WQv = [(P) (Rv) (A)]/12], where:
| |
P = Rainfall depth in inches, using the ninety-percent-storm,
the volume of rainfall for 90% of the storm events which produce runoff
in the watershed annually (e.g., 1.0 inches). (See Appendix A.[1])
| |
A = Project area in acres
| |
Rv = Volumetric runoff coefficient [0.05 + 0.009(I)],
where I is the impervious surface percentage (impervious area ÷
total project area) x 100%
|
Stormwater Credit
|
Description
| |
---|---|---|
Natural area conservation
|
Conservation of natural areas, such as forest,
wetlands, or other sensitive areas in a protected easement, thereby
retaining their predevelopment hydrologic and water quality characteristics.
Using this credit, a designer may subtract conservation areas from
total site area when computing the required water quality volume.
| |
Disconnection of rooftop runoff
|
Credit is given when rooftop runoff is disconnected
and then directed over a pervious area where it may either infiltrate
into the soil or filter over it. Credit is typically obtained by grading
the site to promote overland flow or by providing bioretention on
single-family residential lots. If a rooftop area is adequately disconnected,
the impervious area may be deducted from the total impervious cover.
| |
Disconnection of non-rooftop runoff
|
Credit is given for practices that disconnect
surface impervious cover by directing it to pervious areas where it
is either infiltrated or filtered through the soil. As with rooftop
runoff, the impervious area may be deducted from the total impervious
cover thereby reducing the required water quality volume.
| |
Stream buffer credit
|
Credit is given when a stream buffer effectively
treats stormwater runoff. Effective treatment constitutes capturing
runoff from pervious and impervious areas adjacent to the buffer and
treating the runoff through overland flow across a grass or forested
area. Areas treated in this manner may be deducted from total site
area.
| |
Grass channel (open section roads)
|
Credit may be given when open grass channels
are used to reduce the volume of runoff and pollutants during smaller
storms. If designed according to appropriate criteria, these channels
may meet water quality criteria for certain types of residential development.
| |
Environmentally sensitive rural development
|
Credit is given when a group of environmental
site design techniques are applied to low-density or rural residential
development. This credit eliminates the need for structural practices
to address water quality volume. See Appendix C.[2]
|
Rev = [(S)(Rv)(A)]/12, where:
Rev = Recharge volume
(acre-feet)
S = Soil specific recharge factor (inches)
A = Site area contributing to the recharge facility
(acres)
Rv = Volumetric runoff
coefficient, Rv = 0.05 + 0.009 (I), where I
= percent impervious area; and
S shall be obtained based upon hydrologic soil
group based upon the table below:
|
Hydrologic Soil Group
|
Soil Specific Recharge Factor (S)
| |
---|---|---|
A
|
0.38
| |
B
|
0.25
| |
C
|
0.13
| |
D
|
0.06
|
Table III-1
| |||
---|---|---|---|
Acceptable Computation Methodologies for
Stormwater Management Plans
| |||
Method
|
Method Developed By
|
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 models limitations
| |
HEC - 1
|
U.S. Army Corps of Engineers
|
When full model is desirable or necessary
| |
PSRM
|
Penn State University
|
When full model is desirable or necessary
| |
Rational Method or commercial package based
on Rational Method
|
Emil Kuiching (1889)
|
For sites less than 200 acres
| |
Other methods
|
Various
|
As approved by the Municipal Engineer
|
Table 3.2 Classification of Stormwater
Hotspots
| |
---|---|
The following land uses and activities are deemed
stormwater hotspots:
| |
Vehicle salvage yards and recycling facilities
| |
Vehicle fueling stations
| |
Vehicle service and maintenance facilities
| |
Vehicle and equipment cleaning facilities
| |
Fleet storage areas (bus, truck, etc.)
| |
Industrial sites (based on SIC codes)
| |
Marinas (service and maintenance)
| |
Outdoor liquid container storage
| |
Outdoor loading/unloading facilities
| |
Public works storage areas
| |
Facilities that generate or store hazardous
materials
| |
Commercial container nursery
| |
Other land uses and activities as designated
by an appropriate review authority
|