Figure 15 . Longitudinal cross section of a bioretention system with perforated underdrain adapted from Charleston County ( 2017 )
A layer of hardwood mulch should be incorporated into the design to prevent erosion and provide additional absorption of water . Double shredded hardwood is acceptable , as well as pine straw in some instances . Pine bark nuggets should be avoided as they are prone to floating . Where vegetation is expected to fill in bare areas of the bioretention site , temporary erosion control blankets may be used in lieu of a hardwood mulch layer .
Utility lines should be identified prior to the design and preparation of the site . Property owners or contractors should contact # 811 to make a request for utility lines to be identified .
Installation
Begin by excavating the area to the dimensions , side slopes , and elevations as shown in the approved plans . This should result in the site complementing the natural features within the nearby landscape . Care should be taken to minimize compaction of the underlying soil during excavation , which can impede proper infiltration . The removed soil should be disposed of properly . The inside of the excavated area should be gradually sloping downstream , allowing the underlying drainage to have a minimum slope of 0.5 %. Following excavation , the site should be tilled to a depth of 12 inches and the bottom lined with a Class 2 , Type C nonwoven geotextile fabric .
After excavation , an underdrain system should be placed . To prepare the site for underdrains , a layer of No . 57 aggregate should be placed 3 feet wide and minimum of 3 inches deep on top of the nonwoven filter fabric . The underdrain pipes should then be placed on top of the underlying aggregate layer . The perforated underdrain drain pipe may be connected to a stormwater conveyance system or stabilized outlet . Install observation wells and / or clean-out pipes to perforated pipe sections at the ratio of one minimum per every 1,000 square feet of surface area , as shown on the plans . The wells or cleanouts should extend 6 inches above the mulch layer and closed with a screw cap . The ends of underdrain pipes not terminating in an observation well should be capped as well .
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NATURE-BASED SOLUTIONS MANUAL FOR KIAWAH ISLAND