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Rain Gardens absorbRain water, reduce Gardens runoff, prevent flooding 2 0 0 5 What are rain gardens? Designing and Planting Rain gardens are depressional areas landscaped with Designing and planting a rain garden is perennial flowers and native vegetation that soak up very similar to creating other perennial r a i n w a t e r. They are strategically located to capture gardens, with a few of the following r u n o ff from impervious surfaces, such as roofs and e x c e p t i o n s : streets. Rain gardens fill with a few inches of water after a storm and then water filters into the ground, Location rather than running off to a storm drain. Rain gardens must be located to inter- cept runoff from impervious areas. T h e y Why are rain gardens important? can be placed anywhere good soils with As cities and suburbs grow, increased storm water adequate percolation rates exist. It is r u n o ff from impervious surfaces becomes a prob- best to keep rain gardens away from lem. As more impervious surfaces are added to our building foundations, utilities, and septic communities, it is more important than ever to help systems. rainwater infiltrate. This protects water quality and reduces storm water runoff . Size Rain gardens are typically 5 to 10 per- Storm water runoff from developed areas increases cent the size of the impervious surface flooding potential and carries pollutants from generating the runoff entering the gar- streets, parking lots and lawns into local streams den. Measure the square footage of the and lakes. Rain gardens can absorb most rainfall impervious area (length x width); then e v e n t s . multiply this by 0.07 (7 percent). Continued on back page . This rain garden is strategically placed to capture runoff from the lawn and street. Rain Gardens Rain Gardens . Continued from front page Low Impact Development (LID) Determine a length and width of the rain garden that best fits the A rain garden is an example of the low site. For example, a 2,000 sq./ft. roof, when multiplied by 7 per- impact development (LID) approach to cent, would call for a rain garden 140 sq./ft. in size, or 14’ long storm water management. Traditionally, by 10’ wide. storm water management has involved the rapid conveyance of water via storm sewers Garden Depth to surface waters. Low impact development A typical rain garden is between four and eight inches deep. A r a i n is a different approach that retains and infil- garden less than four inches deep will need too much surface area trates rainfall on-site. The LID approach to provide enough water storage to infiltrate larger storms. Storm emphasizes site design and planning tech- water runoff should spread evenly across the entire rain garden, to niques that mimic the natural infiltration- increase the opportunity for infiltration. based, groundwater-driven hydrology of our historic landscape. Soil Amendments To prepare for a rain garden, remove soil to create a depressional area. Blend in soil, sand, and compost mixture to enhance infiltra- t i o n . Plant Selection While rain gardens are a highly functional way to help protect water quality, they can also be an attractive part of your yard and neighborhood. Choose native plants based on site considerations for light, moisture, and soil. Vary plant structure, height, and flower color for seasonal appeal and butterfly habitat. Mowed grass borders are recommended around the garden. Young plants, or plugs, are best for rain gardens because they are easier to establish and maintain. When laying plants out, randomly Native landscaping adds color, structure, and diversity to the clump individual species in groups of 3 to 5 plants to provide landscape and provides habitat for butterflies. bolder color. Be sure to repeat these individual groupings to create repetition and cohesion in a planting. It is a good idea to place More Information about plant labels next to each individual grouping. This will help identi- fy the young native plants from weeds as you maintain the garden. Rain Gardens Find additional information about rain gar- dens by visiting the following websites: It is important to water rain gardens regularly throughout the first season. Once established, they will thrive without additional www.iowasudas.org watering. A shredded wood mulch is an important part of a rain www.raingardens.org garden. Mulch helps retain moisture and discourages weed seeds www.mninter.net/~stack/rain/ from germinating. www.lowimpactdevelopment.org www.cwp.org www.stormwatercenter.net Rain Gardens The USDA is an equal opportunity provider and employer. How To Design and method for sizing the rain garden (If sod is present dig it up or kill it is to take the sq feet of impervious chemically. Avoid rotor tilling sod Build A Rain surface X 1”/12” (or 0.083) to as root chunks may survive and Garden determine sq feet of surface area. cause competition with rain garden If you provide 1 foot of depth you plants). Determine the location for a will have a depressional rain garden. The location should area that will hold 100% be situated so runoff from the of the runoff from a 1” impervious surface you want to rain. This method pro- manage can be intercepted. You vides extra capacity may see in some information that a because it takes no credit place that ponds water is a good for infiltration rates or for location for a rain garden. This pore space storage in the can be misleading. A rain garden soil profile). must have adequate percolation Now determine a rates. Remember the goal is to length and width for the infiltrate impounded runoff in a rain garden that will equal the sur- If deep topsoil exists (12 inches relatively short period of time (i.e. face area needed and that will fit or more) simply create a 6 inch 12 - 24 hours). A place that ponds the proposed location for the rain depression and plant. If soil quali- water will obviously be a place garden. Long and linear rain gar- ty is less than optimum, add where water moves to but if it dens are generally recommended amendments, like sand and com- ponds water for any length of time because you want to be able to do post, to improve soil quality. then it probably does not have ade- installation and maintenance with- Excavate and save about one quate percolation rates for a rain out having to be inside the bound- inch of topsoil. garden. (See information on bio- ary of the rain garden. Any traffic Excavate additional soil to get a retention cells). (equipment or foot traffic) will hole 12” – 18” deep. Conduct soil investigation to increase compaction within the Mix sand and compost to the ensure you have adequate percola- rain garden. Compaction is the saved topsoil to create a soil matrix tion rates. You should have a mini- enemy of infiltration which the pri- that will be about 50% sand, 30% mum perc rate of 0.5 inches/hour, mary goal of installing a rain gar- compost and 20 % topsoil. but it’s better to have 1 inch/hour den. Since sand and compost is usu- or higher perc rates. (Check with Lay out a rope to delineate the ally sold by the ton you will need to Extension Service or County surface area of the rain garden. calculate the tons of material need- Health Department for informa- Rain gardens can be any shape but ed to this by using the following tion on how to conduct a perc test). make the border of the rain gar- formulas: Measure the impervi- Sand: Multiply the sq ft of surface ous surface (I.S.) that will area of the rain garden by 0.25 ft contribute runoff to the (or 3”) to compute the cubic feet of rain garden in square sand needed. Then divide this by feet. (length [L] x width 27 (cubic ft per cubic yard) to con- [W] = I.S.). vert to cubic yards and then multi- Multiply the sq ft of ply by 1.5 to convert cubic yards to I.S. by 0.07 (7%) to get tons of sand needed. (i.e. 140 sq ft the sq ft of surface area X 0.25 = 35 cu ft divided by 27 = (S.A.) for the rain garden. 1.3 cu yds x 1.5 = ~ 2 ton of sand For instance if you have a needed). 50 ft L x 40 ft W roof the I.S. den blend in with the surrounding Compost: Multiply the sq ft of sur- would equal 2,000 sq ft. x 0.07 = landscape in a pleasing manner. face area of the rain garden by 140 sq ft of S.A. needed for the Remove vegetative cover from 0.167 ft (2”) to compute cu yards rain garden. (An alternative the surface area of the rain garden. needed. Divide by 27 to convert to Bio-Retention Cell L x W x 1’D divided by 27 = cu yds x 1.5 = tons of Construction Steps rock Calculate sand needs by Excavate surface area down to using this formula: 42”… L x W x 2’D divided by 27 Backfill with 12” of clean, = cy yds x 0.6 (60%) x 1.5 washed, 1” aggregate. = tons of sand Place subdrain tile in the rock Calculate compost needs by bed…if the surrounding soil will using this formula: percolate place tile in the upper L x W x 2’ D divided by 27 part of the rock bed…if the sur- = cu yds x .25 (25%) x 1200 cu yards.
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