Why and How; What’s Working and What Isn’t

 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

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

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

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

 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

 Optimize the site layout

 Use pervious surfaces and green roofs where possible

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

 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

One Acre V Rainfall Record Hour #Gage 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

infiltration evapotranspiration

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

 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

 Municipalities can pass implementation costs on to developers  With a 4% sizing factor, nearly all projects can implement bioretention  Costs are reasonable (<<1% of project cost)  Bioretention technology is increasingly well-defined and well-known  Bioretention has a strong track record with hundreds of projects of all types built and in operation

 Bioretention not allowed unless infiltration, evapotranspiration, and harvesting/reuse are infeasible  Poorly targeted exceptions for high- density, “smart growth” infill projects  Regulatory rationale: “We have a feeling”  Mandate for off-site and in-lieu programs  Ever-increasing reporting requirements  Invalid technical specifications written into permits

 Focus on maximizing water quality benefit  Trust and cooperation  Fact-based decision-making  Continuous Improvement  Patience and Fortitude