Canada; Infiltration Rain Garden - British Columbia

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Canada; Infiltration Rain Garden - British Columbia
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  Infiltration Swale Stormwater Source Control Design Guidelines 2005  Greater Vancouver Sewerage & Drainage District This page is intentionally left blank. 42  Stormwater Source Control Design Guidelines 2005 Infiltration Rain Garden  Greater Vancouver Sewerage & Drainage District Photo Credit: Lanarc Consultants Ltd. Informal rain garden, Water Pollution ControlLaboratory, Portland Oregon Photo Credit: Lanarc Consultants Ltd.   Rain garden overflow, Buckman Terrace, PortlandOregon. Infiltration Rain Garden Description The Infiltration Rain Garden is a form of bioretentionfacility, designed to have the aesthetic appeal of agarden, as opposed to a purely functional appearance.Rain Gardens are commonly a concave landscape areawhere treated runoff from roofs or paving is allowed topond temporarily while infiltrating into deep constructedsoils below (See Drawing 3A) .  The surface planting of Rain Gardens is dominated bytrees, shrubs, and groundcovers, with planting designsrespecting the various soil moisture conditions in thegarden. Plantings may also include rushes, sedges andother grass-like plants, as well as sodded lawn areas for erosion control and multiple uses.On subsoils with low infiltration rates, Rain Gardensoften have a drain rock reservoir and perforated drainsystem to collect excess water. (See Drawing 3B &3C).The perforated drain system may connect to a controlstructure in a catch basin that provides overflow whilemaintaining a slow decanting of the water in the raingarden between storms (See Drawing 3D).While usually designed as a ‘standalone’ facility withoutconveyance, new designs are evolving that put a seriesof Rain Gardens along linear areas like roads – withweirs and surface conveyance similar to InfiltrationSwales.Other common terms used are Bioretention and DrySwale with Underdrain (Stephens et al. , 2002) or  Swale /Trench   Element (MUNLV-NRW, 2001). Application and Limitations  Application and Limitations are similar to InfiltrationSwales. Photo Credit: Lanarc Consultants Ltd. Formal rain garden, Buckman Terrace, PortlandOregon. 43  Infiltration Rain Garden Stormwater Source Control Design Guidelines 2005  Greater Vancouver Sewerage & Drainage District Design Guidelines 1.   The Rain Garden area should be 10-20% of the upstreamimpervious area that it serves, preferably sized bycontinuous flow modelling. Common rain garden size isabout 50 m 2 draining 250m 2 of impervious area, althoughthis sizing and proportion will vary by rainfall and soilcharacteristics. Smaller, distributed Rain Gardens arebetter than single large scale facilities.2.   Site Rain Gardens similar to other infiltration facilities –minimum 30m from wells, minimum 3 m downslope of building foundations, and only in areas where foundationshave footing drains.3.   Provide pretreatment erosion control to avoidsedimentation in the garden. Provide non-erodablematerial, sediment cleanout basins, and weir flowspreaders at point-source inlets. Flow to the swale shouldbe distributed sheet flow, travelling through a grassy filter area or grass swale prior to entering the Rain Garden(500 mm minimum, greater than 3000 mm desirablegrassy pretreatment swale length; Claytor and Schueler,1996).4.   Rain Garden bottom (Drawing 3A): flat cross section, witha longitudinal slope of 2% (or 1% by US001, or dished byGE004). Provide a 50mm – 75mm layer of organic mulch– well aged compost, bark mulch or similar weed freematerial. The mulch is important for both erosion controland maintaining infiltration capacity.5.   Rain Garden bottom width: 600mm minimum, 3000mmdesirable, length:width ratio of 2:1 (Gibb et al. , 1999).6.   Rain Garden side slopes 2 horizontal : 1 verticalmaximum, 4:1 preferred for maintenance. Provide organicmulch on side slopes similar to bottom.7.   Maximum ponded level: 150mm (Gibb et al. , 1999),300mm (NRW).8.   Drawdown time for the maximum surface ponded volume:48 hours (72 hours max. - Maryland Dept. EnvironmentalResource Programs, 2001).9.   Treatment soil depth: 450mm minimum (City of Portland,2002), 1200mm desirable (Gibb et al. , 1999). Treatmentsoil should have a minimum infiltration rate of 13mm/hr,with 6mm/hr used for design.10.   Slope of the drain rock reservoir bottom shall be level.11.    A non-erodible outlet or spillway must be established todischarge overflow (Maryland Dept. EnvironmentalResource Programs, 2001). 44  Stormwater Source Control Design Guidelines 2005 Infiltration Rain Garden  Greater Vancouver Sewerage & Drainage District Photo Credit: Lanarc Consultants Ltd.   Flow-thru planter – a formal-shaped rain gardenthat provides water quality treatment and limitedflow attenuation – adapted to a near-buildinglocation at Buckman Terrace in Portland, Oregon. 12.    Avoid utility or other crossings of the Rain Garden. Whereutility trenches must be constructed crossing below thegarden, install trench dams to avoid infiltration water following the utility trench.13.   Rain gardens can be constructed in a variety of shapes,including formal rectalinear flow-through planters(Drawing 3E). Get geotechnical advice prior to siting raingardens closer than 3m to building foundations. Design Options: Infiltration Rain Gardens may take a variety of shapes, frominformal, organically shaped ‘bowls’ to formal, rectilinear planting areas and planters.Drain rock reservoir and underdrain may be deleted whereinfiltration tests by the design professional taken at the level of the base of the proposed construction show an infiltration ratethat exceeds the inflow rate for the design storm. 45
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