WQv
|
=
|
[(c)(P)(A)]/12
| ||
Where:
| ||||
WQv
|
=
|
water quality volume in acre-feet
| ||
c
|
=
|
Rational Method postdevelopment runoff coefficient
for the two-year storm
| ||
P
|
=
|
1.25 inches
| ||
A
|
=
|
Area in acres of proposed regulated activity
|
REv
|
=
|
(0.25) * (I)/12
| ||
Where:
| ||||
REv
|
=
|
Recharge Volume in acre-feet
| ||
I
|
=
|
impervious area in acres
|
Area tributary to infiltration BMP/base area
of infiltration BMP* 100%
| ||
The area tributary to the infiltration BMP shall
be weighted as follows:
| ||
All disturbed areas to be made impervious: weight
at 100%
| ||
All disturbed areas to be made pervious: weight
at 50%
| ||
All undisturbed pervious areas: weight at 0%
| ||
All existing impervious areas: weight at 100%
|
Permeability Range*
|
Thickness Factor
| |
---|---|---|
6.0 to 12.0 inches/hour
|
0.8
| |
2.0 to 6.0 inches/hour
|
1.0
| |
1.0 to 2.0 inches/hour
|
1.4
| |
0.75 to 1.0 inches/hour
|
1.2
| |
0.5 to 0.75 inches/hour
|
1.0
|
*If the permeability rate (as measured by the
adapted 25 PA Code § 73.15 percolation test in Appendix
G) falls on a break between two thickness factors, the smaller thickness
factor shall be used. Sites with soil permeability greater than 12.0
inches per hour or less than 0.5 inches per hour, as measured by the
adapted 25 PA Code § 73.15 percolation test in Appendix
G, are not recommended for infiltration.
|
Return Period
|
24-Hour Rainfall Depth
(inches)
| |
---|---|---|
1-year
|
2.40
| |
2-year
|
3.00
| |
5-year
|
3.60
| |
10-year
|
4.56
| |
25-year
|
5.52
| |
50-year
|
6.48
| |
100-year
|
7.44
|
A graphical and tabular presentation of the
Type II-24 hour distribution is included in Appendix C.[3]
|