20070777 Ver 1_More Info Received_20070622

POST OFFICE BOX 104Cy~
SMITHFIELD, N. C. 27577

June 21, 2007
Ian McMillian
Division of Water Quality
Department of Environmental and Natural Resources
1650 Mail Service Center
Raleigh, NC 27699-1650
RE: 2006/2007 Looping Line Additions
Johnston County, North Carolina
DWQ Project # 07-0777
Dear Mr. McMillian,
Rick J. Hester
County Manager
[phone]
FAX [phone]
Please find enclosed our Stormwater Ordinance approved by DWQ under the Neuse
Nutrient Management Strategy rules and our revised Stormwater Design Manual. The
county currently operates under this very aggressive ordinance and design criteria which
covers all new commercial development and subdivisions. This information is available
to the public via our website (www.johnstonnc.com) or in person at the Department of
Utilities office in Smithfield.
Please accept this as an affidavit that the provisions of the Johnston County Stormwater
Ordinance are fully implemented and enforced under our Stormwater management permit
application and review process.
Sincerely,
<~~
.~
Rick Hester
County Manager
Enclosures
cc: Timothy G. Broome, P. E.
Chris Windley, P. E. -McKim & Creed
Chandra C. Coats, P.E.
Sandy Mills
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ARTICLE 9 STORMWATER MANAGEMENT ORDINANCE
9.1 Purpose
The purpose of this section is to establish minimum criteria to control and minimize
quantitative and qualitative impacts of stormwater runoff from development within the
County, a nutrient management program for new development in accordance with 15A
NCAC 2B .0235 Neuse River Basin -Nutrient Sensitive Waters Management Strategy:
Basinwide Stormwater Requirements and to establish regulations to provide additional
protection within the Environmentally Sensitive Area District (ES). DOOR: 4.5.2
Further, prudent site planning should include special consideration for the purposes of
preserving natural drainage ways, maximizing infiltration, and slowing stormwater runoff
from individual sites in route to streams and rivers by use of effective runoff
management, structural and non-structural best management practices, drainage
structures, and stormwater facilities.
9.2 APPLICABILITY
The provisions of this section shall apply to all areas within the planning jurisdictional
limits of Johnson County. The provisions of the Environmentally Sensitive Area District
shall apply to those areas defined and established on the map entitled "Environmentally
Sensitive Areas of Johnston County, North Carolina." DOCR: 4.5.2.2
9.2.1 EXCEPTIONS TO APPLICABILITY
The provisions of this chapter shall not apply to:
a) Developers/property owners that can demonstrate that they have vested rights as of
the adoption date of the revised Stormwater Ordinance shall be exempt from the
revised Stormwater Ordinance.
b) Exemption from storm attenuation: Developments that meet one of the following
requirements shall be exempt from storm attenuation:
1. The increase in peak flow between pre- and post-development conditions does not
exceed ten percent, or
2. The proposed development meets all of the following criteria: overall impervious
surface is less than 15% outside the ES and 12% inside the ES, and the pervious
portions of the site are utilized to the extent practical to convey and control
stormwater runoff.
Note: Nutrient management and/or reduction shall still be required.

9.3 DEFINITIONS
For the purpose of this section, the following terms, phrases and words, and their
derivatives, shall have the meaning given herein:
9.3.1 Applicant - An owner or developer of a site who executes the Stormwater Permit
Application pursuant to this ordinance is considered the applicant.
9.3.2 Best Management Practices (BMP'S) - A wide range of practices that have been
demonstrated to effectively manage the quality and/or quantity of stormwater runoff and
which are compatible with the planned land use. BMP's can be structural (detention
ponds, wetlands, etc.) or non-structural (reduced road pavement width, cluster
development, etc.).
9.3.4 Channel Bank -The location of the upper edge of the active channel above which the
water spreads into the overbanks on either side of the channel or the elevation of the two-
year frequency storm. Where the channel bank is not well defined, the channel bank
shall be considered the edge of the waterline.
9.3.5 Design Storm -The specific frequency and, if necessary, duration of the rainfall event to
be used in design to meet the criteria established in the Stonnwater Design Manual.
9.3.6 Development: Any of the following actions taken by a public or private individual or
entity:
a) The division of a lot, tract or parcel of land into two (2) or more lots, plots, sites,
tracts, parcels or other divisions by plat or deed, or
b) Any land change, including, without limitation, clearing, tree removal, grubbing,
stripping, dredging, grading, excavating, transporting and filling of land.
9.3.7 Drainage Structures - Shall include swales, channels, storm sewers, curb inlets, yard
inlets, culverts and other structures designed to convey stormwater.
9.3.8 Existing development - An individual non-residential site with site plan approval by the
Planning Dept. OR a non-residential or residential subdivision with preliminary
subdivision approval from the Planning Board.
9.3.9 Impervious Surface - A surface composed of any material that impedes or prevents
natural infiltration of water into the soil. Gravel areas shall be considered impervious.
9.3.10 Illegal discharges - Any unlawful disposal, placement, emptying, dumping, spillage,
leakage, pumping, pouring, or other discharge of any substance other than stormwater
into a stormwater conveyance system, the waters of the State or upon the land such that
the substance is likely to reach a stormwater conveyance system or waters of the State
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constitutes an illegal discharge.
9.3.11 Land Disturbance -Land disturbance is defined as removal of topsoil, grubbing, stump
removal and/or grading.
9.3.12 Natural Drainage Way -Shall mean an incised channel with a defined channel bed and
banks that are part of the natural topography. Construction channels such as drainage
ditches shall not be considered a natural drainage way unless the constructed channel was
a natural drainage way that has been relocated, widened, or otherwise improved.
9.3.13 New Development: For the purposes of the Stormwater Ordinance, new development
shall be defined as:
a) Any activity that disturbs more than one acre of land or creates more than 17,000
square feet of impervious surfaces for a single family or duplex residential
development, recreational facility, or multifamily, institutional, commercial or
industrial development; and
b) New development shall not include mining, agricultural or forestry activities.
9.3.14 Riparian Buffer - An area of trees, shrubs, or other forest vegetation, that is adjacent to
surface waters. For purposes of this Ordinance, surface water shall be present if the
feature is approximately shown on either the most recent version of the Johnston County
soil survey report prepared by the NRCS OR the most recent version of the 1:24,000
scale (7.5 min.) quadrangle topographic maps prepared by the United States Geological
Survey. Riparian buffers adjacent to surface waters that do not appear on either of the
maps shall not be subject to this Ordinance, except as noted in Section 4.5.2,
Environmentally Sensitive Overlay District.
9.3.15 Stormwater: Flow resulting from and occurring after any form of precipitation.
9.3.16 Stormwater Administrator -The person designated by the County Manager of Johnston
County to have authority to review and approve Stormwater Permits and stormwater
management plans. The Stormwater Administrator shall also be responsible for
inspecting development and enforcing the provisions of this ordinance.
9.3.17 Stormwater conveyance system or structure: Any feature, natural or man-made, that
collects and transports stormwater, including but not limited to roadways with collection
systems, catch basins, man-made and natural channels, streams, pipes and culverts, and
any other structure or system designed to transport runoff.
9.3.18 Stormwater Design Manual -The manual of design, performance, and review criteria
adopted by the Johnston County Board of Commissioners for the administration of the
Stormwater Program.
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9.3.19 Vegetative Buffer - An area that has a dense ground cover of herbaceous or woody
species, which provides for diffusion and infiltration of runoff and filtering of pollutants.
9.3.20 Vested Rights for Stormwater -Vested rights shall be based upon the following criteria:
a) Having an outstanding valid building permit in compliance with GS 153A-344.1 or
GS 160A-385.1, or
b) Having an approved site specific or phased development plan in compliance with GS
153A-344.1 or GS 160A-385.1.
Projects that require a state permit, such as landfills, NPDES wastewater discharges, land
application or residuals and road construction activities, shall be considered to have
vested rights if a state permit was issued prior to the effective date of the adoption of the
revised Stormwater Ordinance.
9.3.21 Water Dependent Structures -Those structures that require the access or proximity to, or
sitting within surface waters to fulfill its basic purpose, such as boat ramps, boathouses,
docks, and bulkheads. Ancillary facilities such as restaurants, outlets for boat supplies,
parking lots, and commercial boat storage areas are not considered water-dependent
structures.
9.4 INTERPRETATION
In interpreting and applying this section, the requirements are intended to be minimum
requirements, which are imposed and are to be conformed to, and are in addition to, and
not in lieu of, all other legal requirements.
This section shall not be deemed to interfere with or annul or otherwise affect in any
manner whatsoever any ordinance, rules, regulations, permits, or easements, covenants,
or other agreements between parties, provided., however, that where this Ordinance
imposes greater restrictions and controls with respect to stormwater management, the
provisions of this section shall prevail.
9.5 PERMITS
9.5.1 Stormwater Permit; Except where provided elsewhere, land-disturbing activities shall not
commence without obtaining a Stormwater Permit pursuant to the provisions of this
Ordinance and the Stormwater Design Manual.
The Stormwater Permit Application shall be made by, or on behalf of, the owner(s) or
developer(s) of the site for which the permit is sought. The application shall be filed with
the County on a form supplied by the County and shall be accompanied with the
information identified in the Stormwater Design Manual.
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A Stormwater Permit shall not be issued until the following conditions are met:
a) Approval of the Stormwater Management Plan by the Stormwater Administrator.
b) Submission and approval of any required easements and impervious area statements
on a map to be recorded.
c) Submission and approval of any required inspection and maintenance agreement
and/or escrow account or other legal instrument established to ensure long-term
maintenance of BMP's.
d) Payment of all fees.
If the development requires approval of an Erosion and Sediment Control Plan, the
Stormwater Permit will be conditional upon the owner receiving such erosion and
sediment control approval.
The Stormwater Permit will be valid for one (1) year from the date of issuance or until
significant changes in the development are made that change the intent of the permit.
The Stormwater Administrator shall determine significant changes. If significant
changes are made, the original Stonnwater Permit shall not be valid, and a new permit
shall be required.
9.6 FEES
A list of fees associated with this section is available at the Office of the Clerk to the
Board of Commissioners in the Johnston County Courthouse.
9.7 STORMWATER MANAGEMENT
Stormwater shall be conveyed from development in an adequately designed drainage
system of natural drainage ways, grass swales, storm sewers, culverts, inlets, and
channels. Drainage systems shall be designed, constructed, and maintained to encourage
natural infiltration, control velocity, control flooding, and extend the time of
concentration of stormwater runoff.
The post-development runoff rate for the 1-year storm event shall be attenuated to the
pre-development runoff rate for the 1-year storm.
The nitrogen loading contributed by new development shall be restricted to 3.6 lbs. of
nitrogen per acre per year. Methodologies for determining nitrogen loading are outlined
in the Stonnwater Design Manual.
A developer has the option of offsetting the nitrogen loading from a development by
paying into the State Wetlands Restoration Program. Procedures for offset payments are
5

outlined in the Stormwater Design Manual. When using the offset payment, the total
nitrogen loading from a development shall not exceed 6.0 lbs. per acre per year for
residential development and 10 lbs. per acre per year for non-residential development.
9.7.1 Stormwater Management Plans
Stormwater Management Plans shall:
a) Include drawings, maps, supporting calculatiorn, specifications, and summaries as
outlined in the Stormwater Design Manual.
b) Demonstrate through accepted engineering practices described in the Stormwater
Design Manual the impacts of the proposed development. Impacts of the proposed
development shall include:
1. Effects on existing upstream and/or downstream drainage systems and property;
2. Ability of the natural drainage way to handle additional stormwater runoff; and
3. Site-specific criteria supporting the analysis of any impacts noted in 1) and 2)
above.
c) Demonstrate through accepted engineering practices described in the Stormwater
Design Manual that stormwater runoff is adequately conveyed through the
development in a drainage system designed to meet the criteria described in the
Stormwater Design Manual.
d) Demonstrate through accepted engineering practices described in the Stormwater
Design Manual that stormwater facilities required to control the impacts of the
development are designed to meet the criteria described in the Stormwater Design
Manual.
e) Demonstrate that the nitrogen loading from the new development does not exceed the
limits set forth in Section 9.7.
9.8 Stormwater BMP Inspection, Maintenance and Easement Requirements
9.8.1 Maintenance A ear ement: A written inspection and maintenance agreement in a form
acceptable to the County Attorney and executed by the Applicant and the owner(s) of the
BMP, if different than the applicant, shall be provided prior to receiving a Stormwater
Permit. The agreement shall:
a) Bind the parties thereto and all subsequent owners, successors, and assigns to
maintenance and inspection of the system or structure;
b) State that if the County directs the correction, repair, replacement, or maintenance of
the system or structure in writing and the actions are not satisfactorily performed
within a reasonable time (but not greater than 120 days), the County (or its
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contractors) may, after reasonable notice, enter the land and perform all the necessary
work and may assess the owner(s) of the facility with the oast of the work performed
or the County can seize all or part of the escrow or other fund set aside by the
Applicant for perpetual maintenance.
The owner(s) served by the facility shall be jointly responsible to the County for the
maintenance of the facility and liable for any costs incurred by the County pursuant to
the said agreement. All properties are jointly subject to the imposition of the liens for
said costs.
c) The Inspection and Maintenance Agreement shall be recorded in the Register of
Deeds at the expense of the applicant.
9.8.2 Easements: Easements for stormwater BMP's shall include the area of the BMP, area of
ponded water, and enough area for access and maintenance. The easement shall be
recorded in the Register of Deeds at the expense of the applicant and shall be depicted on
the final plat or recorded map.
9.9 ILLEGAL DISCHARGE AND CONNECTIONS
9.9.1 Illegal Discharge: No person shall cause or allow the discharge, disposal, pouring or
pumping directly or indirectly to any stormwater conveyance structure, stormwater
conveyance system, stream, lake, pond, wetland or other body of water, or upon the land
in proximity to the same, any fluid, solid or other substance (other than stormwater).
Prohibited substances include, but are not limited to oil, anti-freeze, chemicals, animal
waste, paints, garbage and litter. Examples of illegal discharges are:
a) Dumping of oil, anti-freeze, paint or cleaning fluids
b) Untreated commercial carwash wash water
c) Industrial discharges
d) Contaminated foundation drains
e) Cooling waters, unless no chemicals added and has valid NPDES permit
f) Wash water from commercial and industrial activities
g) Chlorinated backwash and draining associated with swimming pools
h) Domestic wastewater
i) Septic system effluent
j) Washing machine discharges
9.9.2 Allowable Discharges: Examples of allowed discharges are:
a) Water line flushing
b) Irrigation
c) Uncontaminated groundwater pumping
d) Street wash water
e) Dechlorinated backwash and drainage associated with swimming pools
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f) NPDES permitted discharges
9.9.3 Illegal Connections: Connections to a stormwater conveyance system or structure that
allow the discharge of non-stormwater are unlawful. Prohibited connections include but
are not limited to:
a) Floor drains
b) Wastewater from washing machines or sanitary sewers
c) Wash water from commercial vehicle washing or steam cleaning
d) Waste water from septic systems
9.9.4 Determination of Connection: Upon determining that said connection:
a) May result in the discharge of hazardous materials, may pose a threat to health and
safety, or is likely to result in immediate injury or harm to human or animal life,
natural resources, to real or personal properly, or habitat, or
b) Was made in violation of any applicable regulation or ordinance,
The Stormwater Administrator shall outline in a Notice of Violation, sent by certified
mail, the time in which the connection shall be removed. Failure to comply with the
terms and deadlines set in the Notice of Violation will constitute a violation of this
ordinance. Reference Section 16 for enforcement procedures.
9.10 RIPARIAN BUFFERS
Fifty foot (50') wide riparian buffers shall be maintained along both sides of a stream,
river or other water body as required by the Neuse River Basin: Nutrient Sensitive Waters
Management Strategy: Protection and Maintenance of Riparian Buffers, Section 3(a-b).
(Reference Section 9.3 Definitions)
Riparian buffers shall be noted on the maps submitted for Stormwater Management Plan
approval and shall be noted on the final, recorded map.
Determinations of exemptions (as noted in 15A NCAC 2B.0233 Neuse River Basin:
Nutrient Sensitive Waters Management Strategy: Protection and Maintenance of Riparian
Buffers, Section 3(a-b)) shall be made by the NCDENR Division of Water Quality.
9.11 RIGHT TO ENTER
Any County personnel, or contractors for the County, shall be permitted to enter upon
public or private property for the purposes of inspection, sampling, monitoring, testing or
otherwise verifying compliance. Should the County personnel, or contractor for the
County, be denied reasonable access to any property, the Stormwater Administrator shall
obtain an administrative search warrant.
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No person shall obstruct, hamper or interfere with any such representative while carrying
out his/her official duties.
9.12 VARIANCES
The Board of Adjustment shall consider all variance requests as set out in Article 13.
9.13 ENFORCEMENT
9.13.1 Violations
Whenever, by the provisions of this section, the performance of any act is required, or the
performance of any act is prohibited, or whenever any regulation or limitation is imposed
on the use of any land, or on the erection, alteration, or the use or change of use of a
structure, a failure to comply with such provisions shall constitute a violation of this
ordinance.
The owner, tenant, or occupant of any land or structure, or part thereof, and any architect,
engineer, builder, contractor, agent, or other person who participates in, assists, directs,
creates, or maintains any situation that is contrary to the requirements of this section may
be held responsible for the violation and be subject to the penalties and remedies
provided herein.
Failure to follow an approved Stormwater Management Plan or Permit shall constitute a
violation of this section and subject to the penalties and remedies provided herein.
9.13.2 Procedures Upon Discovery of Violations
Procedures upon discovery of violations shall be as follows:
a) Upon determination that any provision of this section is being violated, the
Stormwater Administrator shall deliver a written notice by personal service or by
registered or certified mail, return receipt requested, to the person(s) responsible for
such violation, indicating the nature of the violation, ordering the action necessary to
correct it and outlining the timeframe for gaining compliance. Additional written
notices may be sent at the Stonnwater Administrator's discretion.
b) The final written notice, which may also be the initial notice, shall state the
enforcement action the County intends to take if the violation is not corrected, and
shall advise that the Stormwater Administrator's order may be appealed to the Board
of Adjustment as provided in Article 13.
In cases when delay would seriously threaten the effective enforcement of this ordinance,
or pose a danger to the public health, safety, or general welfare, the Stormwater
9

Administrator may seek enforcement without prior written notice by invoking any of the
penalties or remedies contained herein.
9.13.3 Penalties and Remedies
Penalties and remedies shall be as follows:
a) Any violation of any provision of any Section of the Stormwater Management
Ordinance shall constitute a misdemeanor and shall subject the violator to a penalty
of five hundred dollars ($500.00) and/or imprisonment for not more than 30 days.
b) Development that begins land-disturbing activities prior to obtaining a Stormwater
Management Permit shall also be subject to a one-time one thousand dollar
($1000.00) penalty.
c) Illegal discharge: Any designer, engineer, consultant, contractor or person that allows,
acts in concert, participates, directs or assists directly or indirectly in an illegal
discharge shall be subject to civil penalties as follows:
1) For first time offenders if the quantity of the discharge is equal to or less than five
(5) gallons and consists of domestic or household products, said person shall be
assessed a civil penalty not to exceed one hundred dollars ($100.00) per violation
or day for any continuing violation. If the quantity of the discharge is greater than
five (5) gallons or contains non-domestic substances or if the person cannot
provide clear and convincing evidence of the volume and nature of the substance
discharged, said person shall be assessed a civil penalty not to exceed one
thousand dollars ($1000.00) per violation or per day for continuing violation.
2) For repeat offenders, the amount of the penalty shall be double the amount
assessed for the previous penalty, not to exceed ten thousand dollars ($10,000.00)
per violation or per day for any continuing violation.
3) The County Manager or his designee shall take the following into consideration
when determining the civil penalty amount:
a) The degree and extent of harm to the environment, public health, and
property;
b) The cost of remedying the damage;
c) The willfulness of the violation;
d) The duration of the violation;
e) The violator's prior record in complying or failing to comply with this
ordinance; and
f) The amount of money saved by the violator by noncompliance.
If the offender fails to pay the penalty within ten (10) days of receiving final written
notice of a violation, the County in a civil action may recover the penalty. A civil penalty
10

may not be appealed to the Board of Adjustment if the offender received a final written
notice of violation and did not appeal to the Board of Adjustment with 30 days.
Each day that any violation continues after receipt of the final written notice of such
violation shall constitute a separate violation and a separate offense for purposes of the
penalties and remedies specified herein.
In addition to the penalties and remedies above, the County may institute any appropriate
action or proceedings to prevent, restrain, correct, or abate a violation of this ordinance.
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f
Johnston County
Stormwater
Design Manual
August 1998
Revised October 2000
Revised November 2004
Revised May 2005
Revised March 2006
Revised January 2007 (DRAFT)
Johnston County Dept. of Utilities
P.O. Box 2263
Smithfield, NC 27577
209-8333

's
Table of Contents
Section 1 Stormwater Management Permit Application
1.1 Introduction
1.2 Purpose
1.3 Intent
1.4 Disclaimer
1.5 General Procedures
1.5.01 Stormwater Management Plan Permit Application
1.5.02 Stormwater Management Plan Application Fee
1.5.03 Stormwater Management Statement
1.5.04 Riparian Buffers
Section 2 Impervious Area Limitations
2.1 Requirements
2.2 Impervious Area Calculations
2.3 Land Dedication Program
2.3.01 Land Dedication Program Options
2.3.02 Land Dedication Calculation
2.3.03 Land Dedication Rate
2.3.04 Land Dedication Example Calculations
2.3.05 Water Quality Protection Criteria (WQPC)
2.3.06 County Policy
Section 3 Hydrology and Hydraulics
3.1 Peak Discharge Calculation Methods
3.1.01 Rational Method
3.1.02 SCS Peak Discharge Method
3.2 Attenuation
3.3 Design and Check Storm Guidelines
3.4 Channel Design
3.4.01 Channel Design Guidelines
3.4.02 Submittal Requirements for Proposed Channels
3.5 Storm Sewer Design
3.5.01 Stream Crossing Guidelines
3.5.02 Submittal Requirements
3.6 Cross Lot Drainage
3.7 Offsite Stormwater Impacts Analyses
3.7.01 Minimizing Offsite Stormwater Impacts
3.7.02 Existing Facilities
3.7.03 Submittal Requirements
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Section 4 Nitrogen Reduction
4.1 Nitrogen Cycle
4.2 Total Nitrogen Calculations
4.3 Maximum Total Nitrogen Export Limits
4.4 Total Nitrogen Offset Payment Calculation
4.5 Total Nitrogen Calculation for Redevelopment
4.6 BMP Reduction Credits
4.7 Example Calculations
Section 5 Stormwater BMP's
5.1 Selecting BMP's
5.2 BMP Nitrogen Removal Rates
5.3 Non-Structural BMP's
5.4 Structural BMP's
5.4.01 Infiltration BMP's
5.4.02 Filtration BMP's
5.3.03 Stormwater Ponds
5.4.04 Stormwater Wetlands
5.4.05 Open Channel Systems
5.4.05 Vegetated Buffers and Filter Strips
Section 6 Easements
Section 7 Resources and References
Appendices
Appendix A Sample Stormwater Management Statement
Appendix B Stormwater Management Plan Checklist and Application Form
Appendix C Maintenance and Inspection Agreement
Appendix D Johnston County Stormwater Ordinance
Appendix E Environmentally Sensitive Area Map
3
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Section 1 Stormwater Management Permit Application
1.1 Introduction
In December of 1997, the Environmental Management Commission adopted rules to reduce the
amount of nitrogen delivered to the Neuse River Estuary from point and non-point sources by a
minimum of 30% of the 1995-loading rate. Wastewater treatment plants and industrial plants are
considered point source discharges. Point source discharges are regulated under the National
Pollution Discharge Elimination System (NPDES) administered through the NC Department of
Environment and Natural Resources Division of Water Quality. Non-point source discharge
originates from snow melt or rainfall flowing across lawns, streets, parking lots, agricultural fields,
industrial sites, construction sites, etc. In May 1998, the Johnston County Board of
Commissioners adopted the Johnston County Stormwater Management Ordinance. The intent of
this first ordinance was to protect streams from the secondary impacts of development. Later in
1998, the Environmental Management Commission adopted the Neuse River Basin -Nutrient
Sensitive Waters Management Strategy: Basin-wide Stormwater Requirements (Neuse Rules). In
those rules, Johnston County was identified as one of 15 local governments required to adopt a
Stormwater program specifically addressing nitrogen reduction. In 2000, the Johnston County
Board of Commissioners adopted the revised Johnston County Stormwater Management
Ordinance to address the requirements stated in the Neuse River Basin Rules. A copy of the
revised ordinance can be found in Appendix D.
1.2 Purpose
The Johnston County Stormwater Management Ordinance and this design manual shall promote
and insure the design, construction, management, and maintenance of stormwater systems to:
1. Improve and enhance the quality of stormwater runoff from development,
2. Maximize infiltration of stormwater,
3. Collect and transmit excess stormwater flows in a manner to protect human health and
welfare and to protect property, and
4. Protect and preserve downstream natural drainage ways.
The impacts on surface water resources from development shall be mitigated to the extent
feasible.
1.3 Intent
The intent of this design manual is to provide and establish minimum design criteria and
standards and recommend methods of analysis and design. Design criteria included herein are
the general procedures of Johnston County Public Utilities, but they may not be applicable in
every situation. In order to insure the policies and objectives of the stormwater ordinance are
met, occasionally standards that are more stringent could be required by the Stormwater
Administrator.
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1.4 Disclaimer
To the best of their ability, the authors of this manual have insured that the material presented is
accurate and reliable. The planning, design, and construction of stormwater management
facilities require considerable judgment by the engineers/designers. It is the responsibility of the
design professional to insure that any methods, procedures, or techniques presented in this
manual are appropriate for a given situation. Johnston County accepts no responsibility for any
cost, loss, damage, or injury resulting from the use of this manual.
1.5 General Procedures
New construction and development within the planning jurisdiction of Johnston County are
subject to many ordinances and policies of the County. The Johnston County Planning
Department can provide complete information on all applicable County ordinances and policies.
1.5.01 stormwater Management Plan Permit Application
An approved stormwater Management Plan is required for all new development. The
stormwater Management Ordinance defines new development as:
Any activity that disturbs more than one acre of land or creates more than 17,000
square feet of impervious surfaces for a single family or duplex residential
development, recreational facility, or multifamily, institutional, commercial or
industrial development.
A stormwater Management Plan Permit Application shall be submitted on behalf of the
owner or developer of the site where new development is proposed. The owner or
developer must sign the application and provide the required contact and development
information on the application. An application form is included in Appendix B.
Prior to submitting a stormwater Management Plan Permit Application and before
beginning detailed site and stormwater management design, the developer and his/her
consultant are encouraged to schedule apre-design conference with the stormwater
Administrator. Apre-design conference may help in the overall design of development,
by providing the developer and his/her consultant(s) site specific concerns and
suggestions based on the stormwater Management Ordinance and Design Manual. The
County's stormwater management policies and design criteria may be clarified with
respect to the specific development and site issues.
The stormwater Permit Plan Application package shall be accompanied by the following
information:
1. The completed and signed application form.
2. The review fee for the stormwater Management Plan Permit Application. See
section 1.5.02 for more information. All checks should be made payable to
Johnston County.
3. The parcel number or North Carolina PIN should be provided for the tract where
the development is proposed.
4. Impervious area calculations in accordance with Section 2 of this manual.
5. Total Nitrogen Export calculations in accordance with Section 4 of this manual.
Draft Revised 1/07

ti t
6. A vicinity map, showing the location of the development within the County. The
scale of this map must be adequate to show multiple cross streets.
7. An overall map showing the current zoning and land use of all surrounding
property, both up and downstream, which could be directly impacted by the
proposed stormwater management system.
8. One complete set of development plans, signed and sealed by the professional
engineer or land surveyor.
9. One set of site specific stormwater Management Plans and details, signed and
sealed by the design professional. The following information should be included
in the stormwater Management plans as applicable:
a) Detailed existing topographic information for the entire site. A minimum
of 2-foot contours is required. Where existing slopes exceed 12%, a
minimum of 10-foot contours will be accepted.
Note: The County prefers topographic information to extend 50'
beyond the site property lines, where it can be provided.
b) Proposed road profiles with existing and proposed elevations. Profiles
should include storm sewer piping where proposed. Storm sewer
profiles, with existing and proposed ground elevations, should be
provided where storm sewer is not located within the road profile.
c) The Vertical Datum utilized.
d) The 100-year flood plain boundaries and elevations from the most recent
FIRM.
e) All jurisdictional wetland boundaries on site.
f) All water features, as found on the USGS Quadrangles and the Johnston
County Soil Survey. The plans must also show the required Riparian
Buffers surrounding each feature. See section 1.5.04 for additional
information on Riparian Buffers.
g) All proposed stormwater management facilities.
h) All existing stormwater management facilities should be shown. Existing
structures should be labeled with inverts, size, material and ground
elevations.
i) Details of all stormwater management facilities. Specific design
dimensions should be indicated as appropriate.
j) Drainage easements, as discussed in Section 6 of this manual.
k) A construction sequence, to specifically discuss the proposed installation
timing of all stormwater management facilities proposed.
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10. Drainage area maps for the development. Both overall and site specific drainage
area maps may be required. Overall maps are necessary for analyzing
development wide impacts up and downstream, and site specific maps are
necessary for review of the proposed internal stormwater management facilities.
11. An analysis of the stormwater runoff impacts that the proposed development or
construction activity will have on the surrounding properties, both up and
downstream. See Section 3.7 for additional information on Offsite stormwater
Impacts Analyses.
12. Design calculations and details for all proposed stormwater management
facilities. See Section 3 for additional information.
13. Design calculations and details for all proposed Best Management Practices
(BMP's). All BMP's must meet the design guidelines listed in Chapter 5 of this
manual. If other guidelines are utilized, the calculations should document the
source utilized and confirm that the source contains design guidelines acceptable
by Johnston County Public Utilities.
14. Written Inspection and Maintenance agreements, to be recorded, for all
stormwater management facilities proposed as a BMP. Additional maintenance
agreements may be required for other stormwater management facilities.
15. A copy of the approval letter from the NC Department of Environment and
Natural Resources, Land Quality Section for the site sedimentation and erosion
control plan.
16. Where applicable, a revised stormwater Management Statement. Statements
are required for preliminary plat approval, if the proposed development is
modified after the Statement is submitted, a revised Statement should be
included with the stormwater Management Plan application.
Note: The County prefers that all plans submitted for review be folded, not rolled. All
calculations provided should be bound and dated, with revision dates as appropriate. All
plans and calculations should be sealed and signed by the design professional.
A stormwater management permit approval letter will not be provided until:
1. The plan has been approved by Johnston County Public Utilities.
2. The stormwater Management Permit Application Fee has been received.
3. All other fee calculations, (e.g. Nitrogen Offset Payment and Land
Dedication Fee) have been approved. Payment of all fees is required prior
to recordation of the Final Plat. If said fees are not provided, the
development will be in violation of the stormwater Management Ordinance.
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1.5.02 Stormwater Management Plan Application Fee
A review fee will be required at the time of application for a Stormwater Management
Plan Permit. The review fees are as follows (effective July 1, 2006):
Single Family Residential Development
0-10 Acres $500.00
10+ Acres $500.00 plus $30.00 per acre
Other Development-Commercial, Industrial, Institutional and Multifamily
0-5 Acres $500.00
5+ Acres $500.00 plus $75.00 per acre
The review fee is based on total site acreage, rounded up to the nearest acre. All checks
should be made payable to Johnston County.
Example Calculations:
Residential Subdivision with a total area of 5.2 Acres:
5.2 Acres<10 Acres
Therefore, Fee = $500.00
Residential Subdivision with a total area of 17.1 Acres:
$500.00 + (18 Ac ` $30.00/Ac) _ $1,040.00
Commercial Site Plan with a total area of 10.8 Acres:
$500.00 + (11 Ac * $75.00/Ac) _ $1,325.00
1.5.03 Stormwater Management Statement
A preliminary subdivision plat for a commercial or residential subdivision must
include a Stormwater Management Statement. A Stormwater Management Statement
is required for any subdivision plat that must be approved by the Johnston County
Planning Board. Stormwater Management Statements are not required for individual site
plans.
An example Statement is in Appendix A. The example statement can be utilized as a
guideline; however, the required Stormwater Management Statements must provide
detailed site specific information for the development. Generic language should be
avoided and specific descriptions of how the development will address stormwater runoff
impacts should be included.
The purpose of the Stormwater Management Statement is to help the developer and
his/her consultant(s) begin planning the subdivision with consideration given to
stormwater management, nutrient management and stream protection. The Statement
and apre-design conference allow the Stormwater Administrator an opportunity to work
with the developer during the early stages of project planning to identify an economical,
effective and practical stormwater management design.
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The Stormwater Management Statement must include:
1. Development name and location.
2. Developer/Owner and Consultant contact information.
3. A description of the existing site.
4. A description of the proposed development or construction activity.
5. A statement documenting whether or not the site is located within the
Environmentally Sensitive Area.
6. An impervious area calculation, as per Section 2 of this manual.
7. A preliminary estimate of Total Nitrogen Export from the proposed development
and proposed method of reduction as necessary. Additional information on Total
Nitrogen Loading can be found in Section 4 of this Manual.
8. A description of the stormwater impacts the proposed development or
construction activity may have on surrounding properties. This includes
identifying upstream and downstream drainage facilities potentially affected by
this development and the ability of the existing drainage ways to handle any
additional runoff.
9. A description of the proposed stormwater management facilities and how they
will be designed, constructed, maintained and operated to:
a) Minimize the adverse affects on the quality of stormwater runoff from the
development;
b) Provide BMP's, to maximize infiltration, minimize connected impervious
surfaces and minimize concentrated flows;
c) Provide distributed stormwater runoff to minimize offsite impacts and
provide sheet flow into existing vegetated buffers;
d) Extend the time of concentration to the maximum practical level;
e) Preserve and protect natural drainage ways;
f1 Respect the practical limits of public and private drainage facilities;
g) Protect neighboring properties from unreasonable adverse effects
resulting from the development;
h) Prevent flooding within the development and on surrounding properties;
and
i) Limit the impacts of stormwater runoff discharging into or from the site, or
obtain approvals and easements from the affected property owners.
10. A vicinity map, providing the location of the development.
11. A USGS quadrangle, identifying the site and all up or downstream structures that
may be affected by the development.
12. A Johnston County Soil survey identifying the site.
Preliminary subdivision plats that do not include a Stormwater Management
Statement will not be considered complete and may not be included on the
Planning Board agenda.
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1.5.04 Riparian Buffers
On June 22, 1999, The Environmental Management Commission adopted the Neuse
River Basin: Nutrient Sensitive Waters Management Strategy: Protection and
Maintenance of Riparian Buffers (15A NCAC 28.0233). The purpose of the rule is to
protect and preserve riparian buffers in the Neuse River Basin to maintain their nutrient
removal functions. Johnston County requires all site plans and subdivisions submitted for
County review to have all streams identified and riparian buffers noted on the maps
where applicable.
The Neuse River Riparian Buffers rules apply to all existing streams, lakes, ponds or
other bodies of water if the feature is shown on either the most recent version of a
1:24,000 scale (7.5 minute) quadrangle topographic map prepared by the USGS or the
most recent version of a soil survey map prepared by the NRCS. The Neuse River
Riparian Buffer rules implement and pertain to a 50' Riparian Buffer around all water
features, as noted above. Exemptions from the Neuse River Basin Rules for riparian
buffers along surface waters that appear on the maps must be obtained in writing from
the NCDENR Division of Water Quality.
Approval from the Division of Water Quality will be required prior to Johnston County
Stormwater Management Plan approval for any sites that appear to be impacting riparian
buffers. For more information on allowable activities within a riparian buffer or along a
stream or for detailed information for exemptions from the riparian buffer rule, contact the
NCDENR Division of Water Quality Raleigh Regional Office at [phone].
Additional riparian buffers shall apply to areas located inside the Environmentally
Sensitive Area (ESA) (formerly referred to as the Environmentally Sensitive Overlay
District (ESOD)) as set out in Sec. 14-103 of the Johnston County Land Development
Code. Pursuant to Sec. 14-103 intermittent and perennial streams shall be defined as
follows:
Intermittent Stream - A natural drainage way, which shows up as a blue line on
the USGS 7.5 minute quadrangle maps and has a contributing drainage area of
300 acres or more shall be considered an intermittent stream for the purposes of
this ordinance.
Perennial Stream -Perennial streams are streams that have essentially
continuous flows. Perennial streams in the Environmentally Sensitive Area are
specifically designated to be Swift Creek, White Oak Creek, Little Creek (from the
US70 Clayton Bypass to Swift Creek), and Little River (from the County line to
NC Highway 39).
All perennial streams located in the Environmentally Sensitive Area, as listed above,
must provide riparian buffers as per the following guidelines:
Perennial stream buffers shall be measured from the top of the channel bank and
extend landward a minimum distance of 100 feet measured horizontally on a line
perpendicular to the water body, subject to the following conditions:
a. The buffer shall be undisturbed and remain forested if currently forested.
b. If the existing buffer is not forested, it shall be maintained in a natural
state and allowed to revegetate.
c. There shall be no fill allowed within the buffer area.
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All proposed development and construction activity located near or adjacent to a Riparian
Buffer must adhere to the Neuse River Basin Rules. In particular, the following items
must be addressed, where applicable, on all stormwater Management Plans:
1. All surface waters, as shown on the USGS Quadrangles or Johnston County
Soil Survey, must be shown on the proposed plan.
2. All Riparian Buffers must be appropriately labeled on the proposed plan and
Final Plat. Riparian buffers must extend 50 feet (or 100 feet) from the top of
bank for streams, or the edge of normal water level for ponds.
3. Where development requires the disturbance of a Riparian Buffer,
documentation of approval from NCDENR-DWQ is required. If the activity is
considered exempt, documentation or an explanation should be provided
noting this, utilizing the conditions stated in the Neuse River Basin Rules.
4. Diffuse flow must be provided for all stormwater runoff entering the riparian
buffer. The following guidelines should be utilized:
a) Development located near riparian buffers should minimize large
concentrated discharge points. By providing multiple outlets for
stormwater runoff and maintaining natural drainage patterns the
stormwater runoff impacts from new development can be minimized.
b) Documentation must be provided, indicating that the proposed
development has provided sheet flow at all discharge points where
required. Appropriate calculations and details should be included.
c) Additional methods to provide diffuse flow will be reviewed and approved
on an individual basis. Developers and design professionals may
request apre-design conference to determine if a proposed facility will
be accepted.
d} The NCDENR-DWQ Level Spreader Design Guidelines, effective
January 1; 2007, can be utilized.
e) Discharge that will flow into an existing, non-buffered draw or stream,
prior to entering the riparian buffer will be exempt from the diffuse flow
requirement. The discharge point must enter the draw at least 50 feet
upstream of the edge of the riparian buffer to receive this exemption.
5. All stormwater facilities proposed to create sheet flow must be contained
within permanent drainage easements and have Inspection and Maintenance
Agreements and/or guidelines provided in the restrictive covenants. The
Inspection and Maintenance Agreement and the restrictive covenants must
be recorded.
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Section 2 Impervious Area Limitations
2.1 Requirements
All new development shall be held to the following impervious area limits:
Table 2.1 Impervious Area Limits
Impervious Area Limitation
Type of Development
Inside the ESOD Outside the ESOD
Single Family Residential 12% 15%
Other Development- 50% 60%
Commercial, Industrial,
Institutional and Multi-family
Single Family Residential includes all subdivisions where no more than one dwelling unit is
proposed per lot. The development must be subdivided with single-family homes proposed.
Recreation areas and open space associated with asingle-family subdivision is also included in
the total development maximum impervious area allowed.
Other Development includes all commercial, industrial, institutional and multi-family
developments. Residential developments, such as duplexes, town homes and apartments are
considered multi-family and are included in Other Development.
The owner of a new development can exceed these limitations by participating in the Land
Dedication Program outlined in Section 2.3 below. However, the maximum impervious area limit
allowed with Land Dedication is 30% for single-family residential development, 40% for single-
family developments inside the MTD or previously zoned PUD or R10, and 80% for all other
development.
2.2 Impervious Area Calculations
All Stormwater Management Plan Applications must include calculations documenting the total
proposed impervious area within the development. Impervious areas are defined as all surfaces
composed of material that impedes or prevents natural infiltration of water into the soil. Examples
of impervious surfaces include pavement, concrete, gravel and roofed structures.
For single-family residential subdivisions, where the exact building footprint per lot is unknown, a
maximum impervious area for the site or for each lot can be utilized to determine compliance with
the maximum impervious area allowed within the development. The following criteria should be
adhered to when using this option:
1. A reasonable impervious area per lot must be used in the calculations. Documentation
that the area used will be adequate for the intended use is required. Atypical plot plan or
analyses of existing impervious areas in surrounding, comparable developments can be
provided to meet this requirement.
2. The County prefers residential developments utilize a maximum impervious area per lot
as opposed to an average area per lot. If the development must utilize an average
impervious area, reasonable explanation must be provided. If an average impervious
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13
area is used; the developer must track the development internally and provide annual {or
as requested) updates to the County.
3. The County reserves the right to request as built documentation from the developer,
verifying that the proposed maximum impervious areas have not been exceeded.
4. The County encourages additional impervious area per lot be provided far future
expansion by the harneowner (e.g. 100-500 sf per lot for sheds, screened porches or
additions}.
5. All impervious area within the proposed Right-of Way but outside of the edge of roadway
pavement must be included in the allowable impervious area per lot.
6. The maximum impervious area allowed per lot {or average) must be recorded on the
Final Plat and noted in the restrictive covenants recorded for the subdivision. The
following note can be followed as an example to meet this requirement:
• The maximum impervious area allowed per lot within Coats Subdivision is 4,500
sf. Impervious area includes all buildings, sheds: sidewalks, covered porches,
driveways and surfaces such as gravel, concrete, asphalt, brick, slate or stone
that impede the infiltration of water into the soil. The maximum impervious area
allowed per lot should include any impervious area proposed within the portion of
the right-of-way between the edge of roadway pavement and the front lot line.
If a development area includes existing impervious areas, all impervious areas to remain must be
included in the total impervious area calculations. Existing impervious areas that are removed
are not required to be included in the maximum allowable impervious area limits.
2.3 Land Dedication Program
The Land Dedication Program BMP was created to encourage future development to participate
in the preservation of valuable land, creating permanent open space and helping to offset the
impacts ofhigh-density developments within a given watershed. Developers/owners participating
in the Land Dedication program can achieve a higher impervious area limit by dedicating
additional land or providing a Land Dedication Fee to the County. The maximum impervious area
limits while using the Land Dedication Program are noted in Table 2.2:
Table 2.2 Impervious Area Limits using the Land Dedication Program BMP
Type of Development Location Impervious Area Limit
Single Family Residential In MTDs*, or previously 40%
zoned PUDs and R10
Single Family Residential All other areas 30%
Other Development- All areas 80%
Commercial, Industrial,
Institutional and Multi-Family
*Note: MTD stands for Municipal Transition District, as defined by the Johnston County
Development Ordinance.
**Please note the Land Dedication Program does not exempt proposed development from
the stormwater attenuation and total nitrogen reduction requirements as discussed in
Sections 3 and 4.
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2.3.01 Land Dedication Program options:
To participate in the Land Dedication Program, Developers and Owners have two options as
follows:
The developer may identify land to be dedicated into the County Land Dedication
Program. The developer/owner must secure a permanent conservation
easement for the land to be included in the Land Dedication Program. The
method used to calculate how much land is required and determine what
area/land can be used for the Land Dedication Program is outlined below. A
minimum area of 2 Acres is required for use in the County Land Dedication
Program.
In the County's sole discretion, the County may allow a developer to pay a fee
into the County's Land Dedication Program, based on the number of acres of
land required to be dedicated using a Land Dedication Rate of 2.5:1. The County
will set the per-acre fee. The interim per acre fee has been set at $10,000.00 per
acre. The fee is based upon the actual cost of the land identified by the County or
other party for dedication plus program administration cost. The minimum Land
Dedication Fee allowed is $10,000.00.
2.3.02 Land Dedication Calculation:
The amount of land to be dedicated into the County Land Dedication Program is based upon
the proposed increases in impervious area over the Impervious Area Limits as shown in
Table 2.1. The proposed impervious area increases over the allowable impervious area must
be calculated and that area multiplied by a factor of 1.5 or 2.5, depending on if the Land
meets the Water Quality Protection Criteria, to determine the total land area that must be
dedicated into the Land Dedication Program or the number of acres that will be used to
determine the Land Dedication Fee. Please see the example calculations listed below.
2.3.03 Land Dedication Rate:
For land meeting the Water Quality Protection Criteria (WQPC) outlined below, the land
dedication rate is 1.5:1, where 1.5 acres must be put into a permanent conservation
easement (or some other perpetual easement to prohibit development) for each acre of
proposed impervious area over the limit as set in Table 2.1.
For land not meeting the WQPC outlined below, or to pay a fee in lieu, the land dedication
rate is 2.5:1, where 2.5 acres must be put into a permanent conservation easement (or some
other perpetual easement to prohibit development) for each acre of proposed impervious
area over the limit as set in Table 2.1.
2.3.04 Land Dedication Example Calculations:
20 ac. industrial tract outside the ESA:
Impervious area allowed: 12 ac. (60%)
Impervious area allowed with land dedication BMP: 16 ac. (80%)
Area of additional impervious area proposed: 16 ac. -12 ac. = 4 Acres
Land dedication rate of 1.5:1 for land meeting the WQPC: 4 acres x 1.5 = 6 Acres
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20 ac. commercial tract inside the ESA:
Impervious area allowed: 10 ac. (50%)
Impervious area allowed with land dedication BMP: 16 ac. (80%)
Area of additional impervious area proposed: 16 ac. - 10 ac. = 6 Acres
Land dedication rate of 1.5:1 for land meeting the WQPC: 6 acres x 1.5 = 9 Acres
Land dedication payment option: 6 acres x 2.5 x $10,000/acre = $150,000
20 ac. commercial tract inside the ESA:
Impervious area allowed: 10 ac. (50%)
Impervious area allowed with land dedication BMP: 16 ac. (80%)
Area of additional impervious area proposed: 16 ac. - 10 ac. = 6 Acres
Land dedication rate of 2.5:1 for land not meeting the WQPC: 6 acres x 2.5 = 15 Acres
100 ac. residential development inside the ESA, outside the MTD, not previously zoned
PUD or R10:
Impervious area allowed: 12 ac. (12%)
Impervious area allowed with land dedication BMP: 30 ac. (30%)
Area of additional impervious area proposed: 30 ac. - 12 ac. = 18 Acres
Land dedication rate of 1.5:1 for land meeting the WQPC: 18 acres x 1.5 = 27 Acres
100 ac. residential development outside the ESA, inside the MTD, not previously zoned
PUD or R10:
Impervious area allowed: 15 ac. (15%)
Impervious area allowed with land dedication BMP: 40 ac. (40%)
Area of additional impervious area proposed: 40 ac. -15 ac. = 25 Acres
Land dedication rate of 1.5:1 for land meeting the WQPC: 25 acres x 1.5 = 37.5 Acres
100 ac. residential development outside the ESA, inside the MTD, not previously zoned
PUD or R10:
Impervious area allowed: 15 ac. (15%)
Impervious area allowed with land dedication BMP: 40 ac. (40%)
Area of additional impervious area proposed: 40 ac. -15 ac. = 25 Acres
Land dedication rate of 2.5:1 for land not meeting the WQPC: 25 ac. x 2.5 = 62.5 Acres
Land dedication payment option: 25 acres x 2.5 x $10,000/acre = $625,000
Note: No additional water quality BMP can be combined with the Land Dedication program to
achieve. more impervious area than the maximum impervious area limits as set in Table 2.2
(e.g. 80% imperviousness for multifamily residential, industrial, commercial or institutional
development and no more than 30% to 40% imperviousness for single family residential
development).
2.3.05 Water Quality Protection Criteria (WQPC):
The Water Quality Protection Criteria are as follows:
1. In the Environmentally Sensitive Area, land considered meeting the WQPC may
not contain any of the following environmentally sensitive areas: jurisdictional
wetlands, the 100-year floodplain as shown on the most recent FIRM's and the
100-ft riparian buffers for Swift Creek, White Oak Creek, Little Creek, and Little
River (from the county line to NC Highway 39).
2. For areas outside the Environmentally Sensitive Area, land considered meeting
the WQPC may include areas within the 100-year floodplain but may not include
any of the 50-foot Riparian Buffer area, or jurisdictional wetlands.
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3. The minimum length of property line along wetlands, riparian buffers, floodplains,
or streams must be 100' per acre for property to be included in the Land
Dedication Program; the minimum length applies to land both inside and outside
of the ESA.
4. The chosen land must be located within the same watershed as the
development. The watersheds are:
Little River
Buffalo Creek
Neuse River
Swift Creek
Middle Creek
Black Creek
Mingo Swamp
Stone Creek
Hannah Creek
Moccasin Creek
A map of these watersheds is available in the Public Utilities Office and is on the County GIS
system. The Stormwater Administrator will evaluate each watershed every 3 years for total
imperviousness and for stream/river condition.
2.3.06 County Policy:
The County or other party such as the Triangle Land Conservancy will identify potential tracts
of land for setting aside as perpetually undeveloped land in the Land Dedication Program.
Each tract or corridor of land must meet all of the following criteria:
1. The tract must be placed into a perpetual conservation easement that will not
allow development or farming of the land.
2. The tract may be used for passive recreation provided no hardened surfaces are
involved.
3. The land may be sold or transferred to a conservation group or agency.
4. The tract of land cannot previously, or in the future, have been utilized as
dedicated open space as required by the County Zoning ordinance.
5. The tract of land cannot be counted or used for Total nitrogen export calculations
or attenuation calculations.
Dedicated land can be selectively timbered and replanted as forestland. However, no clear
cutting will be allowed and timbering and replanting must be done consistent with all forestry
water quality BMPs as outlined by the NC Div. of Forest Resources. The land cannot be
farmed and must be maintained in a natural. vegetative state.
Funds from land dedication payments shall be used (only) for purchase of environmental
preservation lands. Property will be acquired once funds become available.
The County can enter into options to purchase with landowners, purchase the property
outright, or have a third party such as the Triangle Land Conservancy administer the land
transactions.
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Section 3 Hydrology and Hydraulics
3.1 Peak Discharge Calculation Methods
To calculate the stormwater runoff from a development and to properly design the proposed stormwater
drainage facilities requires some knowledge of the hydrologic behavior of the watershed in question. For
most designs, it is adequate to estimate the peak discharge of the drainage area for the required design
frequencies. Many different methods of estimating peak runoff are available. However, every model has
limitations that. will affect its behavior for different sized watersheds. The designer should understand
each model's limitations. Two methods are identified in this manual: The Rational Method and the SCS
Peak Discharge Method. Both need site-specific as well as region specific information. The following
information is provided to aid the designer.
Reference material for estimating peak runoff can be found in Secfion 7 Reference and Resources.
3.1.01 Rational Method
The Rational Method is an acceptable method for estimating peak discharge in the design of
stormwater facilities for relatively small watersheds (up to 50 acres). The basic rational equation
is:
Q=CiA
where: Q =peak flow for the design storm from the drainage area, cfs
C =coefficient of runoff, dimensionless
i = average rainfall intensity for a storm duration, in/hr, based on the time of
concentration, T~
T~ =time of concentration, minutes, the estimated time for runoff to flow from the most
remote point in the watershed to the point under consideration. It consists of the total
time for overland sheet flow and concentrated flow (channel or pipe).
A =drainage area, acres
The rational equation is based upon the assumption that rainfall is uniformly distributed at a
steady rate over the entire drainage area, causing stormwater runoff to reach a peak rate at the
outlet of the watershed at a given time, Tp, time to peak. The rational method typically gives a
conservative estimate of peak runoff.
For designing stormwater management plans in Johnston County, NG, runoff coefficients as
listed in the following table (3.1.01) can be utilized far drainage areas within the proposed site.
Actual existing and proposed site conditions should be taken into consideration when choosing
runoff coefficients, including soil type, cover condition; and slope. The design professional may
utilize other charts to determine appropriate runoff coefficients as long as a reference copy of the
chart utilized is provided with the application package.
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Table 3.1.01 Johnston Gaunty Runoff Coefficiencts for On-Site Areas*
Land Use "C" Value'
Residential, Single Family Lots 0.25 - 0.40
Lawn Areas 0.10 - 0.30
Commercial/industrial` 0.60 - 0.85
impervious Areas 0.95
Cultivated Agricultural Land 0.20 - 0.40
Meadow 0.05 - 0.30
Woodlands 0.05 - 0.25
Notes:
1 } The designer must use judgment to select the appropriate runoff coefficient within the
ranges provided for each land use. Soil type, slopes, and conditions of each land use
should be taken into account.
2) Runoff coefficient values will vary based on proposed density and open space provided
within each watershed/drainage area.
*Table 3.1.01 has been adopted from "Table 8.03a Value of Runoff Coefficient
(C) for Rational Formula" as found in the NC Erosion and Sediment Control Planning and
Design Manual.
For off-site drainage areas contributing to and/or draining through a proposed development,
runoff coefficients from the following Table (3.1.02) may be utilized.
Table 3.1.02 Runoff Coefficients for Off-Site Areas
Land Use "C" Value
Average residential lot size 10,000 sq. ft. 0.70
Average residential lot size 30,000 sq. ft. 0.50
Average residential lot size 40,000 sq. ft. 0.40
Impervious surfaces (parking lots, roofs,
roadways, etc.) 0.99
Managed lawn area 0.30
Unimproved forested area 0.1 - 0.2
The designer must provide information supporting the chosen C factors in the stormwater
plan design.
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When determining rainfall intensity (i}, the IDF equations may be used. The IDF equation is:

Where i =storm intensity in inches per hour
g&h = empiricaAy derived constants
T =the time of concentration in minutes
19
The following table of g and h values should be utilized far watersheds in Johnston County:
Table 3.1.04 g and h Values for Johnston County
Storm Event g h
1 Year 108 19
2 Years 138 20
10 Years 205 23
25 Years 245 24
100 Years 307 27
3.1.02 SCS Peak Discharge Method
The SCS peak discharge method for calculating runoff was developed by the USDA Natural
Resources Conservation Service (formerly the Soil Conservation Service) and is contained in
NRCS Technical Release No. 55 (TR 55) entitled "Urban Hydrology for Small Watersheds",
Second Ed.; June 1986. TR55 can be found at NRCS' website:
http://www.wcc.nres. usda.gov/hydrolhydro-tools-models-wintr55.html.
The standard equation for the SCS method is:
(P + 0.8S)
where:
S=1000-10
CN
P =precipitation, inches, and
CN =runoff curve number, dimensionless
S is related to the soil and cover conditions of the watershed, while CN is determined by the
hydrologic soil group, cover type, treatment, hydrologic condition, and antecedent runoff
condition. CN ranges from 0 - 100. Full CN tables are available in NRCS' Technical Release 55,
as well as the NC Erosion and Sediment Control Planning and Design Manual. Hydrologic soil
groups for soils found in Johnston County are also available in the NRCS district office and in the
NC Erosion and Sediment Control Planning and Design Manual. For the purposes of developing
a stormwater management plan for Johnston County and obtaining the appropriate precipitation,
use the type II rainfall distribution.
Draft Revised 1/07

r
The following rainfall amounts (P) are specific to Johnston County (courtesy of the NRCS
Smithfield Office):
Storm Event Rainfall (inches)
1 yr, 24 hr 3.2
2 yr, 24 hr 3.7
5 yr, 24 hr 4.9
10 yr, 24 hr 5.8
25 yr, 24 hr 6.7
50 yr, 24 hr 7.5
100 yr, 24 hr 8.4
3.'1.03 Additional Peak Runoff Calculation Notes
Additional methods for computing peak runoff may be acceptable for very large drainage areas
(400+ acres}. Please consult the Stormwater Administrator prior to submitting a Stormwater
Management Plan for more information on available methods.
Please note the same method must be used to calculate both predevelopment and
postdevelopment peak runoff rates for offsite and attenuation comparisons. However, it is
acceptable to utilize different peak runoff calculation methods throughout a site when not
compared directly. For example, the rational method may be used to determine culvert and
channel sizes, while the SCS method is used for the Offsite Stormwater Impact Analysis.
3.2 Attenuation
To reduce stream degradation and to minimize increases in flooding, runoff from all new development
must provide attenuation of stormwater runoff in accordance with the Stormwater Management
Ordinance. Attenuation must be provided to limit the post-development peak runoff rate to that of the
predevelopment peak runoff rate, for the 1-year, 24-hour storm event.
Exemption from the stormwater attenuation requirements is provided to new developments that meet one
of the following criteria:
a) The increase in peak flow between the pre-and post-development conditions does not
exceed 10%, or
b) The new development meets all of the following criteria:
1. The total impervious area on site is less than 15% outside of the ESA and less than
12% inside the ESA: and
2. The remaining pervious portions of the site are utilized to the maximum extent
practical to convey and control the stormwater runoff in a manner that minimizes
concentrated flow and does not create adverse impacts offsite.
When attenuation is required, the following guidelines should be followed:
Stormwater runoff from impervious surfaces should be directed into stormwater attenuation
devices to the greatest extent possible. In particular, runoff from roadways, parking lots and
driveways has the highest priority to receive detention; runoff from roofs/buildings and other
'`detached'' impervious surfaces are of a lower priority. However, if significant portions of
impervious area are not detained, including detached areas, a more detailed attenuation
analysis may be required. See the Offsite Stormwater Impacts Analyses section for mare
details.
20 Draft Revised 1/07

f 1
Attenuation is required for the 1-year storm event. fn addition to this requirement, where
available and appropriate, the County prefers attenuation be provided for larger storm events.
When attenuation is provided for the 10- and 25-year storm events, to the point where
postdevelopment peak runoff from the entire site is within 10% of the predevelopment peak
runoff rate, no additional downstream impact analyses is required.
The predevelopment peak runoff calculated should utilize the most conservative land use for
the property as documented in the preceding 10 years. In addition, the land use should be
viewed as being in goad hydrologic condition.
Where feasible, attenuation should be provided for an overall watershed/drainage area. The
County encourages the use of regional stormwater management facilities. For example,
industrial or commercial parks are encouraged to provide one overall attenuation facility,
instead of proposing individual stormwater management facilities throughout the
development. Large, multi-phased residential developments should also consider regional
stormwater management facilities. When possible, larger attenuation facilities are more
efficient and easier to maintain than multiple smaller facilities.
The following design calculations and details should be provided to document attenuation requirements:
1. Predevelopment and postdevelopment drainage area maps.
2. Predevelopment and postdevelopment peak flow calculations. The calculations should
indicate all assumptions made, including land use and condition and time of concentration.
3. Calculations documenting the runoff coefficient or curve number used.
4. Calculations documenting the Time of Concentration utilized.
5. Storm events analyzed should be based on the Design and Check Storms for Development
(See Table 3.3.01).
6. Predevelopment hydrographs, summary or full output should include input variables.
7. Postdevelopment inflow hydrographs, summary or full output should include input variables.
8. Postdevelopment outflow hydrographs/routed hydrographs. Full hydrograph routing should
be provided for 1-and 10-year storm events. Summaries can be provided for remaining
storm events analyzed.
9. Volume calculations for the attenuation facility.
10. A Summary of the proposed outlet control devices (orifices, weirs, etc.), including size,
elevation and coefficients used.
11. Stage, Storage, Outflow chart for the proposed attenuation facility.
3.3 Design and Check Storm Guidelines
The storm events to be utilized for the design of various conveyance structures required for development
and construction activities, shall conform to the storms events noted in Table 3.3.01 Design and Check
Storms for Development. Where conflicts exist between County and State or Federal design standards,
the more restrictive guideline shall govern.
Table 3.3.01 lists two storm events to be analyzed for every drainage structure. The first storm event is
the design storm event; the second is the check storm event. For the purpose of obtaining stormwater
Management Plan approval, structures should be designed to prevent erosion, flooding and be
adequately sized for the design storm event. The check storm event must be analyzed to confirm safe
hydraulic passage of the peak stormwater runoff without damage or undue risk to human welfare and
without causing property damage.
When calculating peak runoff for drainage facility design, the discharge estimates for the specified design
storms shall be calculated assuming full development of the contributing watershed based on current
zoning, or on existing development, whichever produces the greater rate of runoff.
21 Draft Revised 1/07

Established and documented methods for estimating peak runoff, such as the Rational Method and the
SCS Method are acceptable (see Section 3.1.01 for information specific to Johnston County for use in
peak discharge design). The Stormwater Administrator reserves the right to require that two methods be
checked and the greater rate of discharge utilized in design.
Table 3.3.01 Design and Check Storms for Development
Structure Design Stormy Check Storm
Culvert or structure receiving runoff from a 10 yr. 25 yr.
drainage area < 25 ac. in size
Culvert or structure receiving runoff from a 10 yr. 100 yr.
drainage area > 25 ac. in size
Culvert or structure receiving runoff from a 25 yr. 100 yr.
drainage area > 500 ac. in size
Drainage facilities for local roadways 10 yr. 25 yr.
Drainage facilities for parking lots 10 yr. 25 yr.
Collector or arterial roadway culverts or drainage 25 yr. 100 yr.
facilities
Bridges or box culverts 50 yr. 100 yr.
Spillway structures for impoundments 25 yr. 100 yr.
(e.g. detention/retention facilities)
Major highways, railroads and medium or high 100 yr. 500 yr.
hazard impoundments.2
Drainage facilities for residential developments 2 yr. 10 yr.
accepting lot runoff only (no roadway discharge)
Drainage facilities providing diffuse flow 10 yr. 25 yr.
~ If the design element Is part of a water quality BMP or detention structure, follow the design
storm requirements noted in Section 5 for Stormwater BMPs. The design element must still be
designed for the check storm to insure safe hydraulic passage.
2 Dams impounding 10 ac-ft or more and that are 15 ft. high must have a Dam Safety Permit from
the NCDENR Division of Land Resources, Land Quality Section. Contact the Dam Safety
Program at [phone].
The design and check storms used for any proposed drainage facility should not be less than the storm
events used for the design of any upstream drainage structure.
3.4 Channel Design
The use of natural channels to convey and distribute stormwater runoff is encouraged. Vegetated swales
provide some nutrient and total suspended solids removal, help maintain or increase the time of
concentration and can help to provide distributed, diffuse flow. When properly designed and where
feasible, the use of natural channels can help create a development that has a smaller impact on the
surrounding environment.
22 Draft Revised 1/07

23
3.4.01 Channel Design Guidelines
Every development proposing channels to collect, convey and/or distribute stormwater runoff
must use the Storm Guidelines listed in Section 3.3 above and utilize the guidelines in the NC
Erosion and Sediment Control Planning and Design Manual. In addition, the following standards
must be applied:
1. Vegetated swales are preferred over structural finings (e.g. rip rap). If the use of a
structural lining is required, an explanation must be provided documenting why other
measures cannot be applied. For example, in many cases stormwater runoff can be
redistributed to provide less concentrated runoff and minor regrading can reduce slopes.
Please note, synthetic liners are not considered structural lining when the final product
will be vegetated.
2. Each channel must be designed for an appropriate volume of runoff. Depending on the
location of the channel, maximum peak flaw rates, depths and velocities will be applied
as the Stormwater Administrator deems appropriate.
For example, in a residential development with a channel located between or
within residential lots, the maximum design flow rate should not exceed 30 cfs,
with velocities under 5 fps and flow depths under 1 foot. Due to the number of
variables in channel design, each development will be reviewed independently.
The design professional should review the proposed channel locations and
purpose and provide a design to collect and convey stormwater runoff in a
manner to protect human health and welfare and to protect property.
3. Proposed channels should be designed at subcritical flow. Where this is not possible,
due to existing steep slopes, additional measures to prevent erosion should be provided.
For example, slopes should be reduced, peak runoff redistributed to be less concentrated
and/or erosion control matting proposed to ensure establishment of vegetation.
4. Proposed channels should be designed with a minimum slope of 0.5%.
5. Proposed channels should have a bottom width no greater than 8 feet wide.
6. Proposed channels should have side slopes no steeper than 3:1 (H:V).
7. Proposed channels should be constructed with 0.5' of freeboard above the design storm
depth.
8. Drainage easements for channels located outside of the public Right Of Way should be in
accordance with Section 6 of this manual.
3.4.02 Submittal Requirements for Proposed Channels
The following information should be submitted for review at the time of Stormwater Management
Plan Permit application for all developments with proposed channels:
1. Drainage area maps for each channel. The control point for the drainage area to each
channel should be the most downstream point of the channel, or outlet point.
2. Peak flow calculations, documenting all variables and assumptions made, for the design
and check storm events.
3. Channel calculations, documenting the proposed channel dimensions and design flow
depth and velocity for both the design and check storm events.
4. Channel labeling with dimensions on the plan view and detail, as appropriate.
Draft Revised 1/07

3.5 Storm Sewer Design
Johnston County will review ail proposed storm sewer located outside of existing and future NCDOT right-
of-way. All proposed storm sewer located within a public ROW is the responsibility of the developer and
design professional to insure adequate design has been provided. The County will review all crossings of
existing streams; regardless of location in or outside of a public RQW.
Where storm sewer is required to convey stormwater runoff.. the following design guidelines should be
utilized in Johnston Gounty~
1. The minimum slope for proposed storm sewer is 0.2%. Far pipe sizes 36" and higher, the
minimum slope is 0.15%.
2. The minimum pipe diameter allowed is 15".
3. Minimum cover for all pipes located outside of areas regularly accepting vehicular traffic is
4.5 feet. Minimum cover for all pipes under roadways. parking lots, etc, where vehicular
traffic is proposed is 1.0 foot. Minimum cover is measured from the ground to the top of the
pipe.
4. Af utility crossings: the minimum separation from storm sewer to water lines or sanitary sewer
lines is 1 foot. The distance is measured from outside of pipe to outside of pipe. If less than
12" is proposed, concrete encasement or a concrete cradle may be proposed; but a minimum
separation of 6" must be provided.
5. Drops in structures must be provided when there is a change in pipe size andtar a change in
alignment greater than 45 degrees.
3.5.01 Stream Crossing Guidelines
If a stream crossing is proposed for a development or construction activity, the following
guidelines should be utilized:
1. The crossing should be as perpendicular to the stream as possible. The crossing
should be located at a section where the stream width, riparian buffer and/or
wetlands are the shortest and where the crossing will have the smallest impact on the
existing drainage feature.
2. The culvert or bridge should be designed using the Storm frequency guidelines listed
in Section 3.3 above.
3. For the design storm, a minimum of 2 feet of freeboard should be provided below the
proposed pavement elevation. For the check storm, a minimum of 0.5 feet of
freeboard should be proposed.
4. All culverts and bridges should be designed to handle the full build-out of the
upstream drainage area. Existing zoning and maximum impervious area limits
should be utilized to determine maximum build out.
5. All culverts and bridges proposed for a stream crossing with a drainage area over
100 acres must provide a backwater analysis to document that there are na adverse
impacts up or downstream of the proposed crossing. Existing and proposed
calculations must be provided. Increases in flooding offsite are considered adverse if
the water surface elevation increases over 0.5 feet or if an existing structure is within
1 foot of the proposed water surface elevation.
6. All culverts must provide protection from scour at the downstream side of the
crossing.
24 Draft Revised 1/07

3.5.02 Submittal Requirements
1. Drainage area maps for each inlet and/or pipe should be provided.
2. Peak flow calculations for the design and check storm events should be provided.
3. Pipe inverts, sizes, length, slope and material should be provided on the plan and
profile view.
4. Design calculations should indicate full flow capacity and design velocity for each
pipe.
5. All inlets and pipes should be appropriately labeled on the plan view and the
calculations.
3.6 Cross Lat Drainage
For all proposed single family residential subdivisions, cross lot drainage must be addressed. If
stormwater runoff will drain across three or more lots, drainage facilities to capture and convey the runoff
must be proposed. The stormwater runoff must be directed into an existing or proposed drainage way
and must be kept away from proposed building pad locations.
For aJl other development, cross lot drainage must be kept to a minimum and will be addressed
specifically within each project as necessary.
3.7 Offsite stormwater Impacts Analyses
Offsite stormwater Impact Analyses are required where a development will create an increase in peak
runoff by 10% or more. The analyses are provided to document that upstream and/or downstream
properties will not be adversely affected by the development. Increases in stormwater runoff from
proposed developments can cause excessive erosion, flooding and other negative impacts to surrounding
properties. The development must be designed and constructed in such a manner as to minimize these
impacts.
3.7.01 Minimizing Offsite stormwater Impacts
The following methods should be addressed in every development to minimize offsite impacts:
1. Existing flow patterns should be maintained.
2. Multiple outflow points should be provided.
3. stormwater runoff should be kept diffuse, with minimal concentration.
4. Discharge points should be directed into existing well-defined watervvays.
5. The existing land use on properties downstream of the development should be
considered when locating discharge points.
6. Direct discharges onto adjacent properties, where only sheet flow previously existed
should be avoided.
3.7.02 Existing Facilities
When discharging into an existing drainage facility, the facility must be analyzed to determine if
the increase in peak runoff will create an adverse impact.
• For existing streams and ditches the analysis should focus on increases in velocity
and flooding within the stream.
• For existing culverts the analysis should focus on increases in headwater and
flooding at the structure.
25 Draft Revised 1/07

Where the existing facility has a drainage area that is significantly larger than the drainage area
from the proposed development, no additional analysis is required. The point of insignificance is
reached when the development's drainage area is 10% or less than the existing facilities drainage
area.
3.7.03 Submittal Requirements
The following information must be provided for all Offsite stormwater Impact Analyses:
1. Predevelopment and postdevelopmentdralnage area maps to each existing
stormwater facility or each outlet point, as appropriate.
2. Predevelopment and postdevelopment peak flow calculations. The calculations
should indicate all assumptions made, including land use and condition and time of
concentration.
3. Calculations documenting the runoff coefficient or curve number used for both the
predevelopment and postdevelopment conditions. Please note, the existing
conditions of the watershed should be used for the predevelopment analysis and the
postdevelopment analysis should include changes to the existing condition from the
proposed development.
4. Calculations documenting the Time of Concentration utilized for both the
predevelopment and postdevelopment conditions.
5. Storm events analyzed should be based on the Design and Check Storms for
Development Table 3.3.01. At a minimum the 10-and 25-year storm events should
be provided for most offsite stormwater impact analyses.
6. Existing stormwater facility information.
a. For existing streams, channel characteristics such as slope, bottom width, side
slopes, stream depth and streambed condition should be provided.
b. For existing culverts, pipe material, size, inverts and pavement elevations should
be provided.
7. Predevelopment and postdevelopment hydraulic calculations for each stormwater
facility.
If increases in flooding are found that will flood roadways or cause existing channels to overtop
(where this didn't occur in the predevelopment analysis), additional measures to reduce peak flow
rates may be required. In addition, if velocity increases are over 15%, additional measures to
reduce erosion or reduce peak flow rates may be required.
26 Draft Revised 1/07

Section 4 Nitrogen Reduction
As required by Neuse River Basin -Nutrient Sensitive Waters Management Strategy: Basin wide
Stormwater Requirements (15A NCAC 26 .0235), the Johnston County Stormwater Ordinance
requires new development to reduce nitrogen loading by installing BMPs, both structural and non-
structural, or by offset payments to the Ecosystem Enhancement Program (EEP). The nitrogen
loading limits for each development type, calculations for loading amounts, and method for
payments into the EEP are outlined in this Section.
4.1 The Nitrogen Cycle
Nitrogen is an essential nutrient for plants. However, plant-available nitrogen is scarce under
natural conditions. Most nitrogen exists as Nzgas in the atmosphere. The conversion of Nzto N
compounds and from N compounds back to Nz is called the nitrogen cycle. Figure 4.1.01 shows a
simplified nitrogen cycle in an undisturbed forest.
Figure 4.1.01 Nitrogen Cycle in an Undisturbed Forest
INnURTR114~N~ GIidF'
FIICATIONNH~~ RE-MC)VAI
[j IaLOGI~AL
>1C)1 ATI7lSTI4N FUCAIION
tEH f ILI1f=H KA 114F•ALL . ~ .
1
A tJI IJIA l ~~-
• • • WASTr S =
. ~.
GR dP
LIPTAKf
MINE=HALILA f 10 N
OtiGhNIG
Nligt LIATTI-la
~~ IMMOkfILILh11QN
nnrr~lE-IC n _~
~~3 -u~lvntilrlCnilcr~l
I r•ACrnNr,
In an urban environment, impervious surfaces disconnect rainfall and shallow surface water.
Impervious surfaces cause runoff that would otherwise infiltrate into pervious areas to runoff into
storm drain systems and water bodies. Pollutants such as oils and nutrients are carried into water
bodies with runoff, and the hydrologic and natural nitrogen cycles are altered.
27 Draft Revised 1/07

e ~
Summarizing research conducted on nitrogen loading from urban areas:
1) The transport of atmospheric N from land to surface waters is a major contributor of
nitrogen to urban streams.
2) Reducing transport-related imperviousness in urban areas is likely to play an important role
in reducing the deposited nitrogen that moves from urban land to surface waters.
3) Minimizing the use of curb and gutter with storm sewer will also reduce the deposited
nitrogen that moves from urban land surfaces to surface waters.
4.2 Total Nitrogen Calculations
Every proposed development must provide calculations to determine the total proposed nitrogen
export from the site. The Total Nitrogen Export is based on proposed land use. Two methods are
available to calculate the Total Nitrogen Export as outlined below:
Method 1: For residential development where lots are shown but the actual footprint of buildings
and paved areas are not shown. This method estimates the total amount of impervious surfaces
expected within the development, based on lot size, house size and estimations for driveways and
other impervious areas.
Method 2: For residential and non-residential development showing the entire footprint of the
buildings and other impervious surfaces. Method 2 shall apply to Commercial/Industrial
subdivisions and site plans. This method is more accurate and simpler than Method 1.
Figure 4.2.01 Method 1 for Quantifying TN Export from Residential
Developments when Buildings and Driveway Footprints are not Shown
Step 1: Determine area for each type of land use and enter in Column 2.
Step 2: Total the areas for each type of land use and enter at the bottom of Column 2.
Step 3: Determine the TN export coefficient associated with right-of-way using Graph 1.
Step 4: Determine the TN export coefficient associated with lots using Graph 2.
Step 5: Multiply the areas in Column 2 by the TN export coefficients in Column 3 and
enter in Column 4.
Step 6: Total the TN exports for each type of land use and enter at the bottom of Column 4.
Step 7: Determine the export coefficient for site by dividing the total TN export from uses at
the bottom of Column 4 by the total area at the bottom of Column 2.
(1) (2) (3) (4)
Type of Land Cover Area (acres) TN Export Coeff. TN export from
(Ibs/ac/yr) use (Ibs/yr)
Permanently protected 0.6
undisturbed open space
(forest, un-mown meadow,
etc. )
Permanently protected 1.2
managed open space
(lawn, grass, landscaping,
etc.
Right-of-way (read TN
ex ort from Gra h 1
Lots (read TN export from
Gra h 2
TOTALS _
28 Draft Revised 1/07

Graph 1: Total Nitrogen Export from Right-of-Way,
25.0
F 20-0
a 15:0
_c
r
a 10.0
X
W
5.0
,ioo
,o.oo
s.oo
s.oo
K
h
4.00
~.
0.0 ~-
0°/v
20°h 40°~6 60`Y° $0% 100°fv 120%
Percentage of Ri~M-nt-Way that Is Pavement
Graph 2: Total Nitrogen Export from Lotsz
~ i
( ~
E i 4 t
__ _ __ _ ~
__ ~_.-. _ a.... .-_ _. _~ _ " . _ _ ~ __
i
i
e
0 2 4 6 8 10 12 id ,S ,8 20 Q 2A 26 28 30 32
Number of Dwetting Untts PaAtxo
~ "Graph 1: Total Nitrogen Export from Right-of-Way° was taken from the Neuse River Basin: Model Stormwater Program for
Nitrogen Control.
2 "Graph 2: Total Nitrogen Export from lots" was taken from the Neuse River Basin: Model Stormwater Program for
Nitrogen Control.
o.~
29
Draft Revised 1/07

When using Method 1 to calculate Total Nitrogen Export the following guidelines should be adhered
to when estimating the TN Export Coefficient from Graphs 1 and 2:
1. To determine the percentage of Right-of-Way that is pavement, the entire roadway
surface within the ROW should be used.
2. To determine the Number of Dwelling Units per Acre, only the land area within the
proposed single-family lots should be used. The area should also exclude all land that
is already utilized as undisturbed or managed permanently protected open space.
Figure 4.2.02: Method 2 for Quantifying TN Export from Residential / Non-
residential Developments When Footprints of all Impervious Surfaces Shown
Step 1: Determine area for each type of land use and enter in Column 2.
Step 2: Total the areas for each type of land use and enter at the bottom of column 2.
Step 3: Multiply the areas in Column 2 by the TN export coefficients in Column 3 and enter
in Column 4.
Step 4: Total the TN exports for each type of land use and enter at the bottom of Column 4.
Step 5: Determine the export coefficient for site by dividing the total TN export from uses at
the bottom of Column 4 by the total area at the bottom of Column 2.
(~) (2) (3) (4)
Type of Land Cover Area (acres) TN Export Coeff. TN export from
(Ibs/ac/yr) use (Ibs/yr)
Permanently protected 0.6
undisturbed open space
(forest, un-mown meadow,
etc. )
Permanently protected 1.2
managed open space
(lawn, grass, landscaping,
etc.
Impervious surfaces 21.2
(roads, parking lots,
driveways, roofs, paved or
graveled storage areas,
raveled roads, etc.
TOTALS -
4.3 Maximum Total Nitrogen Export Limits
For all new development, the maximum allowable nitrogen-loading rate shall be held to 3.6
Ibs/ac/yr. For loading rates exceeding 3.6 Ibs/ac/yr, developments have the following three options:
• BMPs can be installed to reduce the nitrogen-loading rate from the
Development to the allowable rate,
• A one-time payment (offset payment) can be paid into the Ecosystem Enhancement Fund,
or
• A combination of BMPs and aone-time offset payment.
The following Tables (Table 4.3.01 and Table 4.3.02) further breakdown how developments can
apply the options described above. Maximum buy down limits do apply and the guidelines are
based on whether or not the development is located within the Environmentally Sensitive Area.
30 Draft Revised 1/07

•
Table 4.3.01 Nitrogen Export Reduction Options for Developments Outside
the Environmentally Sensitive Area (ESA)
Residential Other Develo ment
If the computed export is less than 6.0 If the computed export is less than 10.0
Ibs/ac/yr, then the owner may either: Ibs/ac/yr, then the owner may either:
1. Install BMPs to remove enough nitrogen to 1. Install BMPs to remove enough nitrogen to
bring the development's total nitrogen bring the development's total nitrogen
export down to 3.6 Ibs/ac/yr. export down to 3.6 Ibs/ac/yr.
2. Pay aone-time offset payment as 2. Pay aone-time offset payment as
calculated below to bring the total nitrogen calculated below to bring the total nitrogen
export down to 3.6 Ibs/ac/yr. export down to 3.6 Ibs/ac/yr.
3. Do a combination of BMPs and offset 3. Do a combination of BMPs and offset
payment to achieve a total nitrogen export payment to achieve a total nitrogen export
of 3.6 Ibs/ac/yr. of 3.6 Ibs/ac/yr.
If the computed export is greater than 6.0 If the computed export is greater than 10.0
Ibs/ac/yr, then the owner must use on-site Ibs/ac/yr, then the owner must use on-site
BMPs to bring the development's export down BMPs to bring the development's export down
to 6.0 Ibs/ac/yr. Then the owner may use one to 10.0 Ibs/ac/yr. Then the owner may use one
of the three options noted above to achieve the of the three options noted above to achieve the
reduction between 6.0 and 3.6 Ibs/aG r reduction between 10.0 and 3.6 Ibs/ac/ r
Table 4.3.02 Nitrogen Export Reduction Options for Developments Inside the
Environmentally Sensitive Area (ESA)
Residential Other Develo ment
If the computed export is more than 3.6 If the computed export is less than 8
Ibs/ac/yr, then the owner must: Ibs/ac/yr, then the owner may either:
1. Install BMPs to remove enough nitrogen to 1. Install BMPs to remove enough nitrogen to
bring the development down to 3.6 bring the development's total nitrogen
Ibs/ac/yr. export down to 3.6 Ibs/ac/yr.
2. Pay aone-time offset payment as
calculated below to bring the total nitrogen
Note: As of May 1, 2005, no offset payments export down to 3.6 Ibs/ac/yr.
are allowed for residential developments 3. Do a combination of BMPs and offset
located within the ESA. payment to achieve a total nitrogen export
of 3.6 Ibs/ac/yr.
If the computed export is greater than 8.0
Ibs/ac/yr, then the owner must use on-site
BMPs to bring the development's export down
to 8.0 Ibs/ac/yr. Then the owner may use one
of the three options noted above to achieve the
reduction between 8.0 and 3.6 Ibs/ac/ r
31
Draft Revised 1/07

4.4 Total Nitrogen Offset Payment Calculation
If developments choose the offset payment option to reduce the Total Nitrogen
Export from the site, where allowed, the nutrient offset payment fee is calculated by using the
following equation:
Nitrogen payment = [($11/Ib)(# of Ibs/ac/year)(# of Acres)(30 years)]
Where:
$11/Ib = The cost of mitigation in dollars per pound of nitrogen mitigation.
This number has been established for the current calendar year
and will be adjusted on an annual basis.
# of Ibs/ac/year = The number of pounds of nitrogen exported or discharged per acre
each year for which mitigation is being requested.
# of Acres = The total development area in acres.
The nitrogen offset payment calculation can be simplified to:
$330*(Site Acreage)*(TN export in Ibs/ac/yr)
4.5 Total Nitrogen Calculation for Redevelopment
For proposed development that is utilizing an existing developed site area for expansion or
redevelopment, the following calculation method can be applied for determining the required Tota!
Nitrogen export level to be brought dawn, using either an offset payment or BMP's.
1. The existing Tatal Nitrogen Export amount; as of 1995, is to be calculated as per
Method 2 listed in Section 4.2.
2. The existing TN export (in Ibs/aclyr} is muitiplied by 7Q% (0.7},
3. If 70% of the existing TN export is greater than 3.6 Ibs/actyr than this amount may
replace 3.6 Ibs/ac/yr in the Maximum Total Nitrogen Export Limits, Section 4.3.
Please note, the maximum impervious area limits and attenuation requirements, as discussed in
Sections 2 & 3 still apply to redevelopment projects.
32 Draft Revised 1/07

4.6 BMP Reduction Credits
In October 2004, the NC Division of Water Quality published updated nutrient removal efficiencies
for BMPs as outlined below:
BMP Type TN Removal Rate Appropriate Design
Standards
Wet detention onds 25% NC and MD Desi n Manuals
Constructed wetlands 40% NC and MD Desi n Manuals
O en channel ractices 20% NC and MD Desi n Manuals
50' Restored riparian buffer
w/levels reader 30% Neuse Riparian Buffer Rule (15A
NCAC 26.0233
Vegetated filter strips with level
s reader 20% NC and MD Design Manuals, Johnston
Coun Desi n Manual
Bioretention 35% NC and MD Desi n Manuals
Sand Filters 35% NC and MD Desi n Manuals
D Detention 10%+ Johnston Count Desi n Manual
Proprietary BMPs Varies Per manufacturer subject to County
and State a royal
Other BMPs Varies Sub'ect to Count and State a royal
+ Dry detention is considered primarily an adjunct practice that can provide volume attenuation to
help meet site attenuation requirements. The practice could be employed for this purpose
preceding a grassed Swale or bioretention area, or preceding a level spreader above a vegetated
filter strip or riparian buffer. Available data indicate that it can provide only limited nutrient removal,
as reflected in the efficiency listed.
Additional research data may result in adjustment of these removal efficiencies in the future.
If more than one BMP is installed in series on a development, then the removal rates shall be
determined through serial rather than additive calculations. For example, If a wet detention pond
discharges through a riparian buffer, then the removal rate shall be estimated to be 47.5 percent
(The pond removes 25% and discharges 75% to the buffer. The buffer then removes 30% of the N
discharged from the pond. Which is 22.5%. The sum of 22.5% and 25% is 47.5%).
4.7 Example Calculations
Example 1: Broome Estates S/D
Given:
• Residential subdivision outside the ESA
• 40.2 acres total, 66 lots, 5.3 acres in ROW with a 20' pavement width
• Expected impervious area = 20%, 8.04 Acres
• Calculated impervious area exceeds impervious area limit of 15%, therefore owner must
participate in the land dedication program
• Length of Timothy Branch = 1800' x 50' riparian buffer = 2.1 acres
• Developed as 2 dwelling units per acre
33
Draft Revised 1/07

Detention required?
• Since imperviousness > 15%, detention is required.
Use Method 2
TN Export TN Export

Land Use
Area, ac
Coefficient, ,
Ibslyr
Ibs/ac/ r
Permanently protected,
undisturbed open space 2 1 0
6 1
26
(forest, existing riparian . .
buffers
Permanently protected
managed open space 30.06 1.2 36.07
lawns, landsca ed areas
Impervious surfaces (roofs,
roads, pavement, parking 8.04 21.2 170.45
areas, etc.
Totals 40.2 - 207.78
Total Nexport = 207.78 Ibs/yr / 40.2 ac = 5.17 Ibs/ac/yr
Requirements: If export < 61bs/aGyr, can use BMPs or offset payment to reduce to 3.6 Ibs/ac/yr.
Option 1: Offset payment and wet detention BMP
• Install wet pond, providing 25% reduction = 3.9 Ibs/ac/yr
• Offset payment calculation:
(3.9 Ibs/ac/yr - 3.6 Ibs/ac/yr) = 0.3 Ibs/ac/yr
0.3 Ibs/ac/yr * 40.2 Ac = 12.06 Ibs/yr (pounds of nitrogen for which mitigation is being
requested)
Payment = $330*(40.2 Acres)*(0.3 Ibs/ac/yr) _ $3,979.80
Option 2: Offset payment only
(5.17 Ibs/ac/yr - 3.6 Ibs/ac/yr) = 1.57 Ibs/ac/yr
1.57 Ibs/ac/yr * 40.2 Ac = 63.11 Ibs/yr (pounds of nitrogen for which mitigation is being
requested)
Payment = $330*(40.2 Acres)*(1.57 Ibs/ac/yr) _ $20,827.62
Example 2: Chesson Acres
Given:
• Residential subdivision inside the ESA
• 101.96 developed acreage, 89 lots, ROW acreage = 6.58
• R30 zoning, assumed impervious area = 15% (15.3 Acres)
• Since the calculated impervious area exceeds the impervious area limit for the ESA (12%), the
ownermust participate in the land dedication program
• 1100' along Sunny Creek x 50' riparian buffer = 1.3 acres
34 Draft Revised 1/07

Detention required?
Imperviousness = 15%, therefore detention is required
Use Method 2
Land Use Area
ac TN Export TN Export,
, Coefficient, Ibs/ac/ r Ibs/ r
Permanently protected,
undisturbed open space 1.3 0.6 0.78
(forest, existing riparian
buffers
Permanently protected
managed open space 85.36 1.2 102.43
lawns, landsca ed areas
Impervious surfaces
(roofs, roads, pavement, 15.3 21.2 324.36
arkin areas, etc.
Totals 101.96 - 427.57
Total Nexport = 427.57 Ibs/yr 1101.96 ac = 4.28 Ibs/ac/yr
Requirements: If export > 3.61bs/actyr, must use BMPs to reduce to 3.61bs/ac/yr.
Option is Install wet pond for 25% nitrogen removal
Total NExport = 4.28 Ibs/ac/yr - (4.28 Ibs/ac/yr) (0.25) = 3.21 Ibs/ac/yr
Option 2: Install vegetated filter strips with level spreaders for 20% nitrogen removal
Total NExport = 4.28 Ibs/ac/yr - (4.28 Ibs/ac/yr)(0.20) = 3.42 Ibs/ac/yr
Example 3: Anderson Commons
Given:
• Commercial development outside the ESA
• 7.9 acres, 1 lot, no ROW
• Expected impervious area = 80% (6.32 Acres)
• The calculated impervious area exceeds impervious area limit of 60%, therefore owner must
participate in the land dedication program
• No riparian buffers
Detention required?
• Since imperviousness exceeds 15%, detention is required.
Use Method 2
Land Use Area
ac TN Export TN Export,
, Coefficient, Ibs/ac/ r Ibs/ r
Permanently protected,
undisturbed open space 0 0.6 0
(forest, existing riparian
buffers
Permanently protected
managed open space 1.58 1.2 1.9
lawns, landsca ed areas
Impervious surfaces
(roofs, roads, pavement, 6.32 21.2 .134.0
arkin areas, etc.
Totals 7.9 - 563.47
35 Draft Revised 1/07

Total Nexport = 563.47 Ibs/yr ~ 7.9 ac. = 71.33Ibs/ac/yr
• Nitrogen loading extremely high; redesign
• Try 60% imperviousness
Method 2
Land Use Area, ac TN Export TN Export,
Coefficient, Ibs/ac/ r Ibs/ r
Permanently protected,
undisturbed open space 0 0
6 0
(forest, existing riparian .
buffers
Permanently protected
managed open space 3.16 1.2 3.79
fawns, landsca ed areas
Impervious surfaces
(roofs, roads, pavement, 4.74 21.2 100.49
arkin areas, etc.
Totals 7.9 - 104.28
Total Nexport = 104.28 Ibs/yr ~ 7.9 ac. =13.20 Ibs/ac/yr
Option 1: Wet detention and offset payment
• Wet detention provides 25% reduction = 9.90 Ibs/ac/yr
• Offset payment calculation:
(9.90 Ibs/ac/yr - 3.6 Ibs/ac/yr) = 6.30 Ibs/ac/yr
6:30 Ibs/ac/yr * 7.9 Ac = 49.77 Ibs/yr (pounds of nitrogen for which mitigation is being
requested)
Payment = $330"(7.9 Acres)*(6.3lbs/ac/yr) _ $16,424.10
36 Draft Revised 1/07

Section 5 Stormwater BMPs and Drainage Facility Guidelines
The following section outlines some "typical" best management practices, BMPs, and structural
drainage facilities, but by no means identifies them all. BMPs can be structural (wet detention
ponds, sand filters or bioretention) or non-structural (reduced impervious area, natural area
preservation or land dedication). They are designed for water quality and/or water quantity
control. All development in Johnston County's jurisdiction shall meet the attenuation
requirement, nutrient reduction requirement and impervious area limits, as outlined in
Sections 2, 3 and 4 of this manual. This section contains design criteria and considerations,
maintenance considerations and site suitability checklists for many Drainage Facilities and BMPs
that are used to meet requirements outlined in this Manual.
It is preferred that a development be designed with systems of drainage facilities utilizing BMPs
that encompass a site's specific needs, the requirements of the Johnston County Stormwater
Ordinance and Design Manual and the basic stormwater principles noted below. The County
encourages the development of innovative BMPs meeting the minimum requirements outlined in
Sections 3 and 4 of this manual.
The basic principles behind stormwater BMPs are:
Runoff attenuation. Reduce the peak discharge rate after the development to the pre-
development peak discharge rate. For Johnston County, the post-development peak
discharge rate must be limited to the pre-development peak discharge rate of the 1 yr storm
event. Storm attenuation is required for all development except for the following situations:
a) The increase in peak flow between the pre-and post-development conditions does
not exceed 10%.
b) The proposed new development meets all of the following criteria:
• Overall impervious surface is less than 15%; and
• The remaining pervious portions of the site are utilized to the maximum
extent practical to convey and control the stormwater runoff.
Reduce the source of the runoff at the site. Reduce the amount of imperviousness at the site.
Disconnect impervious areas with vegetation (traffic islands, green space). Plan development
to cluster impervious areas together, thereby reducing the total amount of impervious area
needed. Increase the amount of open space at the site.
Runoff conveyance. Safely deliver stormwater to BMPs. Minimize disturbance of natural
channels, avoid storm drain enclosures and promote filtering and infiltration. Avoid hard-lined
ditches by installing reinforced synthetic liners and grass.
Runoff treatment. Trap or capture solids, such as sediments and nutrients, in forebays and
other pretreatment areas. Combining pre-treatment and primary treatment in BMPs increases
the BMPs overall efficiency. Primary treatment of stormwater involves either nutrient uptake
or infiltration. Grassed filter strips can serve as pre-treatment or primary treatment. Riparian
buffers combine grassed areas, herbaceous shrubs and trees for pre-treatment and plant
uptake for primary. Detention/retention ponds provide both pre-treatment, if a sediment
forebay is included, and primary treatment in the permanent or temporary pool area.
Constructed wetlands provide primary stormwater treatment, allowing plant uptake of
nutrients and infiltration.
System maintenance and repair. Provide a realistic plan for maintaining long-term
performance of the BMP system to prevent failure and to keep stormwater treatment
efficiencies high. All BMPs shall be inspected annually. Also, access to the BMP and outlet
structure must be maintained. The maintenance plan shall address sediment removal and
37
Draft Revised 1107

disposal, maintenance of vegetation and repair of the structural elements of the BMP.
Secondary environmental impacts. Adjust the design to minimize the secondary stream
impacts caused by BMPs, such as stream warming from wetlands or ponds. Re-establish
natural conditions below a BMP in as short a distance as possible, and comply with all state
and federal permit requirements. Account for public safety and nuisance hazards in the
design.
5.1 Selecting BMPs
Stormwater BMPs must be selected with care. Factors such as soils, topography, size
limitations and/or location can prevent awell-engineered Stormwater Management Plan from
working properly.
Take the following factors into consideration when designing a site plan and Stormwater
Management Plan:
Objectives of the Stormwater Management Plan. The plan shall meet the County's quality
(Nutrient reduction) and quantity (post-development discharge limited to the pre-development
1 yr, 24 hr storm) requirements.
Physical Feasibility. Steep slopes and depth to the water table can decrease the efficiency of
some BMPs. Large drainage areas, lack of head, soils and/or bedrock can determine which
BMP can be installed in certain locations.
Location/pennitting considerations. Stormwater BMPs should not be located in natural
wetlands, inside riparian buffers, or in streams. Locating the BMP in a prominent location
increases the likelihood that a community will more easily acceptthe BMP.
The following table summarizes the advantages and disadvantages of some BMPs used in
North Carolina.
Table 5.1 Summary of BMP Advantages and Disadvantages
(NCSU Cooperative Extension Service and the Maryland Stormwater Design Manual)
BMP Advanta es Disadvanta es
Can double as recreational Medium to high cost relative
Wet ponds facility; low to medium to drainage area; safety
maintenance needs; issues
Highest pollutant removal; Medium to high cost relative
Constructed medium to high community to drainage area; land intensive;
Wetlands acceptance; high habitat quality can be difficult to
maintenance
Infiltration Low design and construction Difficult maintenance; limited to
trenches/wells costs; aids groundwater recharge. sandy soils; high potential for
clo in ;little nitro en removal
Good for retrofits in developed Most expensive per square foot
Sand Filters communities; removes parking lot of device; maintenance difficult
ollutants
Aesthetically pleasing; can meet Little local data to prove
Bioretention landscaping requirements and effectiveness; medium cost
pollutant removal relative to drainage area;
medium maintenance needs
Level spreaders with Low construction costs; effective Land-intensive; difficult to
Ri avian buffers ollutant removal; aestheticall correct) install levels reader
38 Draft Revised 1/07

•
leasin
Easy to maintain; high community High construction costs relative
acceptance to drainage area; low habitat
Dry swale quality
D Detention Meets attenuation re uirements Low efficienc
5.2 BMP Nitrogen Removal Rates
The following chart outlines nutrient removal rates for typical BMPs. The removal rates are
median rates, based upon research performed in other areas. Please note that poor
installation, lack of maintenance or lack of repair can greatly affect a BMPs nutrient removal.
These reduction rates are to be used in the design of BMP systems to satisfy the nutrient
removal required and outlined in Section 4 of this Manual.
Table 5.2 Nitrogen Removal by BMP (Schueler, Urban Stormwafer Management Workshop,
June 1998 and Neuse Stormwater Team Meetings)
BMP Median N
Removal Size of Drainage
Area
Wet Pond 25% >25 acres
Constructed Wetlands 40% > 25-acres
Vegetated- Filters strips with
levels readers 20% Varies
Open channel practices
wet swales, d swales 20% < 5 acres
Sand Filters 35% Varies 2 - 10 acres
Bioretention 35% < 2 acres
D Detention 10% Varies
Proprietary BMP Varies Varies; subject to
NCDENR DWQ a royal
Other BMPs Varies Varies; subject to
NCDENR DWQ a royal
5.3 Non-Structural BMPs
Non-structural BMPs can be employed at a development to reduce the need for BMPs. Most
non-structural BMPs reduce impervious area or disconnect impervious areas so some
amount of treatment is allowed. Examples of non-structural BMPs are: reduction of
pavement width and other planning BMPs, natural area conservation, disconnection of
rooftop runoff, and land dedication. Storage and treatment volume and nutrient reduction are
directly related to impervious area. Significant reductions in impervious cover result in
smaller storage volumes, lower BMP construction costs and lower nitrogen offset required.
As noted in Section 4, reducing the amount of impervious area at a site reduces the nitrogen
loading. Many planning tools exist to reduce the overall imperviousness of a site.
• Keep road widths to a minimum, based upon the requirements in the Johnston County
Development Ordinance.
• Cluster buildings and roadways together to reduce the amount of paving needed.
• Install sidewalks on one side of the road, not both.
• Reduce the number of parking spaces for commercial and industrial sites.
• Use porous pavement material for overflow parking.
39
Draft Revised 1/07

• Angle parking spaces for commercial, industrial or institutional development.
It is in the developer's best interest to limit the total impervious area at a site.
With careful planning and design, storage and storm attenuation can be achieved by routing
runoff through longer flow paths. Examining the variables in the SCS Peak Discharge Method
extending the time of concentration and/or the flow length will decrease the peak discharge, if
everything else remains the same. Innovative planning and design can reduce the storage
requirement for BMPs, thus decreasing construction costs.
5.4 Structural BMPs
Structural BMPs can be divided into the following groups: infiltration (infiltration trenches or
basins), filtration (riparian buffers, sand filters, bioretention), Ponds (wet ponds, dry ponds, pocket
ponds), wetlands (pocket wetlands, extended detention wetlands) and open channel systems (dry
swales, wet swales).
To verify that a BMP is designed appropriately, a Professional Engineer shall certify the design
when the Stormwater Management Plan is submitted containing any of the following BMPs:
ponds, wetlands, bioretention, sand or other media tflters, or infiltration BMPs. Swales and
riparian buffers are not required to have an engineer's certification.
To ensure that the BMP will be properly maintained, the County can require that anon-expiring
interest accruing capital reserve fund be established with Johnston County the named as
beneficiary in the event of default of inspection and maintenance requirements. The amount of
the fund shall be 25% of the original construction cost of the impoundment. Documentation of the
BMPs) construction costs shall be provided before the Stormwater Management Plan can be
approved.
An engineer's certification is also required after the BMP is installed to certify that it was installed
according fo the design and approval. The installation certification shall be submitted to the
Stormwater Administrator and shall be accompanied by a site plan showing the locations of each
BMP installed and certified. If the project has multiple stages and multiple BMPs, please correlate
BMP certifications with the site plan.
Each BMP shall be inspected annually and certified by a Professional Engineer, and inspection
reports with the engineer's certification shall be submitted to the Stormwater Administrator. The
following information shall be provided in the inspection report:
1. Site map with specific BMP identified by number or letter
2. Owner name
3. Inspector's name, date
4. Weather at time of inspection
5. Most recent rain event date and approx. amount
6. BMP type
7. Specific information about the condition of the BMP (for example: vegetation sparse, spillway
blocked, cattails growing in wetland). Note signs of vandalism, repair needs, racked concrete,
seepage, ponded water, dead vegetation, algal growth, and/or debris n BMP.
8. Note any repairs needed or made
9. Provide at least 2 pictures of the structure, showing the inflow area and the outflow area.
It shall be the developer's/owner's responsibility to secure the services of a Professional Engineer
for BMP design and installation certification and annual inspection.
40 Draft Revised 1/07

-, ,'
Figure 5.4.01 Infiltration Basin
(Draft 2000 Maryland Stormwater Design Manual. Vol. 1)
~rr _ Concrete
'O
~
~. lav~+l
spreadar
~
Grass lined -~ Emergency
.~ channel r.~r+Fi
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41 Draft Revised 1/07

Figure 5.4.021nfilfrafion Trench
(Draft 2000 Maryland Stormwater Design Manual. Vol.l)
tar ~t
air,
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5.4.01 Infiltration BMPs
Infiltration BMPs reintroduce surface water to groundwater. They capture and temporarily
store the WQY. In limited situations, they can be designed to attenuate the 1-yr. storm event.
However, in most cases, a separate BMP is needed for storm attenuation. Design for
infiltration-type BMPs is basically the same, though two schematics have been provided (for
infiltration trenches and infiltration basins).
Feasibility
To be suitable for infiltration, underlying soils shall have an infiltration rate of 0.52 in/hr or
greater, as initially determined by the NRCS soil texture classification and subsequently
confirmed by field geotechnical tests. The minimum geotechnical testing is one test hole per
42 Draft Revised 1/07

5000 ftz, with a minimum of two borings per facility (taken within the proposed limits of the
facility). Soils should have a clay content of less than 20% and asilt/clay content of les than
40%.
Infiltration cannot be located on slopes greater than 15% or within fill slopes. The bottom of
the infiltration facility shall be separated by at least four feet vertically from the seasonally
high water table, as documented by onsite testing.
Infiltration facilities should be located at least 100 feet horizontally from any water supply well
and should be placed at least 25 feet down gradient from structures.
The maximum contributing drainage area to an individual infiltration practice should be less
than 5 acres.
Conveyance Criteria
A conveyance system shall be included in the design of all infiltration practices in order to
ensure that excess flow is discharges non-erosively.
All infiltration facilities shall be designed to fully de-water the entire WQ~ within 48 hours after
the storm event. If the runoff is delivered by a storm drainpipe or along the main conveyance
system, the infiltration practice should be designed as an off-line practice. Adequate
stormwater outfalls shall be provided for the overflow associated with the ten-year design
storm (non-erosive velocities on the down-slope).
Pretreatment
A minimum of 25% of the WQ~ must be pretreated prior to entry into an infiltration facility. If
the underlying soils have an infiltration rate greater than 2.0 inches per hour, 50% of the WQ~
shall be pretreated prior to entering the infiltration facility. Sedimentation basins, stilling
basins or sumps can be used as pretreatment measures. Exit velocities from pretreatment
shall be non-erosive during the two-year design storm.
To protect the long-term integrity of the infiltration facility, redundant clog-prevention
measures shall be installed and maintained. The following techniques, at least three per
infiltration trench and at least two per basin, shall be installed in every infiltration facility:
• Grass channel
• Grass filter strip
• Bottom sand layer
• Upper sand layer (6" minimum) with filter fabric at the sand/gravel interface
• Use of washed bank run gravel as aggregate
The sides of the infiltration trench shall be lined with an acceptable filter fabric that prevents
soil piping but has greater permeability than the parent soil.
Design for Treatment
Infiltration practices shall be designed to capture the entire WQv, less the pretreatment
volume, through the floor of each practice. Infiltration practices are best use din conjunction
with other BMPs for storm attenuation.
The construction sequence and specifications for each practice shall be clearly outlined.
A dense vegetative cover shall be established on the contributing pervious drainage areas
before diverting the runoff into the infiltration practice. The pretreatment practice cannot be
43
Draft Revised 1/07

used as a sedimentation practice during construction. In addition, the erosion and sediment
control plan must clearly indicate how sediment will be prevented from entering the infiltration
site.
Maintenance
An observation well shall be installed in every infiltration trench, consisting of an anchored
six-inch diameter perforated PVC pipe with lockable cap. Dewatering methods (such as an
underdrain pipe for draw down) should be built into the design in case of failure.
Direct and permanent access shall be maintained to the structure for maintenance and repair.
5.4.02 Filtration BMPs
Practices that capture and temporarily store the WQv then pass it through a filter bed of sand,
organic matter, soil or other media are filtration BMPs. Sand filters and bioretention structures
are examples of filtration BMPs. These BMPs must be combined with other BMPs to meet
storm attenuation requirements.
Figure 5.4.03 Underground Sand Filter
(Draft 2000 Maryland Stormwater Design Manual. Vol. 1)
.. IrtS,FS; CAAtFS • RY6SAWUM ~ 1L•ttft'
n~ i
~~~ ~~, wcK«xc-,
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44 Draft Revised 1/07

Figure 5.4.04 Surface Sand Filter
(Draft 2000 Maryland Stormwater Design Manual, Vol. 1)
UNriF:RrNta~i~ Ex7t.l:C~ f1(1~v
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1',455 11;:t1lV CFC#~'•$Stt}iE F'11.Tt~t ~1"
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~:iT~7[lP+.,+. EY~•t tk"iTtitkk S5'\,Tt M ~~jj~~+yy~~++ x
- a.:5; ~,. ..31 ~•
~ _ __
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ld"[7F~"i R`,t4ti!i~ ~ ` ° , _~~ (~i'Pi'Tt4~
"i'7l'iTiGRBIY 5ANi7 ~.~r, ,w.,-',,. .~: AL.1.$#!31%'~.~PABd'#TTEFM
F
~ :t
f1^»- i7 :: : X
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. Mt . ~~ , iGi w K ~ L?Fi~IIRA[k S'YSZEL1S
45 Draft Revised 1/07

Figure 5.4.05 Bioretention or Rain Garden
(Draft 2000 Maryland Stormwater Design Manual Vol.l)
FA!a~fSt6T71*~~TF1-av
i

GuR! 5'f0Y2 j
1~ 1
'I ~ ~ T T
~

•a~ .Yr+rrYrti!!~•ai !! !t!{-rY.ck•sr-laY~
rt Yt~-YV {MltkY il4+liM ! tY-4tt•Mt•AS •Y•i•{'
Y •si1F!•!!l Fk ii iYti k! •Mb 1l M'Pl Mia{tr la•
CA.456 iH.t'FA
•ii!!lMi~Yklk tlFiY• •iY YlYYi YYi lM Y-•Y{• ~{~
~r'i M!ltlrfir!-lii1 F! t!!!r•flYi3t41lY M!•
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flit! - +*'

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i tJRAMI
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Feasibility
Most Stormwater filters require 2-6 feet of head. However, the perimeter sand filter can be
designed to function with as little as one foot of head.
The maximum contributing drainage area to an individual filtering system is less than 10
acres. Sand and organic filters should be applied to land uses with a high percentage of
impervious surfaces. Drainage areas with imperviousness less than 75% discharging to a
filtering practice shall require full sedimentation pretreatment techniques.
Conveyance Criteria
If runoff is delivered by a storm drainpipe or is along the main conveyance system, the
46 Draft Revised 1/07

filtering practice shall be designed off-line. The outlet of the filtering practice shall be
designed to prevent erosion during the 10-year storm.
Only the WQv shall be directed into the filtering practice. A flow splitter shall be installed to
direct the overflow to a BMP designed to attenuate the 1-year storm.
Stormwater filters shall be equipped with a 4-6 inch perforated underdrain in a gravel layer. A
permeable filter fabric shall be placed between the gravel layer and the filter media.
Pretreatment
Dry or wet pretreatment equivalent to at least 25% of the computed WQY shall be provided
prior to filter media. The typical practice is a sedimentation basin with a length to width ratio
of 2:1. The required minimum surface area for pretreatment can be found using the Camp-
Hazen equation:
As=QoxE9
W
where:
A5 =sedimentation basin surface area (ft2)
Q° =discharge rate from basin = WQ~i24 hr
W =particle settling velocity (ft/sec)
For I < 75%, use 0.0004 ft/sec (particle size=20 microns)
For I > 75%, use 0.0033 ft/sec (particle size=40 microns)
(I =percent imperviousness)
E9 =sediment trapping efficiency constant; for a sediment trapping
efficiency (E) of 90%, E9 = 2.30
The equation reduces to:
ASr= (0.066) (WQ~) ftzfor I [ 75%
ASP = (0.0081) (WQ~) ftz for 1375%
Where:
ASr=sedimentation basin surface area full
ASP =sedimentation basin surface area partial
Adequate pretreatment for bioretention systems is provided when all of the following are
provided: (a) 20' grass filter strip below a level spreader or optional sand filter layer, (b) gravel
diaphragm and (c) mulch layer.
Before runoff can be diverted into the filtering practice, all contributing drainage areas shall
be provided with a dense vegetative ground cover.
Design for Treatment
The entire treatment system (including pretreatment) shall temporarily hold at least 75% of
the WQ~ prior to filtration.
The filter bed typically has a filter bed depth of 18". Filtering practices cannot provide storm
attenuation and must be combined with other practices.
47
Draft Revised 1/07

The filter area for sand filters shall be sized based upon the principles of Darcy's Law. A
coefficient of permeability (k) shall be used as follows:
Sand: 3.5 ft/day (City of Austin 1988)
Peat: 2.0 ft/day (Galli 1990)
Leaf compost: 8.7 ftiday (Claytor and Schueler, 1996)
Bioretention soil: 0.5 ft/day (Claytor and Shueler, 1996)
Bioretention systems shall consist of the following treatment components: (a) 2.5 to 4 foot
deep planting soil bed, (b) surface mulch layer, and (c) a 12" deep surface ponding area.
The required filter bed area (Ar) is computed using the following equation:
Ar= (WQ~) (dr) ~ [(k) (hr+ dr) (tr)]
Where:
Ar=surface area of filter bed (ftz)
WQ~ =water quality volume (fts, converted from ac-ft)
dr=filter bed depth (ft)
k =coefficient of permeability of filter media (ft/day)
hr=average height of water above filter bed (ft)
tr=design filter bed drain time (days); 1.67 days is the recommended
maximum for sand filters, 2.0 for bioretention
Specific Landscaping Criteria for Bioretention
The soil and planting characteristics are as important as the bioretention facility's location,
size and treatment volume. The planting soil must be a sandy loam, loamy sand, loam
(USDA), or a loam/sand mix (should contain a minimum 35 to 60% sand by volume). The
clay content of these soils should be less than 25% by volume. Soils should fall within the
SM, ML, SC classifications or the Unified Soil Classification System. A permeability of at
least 1.0 foot per day (0.5"ihr) is required, though a conservative 0.5 feet should be used in
the design. The soil should be free of any material over 1" in diameter. The soil should also
be free from brush and seeds from noxious plants. Soil should be placed in 12-18" lifts and
lightly compacted.
The mulch layer helps maintain soil moisture and avoids surface sealing, which reduces
permeability. It also serves as a pretreatment layer, trapping finer sediments remaining
suspended after the primary treatment. The mulch layer should be standard landscape style,
single or double shredded hardwood mulch or chips. The mulch layer should be well aged
(stockpiled or stored for at least 12 months), uniform in color and appearance, and free of
other materials, such as noxious weeds, roots, etc. Grass clippings should not be used as
mulch material.
Plant material selected should be based upon the goal to establish a terrestrial forested
community of native species. Bioretention simulates an upland-species ecosystem. The
community should be dominated by trees but include understory trees, shrubs and herbaceous
plants. Create three zones, as shown in Figure 5.4.05. The lowest zone should
be capable of adapted to fluctuating water levels. The middle zone supports plants that like
drier soils but can withstand occasional inundation. The outer zone supports plants that
prefer dry conditions. A random, natural pattern should be used when planting. Native plants
should be specified over exotics. Woody vegetation should not be planted in the vicinity of
inflow locations. Trees should be planted along the perimeter of the bioretention facility.
Stressors like sun, wind, exposure, etc. should be considered when laying out the planting
plan. Aesthetics and visual considerations should be a prime consideration. Existing and
48 Draft Revised 1/07

r
proposed utilities must be identified and considered.
The planting plan should include a construction sequence; planting schedule and installation
specifications; and maintenance requirements.
Figure 5.4.05 Biorention Planting Zones
(Draft 2000 Maryland Stormwater Design Manual, Vol. 1)
5.4.02 Ponds
*NOTE: The Dam Safety Act is administered by the DENR Land Quality Section and
requires Dam Safety review and permitting if an embankment of a pond is 15 feet or higher
from the lowest point on the downstream toe to the highest point on the embankment AND if
the pond has the capacity to store 10 ac-ft or more of water. Contact the Land Quality Section
Raleigh Regional Office for more information at 791-4200.
Stormwater ponds are practices that have a permanent pool or a combination of extended
detention or shallow wetland with a permanent pool equivalent to the entire WQ~. The term
"pocket" refers to a pond or wetland with a small contributing drainage area with little or no
base flow to sustain water levels in dry weather. Water levels instead are influenced and
maintained by a high water table.
Dry detention is considered primarily an adjunct practice that can provide volume attenuation to
help meet site attenuation requirements. The practice can be employed for meeting nitrogen
removal preceeding a grassed swale or bioretention area, or preceeding a level spreader above a
vegetated filter strip or riparian buffer. Available data indicate that it can provide only limited
nutrient removal (approximately 10%).
49
Draft Revised 1/07

Figure 5.4.06 Wet Pond
(Draft 2000 Maryland Stormwater Design Manual. Vol. n
aeaw sE:r~aa ,~
ovkrrFtaN' ~"~ - spus)K..r
~gi.'vw~.~.., ,,.,~ ~ ~ ~~,._...~,,,_ c
.n
~ ~~~~~~~, ti'"'1~
~,aE~ • ttalAd.~r s#y ..
FAD - ~ {
~ ; - ,~
~ S1
• ~; IfgS~UtMF474t fi+AF[
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tl5S17At1
~' ~~,~~
- ~ ~...-~
. ~ ,~
. ~ ~ ~ ~,~
~~~..
~4wi.L~i41EEXHL p~~
i
T__~_____~.
_._~. _,.__r ~,,~,~
,'~ ~` ~ /_
qF1. ~RSR FIFE.... ~~Ms`titt`
£6 f
«~_F
f4t'ERt
~~
50 Draft Revised 1/07

Figure 5.4.07 Extended Detention Wet Pond
(Draft 2000 Maryland Stormwater Design Manual, Vol. ~
at~r~rot Qa ~* i
r
~.~uow
sear~r~•

wi~ta~ *~ ~ ~ ' i.'`"'~" wa~R a+
v M61/GN
~'
51 Draft Revised 1/07

Figure 5.4.08 Pocket Pond
(Draft 2000 Marvland Stormwater Design Manual, Vol. ~
'~ :.. ~>
fjp1~~Y
-crmaar
i'41EMF Y'A41t..
G1k1t1..17
~~WEIt'tlUk[.
•'~ "t
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1
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~' ~ tt
~:
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------ -.,~... ~

.,_-.,..........-.,..,.--- ~
b~ ~t3t5JCT',k~[.
4
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way e+:
~ . ,... .
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,
f'

Np.L1Ef Tilit[ fO~iKr ft ~; °-' T

Feasibility
Stormwater ponds shall have a minimum contributing drainage area of ten acres or more,
unless groundwater is confirmed as the primary water source (e.g., pocket pond).
Stormwater ponds cannot be located in jurisdictional wetlands or on streams unless the
appropriate permits have been obtained prior to Stormwater Management Plan approval.
The design and construction of stormwater ponds shall be per the requirements and
specification of the Dam Safety Law of 1967and as required by the DENR Land Quality
Section Dam Safety Program. Embankments that are exempt from the Dam Safety Law shall
conform to the following:
a) design for a stable outfall using the 10-yr design storm (or the 2-yr storm if the pond
is an offline structure providing WQ~only);
b) provide the riser/barrel structure with anti-flotation, anti-vortex, and trash rack
devices;
c) one foot of freeboard shall be provided above the design high water for the 10-yr
storm;
d) woody vegetation is prohibited on the embankment.
52 Draft Revised 1/07

Conveyance Criteria
The outfall channel immediately below the pond shall be modified to prevent erosion during the
10-yr storm and shall conform to natural conditions in the shortest distance possible.
Pretreatment
Each pond shall have a sediment forebay upstream from the structure. The forebay shall be
separated from the pond and shall be sized to contain 0.1 inches times the impervious acreage of
the contributing drainage. The forebay storage volume counts towards the total WQ~
requirement. Exit velocities from the forebay shall be non-erosive.
Direct maintenance and access to the forebay shall be provided.
The bottom of the forebay can be hardened to aid in sediment removal. A fixed vertical
sediment depth marker should be installed in the forebay to measure sediment deposition
over time.
Design for Treatment
Ponds shall be designed to capture and treat the computed WQv through any combination of
.permanent pool, extended detention or wetland. Water Quality storage can be provided in
multiple cells, enhancing performance by providing longer flow paths, high surface area to
volume ratios, and redundant treatment methods. If an extended detention pond is used,
storage for the WQv shall be computed and routed separately. If a wet pond is used for nitrogen
removal the required WQv should be provided within the permanent pool volume.
The Water Quality Volume (WQv) is utilized when designing several BMPs available for nutrient
reduction. The WQv is the storage needed to capture and treat the runoff from 90% of the
average rainfall, or the "first flush". In numerical terms, it is equivalent to an inch of rainfall
multiplied by the volumetric runoff coefficient, Rv, and site area. The specific rainfall depth is 1.0
inch.
The following equations are used to determine the required WQv in acre-feet of storage:
WQv = (1.0) (R v) (A)
12
where: WQv =water quality volume (ac-ft)
Rv = 0.05 + 0.009(1) where I is the percent impervious cover
(e.g. I = 52 for 52% impervious coverage in the drainage area)
A =area in acres
Flowpaths from inflow points to the outlet structure shall be maximized. Flowpaths of 1.5
length to 1 width and irregular shapes are recommended.
The perimeter of all deep permanent pool areas (4 feet or greater in depth) shall be
surrounded by 2 benches with a combined minimum width of 15':
a) a safety bench extending outward from the normal water edge to the toe of the pond side
slope, with a minimum length of 6 feet and a maximum slope on the bench of 6%
b) an aquatic bench extending inward from the normal shoreline to 12" deep, with a minimum
length of 6 feet and a maximum depth of 18" below the pool water surface elevation.
53
Draft Revised 1/07

A landscape plan shall for the stormwater pond and its buffer shall be prepared to indicate
how aquatic and terrestrial areas will be vegetatively stabilized and established. Wetland
plants should be encouraged in and around the pond, when possible. The best elevations for
establishing wetland plants are within 6 inches of the normal pool elevation.
A 20-foot buffer and setback shall be provided around the pond, extending outward from the high
(100-year) water surface elevation in the pond. No existing or proposed structures are permitted
within the 20-foot buffer, It is recommended that woody vegetation be proposed around ponds to
discourage geese and provide a natural buffer.
Maintenance
Maintenance responsibility for a stormwater pond shall be vested with a responsible party by
means of the legally binding Maintenance and Inspection Agreement (Appendix C) that is
executed as a condition of plan approval.
The principle spillway shall be equipped with a trash rack that provides access for
maintenance. Sediment removal shall occur when the sediment forebay is 50% full.
An access easement shall be provided from a public right of way to the pond. The access
easement shall be at least 12' wide; have a maximum slope of 15%; and be stabilized. A
drainage easement shall be provided around the pond extending outward 20 feet from the high
water surface elevation in the facility.
The low flow orifice shall have a minimum diameter of 3 inches and shall be protected from
clogging by an external trash rack. The orifice diameter can be reduced to 0.5 inch if an
internal orifice is used (e.g. an over-perforated vertical standpipe protected by hardware cloth
and washed stone filter). However, the preferred method is a submerged reverse-slope pipe
that extends downward from the riser to an inflow point 1 foot below the normal pool
elevation. Broad crested rectangular, V-notch weirs or similar devices extending at least 12
inches below the normal pool are also acceptable low flow orifices. The use of horizontal
perforated pipe protected by geotextile fabric and gravel is not recommended.
All outlet pipes from detention facilities must have a minimum pipe size of 15" and anti-seep
collars provided.
Each pond shall have a pond drain that can be easily accessed to completely or partially
drain the pond within 24 hours. The drain shall be one size larger than the calculated size.
The stormwater Administrator shall be notified prior to draining a pond.
5.4.03 stormwater Wetlands
stormwater wetlands are practices that create shallow wetland areas to treat urban runoff
and often incorporate permanent pools and/or extended detention storage to achieve full
WQ~. stormwater wetlands can also be designed to attenuate the 1-yr storm.
54 Draft Revised 1/07

Figure 5.4.09 Shallow Wetland
(Draft 2000 Marvland Stormwater Design Manual, Vol. n

~ ~
~SQ~ ~^~ / ;Pi.kYplY
~ .
' ~
'
/
*~
fir ° _ 'li

(X' i7G1fFAL4

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t ~~y,
'QiR`
/1
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~ "`..
~
n~
iKiF~7RE~tISMlK16CFORiW1MTkT ~ l.01M1fi~pRF4
~xo~m+swr.~a,rF
PLAN V1P

w~~ '-.,
-aa,r~cukv~.
f ~a xu Kcv~ _
air
- Cy,axv~nn~.
El
-
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a
Y~tEa1`•1 M
tt
.
REI1t
° StABL~
~
PCSIIFMY _. ,,

_
9U'EF~
fi~YlOli Y~HL?: ` WJFYa Oi~Mi- ` _.
tAW ltEYES~EPwF -
-llfTS:OF=fiaJu.ARar
iR.TEii p1APHF
~~
55
Draft Revised 1/07

Figure 5.4:10 Pond/Wetland System
(Draft 2000 Marvland Stonnwater Design Manual, Vol. 1)
wa-r n~xa

~ FOWl7 ~ E]5 FEET kty ~'"~ 1 ~
~ ~^~...
.L _ _
+naF nt+

x~aru s~+~€r`+~t.wrs
PC.AN YI~V41
TIE1F (TAPHR+~:kR!
56 Draft Revised 1/07

Figure 5.4.11 Extended Detention Shallow Wetland
(Draft 2000 Maryland Stormwater Design Manual, Vol. 1)
PLAN VIENN
57
Draft Revised 1/07
_.~.,.~__ v..,,.ww
oaa~~s+~r ra ~

Figure 5.4.12 Pocket Wetland
(Draft 2000 Maryland Stormwater Design Manual, Vol. 1)
Feasibility
A water balance must be performed to demonstrate that a stormwater wetland can withstand
a 30-day drought at summer evaporation rates without completely drawing down. Stormwater
wetlands cannot be located within jurisdictional wetlands or on streams unless the appropriate
permits have been obtained prior to Stormwater Management Plan approval.
Conveyance Criteria
Flowpaths from inflow points to the outlet structure shall be maximized. Flowpaths of 1.5
(length) to 1 (width) and irregular shapes are recommended. These paths may be achieved
by constructing internal berms (e.g., high marsh wedges or rock filter cells).
58 Draft Revised 1/07

Pretreatment
Sedimentation regulation is critical to sustaining stormwater wetlands. A sedimentation
forebay shall be located at the inlet and a micropool shall be located at the outlet of the
wetland. Forebays are designed in the same manner as ponds and should encompass
approximately 10% of the total surface area of the wetland. A micropool is a 3 to 6 foot deep
pool used to protect the low flow pipe from clogging and prevent sediment resuspension.
Design for Treatment
The basic design of the spillway systems of a wetland is the same as a wet pond (refer to
section 5.4.03).
The surface area of the entire stormwater wetland shall be at least 1 % of the total drainage
area to the facility (1.5% for the shallow wetland design). At least 25% of the total WQ~ shall
be in deeper zones with a minimum depth of 4 feet (the forebay and micropool may meet this
criteria). A minimum of 35% of the total surface area shall have a depth of 6 inches or less
and at least 65% of the total surface area shall be shallower than 18 inches.
If extended detention is utilized in a stormwater wetland, the extended detention volume shall
not compromise more than 50% of the total wetland design, and the maximum water surface
elevation shall not extend more than 3 feet above normal pool. Storm attenuation storage
can be provided above the maximum WQ~ elevation within the wetland.
To promote greater nitrogen removal, rock beds maybe used as a medium for the growth of
the wetland plants. The rock should be 1 to 3 inches in diameter and placed up to the normal
pool elevation. Rock beds should also be open to flow-through from either direction.
Landscaping Criteria
A landscaping plan shall be provided that indicates the methods used to establish and
maintain wetland coverage. Minimum elements of the plan include: delineation of
pondscaping zones, selection or corresponding plant species, planting configuration, and
sequence for preparing wetland bed (including soil amendments, if needed). Structures such
as facines, coconut rolls, or straw bales can be used to create shallow marsh cells in highenergy
areas of stormwater wetlands.
A wetland buffer should extend 25 feet outward from the maximum water surface elevation
with an additional 15-foot setback to structures.
The most common and reliable technique for establishing an emergent wetland community in
a stormwater wetland is to transplant nursery stock obtained from local aquatic plant
nurseries. The transplanting window extends from late March to early June. Planting after this
date is not recommended, as plants need a full growing season to build root reserves
for over-wintering. The optimal depth requirements for several common species of emergent
wetland plants are often 6 inches of water or less.
5 to 7 species of emergent wetland species should be used to provide for diversity. A listing
of native wetland species can be found in Table 5.4. At least three aggressive species plants
(such as bulrush, pickerelweed, or arrow arum) should be included in the landscape plan.
The wetland area should be sub-divided into separate planting zones of approximately the
same constant depth. Approximately half of the wetland surface should be planted. 1 plant
species should be planted within each flagged planting zone, based on their approximate
depth requirements. Plants should be installed in clumps with individual plants located as
average of 18 inches in center within each clump. Individual plants should be spaced 12
59
Draft Revised 1/07

inches to 24 inches on center.
A wet hydroseed mixture should be used to establish permanent vegetative cover in the
buffer outside of the permanent pool.
Where the wetland area is excavated to sub-soils, the wetland floor should have 3 to 6 inches
of topsoil added.
Wetland zones should be staked after construction and water levels verified to confirm the
original planting zones. It may be necessary to modify the pondscaping plan to reflect the
actual water levels and appropriate wetland plants.
Maintenance
If a minimum coverage of 50% is not achieved in the planted wetland zones after the second
growing season, a reinforcement planting will be required.
60 Draft Revised 1/07

Table 5.4 Herbaceous Wetlands Plants for Planting in the Normal Pool in NC
Scientific Name Inundation Found in
Johnston
Wildlife Value
Notes
Common Name Tolerance Count
Peltandra virginica Up to 1 ft. Y High, berries eaten Full sun to partial
Arrow arum b wood ducks shade
Saggitaria latifolia Up to 1 ft. Y Moderate; tubers and Aggressive colonizer
Arrowhead/Duck seeds eaten by
otato ducks
Andropogon virginicus Up to 3 in. Y High; song birds and Tolerant of
Broomsedge browsers; winter food fluctuating water
and cover levels and partial
shade
Andropogon Up to 1 ft. Requires full sun
glomeratus
Bush beard rass
Typha spp. Up to 1 ft. Y Low except for cover Aggressive; may
Cattail eliminate other
species; volunteer;
high pollutant
treatment
Ceratophyllum Yes Low food value; good Free floating SAV;
demersum habitat and shelter shade tolerant; rapid
Coontain for fish and growth
invertebrates
Scirpus pungens Up to 6 in. Y High; seeds, cover; Fast colonizer; can
Common three-square waterfowl, songbirds tolerate periods of
dryness; full sun;
hi h metal removal
Lemna spp. Yes Y High; food for High metal removal
Duckweed waterfowl and fish
Saururus cernuus Up to 1 ft. Y Low, except for wood Rapid growth; shade
Lizard's tail ducks tolerant
Hibiscus moscheutos Up to 3 in. Y Low; nectar Full sun; can tolerate
Marsh hibiscus eriodic d Hess
Pontederia cordata Up to 1 ft. Y Moderate; ducks, Full sun to partial
Pickerelweed nectar for butterflies shade
Potamogeton Yes Extremely high; Removes heavy
pectinatus waterfowl, marsh and metals
Pond Weed shorebirds
Carex Spp. Up to 3 in. Y High; waterfowl and Many wetland and
Sedges songbirds several upland
s ecies
Scirpus validus Up to 1 ft. Y Moderate; good Full sun; aggressive
soft-stem bulrush cover and food weedy aliens such
as P. erfoliatum
Polygonum spp. Up to 1 ft. Y High; waterfowl, Fast colonizer,
Smartweed songbirds; seeds and tolerant of fluctuating
cover water levels
Juncus effesus Up to 1 ft. Y Moderate Tolerates wet and
Soft rush d conditions
Nuphar luteum Up to 3 ft. Moderate for food Fast colonizer;
Spatterdock but high for cover tolerate of fluctuating
water levels
61 Draft Revised 1/07

The unnamed tributary flows into Swift Creek. This development comprises less than
1 % of the total drainage area to Swift Creek. Because of its relative size, it is believed
that this development will have an insignificant impact on the natural channel of and
drainage structures in or over Swift Creek downstream from this site.
Provide a copy of the USGS Quadrangle Map with the following identified:
1. Development boundaries
2. Downstream drainage structures
3. Drainage areas for the development
4. Drainage areas for downstream drainage structures
Provide a copy of the USDA-NRCS Soil Survey Map with the following identified:
1. Development boundaries
59 Draft Revised 1/07

Sample Stormwater Statement 8
Subdivision Name: Happy Trails
Location: Cornwallis Road, 3400 ft south of it's intersection with SR 1010
Located outside of the Environmentally Sensitive Overlay District
and inside the Municipal Transition District
Developer: Johnston Co. Developers
204 SOUth 2nd St.
Smithfield, NC 27577
Consultant: Jane Smith
204 SOUth 2nd St.
Smithfield, NC 27577
Phone: [phone], Fax [phone]
General Information
The total acreage within this subdivision is 40.2 acres. The County will provide water
and sewer. Approximately 70% of this tract was planted in Bermuda hay, with the other
30%in mixed pines and hardwood trees. Riparian buffers will be maintained along
approximately 1800' of Boys Branch and 2 tributaries within the tract, with approximately
6 acres of wetlands and riparian buffer. Roadways will cross the tributaries. All
necessary permits will be obtained from DWQ and the Corps of Engineers. This site is
not located within the Environmentally Sensitive Overlay District.
Impervious Area
Approximately 2600 ft of new roadway will be created, with a ROW width of 50' and a
pavement width of 20'. Curb and gutter will be installed to direct runoff from the
roadways to drainage easements between lots. Sidewalks will be installed. Lot sizes
will average 12,000 ftz. The average house footprint will be 1600 ftz and will have double
garages. 100 lots are proposed for this subdivision. The expected imperviousness is:
Lots: 2300 ftZ house and 2 car garage
1400 ftZ driveway, walkways, sidewalks, patios, etc.
3700 ftZ per lot
100 lots x 3700 ftz per lot = 370,000 ftz imperviousness in lots
Roadway: 2600 ft x 21 ft = 54,600 ftz
Total imperviousness = 370,000 ftz + 54,600 ftz = 424,600 ftz or 9.75 ac total impervious
area by design (24% imperviousness)
Allowable imperviousness = 40.2 ac x 15% = 6.03 ac
Maximum imperviousness = 40.2 ac x 40% = 16.08 ac.
The designed impervious area will exceed the allowable but will not exceed the
60
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e
maximum.
The following requirements must be met:
1. Detention (since the impervious area exceeds 15%)
2. Downstream analysis (mandatory)
3. Nitrogen reduction (mandatory)
4. Land Dedication (since the impervious area exceeds 15%)
Peak runoff reduction
3 pocket ponds will be installed to attenuate the peak runoff from this subdivision.
Full details and design will be provided in the final stormwater management plan.
Level spreaders will be installed at the outlets of the pocket ponds prior to
discharging into the riparian buffers.
Downstream analysis
This tract drains into Boys Branch, which has a drainage area at the downstream
point of approximately 2 square miles. 2 tributaries to Boys Branch divide the
subdivision. As noted on the accompanying USGS quad map, drainage from this
site into the southwestern tributary will make up 40% of the total drainage area.
Drainage from this site to the northwestern tributary makes up 70% of the total
drainage area. Each tributary will be evaluated for the stormwater Management
Plan for erosion, capacity and flooding. Since the drainage area from the subdivision
to Boys Branch will be less than 10% of the total drainage area and will have an
insignificant impact on the branch.
Nitrogen reduction esfimafe
(2) (3) (4)
(~) Area TN Export Coeff. TN export from
Type of Land Cover Acres (Ibs/ac/yr) use (Ibs/yr)
Permanently protected 6 0.6 3.6
undisturbed open space
(forest, un-mown meadow,
etc.
Permanently protected 24.4 1.2 29.28
managed open space
(lawn, grass, landscaping,
etc.
Impervious surfaces 9.8 21.2 207.76
(roads, parking lots,
driveways, roofs, paved or
graveled storage areas,
raveled roads, etc.
TOTAL 40.2 - 240.64
Total NExport - 240.64 Ibs/yr 4 40.2 ac = 6.0 Ibs/ac/yr
Requirements: Install BMPs or pay offset payment to reduce to 3.6 Ibs/ac/yr
61 Draft Revised 1/07

• Install pocket ponds designed for storm attenuation and WQY
0 6 Ibs/ac/yr - 25% N-reduction = 4.5
• Pay offset fee
o (4.5 Ibs/ac/yr - 3.6 Ibs/ac/yr) x $330 x 40.2 ac = $11,939.40
Land Dedication
Impervious area with no BMPs = 6.0 ac (15%)
Imperviousness with land dedication = 9.75 ac (24%)
Land dedication NOT MEETING WQPC = (9.75 ac - 6.0 ac) x 2.5 = 9.4 ac
Land dedication MEETING WQPC = (9.75 ac - 6.0 ac) x 1.5 = 5.63 ac
Land has not been identified at this time for the land dedication program. The
developers will work with the County to either identify the land or will pay the land
dedication program per acre payment of $10,000.00.
Provide a copy of the USGS Quadrangle Map with the following identified:
1. Development boundaries
2. Downstream drainage structures
3. Drainage areas for the development
4. Drainage areas for downstream drainage structures
Provide a copy of the USDA-NRCS Soil Survey Map with the following identified:
1. Development boundaries
(,2 Draft Revised 1/07

Appendix B
Stormwater Management Plan/Statement Checklists and Application Form
66 Draft Revised 1/07

Stormwater Management Statement Checklist
Every major subdivision, as defined in the Johnston County Development Ordinance,
disturbing more than one acre or with new or modified existing stormwater appurtenances,
approved after June 1998, shall submit a Stormwater Management Statement. The
Statement shall be a narrative with supporting documentation addressing and including
the following information:
^ Development name and location
^ Developer/Owner and Consultant contact information
^ Site description
^ Description of proposed development
^ A statement noting whether the site is located within the Environmentally Sensitive
Overlay District.
^ Impervious Area Calculation
0 Total Nitrogen Calculation and nitrogen reduction method, as appropriate.
^ A description of stormwater impacts the development will have on surrounding up
and downstream properties
^ A description of the proposed stormwater management system(s) and how the
facilities will comply with the Stormwater Management Ordinance
^ A vicinity map, USGS topographic map and Johnston County Soil Survey with the
development area indicated thereon
Preliminary subdivision plats that do not include a Stormwater Management Statement will
not be considered complete and may not be included on the Planning Board agenda.
67 Draft Revised 1/07

r ~
Stormwater Management Plan Review Checklist
^ Completed and signed Stormwater Permit application form and review fee
^ Development location, PIN and vicinity map
^ An overall map of the development
^ Impervious area calculations
^ Total Nitrogen Export Calculations
^ One complete set of development plans
^ One Stormwater Management Plan
^ Drainage area maps for all stormwater facilities
^ Offsite Stormwater Impact Analyses
^ Design calculations for stormwater facilities, including BMP's
^ Written Inspection and Maintenance agreements for all stormwater BMP's, to be
recorded after construction
^ A copy of the approval letter from NC DENR-Land Quality
^ Stormwater Management Statement, where required
Johnston County Department of Utilities
PO Box 2263
Smithfield, NC 27577
[phone]
68 Draft Revised 1/07

Stormwater Permit Application
Development/Site Name:
Owner/Developer Name:
Address:
Phone:
Fax:
Contact Person:
No. of acres in development:
Type of Fee Fee
Develo ment
Residential 0-10 acres* $500.00 10+ acres* $500.00 + $30.00 per acre*
sin le famil
Ot
he 0-5 acres* $500.00 5+ acres* $500.00 + $75.00 per acre*
o
Dev
ment
*The review fee is based on total site acreage, rounded up to the nearest acre. All checks
should be made payable to Johnston County.
I hereby certify that all information contained within this Stormwater Management
application is accurate and complete to the best of my knowledge and conforms to
Johnston County's Stormwater Management Ordinance and stormwater design criteria.
Johnston County has the right to inspect all stormwater facilities on this tract of land.
Typed or Printed Name
Signature of Owner/Developer
Date
I assume responsibility for inspections, installation, maintenance and operation of all
stormwater facilities and Best Management Practices in accordance with the Stormwater
Management Plan enclosed and the Inspection and Maintenance Agreement, as
applicable.
Typed or Printed Name
Acting as an agent for:
Signature
Date
*Note: Responsibility for the continued operation and maintenance of the proposed
stormwater management facilities can be transferred from the developer to an individual
landowner or Home Owner's Association. A copy of a recorded document, indicating who
will be responsible for maintenance of all stormwater management facilities, must be
provided as a condition of the approved Stormwater Management Plan.
Send the completed application package to:
Stormwater Administrator, Johnston County Department of Utilities
P.O. Box 2263
Smithfield, North Carolina 27577
If you have any questions or need more information, contact the Stormwater Administrator
at 209-8333.
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Appendix C
Sample Maintenance and Inspection Agreement
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ti
Inspection and Maintenance Agreement
Pond name or BMP Type:
Owner:
Location:
Monthly Inspection
G A. Remove debris from primary and emergency spillways
G B.-Check groundcover for signs of erosion or failure
1. On embankment
2. In pond
c C. Check draw down pipe for blockage
G D. Inspect the embankment, primary spillway and underdrain for signs of seepage or
erosion
1. If seepage is found, note approximate flow rate, color of discharge, location
of slumps, wetness on slope, etc.
2. Draw a map of the structure noting any erosion, wetness, slumps, etc.
3-Month Inspection:
G A. Remove debris
G B. Check pipes for undercutting. Replace riprap and repair broken pipes.
G C. Reseed grass swales, the channel between the forebay and permanent pool, and the
pond embankment.
6-Month Inspection:
c A. Remove accumulated sediment from the permanent pool, the forebay and the outlet of
the pond.
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12-Month Inspection:
G A. Submit report to the Environmental Protection Administrator with the following
information:
• Site map with specific BMP identified by number or letter
• Owner name
• Inspector name, Professional Engineer registration seal, signature and date
• Weather at time of inspection
• Most recent rain event date and approximate amount
• BMP type
• Specific information about the condition of the BMP (for example: vegetation sparse,
spillway blocked, cattails growing in wetland). Note signs of vandalism, repair needs,
cracked concrete, seepage, ponded water, dead vegetation, algal growth, and/or
debris in BMP.
• Note any repairs needed or made
• Provide at least two (2) pictures of the structure, showing the inflow area and the
• outflow area
General maintenance
G Mow the side slopes for the embankment and the ponded area (not including the shelf)
according to the growing season of the grass
G Keep cattails from clogging the pond and spillways.
Special Maintenance Requirements:
G List
I, ,hereby acknowledge that I am the financially responsible party
for maintenance and inspection of this detention pond. I will perform the maintenance as outlined
above, to comply with the Johnston County Stormwater Management Ordinance and the
Stormwater Management permit received for this project. Furthermore, I attest that this Inspection
and Maintenance Agreement is on file with the Register of Deeds.
Signature
Date
I, , a Notary Public for the state of
County of , do hereby certify that personally
appeared before me this day of , 20 ,and acknowledge the due
execution of the foregoing instrument. Witness my hand and official seal,
My commission expires
Seal
73 Draft Revised 1/07

APPENDIX D
JOHN5TON COUNTY STORMWATER
MANAGEMENT ORDINANCE
LAND DEVELOPMENT CODE
73 Draft Revised 1/07

(2) New development shall not include mining, agricultural or forestry activities.
Riparian buffer means an area of trees, shrubs, or other forest vegetation, that is adjacent
to surface waters. For purposes of this article, surface water shall be present if the feature
is approximately shown on either the most recent version of the county soil survey report
prepared by the NRCS or the most recent version of the 1:24,000 scale (7.5 min.)
quadrangle topographic maps prepared by the United States Geological Survey. Riparian
buffers adjacent to surface waters that do not appear on either of the maps shall not be
subject to this article, except as noted in section 14-103.
Stormwater means flow resulting from and occurring after any form of precipitation.
Stormwater administrator means the person designated by the county manager to have
authority to review and approve Stormwater permits and Stormwater management plans.
The Stormwater administrator shall also be responsible for inspecting development and
enforcing the provisions of this article.
Stormwater conveyance system or structure means any feature, natural or manmade, that
collects and transports Stormwater, including, but not limited to, roadways with collection
systems, catch basins, manmade and natural channels, streams, pipes and culverts, and
any other structure or system designed to transport runoff.
Stormwater design manual means the manual of design, performance, and review criteria
adopted by the board of commissioners for the administration of the sormwater program.
Vegetative buffer means an area that has a dense ground cover of herbaceous or woody
species, which provides for diffusion and infiltration of runoff and filtering of pollutants.
Vested rights for Stormwater shall be based upon the following criteria:
(1) Having an outstanding valid building permit in compliance with G.S. 153A-344.1
or G.S. 160A-385.1; or
(2) Having an approved site specific or phased development plan in compliance with
G.S. 153A-344.1 or G.S. 160A-385.1.
Projects that require a state permit, such as landfills, NPDES wastewater discharges, land
application or residuals and road construction activities, shall be considered to have
vested rights if a state permit was issued prior to the effective date of the adoption of the
revised sormwater ordinance from which this article is derived.
Water dependent structures means those structures that require the access or proximity
to, or sitting within surface waters to fulfill its basic purpose, such as boat ramps,
boathouses, docks, and bulkheads. Ancillary facilities such as restaurants, outlets for boat
supplies, parking lots, and commercial boat storage areas are not considered water-
dependent structures.
(Ord. of 7-10-2000, § 9.3; Ord. of 1-2-2001, § 9.3)
Cross references: Definitions generally, § 1-2.
Sec. 14-385. Interpretation.
In interpreting and applying this article, the requirements are intended to be minimum
requirements, which are imposed and are to be conformed to, and are in addition to, and
not in lieu of, all other legal requirements. This section shall not be deemed to interfere
with or annul or otherwise affect in any manner whatsoever any ordinance, rule,
regulation, permit, or easement, covenant, or other agreement between parties; provided,
however, where this article imposes greater restrictions and controls with respect to
76 Draft Revised 1/07

stormwater management, the provisions of this article shall prevail. (Ord, of 1-2-2001, §
9.4)
Sec. 14-386. Enforcement.
(a) Violations. Whenever, by the provisions of this article, the performance of any act
is required, or the performance of any act is prohibited, or whenever any regulation or
limitation is imposed on the use of any land, or on the erection, alteration, or the use or
change of use of a structure, a failure to comply with such provisions shall constitute a
violation of this article. The owner, tenant, or occupant of any land or structure, or part
thereof, and any architect, engineer, builder, contractor, agent, or other person who
participates in, assists, directs, creates, or maintains any situation that is contrary to the
requirements of this article may be held responsible for the violation and be subject to the
penalties and remedies provided in this section. Failure to follow an approved stormwater
management plan or permit shall constitute a violation of this article and shall be subject
to the penalties and remedies provided in this article.
(b) Procedures upon discovery of violations. Procedures upon discovery of violations
of this article shall be as follows:
(1) Upon determination that any provision of this article is being violated, the
stormwater administrator shall deliver a written notice by personal service or by
registered or certified mail, return receipt requested, to the person responsible for such
violation, indicating the nature of the violation, ordering the action necessary to correct it
and outlining the timeframe for gaining compliance. Additional written notices may be
sent at the stormwater administrator's discretion.
(2) The final written notice, which may also be the initial notice, shall state the
enforcement action the county intends to take if the violation is not corrected, and shall
advise that the stormwater administrator's order may be appealed to the board of
adjustment as provided in article XI of this chapter.
In cases when delay would seriously threaten the effective enforcement of this article, or
pose a danger to the public health, safety, or general welfare, the stormwater
administrator may seek enforcement without prior written notice by invoking any of the
penalties or remedies contained in this section.
(c) Penalties and remedies. Penalties and remedies for a violation of this article shall
be as follows:
(1) Any violation of any provision of any section of this article shall constitute a
misdemeanor and shall subject the violator to a penalty of $500.00 and/or imprisonment
for not more than 30 days.
(2) Development that begins land disturbing activities prior to obtaining a stormwater
management permit shall also be subject to a one-time $1,000.00 penalty.
(3) Illegal discharge. Any designer, engineer, consultant, contractor or person that
allows, acts in concert, participates, directs or assists directly or indirectly in an illegal
discharge shalt be subject to civil penalties as follows:
a. For first time offenders if the quantity of the discharge is equal to or less than five
gallons and consists of domestic or household products, such person shall be assessed a
civil penalty not to exceed $100.00 per violation or per day for any continuing violation.
If the quantity of the discharge is greater than five gallons or contains nondomestic
substances or if the person cannot provide clear and convincing evidence of the volume
77 Draft Revised 1/07

and nature of the substance discharged, such person shall be assessed a civil penalty not
to exceed $1,000.00 per violation or per day for any continuing violation.
b. For repeat offenders, the amount of the penalty shall be double the amount assessed
for the previous penalty, not to exceed $10,000.00 per violation or per day for any
continuing violation.
c. The county manager or his designee shall take the following into consideration when
determining the civil penalty amount:
1. The degree and extent of harm to the environment, public health, and property;
2. The cost of remedying the damage;
3. The willfulness of the violation;
4. The duration of the violation;
5. The violator's prior record in complying or failing to comply with this article; and
6. The amount of money saved by the violator by noncompliance.
If the offender fails to pay the penalty within ten days of receiving final written notice of
a violation, the county in a civil action may recover the penalty. A civil penalty may not
be appealed to the board of adjustment if the offender received a final written notice of
violation and did not appeal to the board of adjustment within 30 days. Each day that any
violation continues after receipt of the final written notice of such violation shall
constitute a separate violation and a separate offense for purposes of the penalties and
remedies specified in this section. In addition to the penalties and remedies of this
section, the county may institute any appropriate action or proceedings to prevent,
restrain, correct, or abate a violation of this article.
(Ord. of 1-2-2001, § 9.13)
Sec. 14-387. Drainage system.
Stormwater shall be conveyed from a development in an adequately designed drainage
system of natural drainageways, grass swales, storm sewers, culverts, inlets, and
channels. Drainage systems shall be designed, constructed, and maintained to encourage
natural infiltration, control velocity, control flooding, and extend the time of
concentration of Stormwater runoff. The post-development runoff rate for the one-year
storm event shall be attenuated to the predevelopment runoff rate for the one-year storm.
The nitrogen loading contributed by new development shall be restricted to 3.6 pounds of
nitrogen per acre per year. Methodologies for determining nitrogen loading are outlined
in the Stormwater design manual. A developer has the option of offsetting the nitrogen
loading from a development by paying into the state wetlands restoration program.
Procedures for offset payments are outlined in the Stormwater design manual. When
using the offset payment, the total nitrogen loading from a development shall not exceed
six pounds per acre per year for residential development and ten pounds per acre per year
for nonresidential development.
(Ord. of 7-10-2000, § 9.5; Ord. of 1-2-2001, § 9.7)
Sec. 14-388. Stormwater permit.
(a) Except where provided elsewhere in this chapter, land disturbing activities shall not
commence without obtaining a Stormwater permit pursuant to the provisions of this
article and the stormwater design manual. The stormwater permit application shall be
made by, or on behalf of, the owner or developer of the site for which the permit is
78 Draft Revised 1/07

sought. The application shall be filed with the county on a form supplied by the county
and shall be accompanied with the information identified in the stormwater design
manual. A stormwater permit shall not be issued until the following conditions are met:
(1) Approval of the stormwater management plan by the stormwater administrator.
(2) Submission and approval of any required easements and impervious area
statements on a map to be recorded.
(3) Submission and approval of any required inspection and maintenance agreement
and/or escrow account or other legal instrument established to ensure long-term
maintenance of BMPs.
(4) Payment of all fees.
(b) If the development requires approval of an erosion and sediment control plan, the
stormwater permit will be conditional upon the owner receiving such erosion and
sediment control approval. The stormwater permit will be valid for one year from the
date of issuance or until significant changes in the development are made that change the
intent of the permit. The stormwater administrator shall determine significant changes. If
significant changes are made, the original stormwater permit shall not be valid, and a new
permit shall be required.
(Ord. of 7-10-2000, § 9.4; Ord. of 1-2-2001, § 9.5)
Sec. 14-389. Fees.
A list of fees associated with this article is available at the office of the clerk to the board
of commissioners in the county courthouse.
(Ord. of 1-2-2001, § 9.6)
Sec. 14-390. Stormwater management plans.
Stormwater management plans shall:
(1) Include drawings, maps, supporting calculations, specifications, and summaries as
outlined in the stormwater design manual.
(2) Demonstrate through accepted engineering practices described in the stormwater
design manual the impacts of the proposed development. Impacts of the proposed
development shall include:
a. Effects on existing upstream and/or downstream drainage systems and property;
b. Ability of the natural drainageway to handle additional stormwater runoff; and
c. Site-specific criteria supporting the analysis of any impacts noted in subsections
(2)a. and (2)b. of this section.
(3) Demonstrate through accepted engineering practices described in the stormwater
design manual that stormwater runoff is adequately conveyed through the development in
a drainage system designed to meet the criteria described in the stormwater design
manual.
(4) Demonstrate through accepted engineering practices described in the stormwater
design manual that stormwater facilities required to control the impacts of the
development are designed to meet the criteria described in the stormwater design manual.
(5) Demonstrate that the nitrogen loading from the new development does not exceed
the limits set forth in section 14-387.
(Ord. of 7-10-2000, § 9.5.1; Ord. of 1-2-2001, § 9.7.1)
79 Draft Revised 1/07

Sec. 14-391. Stormwater BMP inspection, maintenance and easement requirements.
(a) Maintenance agreement. A written inspection and maintenance agreement in a
form acceptable to the county attorney and executed by the applicant and the owner of
the BMP, if different than the applicant, shall be provided prior to receiving a Stormwater
permit. The agreement shall:
(1) Bind the parties thereto and all subsequent owners, successors, and assigns to
maintenance and inspection of the system or structure.
(2) State that if the county directs the correction, repair, replacement, or maintenance
of the system or structure in writing and the actions are not satisfactorily performed
within a reasonable time (but not greater than 120 days), the county (or its contractors)
may, after reasonable notice, enter the land and perform all the necessary work and may
assess the owner of the facility with the cost of the work performed or the county can
seize all or part of the escrow or other fund set aside by the applicant for perpetual
maintenance. The owner served by the facility shall be jointly responsible to the county
for the maintenance of the facility and liable for any costs incurred by the county
pursuant to the agreement. All properties are jointly subject to the imposition of the liens
for such costs.
(3) The inspection and maintenance agreement shall be recorded in the register of
deeds at the expense of the applicant.
(b) Easements. Easements for stormwater BMPs shall include the area of the BMP,
area of ponded water, and enough area for access and maintenance. The easement shall
be recorded in the register of deeds at the expense of the applicant and shall be depicted
on the final plat or recorded map.
(Ord. of 7-10-2000, § 9.6; Ord. of 1-2-2001, § 9.8)
Sec. 14-392. Discharges and connections.
(a) Illegal discharge. No person shall cause or allow the discharge, disposal, pouring or
pumping directly or indirectly to any stormwater conveyance structure, stormwater
conveyance system, stream, lake, pond, wetland or other body of water, or upon the land
in proximity to the same, any fluid, solid or other substance (other than stormwater).
Prohibited substances include, but are not limited to oil, antifreeze, chemicals, animal
waste, paints, garbage and litter. Examples of illegal discharges are:
(1) Dumping of oil, antifreeze, paint or cleaning fluids.
(2) Untreated commercial carwash washwater.
(3) Industrial discharges.
(4) Contaminated foundation drains.
(5) Cooling waters, unless no chemicals are added and have a valid NPDES permit.
(6) Washwater from commercial and industrial activities.
(7) Chlorinated backwash and draining associated with swimming pools.
(8) Domestic wastewater.
(9) Septic system effluent.
(10) Washing machine discharges.
(b) Allowable discharges. Examples of allowed discharges under this article are:
(1) Water line flushing.
(2) Irrigation.
(3) Uncontaminated groundwater pumping.
80 Draft Revised 1/07

ti t
(4) Street wash water.
(5) Dechlorinated backwash and drainage associated with swimming pools.
(6) NPDES permitted discharges.
(c) Illegal connections. Connections to a stormwater conveyance system or structure
that allow the discharge of nonstormwater are unlawful. Prohibited connections include,
but are not limited to:
(1) Floor drains.
(2) Wastewater from washing machines or sanitary sewers.
(3) Wash water from commercial vehicle washing or steam cleaning.
(4) Wastewater from septic systems.
(d) Determination of connection. Upon determining that a connection to a stormwater
conveyance system:
(1) May result in the discharge of hazardous materials, may pose a threat to health and
safety, or is likely to result in immediate injury or harm to human or animal life, natural
resources, to real or personal property, or habitat; or
(2) Was made in violation of any applicable regulation or ordinance.
The stormwater administrator shall outline in a notice of violation, sent by certified mail,
the time in which the connection shall be removed. Failure to comply with the terms and
deadlines set in the notice of violation will constitute a violation of this article.
(Ord. of 1-2-2001, § 9.9)
Sec. 14-393. Riparian buffers.
Fifty-foot wide riparian buffers shall be maintained along both sides of a stream, river or
other waterbody as required by the Neuse River Basin: Nutrient Sensitive Waters
Management Strategy: Protection and Maintenance of Riparian Buffers, Section 3(a-b).
Riparian buffers shall be noted on the maps submitted for stormwater management plan
approval and shall be noted on the final recorded map. Determinations of exemptions (as
noted in 15A NCAC 2B.0233 Neuse River Basin: Nutrient Sensitive Waters Management
Strategy: Protection and Maintenance of Riparian Buffers, Section 3(a-b)) shall be made
by the NCDENR Division of Water Quality.
(Ord. of 7-10-2000, §§ 9.7, 9.8; Ord. of 1-2-2001, § 9.10)
Sec. 14-394. Right to enter.
Any county personnel, or contractors for the county, shall be permitted to enter upon
public or private property for the purposes of inspection, sampling, monitoring, testing or
otherwise verifying compliance. Should the county personnel, or contractor for the
county, be denied reasonable access to any property, the stormwater administrator shall
obtain an administrative search warrant. No person shall obstruct, hamper or interfere
with any such representative while carrying out his official duties.
(Ord. of 1-2-2001, § 9.11)
Sec. 14-395. Variances.
The board of adjustment shall consider all variance requests as set out in article XI of this
chapter.
(Ord. of 7-10-2000, § 9.10; Ord. of 1-2-2001, § 9.12)
Secs. 14-396--14-420. Reserved.
81
Draft Revised 1/07

APPENDIX E
Environmentally Sensitive Area Map
82 Draft Revised 1/07