Table 3-1
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
| |
Win 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
| |
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For development sites less than 200 acres, Tc<60 min. or
as approved by West Donegal Township
| |
EFH2
|
USDA NRCS
|
Applicable in rural and undeveloped areas subject to the program
limits
| |
Other methods
|
Varies
|
Other methodologies approved by West Donegal Township
|
Table 3-2
Aboveground Storage Facility Design Criteria
| ||||
---|---|---|---|---|
Facility Depth
| ||||
Less than 2 feet
|
2 feet to 8 feet
|
Greater than 8 feet
| ||
Embankment geometry
| ||||
Top width (minimum)
|
2 feet
|
5 feet
|
8 feet
| |
Interior side slope (maximum)
|
2:1
|
5:1
|
5:1
| |
Exterior side slope (maximum)
|
2:1
|
3:1
|
3:1
| |
Embankment construction
| ||||
Key trench
|
Not required
|
Required
|
Required
| |
Pipe collar
|
Not required
|
Required
|
Required
| |
Compaction density
|
Not required
|
Required
|
Required
| |
Internal construction
| ||||
Dewatering feature
|
N/A
|
Required
|
Required
| |
Pretreatment elements
|
Not required
|
Required
|
Required
| |
Outlet structure
| ||||
Pipe size (minimum)
|
6 inches
|
12 inches
|
15 inches
| |
Pipe material
|
HDPE, PVC, RCP
|
HDPE, RCP
|
RCP
| |
Anticlogging devices
|
Required
|
Required
|
Required
| |
Antivortex design
|
Not required
|
Required
|
Required
| |
Watertight joints in piping
|
Yes
|
Yes
|
Yes
| |
Spillway requirements
| ||||
Spillway freeboard (minimum)
|
3 inches
|
6 inches
|
12 inches
| |
Width (minimum)
|
Not required
|
10 feet
|
20 feet
| |
Width (maximum)
|
Not required
|
50 feet
|
50 feet
| |
Spillway channel design
|
Not required
|
Required
|
Required
| |
Routing of one-hundred-year storm
|
Permitted
|
Not permitted
|
Not permitted
|
Table 3-3
Subsurface Storage Facility Design Criteria
| |||
---|---|---|---|
Facility Type
| |||
Infiltration and Storage
|
Storage without Infiltration
| ||
Facility geometry
| |||
Depth from surface (maximum)
|
2 feet less than limiting zone
|
N/A
| |
Loading ratio (maximum)
|
Per BMP Manual*
|
N/A
| |
Distribution system requirements
| |||
Pipe size (minimum)
|
4 inches
|
4 inches
| |
Pretreatment
|
Required
|
Required
| |
Loading/balancing
|
Required
|
Not required
| |
Observation/access ports
|
Required
|
Required
|
NOTES:
| ||
---|---|---|
*
|
Unless otherwise determined by professional geologic evaluation.
|
Table 3-4
Conveyance Facility Design Criteria
| ||||
---|---|---|---|---|
Location
|
Within public street rights-of-way
|
Outside public street rights-of-way
| ||
Loading
|
All
|
Vehicular loading
|
Nonvehicular Loading
| |
Pipe design
| ||||
Material
|
HDPE, RCP
|
PVC, HDPE, RCP
|
PVC, HDPE, RCP
| |
Slope (minimum)
|
0.5%
|
0.5%
|
0.5%
| |
Cover
|
One foot to stone subgrade
|
One foot to stone subgrade
|
One foot to surface
| |
Diameter (minimum)
|
15 inches
|
15 inches
|
8 inches
| |
Street crossing angle
|
75° to 90°
|
N/A
|
N/A
| |
Access/ maintenance port frequency (maximum)
|
400 feet
|
400 feet
|
400 feet
| |
Inlet design
| ||||
Material
|
Concrete
|
Concrete
|
N/A
| |
Grate depression
|
2 inches
|
2 inches
|
1 inch minimum
| |
Manhole design
| ||||
Material
|
Concrete
|
Concrete
|
Concrete
| |
Swale design
| ||||
Freeboard (minimum)
|
6 inches
|
N/A
|
6 inches
| |
Velocity (maximum)
|
Stability check
|
N/A
|
Stability check
| |
Slope (minimum)
|
1%
|
N/A
|
1%
| |
Side slopes (residential area)
|
4:1 max
|
N/A
|
4:1 max
| |
Side slopes (nonresidential area)
|
4:1 max
|
N/A
|
3:1 max
| |
Bottom width to flow depth ratio
|
12:1
|
N/A
|
12:1
| |
Outlet design
| ||||
End treatment
|
Headwall/ endwall
|
N/A
|
Headwall/ endwall or flared end section
| |
Energy dissipater
|
Required
|
N/A
|
Required
|