BIORETENTION AREAS
Rain gardens are generally acceptable for providing alleviation of stormwater runoff issues on a small-scale . This typically ranges from buildings and impervious surfaces on residential or commercial lots . Larger drainage areas often require mitigation projects with additional surface coverage and engineering to compensate for the level of runoff to be collected . Bioretention areas are similar to rain gardens , but are considered a green infrastructure alternative that is engineered to manage stormwater within larger drainage areas between 0.5 to 2 acres . Bioretention areas incorporate underdrains , overflow inlets , gravel beds , and engineered soils to further promote infiltration and treatment of stormwater . These can also serve as stand-alone practices or in combination with other stormwater management techniques which redirect and treat runoff before being discharged downstream .
BIORETENTION AREAS
As with rain gardens , these areas collect stormwater to promote infiltration of stormwater into the soil . The incorporation of underdrains , overflow outlets , and gravel beds increases the capacity and reduces the time the application area treats stormwater . Overflow outlets help relieve the amount of water introduced into the area during high-intensity rainfall events to prevent localized flooding and redirect into downstream treatment areas ( i . e ., ponds ). Gravel and underdrains facilitate the movement of water within the lower soil column . Vegetation within the bioretention areas helps with treating stormwater that collects in these areas through natural plant processes ( phytoremediation ). Additionally , the contaminants captured in these areas are held for an extended amount of time to allow their breakdown through various environmental processes ( e . g ., photodegradation , microbial degradation ). Consultation with professional engineers is required when considering the implementation of bioretention areas so that 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
Bioretention areas should be located in areas downstream from impervious surfaces where a majority of stormwater runoff from an area can be collected , such as areas where a large amount of stormwater collects downstream from several nearby roadways and impervious surface cover . Initial field observations of water flow from impervious surfaces in the area should be considered before considering bioretention as an option . Professional engineers or professional landscape architects should be consulted to assess the drainage in the area , develop plans for bioretention areas , and facilitate the permitting process with the necessary agencies . Areas with a perched water table within 3 feet of the soil surface should be avoided if possible .
General Design Criteria and Preparation
Bioretention areas should be designed to fit around the natural topography and complement the surrounding landscape . Bioretention areas can be of any reasonable shape and can be fit around sensitive areas , natural vegetation , roads , driveways , and parking lots . Typical bioretention areas have a minimum width of ten feet and a minimum flow length of 40 feet to establish a strong healthy stand of vegetation . Figure 13 shows a general design of a bioretention area adapted from designs from Charleston County ’ s Standards and Procedures Manual ( 2017 ).
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