7_NCS000442_StormwaterDesignManual_20210708

II , T V 4 s
ROCKYI
N H G A R O L I" 4
Rocky Mount Engineering Department
One Government Plaza
Rocky Mount, NC 27802
City of Rocky Mount, North Carolina
Stormwater Design Manual
Chapter 1: General Design Criteria
December 2006

Table of Contents
Chapter 1: General Design Criteria
1.1 Introduction............................................................................................ 1-1
1.2 Stormwater Management Overview.........................................................................1-1
1.3 Plan Submittal Requirements..................................................................... 1-2
1.4 General Design Criteria.............................................................................1-4
1.5 Stormwater Quantity Control......................................................................1-13
1.6 Stormwater Quality Control........................................................................1-15
Chapter 2: Structural Best Management Practices
2.1
Introduction.............................................................................................2-1
2.2
Selecting the Appropriate BMPs..................................................................2-1
2.3
BMP Operation and Maintenance Requirements ............................................
2-3
2.4
Water Quality Volume and Peak Flow..........................................................2-3
2.5
Wet Detention Ponds................................................................................2-5
2.6
Stormwater Wetlands................................................................................2-11
2.7
Wet Pond/Wetland Series.........................................................................
2-17
2.8
Restored Riparian Buffers.........................................................................
2-20
2.9
Grassed Swales......................................................................................2-21
2.10
Water Quality Swales...............................................................................
2-22
2.11
Vegetative Filter Strips with Level Spreader ..................................................
2-25
2.12
Bioretention (Rain Gardens).......................................................................2-29
2.13
Sand Filters............................................................................................2-34
2.14
Permeable Pavement...............................................................................2-37
Chapter 3: Stormwater Design Calculations
3.1
Introduction............................................................................................ 3-1
3.2
Computer Software..................................................................................3-1
3.3
Peak Discharge.......................................................................................3-2
3.4
Hydrograph Generation............................................................................
3-15
3.5
Stream Routings.....................................................................................
3-16
3.6
Impoundment Routings.............................................................................3-16
3.7
Streets and Gutters.................................................................................
3-24
3.8
Catch Basins and Drop Inlets....................................................................
3-25
3.9
Storm Drainage Pipes..............................................................................
3-33
3.10
Culverts.................................................................................................3-38
3.11
Open Channels.......................................................................................3-46
Appendix A - Acronyms and Definitions.................................................................A-1
Appendix B - Design Forms and Checklists............................................................B-1
Appendix C - References...................................................................................C-1

Disclaimer
To the best of their ability, the authors have insured that material presented in this manual is
accurate and reliable. The design of engineered facilities, however, requires considerable
judgment on the part of designer. It is the responsibility of the designer to insure that
techniques utilized are appropriate for a given situation. The City of Rocky Mount therefore
accepts no responsibility for any loss, damage, or injury as a result of the use of this manual.
City of Rocky Mount
Stormwater Design Manual December 2006

Chapter 1: General Design Criteria
1.1 Introduction
The intent of this manual is to serve as a reference for City staff and
practicing professionals in designing storm drainage facilities within
the City of Rocky Mount (the City) and its extraterritorial jurisdiction. It
is primarily a compilation of the City's accepted design procedures
and practices. Design criteria listed herein are the general policy of
the City of Rocky Mount and may not be applicable in every situation.
Where the designer determines that conformance with this manual
would create an unreasonable hardship or where an alternative
design may be more appropriate, alternative designs may be
accepted upon written authorization from the Director of Engineering
or his designee. In order to insure good engineering design, the City
staff may occasionally require more stringent standards than those
presented here. This manual may also be subject to periodic change
by the City staff. When changes are required, revisions will be made
available via the City of Rocky Mount website.
Engineering construction specifications are contained in a separate
manual entitled "City of Rocky Mount Department of Engineering
Manual of Specifications", which can be obtained from the City
Engineering Department.
Engineering standard details are available via the City of Rocky Mount
website at http://www.ci.rocky-mount.nc.us/engineering/main.html.
The City of Rocky Mount Stormwater Design Manual consists of 3
Chapters. Chapter 1 — General Design Criteria discusses the overall
minimum design criteria for storm drainage systems within the City's
jurisdiction and the stormwater management criteria for new
development. Chapter 2 — Structural Best Management Practices
presents the acceptable structural measures that can be used to
control stormwater along with minimum design and maintenance
criteria. Chapter 3 — Design Calculations presents the acceptable
hydrologic and hydraulic methodologies, parameters, and in some
cases, equations that are required to demonstrate compliance with
the minimum design criteria.
1.2 Stormwater Management Overview
Stormwater management in the City of Rocky Mount addresses
issues related to the proper control of stormwater runoff in both
quantity (site and roadway drainage and flood conveyance) and
quality (nutrients and suspended solids). The City has adopted a
variety of regulations, ordinances and policies that serve as the
City of Rocky Mount
Stormwater Design Manual December 2006 1-1

foundation of the City's design standards for the management of the
quantity and quality of stormwater runoff. These standards are
designed to protect the health and welfare of the residents of Rocky
Mount, to protect the environment, and to protect those who live
downstream from Rocky Mount. The following sections of this chapter
present the minimum standards related to stormwater management
within the City of Rocky Mount.
1.3 Plan Submittal Requirements
The City of Rocky Mount requires a separate site stormwater
management plan submittal for all developments greater than '/2 acre.
The stormwater management plan submittal shall include the
complete storm drainage system and all of the supporting calculations
for review. All stormwater management plans shall include a
completed City of Rocky Mount Stormwater Management Summary
Sheet, unless the project is exempt or receives a written exemption
from the Director of Engineering, and all of the relevant calculation
forms provided in this stormwater design manual.
The stormwater management plan supporting information shall
include the following:
• Location map.
• Overall site map showing the proposed site, surrounding
properties, and zoning information.
• Existing condition drainage area map showing the existing
stormwater outfall locations and existing land use. The drainage
area map shall be the City topographic map unless more detailed
topography is available. The existing drainage area map shall be
large enough to show portions of the upstream and downstream
drainage areas. Soil survey information shall be shown, including
boundaries and hydrologic classification as identified by the NRCS
• Existing storm drainage pipes including offsite storm drainage
pipes that either discharge onto the site, run parallel to the site, or
will receive stormwater runoff from the site.
• Seasonal high water table, as needed for BMP design.
• Proposed condition drainage area map showing the proposed site
improvements and the stormwater outfall locations. The proposed
drainage area map shall be consistent with the catchment and
outfall areas identified in the Stormwater Management Summary
Sheet.
• All storm drainage catch basins, drop inlets, junction boxes and
pipe locations and sizes along with the supporting inlet design
chart, pipe design chart, and hydraulic grade line calculations as
provided in Chapter 3 — Calculations.
City of Rocky Mount
Stormwater Design Manual December 2006 1-2

1.3.1 Record Drawings
• Profiles of all storm drainage pipe systems. The profiles shall
show existing and proposed utility crossings.
• All open channel locations including vegetated swales, trapezoidal
ditches, intermittent streams and perennial streams along with
supporting design calculations.
• All culvert crossings locations and sizes with the supporting
culvert calculation sheets.
• All structural BMPs and supporting calculations including
hydrograph routings, water quality volumes, spillway and volume
rating curves, outlet protection and other calculations identified in
Chapter 2 — Structural BMP Design Criteria.
• Structural BMP operation and maintenance plan.
• All flood zones including those calculated as required by this
manual. The floodplain and floodway boundaries shall be clearly
labeled to identify the boundary source such as FEMA 100-year
floodplain or calculated 100-year floodplain.
• All wooded pervious areas clearly identified with a metes and
bounds description. A written conservation easement for the
wooded pervious area must be executed prior to plan approval
• All proposed Finish Floor Elevations of buildings.
• All riparian or vegetated buffers shall be shown and clearly
labeled.
• All existing and proposed drainage and utility easements.
• All existing wetlands, perennial and intermittent streams.
• All proposed site utilities in plan view.
• If the project is phased, a schedule for implementation of all
proposed water quality BMPs that specifies when the BMP(s) will
be on-line with respect to the development schedule for the
drainage area serviced by the BMP.
• Certification by a North Carolina registered professional engineer,
registered landscape architect, or registered land surveyor who is
qualified in hydrology and hydraulics, stating that the plans comply
with the standards in the City of Rocky Mount Stormwater Design
Manual.
• Peak runoff calculations to each outfall leaving a site and any
required BMP's to meet peak runoff control requirements.
Upon completion of the new construction, the developer is required to
provide "record drawings", certified by a NC registered professional
engineer, landscape architect, or land surveyor, prior to receiving an
occupancy permit for the property.
City of Rocky Mount
Stormwater Design Manual December 2006 1-3

1.4 General Design Criteria
New construction within the Rocky Mount jurisdictional area is subject
to various City requirements. The following general requirements
apply to both private development as well as City projects, unless the
designer requests and receives approval for alternative designs from
the Director of Engineering or his designee.
The City of Rocky Mount must approve all new or revised stormwater
discharges from private property to a publicly maintained storm
drainage system. The owner of the property or the developer may
make written application to the City for stormwater discharge or may
submit a stormwater management plan for City staff review under the
terms of the Zoning Ordinance or General Plan Project Submittal
Policy.
For all non -single family residential projects that are '/2 acre or larger
and single family residential projects that are 1 acre or larger, the
owner or developer shall provide a stormwater management plan and
supporting calculations to the City for review. The City reserves the
right to require a drainage plan and calculations for projects less than
a'/2 acre if deemed necessary by the Director of Engineering.
A conveyance system shall be properly designed and installed when
stormwater runoff is equal to or greater than 5 cubic feet per second
(cfs) for the 10-year storm. A properly designed conveyance system
may include a vegetated swale, open channel, catch basin with pipe,
stream and floodplain, culvert or structural Best Management Practice
(BMP).
When development of an area changes the flow regime from sheet
flow to concentrated flow, the drainage system shall be designed to
minimize impacts of the concentrated flow on adjoining properties by
tying into existing drainage systems using multiple outlets, through
agreements with adjacent owners, or other appropriate means.
No concentrated flow shall be discharged across walkways.
Provisions are to be made through piping or other means to carry the
flow under the walkway.
All stormwater drainage systems that will be maintained by the City of
Rocky Mount and convey between 5 and 50 cfs shall be piped, unless
those systems are Structural BMPs, intermittent or perennial streams.
No stormwater drainage shall be discharged into a sanitary sewer.
No utilities (sewers, power lines, water lines, etc.) shall be located
within or under any stormwater management facility.
City of Rocky Mount
Stormwater Design Manual December 2006 1-4

1.4.1 Easements
No utilities (sewers, power lines, water lines, etc.) shall be located
over a storm drain line and along the same alignment unless
approved by the Director of Engineering.
In no case shall a building be located within an impoundment area or
over a storm drain line.
Where storm drainage lines cross or parallel other utility lines,
appropriate clearances shall be provided according to the City of
Rocky Mount Department of Engineering Manual of Specifications.
Drainage easements are required for all public and private drainage
systems. This includes commercial developments with out -parcels,
phased development and other developments with surrounding land
under the same ownership as the tract being developed.
Drainage easements shall be provided:
• For all culverts, all new or existing open channels or watercourses
that carry water from public rights -of -way or convey water from
adjoining property across the developing property. Width of the
easements shall be per the most recent City of Rocky Mount
Standard Detail.
• For all stormwater BMPs, the BMP shall be located in a
stormwater BMP easement with a minimum width of 15 feet
beyond the top of bank or toe of slope and as necessary to
provide access for maintenance.
• For all new or existing open channels or watercourses with peak
flows of 15 cubic feet per second or more for the 10 year storm;
• At other locations deemed appropriate by the Director of
Engineering or Engineering Department staff.
In addition:
Appropriate drainage easements must be secured prior to the
submission of the construction plat if the easement(s) is entirely or
partially located offsite.
Access easements, dedicated to the City of Rocky Mount, shall be
provided for access and repair of velocity dissipaters, headwalls, and
other structural portions of the drainage system located outside of the
right-of-way (ROW) which are immediately adjacent to and directly
associated with the City owned portion of the drainage system. These
easements are requested to allow City staff access to repair and
maintain those drainage facilities located immediately adjacent to the
right-of-way which would endanger the roadway should they fail.
City of Rocky Mount
Stormwater Design Manual December 2006 1-5

Adequate easements shall be provided to allow access of
construction equipment, taking into consideration the limitations that
may be imposed by embankment slopes or other obstacles.
Drainage easements containing only storm drainage facilities should
be centered over the culvert or watercourse.
All drainage easements should be recorded based on field surveys,
following construction, to insure that the drainage structure or
watercourse is centered within the easement (unless specifically
offset). Where this is not possible, a note shall be added to recorded
plats establishing that easements are to be centered over the pipe or
channel.
All drainage easements shall be designed to tie into existing
easements, existing watercourses or to other appropriate locations
when possible.
When other easements are not present along the property line, 5-foot
easements for drainage and other general amenities are required
around the entire perimeter of each lot adjacent to the lot property
line.
1.4.2 Streets and Gutters
Gutters shall be designed in such a way that the spread of water
during the 2-year storm does not exceed 6 feet into the travel lane.
The travel lane does not include the gutter section.
When the street typical section includes a full shoulder or parking
lane, no encroachment into the travel lane will be allowed.
For new streets located within or near a floodplain, the 100-year water
surface elevation shall be no more than 2 feet above the low point of
the road unless written approval is provided by the Director of
Engineering.
No peak flow greater than 3 cfs during the 10-year storm may run
down a driveway and into the street without the placement of a catch
basin to intercept the flow.
Driveways that discharge 3 cfs or more during the 10-year storm into
a roadside ditch shall provide outlet protection to prevent erosion of
the ditch.
A minimum longitudinal slope of 0.4% shall be utilized unless
approved otherwise by the Director of Engineering. When lesser
slopes are encountered, the gutter shall be warped to provide the
minimum slope.
City of Rocky Mount
Stormwater Design Manual December 2006 1-6

New street crossings in the Tar -Pamlico riparian buffer areas shall be
as close to a perpendicular angle as possible to minimize buffer
disturbance.
No public or private roads are to be constructed on dams without the
approval of the Director of Engineering.
1.4.3 Catch Basins, Drop Inlets
and Junction Boxes
Catch basins shall be provided at street sags, up -grade of
intersections, up -grade of super -elevation crossovers, and where
driveways would discharge more than 3 cfs into a street for the 10-
year storm.
Catch basins shall be provided in streets to intercept flow such that
the spread conditions defined in 1.4.2 are not exceeded for the peak
flow during the 2-year storm event.
Catch basin capacity and bypass shall be designed for the peak flow
during the 2-year storm event with a 5 minute time of concentration.
Inlet capacity at sags, where relief by curb overflow is not provided,
shall allow for debris blockage by providing twice the required
computed opening for the 2-year storm.
No grate only type inlets are allowed in city streets.
Inlets are required on drainage systems discharging to a public street
(including sheet flow) where the stormwater discharge is more than 3
cfs for the 10-year storm.
For combination grates and curb openings, ignore the curb opening
on continuous grades to determine bypass and ignore the grate
opening at the sag.
Limit the depth of ponding to 1 foot above the drop inlet grates located
outside of the ROW during the inlet design storm.
A structure (catch basin, drop inlet or junction box) is required at all
changes in grade or direction or at any pipe junction. Details shall be
provided on the plans for all such structures.
Yard inlets that collect site stormwater runoff and convey the
stormwater to the stormwater drainage system within the right of way
shall be located outside of the right of way unless site conditions
warrant otherwise.
Junction Boxes shall allow for access to the storm drainage system
with a grate, manhole ring and cover, or a lid capable of being
removed. No "blind boxes" are permitted.
City of Rocky Mount
Stormwater Design Manual December 2006 1-7

Catch basins, drop inlets and junctions boxes shall have minimum
drops between the upstream and downstream openings as provided
in Table 1.1 below.
Table 1.1 - Minimum Structure Invert Drops
Change in Alignment
Drop*
0 - 45 degrees
0.1 ft
45 — 90 degrees
0.2 ft
> 90 degrees (reverse flow
conditions)
Only with detailed study and drop
equal to or greater than the
diameter of the pipe out
Change in Pipe Size
Drop*
Increase in pipe size
Match the crown elevations
Decrease in pipe size
Only with a detailed study and
special provisions for maintenance
* Structure invert drops that are less than the required minimum may be
approved at the discretion of the Director of Engineering.
1.4.4 Storm Drainage Pipes
Storm drainage pipes convey stormwater runoff from areas such as
streets, parking lots and grass areas underground to a receiving
channel, stream or structural BMP. When stormwater runoff is
conveyed from one side of a roadway to the other and flooding or
failure can cause flow across the roadway then the conveyance
system shall be designed as a culvert.
Storm drainage pipes shall be designed, at a minimum, to convey the
future conditions peak flow during the 10-year storm event.
The design of storm drainage pipe systems shall consider where
water flows when the storm drainage system cannot handle the
stormwater runoff, such as storm events greater than the design
storm. Section 1.5.2 identifies when floodplain boundaries shall be
determined and shown on the plans.
For design of the storm drainage pipe it shall be assumed that the
catch basin or drop inlet captures 100% of the peak flow during the
10-year storm event.
Minimum slopes for pipes 36 inches in diameter or less shall be 0.5%.
Minimum slopes for pipes greater than 36 inches in diameter shall be
0.3%
Minimum flow velocity in storm drainage pipes during the design
storm event shall be 2.5 feet per second (fps).
City of Rocky Mount
Stormwater Design Manual December 2006 1-8

All storm drainage pipes within the street ROW shall be reinforced
concrete pipe Class III or higher.
All new pipes within easements that are to be maintained by the City
shall be a minimum of 15 inches in diameter or equivalent. Privately
maintained pipes within easements shall be a minimum of 12 inches
in diameter or equivalent.
Maximum slope for reinforced concrete pipes is 12.0%. The Director
of Engineering may approve greater slopes with the submittal of
appropriate detailed structural designs and other supporting
documentation.
Storm drainage pipes shall be designed such that the hydraulic grade
line calculations demonstrate that when all tailwater conditions, pipe
friction loss and all minor structure losses are considered, the
hydraulic grade line remains at least 6-inches below the grate or
gutter line.
Flared end sections are required at the inlet and outlet on all pipes 48
inches or less.
For multiple pipes 48-inches in diameter or less, a headwall may be
used in lieu of multiple flared end sections.
Inlet and outlet headwalls are required on pipes larger than 48 inches
in diameter.
If a pipe is located within the ROW, the minimum distance between
the outside wall of the pipe and the ROW is 5 feet
Cover for pipes within the ROW shall be provided based on the
following table:
Table 1.2 - Minimum Pipe Clearance
Pipe Size (in.)
Clearance Distance (ft)
From Pipe Invert to Subgrade
15
2.4
18
2.7
24
3.3
30
3.8
36
4.4
42
4.9
48
5.4
54
6.0
60
6.5
66
7.0
72
7.6
Minimum cover for pipes outside of the ROW is 1 foot.
City of Rocky Mount
Stormwater Design Manual December 2006 1-9

1.4.5 Open Channels
Open channels shall be designed to convey the future conditions peak
flow during the 10-year storm event. The future condition land use
shall be obtained from the City Planning Department.
New and existing open channels, with a 100-year discharge equal to
or greater than 50 cfs and which are impacted by the proposed
development, shall be designed to pass the 100-year storm unless the
following criteria are met:
o The developer demonstrates that the 100-year discharge will
not flood habitable structures or increase flood elevations on
adjacent properties and the limits of the 100-year floodplain
are determined and recorded.
o Existing natural channels serving do not have to be improved
to carry the 10-year design flow but the limits of the 100-year
floodplain must be established and recorded on the plat.
Private V-swales are acceptable for design discharges between 0 and
10 cfs. The swale shall be properly protected from erosion during the
design event.
Trapezoidal swales are acceptable between 0-50 cfs. The trapezoidal
swale shall be graded to a minimum of 3 feet bottom width with 3:1
side slopes and properly protected from erosion during the design
event.
Open channels that convey more than 50 cfs during the 10-year
design storm shall be designed as a combination channel. A
combination channel shall have a separate low flow channel designed
to adequately convey the 2-year storm event, without erosion, and a
larger channel that can convey the 10-year design storm.
The minimum slope of soil ditches shall be 0.5%.
For open channels, gradual changes in alignment, not to exceed a
minimum radius of 4 times the top width of the channel, is
recommended. Where no other options are available, sharper
changes in alignment may be allowed under the following conditions:
Table 1.3 — Open Channel Alignment
Open Channel Bend
Requirements
20 - 45 degrees
Bank stabilization must be provided
according to tractive force analysis
>45 degrees
Same as for above but in addition,
freeboard superelevation shall be
calculated to demonstrate adequate
channel depth on the outside bank.
City of Rocky Mount
Stormwater Design Manual December 2006 1-10

Side slopes for vegetated open channels in residential areas should
be no greater than 3 to 1 for stability, safety, and ease of
maintenance. Where the channel width must be limited, side slopes
may be increased if suitable vegetative or structural stabilization
techniques (see following table) and safety measures are utilized.
Aesthetics and ease of maintenance should also be considered in the
design.
Table 1.4 — Maximum Side Slopes
Stabilization Type
Maximum Side Slopes
Vegetative*
2:1
Stone
1.5:1
Grid Pavers
1.5:1
Paving**
1:1
Retaining Walls
Vertical
Bioengineering
Varies
Other Methods
Approved by Director of Engineering
*Note: Special consideration must be given to the use of vegetative
linings in channels. In some cases, structural stabilization is required
along the lower portions of the channel bank where continuous or
frequent water contact weakens the soil structure and may impede the
growth of vegetation (recommend protection to a point 2' above the
bottom of the channel or the high water mark for the 2-year storm,
which ever is greater). The designer is directed to the State of North
Carolina "Erosion and Sediment Control Planning and Design Manual"
for the selection of appropriate vegetation based on soil types and
flow velocities.
**Note: Asphalt channel linings are not allowed in the City right-of-
way.
In the interest of preserving existing vegetation (which helps to
stabilize stream banks and provides shade thereby reducing
temperature extremes) and in order to preserve the aesthetics of
natural channels, not all streams have to be altered to protect them
from erosion. However, existing channels which are an integral part
of the development and which may endanger new or existing
structures or other improvements (such as parking lots and tennis
courts) as the result of future stream bank erosion, should be
evaluated for the need for additional erosion protection. In addition,
those existing channels which will be subject to peak flow increases of
100% or more as the result of complete build -out of the contributing
watershed and those existing channels with sharp bends should also
be evaluated for the need for additional erosion protection.
Vegetation and other bioengineering measures are the preferred
method of stabilizing channels. However, calculations must
City of Rocky Mount
Stormwater Design Manual December 2006 1-11

1.4.6 Culverts
demonstrate that design storm velocities do not exceed those
acceptable for the measure.
Culverts are structures, such as pipes or boxes, which convey surface
water and/or stormwater runoff from one side of a roadway to the
other and when the culvert can not handle all of the stormwater runoff,
flooding will occur across the roadway.
Culverts shall be designed to meet the following requirements unless
the Director of Engineering requires more stringent conditions or
waives the minimum requirements identified below.
• Residential Local — Designed to convey the 25-year design
storm with a 1 foot freeboard and a HW/D<1.2
• Residential Collector — Designed to convey the 25-year
design storm with a 1 foot freeboard and a HW/D<1.2
• Commercial Local — Designed to convey the 50-year design
storm with a 0.5 foot freeboard and a HW/D<1.2
• Commercial Collector — Designed to convey the 50-year
design storm with a 0.5 foot freeboard and a HW/D<1.2
• Industrial Local — Designed to convey the 50-year design
storm with a 0.5 foot freeboard and a HW/D<1.2
• Industrial Collector — Designed to convey the 50-year design
storm with a 0.5 foot freeboard and a HW/D<1.2
• Minor Arterial — Designed to convey the 50-year design storm
with a 0.5 foot freeboard and a HW/D<1.2
• Major Arterial — Designed to convey the 50-year design storm
with a 0.5 foot freeboard and a HW/D<1.2
Minimum size of culvert shall be 18-inches in diameter.
Headwater elevations for all roads crossing watercourses for the peak
flows during the design storm and 100-year storms, including weir
calculations, shall be provided for those situations where overtopping
is allowed.
Culverts on intermittent and perennial streams shall be designed to
maintain the integrity of the stream channel. If the 10-year peak
discharge is greater than 50 cfs then the culvert shall be designed to
handle the 2-year storm event within a lower section set at the stream
invert and the design storm in a larger section set at a higher
elevation such that the larger section is utilized when water elevations
exceed the water elevations during the 2-year storm event.
Flared end sections are required at the inlet and outlet on all culverts
48 inches or less.
City of Rocky Mount
Stormwater Design Manual December 2006 1-12

Inlet and outlet headwalls are required on culverts larger than 48
inches in diameter.
1.4.7 Energy Dissipation Design
Energy dissipators shall be employed whenever the velocity of the
flow leaving a storm drainage pipe or culvert exceeds the erosive
velocity of the receiving channel.
Energy dissipators shall be designed per the North Carolina Erosion
and Sediment Control Planning and Design Manual or Hydraulic
Engineering Circular No. 14 "Hydraulic Design of Energy Dissipators
for Culverts and Channels".
1.5 Stormwater Quantity Control
Stormwater quantity control, for purposes of this manual, refers to the
management of the impact new development has on downstream
drainage systems and properties and the management of potential
flooding within the development site. In order to manage the
downstream impact, new development must control the peak flow
rates that leave the site. This control is referred to as stormwater
detention. In order to manage the potential flooding within the
development, the potential floodplain boundaries must be identified.
This is referred to as floodplain management.
1.5.1 Stormwater Detention
New development shall not result in an increase in the peak
stormwater runoff leaving the site from the pre -development
conditions for the following storm events unless the development is
demonstrated to be exempt:
• 1-year, 24 hour storm event — To reduce downstream channel
degradation
• 10-year, 24-hour storm event — To protect downstream drainage
system capacity.
• 25-year, 24-hour storm event — To protect downstream
properties.
A development is exempt from the above control requirements if:
• The overall impervious surface is less than 15 percent of the total
site and the pervious portions of the site are used to the maximum
extent practical to convey and control the stormwater runoff, or;
• The increase in peak flow between the pre -development and post -
development conditions does not exceed 10 percent, or;
• The Director of Engineering makes a determination that peak
stormwater control at this particular location will increase flooding,
City of Rocky Mount
Stormwater Design Manual December 2006 1-13

accelerate erosion or negatively impact existing storm drainage
problems in the area. In such cases, an alternate method of
stormwater quantity control may be required.
The designer shall demonstrate quantity control requirements are
satisfied by routing hydrographs using the acceptable methods
presented in Chapter 3 — Calculations.
If an impoundment is used to control the peak flow, it is the
responsibility of the designer to verify the whether the impoundment is
regulated by the Division of Land Resources under the jurisdiction of
the Dam Safety Act NCAC T15A 02K.0100. An impoundment that is
15 feet high or more and has 10-acre-feet of storage or more must
comply with the North Carolina Dam Safety Act which has specific
spillway and embankment design standards and requires a separate
submission to the Division of Land Resources for review and
approval. Impoundments below the established threshold can be
classified as high hazard by the Division of Land Resources and also
be required to satisfy the rules of the Dam Safety Act.
1.5.2 Floodplain Management
Chapter 9 of the City's Land Development Code identifies the
allowable activities and procedures for any activities within the flood
zones.
In general, drainage systems are not designed to convey all potential
storm events or quantity of stormwater runoff nor is it possible to
guarantee that the system will never experience a debris blockage or
other unpredictable event. For this reason, the designer shall always
take into consideration where the water would flow in the event the
system capacity is exceeded or the system is blocked or failed.
For storm drainage pipe systems and open channels that convey a
peak flow of more than 50-cfs during the 100-year storm event, the
100-year floodplain boundary shall be calculated and shown on the
development plans. When the drainage system includes a
watercourse, the Subdistrict C flood zone shall be shall be shown as
defined in Chapter 9 of the Land Development Code unless
calculations demonstrate otherwise.
For storm drainage pipe systems and open channels that convey a
peak flow of less than 50-cfs during the 100-year storm event, the
design shall consider where water will flow if the system overflows.
The Director of Engineering may require calculation of the 100-year
floodplain boundary if there is concern that system overflows may
cause structural flooding or unsafe roadway or driveway flooding.
All drainage systems and site development shall consider the effects
of the FEMA Floodplain as shown on the most recent Flood Insurance
Rate maps and floodplains defined by the City of Rocky Mount. The
City of Rocky Mount
Stormwater Design Manual December 2006 1-14

floodway and flood plain boundaries must be shown for any mapped
stream.
The designer shall determine floodplain boundaries using acceptable
step backwater calculations as presented in Chapter 3 — Calculations
unless FEMA or the City has developed an existing floodplain
boundary.
1.6 Stormwater Quality Control
These policies implement citywide measurable performance goals for
control of nutrients, total nitrogen and total phosphorous in stormwater
runoff as required by the Tar -Pamlico Rules, NPDES regulations and
others. The control of sediment is also required for construction site
runoff as part of the City's Erosion Control Program, and specific
restrictions and performance -based criteria for controlling total
suspended solids in stormwater runoff exist in the water supply
watershed protection area. This section summarizes the City's
nutrient and total suspended solids control requirements to satisfy the
Tar -Pamlico Nutrient Sensitive Waters and Water Supply Watershed
Protection Rules. Guidelines and design requirements for the City's
Sediment and Erosion Control Program can be found in Chapter 8,
section 802 of the City's Land Development Code.
The City of Rocky Mount encourages satisfying these stormwater
quality requirements through onsite land planning measures to reduce
the disturbed and impervious areas to the maximum extent practical,
as well as the design and installation of as few structural BMPs as
practical.
The City also strongly encourages the use of onsite structural BMPs
to provide both stormwater quantity and quality control.
Garbage dumpsters and apartment or condominium car washing
areas must be located such that runoff from these areas sheet flows
across a densely vegetated area. These facilities cannot be located
in close proximity to streams or other watercourses.
1.6.1 Tar -Pamlico Nutrient Sensitive Waters
Chapter 8, Section 802 of the City's Land Development Code
describes the requirements for the Tar -Pamlico Nutrient Sensitive
Waters in detail. This section provides a brief summary of those
requirements. Nutrient offset payment can be made in lieu of onsite
land planning and/or structural control measures. Any nutrient offset
payments must be in accordance with North Carolina Administrative
Code 15A NCAC 02B .0240 and subsequent amendments.
City of Rocky Mount
Stormwater Design Manual December 2006 1-15

1.6.2 Nutrient Control — One Acre Disturbance (Single -Family Residential)
New single-family residential development that disturbs greater than 1
(one) acre of land to establish, expand, or replace a single family
residential development or recreational facility shall demonstrate the
following nutrient loading requirements are met:
Total Nitrogen (TN) Export is reduced to 4.0 pounds per acre
per year (Ibs/ac/yr).
o If the nitrogen reduction can not be obtained through onsite
land planning measures and/or structural BMPs then an
equivalent mass load reduction shall be obtained through the
treatment of existing offsite areas either in the site structural
BMP, separate offsite structural BMP or regional structural
BMP as long as the Total Nitrogen Export from the site does
not exceed 6.0 Ibs/ac/yr and the offsite facility is located
within the same classified surface water as defined in Chapter
8 of the City's Land Development Code.
Total Phosphorous (TP) Export is reduced to 0.4 Ibs/ac/yr.
o If the phosphorous reduction can not be obtained through
onsite land planning measures and/or structural BMPs then an
equivalent mass load reduction shall be obtained through the
treatment of existing offsite areas either in the site structural
BMP(s), separate offsite structural BMP(s) or regional
structural BMP located within the same classified surface
water as defined in Chapter 8 of the City's Land Development
Code.
Nutrient offset payment for both nitrogen and phosphorous can be
made in lieu of onsite land planning and/or structural control
measures. Any nutrient offset payments must be in accordance with
North Carolina Administrative Code 15A NCAC 02B .0240 and
subsequent amendments.
1.6.3 Nutrient Control — One Half Acre Disturbance (All Other Projects)
New development that disturbs greater than 1/2 (one-half) acre of
land to establish, expand, or replace a multi -family residential
development, commercial, industrial, institutional or any other non-
residential development shall demonstrate the following nutrient
loading requirements are met:
• Total Nitrogen Export is reduced to 4.0 Lbs/acre/year.
o If the nitrogen reduction can not be obtained through land
planning measures and/or structural BMPs then an equivalent
mass load reduction shall be obtained through the treatment of
existing offsite areas either in the site structural BMP, separate
City of Rocky Mount
Stormwater Design Manual December 2006 1-16

1.6.4 Riparian Buffers
offsite structural BMP or regional structural BMP as long as
the Total Nitrogen Export from the site does not exceed 10.0
Ibs/ac/yr and the offsite facility is located within the same
classified surface water as defined in Chapter 8 of the City's
Land Development Code.
Total Phosphorous (TP) Export is reduced to 0.4 Ibs/ac/yr.
o If the phosphorous reduction can not be obtained through
onsite land planning measures and/or structural BMPs then an
equivalent mass load reduction shall be obtained through the
treatment of existing offsite areas either in the site structural
BMP(s), separate offsite structural BMP(s) or regional
structural BMP located within the same classified surface
water as defined in Chapter 8 of the City's Land Development
Code.
Nutrient offset payment can be made in lieu of onsite land planning
and/or structural control measures. Any nutrient offset payments
must be in accordance with North Carolina Administrative Code 15A
NCAC 02B .0240 and subsequent amendments.
The City strongly encourages onsite structural BMPs be designed for
existing offsite development in order to achieve offsite nutrient load
reductions.
When offsite load reduction is provided in a separate structural BMP,
the structural BMP shall be reviewed and approved by the City of
Rocky Mount and all of the necessary offsite easements and
maintenance agreements shall be in place prior to receiving approval
to begin land development activities.
Offsite facilities shall meet the conditions outlined in Chapter 8 of the
City's Land Development Code.
Nutrient Loading Calculations are presented in Chapter 3 —
Calculations. These calculations have been incorporated into a
spreadsheet that must be completed with each development.
The Tar -Pamlico Nutrient Sensitive Waters Rule established a 50 foot
wide riparian buffer on all sides of intermittent and perennial streams,
ponds and lakes shown on the most recent version of either the
Natural Resources Conservation Service Soil Survey or a 1:24,000
scale (7.5 minute quadrangle) topographic map prepared by the U.S.
Geological Survey (USGS) as appropriate. The buffer is measured
from the top of bank or normal pool of an impoundment. The City will
not approve new development plans that include land area within the
riparian buffer unless the development receives approval from DWQ.
City of Rocky Mount
Stormwater Design Manual December 2006 1-17

1.6.5 Water Supply Watershed Protection
Chapter 8, Section 803 of the City's Land Development Code
describes the City's requirements for development within the
designated Watershed Protection Areas. This section briefly
summarizes those requirements.
New development that disturbs more than 1 acre of land located
within the WS-IV-CA or WS-IV-PA as shown on the City's watershed
map, which is available on the City's website, shall meet the following
conditions in addition to satisfying the stormwater quantity and quality
control identified in earlier sections:
WS-IV-CA (Critical Area)
o Maintain the following low density and built upon limits
■ Single family residential developments shall not exceed 2
dwelling units per acre.
■ All other residential and non residential shall not exceed 24
percent built upon area.
■ Maintain a minimum of a 30 foot wide vegetative buffer
along perennial streams.
o Limit the percent imperviousness to 50 percent and
construct structural BMPs that control the runoff from the first
1-inch of rainfall runoff such that 85% TSS is achieved, and
maintain a 100 foot wide vegetative buffer along perennial
waters. The buffer is measured from the top of bank or
normal pool of impoundment.
WS-IV-PA (Protected Area)
o Maintain the following low density and built upon limits
■ Single family residential developments with a curb and
gutter street system shall not exceed 2 dwelling units per
acre.
■ Single-family residential developments without a curb and
gutter street system shall not exceed 3 dwelling units per
acre.
■ All other residential and non residential with a curb and
gutter street system shall not exceed 24 percent built upon
area.
■ All other residential and non residential without a curb and
gutter street system shall not exceed 36 percent built upon
area.
■ Maintain a minimum of a 30 feet wide vegetative buffer on
perennial streams.
o Limit the percent imperviousness to 70 percent and
construct structural BMPs that control the runoff from the first
1-inch of rainfall runoff such that 85% TSS is achieved, and
maintain a 100 foot wide vegetative buffer along perennial
waters. The buffer is measured from the top of bank or
normal pool of impoundment.
City of Rocky Mount
Stormwater Design Manual December 2006 1-18

City of Rocky Mount, North Carolina
Stormwater Design Manual
iltROCKY
MOULTChapter 2: Structural Best Management
Practices
December 2006
Rocky Mount Engineering Department
One Government Plaza
Rocky Mount, NC 27802

Chapter 2: Structural Best Management Practices
2.1 Introduction
Structural best management practices (BMPs) are constructed
stormwater management facilities designed to treat stormwater runoff
and/or mitigate the effects of increased stormwater runoff peak rate,
volume, and velocity due to urbanization.
The structural BMPs discussed in this section have been approved by
both the City of Rocky Mount and the State and can be utilized for
attenuating peak flows and reducing pollutants in stormwater runoff
from new developments. Structural BMPs shall be designed by North
Carolina registered professionals with experience and knowledge of
the hydrologic and hydraulic methodologies presented in this
Stormwater Management Design Manual. Registered professionals
are defined as professional engineers; landscape architects to the
extent that G.S 89A allows; and land surveyors the extent that the
design represents incidental drainage within a subdivision as provided
in G.S. 89C-3(7).
This chapter presents general guidelines for selecting the appropriate
structural BMP, general maintenance requirements, water quality
volume calculations, minimum required design standards and in some
cases design calculations, recommended design standards to
enhance the BMP and minimum operation and maintenance for the
acceptable structural BMPs.
2.2 Selecting the Appropriate BMPs
Structural BMPs are required to be designed and constructed on new
developments to control stormwater runoff in both quantity and
quality. The control requirements for the City of Rocky Mount are
summarized in Chapter 1 of the Stormwater Management Design
Manual. The City of Rocky Mount prefers that structural BMPs be
located such that the maximum amount of site -developed area drains
to the BMP to minimize the number of structural BMPs that require
maintenance and inspection. The City also prefers that structural
BMPs that naturally receive offsite drainage areas be designed to
handle the existing offsite area in lieu of diverting around the structural
BMP.
The following structural BMPs and associated pollutant removal
efficiencies have been approved by both the State and the City of
Rocky Mount. These pollutant removal efficiencies are assuming the
minimum design standards are followed in the design and
construction of the structural BMP. The City encourages
incorporating the recommended design standards to enhance the
overall BMP, however, additional pollutant removal will not be
acknowledged.
City of Rocky Mount December 2006
Stormwater Design Manual 2-1

Table 2.1 - Acceptable Structural BMPs
TN
TP
TSS
Peak
BMP Type
Removal
Removal
Removal
Flow
Control
Wet Detention
25%
40%
85%
High
Ponds
Stormwater
40%
35%
85%
High
Wetlands
Wet
Pond/Wetland
55%
61 %
98%
High
Series
Restored
30%
30%
40%
Low
Riparian Buffers
Grass Swales
20%
20%
35%
Low
Water Quality
30%
30%
35%
Low
Swale
Vegetated Filter
Strip
20%
35%
40%
Low
with Level
Spreader
Bioretention
40%
35%
85%
Low
(rain gardens)
Sand Filters
35%
45%
85%
Low
Permeable
Varies
Varies
Varies
Varies
Pavement
Proprietary
Varies
Varies
Varies
Low
BMPs
Other BMPs
Varies
Varies
Varies
Varies
If more than one BMP is installed in series, then the removal rate 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%:
1. The pond removes 25 percent of the nitrogen and discharges 75
percent to the buffer.
2. The buffer then removes 30 percent of the nitrogen that
discharged from the pond, which is 22.5 percent.
3. The sum of 25 and 22.5 is 47.5. (NOT 25 plus 30, or 55 percent)
Alternative or innovative BMPs, such as manufactured BMPs, will
require prior approval from the Director of Engineering. The designer
shall provide complete documentation on the BMP, including
specifications, performance, and maintenance requirements to be
submitted. The Director of Engineering will determine if the measure
warrants consideration and what, if any, monitoring controls and
performance bonding will be required.
City of Rocky Mount December 2006
Stormwater Design Manual 2-2

2.3 BMP Operation and Maintenance Requirements
The City of Rocky Mount requires the owning entity to operate and
maintain the structural BMP so the intended function such as pollutant
removal and/or peak flow control does not diminish over time.
Chapter 8 Section 804 of the City's Land Development Code presents
the general requirements for the posting of a performance bond,
establishment of a maintenance agreement, recordation of easements
and general operation, maintenance and repair requirements. This
chapter establishes the minimum required maintenance of each of the
structural BMPs. These minimum maintenance requirements shall be
included in the Operation and Maintenance Plan.
The City of Rocky Mount will perform annual inspections of the
structural BMPs to verify the operation and maintenance is being
performed as identified in the operation and maintenance plan.
Chapter 8 of the City's Land Development Code identifies the
measures the City will undertake to ensure structural BMPs are being
properly maintained.
2.4 Water Quality Volume and Peak Flow
The structural BMPs presented in this Chapter, except for the typical
grass swale, all control and treat the water quality volume (WQv).
The WQv, sometimes referred to as the first flush runoff, is the runoff
from the first inch of precipitation, which is generally the portion of the
runoff with the highest concentrations of most conventional nonpoint
source runoff contaminants. The structural BMPs treat the WQv in
various ways to achieve pollutant load reduction. Some BMPs hold
the WQv volume for a period of time to allow pollutants to settle, some
hold the WQv in a permanent surface to allow the water to interact
with vegetation, some allow the WQv to filter through a media or sheet
across filter strip. In all cases, the WQv is the basis for the structural
BMP water quality component and in some cases will drive the overall
size of the structural BMP.
The WQv is directly related to the amount of impervious cover created
at a site. In numerical terms, it is equivalent to an inch of rainfall
multiplied by the volumetric runoff coefficient (Rv) and site area.
The following equation is used to determine the storage volume, WQv
(in acre-feet of storage):
WQv = (1.0 — inchXRvXA)
12
Where:
WQv = water quality volume (in acre-feet)
Rv = 0.05 + 0.009(I) where I is percent impervious cover
A = area in acres draining to the structural BMP
City of Rocky Mount December 2006
Stormwater Design Manual 2-3

Some structural BMPs, such as bioretention areas, sand filters and
level spreaders function best when large storms are bypassed. These
large storms tend to damage these types of structural BMPs and it is
impractical to properly size these facilities to handle them. When the
WQv is diverted to a structural BMP, a flow splitter is required to
handle the WQv peak flow. The WQv peak flow (Qwq) is based on
the NRCS Unit Hydrograph Method presented in Chapter 3. The
RCN is calculated based on the runoff of the WQv event by using the
equation below:
RCN = 1000
10 + (5XP)+ (I OXQ)— (I OXQ' + 1.25(QXP))'
Where:
RCN = Runoff Curve Number
P = The 1 — inch rainfall event
Q = WQv runoff (inches) = (1.OX.05 + .009(I))
Where:
I = the drainage area % imperviousness
The calculated RCN is then used with the graphical method presented
in Chapter 3 to calculate a peak discharge. For this calculation, if the
la/P calculation is less than 0.1, then 0.1 should be used. It is
important to note that this is an approximation of the peak flow and it
is assumed that if the diversion can handle the WQv peak flow then all
the WQv will be diverted to the structural BMP.
When the drainage area is highly impervious (greater than 95%) then
the Qwq can be calculated using:
Qwq=1.48(A)
Where:
Qwq = water quality peak flow in cfs
A = drainage area in acres
City of Rocky Mount December 2006
Stormwater Design Manual 2-4

2.5 Wet Detention Ponds
Wet detention ponds, as shown in Figure 2.1, are ponds that are sized
and configured to provide significant removal of pollutants from incoming
stormwater runoff and can provide peak flow control. They maintain a
permanent pool of water that is designed for a target total suspended
solids (TSS) removal rate according to the size and imperviousness of
the contributing watershed. Above this permanent pool of water, they are
also designed to hold the WQv and release this over a period of two to
five days. These two basic requirements result in a pond where a majority
of the suspended sediment and pollutants attached to the sediment are
allowed to settle out of the water. The wet detention pond outlet device
can be designed to provide peak control of larger storm events through a
combination of orifices, weirs and pipe/barrel sizing. Chapter 3 presents
the basic equations for typical outlet devices.
Minimum Design Standards
• Minimum drainage area of 10 acres unless otherwise approved by
the Director of Engineering.
• Normal pool volume based on Table 2.2. This table was developed
based on Driscoll's model (US EPA, 1986) based on the long-term
average storm retention time.
• Normal pool minimum depth of 3 feet.
• Maximum normal pool depth of 8 feet.
• The normal pool shall have a combination aquatic vegetation and
safety bench that extends at least 10 feet into the normal pool at a
maximum slope of 6%.
• Minimum length to width ratio of normal pool 1.5:1 (preferably
expanding outward toward the outlet).
• A sediment forebay shall be located at the upstream side of the
normal pool and should consist of a separate cell, formed by an
acceptable barrier protected from erosion.
• The volume of the sediment forebay shall be 0.1-inch times the
impervious acreage draining to the forebay. The surface area of
the forebay shall be considered part of the pond surface area.
• A permanent benchmark shall be installed in the vicinity of the
sediment forebay to allow for ready determination of the sediment
depth.
• The WQv shall be stored above the normal pool and slowly
released over at least 48 hours.
• The top of the embankment shall be a minimum 6-inches above the
100-year, 24-hour elevation with 1-foot recommended, and a
minimum of 1.0 foot above the 25-year, 24-hour elevation.
• Earthen embankment side slopes should be no greater than 3:1
and shall have a well -established grass cover.
• A separate emergency spillway shall be provided to convey events
greater than the 25-year storm. In special circumstances the
Director of Engineering may allow all storm events to be conveyed
through the principal spillway.
City of Rocky Mount December 2006
Stormwater Design Manual 2-5

• The principal spillway shall be designed to control the design
events. If the principal spillway is a riser/barrel type, then the
design shall be such that the barrel controls during the storm event
prior to the use of the emergency spillway.
• The principal spillway shall have trash racks, hoods, or other debris
control devices as necessary to prevent clogging.
• Anti -vortex measures shall be incorporated into the trash racks to
prevent a vortex from forming during large storm events.
• The principal spillway shall be checked for buoyancy and
appropriate anti -floatation measures incorporated to maintain a
minimum factor of safety of 1.5.
• Riprap protection or other stilling basin type structure per HEC-14
shall be provided for the principal spillway outlet and all inlet
structures into the pond.
• When the principal spillway is a riser/barrel structure, anti -seep
collars or filter diaphragms shall be provided to reduce failure due
to piping.
• Pipes through an embankment shall be watertight.
• The normal pool shall be able to be drawn down to the elevation of
the outlet invert within 24 hours through the opening of some type
of emergency drain (i.e. sluice gate, drawdown pipe).
• Design the emergency spillway to pass the 100-year storm event.
• Provide for vehicle maintenance access, a minimum of 15 feet wide
to the embankment and sediment forebay along with provisions for
equipment to maneuver.
• Vehicle maintenance access paths shall be at slopes no greater
than 10 percent.
• If the normal pool area is used as temporary sediment basin during
construction, all sediment shall be removed and properly disposed
of prior to final inspection.
Table 2.2 - Normal Pool SA/DA Ratios in Percent
DA
Impervious
PERCENT
3.0
PERMANENT POOL AVERAGE DEPTHS IN FEET
4.0 5.0 6.0 7.0
8.0
10%
0.59
0.49
0.43
0.35
0.31
0.29
20%
0.97
0.79
0.70
0.59
0.51
0.46
30%
1.34
1.08
0.97
0.83
0.70
0.64
40%
1.73
1.43
1.25
1.05
0.90
0.82
50%
2.06
1.73
1.50
1.30
1.09
1.00
60%
2.40
2.03
1.71
1.51
1.29
1.18
70%
2.88
2.40
2.07
1.79
1.54
1.35
80%
3.36
2.78
2.38
2.10
1.86
1.60
90%
3.74
3.10
2.66
2.34
2.11
1.83
City of Rocky Mount December 2006
Stormwater Design Manual 2-6

Recommended Standards
• Inlet and outlet located to maximize flow length. Use baffles if
short-circuiting cannot be prevented with inlet -outlet placement.
Long flow paths and irregular shapes are recommended.
• Design the pond for multi -objective use, such as amenities or flood
control.
• Landscaping management of buffer as meadow.
• Provide a length to width ratio of 3:1 to 4:1 (preferably wedge
shaped).
• Use reinforced concrete instead of corrugated metal for pipes.
• For minor pond inlets, level spreaders through a vegetated area
should be installed to reduce the sediment loading.
• Consider artificial mixing for small sheltered ponds through the
installation of fountains or mixers.
• Impervious soil boundary to prevent drawdown may be needed.
• Shallow marsh area around fringe 25 to 50 percent of area
(including aquatic vegetation) should be established.
• When the normal pool depth is greater than 4 feet, the safety
bench should be increased to 15 feet.
• Include an aquatic bench with the safety bench that extends inward
from the normal shoreline and has a maximum depth of eighteen
inches below the normal pool water surface elevation.
• Minimum 25 foot wide buffer around pool.
• Provide on -site disposal areas for two sediment removal cycles.
These disposal areas should be protected from runoff.
• Provide an oil and grease skimmer on the principal spillway in
areas with a 50% or more impervious roadway and parking.
• Harden the bottom of the forebay (e.g., using concrete, paver
blocks, etc.) to make sediment removal easier.
Unique Calculations
• No unique calculations required.
tion and Maintenance Recommendations
• Maintenance should always include minimizing erosion problems
and pollutant export to the pond from the contributing watershed.
Care should be taken to secure all appropriate legal agreements for
the easement.
• The sediment forebay should be cleaned out when the sediment
volume is 50% of the original volume.
• Sediment within the remaining normal pool should be removed when
20% of the original normal pool volume is filled.
• No woody vegetation should be allowed on the embankment without
special designs.
• Vegetation over 18 inches high should be cut unless it is part of
planned landscaping.
• Debris should be removed from blocked inlet and outlet structures
and from areas of potential clogging.
City of Rocky Mount December 2006
Stormwater Design Manual 2-7

• The outlet control should be kept structurally sound, free from
erosion, and functioning as designed.
• Periodic removal of dead vegetation should be accomplished.
*Inspection requirements should be outlined in the operation and
maintenance manual. The manual shall identify special
maintenance needs and include routine inspections including but
not limited to:
o Debris removed after every major storm
o Routine mowing schedule
o Sediment buildup and the need for removal
o Erosion along the bank and the need for reseeding or
stabilization
o If reseeding is necessary, a reseeding schedule
o Erosion at the inlet and outlet and methods of stabilization
o Seepage through the dam
o Emergency spillway inspection
o Operation of any valves or mechanical components
o Consider chemical treatment by alum if algal blooms are a
problem.
City of Rocky Mount December 2006
Stormwater Design Manual 2-8

Figure 2.1 - Wet Detention Pond
POND
BUFFER
EMERGENCY
OVERFLOW
SPILLWAY
SPILLWAY
HARDENED
PAD
FOREBAY
....
....
IRREGULAR POOL SHAPE
O�
4 -
6 FEET DEEP
OUTFALL
RISER/
. -
BARREL
MAINTENANCE
ACCESS ROAD
RISER IN
EMBANKMENT
AQUATIC BENC
SAFETY BENCH
LANDSCAPING
AROUND POOL
PLAN VIEW
EMBANKMENT\
RISER
EMERGENCY
0 100 YEAR LEVEL SPILLWAY
11= -
=111- 010 YEAR LEVEL
OVERFLOW SAFETY II =1
SPILLWAY AQUATIC BENCH III IIIII
BENCH
INFLOW III = 0 II I III STABLE
All WET POOL
OUTFALL
FOREBAY
POND DRAIN
REVERSE PIPE
BARREL
ANTI -SEEP COLLAR OR
FILTER DIAPHRAGM
PREPILE
Source: Controlling Urban Runoff
City of Rocky Mount December 2006
Stormwater Design Manual 2-9

25-YEAR LEVEL
1-YEAR LEVEL
Figure 2.2 - Spillway Configuration
100-YEAR LEVEL
HOODED
INLET -
INLET
VALVE BOX W/LID "
NORMAL WATER
SURFACE
DRAIN
GATE VALVE-
W/EXTENSION
ANTI-VCRTEX AND
TRASH RACK
EMBAMKMENT
LINK SEAL OR
WQ EQUIVALENT
OUTLET
OUTLET
EMERGENCY
SPILLWAY
GROUT FOR ANTI -SEEP
BUOYANCY COLLAR
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 10

2.6 Stormwater Wetlands
Stormwater wetlands, as shown in Figure 2.3, can be defined as
pocket wetlands which rely on groundwater to maintain adequate
water supply and shallow marsh wetlands, as shown in Figure 2.4,
which have sufficient drainage area to maintain an adequate water
supply through stormwater runoff. For the purposes of this manual,
the pocket wetland and shallow marsh wetland are both considered
stormwater wetlands and provide the same pollutant removal
efficiencies. Stormwater wetlands consist of micropools, deep water
and areas with shallow flooding to support various types of wetland
vegetation. The micropool, deep water and shallow flooding are sized
for the surface area requirement of a 3 foot deep wet pond with
varying zones that promote stormwater wetland vegetation. Above
this permanent pool of water, the stormwater wetland is also designed
to hold the WQv and release this over a period of two to five days.
These two basic requirements result in a stormwater wetland where a
majority of the suspended sediment and pollutants attached to the
sediment are allowed to settle out of the water and different aerobic
and anaerobic zones are created that enhance nutrient removal. The
stormwater wetland outlet device can be designed to provide peak
control of larger storm events through a combination of orifices, weirs
and pipe/barrel sizing. Chapter 3 presents the basic equations for
typical outlet devices.
Minimum Design Standards
• For drainage areas up to 10 acres the micropool and shallow
flooding area must be excavated into the normal groundwater or a
spring shall be present to maintain the permanent water surface
elevation.
• The Director of Engineering may require the micropool and
shallow flooding area be excavated into the normal groundwater
for drainage areas larger than 10 acres based on local knowledge
unless it can be demonstrated through water balance calculations
that adequate water will be provided by stormwater runoff.
• The permanent pool surface area shall be sized using Table 2.2
with an average depth of 3.0 feet.
• .A minimum of 70 percent of the permanent pool surface area shall
be designed as a marsh. The marsh area shall have an almost
equal distribution of low and high marsh areas. The low marsh
areas shall be between 6 and 12-inches deep and the high marsh
area shall be between 0 and 6-inches deep.
• A soil depth of at least 4 inches should be used for shallow wetland
basins
• A micropool that is between 3 and 6 feet deep shall be located near
the outlet structure. The micropool surface area shall be at least
15 percent of the permanent pool surface area.
• The remaining surface area shall be distributed between a
deepwater area, with an average depth of 3 feet and the sediment
forebay(s).
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 11

• The length to width ratio should be at least 2 to 1
• The WQv shall be stored above the permanent pool surface area
and slowly released over a period between 2-5 days.
• A sediment forebay shall be located at the upstream side of the
wetland and at separate inflow points that contribute more than
10% of WQv and should consist of a separate cell, formed by an
acceptable barrier protected from erosion.
• The volume of the sediment forebay shall be 0.1-inch times the
impervious acreage draining to the forebay. The surface area of
the forebay below the permanent pool shall be considered part of
the stormwater wetland permanent pool surface area.
• A permanent benchmark shall be installed in the vicinity of the
sediment forebay to allow for ready determination of the sediment
depth.
• The top of the embankment shall be a minimum 6-inches above the
100-year 24-hour elevation with 1.0 foot recommended and a
minimum of 1.0 foot above the 25-year 24-hour elevation.
• Earthen embankment side slopes should be no greater than 3:1
and shall have a well -established grass cover.
• A separate emergency spillway shall be provided to convey events
greater than the 25-year storm. In special circumstances the
Director of Engineering may allow all storm events to be conveyed
through the principal spillway.
• The principal spillway shall be designed to control the design
events. If the principal spillway is a riser/barrel type, then the
design shall be such that the barrel controls during the storm event
prior to the use of the emergency spillway.
• The principal spillway shall have trash racks, hoods, or other debris
control devices as necessary to prevent clogging.
• Anti -vortex measures shall be incorporated into the trash racks to
prevent a vortex from forming during large storm events.
• The principal spillway shall be checked for buoyancy and
appropriate anti -floatation measures incorporated to maintain a
minimum factor of safety of 1.5.
• Riprap protection or other stilling basin type structure per HEC-14
shall be provided for the principal spillway outlet and all inlet
structures into the pond.
• When the principal spillway is a riser/barrel structure, anti -seep
collars or filter diaphragms shall be provided to reduce failure due
to piping.
• Pipes through an embankment shall be watertight.
• The permanent pool shall be able to be drawn down to the
elevation of the outlet invert within 24 hours through opening some
type of emergency drain (i.e. sluice gate, drawdown pipe, etc).
• Design the emergency spillway to pass the 100-year storm event.
• The deepwater area of the wetland should include the outlet
structure so outflow from the basin is not interfered with by
sediment buildup.
• Stabilize surrounding slopes with vegetation to trap sediments and
other pollutants, preventing them from entering the wetland.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 12

• A maintenance plan should be provided and adequate provision
made for ongoing inspection and maintenance, with more intense
activity for the first three years after construction.
• The wetland should be maintained to prevent loss of water
available for emergent vegetation due to sedimentation and/or
accumulation of plant material.
• Provide for vehicle maintenance access, a minimum of 15 feet wide
to the embankment and sediment forebay along with provisions for
equipment to maneuver.
• Vehicle maintenance access paths shall be at slopes no greater
than 10 percent.
• 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.
Recommended Specifications
• The designer should maximize use of existing- and post -grading
pondscaping design to create both horizontal and vertical diversity
and habitat.
• It is recommended that the frequently flooded zone surrounding the
wetland be located within approximately 10 to 20 feet from the
edge of the permanent pool.
• Soil types conducive to wetland vegetation should be used during
construction.
• The wetland should be designed to allow slow percolation of the
runoff through the substrate (add a layer of clay for porous
substrates).
• As much vegetation as possible and as much distance as possible
should separate the basin inlet from the outlet.
• The water should gradually get shallower about 10 feet from the
edge of the pond.
• The planted areas should be made as square as possible within
the overall design of the wetland, rather than long and narrow.
• The only site preparation that is necessary for the actual planting
(besides flooding the basin) is to ensure that the substrate is soft
enough to permit relatively easy insertion of the plants.
• 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 early April to
mid -June. Planting after these dates is not recommended, as the
wetland plants need a full growing season to build the root reserves
needed to get through the winter. If at all possible, the plants
should be ordered at least three months in advance to ensure the
availability of the desired species.
• The optimal depth requirements for several common species of
emergent wetland plants are often six inches of water or less. To
add diversity to the wetland, 5 to 7 species of emergent wetland
plants should be used. Of these, at least three species should be
selected from "aggressive colonizers" plants such as bulrush,
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 13

pickerelweed, arrow arum, three square and rice cutgrass (MDE,
1986).
Plants should be installed in clumps with individual plants located
an average of 18 inches on center within each clump. Individual
plants should be spaced 12 inches to 24 inches on center.
Wetland mulch, if used, should be spread over the high marsh area
and adjacent wet zones (-6 to +6 inches of depth) to depths of 3 to
6 inches.
Unique Calculations
• No unique calculations required.
Operation and Maintenance Recommendations
• A stormwater maintenance manual is required for each facility.
The maintenance manual should require the owner of the wetland
to periodically clean the structure. The manual should provide for
ongoing inspection and maintenance, with more intense activity for
the first three years after construction.
• The wetland should be maintained to prevent loss of area of
ponded water available for emergent vegetation due to
sedimentation and/or accumulation of plant material.
• Sediment forebays should be cleaned every 2 to 5 years, except
for pocket wetlands without forebays which are cleaned after a six-
inch accumulation of sediment.
• The ponded water area may be maintained by raising the elevation
of the water level in the permanent pond, by raising the height of
the orifice in the outlet structure, or by removing accumulated
solids by excavation.
• Water levels may need to be supplemented or drained periodically
until vegetation is fully established.
• It may be desirable to remove contaminated sediment deposits or
to harvest above ground biomass and remove it from the site to
permanently remove pollutants from the wetland.
• Performance enhancement can be obtained by increasing the size
of the marsh area, by incorporating multiple pools into marsh area,
or by incorporating a network of shallow channels in the marshy
area.
• Remove woody vegetation/trees in excess of 2-inches in diameter.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 14

Figure 2.3 - Pocket Wetland
MAINTENANCE ACCESS— � ; k kv
w�w+rm+r�
W � SWALE
+Y iY eya ;4WW
SAFETY BENCH
MAX€MUM SAFETY STORM LIMIT
Q 100 YEAR LEVEL
Q 25 YEAR LE
swALE=_
wQy LEVEL
FOREBAY
GROUNDWATER LOW W
TABLE
tl J BUFFER HALr KUUNU
TRASH RACK
1--
EMBANKMENT
HIGH MARSH
Cpy LEVEL
HALF ROUND
p TRASH RACK —
IIII
i lIII- ill—IIII`=�
MICROPOOL :I—I�TI—'—
I POND DRAIN
BARREL
ANTI -SEEP COLLAR or�
FILTER DIAPHRAGM
PLAN VIEW
BROAD
CRESTED
WEIR
STABLE
OUTFALL
PROFILE
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 15

Figure 2.4 - Shallow Marsh Wetland
_'SA ETY BFNCH7 EMERGENCY
i
SPILLWAY
i
I% WEIR
I WALL \
FOREBAY
MICROPOOL OUTFACE
WATERFOWL
ISLAND
- - - BARREL
- J RISER IN
EMBANKMENT
MAINTENANCE � — —
ACCESS ROAD
HIGH MARSH
LANDSCAPE BUFFER (LESS THAN 6 INCHES WATER DEPTH)
LOW MARSH
(WATER DEPTH BETWEEN 6 AND 18 INCHES)
PLAN VIEW
HIGH MARSH EMBANKMENT
RISER EMERGENCY
SPILLWAY
100 YEAR LEVEL III=III=
0 10 YEAR LEVEL
WQv LEVEL PERMANENT II III
POOL
INFLOW �l �IIII STABILIZED
II�II I�I�IF II�II�II
III II�II�II �II�II- III — RIPRAP OUTFALL
I III II�II����II�II�IL— III I
PROTECTION II�II� BARREL
FOREBAY �II-
yl
GABION WALL LBW MARSH POND DRAIN
I ;III III 11
REVERSE PIPE
ANTISEEP COLLAR or
FILTER DIAPHRAGM
PR❑FILE
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 16

2.7 Wet PondMetland Series
The City of Rocky Mount prefers that nutrient reduction be obtained
using onsite controls in series. However the City does not want
multiple embankments and unnecessary sediment forebays, therefore
the features of the wet detention pond and the stormwater wetlands
discussed above have been combined into a single device as shown
in Figure 2.5. The basis for this device is to size the wet pond using
the same criteria identified in the wet detention basin and size the
stormwater wetland micropool and shallow flooding using the same
criteria as identified in the stormwater wetland but design a single
sediment forebay, single embankment and single outlet control
structure to minimize maintenance and potential areas of failure. The
pond/wetland series outlet device can be designed to provide peak
control of larger storm events through a combination of orifices, weirs
and pipe/barrel sizing. Chapter 3 presents the basic equations for
typical outlet devices.
Minimum Design Standards
• Minimum drainage area of 10 acres unless otherwise approved by
the Director of Engineering.
• Normal pool volume of the pond based on Table 2.2. This table
was developed based on Driscoll's model (US EPA, 1986) model
based on the long-term average storm retention time.
• Normal pool minimum depth of 3 feet.
• Maximum normal pool depth of 8 feet.
• The normal pool shall have a combination aquatic vegetation and
safety bench that extends at least 10 feet into the normal pool at a
maximum slope of 6%.
• Minimum length to width ratio of normal pool of 1.5:1 (preferably
expanding outward toward the outlet).
• A sediment forebay shall be located at the upstream side of the
normal pool and should consist of a separate cell, formed by an
acceptable barrier protected from erosion.
• The volume of the sediment forebay shall be 0.1-inch times the
impervious acreage draining to the forebay. The surface area of
the forebay shall be considered part of the pond surface area.
• A permanent benchmark shall be installed in the vicinity of the
sediment forebay to allow for ready determination of the sediment
depth.
• The pond shall flow into a stormwater wetland with a permanent
pool surface area sized using Table 2.2 with an average depth of
3.0 feet.
• .A minimum of 70 percent of the wetland permanent pool surface
area shall be designed as a marsh. The marsh area shall have an
almost equal distribution of low and high marsh areas. The low
marsh areas shall be between 6 and 12-inches deep and the high
marsh area shall be between 0 and 6-inches deep.
• A soil depth of at least 4 inches should be used for shallow wetland
basins
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 17

• A micropool that is between 3 and 6 feet deep shall be located near
the outlet structure. The micropool surface area shall be at least
15 percent of the permanent pool surface area.
• The remaining permanent pool surface area shall be distributed in
a deepwater area with an average depth of 3 feet.
• 2 times the WQv shall be stored above the normal pool of the wet
pond and stormwater wetland and slowly released over at least 48
hours.
• The top of the embankment shall be a minimum 6-inches above the
100-year 24-hour elevation with 1.0 foot recommended and a
minimum of 1.0 foot above the 25-year 24-hour elevation.
• Earthen embankment side slopes should be no greater than 3:1
and shall have a well established grass cover.
• A separate emergency spillway shall be provided to convey events
greater than the 25-year storm. In special circumstances the
Director of Engineering may allow all storm events to be conveyed
through the principal spillway.
• The principal spillway shall be designed to control the design
events. If the principal spillway is a riser/barrel type, then the
design shall be such that the barrel controls during the storm event
prior to the use of the emergency spillway.
• The principal spillway shall have trash racks, hoods, or other debris
control devices as necessary to prevent clogging.
• Anti -vortex measures shall be incorporated into the trash racks to
prevent a vortex from forming during large storm events.
• The principal spillway shall be checked for buoyancy and
appropriate anti -floatation measures incorporated to maintain a
minimum factor of safety of 1.5.
• Riprap protection or other stilling basin type structure per HEC-14
shall be provided for the principal spillway outlet and all inlet
structures into the pond.
• When the principal spillway is a riser/barrel structure, anti -seep
collars or filter diaphragms shall be provided to reduce failure due
to piping.
• Pipes through an embankment shall be watertight.
• The normal pool shall be able to be drawn down to the elevation of
the outlet invert within 24 hours through the opening some type of
emergency drain (i.e. sluice gate, drawdown pipe).
• Design the emergency spillway to pass the 100-year storm event.
• Provide for vehicle maintenance access, a minimum of 15 feet
wide, to the embankment and sediment forebay along with
provisions for equipment to maneuver.
• Vehicle maintenance access paths shall be at slopes no greater
than 10 percent.
• If the normal pool area is used as temporary sediment basin during
construction, all sediment shall be removed and properly disposed
of prior to final inspection.
Recommended Standards
• The same recommended standards for the wet detention pond and
stormwater wetland apply to the pond/wetland series.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 18

Unique Calculations
. No unique calculations required.
Operation and Maintenance Recommendations
The operation and maintenance recommendations for the wet
detention pond and stormwater wetland also apply to the
pond/wetland series.
Figure 2.5 - Wet Pond/Wetland Series
POND BUFFER
SEDIMENT FOREBAY
HIGH MARSH
. ►
. . .
INFLOW WET POND
MICRO
.�.�.
y ..
SAFETY BENCH
LOW MARSH
ACCESS ROAD
100-YEAR LEVEL
EMBANKMENT
HIGH MARSH
100 YEAR (0"-6" DEPTH
25 YEAR
0 10 YEAR
2 YEAR -_
Q LEVEL -
SEDIMENT WET POND
FOREBAY � 1 MICROP00
LOW MARSH POND OUTLET BARREL
(6"-18" DEPTH) STRUCTURE
�EMEREGENCY
SPILLWAY
DAM
OUTFALL
RISER IN EMBANKMENT
-MERGENCY SPILLWAY
IN NATURAL GROUND
STABLE
OUTFALL
City of Rocky Mount December 2006
Stormwater Design Manual
2-19

2.8 Restored Riparian Buffers
The Tar -Pamlico Rules require maintenance of existing riparian
buffers and establishes how to restore a riparian buffer. This section
describes how to design the riparian buffer for nutrient reduction.
Details for the riparian buffers can be found in the Tar -Pamlico
Riparian Buffer Rule 15A NCAC 2B. 259.
Minimum Design Standards
• The use of buffers should be limited to drainage areas of 10 acres or
less with the optimal size being less than 5 acres.
• Riparian buffers must meet the Tar -Pamlico Buffer rules.
• Runoff entering the buffer must be sheet flow.
• If the runoff does not enter as sheet flow then a level spreader that
meets the most recent DWQ guidelines.
Recommended Standards
• The most damage to the level spreaders and riparian buffers
occurs during the infrequent large storm events. The City of Rocky
Mount prefers the WQv be diverted to the level spreader, and the
larger storms conveyed to the intermittent and perennial stream in
an adequately designed drainage system to prevent erosion during
the larger storm events.
• Runoff water containing high sediment loads should be treated in a
sediment trapping device before being released to a flow spreader.
• Buffers should not be cleaned of leaf litter or "managed,"
particularly Zone 1 of a regulated riparian buffer.
Unique Calculations
• The flow diversion calculations that demonstrate the WQv peak
flow is controlled through the diversion is required.
Operation and Maintenance Recommendations
• The buffer should be inspected for signs of erosion or concentrated
flow after significant rainfall events and needed repairs made
promptly to maintain sheet flow through the riparian buffer.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 20

2.9 Grassed Swales
Grassed swales are shallow trapezoidal or parabolic earthen channels
covered with a dense growth of a hardy grass such as Reed Canary
or Tall Fescue. Grassed swales are sometimes classed as a type of
biofilter because the vegetation on the swale takes up some pollutants
and helps filter sediment and other solid particles out of the runoff.
These channels convey stormwater and provide some stormwater
management for small storms by retarding peak flow rates, lowering
velocities of runoff and by infiltrating runoff water into the soil. Swales
are used primarily in single-family residential developments, at the
outlets of road culverts, and as highway medians.
Minimum Design Standards
• Longitudinal slope should be in the range of 2 to 4%. If the slope
along the flow path exceeds 4%, then checkdams must be installed
to reduce the effective slope to below 4%.
• Side slopes should be no greater than 3: 1 horizontal to vertical.
• Maximum runoff velocity should be 2 fps for the peak runoff of the 2-
year storm.
• The design must also
the 10-year storm.
non-erosively pass the peak runoff rate from
• The length of swale shall be at least 100 feet per acre of drainage
area.
• A vegetation plan shall be prepared in accordance with the
recommendations found in the Erosion and Sediment Control
Planning and Design Manual.
• Swales should be stabilized within 14 days of swale construction.
Recommended Standards
• Swales should be constructed on permeable, noncompacted soils.
• Swales should be sited in areas where the seasonal high water table
is at least one foot below the bottom of the swale.
• Swales should not carry dry -weather flows or constant flows of
water; and
• Swales should have short contact times or short grass.
Unique Calculations
• No unique calculations required.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 21

Operation and Maintenance Recommendations
• Remove excess sediment at least once annually, especially from
the upstream edge to maintain original contours and grading.
• At least once annually, repair any erosion and regrade the swale
to ensure that the runoff flows evenly in a thin sheet through the
swale.
• At least once annually, inspect vegetation and revegetate the
swale to maintain a dense growth of vegetation.
• Grassed swales shall be routinely mowed such that the maximum
height of vegetation is 6-inches.
2.10 Water Quality Swales
Water quality swales are similar to grass swales except a check dam
is installed to capture the WQv and drain it through engineered soil to
an underdrain. Larger storm events flow over the check dam.
Minimum Design Standards
• Design for the WQv.
• Only dry water quality swales will be permitted in the City and are
not suited for intermittent streams or conditions with high ground
water.
• The swale shall be designed to adequately convey the design
storm of the storm drainage system.
• Velocities during the design storm should be limited to less than 2.5
ft/s.
• The average depth of the engineered soil depth above the
underdrain shall be at least 1.0 foot.
• Bottom slopes of the swale should be graded as close to zero as
drainage will permit.
• Swale slope should not exceed 4 percent (2 percent is preferred).
• Swale cross-section should have side slopes of 3:1 (h:v) or flatter.
• Engineered soil shall have a high permeability (fc > 0.5 inches per
hour) and be of the same quality defined for the bioretention facility.
• Dense cover of a water tolerant, erosion resistant grass should be
established.
Recommended Standards
• As a BMP, water quality swales are limited to residential or
institutional areas where the percentage of impervious area is
relatively small.
• Seasonally high water table should be more than 3 feet below the
bottom of the swale.
• Check dams can be installed in swales to promote additional
infiltration. The recommended method is to sink a railroad tie
halfway into the swale. Riprap stone should be placed on the
downstream side to prevent erosion.
• Maximum ponding time behind a check dam is to be less than 48
hours. Minimum ponding time of 30 minutes is recommended to
meet water quality goals.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 22

Unique Calculations
• The water quality volume storage calculation is required.
tion and Maintenance Recommendations
• A stormwater maintenance manual is required for each facility.
The manual should require the owner of the water quality swale to
periodically clean the underdrain.
• Water quality swale should be maintained to keep grass cover
dense and vigorous.
• Maintenance should include periodic mowing, occasional spot
reseeding, and weed control. Swale grasses should never be
mowed close to the ground. Grass heights in the 4 to 6 inch range
are recommended.
• Fertilization of water quality swale should be done when needed to
maintain the health of the grass, with care not to over -apply the
fertilizer.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 23

Figure 2.6 - Schematic of a Water Quality Swale - Cross Section
ENGINEERED SOIL WATER QUALITY
INFLOW CHECK DAM
GRASS CHANNEL W/ (SEE DETAIL)
EROSION CONTROL MATTING
WQvolume
SEDIMENT FOREBAY—/ 8'' 157 STONE
SEE DETAIL) WRAPPED IN FLITER
FABRIC 6'' UNDERDRAIN PERFORATED
W/ GRAVEL JACKET DOWNSTREAM
PROFILE CRASS CHANT
Figure 2.7 - Schematic of a Water Quality Swale — Profile
VARIES
SHOULDER -
BOTTOM WIDTH W ROADWAY
10 YEAR LEVEL
�'IIII�I 2 YEAR LEVEL Din ���ilg„�
lullll=i, WQV LEVEL allll�l
WATER QUALITY �I 3:1 SLOPE OR FLATTER
CHECK DAM
=W-
II=III_ _
GRASS CHANNEL W/
3:1SLOPE TEROSION CONTROL MATTING
OR FLATTER
YA y� y yA yA y yA yQ ENGINEERED SOIL,
12- INCH AVERAGE DEPTH
6IP" UNDERDRAIN PERFORATED
PE WRAPPED IN FILTER FABRIC_
" 57 STONE - B-INCH DEPTH
WRAPPED FILTER FABRIC
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 24

2.11 Vegetated Filter Strips with Level Spreader
Filter strips are uniformly graded and densely vegetated sections of
land engineered and designed to treat runoff and remove pollutants
through vegetative filtering and infiltration. Filter strips are best suited
to treating runoff from roads and highways, roof downspouts, very
small parking lots, and pervious surfaces. They are also ideal
components of the 'outer zone" of a stream buffer, or as pretreatment
for another structural stormwater control. Filter strips can serve as a
buffer between incompatible land uses, be landscaped to be
aesthetically pleasing, and provide groundwater recharge in areas
with pervious soils. Filter strips are often used as a stormwater site
design credit.
Filter strips rely on the use of vegetation to slow runoff velocities and
filter out sediment and other pollutants from urban stormwater. There
can also be a significant reduction in runoff volume for smaller flows
that infiltrate pervious soils while contained within the filter strip. To be
effective, however, sheet flow must be maintained across the entire
filter strip at a depth of around 1-inch. Once runoff flow concentrates,
it effectively short-circuits the filter strip and reduces any water quality
benefits.
Special provisions must be made to ensure design flows spread
evenly across the filter strip. Therefore, a level spreader must be
included in the filter strip design. Filter strips are susceptible to
damages from large intense storm events. Once damaged, the filter
strip will no longer provide the pollutant removal and could damage
the level spreader structure. Therefore, the City of Rocky Mount
prefers that the WQv be diverted to the vegetated filter strip to reduce
the potential damage from large storm events.
Minimum Design Standards
• Filter strips shall be constructed outside the riparian stream buffer
area.
• Filter strips shall be designed for slopes between 2% and 6%.
Greater slopes than this would encourage the formation of
concentrated flow. Flatter slopes would encourage standing
water.
• Filter strips should not be used on soils that cannot sustain a
dense grass cover with high retardance. Designers should
choose a grass that can withstand relatively high velocity flows at
the entrances, and both wet and dry periods.
• The width of the filter strip shall maintain the WQv peak flow at
maximum depth of 1-inch.
• The length of the filter strip shall be designed such that the 2-year
24-hour storm flow has a minimum travel time of 5 minutes.
• In no case shall the vegetated filter strip be less than 15 feet long
to provide filtration and contact time for water quality treatment.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 25

• The WQv peak flow shall be diverted to the level spreader unless
calculations demonstrate the flow depths for the larger events do
not cause erosive velocities.
• When the WQv is diverted, the flow from the larger storms shall be
conveyed in an adequate drainage system to the receiving stream
or drainage system.
• The length of the level spreader shall be equal to the width of the
vegetated filter strip.
• The level spreader lip shall be constructed at 0% grade and be of
a rigid material that will not deform over time.
• The level spreader shall have a depth of at least 6-inches to allow
for the water to pond up prior to flowing into the vegetated filter
strip.
• The bottom of the level spreader shall have weep holes and a
gravel drain to prevent standing water.
• Pedestrian traffic across the filter strip should be limited through
channeling onto sidewalks.
Recommended Standards
• The vegetated filter strip should be sodded. Establishing a dense
uniform grass from seed is difficult and often requires multiple
growing seasons.
• If the drainage area is expected to be heavy in sediments, a
sediment forebay should be installed prior to the level spreader.
• The preferred diversion structure is a drop inlet with a modified
bottom as shown in Figure 2.8.
OUTFLOW
PIPE
Nd+ — DEPTH OF
DI VERSIDN
Figure 2.8 — Diversion Structure
INLET STRUCTURE
INFLOW
PIPE
TOP OF WATER QUALITY OUTLET TRASH
RACK AT OUTLET PIPE INVERT
WATER QUALITY OUTLET
(DIVER5I0N TO BW r
SHAPE BOTTOM
WITH CONCRETE
OR GROUT
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 26

Unique Calculations
• The filter strip width is based on the maximum discharge loading per
foot of the filter strip. This is determined using the following form of
the Manning's equation:
0.00236 Y513S112
q=
n
Where:
q = discharge per foot of width of filter strip (cfs/ft)
Y = allowable depth of flow (inches)
S = slope of filter strip (percent)
n = Manning's "n" roughness coefficient
(use 0.15 for medium grass, 0.25 for dense grass, and 0.35 or
very dense Bermuda -type grass)
Therefore the minimum width of the filter strip is:
Wfmin=Q
q
Where:
Wf min = minimum filter strip width perpendicular to flow (feet)
Q = WQv peak flow (cfs)
To determine the minimum length of the filter strip, use the overland
travel time equation from the NRCS time of concentration
methodology presented in Chapter 3 and solve for length. The
equation is provided below:
(Tt)1.25 (P )0.625 (S)0.5
L f= 2 24
3.34(n)
Where:
Lf = length of filter strip parallel to flow path (ft)
Tt = minimum travel time through filter strip (minutes)
P2-24 = 2-year, 24-hour rainfall depth (inches)
S = slope of filter strip (percent)
n = Manning's "n" roughness coefficient (use 0.15 for medium
grass, 0.25 for dense grass, and 0.35 for very dense
Bermuda -type grass)
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 27

Minimum Operation and Maintenance Requirements
• Filter strips require similar maintenance to other vegetative
practices. Maintenance is very important for filter strips,
particularly in terms of ensuring that flow does not short circuit the
practice.
• Mow grass regularly (frequently) to maintain a 2 to 4 inch height.
• Inspect level spreader and remove built up sediment.
• Inspect the diversion structure and remove collected debris and
properly dispose.
• Inspect vegetation for rills and gullies and correct. Seed or sod
bare areas.
• Inspect to ensure that grass has established. If not, replace with
an alternative species.
Figure 2.9 - Filter Strip with Level Spreader
4:1 CUT EX GROUND
------- ---- -- / -------------------------------------
LEVEL SPREADER
FILTER STRIP V DITCH
90 100 110 120
3:1 RETURN TO GRADE
130 140 150
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 28

2.12 Bioretention (Rain Gardens)
Bioretention areas, or rain gardens, are structural stormwater controls
that capture and temporarily store the WQ, using soils and vegetation
in landscaped areas to remove pollutants from stormwater runoff as
shown in Figure 2.10. Bioretention areas are engineered facilities in
which runoff is conveyed to the "treatment area," consisting of a
sediment forebay ponding area, organic or mulch layer, planting soil,
and vegetation. The filtered runoff is collected in an underdrain
system and returned to the site drainage system. Although
bioretention areas have some affect on peak flows, it is not
recommended these areas be used for site peak flow reduction
control because this will likely require ponding depths that may
destroy the vegetation.
Bioretention areas are also susceptible to damage from large intense
storm events. These events can overwhelm a facility dislocating or
removing the mulch layer and damaging the vegetation. Once
damaged, the bioretention area does not provide the pollutant
removal and typically becomes a nuisance given that they are
generally located in highly visible areas. Therefore, the City of Rocky
prefers that the WQv be diverted to the bioretention area to reduce
the potential damage from large storm events.
Minimum Design Standards
• Design for the WQv.
• The WQv shall drain through the bioretention within 2 days with the
filter (engineered soil) coefficient of permeability (k) of 0.5 ft/day.
• The engineered soil shall be a minimum depth of 2.5 feet and
consist of sandy loam, loamy sand or loam texture with a clay
content rating from 10 to 25 percent.
• The engineered soil must have an infiltration rate of at least 0.5
inches per day and a pH between 5.5 and 6.5.
• The engineered soil must have a 1.5 to 3 percent organic content
and a maximum 500-ppm concentration of soluble salts.
• The engineered soil must have a Phosphorus Index between 20
and 40.
• The maximum ponding depth above the mulch layer is 6-inches.
• The contributing drainage area must be 5 acres or less, though 0.5
to 2 acres is preferred.
• The WQv shall be diverted to the bioretention area unless
calculations demonstrate the larger storm events do not increase
the ponding depth to more than 9-inches above the mulch layer
and the larger flow events can be adequately dispersed through the
area.
• Sediment forebays shall be designed to handle particles greater
than 40 microns in size.
• Sediment forebays shall be separated from the engineered soil with
an impervious layer of material to prevent short circuiting to the
underdrain system.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 29

• The sediment forebay shall be used to sheet the flow across the
mulch area.
• The mulch layer should consist of approximately 3 inches of
commercially available triple hardwood mulch.
• The underdrain collection system should be equipped with a 6-inch
perforated schedule 40 PVC or greater strength pipe in an 8-inch
gravel layer wrapped in filter fabric. The pipe should have 3/8-inch
perforations, spaced on 6-inch centers with a minimum of 4 holes
per row.
• Underdrain pipes should be spaced at a maximum of 10 feet on
center, and a minimum grade of 0.5% must be maintained.
• The depth to the water table from the bottom of the bioretention
facility to the high water table should be a minimum of 2 feet.
• Runoff captured by the facility must be sheet flow to prevent
erosion of the organic or mulch layer.
• Bioretention areas are designed for intermittent flow and to drain
and aerate between rainfall events. Sites with continuous flow from
groundwater, sump pumps or other areas should be avoided.
• An overflow structure and a non -erosive overflow channel must be
provided to safely pass the flow from the bioretention area that
exceeds the storage capacity to a stabilized downstream area.
The high flow structure within the bioretention area can consist of a
grated drop inlet with the top set 6 inches above the mulch layer.
• If the bioretention area is confined by an earthen berm, the top of
the berm should be set with 6-inches of freeboard above the
anticipated maximum water surface elevation.
Unique Calculations
• The sediment forebay surface area shall be designed using the
Camp -Hazen equation, which accounts for the effects of turbulent
flow. This equation is provided below:
As = Q0 �E')
Where:
As = sedimentation basin surface area (ft)
Qo = discharge rate from basin = (WQv/24 hr)
W = particle settling velocity (ft/sec)
0.0033 ft/sec (particle size=40 microns)
E'= sediment trapping efficiency constant; for a sediment
trapping efficiency (E) of 90%, E' = 2.30
The sediment trapping efficiency constant (E) may be calculated from
the sediment trapping efficiency (E) using the following equation:
E' = — Ink — (Y1001]
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 30

The equation reduces to:
Asf = (0.0081XWQv) for 40 micron particle size
Where:
Asf = sedimentation basin surface area full (square feet)
The required bioretention area is based on the Darcy equation. This
equation calculates the required surface area using the following
equation:
(WQvX df )
Af _ ((kXhf + df Xtf ))
Where:
Af = Surface area of filter bed (ft)
WQv = water quality volume (ft)
df = filter bed depth (ft)
k = coefficient of permeability of filter media (ft/day)
hf = average height of water above filter bed (ft)
tf = design filter bed drain time (days)
Recommended Standards
• Bioretention areas can be incorporated into the landscaping plan
as depressed parking lot islands.
• A dense and vigorous groundcover should be established over the
contributing pervious drainage area before runoff can be diverted
into the facility.
• Use native plants, selected based upon hardiness and hydric
tolerance.
Operation and Maintenance Requirements
• The bioretention area should be periodically cleaned and dead,
dying or diseased plant material replaced.
• Inlets should be inspected for signs of erosion after every
significant rainfall.
• Sediment forebay should be cleaned when 50% full of sediment.
• The sediment forebay lip should be inspected to ensure sheet flow.
• Vegetation should be kept healthy.
• The mulch will need to be replaced and/or replenished on an
annual basis. If the mulch layer becomes substantially clogged
with sediment, remove the mulch and replace.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 31

Figure 2.10 - Bioretention Area Plan View
PROTECTED OUTFALL
DIVERTED INLET
FLOW
PARKING AREA
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 32

Figure 2.11 - Bioretention Area Section View
LANDSCAPE PLANTINGS
2"-3'' MUL
6'' PONDING
WATER
�� ..
QUALITY
PIPE �� �1� �� 2.5 FT. ENGINEERED SOIL
SEDIMENT FOREBAY 4 ..� � ..
6IP" UNDERDRAIN OF PERFORATED
PE WRAPPED IN FILTER FABRIC
CLEAN WASHED AT LOCATION SHOWN ON THE
WRAP STONE WITH FILTER FABRIC «57 STONE DESIGN DRAWING.
BOTTOM
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 33

2.13 Sand Filters
Sand Filters are structural stormwater controls that capture and
temporarily store the WQ, either in open areas (as shown if Figure
2.12) or in underground vaults (as shown in Figure 2.13) and allow the
water to filter through a sand media. These structural BMPs are
similar to Bioretention areas except they can be installed under
ground in vaults. Most sand filters have two chambers. The first
chamber is a sedimentation chamber that removes floatables and
other heavy sediments. The second chamber is the filtration
chamber. This chamber removes additional pollutants by filtering the
runoff through a sand bed. The filtered runoff is collected in an
underdrain system and returned to the site drainage system.
Although sand filters areas have some affect on peak flows, it is not
recommended these areas be used for site peak flow reduction
control.
Sand Filters areas are also susceptible to damage from large intense
storm events. These events can overwhelm a facility dislocating and
shifting the sand media and resususpending trapped sediments.
Therefore, the City of Rocky Mount prefers that the WQv be diverted
to the sand filters to reduce the potential damage from large storm
events.
Minimum Design Standards
• Maximum contributing drainage area to an individual sand filter
should be less than 5 acres.
• Design volume based on WQv.
• Designed to completely empty in 36 hours with a sand bed depth of
at least 18-inches.
• Sand permeability shall be 3.5 ft/day.
• A diversion structure shall be located such that the sediment
chamber and filter only receive the WQv and large storms are
adequately bypassed.
• A sediment chamber shall be designed to capture the 40 micron
particle size. The volume of the sediment forebay should be
subtracted from the WQv when calculating the required surface
area of the media.
• Inlet structure to the sand media should be designed to spread the
flow uniformly across the surface.
• The underdrain collection system should be equipped with a 4 inch
perforated schedule 40 PVC or greater strength pipe and wrapped
in filter fabric. The pipe should have 3/8-inch perforations, spaced
on 6-inch centers with a minimum of 4 holes per row.
• Minimum grade of underdrain piping should be 0.5 %.
• Access for cleaning all underdrain piping should be provided.
• Surface filters may have a grass cover to aid in pollution
adsorption.
• Sand filters shall not be placed into service until the ground cover
of the contributing drainage has been established.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 34

Unique Calculations
• The sediment forebay surface area shall be designed using the
Camp -Hazen equation presented in Chapter 3.
• The filter surface area shall be designed using the Darcy equation
presented in Chapter 3.
Recommended Standards
• Incorporating an outlet device that will also trap oil and grease will
enhance the performance of the sand filter and will decrease the
maintenance frequency required to maintain effective
performance.
• If a surface sand filter is used, incorporate a vegetative cover to
increase nutrient removal potential.
Operation and Maintenance Recommendations
• Scrape off sediment layer buildup during dry periods with steel
rakes or other devices.
• Replace some or all of the sand when permeability of the filter
media is reduced to unacceptable levels, which should be specified
in the design of the facility. A minimum infiltration rate of 0.5 inches
per hour should be used for all infiltration designs.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 35

Figure 2.12 - Surface Sand Filter
Source: Center for Watershed Protection
FLOW DIVERSION UNDERDRAIN COLLECTION SYSTEM
BYPASS STRUCTURE FILTER BED
I
® I® PRETREATMENT OUIjFLO�
SEDIMENTATION
® ® CHAMBER — — _
—� O V ERFLO W
1 SPILLWAY
PLAN VIEW
FLOW DIVERSION PERFORATED STANDPIPE
STRUCTURE DETENTION STRUCTURE
INFLOW
— FILTER BED OVERFLOW
SPILLWAY
PRETREATMENT I
OUTFLOW
II II II II II �I�II Il Alma 11 Il o-ll II III IL
UNDERDRAIN COLLECTION SYSTEM
3` TOPSOIL
FILTER FABRIC
18` CLEAN WASHED
CONCRETE SAND
FILTER FABRIC
6" PERFORATED PIPE/GRAVEL
UNDERDRAIN SYSTEM
TYPICAL SECTION
PR❑TILE
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 36

Figure 2.13 Underground Sand Filter
2.14 Permeable Pavement
Traditional paved surfaces, such as asphalt and concrete, do not
allow water to infiltrate and convert almost all rainfall into runoff. If
designed and implemented correctly, permeable pavement systems,
as shown in Figure 2.14, allow at least a portion of stormwater to
infiltrate, thus reducing peak runoff volumes and flows. Permeable
paving materials include, but are not necessarily limited to, porous
concrete, permeable interlocking concrete pavers, concrete grid
pavers, and porous asphalt. Compacted gravel will not be considered
as permeable pavement.
Design and installation of permeable pavement systems must be
performed by appropriate professionals. The primary factors that
should direct permeable pavement design include the following:
1. Providing adequate infiltration and temporary storage
2. Preventing sediment, oils, and greases from reaching the
permeable pavement surface where they have the potential to
clog
3. Using construction techniques that minimize the compaction of
subsurface soils
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 37

FIGURE 2.14
Various permeable pavement systems
Courtesy of NC State University — Biological and Agricultural Engineering Department
Permeable Interlocking
Concrete Pavers (PICP)
Concrete Grid Pavers
(CP) "Turfstone"
Porous Asphalt (PA)
Porous Concrete (PC)
Plastic Turf Reinforcing
Grids (PTRG)
Minimum Design Standards
• A washed aggregate base must be used, and washed 57-size stone is
generally acceptable. Fine particles from standard "crusher run" will
clog the pores at the bottom of the pavement and will not be allowed.
• Traffic volume must be less than 100 vehicles per day.
• As shown in Figure 2.14 below, seasonally high water table must be at
least 2 ft from the base of the permeable pavement or gravel storage
layer. Water tables approaching the permeable pavement system will
not allow water to exfiltrate.
FIGURE 2.15
Schematic of water table design constraint.
In -situ Soil 24 inches
0
Permeable Pavement surface layer
Gravel Storage Layer
Seasonally high water table
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 38

• The completed permeable pavement must be installed at a grade
less than 0.5%. Steeper slopes will reduce the storage capacity of
the permeable pavement.
• Permeable pavement systems are not allowed in areas, such as
buffers, where impervious surfaces are not permitted.
The construction sequence must be inspected to insure that the
surface installation is planned to be completed after adjacent areas
are stabilized with vegetation. Run-on to the permeable pavement
from exposed areas can cause the system to perform ineffectively.
The in -situ soils beneath the permeable pavement must have
sufficient infiltration capacity for the permeable pavement to drain.
To satisfy this requirement, the following conditions must be met:
o The footprint of the permeable pavement installation must
have a vertical saturated hydraulic conductivity of at least 2
in/hr for the top 3 ft of soil as determined by a soil analysis.
o The top 3 ft of soil must also have no finer texture than
Loamy Very Fine Sand as defined by the United States
Department of Agriculture — Natural Resources Conservation
Service (USDA-NRCS) and as determined by a soil analysis.
o Only 2 ac-ft of soil per acre disturbed can be moved for the
footprint of the permeable pavement. Mass grading can
significantly alter the site's applicability for permeable
pavement. If mass grading occurs and conditions (a) and (b)
are still met, then an exception for this requirement can be
given. However, a soil analysis will be required after the
grading is completed to verify the soil properties.
Recommended Specifications
• Permeable pavement should not be placed where upland land
disturbance is occurring or will potentially occur. Land disturbance
upland of the lot could result in frequent pavement clogging.
• Avoid overhanging trees above the permeable pavement installation.
• During preparation of the subgrade, special care must be made to
avoid compaction of soils. Compaction of the soils can reduce the
infiltration capacity of the soil.
• Permeable pavement should not be designed to receive
concentrated flow from roofs or other surfaces. Incidental run-on
from stabilized areas is permissible, but the permeable pavement
should primarily be designed to infiltrate the rain that falls on the
pavement surface itself. No credit will be given for volume or peak
reduction for run-on from impervious surfaces.
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 39

Unique Calculations
Permeable pavement will not receive direct credit for any pollutant
removal (percent reduction, e.g.). However, for the purpose of
meeting pollutant control requirements, credit received for reducing
percent imperviousness will have the effect of reducing pollutant
loads. The extent of pollutant reduction will depend on the site
configuration.
For permeable pavement systems, credit will be given such that a
portion of the permeable pavement will not be counted as impervious
for built -upon -area calculations. Depending on the type of system
used and its construction (see Table 2.4), a portion of the permeable
pavement will be counted as "managed grass." The remainder will be
counted as impervious.
TABLE 2.4: CREDIT RECEIVED FOR VARIOUS PERMEABLE PAVEMENT
SYSTEMS
Permeable Pavement System Credit as Percent Managed Grass
Permeable concrete without gravel base 40 %
Permeable concrete with at least 6" of gravel base 60 %
Flexible pavements with at least 4" of gravel base 40 %
Flexible pavements with at least 7" of gravel base 60 %
Operation and Maintenance Recommendations
Maintenance requirements are critical for the success of permeable
pavement. The following installation and maintenance requirements
are designed to ensure that the permeable pavement system will work
effectively. A maintenance agreement is required for each permeable
pavement installation to receive credit. The maintenance agreement
should include specific requirements and responsibilities of the
property owner and provide for enforcement.
Inspect monthly for first 3 months after installation.
Inspect annually after first three months.
Inspections should ensure that:
o Permeable pavement surface is free of sediment
o Contributing and adjacent areas are stabilized and mowed
with clippings removed
o There is no deterioration of pavement system
o The permeable pavement system is dewatering between
storm events
Perform repairs of permeable pavements with similar
permeable materials
• Vacuum sweep permeable pavement surface annually
City of Rocky Mount December 2006
Stormwater Design Manual 2 - 40

City of Rocky Mount, North Carolina
Stormwater Design Manual
iltROCKY
MOULT Chapter 3: Stormwater Design
Calculations
December 2006
Rocky Mount Engineering Department
One Government Plaza
Rocky Mount, NC 27802

Chapter 3: Stormwater Design Calculations
3.1 introduction
The design of properly sized storm drainage systems requires some
knowledge of the hydrologic behavior of the watershed in question
and hydraulic principles of fluids. For adequate design of gutters,
catch basins, inlets, storm drainage pipes, open channels and culverts
it is appropriate to estimate the peak discharge of the drainage area
for the required design frequency. The peak discharge is then used to
calculate the capacity of the storm drainage system based on the
system's hydraulic characteristics.
When a watershed is large and complicated, it may be necessary to
generate, route and add hydrographs to determine the peak
discharge. When it is necessary to control the peak flow, hydrographs
must be generated and routed to demonstrate the peak flow
reduction. When routing hydrographs, it is easiest and most reliable
to use an established computer model as long as the model utilizes
the acceptable methodologies presented in this manual.
This chapter presents the accepted hydrologic and hydraulic
calculations and methodologies to generate peak discharges and
hydrographs for use in the design of stormwater drainage systems
within the City of Rocky Mount. This section is not intended to be a
design reference manual and the designers are expected to be
familiar with the identified source publications.
3.2 Computer Software
The City of Rocky Mount Engineering Department utilizes the
Hydraflow software packages to perform internal designs and
independent design verifications on peak discharges, hydrograph
generation, impoundment routings, storm drainage pipe design,
spread calculations and culvert design. The City of Rocky Mount
acknowledges that there are numerous software packages that
adequately perform the calculations described in this chapter and
some calculations can be adequately performed without the use of
software packages. The software packages listed in Table 3.1 are
accepted by the City of Rocky Mount as long as the methods and
parameters described in this Chapter are followed.
City of Rocky Mount
Stormwater Design Manual December 2006 3-1

3.3 Peak Discharge
Table 3.1 — Computer Software
Software
Typical Use
H draflow H dro raphs
Peak Flows, H dro raph Routings
Hydraflow Storm Sewers
Storm Drainage Pipes, HGL's, Gutter
spread, Inlet Design
H draflow HEC-RAS
Culverts, Floodplains, Open Channels
HEC-HMS
Peak Flows, H dro raph Routings
HEC-RAS
Culverts, Irregular Channels
Pond Pack
Peak Flows, Hydrograph Routings
Storm CADD
Storm Drainage Pipes, HGL's, Gutter
Spread, Inlet Design
TR-20
Peak Flows, H dro raph Routings
HY-8
Culvert
HEC-12
Gutter Spread, Inlet Design
In order to maintain consistency with design calculations, the City has
provided design forms and checklists in Appendix B that must be
completed by the designer. The designer must also provide the City
with a copy of the relative computer outlet, inlet and output design
files.
In special scenarios, other software packages or methods described
in this chapter are more appropriate. If the designer prefers to use a
computer model other than that described here, the designer must
receive written authorization from the Director of Engineering. When
requesting the use of another model, the designer must provide a
written description for the need to use the computer software or other
methodology along with supporting technical information to evaluate
the request.
The City of Rocky Mount allows peak discharges to be calculated
using either the Rational Method or the NRCS (SCS) Method. These
two methods are presented below.
3.3.1 Rational Equation
The rational formula estimates the peak rate of runoff at any location
in a watershed as a function of the drainage area, runoff coefficient,
and mean rainfall intensity for a duration equal to the time of
concentration (the time required for water to flow from the most
remote point of the basin to the location being analyzed). The rational
formula is expressed as follows:
City of Rocky Mount
Stormwater Design Manual December 2006 3-2

Rational Equation Formula
Q = (CXI X A)
Where:
Q = peak flow from the drainage area (cfs)
C = coefficient of runoff (dimensionless)
I = rainfall intensity for a given time to peak (in/hr)
A = drainage area (acres)
The rational equation is based on the assumption that rainfall is
uniformly distributed over the entire drainage area at a steady rate,
causing flow to reach a maximum at the outlet to the watershed at the
time to peak (Tp). The rational method also assumes that all land
uses within a drainage area are uniformly distributed throughout the
area. If it is important to locate a specific land use within the drainage
area, then another hydrologic method should be used where
hydrographs can be generated and routed through the drainage
system.
The rational equation shall only be used for drainage areas less than
100 acres.
3.3.1.1 Runoff Coefficient
The runoff coefficient (C) is the variable of the rational method least
susceptible to precise determination and requires judgment and
understanding on the part of the design engineer. While engineering
judgment will always be required in the selection of runoff coefficients,
typical coefficients represent the integrated effects of many drainage
basin parameters. Table 3.1 gives the recommended runoff
coefficients for the rational method.
City of Rocky Mount
Stormwater Design Manual December 2006 3-3

Table 3.1 Recommended Runoff Coefficient Values
(Sources: North Carolina Erosion and Sediment Control Planning and Design
Manual and The City of Rocky Mount's Minimum Storm Drainage Design
Requirements)
Description of Area Runoff Coefficient, C
Woodlands 0.20 - .025
Parks, cemeteries 0.25
Playgrounds 0.30
Lawns and Cropland:
Sandy soil, flat, 2%
0.10
Sandy soil, average, 2 - 7%
0.15
Sandy soil, steep, > 7%
0.20
Clay soil, flat, 2%
0.17
Clay soil, average, 2 - 7%
0.22
Clay soil, steep, > 7%
0.35
Graded or no plant cover
Sandy soil, flat, 0-5%
0.30
Sandy soil, average, 5 - 10%
0.40
Clay soil, flat, 0-5%
0.50
Clay soil, average, 5-10%
0.60
Residential
R-15, very low density
0.50
R-10. low density
0.50
R-8, manufactured
0.55
R-6, single family
0.55
R-6MFA, medium density multi -family
0.60
MFA, multi -family
0.70-0.75
MHP, mobile home park
0.75
Business:
O & I and all B Zones 0.85
All Industrial Zones 0.85-0.95
Commercial/Shopping Centers 0.85-0.95
Streets
Gravel areas 0.50
Drives, walks, roofs 0.95
Asphalt and Concrete 0.95-1.00
City of Rocky Mount
Stormwater Design Manual December 2006 3-4

It is often desirable to develop a composite runoff coefficient based on
the percentage of different types of surfaces in the drainage areas.
Composites can be made with the values from Table 3.1 by using
percentages of different land uses, as illustrated in the Composite C
Equation below. In addition, more detailed composites can be made
with coefficients for different surface types such as roofs, asphalt, and
concrete streets, drives and walks. The composite procedure can be
applied to an entire drainage area or to typical "sample" blocks as a
guide to the selection of reasonable values of the coefficient for an
entire area.
Composite C Equation:
CompositeC - (C1XA1)+ (C1XA2)+ ....(CxX Ax)
Al+A2+..... Ax
3.3.1.2 Rainfall Intensity
The rainfall intensity (1) is the average rainfall rate in inches/hour for a
duration for a selected return period. The duration is equal to the time
of concentration (Tc) for the drainage area. Acceptable time of
concentration methods is presented in section 3.1.3. Once a
particular return period has been selected for design and a time of
concentration calculated for the drainage area, the rainfall intensity
can be determined from the intensity -duration -frequency (IDF) data for
the City of Rocky Mount given in Table 3.2.
Table 3.2 Intensity - Duration - Frequency Table
City of Rocky Mount, NC
Tc
(duration)
Frequency (Yrs)
1
2
5
10
25
50
100
5 mins
4.48
5.76
6.58
7.50
8.19
8.96
9.72
10
3.73
4.76
5.54
6.13
7.01
7.71
8.40
15
3.20
4.04
4.74
5.25
6.03
6.64
7.24
20
2.80
3.47
4.12
4.64
5.42
5.93
6.47
30
2.49
2.70
3.28
3.71
4.32
4.80
5.28
40
1.87
2.28
2.77
3.15
3.70
4.08
4.48
50
1.60
1.94
2.38
2.71
3.19
3.53
3.88
60
1.40
1.70
2.12
2.41
2.84
3.17
3.50
90
1.02
1.22
1.52
1.74
2.06
2.29
2.53
2 hr
0.80
0.95
1.20
1.37
1.62
1.81
2.00
3
0.56
0.71
0.89
1.02
1.21
1.35
1.50
6
0.30
0.44
0.56
0.65
0.77
0.86
0.96
12
0.15
0.26
0.33
0.39
0.46
0.52
0.57
24
0.08
0.15
1 0.19
0.22
1 0.27
1 0.30
1 0.33
City of Rocky Mount
Stormwater Design Manual December 2006 3-5

3.3.2 NRCS (SCS) Method
3.3.2.1 Runoff Volume
The NRCS (SCS) method utilizes the Runoff Curve Number (RCN),
Type II distribution storm event, and the SCS Dimensionless Unit
Hydrograph to calculate a peak discharge. This method has been
incorporated into many computer software packages and is the
preferred method in the City of Rocky Mount when comparing pre -
developed and post -developed peak discharges for the various
frequency events. The graphical method to calculate peak discharges
is summarized in this section and is described in detail in the SCS
Urban Hydrology for Small Watersheds, Technical Release No. 55,
Second Edition.
The graphical method is limited to the family of la/P curves presented
later. This method should not be used when results are outside the
family of curves provided. In this case, a computer model should be
used as long as the computer model parameters fall within the time
step limitations to adequately model the entire storm event.
The NRCS method requires the calculation of the amount of water
during a given rainfall event that will not soak into the ground or fill up
small voids in the surface. This amount of runoff is based on the
RCN. The amount of runoff is converted to a peak discharge based
on the relationship between the family of curves developed by the
SCS using the Type II storm distribution and the SCS Dimensionless
Unit Hydrograph.
The amount of rainfall that turns into runoff is based on the land cover
type, soil type, and antecedent moisture content (AMC). NRCS
developed RCN's based on these three parameters. Table 3.3
summarizes the RCN's for the AMC II conditions, which is considered
normal. The soil types are divided into four major hydrologic soil
groups denoted by the letters A through D. A soils are those which
have high infiltration capacity and subsequently low runoff rates. D
soils are those with very low infiltration capacity and very high runoff
rates. A list of soils common in Nash and Edgecombe Counties can
be obtained from the respective counties local NRCS field office.
City of Rocky Mount
Stormwater Design Manual December 2006 3-6

1
Table 3.3 Runoff Curve Numbers
Cover Description
Curve Numbers for Hydrologic
Soil Groups
Covertype and hydrologic condition
A
B
C
D
Cultivated land:
without conservation treatment
72
81
88
91
with conservation treatment
62
71
78
81
Pasture or range land
poor condition
68
79
86
89
good condition
39
61
74
80
Meadow:
good condition
30
58
71
78
Wood or forest land:
thin stand, poor cover
45
66
77
83
good cover
25
55
70
77
Open Space (lawns, parks, golf courses,
cemeteries, etc.) 2
Poor condition (grass cover <50%)
68
79
86
89
Fair condition (grass cover 50% - 75%)
49
69
79
84
Good condition (grass cover >75%)
39
61
74
80
Impervious areas:
Paved parking lots, roofs, driveways, etc.
98
98
98
98
(excluding right-of-way)
Streets and Roads:
Paved; curbs and storm drains (excluding right-
98
98
98
98
of -way)
Paved; open ditches (including right-of-way)
83
89
92
93
Gravel (including right-of-way)
76
85
89
91
Dirt (including right-of-way)
72
82
87
89
Developing urban areas and newly graded
77
86
91
94
areas (pervious area only, no vegetation)
Urban districts by zoning:
O& 1 and all B-Zones
96
97
98
98
Industrial Zones
98
98
98
98
Commercial/Shopping Centers
Residential districts by zoning:
R-15, very low density
61
75
83
87
R-10. low density
61
75
83
87
R-8, manufactured
71
80
87
92
R-6, single family
71
80
87
92
R-6MFA, medium density multi -family
80
85
90
95
MFA, multi -family
86
90
93
96
MHP, mobile home park
92
94
96
97
1 Average runoff condition, and la = 0.2S
2 CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type.
Assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and
pervious areas are considered equivalent to open space in good hydrologic condition. If the impervious area is not connected, the
NRCS method has an adjustment to reduce the effect.
City of Rocky Mount
Stormwater Design Manual December 2006 3-7

3.3.2.2 Runoff Volume Equation
The volume of flood runoff can be calculated by the following
equation.
(P — Ia)2
Q _ (P—Ia)+S
Where:
Q = accumulated direct runoff (in.)
P = accumulated rainfall (potential maximum
runoff) (in.) found in Table 3.4
Ia = initial abstraction including surface storage,
interception, and infiltration prior to runoff (in.)
= 0.2*S
S = potential maximum soil retention (in.)
1000 10
RCN
)—
Table 3.4 Accumulated 24-hour Rainfall Frequency
City of Rocky Mount, NC
Duration Frequency (Yrs)/Rainfall (inches)
1 2 5 10 25 50 100
24-hour 1 3.20 3.60 4.56 5.28 6.48 7.20 8.0
3.3.2.3 NRCS Peak Discharge
The peak discharge equation used by the Natural Resources
Conservation Services has the form:
Qp = (QuXAXQXFp)
Where:
Qp =
peak discharge (cfs)
Qu =
unit peak discharge found from Figure 3.1 (csm/in)
A =
drainage area (sq mi)
Q =
runoff depth (in)
Fp =
pond and swamp adjustment factor from Table 3.5
City of Rocky Mount
Stormwater Design Manual December 2006 3-8

Table 3.5. Swamp Correction Factors
Percentage of pond or swamp areas
Fp
0
1.00
0.2
0.97
1.0
0.87
3.0
0.87
5.0
0.72
Note that swamp correction factors should only be used if the area will
remain in place. If there is a possibility the area will be re -graded in
the future, no correction should be made.
Figure 3.1 NRCS Type II Unit Peak Discharge Graph
1�
y
O O O O p O Op 00 O O CD
O � tD to V Cl)CO tD In
ui/ws: °(nb) a6.1Vg3SLp 12ad ;iun
N
L
7
N L
a I.-
N
(Source: NRCS TR-55 Urban Hydrology for Small Watersheds, Second Edition, June 1986)
City of Rocky Mount
Stormwater Design Manual December 2006 3-9

3.3.3 Time of Concentration
Use of the rational formula and the NRCS Unit Hydrograph requires
the time of concentration (tc) for each design point within the drainage
basin. The time of concentration is considered the longest time for
which the stormwater runoff has to travel to the design point. The
time of concentration typically consists of an overland flow (sheet
flow) time, shallow concentrated time and channel flow time. The
overland flow time is sheet flow and generally does not last more than
200 feet in an undisturbed wooded or grass area. After 200 feet or in
some basins shorter, the flow becomes shallow concentrated in a
gutter section, vegetated swale, pipe, etc. The flow may then be
conveyed to a larger system or stream where it is conveyed to the
design point.
Some general guidelines when performing time of concentrations are
given below:
• The minimum time of concentration for drainage area is 5 minutes.
• In some cases runoff from a portion of the drainage area which is
highly impervious may result in a greater peak discharge than
would occur if the entire area were considered. In these cases,
adjustments can be made to the drainage area by disregarding
those areas where flow time is too slow to add to the peak
discharge.
• When designing a drainage system, the overland flow path is not
necessarily the same before and after development and grading
operations have been completed. Selecting overland flow paths in
excess of 100 feet in urban areas and 300 feet in rural areas
should be done only after careful consideration. Except in very
flat areas, overland flow time should not be greater than the pipe
or channel flow time.
There are several acceptable methods for calculating the time of
concentration. The City of Rocky Mount prefers the use of either the
Kirpich or NRCS methodology as described below. If the designer
has reason to use another method the designer must submit the
method and supporting technical information for review and approval
by the Director of Engineering.
3.3.3.1 Kirpich Equation
The Kirpich equation is based on empirical data and observation and
does not break down the time of concentration flow path into different
segments. Although it has no analytical basis, it has proven an
effective method in many years of use. It is therefore widely
considered an acceptable method for estimating time of concentration
for small drainage areas of up to 10 acres. The basic form of the
equation is:
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 10

3.3.3.2 NRCS Method
Kirpich Equation
3 0.385
L
H
TC =
128
Where:
TC = time of concentration (min)
H = height of the most remote point on the watershed above
the outlet (ft)
L = length of flow from the most remote point on the
watershed to the outlet (ft)
(Civil Engineering, Vol. 10, No. 6, June 1940, p.362.)
The following adjustments are commonly made to Kirpich Equation to
compensate for channelization.
• For well-defined natural channels, use Tc.
• For overland flow on grassy surfaces, use Tc * 2.
• For overland flow on paved surfaces, use Tc* 0.4.
• For concrete channels, use Tc * 0.2.
The time of concentration can be broken into three types of flow,
sheet flow, shallow concentrated flow, and channel flow (or pipe flow).
Sheet flow is assumed to be no longer than a few hundred feet and
can be described by Manning's kinematic solution:
Sheet Flow Equation
TV = (0.007XnL)"'
(P2)0.5 (S)0.4
Where:
Tsf = travel time for sheet flow(hours)
n = Manning roughness coefficient
L = flow length (ft)- Maximum length = 100 feet
P2 = 2-yr 24 hour rainfall (in)
S = ground slope (ft/ft)
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 11

Table 3.6 Manning's "n" Value for Sheet Flow
(Source: North Carolina Erosion and Sediment Control Planning
and Design Manual)
Description
"n"
Smooth surfaces:
Concrete, asphalt
0.011
Bare soil, gravel
0.011
Sparse grasses
0.150
Dense grasses
0.240
Bermuda grass
0.410
Woods, light underbrush
0.40
Dense underbrush
0.80
Sheet Flow Equation
Shallow concentrated flow travel time is best estimated by calculating
the average flow velocity from the figure on the following page. The
travel time is estimated as the average flow velocity multiplied by the
flow length.
Tscf = (L)
(V)3600
Where:
Tscf = travel time for shallow concentrated flow (hours)
L = length of shallow concentrated flow path (ft)
V = velocity (fps)
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 12

Figure 3.2 Average Velocity
(Source: NRCS TR-55 Urban Hydrology for Small Watersheds, Second Edition, June 1986)
.50 —
.20 —
41 .10 —
4-
4 _
c .06
N -
N
.04 —
0
U
L
W -
3
.02 —
.01 —
.005 —
t
1
1 t t t t t t i t t
2 4 6 10 20
Average velocity, ft/sec
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 13

At the point where a defined channel or pipe system begins, the flow
velocity can be estimated by the Manning equation. For open
channels the equation has the form:
Open Channel and Pipe Flow Equation
Toc = (L)
(V)3600
Where:
Tof = travel time for open channel and pipe flow (hours)
L = length of open channel and pipe flow path (ft)
V = velocity (fps)
Flow in an Open Channel
V = (1.49XR)1.661 (S,)o.s
n
Where:
V = average flow velocity (fps)
R = hydraulic radius (ft)
S = channel slope (ft/ft)
n = Manning's roughness coefficient
Hydraulic Radius Equation
R_A
P
Where:
A = cross -sectional area (sq. ft)
P = wetted perimeter (ft)
For pipe systems the flow velocity can be estimated by the Manning
equation as well. Assuming the pipe is circular and is flowing just full,
the equation simplifies to the form:
Flow in a Pipe
V _ (0.59XD)1.661 (S,)o.s
n
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 14

Where:
D = pipe diameter (ft)
Other variables are as defined previously
Time of Concentration
The Tc is the total travel time and the sum of the sheet flow, shallow
concentrated flow, open channel and pipe flow.
Tc = Tsf + Tscf + Tof
3.4 Hydrograph Generation
Hydrographs are a graph of the discharge at a particular location in a
watershed. The hydrograph represents the discharge rate versus
time for a given rainfall event and the volume underneath the
hydrograph equals the runoff volume. The highest discharge rate on
the hydrograph is the peak discharge.
The City of Rocky Mount prefers hydrographs be generated using
standard computer software with the NRCS Unit Hydrograph
methodology, appropriate design storms presented in the Peak
Discharge section of this chapter with the NRCS Type II rainfall
distribution. This method uses the NRCS dimensionless unit
hydrograph, rainfall distribution, time of concentration converted to lag
time and the runoff excess from the NRCS RCN method to generate a
hydrograph. The unit hydrograph was developed by the NRCS based
on multiple gage sites and represents the runoff pattern from a typical
watershed. The designer shall not modify the peak rate factor unless
it can be demonstrated through model calibration that this adjustment
is warranted.
However, if the computer model limitations do not apply to the site
condition then the following method, developed by Dr. Rooney
Malcolm and presented in Elements of Urban Drainage Design, can
be utilized. This method calculates the discharge at a given time (t)
based on the step function given below and can be readily
incorporated into a spreadsheet.
Step Function Equation (1)
For 0 < t < 1.25tp
Q = Qp 1— cos II x t in radians
2 tp
Step Function Equation (2)
For t > 1.25 tp
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 15

3.5 Stream Routings
1.3 r
Q = 4.34Qpe `p in radians
Time to Peak
Volume
Tp _ (1.39XQpXTp)
Where:
Tp = time to peak of the hydrograph
Volume= volume of runoff from design storm in cubic feet
= runoff from section 3.3.2.2 multiplied by drainage area
Qp = peak discharge from section 3.1
On large complex basins where multiple subbasins and hydrographs
are required to adequately reflect the watersheds, hydrographs from
individual subbasins need to be routed from one design point to the
next taking into account the affects of the floodplain storage. This is
referred to in this manual as stream routing.
The City of Rocky Mount requires the use of acceptable computer
programs to generate, route and combine the hydrographs.
Acceptable stream routing methods include Muskingum-Cunge,
Modified Puls and ATT-KIN.
3.6 Impoundment Routings
Hydrograph routing is required when City standards require that some
form of impoundment, either detention or retention, be used for new
developments. The type and size of facility required will usually
depend on the size of the proposed development, its impact on the
downstream watercourse and whether or not downstream water
quality is of primary concern. This type of routing is referred to as
impoundment routing.
When an impoundment is required to control peak discharges, the
impoundment outlet device controls the rate at which water can leave
the impoundment. When the inflow discharge is greater than the
outlet device discharge, the excess water is stored in the
impoundment. As water is stored and released, the water surface in
the impoundment increases until the hydrograph is completed. The
quantity of water that must be detained or stored in order to
adequately reduce the peak discharge is referred to as the Required
Storage Volume. This is the volume that must be available in the
facility without exceeding the maximum permissible release rate.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 16

Although the required volume can only be found by routing the design
storm hydrograph through the proposed facility, it is adequate to
estimate the volume by subtracting the permissible outflow peak flow
from the basin from the inflow peak flow for the critical storm duration.
Approximate Storage Volume Equation
S = (Qp — MPRRXTp)
Where:
S = estimated storage volume (cf)
Qp = peak inflow (cfs)
MPRR = maximum permissible release rate (cfs)
Tp = time to peak (seconds)
This is only a good initial estimate and must be verified by routing the
design storm through the proposed facility.
3.6.1 Impoundment Outlet Devices
For purposes of this manual, a stormwater impoundment is a facility
that is constructed to pond the stormwater during all storm events
either temporarily or permanently. These impoundments typically
have outlet devices that consist of a principal spillway and a separate
emergency spillway. The principal spillway is the outlet device that
controls the peak flows of the design storm events whereas the
emergency spillway is designed to pass the 100-year or larger storm
event in a manner which minimizes the impoundment failure. In
special conditions the principal spillway will be designed to pass all of
the anticipated storm events and a separate emergency spillway will
not be provided.
In order to route a hydrograph through an impoundment, an elevation -
discharge rating curve is required to represent the principal and
emergency spillway capacities. This section is a general description
of some common outlet control devices and acceptable parameters
used in impoundment facilities to generate the typical elevation -
discharge rating curve.
Because controlling multiple design storms may be required, some
rather imaginative outlet devices may result. To the extent possible,
outlet devices should be kept simple. This may require an optimal
design for one storm frequency and an over design for the other storm
event.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 17

Riser- Barrel Outlet
A riser -barrel outlet, as shown in Figure 3.3, is the most common type
of principal spillway. The riser controls the water surface elevations in
the impoundment and the barrel conveys the water through the
impoundment structure. The riser -barrel can be a combination of
several types of outlet devices. At different water surface elevations,
different parameters will control the discharge. Small pipes or outlet
holes in the riser will typically act as orifices, substantially limiting the
amount of water that can be discharged through the barrel. These
small openings are used to control the WQv and 1-year 24-hour peak
discharge. When the water reaches the top of the riser, the water will
spill over the edge, which acts as a weir. The length of the weir will
control how much water passes over the edge for a given depth. This
length will be set to control the 10-year, 24-hour peak discharge. As
the water rises, more water passes over the edge of the riser and
through the orifice openings and through the barrel. At some point the
barrel starts to flow full and begins controlling how much water can
pass through the impoundment. The barrel is typically sized to control
the 25-year, 24-hour storm event. When the water surface rises
above the 25-year elevation, the emergency spillway will then convey
a majority of the larger events.
Each of the components summarized above work to control the peak
discharges. The remaining part of this section presents the equations
and acceptable parameters for orifices, weirs, and barrels used to
develop the elevation discharge rating curve.
Figure 3.3 Cross Section of a Riser Barrel
100-YR 24-HR EVENT
25-YR 24-HR EVENT #
10-YR 24-HR EVENT
1-YR 24-HR EVENT f
30
SLOPES
4" DRAIN
4" GATE VALVE
W/EXTENSION
ANTI -VORTEX AND
TRPSH RAOI(
WQ STD. B40.14
DEPTH W/O FRAME AN❑ GRATE
4R EQUIVALENT
WQ LINK SEAL OR
UTLET EQUIVALENT
OUTLET PIPE
DIAMETER
h
GROUT FOR
BUOYANCY
SECTION MEW
NOT TO SCALE
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 18

3.6.1.1 Orifices
3.6.1.2 Weirs
Small openings or pipes are the most common method of controlling
the release of small amounts of water and are typically used to draw
down the WQv in the required 48-hour period and also may be used
to control the 1-year, 24 hour storm event. This is because as the
depth of water increases over the orifice, the amount of water that
passes through the orifice doesn't substantially change.
The discharge through an orifice can be described by an energy
balance analysis. Assuming the upstream velocity is negligible (i.e. a
reservoir) and the water surfaces both upstream and downstream are
free surfaces, the energy balance can be simplified to what is referred
to as the orifice equation.
Orifice Equation
Q = (Cd XAX2gh)0.5
Where:
Q = discharge (cfs)
A = cross -sectional area of the orifice (sq ft.)
g = gravitational acceleration
h = driving head to the centroid of the orifice
(where h > D/2)
Cd = coefficient of discharge (usually 0.50-0.70)
The orifice equation is only appropriate when the headwater depth is
above the top of the orifice (HW>D). When the flow through the
orifice is lower than the top of the orifice, other forms of analysis such
as a modified Weir Equation are required. For manual computations
of discharge, the charts used for the inlet control for culverts may also
be helpful. These charts are similar to the orifice equation but were
developed using empirical data. In many cases they include
discharges for depths as low as half the orifice diameter (HW/D =
0.5).
Weirs control water by limiting the available length allowed for water
to spill over. However, unlike the orifice, as the water rises the weir
allows substantially more water to pass. Most weirs used in
impoundments will fall into one of two categories; sharp -crested weirs
such as flow over a standpipe, or broad -crested weirs such as
emergency overflows in basins. Although considerable research has
been conducted in the modeling of weirs, a simple expression can be
applied to most weirs used in stormwater impoundments. The
equation is usually expressed as:
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 19

3.6.1.3 Barrel
Weir Equation
Q = (CwXLXH)Is
Where:
Q = discharge (cfs)
Cw = weir coefficient
L = length (ft)
H = height of water above the crest of the weir (ft)
For sharp -crested weirs, Cw is usually taken to be about 3.33. For
broad -crested weirs, 3.0 is generally used. Cw is not a true constant,
but rather a function of flow depth and geometry. For horizontal weirs
used in storm drainage, these values will usually suffice.
In some situations it is necessary to notch a riser and create multiple
weirs. The notch is one weir and when the water rises and begins to
spill over the top this becomes a second weir. Another situation
where multiple weirs occur is when the emergency spillway can be
defined as a weir. When multiple weirs are used, each of the weirs
will have different depths of flows and therefore should be calculated
separately and then added together.
As water rises above the top of the riser and more water passes over
the edge, at some point the riser cross sectional opening area may
become more restrictive than the weir length. Calculating the
allowable discharge using the orifice equation and the allowable
discharge using the weir equation at the same water surface elevation
should be performed to determine which condition controls. It is
strongly recommended not to allow the riser to control as an orifice
because this indicates that there is a pocket of air trapped between
the headwater created by the barrel and the water surface in the
pond. This trapped air has the potential for creating destructive forces
on the riser -barrel structure.
The barrel is just like a culvert except that instead of a headwall, there
is a riser section on the upstream side. The headwater elevation for a
given discharge will be the height of the water inside the riser barrel.
Once the headwater for the barrel is above the top of riser, the
controlling flow out of the impoundment will be either the riser or the
barrel.
The barrel sections are typically short sections of pipe and the
capacity and headwater can be calculated using the orifice flow
equation:
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 20

Q = (Cd �AX 2gh)os
Where:
Q = discharge (cfs)
A = cross -sectional area of the orifice (sq ft.)
g = gravitational acceleration
h = driving head to the centroid of the barrel opening (ft)
(where h > D/2)
Cd = coefficient of discharge (usually 0.60-0.70)
In some situations, the downstream tailwater conditions during higher
discharges can control the barrel. Therefore, the designer should
consider checking the barrel for outlet control as described in the
culvert section.
3.6.1.4 Elevation -Discharge Rating Curve
The elevation -discharge rating curve for an impoundment defines the
discharge that will be conveyed through the impoundment when the
water surface is at a given elevation. This rating curve is used to
route the hydrograph through the impoundment and determine the
impoundment water surface elevation for the various storm events.
For a conservative estimation of the impoundment water surface
elevation, the elevation can be determined from the rating curve using
the unrouted peak discharge.
The elevation -discharge rating curve should compile all the various
components of the impoundment outlet devices and determine the
controlling discharge at a given impoundment water surface elevation.
A sample rating curve table is provided below.
Table 3.7 - Typical Elevation -Discharge Curve
Elevation
Low
wf lice ow
Principal Weir
Qp
Barrel
Emergency Spillway
Rating Curve
OriFirst
total
Notch
Top of Riser
Inlet
Outlet
Top
Avg.
weir
weir
Feet
h (feet)
Q (cfs)
h
Q
h
Q
h
Q
Q
h
length
length
Q
Elevation
Q (cfs)
(ft.)
(ft.)
89.125
0.0
0.0
0.0
0.0
0.0
0.0
0.0
NA
0.0
0.0
NA
NA
NA
0.0
89.1
0.0
89.500
0.4
0.1
0.0
0.0
0.0
0.0
0.1
NA
0.0
0.0
NA
NA
NA
0.0
89.5
0.1
90.000
0.9
0.2
0.0
0.0
0.0
0.0
0.2
NA
0.0
0.0
NA
NA
NA
0.0
90.0
0.2
90.500
1.4
0.3
0.0
0.0
0.0
0.0
0.3
NA
0.0
0.0
NA
NA
NA
0.0
90.5
0.3
91.000
1.9
0.3
0.0
0.0
0.0
0.0
0.3
4.3
48.9
51.7
NA
NA
NA
0.0
91.0
0.3
91.500
2.4
0.4
0.5
0.0
0.5
10.9
11.3
4.8
51.7
55.6
NA
NA
NA
0.0
91.5
11.3
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 21

92.000
2.9
0.4
1.0
0.0
1.0
30.9
31.3
5.3
1 54.3
59.2
0.0
10.0
10.0
0.0
92.0
31.3
92.500
3.4
0.4
1.5
0.0
1.5
56.8
57.2
5.8
56.8
62.7
0.5
13.0
11.5
10.8
92.5
67.6
93.000
3.9
0.5
2.0
0.0
2.0
87.4
87.9
6.3
59.3
65.9
1.0
16.0
13.0
34.5
93.0
93.7
93.500
4.4
0.5
2.5
0.0
2.5
122.1
122.6
6.8
61.6
69.0
1.5
19.0
14.5
70.6
93.5
132.2
94.000
4.9
0.5
3.0
0.0
3.0
160.6
161.1
7.3
63.8
72.0
2.0
22.0
16.0
119.9
94.0
183.7
94.500
5.4
0.5
3.5
0.0
3.5
202.3
202.9
7.8
65.9
74.8
2.5
25.0
17.5
183.3
94.5
249.3
3.6.2 Elevation Storage Rating Curve
The elevation storage rating curve defines the available storage at a
given elevation. These curves can be generated by a computer
program which typically uses the conic method to calculate the
volume between two elevations with known surface areas. Another
acceptable method is to use the average end area. In this method
average surface area between two elevations multiplied by the
difference in elevations determines the incremental volumes.
3.6.3 Storage Indication Routing
The storage indication method is used by most standard computer
software applications to route the hydrograph through an
impoundment. This method uses the inflow hydrograph, elevation -
discharge and elevation -storage rating curves to determine the
outflow hydrograph and elevation within the impoundment. The City
of Rocky Mount prefers hydrographs be routed through
impoundments using standard computer software.
3.6.4 Chain Saw Routing
For simple impoundments and conditions when the computer model
limitations apply to the site the Chain Saw Routing developed by Dr.
Rooney Malcolm and presented in Elements of Urban Drainage
Design can be utilized. This method uses the hydrograph defined by
the step function described earlier.
For routing a storm by the Chain Saw Routing method in a
spreadsheet or by hand, it is necessary to formulate an expression for
the stage -storage relationship. For routing by hand, a plot of the
relationship is adequate. For computer application, the relationship
can usually be expressed by a power curve. The simplest ways to
determine the volume is to planimeter (or digitize) a topographic map
of the basin and calculate the storage using the average end areas
method.
The resulting plot of stage vs. storage may be used for routing by
hand or a "best fit" equation of the points may be used. The best fit is
usually of the form:
Storage = (KXStage)'
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 22

Where:
Storage = accumulated volume at the stage (ft)
Stage = Depth or elevation in the impoundment
K= constant for the best fit line. Typically determined using
the spread sheet function
b = constant exponent of the best fit line. Typically determined
using the spreadsheet function
Therefore, to calculate the stage for the associated storage volume:
Stage = (KXStorage)llb
The components required for the Chain Saw Routing method are
similar to those of storage -indication method. The method is an
incremental tabular application of the same differential equation but
simplified to the form:
Si = (Ii — OiXTi — Tj
Where:
Si = incremental change in storage at time i (sec)
Ii = inflow at time i (cfs), using the step function defined in
Section 3.3.
Oi = outflow at time i (cfs)
(Ti —Tj)= time step (sec)
The Chain Saw Routing method may not be as intuitively satisfying as
other methods since the outflow at any time is based on the storage
volume prior to that time step. The method does however lend itself
to spreadsheet application and with sufficiently short time steps
provides reasonable results. Here again the method is best explained
by example.
3.7 Street and Gutters
Effective drainage of street and roadway pavements is essential to the
maintenance of the roadway service level and to traffic safety. Water
on the pavement can interrupt traffic flow, reduce skid resistance,
increase potential for hydroplaning, and limit visibility due to splash
and spray, and cause difficulty in steering a vehicle when the front
wheels encounter puddles. Surface drainage is a function of
transverse and longitudinal pavement slope, pavement roughness,
inlet spacing, and inlet capacity. The design of these elements is
dependent on storm frequency and the allowable spread of
stormwater on the pavement surface.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 23

This section presents design guidance for gutter flow hydraulics
originally published in HEC-12, Drainage of Highway Pavements and
AASHTO's Model Drainage Manual. For more complex gutter
sections, the design should refer to the manual for appropriate
methodologies.
The following form of Manning's Equation should be used to evaluate
gutter flow hydraulics:
Q _ (0.56 �Sx)513 (S), /2 (T)' /3
n
Where:
Q =
gutter flow rate, cfs
Sx =
pavement cross slope, ft/ft
n =
Manning's roughness coefficient
S =
longitudinal slope, ft/ft
T =
width of flow or spread, ft
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 24

Table 3.8 Manning's n Values for Gutter Sections
City of Rocky Mount, NC
Type of Gutter or Pavement
Range of Manning's n
Concrete gutter, troweled finish
0.012
Asphalt pavement:
Smooth texture
0.013
Rough texture
0.016
Concrete gutter with asphalt pavement:
Smooth
0.013
Rough
0.015
Concrete pavement:
Float finish
0.014
Broom finish
0.016
For gutters with small slopes, where
sediment may accumulate, increase
0.002
above values of in by
Note: Estimates are by the Federal Highway
Administration
Source: USDOT, FHWA, HDS-3 (1961).
3.8 Catch Basins and Drop Inlets
The City of Rocky Mount requires the completion Catch Basin Design
Data Sheet provided in Appendix B. This section presents the
equations and charts necessary to complete the sheet and
demonstrate that the catch basins are properly located and designed
according to Chapter 1 requirements.
The capacity of a catch basin or drop inlet depends upon its geometry
and the cross slope, longitudinal slope, total gutter flow, depth of flow
and pavement roughness. The depth of water next to the curb is the
major factor in the interception capacity of both gutter inlets and curb
opening inlets. At low velocities, all of the water flowing in the section
of gutter occupied by the grate, called frontal flow, is intercepted by
grate inlets, and a small portion of the flow along the length of the
grate, termed side flow, is intercepted. On steep slopes, only a portion
of the frontal flow will be intercepted if the velocity is high or the grate
is short and splash -over occurs. For grates less than 2 feet long,
intercepted flow is small. A parallel -bar grate is the most efficient type
of gutter inlet. However, when crossbars are added for bicycle safety,
the efficiency is greatly reduced.
The ratio of frontal flow to total gutter flow, Eo, for straight cross slope
is expressed by the following equation:
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 25

Eo=Qw/Q=1—1— T W )2.67
Where:
Eo = ratio of the frontal flow to total gutter flow
Q = total gutter flow, cfs
Qw = flow in width W, cfs
W = width of depressed gutter or grate, ft
T = total spread of water in the gutter, ft
Figure 3.4 can be used to determine Eo.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 26

Figure 3.4 Ratio of Frontal Flow to Total Gutter Flow
(Source: AASHTO Model Drainage Manual, 1991)
lo
901
0.6
CO
co
0
w 0.4
0_
T
�- - W
0.2 0.4 0.6 018 1.0
W/T
The ratio of frontal flow intercepted to total frontal flow, Rf, is
expressed by the following equation:
Rf =1— (0.09XV — Vo)
Where:
Rf= ratio of frontal flow intercepted by the catch basin grate
V= velocity of flow in the gutter, ft/s
Vo= gutter velocity where splash -over first occurs, ft/s (from
Figure 3.5)
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 27

Figure 3.5 Grate Inlet Frontal Flow Interception Efficiency
(Source: HEC-12, 1984)
a
i
I
� \ i
`a
ti i
i
y
LU
0
� a
Mar
8C J } AC
W Y
t
Ui
S } Z
W
(81JJ) •A AL13011A H3AO—HSY7dg
The ratio of side flow intercepted to total side flow, Rs, or side flow
interception efficiency, is expressed by:
Rs =
1
I1+ (0.15XV)"
( (SxXL)2.3
Where:
Rs = ratio of the side flow intercepted by the catch basin grate
L = length of the grate, ft
Figure 3.6 provides a solution to the equation.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 28

RS
I
08
0.6
04
03
02
0.1
0.06
0.06
004
0.03
0.02
02
0.1
a.08
0.06
sx
0.04
Q03
62!
Figure 3.6 Grate Inlet Side Flow Interception Efficiency
(Source: HEC-12, 1984)
EXAMPLE;
GIYEFF:
$x = 0.026
--- L_a FT-----'-_
Y = 4 FT/3
P1 ND: Ra ; 0.063
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 29

The efficiency, E, of a grate is expressed as:
E = (Rf XEo)+ (RsXl — Eo)
The interception capacity of a grate inlet on grade is equal to the
efficiency of the grate multiplied by the total gutter flow:
Qi = (EXQ) = Q[(RfXEo)+ (RsXl — Eo)]
3.8.1 Grate Inlets in Sag
A grate inlet in a sag operates as a weir up to a certain depth,
depending on the bar configuration and size of the grate, and as an
orifice at greater depths. For a standard gutter inlet grate, weir
operation continues to a depth of about 0.4 feet above the top of grate
and when depth of water exceeds about 1.4 feet, the grate begins to
operate as an orifice. Between depths of about 0.4 feet and about 1.4
feet, a transition from weir to orifice flow occurs.
The capacity of grate inlets operating as a weir is:
Qi _ (CXPyd) s
Where:
P = perimeter of grate excluding bar widths and the side against
the curb, ft
C = 3.0
d = depth of water above grate, ft
The capacity of grate inlets operating as an orifice is:
Qi = (CXAX2gd)os
Where:
C = 0.67 orifice coefficient
A = clear opening area of the grate, ft2
g = 32.2 ft/s2
d = depth of water above grate, ft
Both calculations should be computed at given depths. The lowest
Qi will control the depth of ponding above the grate.
3.8.2 Curb Inlets on Grade
Following is a discussion of the procedures for the design of curb
inlets on grade. Curb -opening inlets are effective in the drainage of
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 30

highway pavements where flow depth at the curb is sufficient for the
inlet to perform efficiently. Curb openings are relatively free of
clogging tendencies and offer little interference to traffic operation.
They are a viable alternative to grates in many locations where grates
would be in traffic lanes or would be hazardous for pedestrians or
bicyclists.
The length of curb -opening inlet required for total interception of gutter
flow on a pavement section with a straight cross slope is determined
using Figure 3.7.
The efficiency of curb opening on grade with inlets shorter than the
length required for total interception is determined using Figure 3.7.
The length of inlet required for total interception by depressed curb -
opening inlets or curb -openings in depressed gutter sections can be
found by the use of an equivalent cross slope, Se, in the following
equation:
Se = Sx + (S'wXEo)
Where:
Eo = ratio of flow in the depressed section to total gutter flow
S'w= cross slope of gutter measured from the cross slope of the
pavement, Sx
S,w_ a
(WX12)
Where:
a= gutter depression, in
W= width of depressed gutter, ft
It is apparent from examination of Figure 3.7 that the length of curb
opening required for total interception can be significantly reduced by
increasing the cross slope or the equivalent cross slope. The
equivalent cross slope can be increased by use of a continuously
depressed gutter section or a locally depressed gutter section.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 31

Figure 3.7 Curb -Opening and Slotted Drain Inlet Length for Total Interception
(Source: HEC-12, 1984)
LT=(),600.42 0.3(I/n
FOR COMPOSITE CROSS SLOPES, USE % FOR SM.
5 Sk +S,,, Eo , SY=CON ..
v
0_bbI �
n 2
r 0,01
0.01
0-02
0-1
Sx, Se
0.01
M 0-03
0.04
OD6
0.08
0.1
0_2
EXAMPLE. -
GIVEN: f1=0.0IS sraol
SA=0,0 0-4 FT315
FIND: L-r = 34 FT
0
(FT31S)
50
40
LT
{ FT) 2C
Do
TO
60
50 10
6
40
5
.20 3
0.3
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 32

3.8.3 Curb Inlets in Sump
3.8.4 Drop Inlets
For the design of a curb -opening inlet in a sump location, the inlet
operates as a weir to depths equal to the curb opening height and as
an orifice at depths greater than 1.4 times the opening height. At
depths between 1.0 and 1.4 times the opening height, flow is in a
transition stage.
Drop Inlets are typically located in natural or graded sump locations.
The capacity and depth of water shall be calculated using the weir
and orifice equations identified in the Grate Inlets In Sags section
above.
3.9 Storm Drainage Pipes
Storm Drainage Pipes are located between the catch basins and drop
inlets and ultimately convey the water to a receiving channel or
stream. The City of Rocky Mount requires the Storm Drain Design
Computations Sheet, provided in Appendix B, be completed for all
pipe systems to demonstrate the storm drainage pipe has the capacity
to convey the design discharge assuming normal depth. The City of
Rocky Mount also requires the Hydraulic Grade Line Calculation
Sheet, provided in Appendix B, be completed for all pipe systems to
demonstrate the storm drainage system is adequate when
considering all of the tailwater conditions and energy losses. This
section presents the equations and allowable parameters to complete
these calculation sheets.
3.9.1 Storm Drain Calculation Sheet
The storm drain calculation sheet requires the designer to calculate
the design peak discharge at each catch basin, drop inlet and junction
box using the rational equation and accumulated time of
concentration. For each storm drainage system, the sheet should be
completed beginning at the furthest upstream inlet. When a storm
drainage system includes multiple branches, then each branch should
be treated as a separate system.
The pipe diameter shall be designed to handle the design discharge
assuming normal depth and full flow capacity of the pipe. The most
widely used formula for determining the hydraulic capacity of storm
drainage pipes for gravity and pressure flows is the Manning's
Formula, expressed by the following equation:
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 33

V = (1.486XR)"'(S)"'
n
Where:
V = mean velocity of flow, ft/s
R the hydraulic radius, ft — defined as the area of flow divided by
the wetted flow surface or wetted perimeter (A/WP)
S = the slope of hydraulic grade line, ft/ft
n = Manning's roughness coefficient, see Table 3.9
In terms of discharge, the above formula becomes:
Q — (1.486XAXR)"'(S)"'
n
Where:
Q = rate of flow, cfs
A cross sectional area of flow, ft2
For pipes flowing full, the above equations become:
V = (0.590XD)2/3(S)1/2
n
Q = (0.463XD)s13(S)l12
n
Where:
D diameter of pipe, ft
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 34

Table 3.9 Manning's n Values
Type of Conduit
Wall & Joint Description
Manning's in
Concrete Pipe
Good joints, smooth walls
0.012
Good joints. rough walls
0.016
Poor joints, rough walls
0.017
Concrete Box
Good joints, smooth finished walls
0.012
Poor joints, rough, unfinished walls
0.018
Corrugated
2 2/3- by'/�-inch corrugations
0.024
Metal Pipes and
6- by 1-inch corrugations
0.025
Boxes Annular
5- by 1-inch corrugations
0.026
Corrugations
3- by 1-inch corrugations
0.028
6-by 2-inch structural plate
0.035
9-by 2-1/2 inch structural plate
0.035
Corrugated Metal
2 2/3-by 1/2-inch corrugated
Pipes, Helical
24-inch plate width
0.012
Corrugations, Full
Circular Flow
Spiral Rib Metal
3/4 by 3/4 in recesses at 12 inch
Pipe
spacing, good joints
0.013
High Density
Polyethylene (HDPE)
Corrugated Smooth Liner
0.015
Corrugated
0.020
Polyvinyl Chloride
(PVC)
O.Oi l
Source: HAS No. 5. 1985
When sizing pipe diameters, the following general rules shall apply:
The pipe diameter shall not be reduced downstream
regardless of the hydraulic capacity.
Minimum pipe diameters shall be as identified in Chapter 1.
Minimum Time of Concentration shall be 5 minutes.
3.9.2 Hydraulic Grade Line Calculation Sheet
The storm drain calculation sheets assume that the flow in the storm
drain system is not affected by downstream conditions, such as
tailwater or hydraulic losses through the structures. These conditions
are considered in the Hydraulic Grade Line Calculation Sheet. The
hydraulic grade line begins at the outlet of the storm drainage system
and progresses upstream to the first inlet. When a system consists of
multiple branches, separate hydraulic grade lines are calculated for
each branch beginning at the common structure and progressing
upstream to the branch first inlet. The hydraulic grade line computes
the potential water surface elevations, under design conditions in the
various inlets, catch basins, manholes, junction boxes, etc. If the
potential water surface elevation in the structures does not satisfy the
design criteria identified in Chapter 1, then adjustments to the storm
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 35

3.9.2.1 Friction Loss
drain design will be required. These adjustments may include
increasing the pipe diameter or relocating the system.
The hydraulic grade line calculations begin at the outlet of the system
from a known water surface elevation, which is typically the depth of
flow in the receiving channel or water surface elevation within a
structure. The hydraulic losses to the next upstream structure are
then added to the known water surface elevation to determine the
potential water surface elevation. These hydraulic losses are the
friction loss of the pipe and the junction losses within the upstream
structure. These losses are summarized below.
The hydraulic loss caused by the roughness of the pipe material.
Hf = (SfXL)
Where:
Hf= friction loss (ft.)
L= length of the pipe between structures (ft.)
SIL friction slope
3.9.3 Contraction Loss
Where:
z
Sf = Q
Where:
Q = the design peak discharge (cfs)
K = pipe conveyance
Where:
K _ 1.486 (AXR)z13
n
The hydraulic loss caused by the contraction of flow within the
structure to the outlet pipe opening.
Ho=(0. r Voz
,
2g
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 36

3.9.4 Expansion Loss
3.9.5 Bend Loss
Where:
Ho = contraction loss (ft.)
Vo = velocity in the outlet pipe assuming full fow (ft/s)
g = 32.2 (ft/sZ)
The hydraulic loss is caused by expansion of the flow within the
structure. When multiple pipes enter the structure, the system with
the largest momentum will be used to determine the expansion loss.
Pipe with inflows of less than 10% of the mainline outflow can be
neglected.
He=(0.35
Vi 2
2g
Where:
He = expansion loss (ft.)
Vi = velocity in the inlet pipe with the largest momentum M (ft/s)
g = 32.2 (ft/sZ)
Where:
M = p(QiXVi)
Where:
Qi = discharge for the influent pipe (ft.)
Vi = velocity in the inlet pipe with the largest momentum M (ft/s)
p = density of water. This can be ignored for the purpose of
comparing inflows.
The hydraulic loss caused by the change in direction within the
structure.
Hb = (K Vi 2
2g
Where:
Hb = bend loss (ft.)
Vi = velocity in the inlet pipe (ft/s)
g = 32.2 (ft/sZ)
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 37

3.10 Culverts
K= bend loss coefficient based on the bend angle. Provided on the
Hydraulic Grade Line Calculation Sheet.
A culvert is a short, closed (covered) conduit that conveys stormwater
runoff under an embankment, usually a roadway. The primary
purpose of a culvert is to convey surface water through the
embankment and protect the embankment from failure, protect traffic
during the design storms events and protect the downstream channel
from the contraction of the floodplain flows. Culverts shall be
designed per the FHWA Hydraulic Design Series No. 5 — Hydraulic
Design of Highway Culverts (HDS-5). This section provides a brief
summary of culvert designs and supporting calculations for typical
culvert design. The designer is expected to be aware of when the
typical design assumptions and methods are not appropriate and
utilize the appropriate design methodology presented in HDS-5.
The City of Rocky Mount requires the Culvert Design Form provided
in Appendix B be completed for each culvert. The City of Rocky
encourages the use of computer programs such as HEC-RAS to
perform the calculations because this software readily computes the
tailwater conditions and roadway overtopping depths. If the designer
uses an acceptable computer program to perform the calculations, the
designer shall complete the form using the results from the computer
program and submit the computer program with the supporting
calculations. If the designer performs the calculations by hand, then
the designer shall submit all of the supporting calculations including
normal depth, weir flow, nomographs, friction losses, etc.
3.10.1 Types of Flow Control
There are two types of flow conditions for culverts that are based
upon the location of the control section and the critical flow depth.
Proper culvert design and analysis requires checking for both inlet
and outlet control to determine which will govern particular culvert
designs.
Inlet Control — Inlet control occurs when the culvert barrel is capable
of conveying more flow than the inlet will accept. This typically
happens when a culvert is operating on a steep slope. The control
section of a culvert is located just inside the entrance. Critical depth
occurs at or near this location, and the flow regime immediately
downstream is supercritical.
Outlet Control — Outlet control flow occurs when the culvert barrel is
not capable of conveying as much flow as the inlet opening will
accept. The control section for outlet control flow in a culvert is located
at the barrel exit or further downstream. Either subcritical or pressure
flow exists in the culvert barrel under these conditions.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 38

Figure 3.8 Culvert Flow Conditions
(Adapted from: HDS-%, 1985)
Inlet Control Flow Condition
Water Surface
-- Water Surface
HW
TW
d, [Control Section]
HW —Headwater
TW—Tailwater
d� —Critical depth
H — Lasses Through Culvert
3.10.2 Inlet Control
Outlet Control Flow Condition
Water Surface
Water Surface
H
E Control
L — — TW Section
Downstream
A. Submerged
Water Surface
B. Unsubmerged d� [Control Section]
HDS —5 includes numerous nomographs for the inlet condition for
various pipe sizes and entrance conditions. The City of Rocky Mount
prefers these nomographs be used to determine the inlet control
HW/D column in the Culvert Design Form. The two most common
inlet forms are provided below. If other types of inlets are used, the
designer shall include a copy of the inlet nomograph with the Culvert
Design Form.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 39

Figure 3.9 Headwater Depth for Concrete Pipe Culvert with Inlet Control
ISO
10,000
I68
6,000
EXAMPLE
(2)
(3)
6.
156
000
0.41: iwah« (s s f«t)
5.
144
5,000 a-lxa efe
4'000
!!$ • liw
3.
132
fe.t
�.
3,000
(6) 2.5 10.111
�'
4.
12a
(2) 2.1 7.4
2,000
(3) Y.2 7,7
4•
108
"o is feat
3'
96
1000
B00
94
6010g-
500
72
400
Q,
E'
=
300
x
1.S
Z03an
Lf
1.5
60
%6
200
F
1.5
Z
_
Z
W
54
a
8
a
49
100
=
J
60
Q
42
+�
a
50 HW
ENTRANCE
D
40
SCALE
0 TYPE
ti
1.0
W
36
30
(11 5gders edge with
9
9
W
d
33
headwall
Z
9
20
191 Brow* end with
C
30
headwall
.8
(31 araara and
•�
7
Projecting
10
6
.7
_T—
I24
.7
6
TO ash scale (2) or (3) PF01W
Pt
5
harliomially to sidle (t)"thed
4
Men siralghi Inclined line thro"le
0 and 0 scales. or reverse so
5
3
illdstrdiad.
i8
2
15
5
.5
.3
1.0
Lit HEADWATER DEPTH FOR
HEADWATER SCALES 253 CONCRETE PIPE CULVERTS
WEVISED MAY 0 4 WITH INLET CONTROL
auREnu of PUBUG ROAos �lsa3
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 40

Figure 3.10 Headwater Depth for Concrete Box Culvert with Inlet Control
11
600
(1)
(2)
(3)
50o
EXAMPLE
g
9
10
10
400
Tx2'Box Q=75ds
7
8
[SIB = 15 cfslR
6
7
8
9
300
Inlet NW HW
6
7
p (ft)
5
5
8
$
(1) 1.75 3.5
5
200
(2) 1.90 3.8
'4
(3) 2.05 4.1
4
4
7
3
3
$
100
3
m
80
2
?
2
5
F
s0
=
2
p+
50
2r
2
1.5
�-�
d
40
p
1.5
W
�
1.5
x
4
30
r
EXAMPLE �
LL
20
x
a
O
1--
a
ILU
1.0
til
_
cc '
W
0.9
3
f
'
Angle of t-
Wingimli —
Flara
10
0.$
0.9
0.9
0,7
HW SCALE WINGWAiLL
0.7
0.7
4
D FLARE
06
2
(t) 30' to 7$°
3
(2) 900and 16°
0.6
0.6
(3) 0(extensions
0•$
2
of side s)
0.5
0.5
7o use scale (2) or (9) project
horizontally to scale (1), then
use straight Inclined line through
0.4
1
D and 4 scales, or reverse as
illustrated.
0.8
0.4
0.4
0.6
1
0.5
0.34
D.35
D.35
HEADWATER
DEPTH
FOR BOX CULVERTS
BUREAU OF PUBLIC ROADS JAN. 1063
WITH INLET CONTROL
City of Rocky Mount
Stormwater Design Manual
December 2006
3 - 41

3.10.3 Outlet Control
HDS —5 includes numerous nomographs for the full flow condition for
various pipe sizes and entrance conditions. The City of Rocky Mount
does not want these nomographs to be used and requires the outlet
control conditions be calculated using the acceptable entrance losses
and Manning's "n" values. Outlet Control requires the calculation of
the tailwater depth and the energy losses associated with the culvert.
These losses are similar to the storm drainage pipe losses required to
establish the hydraulic grade line. The tailwater depth should reflect
the expected water surface elevation in the downstream channel and
floodplain. The tailwater depth can be calculated using the methods
identified in the open channel section or taken from an existing flood
study. The designer shall determine if the tailwater depth will be
affected by downstream conditions and perform the necessary
calculations to reflect these conditions. The basic equations for the
outlet control are provided below:
Hw= H+ho—(LXS)
Where:
Hw = headwater depth above the upstream invert elevation (ft)
H= energy loss in feet through the culvert (ft)
ho = tailwater depth above the downstream invert elevation (ft)
L = culvert length (ft)
S = culvert slope (ft/ft)
Where:
( lz( ll z
H= 1+ke+`29Rn /3`LJ� (2g
Where:
I = exit loss coefficient
ke= entrance loss coefficient from the table
When the tailwater depth is below the top of the culvert, then the
tailwater depth should be compared to the following equation and the
higher depth used to calculate outlet control:
ho=dc+D
2
Where:
do = critical depth in the culvert (ft)
D = inside depth of the culvert (ft)
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 42

Table 3.9 Inlet Coefficients
Type of Structure and Design of Entrance Coefficient Ke
Pipe, Concrete
Projecting from fill, socket end (grove -end)
0.2
Projecting from fill, square cut end
0.5
Headwall or headwall and wingwalls
Socket end of pipe (groove -end)
0.2
Square -edge
0.5
Rounded [radius = 1/12(D)
0.2
Mitered to conform to fill slope
0.7
*End -Section conforming to fill slope
0.5
Beveled edges, 33.70 or 45' bevels
0.2
Side- or slope -tapered inlet
0.2
Pipe, or Pipe -Arch, Corrugated Metal,
Projecting from fill (no headwall) 0.9
Headwall or headwall and wingwalls square- 0.5
edge
Mitered to fill slope, paved or unpaved slope 0.7
*End -Section conforming to fill slope 0.5
Beveled edges, 33.7 o or 45 o bevels 0.2
Side- or slope -tapered inlet 0.2
Box, Reinforced Concrete
Headwall parallel to embankment (no wingwalls)
Square -edged on 3 edges
0.5
Rounded on 3 edges to radius of [1/12(D)] or
0.2
beveled edges on 3 sides
Wingwalls at 30o to 750 to barrel
Square -edged at crown
0.4
Crown edge rounded to radius of [1/12(D)] or
0.2
beveled top edge
Wingwalls at 100 to 250 to barrel
Square -edged at crown
0.5
Wingwalls parallel (extension of sides)
Square -edged at crown 0.7
Side- or slope -tapered inlet 0.2
1 Although laboratory tests have not been completed on Ke values for High -Density
Polyethylene (HDPE) pipes, the Ke values for corrugated metal pipes are recommended for
HDPE pipes.
Note: End Section conforming to fill slope, made of either metal or concrete, are the sections
commonly available from manufacturers. From limited hydraulic tests they are equivalent in
operation to a headwall in both inlet and outlet control.
Source: HDS No. 5, 1985
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 43

Figure 3.11 Critical Depth for Circular Pipes
0 10 20 30 40 50 60
DISCHARGE, CQ {cfs]
6
5
U
F-
w 4
J
a
U
3
U
2L
0
14
12
4 �
0
do CANNOT EXCEED TOP OF PIPE
70 80 90 100
8
7
U
do CANNOT EXCEED TOF' U{ IwL
100 200 300 400 500 600 700 800 900 1000
DISCHARGE, (Q (cfs)
1.000 2,000
DISCHARGE, Q (cfs)
BUREAU OF PUBLIC ROADS JAN. 1964
do (:ANNO I EXCEED TOP OF P11'E
3,000
CRITICAL DEPTH
CIRCULAR PIPE
4,000
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 44

3.10.4 Roadway Overtopping
To complete the Culvert Design Form, the roadway overtopping
should be analyzed for the larger events. The elevation over the road
for a given discharge will be when the discharge over the road plus
the discharge through the culvert add up to the given discharge. This
can be done through a trial and error process of assuming a
headwater elevation and calculating the two discharges or can be
determine by generating an elevation -discharge curve similar to the
curve used for outlet devices. This curve can then be used
graphically or through interpretation to determine the headwater
elevation. Discharge over the road can be determined using the weir
flow equation.
Qr = (Cd XLXH)Is
Where:
Qr = overtopping flow rate (cfs)
Cd = overtopping discharge coefficient
L = length of roadway (ft)
H= depth of water above the road (ft)
Figure 3.10 Discharge Coefficients for Roadway Overtopping
(Source HDS No. 5, 1985)
HwF
FLOW
h i
Lr
3.10
p►Y
3.00
OF
2.90
0115 020 0.24 O.zB 0.32
Al DISCHARGE COEFFICIENT FOR
NW, IL,y 0.15
3.10
PaVEG
3.00
R. 90
Cr 2.00
2.70
2.60
2.50
O 1.0 2.0 3.0 4.0
HWr F't.
01 DISCHARGE COEFFICIENT FOR
HwF 1L,.90.15
Cd'11FCr
01 - Cd L H WF1.5
L OD
✓1VEG
0.90
GRAVEL
0.80
0.70
0.80
0.50
0,6 0.7 DA 0.9 I.❑
hI/HWr
CI SUBMERGENCE FACTOR
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 45

3.11 Open Channels
Vegetative and rip rap open channels shall be designed in
accordance with the procedures identified in the North Carolina
Erosion and Sediment Control Planning and Design Manual. For
irregular shaped channels, the City of Rocky Mount prefers the use of
standard computer programs such as HEC-RAS.
City of Rocky Mount
Stormwater Design Manual December 2006 3 - 46

II , T V 4 s
ROCKYI
N H G A R O L I" 4
Rocky Mount Engineering Department
One Government Plaza
Rocky Mount, NC 27802
City of Rocky Mount, North Carolina
Stormwater Design Manual
Appendices
October 2006

APPENDIX A: ACRONYMS AND DEFINITIONS
Bioretention - An engineered means of managing stormwater runoff, using chemical, biological
and physical processes via a natural, terrestrial -based community of plants, microbes and soil.
Bioretention provides two important functions: (1) water quantity (flood) controls; and (2)
improves water quality through removal of pollutants and nutrients associated with runoff.
Catch Basin - A structure located within a curb and gutter section that allows water to enter into
the storm drainage system. The catch basin has an opening in the curb and may or may not
have an opening in the gutter section covered by a grate.
Design Storm - A theoretical storm of a given frequency that will produce a simulated runoff
peak and volume having the same return frequency. Thus, a 100-year design storm should
produce a 100-yr runoff and volume.
Drop Inlet - A vertical inlet to a buried culvert or storm drainage pipe with a flat grate inlet.
DWQ — North Carolina Division of Water Quality.
Easement - A right to use the land of another for a specific purpose, such as for a right-of-way
or utilities.
Forebay - Excavated settling basin or a section separated by a low weir at the head of the
primary impoundment. The forebay serves as a repository for a large portion of sediment and
facilitates draining and excavating the basin.
Grass Swa/es - A series of vegetated, open channels that are designed to treat and attenuate
stormwater runoff for a specified water quality volume. As stormwater runoff flows through the
channels, it is treated through filtering by vegetation in the channel, filtering through a subsoil
matrix, and/or infiltration into the underlying soils.
Grate Inlet - Depressions or cavities in the pavement or ground that are covered by a steel grate
and designed to collect and covey stormwater. Grate inlets can be found in parking lots,
roadway medians and along town streets.
Illicit Connection - Any discharge to a municipal separate storm sewer that is not composed
entirely of stormwater (some discharges may be authorized by an NPDES permit) and
discharges resulting from fire fighting activities.
Impervious Surface - Surfaces providing negligible infiltration such as pavement, buildings,
recreation facilities(e.g. tennis courts, etc.), and covered driveways. This will include porous
pavement, gravel roads, parking areas and precast concrete, but does not include wooden
slatted decks or the water surface area of swimming pools.
Junction Box — Where stormwater drain lines join or intersect, a box installed to accommodate
changes in flow direction, pipe diameter and elevation.
City of Rocky Mount
Stormwater Design Manual October 2006 Appendix A - 1

Level Spreader - A device used to spread out stormwater runoff uniformly over the ground
surface as sheetflow (i.e., not through channels). The purpose of a level spreader is to prevent
concentrated, erosive flows from occurring and to enhance infiltration.
NCDENR — North Carolina Department of Environment and Natural Resources.
New development - shall be defined as to include the following: 1) any activity that disturbs
greater than one acre of land in order to establish, expand or modify a single family or duplex
residential development or a recreational facility; 2) any activity that disturbs greater than one-
half an acre of land in order to establish, expand or modify a multifamily residential development
or a commercial, industrial or institutional facility; and 3) does NOT include agriculture, mining or
forestry activities. Land disturbance is defined as grubbing, stump removal and/or grading.
NPDES — National Pollutant Discharge Elimination System.
Open Channel - A long, narrow, open trench dug into the ground usually at the side of a road or
field, which is used especially for supplying or removing water, or for dividing land.
Plug Flow - Fluid particles pass through the basin and are discharged in the same sequence in
which they enter. The particles remain in the system €or a time equal to the theoretical
detention time. This type of flow is especially appropriate for basins with high length -to -width
ratios (Metcalf and Eddy, Inc., 1979).
Record Drawings - The primary outlet is often constructed of a rised barrel assembly and
Principal Spillway - The primary outlet is often constructed of a rised barrel assembly and
provides flood protection (ie. for the 10-yr. storm) or reduces the frequency of the operation of
the emergency spillway.
Riparian Buffer - an area of trees, usually accompanied by shrubs and other vegetation, that is
adjacent to a body of water and which is managed to maintain the integrity of stream channels
and shorelines, to reduce the impact of upland sources of pollution by trapping, filtering, and
converting sediments, nutrients, and other chemicals, and to supply food, cover, and thermal
protection to fish and other wildlife.
Spillway - A sluiceway or passage for excess water in a reservoir, to prevent too much
pressure on the dam.
Storm Drainage System — Natural or man-made individual structures, designed in combination,
with the express purpose of conveying stormwater to larger water bodies.
Storm Event - A rainfall event that produces more than 0.1 inch of precipitation and is separated
from the previous storm event by at least 72 hours of dry weather.
Stormwater Wetlands - Manmade structure that is regularly saturated by surface or groundwater
and subsequently characterized by a prevalence of vegetation that is adapted for life in
saturated soil conditions.
Travel Lane - A strip of roadway intended to accommodate the forward movement of a single
line of vehicles. A solid or broken line is used to separate individual traffic lanes from each
other and from the shoulder of the road.
City of Rocky Mount
Stormwater Design Manual October 2006 Appendix A - 2

Vegetated Filter Strips - Strips of vegetation separating a water body from a land use that could
act as a non -point pollution source. Vegetated buffers are variable in width and can range in
function from vegetated filter strips to wetlands or riparian areas.
Wet Detention Pond — Detention basins are excavated areas or natural depressions designed to
detain stormwater runoff. These structures detain or impede flow by storing runoff and
releasing the stored volume at a reduced rate.
City of Rocky Mount
Stormwater Design Manual October 2006 Appendix A - 3

APPENDIX B: DESIGN FORMS AND CHECKLISTS
City of Rocky Mount
Stormwater Design Manual October 2006 Appendix 8 - 1

Design Checklist: Wet Detention Pond
Project:
Form Completed By:
Form Checked By:
Date:
Date:
PRELIMINARY HYDROLOGIC CALCULATIONS
1. Water Quality Volume
Runoff Coefficient, R Rv _
WQv WQV = acre-ft
Average release rate over 48-hour period Rate = cfs
2. 1-Year Detention Requirements
Existing Condition 1-year discharge
1 -year =
cfs
3. Flood Detention Requirements
Existing Condition 10-year discharge
10-year =
cfs
Existing Condition 25-year discharge
25-year =
cfs
D DESIGN
1. Surface Area of Normal Pool
Drainage Area
DA =
acre(s)
Percent Impervious
Impervious =
%
Depth
Depth =
ft
Surface Area of Normal Pool
SA =
acre
2 Sediment Forebay
Volume
Volp. =
cu. ft.
3. Pond Design Characteristics
Normal Pool Elevation
Elevation =
ft
Normal Pool Volume
Volume =
cu. ft.
Top of Embankment
Top =
ft
WQv Elevation
WQv Elev. =
ft
WQv Volume
WQv Vol. =
cu. ft.
1-year peak elevation
ft
1-year outlet discharge
cfs
10-year peak elevation
ft
10-year outlet discharge
cfs
25-year peak elevation ft
25-year outlet discharge cfs
100-year peak elevation ft
100-year outlet discharge cfs
4. Elevation -Discharge Rating Curve Separate Sheet
5. Elevation -Storage Rating Curve Separate Sheet
6. Hydrograph Routing Separate Sheet
Notes:

Design Checklist: Stormwater Wetland
Project:
Form Completed By: Date:
Form Checked By: Date:
PRELIMINARY HYDROLOGIC CALCULATIONS
1 Surface Area Required for Wetland
% imperviousness of drainage area
Drainage Area
SA/DA from Table
Surface Area Required for Wetland
2. Water Quality Volume
Runoff Coefficient, Rv
WQv
Average Release Rate Over 48-hour Period
3. 1-Year Detention Requirements
Existing Condition 1-year discharge
4. Flood Detention Requirements
Existing Condition 10-year discharge
Existing Condition 25-year discharge
STORMWATER WETLAND DESIGN
1. Wetland Design
Micropool Area
Sediment Forebay Area
Pool/Deepwater Wetland Zone (1.5 - 6 feet deep)
Low Marsh Wetland Zone (6-12 inches deep)
High Marsh Wetland Zone (0-6 inches deep)
2. Sediment Forebay
Volume
Drainage Area
Impervious Area
3. Wetland Final Design Characteristics
Normal Pool Elevation
Top of Embankment
WQv Elevation
WQv Volume
1-year peak elevation
1-year outlet discharge
10-year peak elevation
10-year outlet discharge
25-year peak elevation
25-year outlet discharge
100-year peak elevation
100-year outlet discharge
4. Elevation -Discharge Rating Curve
5. Elevation -Storage Rating Curve
6. Hydrograph Routing
Notes:
DA =
SA/DA =
SA =
R =
WQV _
Rate =
1-year =
10-year =
25-year =
Areamp =
Areamp =
Areadw =
Area,. , _
Areahi,h =
Volpre -
DA =
Imperv. _
acres
sq. ft.
acre-ft
cfs
cfs
cfs
cfs
sq. ft., % _
sq. ft., % _
sq. ft., % _
sq. ft., % _
sq. ft, % _
F = 100.00%
cu. ft.
acres
acres
ft
ft
ft
cu. ft
ft
cfs
ft
cfs
ft
cfs
ft
cfs
Separate Sheet
Separate Sheet
Separate Sheet

rDesignChecklist: Riparian Buffer MW
Project.
Form Completed By:
Form Checked By:
Date:
Date:
1. Computed WQv
WQv
WQ =
acre-ft
Qp
Qp =
cfs
2. Drainage Area
A =
acre(s)
3. Diversion structure
Low Flow Orifice - Orifice Equation
A =
ft2
Orifice Diameter
D =
in
4 Level Spreader
Entrance Width
Enter W =
ft
End Width
Exit W =
ft
Depth
Depth =
ft
Notes:

Design Checklist: Grassed Swales
Project:
Form Completed By:
Form Checked By:
Date:
Date:
1. Computed WQv I WQ' = acre-ft
2. Drainage Area A = acre(s)
3. Peak Runoff
Peak Runoff, 10-year event QP_10 = acre-ft
Velocity, 10-year event Vp_10 = ft/s
4. Swale Dimensions
Length Length = ft
Width Width = ft
Longitudinal Slope S = ft/ft
Side Slopes Side Slopes = (h:v)
Notes:

Design Checklist: Water Quality Swale
Project:
Form Completed By:
Form Checked By:
Date:
Date:
1.
Computed WQv
WQv
WQ =
acre-ft
2.
Computed QP-10
QP-10
QP-10 =
acre-ft
VP-10
VP-10 =
ft/s
3.
Sediment Forebay Volume
Volume
Volpe =
acre-ft
4.
Swale Dimensions
Length
Length =
ft
Width
Width =
ft
Side Slopes
Side Slopes =
(h:v)
Area
Area =
ftz
Longitudinal Slope
S =
ft/ft
5.
Check Dams
Depth
Depth =
ft
Spacing Distance
Distance =
ft
Number of Check Dams
No. =
6.
Filter
Area
AF =
ft2
Depth
Depth =
in
Draw Down Time
Time =
hr
Permeability
Fc =
in/hr
Notes:

Design Checklist: Vegetated Filter Strip With Level Spreader
Project:
Form Completed By:
Form Checked By:
Date:
Date:
1.
Computed WQv
WQv
WQv =
acre-ft
Qp
Qp =
cfs
2.
Drainage Area
Area
A =
acre(s)
3.
Diversion structure
Low Flow Orifice - Orifice Equation
A =
ft2
Orifice Diameter
diam. =
in
4.
Filter Strip
Length
Lf =
ft
Width
W =
ft
Slope
S =
ft/ft
Level Spreader Width
Wf =
ft
5.
Level Spreader
Length
L =
ft
Depth
D =
ft
Notes:

Design Checklist: Bioretention (Rain Gardens)
Project:
Form Completed By:
Form Checked By:
BIORETENTION DESIGN
1. Compute WQ volume requirements
2. Drainage Area
3. Bioretention Filter
Filter Depth
Filter Length
Filter Width
4. Engineered Soil
Depth of Soil
Clay Content
Infiltration Rate
pH
Organic Content (%)
Soluble Salts
Phosphorus Index
5 Conveyance to Bioretention Facility
6 Depth of Pond for -year Event
Ponding Depth Above Filter
Design Year Event
7 Sediment Forebay Volume (if required)
Notes:
1
Date:
Date:
WQv _
A=
Af =
Depth =
Length =
Width =
Depth =
Clay =
Rate =
pH =
Organics =
Salts =
P=
acre-ft
acre
ftZ
in
ft
ft
in
in/hr
ppm
online or offline (circle one)
Depth = in
Design = - year event
Volpre = acre-ft

Design Checklist: Sand Filter
Project:
Form Completed By:
Form Checked By:
Date:
Date:
1. Computed WQv
WQv
WQv =
acre-ft
2. Drainage Area
Area
A =
acre(s)
3. Diversion Structure
Low Flow Orifice - Orifice Equation
A =
ft2
Orifice Diameter
diam. =
in
4. Filtration Area
Area - Darcy's Law
Af =
ft2
Filter Length
L =
ft
Filter Width
W =
ft
Sand Depth
D =
in
5. Sediment Forebay
Area - Camp -Hazen Equation
As =
ft2
Length
L =
ft
Notes:

Hydraulic Grade Line Computations
Project Name
Location
Designed by
Checked by
QA /QC Verification
Catch Basin, Drop
Inlet or Junction No.
Outlet W.S.
Elevation
Pipe
diameter
(inches)
Discharge
(cfs)
Pipe Length
(feet)
Head Losses (feet)
Inlet W.S.
Elevation
RIM
Elevation
Remarks
H
H e
H e
H
Total
BEND LOSS COEFFICENTS
Degrees K
Degrees K
Degrees K
0 0.00
15 0.10
20 0.16
25 0.22
30 0.28
40 0.38
50 0.47
60 0.55
70 0.61
80 0.66
90 0.70

Storm Drain Design Computations
Project Name
Location
Designed by
Checked by
QA /QC Verification
STRUCTURE
DISCHARGE
Pi
3e Design
w
F-
U
LnU
y
bA
a
w
�
bo
C�
rx .
C�a
O
cn
Q
V
a
w>
�°
w
(Acres)
(feet)
[min.]
[min.]
[min.]
(in/hr)
(cfs)
(ft/ft)
(inches)
(cfs)
[fps]
(feet)

=k Culvert Design Form
Date Designed By
Project Name:. Checked By
Culvert Location
Skew
Size/Type Pipe
Type Entrance
Direction of Flow
Hydrologic Method
H.W. Control Elev.
Shoulder Elev. = Grade Point Elev. =
�-
median ' ch elev.=
HW HI
+
TW
Lso inv. elev.
1
So =
L=
SUMMARY DATA
Drainage Area =
Design Frequency =
Design Discharge =
Design H.W. Elev =
Q100 Discharge =
Q100 H.W. Elev. _
Depth above road =
Size (ft)
D B
No.
"n"
Type
Pipe
Freq. Total Q
Nat.
H.W.
Allow.
H.W.
T.W.
Elev.
INLET CONTROL
OUTLET CONTROL
H.W.
Elev.
Vo
(fps)
Comments
(yrs)
(cfs)
HW/D
H.W.
Ke
do
do+D/2
ho
H
Lso
H.W.
SUMMARY AND RECOMMENDATIONS:
Page 1

APPENDIX C: REFERENCES
City of Raleigh, Stormwater Management Design Manual - Draft, Central Engineering
Department, City of Raleigh, North Carolina, January 2002.
Charlotte Mecklenburg, Storm Water Design Manual, City of Charlotte and Mecklenburg
County, North Carolina, July 1993.
Georgia Stormwater Management Manual, Volumes 1 and 2, August 2001
2000 Maryland Stormwater Design Manual, Volumes 1 and 2, Center for Watershed Protection
and Maryland Department of the Environment
Guidelines for Drainage Studies and Hydraulic Design, North Carolina Department of
Transportation, 1999
Elements of Urban Stormwater Design, Dr. H. Rooney Malcolm, North Carolina State University,
1989.
North Carolina, Erosion And Sediment Control Planning And Design Manual, North Carolina
Sedimentation Control Commission, North Carolina Department of Natural Resources
And Community Development, 1988.
North Carolina, Tar -Pamlico Basin: Model Stormwater Program, North Carolina Division of
Water Quality, North Carolina Department of Natural Resources And Community
Development, 1999
American Association Of State Highway And Transportation Officials, Model Drainage Manual,
1992.
Federal Highway Administration. 1991. HYDRAIN Documentation.bb
Georgia Soil and Water Conservation Commission, Manual For Erosion And Sediment Control
In Georgia, Fourth Edition, P.O. Box 8024, Athens, Georgia 30603, 1996.
Hershfield, D. M., "Rainfall Frequency Atlas of the United States", Technical Paper 40, 1961.
Maestri, B. and others, "Managing Pollution From Highway Stormwater Runoff',
Transportation Research Board, National Academy of Science, Transportation
Research Record Number 1166, 1988.
Metropolitan Washington Council of Governments, A Current Assessment Of Urban Best
Management Practices - Techniques for Reducing Non -Point Source Pollution in the
Coastal Zone, 777 North Capital Street, Suite 300, Washington, D.C., 1992.
Metropolitan Washington Council of Governments, Controlling Urban Runoff: A Practical Manual
for Planning and Selecting Urban BMPs, 777 North Capital Street, Suite 300,
Washington, D.C., 1987.
City of Rocky Mount
Stormwater Design Manual October 2006 Appendix C - 1

NOAA, "Five- to 60-Minute Precipitation Frequency for the Eastern and Central United States",
NOAA Tech. Memo. NWS HYDRO-35, 1977.
Overton, D. E. and M. E. Meadows. 1976. Storm water modeling. Academic Press. New York,
N.Y. pp. 58-88.
U. S. Department of Agriculture, Soil Conservation Service, Engineering Division. 1986. Urban
hydrology for small watersheds. Technical Release 55 (TR-55).
U. S. Department of Transportation, Federal Highway Administration. 1984. Hydrology.
Hydraulic Engineering Circular No. 19.
Water Resources Council Bulletin 17B. 1981. Guidelines for determining flood flow frequency.
Wright -McLaughlin Engineers. 1969. Urban storm drainage criteria manual. Vol. I and 11.
Prepared for the Denver Regional Council of Governments, Denver, Colorado.
City of Rocky Mount
Stormwater Design Manual October 2006 Appendix C - 2