the final building plans or determined with as-built plats of the property . If a downspout collects water from one side of the roof and the upper portion of the driveway , include the approximate surface area of these surfaces in the calculations . Below is the recommended calculation for determining the appropriate size of a rain garden ( s ) to intercept stormwater runoff on a lot :
Rain Garden Size ( sq ft ) = Total Impervious Coverage ( sq ft ) × 0.30
For example , a residential lot on Kiawah Island with 2000 square feet of total upstream impervious surface area would need a rain garden ( s ) with an area of at least 600 square feet , or 30 % of the impervious surface cover . This is roughly the size of four standard parking spaces , which can be placed in several locations of the property to intercept the majority of stormwater runoff .
Equation 1 . Example calculation for rain garden size on a lot with 2000ft2 in total impervious surface cover
Materials and Preparation
Soil needs to be tested to determine the ability to naturally infiltrate the water . This can be done with a simple percolation test . This is conducted by digging a 1-foot hole with a post hole digger or hand trowel , completely filling the hole with water (~ 2.5 gallons with 8-inch diameter ), and timing how long it takes for the water to drain . According to the last United States Department of Agriculture ( USDA ) Natural Resource Conservation Service ( NRCS ), Kiawah Island consists of four main soil hydrologic groups . This ranking classifies runoff characteristics of each hydrologic group from smallest runoff potential ( A ) to largest ( D ). The designation A / D and C / D indicate soil groups that are predominately classified as A and C , respectively , with certain conditions causing these soils to have high runoff potential ( D ). The hydrologic groups present on Kiawah Island are : A (~ 34 %), A / D (~ 17 %), B (~ 4 %) and C / D (~ 45 %).
Loamy fine sand , which is found on the majority of Kiawah Island ’ s upland areas , drains at a rate of 5-10 minutes per inch . If it takes longer to percolate the water into the ground , the site would require additional soil amendments to facilitate drainage . Areas with a perched water table within 18 inches of the soil surface should be avoided , if possible . This will be apparent when the soil does not drain during a percolation test . Several percolation tests may need to be performed depending on the size of the rain garden and the best time to do the test is at least 48 hours after a rainfall event to avoid saturated soils ( Erin Stevens , personal communication , 2020 ). Also , it may be beneficial to conduct the percolation test during the months of January-March when the seasonal water table is highest .
Soil amendments are needed for the area inside of the rain garden . If during the percolation test , water drains in under six hours , the cell does not require the addition of sand . Areas with increased organic matter , characterized by rich dark black and brown soils , typically have slower drainage rates . The desirable ratio for soil components in the end result is around 60 % sand , 20 % topsoil , and 20 % compost . If a soil amendment is needed , this will be the
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NATURE-BASED SOLUTIONS MANUAL FOR KIAWAH ISLAND