RAIN GARDENS
A major contributor to flooding associated with rainfall is the amount of stormwater runoff from impervious surfaces . The inclusion of impervious surfaces ( e . g ., buildings , roadways , driveways ) decreases the ability of the stormwater to infiltrate , resulting in increased stormwater runoff , nutrient loading and contamination of receiving waters . Capturing stormwater downstream from these surfaces or redirecting runoff helps mitigate localized flooding and improve infiltration . The forest floor in natural habitats typically fulfills this role by acting like a “ sponge ,” which absorbs accumulated stormwater and gradually releases it over time . This can also be problematic in instances where there are multiple days of rainfall and the soil surface becomes too saturated to accept any more runoff .
RAIN GARDENS
Rain gardens are designed to mimic natural processes by retaining stormwater and allowing it to infiltrate into the soil . They are designed to intercept stormwater runoff from impervious surfaces in order to keep it from directly being discharged into ponds and open waterways . This process is accomplished by trapping stormwater runoff in a bowl-like depression where it is allowed to naturally infiltrate into the ground . The inclusion of vegetation in these areas helps improve transpiration of collected water , facilitate the phytoremediation of any chemicals and sediments transported by stormwater , and provides habitat for beneficial wildlife .
This landscaping practice is ideal for handling stormwater runoff from individual residential properties . Larger drainage areas with impervious surface coverage between 0.5 to 2 acres should refer to the following section on bioretention areas , which require the use of professional engineers for permitting and contracting . Numerous rain gardens implemented across a watershed can significantly reduce the amount of stormwater and contaminants introduced into the local drainage infrastructure . They can also be incorporated into drainage areas , which collect stormwater from nearby roadways .
Site Selection
Residential rain gardens should be located in areas downstream from impervious surfaces where the most stormwater runoff can be collected . For example , if water flows away from a residential structure and into the street , the rain garden should be located in an area where it can intercept the runoff before water enters the road . Generally , areas where a “ sheet ” of water flows across the soil surface and does not collect are preferable sites for these gardens . They should also be placed in areas that do not naturally collect water , but allow for drainage to permeate the ground . Observing water flow during and following rainfall events is the best way to determine the flow of stormwater away from impervious surfaces on a property . Multiple rain gardens can be placed throughout an area to maximize the capture of stormwater runoff from impervious surfaces .
It is required to place rain gardens at least 10 feet away from structures and foundations to prevent adverse impacts to built infrastructure . Rain gardens should also be placed at least 25 feet away from septic drain fields , if present . Additionally , it is recommended to avoid areas where the water table is less than 18 inches below the surface to allow for proper infiltration .
The size of the garden depends on the amount of runoff being collected . The County recommends the sizing of bioretention areas , considered engineered rain gardens , to be 3-8 % of the total drainage area . When considering just impervious surfaces , Clemson Extension recommends sizing rain gardens in well-drained , sandy soils to 20 % of the impervious surface , or 30-50 % for loamy and poorly drained soils ( Kim Morganello , personal communication , 2021 ). The cumulative impervious surface ( or footprint ) from buildings , sidewalks , and driveways , among others , which introduces runoff into a specific site should be included in these calculations . This can generally be found on
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