Table III-1
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
---|---|---|---|
Method
|
Method Developed By
|
Applicability
| |
TR-20*
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
Win 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 hydrology computer model is desirable
or necessary
| |
Rational Method*
|
Emil Kuichling (1889)
|
For development sites less than 200 acres, Tc < 60 min. or
as approved by the Township
| |
EFH2
|
USDA NRCS
|
Applicable in rural and undeveloped areas subject to the program
limits
| |
Other methods
|
Varies
|
Other methodologies as approved by the Township
|
*Includes commercial computer packages based on the model
|
Length of Pervious Flow Path*
(feet)
|
Roof Area Treated as Disconnected
|
DIA Credit Factor
| |
---|---|---|---|
Less than 15 feet
|
0%
|
1.0
| |
15 feet to less than 30 feet
|
20%
|
0.8
| |
30 feet to less than 45 feet
|
40%
|
0.6
| |
45 feet to less than 60 feet
|
60%
|
0.4
| |
60 feet to less than 75 feet
|
80%
|
0.2
| |
75 feet or more
|
100%
|
0
|
*Pervious flow must be at least 15 feet from any impervious
surface and cannot include impervious surfaces.
|
Aboveground Storage Facility Design Criteria
| |||
---|---|---|---|
Facility Depth
| |||
Less than 2 Feet
|
2 Feet to 8 Feet
|
Greater Than 8 Feet
| |
(a) Embankment geometry
| |||
[1] Top width (minimum)
|
2 feet
|
5 feet
|
8 feet
|
[2] Interior side slope (maximum)
|
2:1
|
3:1
|
5:1
|
[3] Exterior side slope (maximum)
|
2:1
|
3:1
|
3:1
|
(b) Embankment construction
| |||
[1] Key trench
|
Not required
|
Required
|
Required
|
[2] Pipe collar
|
Not required
|
Required
|
Required
|
[3] Compaction density
|
Not required
|
Required
|
Required
|
(c) Internal construction
| |||
[1] Dewatering feature
|
N/A
|
Required
|
Required
|
[2] Pretreatment elements
|
Not required*
|
Required
|
Required
|
(d) Outlet structure
| |||
[1] Pipe size (minimum)
|
6 inches
|
12 inches
|
15 inches
|
[2] Pipe material
|
SLHDPE, PVC, RCP
|
SLHDPE, RCP
|
RCP
|
[3] Anticlogging devices
|
Required
|
Required
|
Required
|
[4] Antivortex design
|
Not required
|
Required
|
Required
|
[5] Watertight joints in piping?
|
Yes
|
Yes
|
Yes
|
(e) Spillway requirements
| |||
[1] Spillway freeboard (minimum)
|
Not required
|
3 inches
|
6 inches
|
[2] Width (minimum)
|
5 feet
|
10 feet
|
20 feet
|
[3] Width (maximum)
|
50 feet
|
50 feet
|
50 feet
|
[4] Spillway channel design
|
Not required
|
Required
|
Required
|
[5] Routing of 100-year storm
|
Permitted
|
Permitted
|
Permitted
|
*Pretreatment required for infiltration BMPs unless shown to
be unnecessary
|
N/A = Not applicable
|
SLHDPE = Smooth-lined high density polyethylene pipe; PVC =
Polyvinyl chloride;
|
RCP = Reinforced concrete pipe
|
Subsurface Storage Facility Design Criteria
| ||||
---|---|---|---|---|
Facility Type
| ||||
Infiltration and Storage
|
Storage without Infiltration
| |||
(a) Facility geometry
| ||||
[1] Depth from surface (maximum)
|
2 feet less than limiting zone
|
N/A
| ||
[2] Loading ratio (maximum)
|
Per BMP Manual*
|
N/A
| ||
(b) Distribution system requirements
| ||||
[1] Pipe size (minimum)
|
4 inches
|
4 inches
| ||
[2] Pretreatment
|
Required
|
Required
| ||
[3] Loading/balancing
|
Required
|
Not required
| ||
[4] Observation/access ports
|
Required
|
Required
|
*Unless otherwise determined by professional geologic evaluation.
|
Conveyance Facility Design Criteria
| ||||
---|---|---|---|---|
Location
|
Within Public Street Right-of-Way
|
Outside Public Street Right-of-Way
| ||
Loading
|
All
|
Vehicular Loading
|
Non-Vehicular Loading
| |
(a) Pipe design
| ||||
[1] Material
|
SLHDPE, RCP
|
PVC, SLHDPE, RCP
|
PVC, SLHDPE, RCP
| |
[2] Slope (minimum)
|
0.5%
|
0.5%
|
0.5%
| |
[3] Cover
|
1 foot to stone subgrade
|
1 foot to stone subgrade
|
1 foot to surface
| |
[4] Diameter (minimum)
|
15 inches
|
15 inches
|
8 inches
| |
[5] Street-crossing angle
|
75° to 90°
|
N/A
|
N/A
| |
[6] Access/maintenance port frequency (maximum)
|
400 feet
|
400 feet
|
600 feet
| |
(b) Inlet design
| ||||
[1] Material
|
Concrete
|
Concrete
|
N/A
| |
[2] Grate depression
|
2 inches
|
2 inches
|
1 inch minimum
| |
(c) Manhole design
| ||||
[1] Material
|
Concrete
|
Concrete
|
Concrete
| |
(d) Swale design
| ||||
[1] Freeboard (minimum)
|
6 inches
|
N/A
|
6 inches
| |
[2] Velocity (maximum)
|
Stability check
|
N/A
|
Stability check
| |
[3] Slope (minimum)
|
1%
|
N/A
|
1%
| |
[4] Side slopes (residential area)
|
4:1 max
|
N/A
|
4:1 max
| |
[5] Side slopes (nonresidential area)
|
4:1 max
|
N/A
|
3:1 max
| |
[6] Bottom width to flow depth ratio
|
12:1 max
|
N/A
|
12:1 max
| |
(e) Outlet design
| ||||
[1] End treatment
|
Headwall/ endwall
|
N/A
|
Headwall/ endwall or flared end section
| |
[2] Energy dissipater
|
Required
|
N/A
|
Required
|
N/A = Not applicable or no criteria specified
|
SLHDPE = Smooth-lined high-density polyethylene pipe;
|
PVC = Polyvinyl chloride;
|
RCP = Reinforced concrete pipe
|