Bioretention areas are designed to efficiently remove at least 80 % of the average annual post-development total suspended solids from stormwater runoff . To reach this removal efficiency , bioretention areas are designed to have a water quality volume ( amount of stormwater runoff from any given storm that should be captured and treated ) equal to :
›››› 1 inch of runoff from impervious areas located on the Project site , or
BIORETENTION AREAS
›››› 1.5 inches of runoff from built-upon portions of the Project if located within 1,000 feet of shellfish beds .
Bioretention areas are required to be designed to treat the water quality volume of runoff from the entire drainage basin . Bioretention areas work best when constructed off-line , capturing only the water quality volume . Excess runoff should be diverted away from the bioretention area or collected with an overflow catch basin . Bioretention areas should fully de-water within a 24 to 48-hour period depending on the dimensions , filter media , and underdrain system incorporated within the design . In order to allow for proper pollutant removal , the system should be designed to permit the ponded runoff above the bioretention area surface to drain in a maximum of 12 hours . It should be designed for runoff within the filter media to drain within 48 hours . Refer to Appendix A within the County ’ s Permitting Standards and Procedures Manual ( 2017 ) for the general equation used to determine the draw down time .
Table 3 . Design characteristics for bioretention areas ( Charleston County , 2017 )
FALL 2022
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