ENHANCED BIOSWALES
Much like bioretention areas , bioswales are considered a gray-green infrastructure alternative that is engineered to improve drainage and infiltration in the application area . The primary difference between bioretention areas and bioswales lies in their construction . Bioswales are linear , vegetated ditches which allow for the collection , conveyance , filtration , and infiltration of stormwater ( SUNY College of Environmental Science and Forestry , 2021 ). They are typically found along roadsides and include low-lying vegetation like grasses , shrubs , and forbs . Bioswales can serve as a stand-alone feature or can be designed to combine with other practices , which redirect and treat stormwater before being discharged to receiving waters . The term " Bioswale " is sometimes used as a catch-all term for vegetated swales , grass swales , or filter strips ; however , each of these stormwater management techniques have slight differences in design . Charleston County outlines specific design criteria for “ Enhanced Bioswales ” in their 2017 Permitting Standards and Procedures Manual . Enhanced bioswales are engineered landscape features that utilize porous media at the bottom of the channel to improve water quality , capture suspended sediments , and reduce runoff volumes and flow rate .
ENHANCED BIOSWALES
This stormwater management technique also incorporates underdrains , overflow outlets , gravel beds , and engineered soils to further promote infiltration and treatment of stormwater . Flow rate can be further controlled by adding rock check dams at specified intervals along the conveyance to help slow runoff within the swale . The overflow outlets help control the amount of water contained in the bioswale during high-intensity rainfall events to prevent localized flooding and redirect into downstream treatment areas ( e . g ., ponds ). Gravel and underdrains facilitate infiltration and provide for additional storage while vegetation helps with treating stormwater that collects in these areas through natural plant processes ( phytoremediation ). Enhanced bioswales can serve as a stand-alone feature or built-in combination with other practices which redirect and treat stormwater before being discharged to receiving waters . Consultation with professional engineers is required when considering the implementation of bioswales so they are designed in accordance with Charleston County Permitting Standards . Most of the specifications and recommendations herein originate from the County Stormwater Program ’ s second edition of the Permitting Standards and Procedures Manual produced in 2017 .
Site Selection
Enhanced bioswales should be located in areas immediately downstream from impervious surfaces where stormwater runoff can be intercepted . Common examples include areas adjacent to roads , driveways and parking lots . This can also include areas where stormwater can easily be redirected away from an area where water already collects . It is recommended to place bioretention areas at least 20 feet away from any structures to prevent impacts to the foundation . They should also be placed at least 25 feet away from septic systems , if present .
The size of the bioswale will also be dependent on the amount of runoff being collected . Charleston County specifies that bioswales be located parallel to roadways and sized at 100 feet in length for a road length of 300 feet . ( Table 6 ).
Materials and Preparation
Property owners or contractors should consult with an engineering firm or professional engineer with experience in constructing these systems if they are interested in the viability of this stormwater management practice in their area . 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 .
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