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.
|
Rational Method Runoff Coefficients ("C") for the Design
of Storm Sewer
| ||
---|---|---|
Description of Area
|
Runoff Coefficient ("C")
| |
Residential
| ||
Two-acre lots
|
0.4
| |
One-acre lots
|
0.45
| |
One-half-acre lots
|
0.5
| |
One-fourth-acre lots
|
0.6
| |
Townhouses, multifamily
|
0.7
| |
Commercial
|
0.75
| |
Industrial
|
0.8
| |
Parks, cemeteries
|
0.35
| |
Meadow
|
0.3
| |
Unimproved (forest, brush, meadow, mixed)
|
0.25
|
Rational Method Runoff Coefficients ("C") for the Design
of Storm Sewer (Smaller Watersheds)
| ||
---|---|---|
Description of Area
|
Runoff Coefficient ('C')
| |
Impervious
|
0.95
| |
Grass
|
0.35
| |
Forest
|
0.25
|
Where practicable and appropriate the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to one
inch of runoff (I) over all proposed impervious surfaces.
|
The Rev required shall be computed as:
| ||||
Rev = (1/12) * (I)
| ||||
Where:
| ||||
Rev = Recharge Volume (cubic feet)
| ||||
I = Impervious Area within the limits of earth disturbance (square
feet)
| ||||
An asterisk (*) in equations denotes multiplication.
|
Table 408.1
| |||
---|---|---|---|
Peak Rate Control Standards for All Sites in Horsham Township
| |||
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
Reduce To
| |||
Two-year
|
One-year
| ||
Five-year
|
Two-year
| ||
Ten-year
|
Five-year
| ||
Twenty-five-year
|
Ten-year
| ||
Fifty-year
|
Twenty-five-year
| ||
One-hundred-year
|
Fifty-year
|
NOTES:
| |
---|---|
Projects that are required to obtain a NPDES permit for stormwater
discharges associated with construction activities are required to
show no increase in peaks from existing conditions.
|
Table 409.1
| |||
---|---|---|---|
Acceptable Computation Methodologies For Stormwater Management
Plans
| |||
Method
|
Method Developed By
|
Applicability
| |
WINTR-20
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
WINTR-55
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites less than two 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
|
NOTES:
| |
---|---|
Successors to the above methods are also acceptable. These successors
include WinTR55 for TR-55 and WinTR20 for TR-20.
|