Soil-Cover-Complex Method
|
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, la).
|
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 inches runoff/12 inches) x Impervious
Surface (square feet)
|
Volume (cubic feet) = (1 inch runoff/12 inches) x Impervious
Surface (square feet)
|
Stormwater Management Site Area = [Total Site Area (for both
pre- and post-development conditions) – Protected Area]
|
Required Volume Control (cubic feet)
|
—
|
Nonstructural Volume Control (cubic feet)
|
=
|
Structural Volume Requirement (cubic feet)
|
Table 353-13A
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
|
Where:
| |||
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 353-14C
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
|
United States 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 Borough
of Chalfont and/or the Borough of Chalfont Engineer
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer
|
Q = CiA
|
Where:
| |||
Q
|
=
|
Peak runoff rate, in cubic feet per second (cfs).
| |
C
|
=
|
Runoff coefficient equal to the ratio of the peak runoff rate
to the average rate of rainfall over a time period equal to the time
of concentration.
| |
i
|
=
|
Average rainfall intensity in inches per hour for a time equal
to the time of concentration.
| |
A
|
=
|
Drainage area in acres.
|
NOTE: Appropriate values for the runoff coefficient and rainfall
intensity can be found in the Commonwealth of Pennsylvania Department
of Transportation Design Manual, Part 2, as amended and updated.
|