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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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Presentation Transcript
low impact development

Low Impact Development

Why and How;

What’s Working and What Isn’t

topics
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
conventional urban drainage
Conventional Urban Drainage
  • Features
    • Impervious surfaces: roofs and pavement
    • Catch basins and piped drainage
    • “Collect and convey” design objective
lid drainage principles
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
lid design steps
LID Design Steps
  • Optimize the site layout
lid design steps1
LID Design Steps
  • Optimize the site layout
  • Use pervious surfaces and green roofs where possible
lid design steps2
LID Design Steps
  • Optimize the site layout
  • Use pervious surfaces and green roofs where possible
  • Disperse runoff to landscaping
lid design steps3
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
sizing treatment facilities
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

bioretention
Bioretention

evapotranspiration

infiltration

sizing criterion
Sizing Criterion

i = 0.2 inches/hour

BMP Area/Impervious Area =

0.2/5 = 0.04

Surface Loading Rate

i = 5 inches/hour

harvesting and reuse
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

why is lid controversial
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
messing with success
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
lid what we need now
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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