GA: Rain Gardens - University of Georgia


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GA: Rain Gardens - University of Georgia

  1. 1. Rain Gardens Sheryl Wells, Public Service Representative Biological and Agricultural Engineering Frank Henning, Region IV Land Grant Universities Water LiaisonIntroductionA rain garden is a natural or man-made depression in the landscape that captures stormwaterrunoff from impervious surfaces like rooftops and driveways. A rain garden is a best manage-ment practice that is designed to be aesthetically pleasing and naturally integrated into thelandscape (Figure 1). Surface runoff from a roof or other collection surfaces is channeled via raingutters, pipes, swales or curb openings to the rain garden. Rain gardens reduce runoff and treat pollutants on-site while re- charging groundwater by allow- ing the runoff to percolate into the soil where it is filtered and treated through natural processes. Properly designed rain gardens allow water to soak into the ground within 12 to 48 hours and discourage mosquito breeding areas. A rain garden may be referred to as a bioinfiltration or bioretention cell; however, bioreten- tion cells are typically larger and constructed on commercial sites that usually require an engineered design from a professional engineerFigure 1. Rain garden (PE) or registered landscape archi-(Dietz and Filchak 2004) Connecticutraingardenbroch.jpg tect (RLA). History In 1990, in Prince George’s County, Maryland, a developer named Dick Brinker was building a new housing subdivision and had the idea to replace the traditional retention ponds with rain gar- dens. This idea developed into a successful project for Somerset, a residential subdivision, which has a 300- to 400-square-foot rain garden on each house’s property ( feb03/run.htm#one). This rain garden system was highly cost-effective and resulted in a 75 to 80 percent reduction in stormwater runoff during regular rainfall events. This community was instru- mental in implementing low-impact development (LID), which is an ecologically sensitive design approach to stormwater management. Many successful LID case studies were done in this commu- nity (
  2. 2. BenefitsExpanded development of impervious surfaces such as streets, buildings, parking lots, drive-ways, roofs and patios increases the quantity of surface water that must be managed. A 1-inchrainfall event will produce more than 600 gallons of stormwater runoff for every 1,000 squarefeet of impervious surface (Van Giesen and Carpenter, 2009). Stormwater runoff can cause flood-ing, increase erosion and sedimentation and pollute surface and groundwater (Figure 2). Raingardens are an attractive option for areas in which the property owner can mitigate the impact oftheir impervious surfaces.Rain gardens are designed to capture stormwaterrunoff and allow it to slowly seep into the ground. Raingardens remove pollutants such as nutrients, chemi-cals, bacteria, sediments, litter, oil metals and othercontaminants through physical, chemical and biologi-cal processes. Some of these mechanisms include: ab-sorption, microbial processes, plant uptake, sedimen-tation and filtration. By slowing stormwater so that itcan infiltrate into the soil, rain gardens enhance sedi-mentation and filtration. Vegetation in rain gardenshas the potential to reduce or remove dissolved nutri-ents and pollutants through plant uptake and adsorp-tion. Dissolved metals and nutrients bind or adsorb to Figure 2. Stormwater runoff.soil particles such as clay and organic matter, whichreduces their impact on surface and groundwater sources. Soil microorganisms help break downpollutants into less detrimental compounds and use them as food sources.Well-designed rain gardens provide more than water quality benefits. Because stormwater andthe nutrients it contains are diverted to rain gardens, additional irrigation and fertilization maynot be necessary after initial plant establishment. This reduces energy inputs since the needto operate irrigation systems and to manufacture fertilizers is lower. Desirable wildlife such asbirds, butterflies, bees and other important pollinators are attracted to diverse habitats. Aquaticecosystems are also improved with reduced pollutant loads that improve water quality. Rain gar-dens can be an attractive addition to any property and can help increase the property’s value. Rain gardens have many benefits, including: • Stormwater runoff is treated on-site instead of entering streams • Stormwater runoff is reduced • Water infiltration increases, resulting in more water entering the soil and less pollution entering streams • Water and energy conservation • Wildlife habitats are created and aquatic ecosystems are improved • Attractive gardens can increase property valuesRain Gardens 2 University of Georgia Cooperative Extension Bulletin 1380
  3. 3. If you live in an urban area, pay attention to the water levels in the local streams and riversshortly after a rainfall. Impervious surfaces like pavement, curbs, roofs and gutters efficientlytransport stormwater to our streams. Because water reaches streams faster, less of it is ab-sorbed, adding to the impact on stream bank erosion. The water levels in local streams can peakand fall to base flow in hours instead of days or weeks. Rainfall and stream events in more natu-ral areas are less erosive and in-stream flow is usually more consistent. Rain gardens help returnthe hydrologic function, water treatment capacity and other environmental services of a devel-oped landscape back to pre-developed conditions.Location/SiteBefore selecting a site in the landscape for a rain garden, it is best to observe the direction of sur-face runoff during a rainfall event. Choosing an area downslope from a roof downspout or drive-way area is ideal. Often, natural depressions in the landscape can be used successfully. Alwayslocate wells, septic system tanks, drain lines and any underground utilities by contacting yourburied utility locating service (dial 811). Also, check your local building codes before installing arain garden.These guidelines will help ensure the garden is optimally located:• Construct the rain garden at least 10 feet from buildings to prevent seepage into foundations (Figure 3).• Construct the rain garden at least 25 feet from a septic tank, septic drain field or well head.• Avoid locations with slopes greater than 12 percent.• Avoid low areas in the landscape that retain water or where ponding occurs frequently.• Avoid soils that have low or extremely slow infiltration.• Avoid placing the rain garden on top of underground utilities.• Avoid placing the garden over shallow water tables. In areas with elevated water tables, con- sider a wetland garden.• Choose a location with full or partial sun.• Do not place the rain garden directly under trees since the root system may create competi- tion for other plant material and the canopy may create a shading problem. Figure 3. Rain garden location (Dietz and Filchak 2004) foundation.jpgUniversity of Georgia Cooperative Extension Bulletin 1380 3 Rain Gardens
  4. 4. DesignRain garden design can vary considerably depending on the site constraints and owner/designerpreferences; however, it should naturally complement the layout of the landscape. Commonshapes include oval, round, oblong and kidney bean (Figure 4). The longest side of the rain gar-den should be perpendicular to the slope of the property. Figure 4. Rain garden design (Dietz and Filchak)http://dnr.wi.jpgSizeResidential rain gardens are typically 100 to 300 square feet. There are different methods of siz-ing rain gardens. A common method is to determine the size of the collection area draining to thegarden, the volume of water it will need to temporarily store (ponding depth), and the type of soillocated under the garden.Most contaminants are found in the first inch or first-flush of runoff. Properly designed raingardens are constructed to catch the first-flush. In the event a storm produces more than 1 inchof rainfall in a 24-hour period, an overflow area or berm should be installed to safely divert thewater out of the garden in a manner that minimizes erosion or other damage. The overflow fromrain gardens is often delivered directly to nearby streams. The Natural Resource ConservationService uses a curve number to calculate the size of a rain garden. The curve number representsa measure of how much water will infiltrate versus runoff during a storm. For an in-depth sizingdiscussion on this method refer to ( Gardens 4 University of Georgia Cooperative Extension Bulletin 1380
  5. 5. A soil survey or classification analysis provides useful information about the soil under a raingarden. Sandy loam or loamy sand is ideal because it typically results in a permeability rate of1 to 6 inches per hour. Infiltration can often be improved by either 1) amending the existing soilwith organic matter, or 2) aerating the soil to reduce compaction.An ideal soil mixture should contain 50 to 60 percent sand, 20 to 30 percent topsoil and 20 to 30percent compost (see: Coarse, washed sand isrecommended. River sand is less desirable due to its rounded shape and because it may containsilt.The following table is used to calculate the size of a rain garden based on soil type, imperviousdrainage area and ponding depth ( Rain garden ponding depth Soil type 4-5 inches 6-7 inches 8-9 inches Clay 0.19 0.15 0.08 Silt 0.34 0.25 0.16 Sand 0.43 0.32 0.20 Example: A 1,000 ft2 roof drains into a clay soil rain garden with a 4” ponding depth Solution: 1,000 ft2 x 0.19 = 190 ft2 rain gardenA soil percolation test should be conducted to determine infiltration rate. Conduct a simple perco-lation test by digging a hole with a shovel or post-hole digger. The hole should be dug to approxi-mately the same depth as the deepest area that will be excavated during rain garden construc-tion. Saturate the hole by filling it with water and allowing it to drain. Place a yardstick in thehole then refill it with water to determine how many inches of water infiltrates in an hour ( The hole should drain within 12to 48 hours. In the event that there is still water in the hole after 48 hours, the site will probablynot be suited for a basic rain garden.Sites with poor drainage may require either a rain garden with an under-drain system or theymay be better suited for wetland gardens. Under-drain installation information can be found Wetland gardeninformation can be found at Signs of impermeable soils: • The site remains saturated or ponds water for several days after a storm event. • After digging for a percolation test, water fails to drain from the hole for more than 48 hours, provided it has not rained. • Signs of a wetland soil are evident within 1 foot of the surface. Wetland soils are often gray with ribbons or areas of brown color.University of Georgia Cooperative Extension Bulletin 1380 5 Rain Gardens
  6. 6. Slope and Ponding DepthPonding depth is a term used to describe the depression area between the top of the mulch layerand the bottom of the overflow outlet. Rainwater is captured in the depression and held thereuntil it infiltrates into the soil. The slope of the garden and the underlying soils determines theponding depth. Typical rain gardens are normally 4 to 9 inches deep (Figure 5). A rain gardenthat is less than 4 inches deep will need an excessive amount of surface area to provide enoughstorage area for large storm events to infiltrate properly. A garden with a depth of more than 9inches might pond water too long. Regardless of the ponding depth, the goal is to keep the raingarden level. Figure 5. Rain garden depth. (Wisconsin Department of Natural Resources 2008) SizingaRainGarden.jpgThe slope of the lawn determines the rain garden’s depth. To determine the slope of the lawn, fol-low these steps: • Pound a stake in the ground at the uphill and downhill end of the rain garden. • Tie a string to the bottom of the uphill stake and run the string to the downhill stake. • Place a level on the string and tie the string to the downhill stake at the position in which the string is level. • Measure the width between the stakes. • Measure the height on the downhill stake between the ground and the string. • Divide the height by the width and multiply by 100 to find the percent slope of the lawn. Areas with a slope of 12 percent or more are not recommended for rain gardens.Use the slope of the lawn to select the ponding depth from the following options: • Slope less than 4 percent = ponding depth 4 to 5 inches deep. • Slope between 5 and 7 percent = ponding depth of 6 to 7 inches. • Slope between 8 and 12 percent = ponding depth of 8 to 9 inches.Rain Gardens 6 University of Georgia Cooperative Extension Bulletin 1380
  7. 7. Example:The length of the string between the stakes is 150 inches and the height is 8 inches. Height/width x 100 = % slope. Therefore, 8/150 x 100 = 5% slopeWith a 5 percent slope, the rain garden should be approximately 6 inches deep.Connecting / Conveyance of Rainwater Harvesting Systems to Rain GardensPartnering rain gardens with rainwater harvesting systems is a good way to capture irrigationwater for the rain garden as it is establishing and to reduce the amount of sediment enteringthe garden. The cistern will act as a settling basin as sediment enters the tank and sinks to thebottom. The cistern can drain into the garden via a gutter downspout extension pipe that can beleft on top of the ground or buried. Water can also be channeled through downspouts that flowthrough grass or stone swales. The stone will stabilize the swale and prevent soil erosion. Theswales or flow channels should have a minimum 2 percent slope. The width/depth of the flowchannel should be a 2:1 ratio. For example, if the depth of the channel is 1 foot, the width of thechannel should be 2 feet. The channel will act as a pretreatment filter for pollutants before theyenter the garden.Garden Construction and InstallationIf there is existing vegetation where the garden will be located it will need to be removed. Achemical can be used, but a more environmental approach is to use black plastic to cover thelawn or grass until it dies. After choosing a location, the shape and size of the garden can be out-lined with paint or a rope. This can be a boundary outline while digging the garden. Some of thesoil that is removed from the depression can be used to create a berm along the downhill slopeand on the sides if necessary to help to retain water (Figure 5). Excavate the rain garden depres-sion 2 to 3 inches deeper than the designed final depth in order to account for mulch. The bermon the downhill side of the rain garden should be the same level as the uphill side. Soil thatis not needed for the raingarden’s berm should beremoved and utilized else-where in the landscape.The bottom of the raingarden can be tilled and/or amended with organicmatter in order to facilitateplanting and improve infil-tration (planting depth istypically 2 to 3 feet). Oncethe rain garden construc-tion phase is complete, in-stall plant material (Figure6) and mulch the rain gar-den area with 2 to 3 inchesof hardwood mulch. The Figure 6. Soil amendmentrain garden berm should (Dietz and Filchak 2004) http://www.tredyffrin.jpgbe covered with mulch,rock or stone to prevent erosion.University of Georgia Cooperative Extension Bulletin 1380 7 Rain Gardens
  8. 8. PlantsPlants adapted to regional climatic conditions are recommended. Native, non-invasive plantspecies are good choices for rain gardens. They will reduce the introduction of non-native plantsto the natural riparian areas that are found along streams. For a list of Georgia native plantssee University of Georgia Cooperative Extension Bulletin 987, Native Plants for Georgia Part I:Trees, Shrubs and Woody Vines (Wade et al, 2008) and ( Plants that provide ahabitat for wildlife are also recommended.A variety of plants that are resistant to stress from periods of water pooling and periods ofdrought should be used (Figure 7). Plants that tolerate both wet and dry conditions are prefer-able for the bottom of the garden. Plants that tolerate dry conditions should be used toward theedges of the garden. Small trees can be used on the edges or berms. Large trees are not recom-mended for small rain gardens since the root system will create competition for other plants andcould interfere with under-drains. Leaf debris and shade can also be a problem with large trees.Below is a partial plant list with soil moisture recommendations.Partial plant list for rain gardensScientific name Common name Soil MoistureShrubsCallicarpa americana American Beauty Berry M-DCalycanthus floridus Sweet Shrub MClethera alnifolia Clethra MMyrica ceifera Southern Wax Myrtle AItea virginica Sweetspire MPerennialsAster spp Aster M-AEchinacea angustiflora Purple Cone Flower A-DIris versicolor Blue Flag Iris MRuellia spp Mexican Petunia A-DRudbeckia Black Eyed Susan M-AMonarda dydima Bee Balm M-AIris hexagona Louisiana Iris MHemerocallis Daylilly M-DHibiscus Rose mallow A-DHelianthus angustafolius Swamp Sun Flower M-DLantana ‘New Gold’ Lantana (annual/perennial) M-DOrnamental GrassesPanicum virgatum Switchgrass M-AMiscanthus sinensis ‘Zebrinus’ Zebra grass M-DMiscanthus sinensis ‘Gracillimus’ Maiden grass M-DSmall TreesIlex vomitoria Yaupon Holly M-DCrataegus spathulata Littlehop Hawthorn M-AVitex agnus-castus Chaste Tree A-DLagerstromia indica Crape Myrtle M-DM = Wet soil, A = Average soil moisture, D = Dry soilRain Gardens 8 University of Georgia Cooperative Extension Bulletin 1380
  9. 9. Figure 7. Plant placement(Glen 2011)http://www.bae.ncsuplant placement.bmp 1 1 small tree or tree form shrub 2 3 perennials 3 6 small perennials or groundcovers 4 3 perennials 5 3 large perennials or ornamental grasses 6 3 small shrubs * Each square equals 1 square foot.MaintenanceRegular visual inspections of the garden should be conducted to check for erosion, excessivesediment deposits and dead or diseased vegetation. The plants in the rain garden will need tobe watered during establishment (usually the first growing season) to encourage a healthy rootsystem. Once established, plants may only need to be watered when they show signs of drought.Annual mulching, with hardwood chips, is recommended. To maintain the proper functioningof the rain garden, the ponding zone must be maintained as designed and not reduced with theaddition of mulch. Pruning is typically needed and will depend on the owner’s preference. Handpulling of weeds may be necessary. Herbicides are seldom recommended; if necessary, those withlow impacts on aquatic organisms should be chosen. Fertilizers are usually not necessary afterestablishment. If for some reason water ponds in the garden for more than 72 hours, mosquitobriquettes may need to be incorporated until the ponding problem can be corrected.University of Georgia Cooperative Extension Bulletin 1380 9 Rain Gardens
  10. 10. ReferencesClemson Public Service. Carolina Clear. 2008. Rain Gardens. A rain garden manual for South Carolina. Green solutions to stormwater pollution. Information Leaflet 87. www. raingardenmanual_022709b.pdfDietz, M. and K. Filchak. 2004. Rain gardens in Connecticut: A design for homeowners. Univer- sity of Connecticut Cooperative Extension Service. 51130-03108. “Stormwater Case Studies” Native Plant Society. www.gnps.orgGlen, C. 2011. Oval rain garden. www.ncstate-plants.netHunt, W. F. and N. White. 2006. Designing rain gardens. (Bio-retention areas). North Carolina Cooperative Extension Service. AG-588-3. Resource Conservation Service. Rain garden site and soil assessment. Carolina State University Cooperative Extension. Backyard rain gardens. Giesen, E. and F. Carpenter. 2009. Georgia rainwater harvesting guidelines. dlns.040209.pdfWade, G., E. Nash, E. McDowell, B. Beckham, and S. Crisafulli. 2008. Native Plants of Georgia. Part I. Trees, Shrubs and Woody Vines. University of Georgia Cooperative Extension B-987.Wisconsin Department of Natural Resources. 2003. Rain Gardens. A how-to manual for hom- eowners. University of Wisconsin-Extension. UWEX Publication GW0037. Natural Resources. 2003. “Rain gardens made one Maryland community famous.” Gardens 10 University of Georgia Cooperative Extension Bulletin 1380
  11. 11. University of Georgia Cooperative Extension Bulletin 1380 11 Rain Gardens
  12. 12. Bulletin 1380 / February 2011The University of Georgia and Ft. Valley State University, the U.S. Department of Agriculture and counties of the statecooperating. Cooperative Extension, the University of Georgia College of Agricultural and Environmental Sciences, offerseducational programs, assistance and materials to all people without regard to race, color, national origin, age, gender ordisability. An Equal Opportunity Employer/Affirmative Action Organization Committed to a Diverse Work Force