An underdrain system should be incorporated beneath the filter media to provide adequate filtration of runoff volume . The underdrain should contain a continuous closed-joint perforated plastic pipe with a minimum 4-inch diameter , an 8-inch minimum gravel filter layer , a nonwoven geotextile filter fabric to separate the gravel from the native soils and the gravel from the filter media , and a minimum 4-inch diameter non-perforated PVC clean out wells . The underdrain pipes should be designed to lie on top of a 3-inch layer minimum of underlying aggregate and graded downstream with a minimum 0.5 % longitudinal slope . The maximum spacing of pipe underdrains is 10 feet . The underdrain system is designed to safely pass the calculated peak drawdown rate .
Where nitrogen or phosphorus is a concern , create a 90-degree elbow in the underdrain system from the bottom of the Bioretention area to create an Internal Water Storage Zone to encourage the denitrification process . If enhanced nitrogen and phosphorus removal is required , plans should include an appropriately sized internal water storage zone to handle water quality volume . This zone should be designed to hold the water quality volume below the outlet of the underdrain system , as well as treat the water quality volume of runoff from the entire drainage basin . A nonwoven geotextile fabric needs to be placed between the internal water storage zone and the underlying native soil . However , placement of the nonwoven geotextile fabric is not required between this zone and the underdrain system . The internal water storage zone consists of a mixture of the filter media and the stone used in the underdrain system . Suitable carbon sources , like wood chips , added to the gravel in the internal water storage zone can enhance the denitrification process by providing a nutrition source for anaerobic microbes . To calculate the surface area of the internal water storage zone area , divide the water quality volume by the ponding depth ( minimum 12 inches ). Provide a minimum of 12 inches of Filter Media above the maximum ponding height of the internal water storage zone . A valve for dewatering the internal water storage zone should be installed if prolonged standing water occurs .
The filter media for the site should consist of loamy sand or sandy loam soil . Soils originating on site or from excavation should not be used in the filter media . The filter media should be level across the surface to allow uniform distribution of temporary ponding across the site . The maximum ponding depth above the filter media is 9-12 inches to allow the area to drain within a reasonable time and to prevent long periods of plant submergence . The minimum infiltration rate of the filter media should be between 1-6 inches per hour , with an average total porosity of approximately 45 % as a function of depth and volume . The filter media being used needs to be tested in order to determine if it meets the textural classifications and chemical specifications required through County permitting .
Table 4 . Bioretention underdrain material specifications ( Charleston County , 2017 )
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NATURE-BASED SOLUTIONS MANUAL FOR KIAWAH ISLAND