Contributing drainage area to stormwater filtration BMPs

Stormwater filtration BMPs - contributing drainage area
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Stormwater BMP	Recommended contributing area	Notes
Dry swale or step pool	5 acres or less	A dry swale or step pool must be able to drawdown within 48-hours in order to preserve vegetation.
Wet swale	5 acres or less	The boundary between upland and wetland type vegetation should be considered in the design. Different plant species will be tolerant of different periods of inundation.
Biofiltration Basin	5 acres or less	Biofiltration basins must meet the required 48 hour drawdown time and must be sized in order to allow for adequate maintenance. It is HIGHLY RECOMMENDED that biofiltration basins be designed to prevent high levels of bounce as submerging vegetation may inhibit plant growth. A maximum wet storage depth of 1.5 feet is HIGHLY RECOMMENDED.
Permeable Pavement (with underdrain)	It is RECOMMENDED that external contributing drainage area not exceed the surface area of the permeable pavement. It is HIGHLY RECOMMENDED that external contributing drainage area not exceed twice the surface area of the permeable pavement	It is RECOMMENDED that external drainage area be as close to 100% impervious as possible. Field experience has shown that drainage area (pervious or impervious) can contribute particulates to the permeable pavement and lead to clogging. Therefore, sediment source control and/or pretreatment should be used to control sediment run-on to the permeable pavement section.
Tree Trench/Tree Box (with underdrain)	up to 0.25 acres per tree
Green roof	Not applicable	Contributing drainage from conventional roofs must be no larger than the surface area of the green roof

References: Virginia, North Carolina, West Virginia, Maine, Lake Tahoe, Connecticut, Massachusetts, New York, Wisconsin, Vermont, New Hampshire, Ontario, Pennsylvania