Table III-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
| |
WinTR-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 Elizabeth Township
| |
EFH2
|
USDA NRCS
|
Applicable in rural and undeveloped areas subject to the program
limits
| |
Other methods
|
Varies
|
Other methodologies approved by Elizabeth Township.
|
Table III-2. Aboveground storage facility design criteria.
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 100-year storm
|
Permitted
|
Not permitted
|
Not permitted
|
Table III-3. Subsurface storage facility design criteria:
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
|
*
|
Unless otherwise determined by professional geologic evaluation.
|
Table III-4. Conveyance facility design criteria:
Conveyance Facility Design Criteria
| |||
---|---|---|---|
Location
|
Within Public Street Right-of-Way
|
Outside Public Street Right-of-Way
| |
Loading
|
All
|
Vehicular Loading
|
Non-Vehicular Loading
|
Pipe design
| |||
Material
|
HDPE, RCP
|
PVC, HDPE, RCP
|
PVC, HDPE, RCP
|
Slope (minimum)
|
0.5%
|
0.5%
|
0.5%
|
Cover
|
1 foot to stone subgrade
|
1 foot to stone subgrade
|
1 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 maximum
|
N/A
|
4:1 maximum
|
Side slopes (nonresidential area)
|
4:1 maximum
|
N/A
|
3:1 maximum
|
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
|