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# WI: Madison: Sizing a Rain Garden

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Madison: Sizing a Rain Garden

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### WI: Madison: Sizing a Rain Garden

1. 1. Sizing a Rain Garden Background Activity Overview Determining the size of a rain garden is not an exact science. Experts have Students calculate the size of a rain different opinions. With that said, how big will you build your rain garden? garden using measuring tools, tables, There are two answers: any size you want, or if you want 100% runoff con- and formulas. trol, you need to make calculations. The first answer may seem like an odd Objectives response. But it means that a rain garden of any size will reduce the amount Students will: of precipitation that becomes surface runoff. Each water drop that stays on • Determine the drainage area the school property is one less drop that potentially carries pollutants to through observation and data lakes, rivers and streams. The second answer involves learning how large the collection. drainage area is, what type of soil is in the garden, and what is the percent • Measure the area draining slope of the site. Next take this information and plug it into formulas to ob- into the rain garden using tain a square foot area for the garden. The following activity description cov- measuring tools. ers each step of the process for sizing a rain garden. • Use strategies in real-world situations to solve multiplica- Activity Description tion problems. Calculate Drainage Area Subjects Covered 1. Go out to the area designated for the rain garden. Identify the section of Math, Science, and Geography roof that will drain into the downspout. Measure the length and width of this area. Take measurements of other hard surfaces such as driveways, Grades parking areas, or sidewalks that may drain into the rain garden. Repeat 4 through 12 measurements of lawn that noticeably drain into the proposed rain gar- Activity Time den. (It is not necessary to include the lawn area between the building Two 50-minute class periods and the garden site. Insert these numbers in the “Sizing a Rain Garden” worksheet. Season Any 2. Calculate areas draining into the rain garden. See Figure 1 showing the drainage area for one downspout on a school roof. Materials Measuring tapes, map of schoolground including location of buildings and paved surfaces; information about Bird’s Eye View of a School soil type (from Earth Partnership for Schools activity, “Getting to Know Length = 150 feet Your Soil for Rain Gardens” or “In- Width = 100 feet L X W = 15,000 feet filtration Test: Exploring the Flow of 15,000 sq ft draining Water Through Soils”), slope (from to rain garden Earth Partnership for Schools activity, “Measuring Slope for Rain Gardens”); sizing a rain garden worksheet Downspout W Area of roof i draining to State Standards d downspout Math: A.4.1, A.4.3, A.4.5, A.8.1, t A.12.1, B.4.5, B.8.1, B.8.7, D.4.1, h Length D.4.2, D.4.3, D.4.4, D.4.5, D.8.1, D.8.2, D.8.3, D.8.4 Figure 1: Drainage area for a section of roof leading to one downspout. Science: C.4.3, C.4.5, C.8.1© Earth Partnership for Schools • University of Wisconsin – Madison Arboretum Plan a Rain Garden 3-1