Soil Cover Complex Method:
| |
---|---|
Step 1: Runoff (inches) = Q = (P - 0.2S)2/(P + 0.8S)
|
Where:
| ||||
P
|
=
|
Two-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 inches 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 pre- and post-development conditions)
- Protected Area}
| |
Required
|
Nonstructural
|
Structural
| |
---|---|---|---|
Volume Control (ft3) -
|
Volume Control (ft3) -
|
Volume Requirement (ft3)
|
Table 304.1
| |||
---|---|---|---|
Peak Rate Runoff Control Standards by Stormwater Management
Districts in the Neshaminy Creek Watershed
| |||
District
|
Design Storm Post-Development (Proposed Conditions)
|
Design Storm Predevelopment (Existing Conditions)
| |
A
|
2-year
|
1-year
| |
5-year
|
5-year
| ||
10-year
|
10-year
| ||
25-year
|
25-year
| ||
50-year
|
50-year
| ||
100-year
|
100-year
| ||
B
|
2-year
|
1-year
| |
5-year
|
2-year
| ||
10-year
|
5-year
| ||
25-year
|
10-year
| ||
50-year
|
25-year
| ||
100-year
|
50-year
| ||
C
|
2-year
|
2-year
| |
5-year
|
5-year
| ||
10-year
|
10-year
| ||
25-year
|
25-year
| ||
50-year
|
50-year
| ||
100-year
|
100-year
|
Table 304.2
| |||
---|---|---|---|
Peak Rate Runoff Control Standards by Stormwater Management
Districts in the Delaware River South Watershed
| |||
District
|
Design Storm Post-Development
(Proposed Conditions)
|
Design Storm Predevelopment
(Existing Conditions)
| |
A
|
2-year
|
1-year
| |
5-year
|
5-year
| ||
10-year
|
10-year
| ||
25-year
|
25-year
| ||
50-year
|
50-year
| ||
100-year
|
100-year
| ||
B
|
2-year
|
1-year
| |
5-year
|
2-year
| ||
10-year
|
5-year
| ||
25-year
|
10-year
| ||
50-year
|
50-year
| ||
100-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
|
To achieve this goal, the following criterion is established:
| |
The following calculation formula is to be used to determine
the water quality storage volume (WQv) in acre-feet of storage for
the Delaware River South watershed:
|
Equation: 307.1
| |
WQv = [(P)(Rv)(A)]/12
| |
Where:
| |
WQv = Water quality volume (acre-feet).
| |
P = Rainfall amount equal to 90% of events producing this rainfall
(inches).
| |
A = Area of the project contributing to the water quality BMP
(acres).
| |
Rv = 0.05 + 0.009(I), where I is the percent of the area that
is impervious surface (impervious area/A*100).
|
The P value for the five PennDOT rainfall regions is shown in
Figure B-2 in Appendix B of the Model Ordinance within the Plan. Since
the Delaware River South is in PennDOT Region 5, the P value to be
utilized to meet this requirement is 2.04 inches.
|
The following criteria shall apply.
| |
The NRCS runoff shall be utilized to calculate infiltration
requirements (P) in inches.
|
For zero runoff:
| ||||
Where:
| ||||
Equation: 308.1
| ||||
P
|
=
|
I (Infiltration) = (200/CN) - 2
| ||
P
|
=
|
I = infiltration requirement (inches)
| ||
CN
|
=
|
SCS (NRCS) curve number of the existing conditions contributing
to the recharge facility
|
This equation can be displayed graphically in, and the infiltration
requirement can also be determined from, Figure 308-1.
| |
The recharge volume (Rev) required would
therefore be computed as:
| |
Equation: 308.2
| |
Rev = I * impervious area (SF) /12 =
cubic feet (CF)
|
It has been determined that infiltrating 0.5 inch of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of § 25-308, Subsection 1A(2)(a) cannot be achieved, then 0.5 inch of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions' curve number of 81. Above a curve number of 81, Equation 308.1 or the curve in Figure 308.1 should be used to determine the infiltration requirement.
|
Where:
| ||||
I
|
=
|
0.5 inch
| ||
The recharge volume (Rev) required would
therefore be computed as:
| ||||
Rev
|
=
|
I * percent impervious area (SF)/12 = cubic feet (CF)
|