Step 1: Runoff (inches) = Q = (P - 0.2S)2/(P + 0.8S)
|
Where:
| ||||
---|---|---|---|---|
P
|
=
|
2-year rainfall (inches)
| ||
S
|
=
|
(1,000/CN) - 10, the potential maximum retention (including
initial abstraction, Ia)
|
Step 2: Runoff Volume (cubic feet) = Q x Area x 1/12
|
Where:
| ||||
---|---|---|---|---|
Q
|
=
|
Runoff (inches)
| ||
Area
|
=
|
SWM Area (square feet)
|
Volume (cubic feet) = (2-inch runoff/12 inches) * impervious
surface (square feet)
|
Volume (cubic feet) = (1-inch runoff/12 inches) * impervious
surface (square feet)
|
Stormwater Management Site Area =
Total Site Area (for both predevelopment and post-development
conditions) - Protected Area
|
Required Volume Control (cubic feet)
|
—
|
Nonstructural Volume Control (cubic feet)
|
=
|
Structural Volume Requirement (cubic feet)
|
Table 304.1
Peak Rate Runoff Control Standards by Stormwater Management
Districts
in the Neshaminy Creek Watershed
(includes Little Neshaminy Creek)
| |||
---|---|---|---|
District
|
Design Storm, Post Development
(proposed conditions)
|
Design Storm, Predevelopment
(existing conditions)
| |
A
|
2-year
5-year
10-year
25-year
50-year
100-year
|
1-year
5-year
10-year
25-year
50-year
100-year
| |
B
|
2-year
5-year
10-year
25-year
50-year
100-year
|
1-year
2-year
5-year
10-year
25-year
50-year
| |
C
|
2-year
5-year
10-year
25-year
50-year
100-year
|
2-year
5-year
10-year
25-year
50-year
100-year
|
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.
|
Table 305.1
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
---|---|---|---|
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
|
U.S. 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 Kuichling (1889)
|
For sites less than 200 acres, or as approved by the municipality
and/or Municipal Engineer.
| |
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer.
|