Figure 13 . General design of a bioretention area adapted from Charleston County ( 2017 )
The size of the bioretention area will also be dependent on the amount of runoff being collected . The County ’ s recommended target drainage area for bioretention areas is between 0.5 to 2 acres . The resulting surface area for bioretention areas should be 3-8 % of the contributing watershed , depending on the amount of impervious surface in the drainage area . The USEPA recommends for bioretention areas to be sized to at least 5-10 % of the surface area which introduces stormwater , with 7 % being the average size ( EPA , 1999 ). The runoff coefficient generated for an individual watershed area should be used to adjust these generalized percentages . Charleston County places the following constraints on the design of bioretention areas ( Charleston County , 2017 ; see Table 3 and Table 4 ):
›››› Space Required : Minimum surface area range : 50 to 200 feet 2 , Minimum width : 5 to 10 feet , Minimum length : 10 to 20 feet , Minimum depth : 2 to 4 feet . Typical bioretention areas have a minimum width of 10 feet and a minimum flow length of 40 feet to establish a strong healthy stand of vegetation .
›››› Slopes : 4:1 preferred , 3.5:1 maximum .
›››› Water Table : 12 inch clearance above the water table recommended . Avoid areas with shallow water tables that are within 3 feet from the bottom of the bioretention area .
›››› Proximity to building foundations : Minimum distance of 10 feet down gradient from buildings and foundations . Furthermore , they should also be placed at least 25 feet away from septic systems . ( Clemson Extension ).
›››› Maximum depth : 2 to 4 foot depth depending on soil type
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NATURE-BASED SOLUTIONS MANUAL FOR KIAWAH ISLAND