The underdrain system should be covered with No . 57 aggregate a minimum depth of 8 inches . Place a Class 2 , Type C nonwoven geotextile fabric between the boundary of the gravel and the filter media to prohibit the filter media from filtering down to the perforated pipe underdrain . Place an outflow structure at the elevation of the maximum 9-inch to 12-inch ponding depth of the bioretention area to carry excess runoff from the bioretention area downstream .
The filter media consisting of loamy sand or sandy loam should be placed over the aggregate . Soils originating on site 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 porosity of approximately 45 % as a function of volume and depth .
BIORETENTION AREAS
The appropriate plants should be placed throughout the bioretention site following the development of an approved landscape plan . Plantings should conform to the current standards of the American Standard for Nursery Stock . Native grasses and perennials should be planted in clusters 12-18 inches apart and staggered . Native shrubs incorporated in the plantings should be a minimum of 2 feet in height ( generally 5-gallon containers or larger ). Plant shrubs of the same species in clusters 10 feet on-center . Do not plant shrubs near the inflow and outflow points of the Bioretention area . Any trees installed on site should be greater than 1 inch in diameter , free of defects , have a strong central leader to prevent future structural weaknesses , and be placed along the perimeter . Trees should be planted at a density of one tree every 250 square feet , avoiding placement directly above or near underdrains . Depending on the site designs , tall turfgrass can also be installed at this step .
Additional compost can be added when planting to help retain moisture and provide a gradual source of nutrients . Finally , a layer of hardwood mulch is placed on top of the exposed soil 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 .
Maintenance and Monitoring
Charleston County suggests the maintenance requirements of bioretention areas are low . A summary of maintenance requirements can be found on the following page ( Charleston County , 2017 ; see Table 5 ). The property owner or designated entity should conduct general landscape maintenance of the site post-installation . Generally during the first year , irrigation should be installed in order to allow the transplanted vegetation to establish . The first few years while the plants are being established , the bioretention area will need to be weeded occasionally . After plants establish and begin to fill in the area , the frequency of weeding is expected to be reduced . Irrigation should be gradually reduced over the establishment year and eventually turned off , only to be used in the future during periods of drought .
FALL 2022
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