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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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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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