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Why and How; Whats Working and What Isnt

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How Low Impact Development can mitigate effects of urban drainage Applying design criteria for bioretention and harvesting/reuse Why is LID controversial? The LID mandate: Problems and possible solutions

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Features Impervious surfaces: roofs and pavement Catch basins and piped drainage Collect and convey design objective

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Site ScaleWatershed and Stream Scale Higher peak flowsFlooding and scouring of stream beds Lower time of concentrationFlash flows Runoff from small stormsDischarge when runoff did not previously occur Increased runoff durationsStream erosion at moderate stream flow rates Greater runoff volumesHigher pollutant loading Greater runoff energyConveys trash and gross pollutants to streams Decreased infiltrationLower and less frequent stream base flow Dry weather dischargesHigh pollutant concentrations during low flows

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Watershed and Stream ScaleSite scale Reduce peak flowsDetain runoff on site Increase time of concentrationSlow runoff from leaving site No runoff from small stormsInfiltrate, evapotranspirate and reuse Reduce duration of moderate flowsLet runoff seep away very slowly Reduce runoff volumeInfiltrate and reuse where possible Reduce runoff energyDetain and slow flows Increase groundwater storage and stream base flows Facilitate infiltration Reduce pollutants in runoffDetain and filter runoff Protect against spills and dumpingDisconnect drainage and filter runoff

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Instead of collect and convey, slow it, spread it, sink it. Avoid concentrating flows Keep drainage areas small Promote infiltration Detain Treat Route high flows so they flood safely

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Optimize the site layout

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Use pervious surfaces and green roofs where possible

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Optimize the site layout Use pervious surfaces and green roofs where possible Disperse runoff to landscaping

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Optimize the site layout Use pervious surfaces and green roofs where possible Disperse runoff to landscaping Direct drainage from impervious surfaces to bioretention facilities, flow-through planters, or dry wells

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One Acre V Rainfall Record Hour #Gage 10.00 20.02 30.01 48-hour drawdown Vary V until 80% is detained and 20% overflows Largest storm retained 85 th percentile, 24 hour 0.5" - 1.0" storm depth 0.2 inches per hour

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infiltration evapotranspiration

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Surface Loading Rate i = 0.2 inches/hour i = 5 inches/hour BMP Area/Impervious Area = 0.2/5 = 0.04

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Facility must fully drain within specified drawdown time No way to credit removal of a portion for reuse unless drawdown is regular and predictable Water Quality Volume

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Municipalities can pass implementation costs on to developers With a 4% sizing factor, nearly all projects can implement bioretention Costs are reasonable (