The following information is from the Department of Environmental Protection, Bureau of Watershed Management. Pennsylvania Stormwater Best Management Practices Manual,
Chapter 5: Non-Structural BMPs and Chapter 8: Stormwater Calculations and Methodology. December, 2006.
Chapter 5: Non-Structural BMPs and Chapter 8: Stormwater Calculations and Methodology. December, 2006.
PRESERVATION OF NATURAL RESOURCE FEATURES OF THE DEVELOPMENT SITE
(e.g., FLOODPLAINS, WETLANDS, PRAIRIES, AND WOODLANDS)
Preserved natural resource features are not to be included in the Runoff Volume calculation
Stormwater Management Area = (Total Area - Preserved Area)
Runoff from the Preserved Areas may be excluded from peak rate calculations for runoff control, provided that the runoff from the Preserved Area is not conveyed to and/or through stormwater management control structures. If necessary, runoff from Preserved Areas should be directed around BMPs and stormwater pipes and inlets by means of vegetated swales or low berms that direct flow to natural drainage ways.
PRESERVATION OF THE EXISTING NATURAL STREAMS, CHANNELS, AND DRAINAGE WAYS
A volume reduction may be credited based upon the area of the natural drainage feature that is vegetated.
Volume Reduction (ft
3) = Area x ¼" runoff
= Vegetated Area of Natural Drainage Feature (ft2) x ¼"/12
The peak rate is reduced by a longer travel time of runoff through natural drainage features. The time of travel (Tt) after development may be considered the same as the Tt before development for flows through natural drainage features. When calculating flow rates:
Tt_before = Tt_after
MINIMIZING IMPERVIOUS SURFACES
Minimizing impervious surfaces is "self-crediting" in that the use of this BMP automatically provides a reduction in impervious area and a corresponding reduction in stormwater impacts.
THE USE OF NATURAL LANDSCAPING AS AN ALTERNATIVE TO TURF GRASS
A volume reduction may be credited in the same manner as for preservation:
Volume Reduction (ft3) = Area x ¼" runoff
= Vegetated Area of Natural Landscaping (ft2) x ¼"/12
The peak rate for runoff will be reduced by using the reduced volume as calculated above.
THE USE OF OPEN VEGETATED CHANNELS, FILTER STRIPS, AND INFILTRATION (BASINS, TRENCHES, FLOODPLAIN RESTORATION, ETC.)
TO CONVEY, FILTER, AND INFILTRATE STORMWATER RUNOFF
TO CONVEY, FILTER, AND INFILTRATE STORMWATER RUNOFF
Storage volume equals the amount of runoff the facility can hold.
PRESERVATION OF THE NATURAL INFILTRATION AND STORAGE CHARACTERISTICS OF THE SITE (E.G. DISCONNECTION OF IMPERVIOUS COVER,
ON-LOT BIORETENTION FACILITIES, ROOFTOP DETENTION, PARKING LOT DETENTION)
ON-LOT BIORETENTION FACILITIES, ROOFTOP DETENTION, PARKING LOT DETENTION)
A volume reduction may be credited based upon the area that will be disconnected from a storm sewer or other structural facility and drain into an infiltrating area instead.
Volume Reduction (ft3) = Disconnected area (ft2) x ¼"/12
The peak rate for runoff will be reduced by using the reduced volume as calculated above.
STRUCTURAL MEASURES THAT PROVIDE WATER QUALITY AND QUANTITY CONTROL
(STORMWATER WETLANDS, WET DETENTION FACILITIES, SEDIMENTATION TRAPS, ETC.);
(STORMWATER WETLANDS, WET DETENTION FACILITIES, SEDIMENTATION TRAPS, ETC.);
Storage volume equals the amount of runoff the facility can hold.
STRUCTURAL MEASURES THAT PROVIDE ONLY QUANTITY CONTROL AND CONVEYANCE.
Storage volume equals the amount of runoff the facility can hold.
(Ord. 9097, passed 5-16-17)