LOW IMPACT DEVELOPMENT IN COMPREHENSIVE FLOOD RISK MITIGATION PLANNING

Lisa Hair, PE, USEPA 2012 USACE Silver Jackets Workshop

Reduce Federal Expenditures And Improve Resiliency

Flood mitigation activities are FEMA, USACE

Protection of Water Quality is EPA’s goal Resiliency includes water supply

assurance and adaptability to growth and climateSmall storm retention in local

ordinances for new development can help achieve

these.

LID reduces urban pollutants

2007: EPA Commissioned National Research Council

to review Stormwater Program

Result: Current methods not effective

Runoff volume control needed - not just

concentration of pollutants in runoff

• Natural approaches - infiltration and evapotranspiration – and harvest-and-use

• Distributed small-scale hydrologic controls retain rainfall close to the source

•Replicates the pre-development hydrologic regime: reducing runoff volume compared to impervious surfaces

Modified from Prince George's County, Maryland. Larry Coffman et al. (1999).

Low Impact Development” (LID)

Adaptable LID Practices Reduce the Impact of Imperviousness

USGS equations predict increases in flooding with watershed imperviousness

The National Flood Frequency Program, USGS, 2002

Imperviousness = Higher Flood Peaks

Impervious Surfaces Increase Need for Mitigation

Comprehensive Flood Risk Mitigation

Starts with Stormwater OrdinancesMotivated by, flooding, drought, stream erosion,

beach or shellfish contamination –not usually

water quality!

“But we have detention pond requirements”: Why Detention Does Not Work

Extending hydrograph duration results in overlap and more flooding, scouring

Limiting flow rate with larger volumes extends the duration of that “peak” flow

PROVEN BENEFITS OF FLOOD REDUCTION,COST SAVINGS, ENVIRONMENTAL

Nashville: USACE, Mill Creek – LID could reduce flood losses (reservoirs unacceptable); save endangered species as a side-benefit

Omaha: Papio Creek Watershed Plan - LID adoption multi-jurisdiction for flood control; WQ side-benefit

IL: Kane, Lake Counties - LID adopted for flood control; WQ and Groundwater Recharge side benefits

Maricopa County (Phoenix) - LID for flood control since 1985; groundwater recharge side benefit

MN: Capital Region Watershed District - LID cheaper flood control option; water quality improvement for popular lake

Los Angeles’ Sun Valley Watershed - LID flood control benefits changed LA’s approach to overall stormwater management

NC: Asheville Flood Task Force - Ordinances adopting LID for flood control, will see WQ benefits

Small storms cause repititive losses - How much flooding can LID reduce?

• Typical LID design retains 0.5 to 2”; can be designed for more

• Flood reduction effect is large for small events

• Not noticeable at major storms such as >5”

• BUT – 80% to 90% of annual rainfall is <1.5”

• AND - 80% to 90% of annual pollution is reduced

Calculate Benefits $$

Annualized Avoided Losses,

-San Antonio, TX: $6 M/yr

-Richmond, VA: $2M/yr

EPA is not proposing LID for flood control, these are side-benefits to LID for water quality

EPA modeled HUC-8’s from 2018 to 2040: Adopted LID practices on new development and redevelopment:

Atkins, 2012

USACE Approach for Reducing Future Federal Expenditures

Policy Guidance Letter (PLG 52) requires recipients of Corps projects to protect against future flood risk

Adoption of local ordinances for “no net increase in runoff” from new

development is recommended

FEMA NFIP looking to LID for Resiliency and Environment

FEMA CRS scoring credits for LID

NFIP EIS alternative: integrating future conditions –biggest impact may be future imperviousness

FEMA has LID in “Model Ordinances” for environmental protection, and funded LID pilots because of repetitive losses

Conclusions LID is necessary for water quality/stream health

Cost-effective for small storm flood reduction

Adds community resiliency via groundwater recharge, maintaining stream baseflow, enables growth w/o flood damage increase, eases climate change impacts

Reduces future federal expenditures, protects existing federal investments in flood control

LID is essential part of comprehensive mitigation planning – include LID in alternative selection

For more information Google EPA Green Infrastructure