Rain Garden Design and Construction Guidelines 3 LIST OF FIGURESFigure 1 Typical rain garden capturing rooftop runoff from the downspout. This phototaken by Roger Bannerman in Dane County, Wisconsin (WDNR, 2002). ........................ 4Figure 2 Example rain garden sizes, shapes, and positioning for a homeowner’s rooftoprunoff. The three areas darkened in black are the rain gardens; dashed lines below eachgarden indicate relative depth. (PGCM, 2006)................................................................. 5Figure 5 Root Systems of Prairie Plants as diagrammed by Heidi Natura demonstratingthe proportional length of various native prairie plants and turf grass (far left) (1995).The longest roots shown are 15 feet long......................................................................... 9Figure 6 Demonstration of a curb cut for runoff capture from street. This picture is froma project in Burnsville, MN, designed by Barr Engineering Company (2004). ............... 14Figure 7 Infiltration and recharge facility for enhanced infiltration designed by PrinceGeorge’s County, MD (2006)........................................................................................ 15Figure 8 Filtration and partial recharge facility designed by Prince George’s County, MD(2006). .......................................................................................................................... 16Figure 9 Infiltration, filtration, and recharge facility designed by Prince George’s County,MD (2006). ................................................................................................................... 16Figure 10 A filtration-only bioretention cell designed by Prince George’s County, MD(2006). .......................................................................................................................... 17Figure 11 Sizing of a bioretention cell designed by Prince George’s County, MD (2006),shown for comparison to the LID Center design. ........................................................... 18Figure 12 Sizing and specifications of a bioretention cell designed by the LID Center(USEPA, 2003). ............................................................................................................ 18 LIST OF TABLESTable 1 Plant list for rain gardens with silt and sandy soils in the southern Wisconsinregion provided by Applied Ecological Services, Inc., Brodhead, WI, and published inthe Wisconsin DNR’s Rain Gardens: A how-to manual for homeowners (2003)............ 10
Rain Garden Design and Construction Guidelines 41 IntroductionLow Impact Development (LID) is an ecologically sensitive design approach tostormwater management. Prince George’s County Department of EnvironmentalResources, a pioneer in LID design and implementation, has developed a heavilyreferenced Low Impact Development Design Manual. The manual identifies the goal ofLID design to maximize onsite storage and infiltration at the parcel level (PGCM, 1997).Bioinfiltration cells, rain gardens in the vernacular, are a rising component of LID designin suburban and, in some cases, urban United States. Bioinfiltration of stormwater issource control mitigation of overburdened storm sewers. Additional benefits includemaintaining the natural hydrologic regime and restoring elements of lost ecologicalsystems. To date, most designs strive to accept a certain initial depth of any storm,sometimes called the ‘first flush’ (Traver, 2004). 1.2 DefinedIn the most general sense, bioinfiltration cells are shallow depressions in the soil to whichstormwater is directed to maximize infiltration (Figure 1). They are most often mulchedand planted with native vegetation that contributes to water capture capacity viaevaporation and transpiration. Figure 1 Typical rain garden capturing rooftop runoff from the downspout. This photo taken by Roger Bannerman in Dane County, Wisconsin (WDNR, 2002).Several types of bioinfiltration systems exist: bioretention cells, bioinfiltration cells,vegetated biofilters, rain gardens, grass swales, infiltration trenches, buffer strips. Slightvariations in design or function arguably justify the need for different nomenclature.Bioretention might include the use of an underdrain, substantial aggregate backfill, orgeotextile lining. Vegetated swales are often a linear design providing moderateconveyance, often installed along a roadway. The terms bioinfiltration cell and raingarden will be used interchangeably within this technical brief. 1.3 Need/ApplicationBioinfiltration has come about due to negative effects of traditional stormwatermanagement systems and resulting state and federal regulations. Stormwater pipingnetworks rapidly convey stormwater away from its source preventing groundwater
Rain Garden Design and Construction Guidelines 5recharge and potentially causing flooding, erosion, and combined sewer overflowsdownstream. Traditional detention basins are not designed for volume reduction, andEmerson et al. (2005) found only a 0.3% watershed-wide reduction in peak storm flowaccording to the modeling of 100 detention basins. Studies like this demonstrate the needto address runoff through source control.The leading regulation driving site-scale, source control stormwater management is therecent addition of Phase II of the National Pollutant Discharge Elimination System(NPDES) to the 1972 Clean Water Act. It requires small municipalities with separatestorm sewer systems (MS4s) to address stormwater runoff from their site with therecommended use of “structural BMPs [Best Management Practices] such as grassedswales or porous pavement” (USEPA, 2000).Rooftop runoff, in particular, can be a substantial contributor to municipal storm sewersystems. Pitt et al. (2002) identified one case where directly connected residential roofsconstitute an estimated 30-35% of annual runoff volume in the cities of Phoenix, Seattleand Birmingham. Runoff source control by homeowners can be valuable to the overallgoal of stormwater runoff and pollution reduction. 1.4 Report ContentsThis technical brief strives to provide the homeowner with a guide to construction of aneffective rain garden for rooftop runoff in suburban, urban, and rural areas (Figure 2).This is a conservative design approach to capture a designated depth of rainfall from eachrain event. The design emulates pre-settlement, natural hydrologic conditions. Thedesign is easily adapted to an underdrain system (Section 8.3) for sites with lowpermeability soils or where water treatment is the main objective.Figure 2 Example rain garden sizes, shapes, and positioning for a homeowner’s rooftop runoff. Thethree areas darkened in black are the rain gardens; dashed lines below each garden indicate relative depth. (PGCM, 2006)This technical brief appropriately references existing, reputable design manuals andstrikes a balance between oversimplified designs and technically complex manualswritten for governments, municipal planners, and licensed professional engineers. In
Rain Garden Design and Construction Guidelines 6particular, it strives to incorporate the most recent design recommendations from researchpublished in the scientific literature.2 Tools and Materials ListThe following list describes the materials needed for the installation of a rain gardendesigned using this manual. Characteristics and quantities of certain materials are furtherspecified in related sections. • Shovel or optional compact excavator (Section 7) • ~50/50 sand / soil mixture, soil consisting of a sandy loam or silt (Section 4) • Optional soil specific to planted vegetation (Section 4) • Mulch (Section 4) • Additional fine sand (Section 4) • Additional soil (Section 4) • A level (Section 7) • Native mesic plants (Section 5)3 Site Selection 3.1 Physical RationaleAn ideal site will aid conveyance to the system and contribute to the hydrologic needs(though minimal) of the vegetation during dry periods. 3.2 DesignSelect a site at least 10 feet from the foundation of the house to prevent water seepageinto the foundation (WDNR, 2003). Select a site with a gentle downward slope (but lessthan 12%) leading away from the house and into the garden, if possible (WDNR, 2003).Determine whether you will pipe the water from the gutter and pipe directly into thegarden or whether the water will flow over land before reaching the rain garden. Thisdecision will influence the amount of pipe needed (contributing to the cost), and,therefore, the distance from the house.4 Infiltration Media 4.1 Physical RationaleSoils in urban settings get compacted during site construction. Soil compactionsignificantly reduces infiltration rates compared to native soils. For example, Pitt et al.(2002) compared infiltration rates of non-compacted and compacted sandy soils andobtained a reduction from 13 in/hr to 1.4 in/hr due to compaction. For this reason, a non-compacted, moderate to high infiltration media will backfill the bioinfiltration cell. This
Rain Garden Design and Construction Guidelines 7media also provides water treatment through sedimentation, filtration, sorption1, andprecipitation2 (Hseih and Davis, 2005). Settling occurs in the shallow ponding area,filtering occurs through the layers of media, adsorption3 and cation exchange4 occurwithin the biologically active organic materials in the filter, and phytoremediation5 occursby the plants (Clar et al., 2004: PGCM, 2006). Total suspended solids (TSS) in the waterentering the rain garden are filtered out in this design. TSS are a concern because theycan promote clogging of the media, restricting the hydraulic conductivity6 and rate ofinfiltration (Beach et al., 2005).It is important to obtain and retain a high enough infiltration rate that the rain gardendrains before mosquitoes are able to propagate. They require 7 – 12 days to lay and hatcheggs (WDNR, 2003). The USEPA recommends a conservative drainage time of 48 hoursor less to prevent propagation of mosquitoes (1999). This constraint is also satisfied inthe design. 4.2 DesignHseih and Davis (2005) recommend two possibilities for optimum water treatment. Dueto ASCE copyright, they may not be copied here but are accessible athttp://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JOEEDU000131000011001521000001&idtype=cvips&gifs=yes orhttp://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=JOEEDU000131000011001521000001&idtype=cvips&prog=normal if access is approved or on page 1530,Figures 4a and 4b of the referenced journal publication. In this technical brief, thedesigns will be referred to as Hseih and Davis (2005) Figures 4a and 4b.In both designs, the mulch layer should be of high permeability (d107>0.1mm, 0.00039in)and appropriate uniformity (a d60/d10 value less than 4) to filter TSS (Hseih and Davis,2005).Hseih and Davis Figure 4a demonstrates a single-layer media design for maximum waterquality treatment consisting of 20-70% sandy soil (sandy loam texture) mixed with coarsesand (e.g. d10>0.3mm). Plant species requirements will inform the chosen percentage.The suggested depth is 55-75 cm for best water quality treatment. Infiltration rates were1 Sorption includes several processes through which solutions (e.g. water and chemical constituents) bind tosolids including absorption and adsorption.2 The precipitation of chemical constituents is the separation of the constituent from solution as a solid.3 Adsorption is the process at the solution-soil interface in which the soil attracts and holds to its surface asolution.4 Cation exchange is the switching places of cations (positively charged ions) in solution with cationsadsorbed to the soil.5 Phytoremediation is the neutralization or removal of chemical constituents through vegetation.6 Hydraulic conductivity is a measure of the soil’s ability to transmit water. This varies with respect to soilmoisture content and pore water pressure.7 The designation dx represents the diameter of particles in an aggregate for which at least x percent are thatdiameter or finer. This number can be provided by the supplier.
Rain Garden Design and Construction Guidelines 8found to be between 1.2 – 5.4 cm/min at 15 cm water head8. This design requiresminimal construction and maintenance as compared to the design in Hseih and DavisFigure 4b.Hseih and Davis (2005) Figure 4b illustrates a multilayer design for maximum waterquality treatment that incorporates a vegetation layer for optimum plant survival and afilter layer for treatment. The filter layer serves as a back-up to the initial vegetationlayer. The vegetation layer (25 – 30 cm in depth) is specific to the region of installation.Consult a local naturalist or plant nursery for guidance as you obtain from them theplants. The filter layer consists of the same coarse sand (e.g. d10>0.3mm) and sandy loamsoil as recommended in the single layer design, but existing in a 50/50 sand/soil ratio inthis case. The Hseih and Davis (2005) Figure 4b design requires more construction andmaintenance than the single layer design (Hseih and Davis (2005) Figure 4a).In the latter design, total phosphorus, nitrate, and ammonium removal is expected to begreater by about 50%, 3%, and 9%, respectively (Hseih and Davis, 2005). This relative,additional treatment can be weighed against increased design complexity to inform yourdecision about which design to install. 4.2.1 Volume of Infiltration MediaIn order to account for minor compaction during construction, obtain 10% more materialthan that required according to the diagram’s dimensions.5 Vegetation 5.1 Physical RationaleIn general, vegetation helps to maintain the infiltration capacity of the bioretention cellmedia (Clar et al. 2004). Bioretention plants are also a component of water treatmentbecause they can uptake some nutrients and heavy metals from the media (Hsieh andDavis, 2005). This is referred to as phytoremediation.In general, native plants are preferred over nonnative plants due to their adaptation to theregional climatic trends. The author’s experience comes from the Midwest where mesic9prairie species are the plants of choice for rain gardens. Prairie species demonstrate theadaptive nature of plants to their native regions.Figure 3 demonstrates the proportional root depth of a variety of native prairie species ascompared to turf grass (far left in the diagram). The longest roots shown are 15 feet long.Long roots and large root masses enable the plants to locate water in periods of drought,long after the turf grass has wilted. The plants are adapted to the trends of rain storms8 Water head refers to the height of water above the point of interest. Due to gravity, water head increasesthe downward force of water moving into the soil.9 The term mesic is a designation for an ecosystem’s level of dryness. It falls in the middle of wet and dry(e.g. in decreasing order of average soil moisture content - wet prairie, mesic prairie, and dry prairie).
Rain Garden Design and Construction Guidelines 9and dry spells characteristic of their native region. The plants’ physiology increases thecapacity for evapotranspiration and precludes the need for irrigation. As such, they areideal for bioinfiltration cells for water quantity control.Figure 3 Root Systems of Prairie Plants as diagrammed by Heidi Natura demonstrating theproportional length of various native prairie plants and turf grass (far left) (1995). The longest rootsshown are 15 feet long.The effect that native plants have on water quality control is a growing area of study.The diversity that a native matrix of plants can provide is, conceptually, beneficial towater quality. Plant species with cellular level variations are likely to be conducive touptake of a variety of constituents. 5.2 DesignPlant lists for the region of southern Wisconsin were supplied by Applied EcologicalServices and published in the Wisconsin Department of Natural Resources’ (WDNR)design manual (Table 1) (2003). This list merely exemplifies the diversity andapproximate number of plant species to obtain. For the Midwest region, it is a goodstarter list that may be presented to a local native mesic prairie plant grower who canaugment and adjust the list to fit your ecological niche.
Rain Garden Design and Construction Guidelines 10 Table 1 Plant list for rain gardens with silt and sandy soils in the southern Wisconsin region provided by Applied Ecological Services, Inc., Brodhead, WI, and published in the Wisconsin DNR’s Rain Gardens: A how-to manual for homeowners (2003). SPECIES NAME COMMON NAME Full to Partial Shade Aquilegia canadensis Wild columbine Aster macrophyllus Big-leaved aster Carex vulpinoidea Fox sedge Eupatorium maculatum Spotted Joe-Pye weed Geranium maculatum Wild geranium Phlox divaricata Woodland phlox Rudbeckia subtomentosa Sweet coneflower Schizachyrium scoparium Little blue stem Solidago flexicaulis Zig zag goldenrod Tradescantia ohiensis Spiderwort Zizia aurea Golden Alexander Full to Partial Sun Aster navae-angliae New England aster Carex vulpinoidea Fox sedge Coreopsis lanceolata Sand coreopsis Eupatorium perfoliatum Boneset Euphorbia corollata Flowering spurge Liatrix aspera Rough blazing star Monarda fistulosa Wild Bergamot Physostegia virginiana Obedient plant Rudbeckia subtomentosa Sweet coneflower Schizchyrium scoparium Little blue stem Solidago Riddelli Riddells goldenrod Tradescantia ohiensis Spiderwort Zizia aurea Golden AlexanderFor other regions throughout the United States, the Brooklyn Botanic Garden ofBrooklyn, New York, provides starter lists of rain garden plants. These lists can beaccessed at http://www.bbg.org/gar2/topics/design/2004sp_raingardens.html (BBG,2006).It is likely that native plant growers in your region are familiar with rain gardeninstallations and will be able to identify the required vegetation to refine these starterlists. (NOTE: It is a misconception that wetland vegetation is used in these installations.Wetland plants would require irrigation every dry season.)6 Sizing 6.1 Physical Rationale
Rain Garden Design and Construction Guidelines 11An underdrained swale system with backfill similar to a bioretention cell provides insightinto critical characteristics that influence reduction of runoff, storage, and infiltration.Effectiveness depends on the size of the bioretention cell in addition to other contributingfactors. The reduction in runoff will depend on the temporal distribution of runoffcontributions to the rain garden and the ability of the soil to infiltrate water. The storageand infiltration will also depend largely on the intensity of the storm and the inflow(Barber et al., 2003). The infiltration rates influencing runoff reduction and storage havealready been addressed (Sections 4 and 5). This section addresses the sizing.The sizing method used here is a conservative calculation. It is sized to have anaboveground storage capacity equivalent to a determined depth of runoff from the roofarea. This design was used by Dietz and Clausen (2006) in a reputable study on raingarden flow and pollutant retention. These authors found it to be compatible with the1993 version of The Bioretention Manual developed by Prince George’s County, MD(1993). The updated manual continues to be the most referenced manual and a leaderinternationally in LID design methods (PGCM, 2006). It utilizes the Natural ResourceConservation Service Curve Number to account for sites with variable land-use. Thedesign discussed here assumes one surface type, the rooftop, and allows for simplifiedcalculations suited to the homeowner’s application. 6.2 Design (1) Determine the depth of rainfall desired for capture. There are a few options for determining this value: a. At a minimum, you can use a 0.5-inch depth of rainfall which could be considered capturing the ‘first-flush’ of the rainfall event. Capturing the first-flush is especially important in roadway runoff applications. In rooftop drainage, areas with considerable atmospheric deposition would cause a contaminated first-flush of runoff; b. Access the National Weather Service’s historic precipitation data from the rain gage nearest you. Locate the total depth of each rain storm for the past several years. From this data determine a constant depth of rainfall from each storm that constitutes 80% of the annual runoff. This will be your desired rainfall capture depth; c. Or you may wish to maximize the acreage of your rain garden to capture as much rainfall as is feasible on your lawn. In this case, you would design on this basis. If desired, you can back calculate your theoretical capture volume. (2) Calculate the plan view (bird’s eye view) area of the rooftop which will drain into the bioretention facility. This is easily based on the dimensions of the foundation of the house. (3) Multiply the plan view area of the roof by the desired rainfall capture depth (based on item (1)) to attain the approximate volume of runoff which the bioretention cell will need to accept.
Rain Garden Design and Construction Guidelines 12 (4) Divide by 4 inches the volume of runoff desired for capture. This is the required surface area of your garden. Four inches is determined to be the maximum storage depth10 of your garden. (5) Determine the shape of your rain garden satisfying the calculated surface area requirements. A good shape is approximately twice as long as it is wide for optimum distribution of inflow throughout the rain garden. The longest dimension should be oriented perpendicular to the slope.Using this technique, the intention is that the bioretention cell will infiltrate the desiredvolume of runoff. Excess runoff will overflow and run down the lawn to the storm sewersystem as it would have before bioretention cell installation. 6.3 Design AssumptionsThis is a conservative design. The design assumes that all of the desired runoff will fillthe basin at one time. In reality, infiltration will occur before the desired depth ofprecipitation has fallen and the rain garden’s capacity will be greater than the volume forwhich it was designed.The design also makes a reasonable assumption that runoff from the impervious roof ismagnitudes larger than the runoff from the short section of pervious lawn between theroof and the bioretention cell. Runoff from the lawn is considered negligible.7 ConstructionThe following guidelines are for the construction of the two Hseih and Davis (2005)diagrams referenced in Section 4 as Figures 4a and 4b. All specifications are providedbelow except for those cases where greater detail is provided in previous sections. 7.1 ExcavationExcavate to a depth of 125 cm (49 inches, 4.1 feet) measured from the uphill edge of therain garden. This depth is based on maximum media depths as shown in the Hseih andDavis (2005) diagrams (referenced in Section 4 as Figures 4a and 4b) including 4 inchesof above-ground storage.Side walls can be close to vertical to maximize the volume of infiltration media backfilland to simplify the calculations of needed materials. (NOTE: Do NOT dig deeper than 5feet without competent personnel trained in excavation safety. In 1993 the U.S.Occupational Safety and Health Administration (OSHA) instituted a law requiring trainedpersonnel on site and proper protection of trench walls for trenches greater than 5 feetdeep. Protection from collapse includes sheeting/bracing, shoring or sloping. Go tohttp://www.osha.gov for more information.)10 The native vegetation referred to in Section 5 will limit the pooling depth to around 4 inches forsatisfactory growing conditions (WDNR 2003). Consultation with a local supplier may adjust this depth.
Rain Garden Design and Construction Guidelines 13If using heavy equipment, keep it outside the perimeter of the bioretention cell ifpossible. Minimize soil compaction to preserve infiltrative capacity of the soil. 7.2 BermCreate a berm on the outside edge of the downhill portion of the perimeter of the raingarden. The height of the berm should be equal in elevation to the height of the uphilledge and around 10 inches high. Compact the berm firmly; manually is acceptable. Thiscan be executed by jumping on it aggressively. To protect the berm against erosion, itmust have very gently sloping sides and some type of cover. Cover the berm with mulch,or plant it with grass or dry-tolerant plant species. If planting, use straw or erosion-control mat for stabilization in the interim time before germination (WDNR, 2003). 7.3 Backfill and Compaction of Bioretention SoilBackfill the bioretention cell in successive horizontal layers (lifts) of 12 inches or lesswith the prescribed soil mixture in Section 4. Compact each lift by supersaturating theentire area of the rain garden (USEPA, 2003). If pooling occurs, wait until water isdrained before placing the next lift. Throughout the process, be sure the layers stay levelto facilitate even distribution of inflow and maximum infiltration. For best results, placea level on top of a two by four that extends a greater distance across the surface of thegarden. 7.4 PlantingPlant the plants at a rate of one plant every 1 to 2 feet on center. For more rapid filling inof plant materials, plant them closer to 1 foot apart. Some installations may includeshrubs or other plants that require larger spacing. Spacing may be maintained foraesthetic purposes. However, the natural system, including the mesic prairie system, maybe characterized by a dense network of vegetation. To maximize natural ecosystembenefits, this density is recommended, though not requisite. 7.5 MulchLay the mulch between the plants at the depth detailed in the mulch specifications. Thiswill require careful placement to avoid dismembering or covering the plants. Be sure thefinal depth between the mulch and the perimeter (unexcavated soil and berm) is 4 inchesand the mulch is level throughout.Mulch may be replaced with compost. Some tests show it has a greater capacity to treatpollutants and higher evapotranspiration rates (Barber et al., 2003). Effectively, anycombination of mulch and compost may be used. 7.6 Piping to Rain Garden
Rain Garden Design and Construction Guidelines 14Using flexible, light-weight, corrugated polyethylene pipe, extend the downspout fromcontributing roof areas to the uphill edge of the rain garden perimeter. The piping maybe aligned with room for an extension (a flared end section or an apron) that disperses thewater at the perimeter. If necessary (i.e. erosion occurs at the inlet to the garden), smallcobble may be used at the inlet to dissipate the force of the water entering the rain garden.8 Design Variations 8.1 On-Site LocationRain gardens may be located at the end of the yard along the street curb rather than closeto the house. This location has two advantages. Assuming a downward sloping yard, itcan capture more on-site runoff. In addition, curb cuts could be made to accept, infiltrate,and treat stormwater runoff from the road (Figure 4). Figure 4 Demonstration of a curb cut for runoff capture from street. This picture is from a project in Burnsville, MN, designed by Barr Engineering Company (2004).For a successful case study of an entire neighborhood’s project and explanatory pictures,see Barr Engineering Company’s installation in Burnsville, MN, as documented in Landand Water magazine (2004) available at www.landandwater.com. 8.2 On-Site Soil Infiltration CapacityTwo simple, low-technology tests may be performed to determine pre-constructioninfiltration rates (refer to page 9 of the manual mentioned in this section). If on-site soilshave infiltration rates greater than 0.5 in/hr you can design and build the rain gardenaccording to the Wisconsin Department of Natural Resources’ manual, Rain Gardens: AHow-to Manual for Homeowners (WDNR, 2003). This manual involves minimalexcavation, is user friendly, and is commonly referenced among Midwestern, non-profitorganizations that encourage these installations. It is available online athttp://www.dnr.state.wi.us/org/water/wm/nps/rg/index.htm. 8.3 Alternative Design Objectives
Rain Garden Design and Construction Guidelines 15If water quality treatment is prioritized over infiltration and groundwater recharge, anunderdrain approach is becoming the standard. The drain pipe allows water to beconveyed through the system when infiltration below and surrounding the cell is too slow(Barber et al., 2003).Below is a collection of cross sections and descriptions delineating variations on thedesign and their respective functions. In all cases (except for the first), the designincorporates a perforated pipe for the underdrain, a cleanout/observation well11, coarseaggregate, and an optional geotextile covering the entire excavated perimeter or someportion of it. Construction may be done independently or with on-site technical aid.Specifications suitable for a contractor or regulatory approval (if, in the rare case,connecting to a storm sewer) may be required. In all cases, the underdrain system isexpected to limit ponding to less than half an hour (PGCM, 2006). All pictures andsummarized descriptions are drawn from the Bioretention Manual developed by PrinceGeorge’s County, MD (2006).The bioretention cell design in Figure 5 facilitates high recharge of groundwater. Themanual recommends in-situ soils with infiltration rates of at least 1 in/hr and a depth of atleast 2.5 feet for adequate filtration. Figure 5 Infiltration and recharge facility for enhanced infiltration designed by Prince George’s County, MD (2006).The bioretention cell in Figure 6 facilitates high filtration and partial recharge of runoff.The underdrain location ensures a desired rate of drainage. Again, the depth is at least2.5 feet.11 A cleanout/observation well is a vertical pipe connecting to the underdrain and protruding slightly abovethe soil surface. It facilitates monitoring how quickly water is transmitted through the system and cleaningout the underdrain from possible build-up of biological materials. (PGCM, 2006)
Rain Garden Design and Construction Guidelines 16 Figure 6 Filtration and partial recharge facility designed by Prince George’s County, MD (2006).The bioretention cell in Figure 7 is designed for maximum water quality treatment. It isdesigned to handle higher nutrient loadings by facilitating a fluctuatingaerobic/anaerobic12 zone in the layer below the underdrain. The area below theunderdrain simultaneously provides a storage area and recharge zone. Figure 7 Infiltration, filtration, and recharge facility designed by Prince George’s County, MD (2006).The bioretention cell in Figure 8 is designed for pre-treatment of highly contaminatedwater before discharge at an outlet pipe. The liner prevents groundwater contamination.12 Aerobic and anaerobic conditions refer to the presence or absence of oxygen, respectively, within the soilsystem. The type of bacteria functioning within the soil (contributing to water treatment) is directly relatedto which condition is present.
Rain Garden Design and Construction Guidelines 17 Figure 8 A filtration-only bioretention cell designed by Prince George’s County, MD (2006).The Department of Environmental Resources of Prince George’s County, MD, hasdeveloped a comprehensive guide (from which the above diagrams and descriptionsoriginated) to understanding and designing a bioretention system that utilizes anunderdrain. The design criteria and conditions are specified for implementation in aresidential community. The manual provides complete instruction and examples on howto develop the bioretention plans as well as a corresponding grading plan and sedimentand erosion control plan. The document identifies issues and responsibilities for thehomeowner, developer, designer, and inspector from the concept phase through themaintenance and operation phases of the bioretention cell installation. It also includesguidance for construction, community involvement, maintenance, and additional notes oncompaction. It is available on the County’s website athttp://www.co.pg.md.us/Government/AgencyIndex/DER/ESD/Bioretention/bioretention.asp.For purposes of comparison to the design recommended in this technical brief, Figure 9 isa cross section delineating basic dimensions as recommended by Prince George’sCounty. The two designs are comparable in this regard.
Rain Garden Design and Construction Guidelines 18 Figure 9 Sizing of a bioretention cell designed by Prince George’s County, MD (2006), shown for comparison to the LID Center design.An additional set of bioretention cell specifications is notable. The non-profit, LowImpact Development Center in Beltsville, MD, has posted on its website a bioretentionspecification developed through a Cooperative Assistance Agreement under the US EPAOffice of Water 104b(3) Program. It is developed for “local governments, planners, andengineers for developing, administering, and incorporating Low Impact Development(LID) into their aquatic resource protection programs” (USEPA, 2003). Figure 10 is theplan designed for the application discussed here. The specifications can be found onlineat http://www.lowimpactdevelopment.org/epa03/biospec.htm.Figure 10 Sizing and specifications of a bioretention cell designed by the LID Center (USEPA, 2003).NOTE: If installing an underdrain according to these design variations, the infiltrationmedia diagrammed in Hseih and Davis (2005) diagrams (referenced in Section 4 as
Rain Garden Design and Construction Guidelines 19Figures 4a and 4b) can be a more specific designation of the ‘bioretention soil’ asdiagrammed in the underdrain cross section (Figure 10).9 Maintenance 9.1 VegetationIn its first 2-3 years of growth, required maintenance will be intensive with regard to thevegetation. Until a dense network of vegetation is established, weeding non-natives andespecially invasive13 plants will be a necessity. Mulch will help prevent weeds but maysimultaneously stall expansion of the planted vegetation. Once the matrix is established,it will be an effective defense against the germination of invasives (Vanderpoel, 2003).Smaller rain gardens can be hand-weeded. Spot herbicide application using state-approved chemicals is an option for treatment. In the case of herbicide application, itshould be used sparingly and in dry, calm conditions only. At no point shouldapplication occur in standing water. In the case of wind, drift from the spray may stuntthe growth of (or kill) downwind plants. 2.2 Clogging PreventionThe rain garden should increase in infiltrative capacity during the initial establishment ofthe root network of the plants due to the resulting organics in the soil and the increasedtranspiration. However, rain gardens may be susceptible to clogging after a certainperiod of time, dependent upon the contaminant and solids loading. An option to restorethe infiltrative capacity and break up the biomat14 may be to aerate and loosen up the soilannually. This must be done without uprooting or disturbing the plants. In the dormantseason, piercing the soil with a pitch fork in the vertical direction (careful not to overturnthe soil) may be the best remedy at the lowest risk of damage. Although untested, it maybe effective to do this with care throughout the growing season.10 Final RemarksAdding a rain garden to your property helps to restore the natural hydrology of the area.It reduces contributions to storm sewers, mitigates downstream flooding, and provides anoutlet for education and native ecosystem restoration.Rain gardens also contribute aesthetically to your property, raising your property value.Prince George’s County, Maryland, has identified properties with rain gardens as havingincreased real estate values by up to 20% (PGCM, 2006). In Prairie Crossingsubdivision, they roughly estimated that their homes command a 30% premium overother typical nearby communities (Prairie Crossing Information and Sales Center, 2006).13 Invasive plants are characterized by their aggressive propagation and prevention of native plantestablishment. Usually invasives are nonnative and natives are noninvasive. However, this is not alwaysthe case.14 Biomats are formed as a result of biological buildup (or clogging) in a horizontal zone within the soil.
Rain Garden Design and Construction Guidelines 20To see a community development utilizing comprehensive Low Impact Developmenttechniques such as rain gardens on a large scale, go to www.prairiecrossing.com.11 ResourcesBarr Engineering Company. (September/October 2004). “Burnsville Rainwater Gardens.”Land and Water, 48(5), 47.Barber, M. E., King, S. G., Yonge, D. R., Hathorn, W. E. (2003). “Ecology Ditch: A BestManagement Practice for Storm Water Runoff Mitigation.” J. Hydrologic Egrg, 8(3),111.Beach, D.N.H., McCray, J.E., Lowe, K.S., Siegrist, R.L. (2005). “Temporal Changes inHydraulic Conductivity of Sand Porous Media Biofilters During Wastewater InfiltrationDue to Biomat Formation.” J. Hydrology, 311, 230-243.Bouwer, H. (2002). “Artificial Recharge of Groundwater: Hydrogeology andEngineering.” Hydrogeology Journal, 10, 121-142.Brooklyn Botanic Garden (BBG) (2006). “Rain Garden Plants.”http://www.bbg.org/gar2/topics/design/2004sp_raingardens.htmlLast accessed 12 April 2006.Christianson, R.D., Barfield, B. J., Hayes, J. C., Gasem, K., Brown, G. O. (2004).“Modeling Effectiveness of Bioretention Cells for Control of Stormwater Quantity andQuality.” ASCE Conf. Proc., Critical Transitions in Water and Environmental ResourcesManagement, 37.Clar, M. L., Barfield, B., O’Connor, T. (2004). “BMP Design Guidelines: VegetativeBiofilters.” ASCE Conf. Proc., Critical Transitions in Water and EnvironmentalResources Management, 66.Dietz, M. E., Clausen, J. C. (2006). “Saturation to Improve Pollutant Retention in a RainGarden.” Environmental Science and Technology, 40(4), 1335-1340.Emerson, C. H., Welty, C., Traver, R. G. (2005). “Watershed-Scale Evaluation of aSystem of Storm Water Detention Basins.” J. Hydrologic Engrg., 10(3), 237-242.Hsieh, C-h, Davis, A.P. (2005). “Evaluation and Optimization of Bioretention Media forTreatment of Urban Storm Water Runoff.” J. of Environ. Engrg., 131(11), 1521-1531.Natura, Heidi. 1995. “Root Systems of Prairie Plants.” Available athttp://www.livinghabitats.com/Pitt, R., Chen, S-E, Clark, S. (2002). “Compacted Urban Soils Effects on Infiltration andBioretention Stormwater Control Designs.” ASCE Conf. Proc., Urban Drainage, 14.
Rain Garden Design and Construction Guidelines 21Prairie Crossing Information and Sales Center. Email Correspondence. 11 March 2006.Prince George’s County, Maryland (PGCM), Department of Environmental Resources.The Bioretention Manual; Watershed Protection Branch, MD Department ofEnvironmental Protection: Landover, MD, 1993.Prince George’s County, Maryland (PGCM), Department of Environmental Resources.The Bioretention Manual; Watershed Protection Branch, MD Department ofEnvironmental Resources: Largo, MD, 2006.Available athttp://www.goprincegeorgescounty.com/government/agencyindex/der/esd/bioretention/bioretention.aspPrince George’s County, Maryland (PGCM), Department of Environmental Resources.1997. Low Impact Development Design Manual. Prince George’s County, MD.Traver, R.G. (2004). “Infiltration Strategies for LID.” ASCE Conf. Proc., World WaterCongress, Critical Transitions in Water and Environmental Resources Management, 83.United States Environmental Protection Agency (USEPA) (2003). Drainage-Bioretention Specification. Cooperative Assistance Agreement, Program 104b(3), Officeof Water. http://www.lowimpactdevelopment.org/epa03/biospec.htmLast accessed 30 March 2006.United States Environmental Protection Agency (USEPA). (2000, revised 2005).“Stormwater Phase II Final Rule: Small MS4 Stormwater Program Overview.”http://www.epa.gov/npdes/pubs/fact2-0.pdfLast accessed 22 March 2006.United States Environmental Protection Agency (USEPA). (1999). “Storm WaterTechnology Fact Sheet: Bioretention. EPA 832-F-99-012. Office of Water, Washington,D.C.Vanderpoel, T. (2003). Restoration Co-Chair, Citizens for Conservation, Barrington, IL.Wisconsin Department of Natural Resources and University of Wisconsin-Extension(WDNR) (2002). “Rain Gardens: A Household Way to Provide Water Quality in YourCommunity.” Board of Regents of the University of Wisconsin System.Wisconsin Department of Natural Resources and University of Wisconsin-Extension(WDNR) (2003). “Rain Gardens: A How-to Manual for Homeowners.” Board ofRegents of the University of Wisconsin System.