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
|
US 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 Borough
| |
EFH2
|
USDA NRCS
|
Applicable in rural and undeveloped areas subject to the program
limits
| |
Other methods
|
Varies
|
Other methodologies as approved by the Borough
|
*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?
|
No
|
Yes
|
Yes
| |
(e)
|
Spillway requirements
| ||||
[1]
|
Spillway freeboard (minimum)
|
Not required
|
3 inches
|
6 inches
| |
[2]
|
Width (minimum)
|
Not required
|
10 feet
|
20 feet
| |
[3]
|
Width (maximum)
|
Not required
|
50 feet
|
50 feet
| |
[4]
|
Spillway channel design
|
Not required
|
Required
|
Required
| |
[5]
|
Routing of 100 year storm
|
Permitted
|
Permitted
|
Permitted
|
NOTES:
|
---|
*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
|
NOTE:
|
---|
*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
|
Nonvehicular 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
|
600 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 (non-residential area)
|
4:1 max
|
N/A
|
3:1 max
| |
[6]
|
Bottom width to flow depth ratio
|
12:1
|
N/A
|
12:1
| |
(e)
|
Outlet design
| ||||
[1]
|
End treatment
|
Headwall/endwall
|
N/A
|
Headwall/endwall or flared end section
| |
[2]
|
Energy dissipater
|
Required
|
N/A
|
Required
|
NOTES:
|
---|
N/A = Not applicable or no criteria specified
|
SLHDPE = Smooth lined high density polyethylene pipe; PVC =
Polyvinyl chloride;
|
RCP = Reinforced concrete pipe
|