WQv = [(P)(Rv)(A)]/12]
|
Where:
| ||||
---|---|---|---|---|
P
|
=
|
Rainfall depth in inches, using the "90% 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)
| ||
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 nonrooftop runoff
|
Credit is given for practices that disconnect surface impervious
cover by directing it to pervious areas where it is either infiltrated
or filtered though 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 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 E.[1]
|
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
|
If more than one hydrologic soil group (HSG) is present at a
site, a composite recharge volume shall be computed based upon the
proportion of total site area within each HSG.
|
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 model's 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 Borough Engineer
|