Return Period
(years)
|
24-Hour Storm
(inches)
|
---|---|
1
|
2.00
|
2
|
2.35
|
5
|
2.92
|
10
|
3.30
|
25
|
3.91
|
50
|
4.48
|
100
|
5.00
|
Ground Cover
|
Hydrologic Soil Group Curve Numbers
| |||
---|---|---|---|---|
A
|
B
|
C
|
D
| |
Woodland
|
30
|
55
|
70
|
77
|
Meadow
|
30
|
58
|
71
|
78
|
Grass
|
39
|
61
|
74
|
80
|
Surface Description
|
"n" Value
|
---|---|
Impervious surfaces
|
0.011
|
Agricultural lands
|
0.17
|
Grass, lawn, or open space
|
0.24
|
Wooded areas
|
0.40
|
Table 1
| ||
---|---|---|
Acceptable Computation Methodologies for Stormwater Management
Plans
| ||
Method
|
Source
|
Applicability
|
TR-20 or commercial package based on TR-20
|
USDA - NRCS
|
When use of full model is desirable or necessary
|
TR-55 or commercial package based on TR-55
|
USDA - NRCS
|
Applicable for plans within the model's limitations
|
HEC - HMS
|
U.S. Army Corps of Engineers
|
When full model is desirable or necessary
|
PSRM
|
Penn State University
|
When full model is desirable or necessary
|
VT/PSUHM
|
Virginia Polytechnic Institute and Penn State University
|
When full model is desirable or necessary
|
HEC-1
|
U.S. Army Corps of Engineers
|
When full model is desirable or necessary
|
SWMM or commercial package based on SWMM
|
U.S. EPA
|
Most applicable in urban areas
|
Small Storm Hydrology Method (as included in SLAMM)
|
PV and Associates, or the website www.winslamm.com
|
Calculation of runoff volume from urban and suburban areas
|
Q = Peak flow rate, cubic feet per second (CFS)
|
C = Runoff coefficient, dependent on land use/cover
|
I = Design rainfall intensity, inches per hour
|
A = Drainage area, acres.
|
WQv = [(P)(Rv)(A)]/12 (acre-feet)
|
Where:
| ||
P
|
=
|
rainfall depth in inches (set to 1 inch)
|
Rv
|
=
|
volumetric runoff coefficient, 0.05 + 0.009(I) where I is percent
impervious cover
|
A
|
=
|
site area (acres)
|
Recharge Volume (Rev), (acre-feet)
Fraction of WQv, depending on soil hydrologic
group.
Rev = (S)(Ai)
Where; S = soil specific recharge factor in inches
Ai = the measured impervious cover
|
Hydrologic Soil Group
|
Soil Specific Recharge Factor (S)
|
---|---|
A
|
0.40 inches of runoff
|
B
|
0.25 inches of runoff
|
C
|
0.10 inches of runoff
|
D
|
0.05 inches of runoff
|
Volume Reduction Method #1: Natural Area Conservation
| |||
A water quality volume reduction can be taken when undisturbed
natural areas are conserved on a site, thereby retaining their pre-development
hydrologic and water quality characteristics. Under this method, a
designer would be able to subtract the conservation areas from the
total site area when computing the water quality protection volume.
An added benefit is that the post-development peak discharges will
be smaller, and hence, water quantity control volumes will be reduced
due to lower post-development curve numbers or rational formula "C"
values.
| |||
Rule: Subtract conservation areas from total site area
when computing water quality protection volume requirements.
| |||
Criteria:
| |||
•
|
Conservation area cannot be disturbed during project construction
and must be protected from sediment deposition. The conservation area
shall be protected with a safety fence until construction has been
completed. After construction the area shall be posted with signage
indicating that it is a conservation area.
| ||
•
|
Shall be protected by limits of disturbance clearly shown on
all construction drawings.
| ||
•
|
Shall be located within an acceptable conservation easement
instrument that ensures perpetual protection of the proposed area.
The easement must clearly specify how the natural area vegetation
shall be managed and boundaries will be marked. [Note: managed turf
(e.g., playgrounds, regularly maintained open areas) is not an acceptable
form of vegetation management.]
| ||
•
|
Shall have a minimum contiguous area requirement of 10,000 square
feet.
| ||
•
|
Rv is kept constant when calculating
WQv.
| ||
•
|
Must be forested or have a stable, natural ground cover.
| ||
Volume Reduction Method #2: Stream Buffers
| |||
This reduction can be taken when a stream buffer effectively
treats storm water runoff. Effective treatment constitutes treating
runoff through overland flow in a naturally vegetated or forested
buffer. Under the proposed method, a designer would be able to subtract
areas draining via overland flow to the buffer from total site area
when computing water quality protection volume requirements. The design
of the stream buffer treatment system must use appropriate methods
for conveying flows above the annual recurrence (two-year storm) event.
| |||
Rule: Subtract areas draining via overland flow to the
buffer from total site area when computing water quality protection
volume requirements.
| |||
Criteria:
| |||
•
|
The minimum undisturbed buffer width shall be 50 feet from top
of bank.
| ||
•
|
The maximum contributing length shall be 150 feet for pervious
surfaces and 75 feet for impervious surfaces.
| ||
•
|
The average contributing slope shall be 3% maximum unless a
flow spreader is used. In no case shall the average contributing slope
be greater than 10%.
| ||
•
|
Runoff shall enter the buffer as overland sheet flow. A flow
spreader can be installed to ensure this.
| ||
•
|
Buffers shall remain as naturally vegetated or forested areas
and will require only routine debris removal or erosion repairs.
| ||
•
|
Rv is kept constant when calculating
WQv.
| ||
•
|
Not applicable if overland flow filtration/groundwater recharge
reduction is already being taken.
| ||
Volume Reduction Method #3: Enhanced Swales
| |||
This reduction may be taken when enhanced swales are used for
water quality protection. Under the proposed method, a designer would
be able to subtract the areas draining to an enhanced swale from total
site area when computing water quality protection volume requirements.
An enhanced swale can fully meet the water quality protection volume
requirements for certain kinds of low-density residential development
(see Volume Reduction Method #5). An added benefit is the post-development
peak discharges will likely be lower due to a longer time of concentration
for the site.
| |||
Rule: Subtract the areas draining to an enhanced swale
from total site area when computing water quality protection volume
requirements.
| |||
Criteria:
| |||
•
|
This method is typically only applicable to moderate- or low-density
residential land uses (three dwelling units per acre maximum).
| ||
•
|
The maximum flow velocity for water quality design storm shall
be less than or equal to 1.0 feet per second.
| ||
•
|
The minimum residence time for the water quality storm shall
be five minutes.
| ||
•
|
The bottom width shall be a maximum of six feet. If a larger
channel is needed, use of a compound cross section is required.
| ||
•
|
The side slopes shall be 3:1 (horizontal:vertical) or flatter.
| ||
•
|
The channel slope shall be 3% or less.
| ||
•
|
Rv is kept constant when calculating
WQv.
| ||
Volume Reduction Method #4: Overland Flow Filtration/Groundwater
Recharge Zones
| |||
This reduction can be taken when "overland flow filtration/infiltration
zones" are incorporated into the site design to receive runoff from
rooftops or other small impervious areas (e.g., driveways, small parking
lots, etc.). This can be achieved by grading the site to promote overland
vegetative filtering or by providing infiltration or "rain garden"
areas. If impervious areas are adequately disconnected, they can be
deducted from total site area when computing the water quality protection
volume requirements. An added benefit will be that the post-development
peak discharges will likely be lower due to a longer time of concentration
for the site.
| |||
Rule: If impervious areas are adequately disconnected,
they can be deducted from total site area when computing the water
quality protection volume requirements.
| |||
Criteria:
| |||
•
|
Relatively permeable soils (hydrologic soil groups A and B)
should be present.
| ||
•
|
Runoff shall not come from a designated hotspot.
| ||
•
|
The maximum contributing impervious flow path length shall be
75 feet.
| ||
•
|
Downspouts shall be at least 10 feet away from the nearest impervious
surface to discourage reconnections.
| ||
•
|
The disconnection shall drain continuously through a vegetated
channel, swale, or filter strip to the property line or structural
storm water control.
| ||
•
|
The length of the disconnection shall be equal to or greater
than the contributing length.
| ||
•
|
The entire vegetative disconnection shall be on a slope less
than or equal to 3%.
| ||
•
|
The surface impervious area tributary to any one discharge location
shall not exceed 5,000 square feet.
| ||
•
|
For those areas draining directly to a buffer, reduction can
be obtained from either overland flow filtration or stream buffers
(see Method #2).
| ||
•
|
Rv is kept constant when calculating
WOv.
| ||
Volume Reduction Method #5: Environmentally Sensitive
Large Lot Subdivisions
| |||
This reduction can be taken when a group of environmental site
design techniques are applied to low- and very-low-density residential
development (e.g., one dwelling unit per two acres [du/ac] or lower).
The use of this method can eliminate the need for structural storm
water controls to treat water quality protection volume requirements.
This method is targeted towards large lot subdivisions and will likely
have limited application:
| |||
Rule: Targeted towards large lot subdivisions (e.g., two-acre
lots and greater). The requirement for structural facilities to treat
the water quality protection volume may be waived.
| |||
Criteria:
| |||
For Single Lot Development:
| |||
•
|
Total site impervious cover is less than 15%.
| ||
•
|
Lot size shall be at least two acres.
| ||
•
|
Rooftop runoff is disconnected in accordance with the criteria
in Method #4.
| ||
•
|
Grass channels are used to convey runoff versus curb and gutter.
| ||
For Multiple Lots:
| |||
•
|
Total impervious cover footprint shall be less than 15% of the
area.
| ||
•
|
Lot areas should be at least two acres, unless clustering is
implemented. Open space developments should have a minimum of 25%
of the site protected as natural conservation areas and shall be at
least a half-acre average individual lot size.
| ||
•
|
Grass channels should be used to convey runoff versus curb and
gutter (see Method # 3).
| ||
•
|
Overland flow filtration/infiltration zones should be established
(see Method # 4).
|