Standards Oversight Council



Roger BannermanJudy HorwatichMarch 28, 2017Sizing Factors for Determining the Surface Area of Rain GardensPurposeTo develop a series of sizing factors that can be multiplied times a drainage area to determine the surface area of a rain garden.MethodDevelop sizing factors using WinSLAMM. The sizing factor is the ratio of the surface area of a rain garden to the size of the drainage area. Each sizing factor is unique to a percent volume reduction value. Multiple model runs produced increasing percent reductions associated with increasing surface areas. Each set of model runs is unique because the size of the drainage area and the depth is kept constant. The percent volume reduction values are a function of the rain garden surface area. The drainage area size is divided into the surface area of the rain garden for each percent reduction to produce the sizing factors. For example, a sizing factor of 0.30 is a 100% reduction for a rain garden depth of 4 inches with loam soils (Fig 1). The 0.30 is a result of dividing the assumed drainage area of 600 sq. ft. into 177.5 sq. ft., which is the calculated size of the rain garden (Equation 1 and2). Figure 1. Size factors for loam soils. Equation 1: 177.5/600 = 0.30 sizing factor for 4 in depth, clay loam, and 100% ControlEquation 2: 139.0/600 = 0.23 sizing factor for 4 in depth, clay loam, and 90% ControlModeling AssumptionsThe following assumptions were selected to make the WinSLAMM runs:Use average Madison rainfall yearDrainage area of 600 sq. ft. of impervious area (roof top).Infiltration rates are 0.15 (Clay Loam), 0.30 (Silt Loam), 0.50 (Loam), 1.0 (Loamy Sand), and 2.0 (Sand) inches/hour.Depths of rain gardens are 4, 6, and 8 inchesSizing factors are not affected significantly by different size drainage areas.Reduction goals of 75, 90 100%Results The following tables represent sizing factors for three volume reduction goals as a function of soil types and rain garden depths.Table 1. Sizing Factors for Volume Reduction Goal of 75 PercentRain Garden Depth, inchesSoil TypeClay LoamSilt LoamLoamLoamy SandSand3-5 0.150.100.080.070.046-70.120.090.070.050.0380.110.080.060.040.03 Table 2. Sizing Factors for Volume Reduction Goal of 90 PercentRain Garden Depth, inchesSoil TypeClay LoamSilt LoamLoamLoamy SandSand3-50.230.190.150.120.076-70.180.140.120.100.0680.150.120.100.070.05Table 3. Sizing Factors for Volume Reduction Goal of 100 PercentRain Garden Depth, inchesSoil TypeClay LoamSilt LoamLoamLoamy SandSand3-50.410.350.300.200.176-70.300.300.200.170.1280.250.230.180.130.11 What Range of Sizes Do the Sizing Factors Produce?People want to install reasonable sized rain gardens. A reasonable size garden will have more modest cost, be easier to build and maintain. Most installations seem to be in the range of 100 sq. ft. to 500 sq. ft. A rain garden should be big enough to have the appearance of a garden with a variety of plant species. A garden over 500 sq ft might be more challenging to build and the effective infiltration area might be reduced. The following tables show the size of rain gardens assuming 600 square feet or 3000 sq. ft. of impervious drainage area.Table 4. Surface Area of Rain Gardens Assuming Clay Loam Soils.Rain Garden Depth, inchesSurface Area of Rain Garden with Impervious Drainage Area of 600 sq. ft.Surface Area of Rain Garden with Impervious Drainage Area of 3000 sq. ft.75%90%100%75%90%100%3-59013821045069012306-77210818036054090086690150330450750Table 5. Surface Area of Rain Gardens Assuming Sandy Soils.Rain Garden Depth, inchesSurface Area of Rain Garden with Impervious Drainage Area of 600 sq. ft.Surface Area of Rain Garden with Impervious Drainage Area of 3000 sq. ft.75%90%100%75%90%100%3-525421021202105106-718367290180360818306690150330Table 5. Surface Area of Rain Gardens Assuming 1000 sq. ft. of Drainage Area.Rain Garden Depth, inchesSurface Area of Rain Garden with Clay Loam SoilsSurface Area of Rain Garden with Sandy Soils75%90%100%75%90%100%3-515023041040701706-7120180300306012081101502503050110Suggestion: Should we set a minimum and maximum rain garden size? A minimum of 100 sq. ft. and a maximum of 600 square feet. Anything over 600 sq. ft. must be split into smaller cells.Drawn Down Time Most literature describing criteria for infiltration devices suggest the drawn down time for the surface pool of water be less than 24 hours. The concern is the plants might not survive being submerged for more than a day. The following tables show the drawn down times for clay loam and silt loam soils for different depths of rain gardens.Table 6. Draw Down Times for 8 inch Deep Rain Gardens with Clay Loam and Silt Loam Soils.Rain Garden Depth, inchesDrawn Down Times for Clay Loam Soils, HoursDraw Down Times for Silt Loam Soils, Hours75%90%100%75%90%100%3-5434334136-7575749208707062444427Suggestions: Do we require some soil amendment to improve infiltration rates for clay loam soils? Do we assume soil structure will improve with mature prairie plants? Do we require plugs for clay soils to speed up improvement in infiltration rates. Do we require infiltration test and if it is over 0.3 in/hr it is not a problem?Plots for Selecting Volume Reduction Goals Multiple model runs were made for each depth and soil type using a batch file process. Therefore over 20 values of sizing factors and percent reductions were produced for each depth and soil type. The following plots represent the numbers generated by the model. ................
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