
RS Means. 2001. Facilities Construction Cost Data. Robert Snow Means Company,
Inc., Kingston, Massachusetts.
RS Means. 1990. Means Estimating Handbook. Robert Snow Means Company, Inc.,
Kingston, Massachusetts.
Rouge River. 1997. Planning and Cost Estimating Criteria for Best Management
Practices, 1st edition. Rouge River National Wet Weather Demonstration Project,
Wayne County, Michigan.
O TETRATECN.INC
11

Cost Estimation for Medium Density Residential Development Example October, 2002
Rouge River. 2001. Planning and Cost Estimating Criteria for Best Management
Practices (Update). Rouge River National Wet Weather Demonstration Project, Wayne
County, Michigan. Apri12001. TR-NPS25.00.
SEWRPC. 1991. Costs of Urban Nonpoint Source Water Pollution Control Measures.
Southeastern Wisconsin Regional Planning Commission.
US EPA. 1999. Preliminary Data Summary of Urban Stormwater Best Management
Practices. Report EPA-821-R-99-012. United States Environmental Protection Agency,
Office of Water. Washington, DC. [Online] Available:
http://www. epa.gov/O ST/stormwater/
Weinstein, Neil. 2001. Columns Subdivision LID cost estimates. Low Impact Design
Center. Beltsville, Maryland.
Weinstein, Neil. 2002. Low Impact Design Center. Beltsville, Maryland (personal
communication).
Wiegand, C., T. Schueler, W. Chittenden and D. Jellick. 1986. "Cost of Urban Runoff
Controls." pp. 366-380. In: Proceedings of an Engineering Foundation Conference.
Urban Water Resources. ASCE. Henniker, New Hampshire June 23-27, 1986.
O TETRATECH.INC.
12

TECHNICAL MEMORANDUM:
Cost Comparison of Conventional and Low Impact
Development Designs for a Commercial Site in
Mecklenburg County, North Carolina
January 2003
Prepared for
Mecklenburg County Land Use and
Environmental Services Agency
Prepared by
Tetra Tech, Inc.
P.O. Box 14409
3200 Chapel Hill -Nelson Highway, Suite 105
Research Triangle Park, NC 27709
[phone]

Cost Estimation for Commercial Development Example -Revised January, 2003
BACKGROUND AND INTRODUCTION
Tetra Tech, Inc. compared the costs of conventional and Low Impact Development (LID) designs
for a proposed commercial development site in Huntersville, North Carolina. This memo presents
the preliminary design and cost comparison.
For purposes of conducting this cost comparison, the Town of Huntersville provided conceptual site
plans and information for the subject development (Figure 1). Mecklenburg County estimated the
dimensions of the conventional stormwater system for this site. Tetra Tech, with design support
from Soil and Environmental Consultants (S&EC), then provided a conceptual design of an LID
stormwater system (Figure 2). It should be noted that the LID design was constrained by the Town
and County to work with existing building footprints, road configurations, parking areas, and use of
curb and gutter. The conceptual nature of the designs also limited precise sizing of stormwater
systems.
The LID design achieves more stringent water quantity and quality guidelines than the conventional
design. The conventional site complies with the existing Mecklenburg County requirements, which
are to remove 85% of Total Suspended Solids (TSS) from the runoff of the first inch of rainfall and
to approximate pre-development peak discharge runoff rates for the 2-year and 10-year storms. The
LID design complies with Huntersville's proposed water quality regulations for the Transit-
Oriented Development, Town Center, and Highway Commercial Zoning Districts. The proposed
requirements for these high-density districts are to meet the existing requirements described above
and to control and treat the post-development minus pre-development runoff volume for the 1-year,
24-hour storm. By complying with more stringent guidelines, the LID design controls nearly 1.4
acre-feet more runoff volume and removes more pollutants than the conventional design. ~
This analysis shows that the LID design would cost about $10,000 more than the conventional
design. The difference in cost is due, in part, to the difference in stormwater criteria and the design
constraints. The accuracy of this study is limited due to the conceptual nature of the designs.
/1~Te~tra Tech will provide a detailed comparison of pollutant removal and runoff retention.
L~j_ ~ TETRATECH.INC.

Cost Estimation for Commercial Development Example -Revised January, 2003
Figure 1. Conventional Site Plan for the Subject Development
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Figure 2. Low Impact Development Plan for the Subject Development
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O TETRATECH,INC.

Cost Estimation for Commercial Development Example -Revised January, 2003
ESTIMATION SOURCES AND METHODS
This study was designed to use a development site as an example for potential, future development
of Huntersville. All prices, both national averages and non-local prices, were converted to
Charlotte prices for the year 2003. Unit prices were obtained from S&EC, RS Means Manuals, and
Yarbrough-Williams and Houle, Inc., a development engineering firm local to Huntersville. The
sites did not differ in pavement or curb and gutter.
Storm drain prices were obtained from Yarbrough-Williams and Houle, Inc. and RS Means (2001)
(Table 1). Mecklenburg County designed a conceptual layout of the conventional storm drain
system. The County then estimated that bioretention in the LID Design would reduce flow through
the storm drain system by about 5% to 15%, depending on the size and slope of each parking lot;
therefore, storm drain pipe costs for LID were estimated as a median of 10% less than conventional
pipe costs. The cost of manholes, inlets, end sections, and head walls were assumed to be equal for
the LID and conventional designs.
Table 1. Cost Estimation of Storm Drain Systems
Component Quantity Unit Price Conventional LID' Source
15-inch Reinforced Concrete
Pipe (RCP) 2090 $15/ LF $31,350 $28,215 Yarbrough-Williams and Houle, Inc.
18-inch RCP 1550 $18/ LF $27,900 $25,110 Yarbrough-Williams and Houle, Inc.
24-inch RCP 880 $24/ LF $21,120 $19,008 Yarbrough-Williams and Houle, Inc.
30-inch RCP 610 $30/ LF $18,300 $16,470 Yarbrough-Williams and Houle, Inc?
36-inch RCP 1020 $36/ LF $36,720 $33,048 Yarbrough-Williams and Houle, Inc.
48-inch RCP 150 $55/ LF $8,250 $7,425 RS Means, 2001
60-inch RCP 50 $88/ LF $4,400 $3,960 RS Means, 2001
Inlets/ Manholes 80/ 4 $1,100/ unit $92,400 $92,400 Yarbrough-Williams and Houle, Inc.
24-inch Flared End Section 1 $400/ unit $400 $400 Yarbrough-Williams and Houle, Inc.
36-inch Flared End Section 2 $675/ unit $1,350 $1,350 Yarbrough-Williams and Houle, Inc.
60-inch Head Wall 1 $1,600/ unit $1,600 $1,600 Yarbrough-Williams and Houle, Inc.Z
Total Cost $243,790 $228,986
' LID pipes are assumed at 90% of the conventional cost based on a median 10% reduction in flow.
2 Price was extrapolated from Yarbrough-Williams and Houle, Inc. cost data and confirmed with RS Means (2001).
The costs of the stormwater ponds and bioretention included excavation, hauling, soil fill, and outlet
structures. Since this analysis was conducted at a conceptual level, S&EC based the price of the
pond outlet structures2 on similar project experience and not site-specific routing data. As a
conservative estimate, the outlets were assumed to be equal in cost; however, in the final site
design, the LID design outlet is likely to cost less than the conventional design outlet.
It was assumed that pond and bioretention landscaping would fulfill some of the landscaping
z The pond outlet structures would include the following components: riser, barrel, anchor slab, flared end section (or
/head wall), riprap outlet protection apron, erosion protection blanket, and seepage control device.
TETRATECH, INC. 3

Cost Estimation for Commercial Development Example -Revised January, 2003
requirements of the development, and, therefore, planting costs were not considered. Design and
engineering costs were estimated as 25% of the total construction costs (US EPA, 1999).
The cost estimations of the stormwater ponds and bioretention are presented in Tables 2 through 4.
The conventional design achieves existing requirements with a 1-acre wet detention pond (Table 2).
The original conventional plan allocated 0.5 acres for a stormwater pond; therefore, the street and
building layout would need to be altered to accommodate a 1-acre pond. Effectively, this redesign
would likely result in less building, parking, or landscaping square footage, or the creation of a
more urban design with increased building height and/or parking decks. The cost of this redesign is
not considered in this estimation.
In the LID design, bioretention areas (Table 3) retain the volume of the 1-year storm post-
development minus pre-development volume and partially control peak discharge. A dry detention
pond controls the additional peak discharge in the LID design (Table 4).
Table 2. Cost Estimation of Wet Detention Pond (Conventional Design)
Component Quantity Unit Price Total Source
_
Excavation CY 22,741 $3.74 $85,050 RS Means, 2001
Haulin ' CY 13,644 $3.56 $48,574 RS Means, 2001
Backfill CY z 6,822 $0.90 $6,152 RS Means, 2001
Com action CY 2 6,822 $2.00 $13,644 RS Means, 2001
Outlet Unit 1 $100,000 $100,000 S&EC
Desi nand En ineerin 25% $63,355 USEPA, 1999
Total $316,776
~ Quantity assumes 60% of excavated material is dumped offsite; Price refers to hauling distance of 2 miles.
z Quantity assumes 30% of excavated material is used in earthworks, and 10% is spread elsewhere onsite.
Table 3. Cost Estimation for Bioretention Areas (LID Design)
Components Quantity Price Total ..: - - Source
Excavation (CY) 6,82 $3.7 $25,52 RS Means, 2001
Hauling (CY) 6,14 $3.5 $21,86 RS Means, 2001
Soil (CY) 6,06 $15.0 $90,99 S&EC
Subdrain (LF) 2,30 $3.5 $8,05 S&EC
Design and Engineering 25% $36,60 USEPA, 1999
otal $183,041
~ Quantity assumes 90% of excavated material is dumped offsite; price refers to hauling distance of 2 miles.
O TETRA 7ECN.INC.

Cost Estimation for Commercial Development Example -Revised January, 2003
Table 4. Cost Estimation for Dry Detention Pond (LID Design)
Component Quantity Unit Pnce __ Total_ Source
Excavation CY 3,950 $3.74 $14,773 RS Means, 2001
Haulin ' CY 2,370 $3.56 $8,437 RS Means, 2001
Backfill CY z 1,185 $0.90 $1,069 RS Means, 2001
Com action CY z 1,185 $2.00 $2,370 RS Means, 2001
Outlet Unit 1 $100,000 $100,000 S&EC
Desi nand En ineerin 25% $31,662 USEPA, 1999
Total $158,311
~ Quantity assumes 60% of excavated material is dumped offsite; price refers to hauling distance of 2 miles.
z Quantity assumes 30% of excavated material is used in earthworks, and 10% is spread elsewhere onsite.
RESULTS
The net cost for choosing LID over conventional design was calculated by subtracting the LID costs
from the conventional costs. The net cost was estimated at about $10,000 (Table 5). The difference
in cost is due, in part, to the difference in stormwater criteria and the design constraints. The
accuracy of this study is limited due to the conceptual nature of the designs.
Table 5. Net Cost to the Developer for Choosing LID over Conventional Design
Component Conventional LID Net
torm Drain System $243,79 $228,98 $14,80
Dry Detention Pond $316,77 $158,311 $158,46
Bioretention Areas $ $183,041 $183,041
otal $560,56 $570,33 $9,772
In essence, the LID design costs more than the conventional design because the LID design controls
nearly 1.4 acre-feet more runoff than the conventional design. The conventional design controls
about 1.5 acre-feet while the LID design controls about 2.9 acre-feet.
It also should be noted that the conventional costs do not include the cost of allocating an additional
one-half acre to a wet detention pond to meet the existing requirements. This cost to the developer
may be substantial since the pond would likely displace parking, building, and/or landscape square
footage.
A constraint on the size of the dry detention pond also contributed to the net cost of the LID design.
The dry detention pond was used to control the peak discharge of the 2-year and 10-year storms.
Since the dry detention pond was limited to one-half acre by the original design, the bioretention
cells had to be designed to handle more volume to help control peak discharge for the 10-year storm
event.
Several small areas of the site will need to be graded to ensure proper drainage. Alternatively, in
the LID design, S&EC suggested the possible use of vegetated filter basins for these areas (Figure
2).
5
TETRA TECN INC.

Cost Estimation for Commercial Development Example -Revised January, 2003
REFERENCES
RS Means. 2001. Facilities Construction Cost Data. Robert Snow Means Company, Inc., Kingston,
Massachusetts.
US EPA. 1999. Preliminary Data Summary of Urban Stormwater Best Management Practices. Report
EPA-821-R-99-012. United States Environmental Protection Agency, Office of Water. Washington, DC.
[Online] Available: http://www.epa.gov/OST/stormwater/
O TETRA TECH~INC.

TECHNICAL MEMORANDUM:
Cost Comparison of Conventional and Low Impact
Development Designs for a Proposed Elementary
School Site in Mecklenburg County, North
Carolina
January, 2003
Prepared for
Mecklenburg County Land Use and
Environmental Services Agency
Prepared by
Tetra Tech, Inc.
P.O. Box 14409
3200 Chapel Hill -Nelson Highway, Suite 105
Research Triangle Park, NC 27709
[phone]

Cost Estimation for Proposed School Site Example -Final Version, Revised January, 2003
BACKGROUND AND INTRODUCTION
Tetra Tech, Inc. compared the costs of traditional and Low Impact Development (LID) designs for a
proposed elementary school in Huntersville, North Carolina. This memo presents the preliminary
design and cost comparison.
For purposes of conducting this cost comparison, the Town of Huntersville provided site plans and
information for the proposed school site using a conventional stormwater approach. Tetra Tech,
with design support from Soil and Environmental Consultants (S&EC), then provided a redesign of
the property using an LID approach. It should be noted that the LID design was constrained by the
Town and County to work with existing building footprints, road configurations, parking areas, and
use of curb and gutter.
The LID design achieves stricter water quantity and quality guidelines than the traditional design.
The traditional site is designed to comply with the existing Mecklenburg County requirements,
which are to remove 85% of Total Suspended Solids (TSS) from the first inch of runoff and to
approximate pre-development peak discharge runoff rates for the 2-year and 10-year storms. The
traditional plan achieves these requirements with a curb and gutter system that directs runoff into a
stormwater pond.
The LID design complies with Huntersville's proposed regulation to retain the post-development
minus pre-development runoff volume for the 2-year storm. By complying with stricter guidelines,
the LID design retains far more runoff onsite and removes considerably more pollutants than the
traditional design.' While the LID plan uses curb and gutter, the runoff is directed into bioretention
areas throughout the site. The bioretention areas retain the excess volume of the 2-year storm post-
development minus pre-development volume and partially control peak discharge. A dry detention
pond controls the additional peak discharge.
This analysis shows that the current LID design costs about $50,000 more than the traditional
design. The higher cost is due, in part, to the high amount of impervious surface onsite and the
resulting volume of runoff. A medium or low-density development would offer more options for
reducing infrastructure costs than a highly impervious development like the proposed school site.
The LID design also costs more than the conventional design because the proposed regulations
require more expensive stormwater controls than the existing ordinance.
1 Tetra Tech will provide a detailed comparison of pollutant removal and runoff retention.
TETRATECN INC.

Cost Estimation for Proposed School Site Example -Final Version, Revised January 2003
Figure 1. Traditional Site Plan for Proposed School
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Figure 2. Low Impact Development Plan for Proposed School
~~Huildings 2.1 AC

' 1 r't. _4 sS'AS"d~1 Sidewalks 0.7 AC
'
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-R g Nmual Areas 2.5 AC

dsceped Areas 7.5 AC
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p~-Buildings 2.1 AC
f 'ISidewalks 0.7 AC
r _~Play Areas 0 3 AC
~IParking 1 U AC
7Roads 1.0 AC
Wet Detrntion Pond 0.6 AC
~Remaming Natural Areu 2.0 AC
~!~LTETIIA TECH.INC.

Cost Estimation for Proposed School Site Example -Final hersion, Revised January, 2003
ESTIMATION SOURCES AND METHODS
This study was designed to use a development site as an example for potential, future development
of Huntersville. All prices, both national averages and non-local prices, were converted to
Charlotte prices for the year 2002. Infrastructure prices (Table 1) were obtained from Yarbrough-
Williams and Houle, Inc., a development engineering firm local to Huntersville. Pond and
bioretention costs were estimated with unit prices obtained from S&EC and RS Means Manuals.
Table 1. Prices for Infrastructure
-- --
Component Price Source ~ Notes
Street and Parking Lot $9.50 Yarbrough-Williams 8" & 2'/z"
Paving (per SY) and Houle, Inc.
Storm Drain (per LF) $24.00 Yarbrough-Williams 24" Reinforced Concrete Pipe
and Houle, Inc.
Catch Basins (per unit) $1,100 Yarbrough-Williams Single
and Houle, Inc.
The costs of the stormwater ponds and bioretention areas included excavation, hauling, soil fill, and
outlet structures (Tables 2 through 4). Since this analysis was conducted at a conceptual level,
S&EC based the price of the pond outlet structure2 on similar project experience and not site-
specific routing data. Therefore, the price of the outlet may vary considerably depending on the
stormwater routing of the final site design. It was assumed that pond and bioretention landscaping
would fulfill some of the landscaping requirements of the development, and, therefore, planting
costs were not considered. Design and engineering costs were estimated as 25% of the total
construction costs (US EPA, 1999).
Table 2. Cost Estimation of Wet Detention Pond (Conventional)
Component Quantity Unit Price Total Source.; _
Excavation (CY) 9,680 $3.74 $36,203 RS Means, 2001
Hauling" (CY) 4,840 $3.56 $17,230 RS Means, 2001
Outlet (Unit) 1 $75,000 $75,000 S&EC
Design and Engineering 25% $32,108 US EPA, 1999
Total $160,542
*Quantity assumes half of excavated material is dumped offsite; Price refers to hauling distance of 2 miles.
z The pond outlet structures would include the following components: riser, barrel, anchor slab, flared end section (or
head wall), riprap outlet protection apron, erosion protection blanket, and seepage control device.
TETRATECN INC.

Cost Estimation for Proposed School Site Example -Final Version, Revised January, 2003
Table 3. Cost Estimation for Bioretention Areas (LID)
Components Quantity Price Total Source
Excavation (per CY) 5,082 $3.74 $19,007 RS Means, 2001
Hauling' (per CY) 2,541 $3.56 $9,046 RS Means, 2001
Soil (per CY) 4,518 $15.00 $67,763 S&EC
Subdrain (per LF) 1,278 $3.50 $4,473 S&EC
Total $100,288
*Quantity assumes half of excavated material is dumped offsite; Price refers to hauling distance of 2 miles.
Table 4. Cost Estimation for Dry Detention Pond (LID)
Component Unit Quantity Unit Price Total ~ Source
Excavation CY CY 1,500 $3.74 $5,610 RS Means, 2001
Haulin CY CY 750 $3.56 $2,670 RS Means, 2001
Backfill, Structural Common Earth CY 1250 $0.90 $1,127 RS Means, 2001
Com action CY 1250 $2.00 $2,500 RS Means, 2001
Outlet Unit unit 1 $60,000 $60,000 S&EC
Total $71,907
*Quantity assumes half of excavated material is dumped offsite; Price refers to hauling distance of 2 miles.
O TETRA TECH,INC. 4

Cost Estimation for Proposed School Site Example -Final Version, Revised January, 2003
RESULTS
The net cost for choosing LID over conventional design was calculated by subtracting the LID costs
(Table 5) from the infrastructure costs (Table 6). The net cost was estimated at about $50,000.
Table 5. Total Costs of Low Impact Design for Proposed School
Component
Bioretention Cost 1
$100,288.14
Dry Detention $71,907.21
Design and Engineering $43,048.84
Total $215,244.18
Table 6. Net Cost to the Developer for Choosing LID over Conventional Design.
Component LID Traditional Net
Street and Parking Lot Paving $128,744 $128,744 -
Storm Drain Piping $55,392 $57,216 $1,824
Catch Basins $19,800 $24,200 $4,400
Stormwater Pond - $160,542 $160,542
Subtotal $203,936 $370,702 $166,766
LID costs $215,244 - $(215,244)
Net Cost $(48,478)
O TETgA7ECN.IN[.

Cost Estimation for Proposed School Site Example -Final Version, Revised January 2003
REFERENCES
RS Means. 2001. Facilities Construction Cost Data. Robert Snow Means Company, Inc.,
Kingston, Massachusetts.
US EPA. 1999. Preliminary Data Summary of Urban Stormwater Best Management Practices.
Report EPA-821-R-99-012. United States Environmental Protection Agency, Office of Water.
Washington, DC. [Online] Available: http://www.epa.gov/OST/stormwater/
O TETRATECN,INC.

APPENDIX F
PLANT LIST

SPECIES WATER EXPOSURE HEIGHT
Large Maturing Deciduous Trees TOLERANCE
Red Maple, Swamp Maple High Sun to shade 60'-90'
Acer rubrum
Silver Maple Medium Sun 60'-80'
Acer saccharinum
River Birch High Sun 50'-70'
Betula ni ra
Common Hackberry Medium Sun 40'-55'
Celtis laevi ata
Persimmon Medium Sun 40'-50'
Dios ros vir iniana
White Ash High Sun 60'-80'
Fraxinus americana
Green Ash High Sun 50'-70'
Fraxinus enns lvanica
Honeylocust Medium Sun 50'-75'
Gleditsia triacanthos
Tulip Poplar Low Sun 60'-150'
Liriodendron tuli i era
Sweet Gum High Sun 75'-100'
Li uidambar s raci ua
Black Gum Medium Sun or shade 50'-100'
N ssa s lvatica
Sycamore Medium Sun to part 75'-100'
Platanus occidentalis Shade
Swamp White Oak High Sun to part 50' - 60'
uercus michauxii shade
Water Oak High Sun to part 50'-75'
uercus ni ra Shade
Willow Oak Medium Sun 60'-80'
uercus hellos
American Elm, White Elm
Ulmus americana Medium Full sun 75'-125'
F-1

SPECIES WATER EXPOSURE HEIGHT
Small Maturin Deciduous Trees TOLERANCE
Box Elder High
Acer ne undo
Paw paw Medium Shade to part 15' - 20'
Asimina triloba shade
American Hornbeam Medium Sun 25'
Car inus caroliniana
Hop Hornbeam Low 25'
Ost a vir iniana
Sourwood Low Sun or shade 20'-30'
Ox dendrum arboreum
Black Willow High Sun 35'
Salix ni ra
Evergreen Trees
Eastern Red Cedar Low Sun 40'-60'
Juni erus vir iniana
Carolina Hemlock Medium Shade or part 30'-80'
Tsu a caroliniana shade
Deciduous Shrubs
Buckeye, Painted Buckeye Medium Sun to shade 9'
Aesculus s lvatica
Tag Alder High Sun to shade 15'
Alnus serrulata
American Beautyberry Low -Medium Sun to part 5' - 10'
Calicar a americana shade
Buttonbush High Sun to shade 3'-6'
Ce halanthus occidentalis
Silky Dogwood High Sun to part 10'-12'
Corpus amomum shade
Hearts-a-bustin', Strawberry bush Medium Sun to part 3' - 5'
Euon mus americana shade
Witch Hazel Medium Sun to shade 5'-6'
Hamamelis vir iniana
Virginia-willow Low Sun to shade 6'
Itea vir inica
Deciduous holly, Possum haw High Sun to part 20'
Ilex decidua shade
Winterberry High Sun to part 6'-12'
Ilex Montana shade
Spicebush High Shade 6'-12'
Lindera benzoin
Pinxter Azalea Medium Sun to part 3' - 6'
Rhododendron nudi orum shade
F-2

SPECIES WATER EXPOSURE HEIGHT
TOLERANCE
Red chokeberry High Sun to shade 6'-9'
Sorbus arbuti olia
Sparkleberry Medium Sun to part 6'-10'
Vaccinium arboreum shade
Evergreen Shrubs
St. John's Wort Medium Sun to part 1'-3'
H ericum h ericoides shade
Rhododendron, Rosebay, Great Laurel Medium Sun or part 4'-6'
Rhododendron maximum shade
Herbs and Flowers
Arrow arum High Sun 1'-2'
Peltandra vir inica
Cardinal flower High Sun to part 1'-5'
Lobelia cardinalis shade
Christmas fern Medium Sun to part 2'
Pol stichum acrostichoides shade
Cinnamon fern High Sun to part 2' - 4'
Osmunda cinnamomea shade
Clearweed High Part shade 1.5'
Pilea umila
Common or broad-leaved cattail High Sun 6'-9'
T ha lati olia
Dwarf Iris Low Full sun <1'
Iris cristata
Jewelweed, Touch-me-not High Part shade 2'-4'
Im atiens ca ensis
Lizard's Tail High Sun to part 1'-3'
Saururus cernuus shade
Marsh mallow Medium Sun 2' - 4'
Hibiscus moscheutos
Pickerelweed High Sun to part I'- 3'
Pontederia cordata shade
Southern lady fern Medium Part shade 1'-3'
Ath rium as lenioides
Tickseed Sunflower High Sun 1'-3'
Bidens aristosa
Grasses and Sedges
Barnyard grass High Sun 1.5'
Echinochloa crus alli
Bulrush High Sun 1'
Scir us atrovirens
F-3

SPECIES WATER EXPOSURE HEIGHT
TOLERANCE
Carex or sedge High Sun 1'-1.5'
Carex
Rice cutgrass High Sun to part 3'-5'
Leersia o zoides shade
Switch rass Low Sun 4' - 5'
River oats, Spikegrass
Chasmanthium lati olium Medium Sun 1'-3'
Rush High Sun 1'-1.5'
Juncus
Ground Covers
Phlox Low Sun to part 0.5'
Phlox nivalis shade
IMPORTANT NOTE: Variations from the above plant list are allowed provided they
are indicated on Planting Plans and approved by Mecklenburg County Land Use and
Environmental Services. However, the following plant species are considered invasive
exotics for the southeast and should not be used. This list was compiled by Allison
Schwarz and Johnny Randall with the N.C Botanical Gardens and is available at
www.ncb~unc.eduiplants-to-avoid-S EUS.htm
Acer platanoides -Maple, Norway - Aceraceae
Ailanthus altissima -Tree of heaven - Simaroubaceae
Ajuga reptans - Bugleweed; Common bugle - Lamiaceae
Akebia quinata -Chocolate vine; Fiveleaf akebia - Lardizabalaceae
Albizia julibrissin -Mimosa; Silk tree - Fabaceae
Alliaria officinalis (See Alliaria petiolata)
Alliaria petiolata -Garlic mustard - Brassicaceae
Allium vinale -Wild garlic; Field garlic; Crow garlic - Liliaceae
Alternanthera philoxeroides - Alligatorweed - Amaranthaceae
Ampelopsis brevipedunculata -Amur peppervine; Porcelain-berry - Vitaceae
Ampelopsis heterophylla (See Ampelopsis brevipedunculata)
Arrhenatherum elatius - Oatgrass, Tall - Poaceae
Artemisia vulgaris -Common wormwood; Mugwort - Asteraceae
Arundo donax -Reed, Giant or Elephant grass - Poaceae
Berberis thunbergii -Barberry, Japanese - Berberidaceae
Broussonetia papyrifera -Mulberry, Paper - Moraceae
Cardiospermum halicacabum -Balloon vine; Love in a puff - Sapindaceae
Carduus vulgaris (See Cirsium vulgare)
Carex kobomugi -Sedge, Japanese or Asiatic sand sedge - Cyperaceae
Cassia obtusifolia (See Senna obtusifolia)
Celastrus orbiculatus -Bittersweet, Oriental - Celastraceae
Chrysanthemum leucanthemum (See Leucanthemum vulgare) .
Clematis paniculatus (See Clematis terniflora)
F-4

Clematis terniflora -Clematis, Leatherleaf; Sweet autumn virgin's bower -
Ranunculaceae
Commelina communis -Dayflower, Common - Commelinaceae
Dioscorea batatas (See Dioscorea oppositifolia)
Dioscorea oppositifolia -Yam, Chinese or Cinnamon vine - Dioscoreaceae
Dipsacus fullonum -Teasel, Fuller's -Dipsacaceae
Dipsacus fullonum ssp. sylvestris -Teasel, Common or Wild -Dipsacaceae
Dipsacus laciniatus -Teasel, Cutleaf -Dipsacaceae
Dipsacus sylvestris (See Dipsacus fullonum ssp. sylvestris)
Egeria densa -Brazilian waterweed; Elodea -Hydrocharitaceae
Elaeagnus angustifolia -Olive, Russian - Oleaceae
Elaeagnus pungens -Olive, Thorny - Oleaceae
Elaeagnus umbellata -Olive, Autumn - Oleaceae
Eleutherococcus pentaphyllus -Ginseng shrub; Fiveleaf Aralia -Araliaceae
Elodea densa (See Egeria densa)
Eragrostis curvula - Lovegrass, Weeping -Poaceae
Euonymus alata - Spindletree, Winged or Burning Bush - Celastraceae
Euonymus fortunei - Wintercreeper or Climbing Euonymus - Celastraceae
Euphorbia esula -Leafy spurge or Wolf smilk - Euphorbiaceae
Foeniculum vulgare -Fennel, Sweet - Apiaceae
Glechoma hederacea - Groundivy - Lamiaceae
Hedera helix -Ivy, English -Araliaceae
Hesperia matronalis -Dames rocket - Brassicaceae
Humulus japonicus - Hops,J apanese - Cannabaceae
Hydrilla verticillata - Waterthyme -Hydrocharitaceae
Imperata arundinaceae (See Imperata cylindrica)
Imperata cylindrica - Cogongrass -Poaceae
Ipomoea coccinea -Morningglory, Red or Redstar - Convolvulaceae
Ipomoea hederacea -Morningglory, Ivyleaf - Convolvulaceae
Ipomoea purpurea -Morningglory, Common or Tall - Convolvulaceae
Iris pseudacorus -Yellow flag or Paleyellow iris - Iridaceae
Lapsana communis - Nipplewort, Common - Asteraceae
Lespedeza bicolor -Lespedeza, Bicolor -Fabaceae
Lespedeza cuneata -Lespedeza, Chinese or Sericea -Fabaceae
Lespedeza sericea (See Lespedeza cuneata)
Ligustrum obtusifolium -Privet, Border or Blunt-leaved - Oleaceae
Ligustrum sinense -Privet, Chinese - Oleaceae
Ligustrum villosum (See Ligustrum sinense)
Ligustrum vulgare -Privet, European or Common - Oleaceae
Lonicera xBella -Honeysuckle, Whitebell or Bell's -Caprifoliaceae
Lonicera fragrantissima - J anuary jasmine or Sweet-breath-of-spring -Caprifoliaceae
Lonicera japonica - Honeysuckle, J apanese -Caprifoliaceae
Lonicera maackii -Honeysuckle, Amur -Caprifoliaceae
Lonicera morrowii -Honeysuckle, Morrow's -Caprifoliaceae
Lonicera standishii -Honeysuckle, Standish's -Caprifoliaceae
Lonicera tatarica -Honeysuckle, Tartarian -Caprifoliaceae
F-5

Lotus corniculatus -Birdsfoot deervetch or Birdsfoot trefoil -Fabaceae
Ludwigia uruguayensis - Primrosewillow or Hairy water-primrose - Onagraceae
Lysimachia nummularia - Moneywort or Creeping J envy - Primulaceae
Lythrum salicaria -Loosestrife, Purple - Lythraceae
Lythrum virgatum -Loosestrife, European wand - Lythraceae
Mahonia bealei -Oregon grape - Berberidaceae
Melia azedarach -Chinaberry - Meliaceae
Melilotus alba -Clover, White sweet -Fabaceae
Microstegium vimineum - Nepalgrass, J apanese grass -Poaceae
Miscanthus sinensis -Chinese Silvergrass -Poaceae
Morus alba -Mulberry, White or Common - Moraceae
Morus papyrifera (See Broussonetia papyrifera)
Mosla dianthera -Miniature beefsteak -Lamiaceae
Murdannia keisak -Asian spiderwort or Aneilima - Commelinaceae
Myriophyllum aquaticum - Watermilfoil, Parrotfeather -Haloragaceae
Myriophyllum spicatum - Watermilfoil, European or Spike -Haloragaceae
Nasturtium officinale (See Rorippa nasturtium-officinale)
Pastinaca sativa -Parsnip, Wild - Apiaceae
Paulownia tomentosa -Empress or Princess tree - Scrophulariaceae
Perilla frutescens -Beefsteak plant -Lamiaceae
Phalaris arundinacea - Canarygrass, Reed -Poaceae
Phragmites australis -Reed, Common -Poaceae
Phyllostachys aurea -Bamboo, Golden -Poaceae
Polygonum cespitosum - K notweed, Bunchy or Oriental ladysthumb -Polygonaceae
Polygonum cuspidatum - K notweed, Japanese -Polygonaceae
Polygonum perfoliatum -Mile-a-minute -Polygonaceae
Polygonum sachalinense - K notweed, Giant -Polygonaceae
Populus balsamifera ssp. balsamifera -Poplar, Balsam or Balm of Gilead - Salicaceae
Potamogeton crispus - Pondweed, Curly - Potamogetonaceae
Pseudosasa japonica -Bamboo, Arrow -Poaceae
Pueraria montana (See Pueraria lobata)
Pueraria lobata - K udzu -Fabaceae
Quercus acutissima -Oak, Sawtooth - Fagaceae
Ranunculus ficaria - Celandine, Lesser or Fig buttercup - Ranunculaceae
Raphanus raphanistrum -Wild radish or Jointed charlock -Brassicaceae
Rhamnus alms -Buckthorn, Glossy - Rhamnaceae
Rhamnus cathartica -Buckthorn, Common - Rhamnaceae
Rhamnus frangula (See Rhamnus alms)
Rorippa nasturtium-aquaticum -Watercress -Brassicaceae
Rosa multiflora - Multiflora rose -Rosaceae
Rubus phoenicolasius - Wineberry or Wine raspberry -Rosaceae
Rumex acetosella -Sorrel, Red or Common sheep sorrel -Polygonaceae
Senna obtusifolia - Coffeeweed or Sicklepod -Fabaceae
Setaria faberi - Bristlegrass, J apanese or Giant foxtail -Poaceae
Setaria pumila -Bristlegrass, Yellow or Smooth Millet -Poaceae
Setaria viridis -Bristlegrass, Green or Green Millet -Poaceae
F-6

Solanum viarum -Tropical soda apple or Tropical nightshade - Solanaceae
Sorghum halepense - J ohnsongrass - Poaceae
Spiraea japonica -Japanese meadowsweet - Rosaceae
Stellaria media -Chickweed, Common - Caryophyllaceae
Torilis arvensis -Hedge-parsley - Apiaceae
Trapa natans - Water chestnut - Trapaceae
Tribulus terrestris - Puncturevine - Zygophyllaceae
Tussilago farfara - Coltsfoot - Asteraceae
Verbascum thapsus -Mullein, Common - Scrophulariaceae
Veronica hederaefolia -Speedwell, Ivyleaf - Scrophulariaceae
Viburnum dilatatum - Arrowwood, Linden - Caprifoliaceae
Vinca major -Periwinkle, Bigleaf -Apocynaceae
Vinca minor -Periwinkle, Common -Apocynaceae
Wisteria floribunda - Wisteria, J apanese -Fabaceae
Wisteria sinensis -Wisteria, Chinese -Fabaceae
F-7

Resources:
• Prince George's County Department of Environmental Resources
• Plants of the Southeast
• Trees, Taylor's Guides, by Susan A. Roth
• Waterscaping, by Judy Glattstein
• www.esb.enr.state.nc.us/Wetplant/introduction.htm
F-8

APPENDIX G
Sample Maintenance Covenant

DECLARATION OF COVENANTS
For Maintenance of Water Quality Control Structures
THIS DECLARATION OF COVENANTS, made this day
of , 20 , by
hereinafter referred to as the "Covenantor(s)" to and for the benefit of the Town
of Mint Hill and its successors and assigns.
WITNESSETH:
WHEREAS, the Town of Mint Hill is authorized to prevent surface water
quality degradation from development or redevelopment activities within its
jurisdiction as set forth in Chapter 8.17 of the Mint Hill Zoning Ordinance: and
WHEREAS, Covenantors) is (are) the owner(s) of a certain tract or parcel
of land more particularly described as:
being all or part of the land which it acquired by deed dated
from grantors, and recorded among the
Land Records of (governing body), in Liber at Folio
such property being hereinafter referred to as
the "the property;" and
WHEREAS, the Covenantors) desires to construct certain improvements
on its property regulated by Chapter 8.17 of the Mint Hill Zoning Ordinance; and
WHEREAS, in order to construct certain improvements on its property,
the Covenantors) desires to build and maintain at its expense, a water quality
control structure more particularly described and shown on plans titled
and further identified under approval number ;and
WHEREAS, the Town of Mint Hill or its designee have reviewed and
approved these plans subject to the execution of this agreement.
NOW THEREFORE, in consideration of the benefits received by the
Covenantors) as a result of approval by the Town of Mint Hill or its designee of
these plans, the Covenantor(s), with full authority to execute deeds, mortgages,
other covenants, and all rights, title and interest in the property described above,
do hereby covenant with the Town of Mint Hill as follows:
Covenantors) shall construct and perpetually maintain, at its sole expense,
the above-referenced water quality control structure in strict accordance
with the plan approval granted by the Town of Mint Hill or its designee.
G-1

Covenantors) shall, at its sole expense, make such changes or
modifications to the water quality control structure as may, at the
discretion of the Town of Mint Hill or its designee, be determined
necessary to insure that the facility and system is properly maintained and
continues to operate as designed and approved.
The Town of Mint Hill, its agents, employees and contractors shall have
the perpetual right of ingress and egress over the property of the
Covenantors) and the right to inspect at reasonable times and in
reasonable manner, the water quality control structure in order to insure
that it is being properly maintained and is continuing to perform in an
adequate manner.
4. The Covenantors) agrees that should it fail to correct any defects in the
above-described water quality control structure within ten (10) days from
the issuance of written notice, or shall fail to maintain the structure in
accordance with the approved design standards and with the law and
applicable executive regulation or, in the event of an emergency as
determined by the Town of Mint Hill or its designee in its sole discretion,
the Town of Mint Hill or its designee is authorized to enter the property to
make all repairs, and to perform all maintenance, construction and
reconstruction as the Town of Mint Hill or its designee deems necessary.
The Town of Mint Hill or its designee shall then assess the Covenantors)
and/or all landowners served by the water quality control structure for the
cost of the work, both direct and indirect, and applicable penalties. Said
assessment shall be a lien against all properties served by the facility and
may be placed on the property tax bills of said properties and collected as
ordinary taxes by the Town of Mint Hill.
5. Covenantors) shall indemnify, save harmless and defend the Town of
Mint Hill or its designee from and against any and all claims, demands,
suits, liabilities, losses, damages and payments including attorney fees
claimed or made by persons not parties to this Declaration against the
Town of Mint Hill or its designee that are alleged or proven to result or
arise from the Covenantors) construction, operation, or maintenance of
the water quality control structure that is the subject of this Covenant.
6. The covenants contained herein shall run with the land and the
Covenantors) further agrees that whenever the property shall be held, sold
and conveyed, it shall be subject to the covenants, stipulations, agreements
and provisions of this Declaration, which shall apply to, bind and be
obligatory upon the Covenantors) hereto, its heirs, successors and assigns
and shall bind all present and subsequent owners of the property served by
the water quality control structure.
7. The Covenantors) shall promptly notify the Town of Mint Hill or its
designee when the Covenantors) legally transfers any of the
Covenantors) responsibilities for the water quality control structure. The
G-2

Covenantors) shall supply the Town of Mint Hill or its designee with a
copy of any document of transfer, executed by both parties.
The provisions of this Declaration shall be severable and if any phrase,
clause, sentence or provisions is declared unconstitutional, or the
applicability thereof to the Convenantor is held invalid, the remainder of
this Covenant shall not be affected thereby.
9. The Declaration and the exact boundary of all water quality control
structures (as shown on final plats prepared by a registered surveyor) shall
be recorded at the Mecklenburg County Register of Deeds Office at the
Covenantors) expense.
10. In the event that the Town of Mint Hill or its designee shall determine at
its sole discretion at future time that the water quality control structure is
no longer required, then the Town of Mint Hill or its designee shall at the
request of the Covenantors) execute a release of this Declaration of
Covenants which the Covenantors) shall record at its expenses.
G-3

IN WITNESS WHEREOF, the Covenantors) have executed this
Declaration of Covenants as of this day of
20
ATTEST:
(Signature)
(Signature)
(Printed Name) (Printed Name and Title)
STATE OF
COUNTY OF
On this day of , 20 ,before me, the
undersigned officer, a Notary Public in and for the State and County aforesaid,
personally appeared ,who
acknowledged himself to be , of
,and he as such authorized to do so, executed
the foregoing instrument for the purposes therein contained by signing his name
as for said
WITNESS my hand and Notarial Seal
My commission expires
Notary Public
Seen and approved
(Governing Body)
FOR THE COVENANTORS)
G-4

CESAW-RG/Walker
September 29, 2005
MEMORANDUM FOR: Charles Jones, Director, North Carolina Division of Coastal
Management
FROM: Tom Walker, Project Manager, Corps of Engineers, Wilmington District
SUBJECT: AID 200110096 Potash Company of Saskatchewan (PCS) Phosphate
Division mine advance, South Creek and tributaries, Beaufort County, North Carolina.
a. On 30 October 2000, PCS Phosphate submitted a permit application
(attached) to the Regulatory Division requesting authorization to expand their existing
mining operation. Adverse environmental impacts associated with this request equaled
2,500 acres of wetlands and 49 acres of waters of the United States, including navigable
waters. This application was accepted and a Public Notice was issued on 9 January 2001
b. Based on comments received during the scoping process, PCS Phosphate
elected to revise its application such that impacts to waters and wetlands were reduced to
2,394 acres, including 4 acres of estuarine waters. An additional Public Notice
describing this revision was issued on 04 October 2001.
c. By correspondence dated 20 June 2003, PCS Phosphate requested that the
Corps consider the boundary included in their 30 October 2000, application as an
alternative to be evaluated in the Environmental Impact Statement (EIS).
d. The Corps, in a memo dated 22 Apri12005 and circulated to PCS staff,
identified 11 alternatives to be addressed in the draft environmental impact statement
(DEIS). The boundary included in PCS's 30 October 2000, application has not, at this
time, been identified as an alternative to be included in the EIS.