Equation 129-16.1
|
Rev = [(S)(Rv)(A)]/12 (inches/foot)
|
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, "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
|
0.26
|
C
|
0.14
|
D
|
0.07
|
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.
|
Equation 129-17.1
|
WQv = [(P)(Rv)(A)]/12 (inches/foot)
|
Where:
| |||
P
|
=
|
Rainfall amount equal to 90% of events producing this rainfall
(inches): the volume of rainfall for 90% of the storm events which
produce runoff in the watershed annually.
| |
A
|
=
|
Area of the project contributing to the water quality BMP (acres).
| |
Rv
|
=
|
Volume runoff coefficient, 0.05 + 0.009(I), where I is the percent
of the area that is impervious surface (impervious area ÷ total
project study area) x 100%.
|
[NOTE: Source of the following design criteria
is the Pennsylvania Department of Environmental Protection, Bureau
of Soil and Water Conservation Publication, Erosion and Sediment Pollution
Control Program Manual (Document No. 363-2134-008).]
|
Allowable Velocity
| ||
---|---|---|
Material
|
Velocity
(feet per second [fps])
| |
Well established grass on good soil
| ||
Short Pliant bladed grass
|
4.0 to 5.0
| |
Bunch grass - soil exposed
|
2.0 to 3.0
| |
Stiff stemmed grass
|
3.0 to 4.0
| |
Earth without vegetation
| ||
Fine sand or silt
|
1.0
| |
Ordinary firm loam
|
2.0 to 3.0
| |
Stiff clay
|
3.0 to 5.0
| |
Clay and gravel
|
4.0 to 5.0
| |
Coarse gravel
|
4.0 to 5.0
| |
Soft shale
|
5.0 to 6.0
| |
Shoulders
| ||
Earth
|
(as defined above)
| |
Stabilized
|
6.0
| |
Paved
|
10.0 to 15.0
|
Q = CIA
|
Where:
| |||
Q
|
=
|
Peak runoff rate measured in cubic feet per second (cfs).
| |
C
|
=
|
Runoff coefficient: The coefficient of stormwater runoff includes
many variables, such as ground slope, ground cover, shape of drainage
area, etc.
| |
I
|
=
|
Intensity: Average rainfall intensity in inches per hour for
a time equal to the time of concentration.
| |
A
|
=
|
Area: Drainage area in acres.
|
Values for the rainfall intensity shall be based on NOAA Atlas
14, Volume 2, Version 3.0, rain data found in Table A-3 and Figure
A-2 of Appendix A of this chapter.[1]
|
Table 129-20.1 Acceptable Computation Methodologies for Stormwater
Management Designs
| ||
---|---|---|
Method
|
Method Developed By
|
Applicability
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
|
HEC-1, HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichiling (1889)
|
Applicable sites less than 50 acres, or as approved by the Township
engineer
|
Other methods
|
Varies
|
Other computation methodologies approved by the Township engineer
|