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Low Impact Development

Low Impact Development. Why and How; What’s Working and What Isn’t. Topics. 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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Low Impact Development

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  1. Low Impact Development Why and How; What’s Working and What Isn’t

  2. Topics • 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

  3. Conventional Urban Drainage • Features • Impervious surfaces: roofs and pavement • Catch basins and piped drainage • “Collect and convey” design objective

  4. Drainage Impacts

  5. LID Design Objectives

  6. LID Drainage Principles • 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 theyflood safely

  7. LID Design Steps • Optimize the site layout

  8. LID Design Steps • Optimize the site layout • Use pervious surfaces and green roofs where possible

  9. LID Design Steps • Optimize the site layout • Use pervious surfaces and green roofs where possible • Disperse runoff to landscaping

  10. LID Design Steps • 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

  11. Sizing Treatment Facilities One Acre Vary V until 80% is detained and 20% overflows V 48-hourdrawdown Largest storm retained  85th percentile, 24 hour  0.5" - 1.0" storm depth  0.2 inches per hour

  12. Bioretention evapotranspiration infiltration

  13. Sizing Criterion i = 0.2 inches/hour BMP Area/Impervious Area = 0.2/5 = 0.04 Surface Loading Rate i = 5 inches/hour

  14. Harvesting and Reuse • Facility must fully drain within specified drawdown time • No way to “credit” removal of a portion for reuse unless drawdown is regular and predictable WaterQualityVolume

  15. Harvesting and Reuse

  16. Why is LID Controversial? • 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

  17. Messing with Success • 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

  18. LID: What we need now • Focus on maximizing water quality benefit • Trust and cooperation • Fact-based decision-making • Continuous Improvement • Patience and Fortitude

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