Table 132-13-1 Allowable Velocity
| |||
---|---|---|---|
Allowable Velocity
| |||
Material
|
Velocity in Feet per Second
(fps)
| ||
Well established grass on good soil
| |||
Short pliant bladed grass
|
4.0 to 5.0
| ||
Bunch grass - soil exposed
|
2.0 to 3.0
| ||
Stiff stemmed grass
|
3.0 to 4.0
| ||
Earth without vegetation
| |||
Fine sand or silt
|
1.0
| ||
Ordinary firm loam
|
2.0 to 3.0
| ||
Stiff clay
|
3.0 to 5.0
| ||
Clay and gravel
|
4.0 to 5.0
| ||
Coarse gravel
|
4.0 to 5.0
| ||
Soft shale
|
5.0 to 6.0
| ||
Shoulders
| |||
Earth
|
(as defined above)
| ||
Stabilized
|
6.0
| ||
Paved
|
10.0 to 15.0
|
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.
|
Table 132-14-1
| |||
---|---|---|---|
Acceptable Computation Methodologies for Stormwater Management
Designs
| |||
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)
|
Applicable sites less than 200 acres, or as approved by the
Township Engineer
| |
Other methods
|
Varies
|
Other computation methodologies approved by the Township Engineer
|
WQv = [(P)(Rv)(A)]/12,
where:
|
P
|
=
|
Rainfall depth in inches, using the "90% storm," the volume
of rainfall for 90% of the storm events which produce runoff in the
watershed annually.
| |
A
|
=
|
Project area in acres.
| |
Rv
|
=
|
Volume runoff coefficient [0.05 + 0.009(I)], where I is the
impervious surface percentage (impervious area ÷ total project
area) x 100%.
|