Drainage Design Manual Draft-1998

DRAFT
MAY 26, 1998
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Division of Coastal Management
CRAVEN COUNTY
DRAINAGE DESIGN MANUAL
The preparation of this document was financed in part through a grant provided by the
North Carolina Coastal Management Program, through funds provided by the Coastal
Zone Management Act of 1972, as amended, which is administered by the Office of
Ocean and Coastal Resource Management, National Oceanic and Atmospheric
Administration.

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DRAFT DESIGN MANUAL
MAY 26, 1998
CRAVEN COUNTY
DRAINAGE DESIGN MANUAL
Introduction.........................................................................................................1
1.1
Statement of Intent...................................................................................1
1.2
Disclaimer................................................................................................1
Definitions...........................................................................................................1
SitePlans............................................................................................................3
3.1
Applicability. 3
3.2
Site Plan.................................................................................................. 3
3.3
Driveway Pipes........................................................................................ 3
3.4
Water Quality Measures.......................................................................... 4
Stormwater Management Plans.......................................................................... 5
4.1
Applicability..............................................................................................5
4.2
Drawings..................................................................................................5
4.3
Supporting Calculations...........................................................................6
4.4
Stormwater Conveyance......................................................................... 6
4.5
Floodplain Limits......................................................................................7
4.6
Point of Discharge................................................................................... 8
4.7
Water Quantity Control............................................................................ 8
4.8
Water Quality Control.............................................................................. 9
Drainage Easements.........................................................................................11
5.1
Applicability............................................................................................11
5.2
Swales and Channels............................................................................11
5.3
Storm Sewers and Culverts...................................................................11
5.4
Stormwater Management Facilities........................................................11

' DRAFT DESIGN MANUAL
MAY 26, 1998
1 1 INTRODUCTION
1.1 Statement of Intent. The intent of the Craven County Drainage Design
Manual is to establish the requirements and supporting information that
the County staff will use to determine if a proposed development is
consistent with the Craven County Stormwater Management Ordinance.
The Craven County Stormwater Administrator will consider the County
staff recommendations prior to issuing a Stormwater Permit. Craven
County encourages the designer to propose other measures that may
' not be identified in this manual that meet or exceed the intent of the
Stormwater Management Ordinance. The Stormwater Administrator will
have the authority to deny or accept procedures and methods that are
not specifically identified in this manual.
1.2
Disclaimer. The Manual is established to provide the County staff,
developers, engineers, surveyors, and builders a better understanding of
acceptable methods to meet the intent of the Stormwater Management
Ordinance. Design of stormwater management for development
requires considerable judgment by the designer. Craven County accepts
no responsibility for any loss, damage , or injury as a result of the use of
this manual.
2 DEFINITIONS
2.1
Constructed Channel - shall mean a relocated natural drainage way or
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constructed channel with a defined bed width for the purpose of
conveying stormwater.
2.2
Development - should. generally mean any of the following actions taken
by a public or private individual or entity:
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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,
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or
any land change, including, without limitation, clearing, tree
removal, grubbing, stripping, dredging, grading, excavating,
transporting and filling of land.
2.3
Drainage System - Shall include natural drainage way, swales, channels, -.. _-
storm sewers, curb inlets, yard inlets, culverts and other measures
intended to convey stormwater.
2.4
Invert - shall mean the lowest elevation of the inside perimeter of a pipe
or box culvert.

DRAFT DESIGN MANUAL
MAY 26, 1998
2.5 Land disturbing activity - shall mean any use of the land by any person
that results in a change in the natural cover or topography that may alter
' the quantity and/or quality of the stormwater runoff.
2.6 Natural Channel - shall mean the well defined portion of a natural
drainage way that conveys the base flow and/or stormwater runoff from
a majority of the rainfall events. A natural channel has not been altered
or relocated.
' 2.7 Peak Discharge - the highest stormwater quantity expressed in cubic
feet per second (cfs) for a given storm event.
2.8 10-year frequency storm - the storm that has a 10 percent chance of
occurring in any given year.
2.9 100-year frequency storm - the storm that has a 1 percent chance of
occurring in any given year.
2.10 Point of discharge - The point or points where concentrated stormwater
runoff leaves the site.
2.11
Post developed conditions - shall mean the conditions that will exist on a
site after the site is developed.
2.12
Pre -developed conditions - shall mean the conditions that exist on a site
to the site being developed.
prior
2.13
Stormwater Management Facilities - Shall include devices designed
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specifically to detain or retain stormwater for water quantity and/or water
quality control. These devices shall not include those drainage
structures that provide incidental water quantity or water quality control
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such as grassed swales. These devices include but are not limited to
wet ponds, dry ponds, bioretention areas, filter strips, infiltration
trenches.
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2.14
Site - shall mean the property that a proposed development takes place.
2.15 Swales - shall mean a constructed depression with no defined bottom
width that's main purpose is to convey stormwater runoff.

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MAY 26, 1998
3 SITE PLANS
' 3.1 Applicability. Land disturbing activities that require a site plan as
identified in the Craven County Stormwater Management Ordinance
shall submit a site plan with the stormwater permit application.
3.2 Site Plan. The Owner shall provide a drawing at a scale of no greater
than 1 inch = 50 feet referred to as the Site Plan. The Site Plan shall
include at minimum the following information:
3.2.1 Address or Vicinity Map showing the location of the activity.
3.2.2 Subdivision Name and the date of the approved subdivision plat,
if available.
3.2.3 The date of the subdivision's approved Stormwater Permit, if
available.
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3.2.4 The site boundaries.
3.2.5 Street Right of Way
3.2.6 Street Name and State Road Number
3.2.7 Existing roadway width and pavement type
3.2.8 Street Drainage System type (roadside ditch or curb & gutter)
3.2.9 Existing Structures
3.2.10 Proposed Structures and finish floor elevations
3.2.11 Driveway location and type (gravel, asphalt, concrete, etc.)
3.2.12 Driveway pipe location and inverts
3.2.13 Indicate the general drainage patterns on the lot or provide 1 foot
contour intervals.
3.2.14 Show any easements
3.2.15 Show any natural drainage ways and direction of flow
3.2.16 Show any flood boundaries and/or elevations
3.2.17 Show any phasing of land disturbing activities. If needed, a
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separate drawing can be provided for each phase.
3.3 Driveway Pipes. The driveway pipes shall be installed to maintain the
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roadside ditch drainage.
3.3.1 If the lot is within a subdivision that has an approved NCDENR
Sediment and Erosion Control Plan, the driveway pipe inverts
shall be consistent with the roadside ditch shown on the
approved plan.
3.3.2 If the subdivision does not have an approved NCDENR Sediment
and Erosion Control Plan, the pipe inverts shall be set to maintain
the existing roadside ditch grade. The site plan shall
demonstrate that the driveway pipe is consistent with the existing
roadside ditch grade by providing the next downstream and
upstream pipe inverts or ditch inverts 100 feet downstream and
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upstream, whichever comes first.
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DRAFT DESIGN MANUAL
MAY 26, 1998
' 3.3.3 The County reserves the right to require specific driveway pipe
inverts to be consistent with future plans.
' 3.3.4 The driveway pipe inverts shall be inspected prior to receiving an
occupancy permit.
' 3.4 Water Quality Measures. The site plan shall demonstrate that adequate
measures will be taken prior to, during, and after the land disturbing
activity to minimize the amount of pollutants that will leave the site or
reach a drainage system. All stormwater runoff from either a disturbed
' area or an area where construction activities are taking place shall pass
through a water quality measure prior to leaving the site or entering a
drainage system. A description of the water quality measures to be
taken shall be provided with the Stormwater Permit. Acceptable
measures shall include, but not limited to:
3.4.1 Undisturbed buffers. An undisturbed buffer is defined as an area
with a minimum width of 10 feet and a stable vegetative or
wooded cover such that stormwater runoff must sheet through
the buffer prior to leaving the site or entering the drainage
system. No activities, including the storage of material, shall be
allowed within the undisturbed buffer. The buffer shall be
maintained such that stormwater sheets through the undisturbed
buffer area. If the buffer is disturbed for the installation of
utilities, the area disturbed shall be immediately stabilized or
another water quality measure installed to prevent pollutants from
leaving the site or entering the drainage system.
3.4.2 Phasing. The undisturbed buffers can be phased on a project
such that portions of the lot are disturbed at different times in
order to satisfy the intent of the Stormwater Management
Ordinance. If phasing is utilized, the site plan shall indicate the
areas of the various phases and the written narrative shall
describe how the phasing will occur. A buffer area can not be
disturbed until the area which drains to the buffer is stabilized or
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other water quality measures are taken prior to disturbing the
buffer.
3.4.3 Temporary Structural Measures. Structural measures that filter
stormwater runoff and reduce the pollutants from leaving the site
or entering the drainage system until the construction activities
are completed. Temporary Structural measures include, but are
not limited to silt fence, check dams, sediment traps, and inlet
protection.
3.4.3.1 Temporary structural measures shall be installed per the
North Carolina Erosion and Sediment Control Handbook.
Supporting calculations are not required, however, the
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' DRAFT DESIGN MANUAL
MAY 26, 1998
County reserves the right to check the adequacy of the
measures using the design procedures if the measures
are not adequately performing.
3.4.3.2 Temporary structural measures shall not be removed until
the site has established vegetation. When the measures
t are removed, trapped pollutants shall be properly
disposed of either offsite or onsite. The permanent
ground cover shall be installed immediately.
' 4 Stormwater Management Plans
4.1 Applicability. Land disturbing activities that require a stormwater
management plan as identified in the Craven County Stormwater
Management Ordinance shall submit a stormwater management plan
with the stormwater permit application. The stormwater management
plan shall be prepared by a registered professional engineer or surveyor
qualified in the area of stormwater.
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4.2 Drawings. The Stormwater Management Plan shall include drawings to
scale. The drawings shall include the following minimum information:
i4.2.1
Vicinity map showing the location of the project within Craven
County.
4.2.2 Property boundaries, rights of way, existing and proposed
easements, and adjacent property owners.
4.2.3 Existing topography with a minimum 1 ft. contour intervals,
existing tree lines, roads, drainage features, utilities, buildings
and other notable features.
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4.2.4 Proposed improvements, including proposed grading, parking,
roads, building locations and size, drainage structures including
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swales, pipes, inlets, ponds, infiltration trenches, level spreaders
or other notable features.
' 4.2.5 Existing perennial and intermittent streams and lakes along with
associated buffers and floodplain limits.
' 4.2.6 100-year floodplain boundaries as required per this section.
4.2.7 Finished floor elevations for those structures adjacent to or within
the 100-year floodplain limits.
4.2.8 Construction details and/or specifications, prepared by a
registered professional engineer or surveyor qualified in the area
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' DRAFT DESIGN MANUAL
MAY 26, 1998
of stormwater, for drainage structures or facilities on the drawings
that are not included in the NCDOT Roadway Standard Drawings
' or the Carolina Erosion and Sediment Control Planning and
Design Manual. The details and/or specifications shall show the
type of material, elevations, quality control requirements and
other information to ensure the drainage structures and facilities
are properly constructed.
4.3 Supporting Calculations. The calculations should include those that
were utilized to determine the size of the drainage structures and
associated features. These do not include the calculations that are
required to be submitted with the sediment and erosion control permit
unless the calculations are required to show compliance with this
section.
4.4 Stormwater Conveyance. Stormwater shall be conveyed through the
development in an adequately designed drainage system of swales,
storm sewers, inlets, and channels. The drainage system shall be
' considered adequately designed if the 10-year frequency storm can be
conveyed with the following conditions:
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4.4.1 Swales and channels shall have the capacity to pass the 10-year
frequency storm event without overtopping of banks
4.4.2 Storm sewers shall convey the 10-year frequency storm such that
the hydraulic gradient is less than 6 inches from the ground
elevation.
4.4.3 The hydraulic gradient on storm sewer systems shall be less than
5 feet above the crown of the pipe.
4.4.4 Swales and channels shall be designed per procedures in the
North Carolina Erosion and Sediment Control Planning and
Design Manual.
4.4.5 Energy grade lines shall be determined per procedures in the
NCDOT Guidelines for Drainage Studies and Hydraulic Design.
4.4.6 Curb inlets shall be designed to meet spread criteria of the 4-inch
per hour rainfall event. Spread criteria shall be as follows:
4.4.6.1 Spread shall be no more than Y2 a through lane.
4.4.6.2If the typical section provides a full shoulder or parking
lane, spread shall not encroach on the travel lane.

DRAFT DESIGN MANUAL
MAY 26, 1998
4.4.6.3 Curb inlets at sag points shall allow for debris blockage by
providing twice the required opening. If relief by curb
overflow is not provided, the inlets at sag points shall be
designed for the 25-year frequency storm.
' 4.4.7 Culverts shall be considered pipes or box culverts that convey
stormwater under public roadways and railroad. Culverts shall be
designed to convey the 25-year frequency storm without
inundation of the roadway shoulder. Headwater depths shall be
' determined using procedures in the latest edition of the NCDOT
Guidelines for Drainage Studies and Hydraulic Design.
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4.5 Floodplain Limits. The 100-year floodplain limits shall be shown on
stormwater management plans.
4.5.1 The 100-year floodplain elevations shall be obtained from the
effective Federal Emergency Management Agency (FEMA) Flood
Insurance Study (FIS). If the FEMA FIS does not include the
stream and the stream has a drainage area greater than 100
acres, then a 100-year floodplain shall be determined utilizing
acceptable engineering methods. Acceptable engineering
methods shall include but not be limited to:
4.5.1.1 Peak discharges shall be determined for the 100-year
frequency storm using one of the methods given below
unless prior approval is obtained from the Stormwater
Administrator:
• The SCS Runoff Curve Number Method per
Technical Release No. 55 "Urban Hydrology for
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Small Watersheds" utilizing the 24-hour Type III
storm distribution. This method can be utilized
with acceptable hydrologic models such as HEC-1
or TR-20.
• USGS Regional Equations with appropriate
adjustment factors
• Existing gage information and frequency analysis.
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4.5.1.2The hydrologic calculations shall be performed for
expected (future) conditions land use as defined by the
Craven County Land Use Plan unless existing gage
information is utilized.
4.5.1.3 Water Surface Elevations shall be calculated using
standard step backwater calculations. Acceptable
hydraulic models shall be HEC-2, HEC-RAS, or WSPRO.
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IDRAFT DESIGN MANUAL
MAY 26, 1998
If the 100-year floodplain can be defined using one cross
section, then a normal depth calculation can be utilized to
' determine the water surface elevation.
4.5.1.4 The cross sections used to establish the 100-year
floodplain elevation(s) shall be either field surveyed or
' taken from a 1 foot interval contour map prepared by a
registered land surveyor.
' 4.5.2 The 100-year water surface elevation shall also be determined
for culverts that have a contributing area of more than 50 acres.
The 100-year water surface elevation shall be plotted on the
stormwater management plans.
4.5.3 All finish floors shall be constructed 1 foot above the 100-year
water surface elevation.
4.6 Point of Discharge. Stormwater shall leave the development at a
location where there is an existing drainage system or facility except for
those areas of the development where stormwater is not concentrated.
If improvements are required off -site to reach an existing drainage
system or facility, an easement for such improvement shall be obtained
prior to receiving the Stormwater Permit
' 4.7 Water Quantity Control
4.7.1 A hydrologic and hydraulic study shall be performed and proper
drainage structures and facilities shall be designed such that the
' development does not cause an increase in flooding upstream or
downstream during the 10-year frequency storm.
' 4.7.2 To demonstrate the development does not cause an increase in
flooding upstream or downstream, the hydrologic and hydraulic
analysis shall determine the pre -developed 10-year peak
' discharge and associated water surface elevation where the
stormwater enters and/or leaves the site. The hydrologic and
hydraulic analysis shall also determine the post -developed 10-
year peak discharge and associated water surface elevation at
the same locations.
4.7.3 If the 10-year post -developed peak discharge at a downstream
point of discharge increases 10 percent or less, then the
development is assumed to not increase the downstream
flooding.
4.7.4 If the 10-year post developed peak discharge from the site is less
' than 10 percent of the peak discharge of the receiving drainage
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DRAFT DESIGN MANUAL
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4.8
MAY 26, 1998
way, then the development is assumed to not increase the
downstream flooding.
4.7.5 If the development provides on -site detention of the 10-year, 24-
hour storm such that there is no increase in the pre -developed
peak discharge for the 10-year, 24-hour storm at the point(s) of
discharge. It is not required to determine the pre -developed
downstream water surface elevations.
4.7.6 If the development provides on site detention described above
and does not encroach on the drainage way floodplain, then it is
not required to determine the pre -developed upstream water
surface elevation.
4.7.7 Wet and dry detention ponds shall safely pass the 100-year, 24-
hour storm without overtopping the top of dam. Overland
emergency spillways shall not be located within the dam
embankment. If a facility is under the jurisdiction of the North
Carolina Dam Safety Act, then the dam and spillway shall be
designed per the Dam Safety requirements in addition to
satisfying the detention requirements per this ordinance.
Water Quality Control.
4.8.1 The Craven County Stormwater Ordinance requires that all
development that requires a stormwater management plan
implement water quality measures similar to the NCDENR
Division of Water Quality stormwater requirements for Coastal
Counties.
4.8.2 A development which is required to receive a Stormwater
Management Permit from the NCDENR Division of Water Quality
is considered to have met the Craven County water quality
requirements.
4.8.3 Craven County encourages the use of low maintenance water
quality measures such as low density development, grassed
swales, infiltration basins, bioretention areas, and dry detention
ponds to meet the water quality requirements of the this section.
4.8.4 Low Density Development. A development shall be considered
low density when the built upon area is no greater than 30
percent of the site. The area that is not built upon shall be
identified on the plat and not disturbed. If the area is disturbed in
the future and the overall site has a built upon area greater than
30 percent, then the entire site, even the area previously built
shall meet the high density development water quality goals.
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DRAFT DESIGN MANUAL
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4.8.5 High Density Development. A development shall be considered
' high density if it does not meet the low density requirements
above. High density development shall incorporate water quality
measures to reduce the amount of pollutants that leave the site
or enter a drainage system. Stormwater from the impervious
areas shall be conveyed through one or a combination of the
following water quality measures prior to leaving the site:
' 4.8.5.1 Vegetative Swales. The stormwater is conveyed for a
minimum of 100 feet in a maintained vegetative swale.
1 4.8.5.2 Vegetative Filter Strip. The stormwater passes through a
maintained vegetative filter strip in sheet flow.
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4.8.5.3Infiltration Basins or Trenches. The runoff from the first
one inch of rainfall infiltrates into the groundwater.
4.8.5.4 Sand Filters. The first one -inch of runoff passes through
a sand filter prior to entering the drainage system.
4.8.5.5 Bioretention Areas. - The first one -inch of stormwater is
diverted to a landscaped area that will hold the water and
allow it to infiltrate and be taken up by the plants. The
remaining stormwater discharges to the drainage system.
4.8.5.6 Dry Extended Detention Pond. The first one -inch of
' stormwater is detained for a period of 2 to 5 days. The
remaining stormwater is discharged to the drainage
system.
' 4.8.5.7 Wet Detention Pond. The first one -inch of stormwater is
stored above the normal pool for a 24 hour period.
' 4.8.6 The water quality measures shall be designed in accordance with
the latest edition of the North Carolina Division of Water Quality
"Stormwater Best Management Practices."
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DRAFT DESIGN MANUAL
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5 Drainage Easements
' 5.1 Applicability. Drainage Easements shall be required on all swales,
constructed channels, natural channels, pipes, or culverts that convey
stormwater runoff from areas other than within the site boundary. If the
' site consists of multiple lots or parcels that can be sold to separate
owners then a drainage easement is required on all Swales, constructed
channels, natural channels, pipes, or culverts that convey stormwater
runoff from areas other than within the individual lot or parcel.
' 5.1.1 All drainage easements shall provide for vehicle access to the
easement from a public right of way.
' 5.1.2 All drainage easements shall be recorded on the plat and shall
clearly identify who has the maintenance responsibility. The
' maintenance responsibility shall be either the individual property
owner or a legally formed homeowners association.
' 5.2 Swales and Channels. Swales and channels that require drainage
easements shall have a drainage easement that includes the normal
swale or channel (top of bank to top of bank) and includes a 12 feet wide
maintenance access along one side 'of the normal channel.
5.3 Storm Sewers and Culverts. Storm Sewers that require a
' drainage easement shall have minimum easements widths shown
below:
5.3.1 Pipes 24-inches in diameter or less shall have a 15 feet
' wide easement.
5.3.2 Pipes over 24-inches and less than 54-inches in diameter
' shall have a 20 feet wide easement
5.3.3 Pipes 54 inches in diameter and greater shall have a 30
' feet wide easement or greater if needed.
5.4 Stormwater Management Facilities. Stormwater Management
' Facilities shall be included in an easement. The easement shall
include the area of the facility, area of ponded water up to the 10-
year frequency storm, and enough area for access and
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maintenance around the entire facility.