LID and Flood Control
Presented
by: by:
Presented
The
Low
Impact
Development
Center, Inc.
The
Low
Impact
Development
Center, Inc.
A non-profit
waterwater
resources
and sustainable
design organization
A non-profit
resources
and sustainable
design organization
www.lowimpactdevelopment.org
www.lowimpactdevelopment.org
The Low Impact Development Center, Inc. has met the
standards and requirements of the Registered Continuing
Education Program. Credit earned on completion of this
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© Low Impact Development Center, 2012
Purpose and Learning Objectives
The purpose of this presentation is to discuss how Low
Impact Development fits into flood control programs.
At the end of this presentation, you will be able to:
• Explain where LID fits into flood control
• Discuss how LID can reduce flooding
Multi-Objective Solutions
Improve Cost Effectiveness
• Flood mitigation activities are FEMA, USACE
• Protection of Water Quality is EPA’s goal
• Stormwater management affects both
Small storm retention in local ordinances for new development
and redevelopment can help achieve both.
Stormwater ordinances
Local drivers vary: water quality, flooding,
beach and shellfish contamination
LID is the most reliable approach to reduce pollutant
loading via volume reduction and soil filtration
EPA Commissioned NRC in 2007 to review
NPDES Stormwater Program
Result: Current methods not effective
Stormwater is #1 growing quality concern
Impairment wide-spread; increasing
Volume control needed not just concentration
Stormwater Rulemaking Underway
Why Detention Does Not Work
Post-Development Condition
Q
Pre-Development Condition
t
Figure 3. Post-Development Hydrograph.
(Q = volumetric flow rate; t = time)
Increased rate and volume
runs off an impervious site
Limiting flow rate with the
same volume extends the
duration of flow impact on
the stream
LID Practices: Infiltration,
evaportranspiration, harvest
and use to restore the runoff
volume of predevelopment
conditions
Does not replace treatment
where needed
Watershed-wide LID can reduce small storm
flood losses from development – less runoff
 Corps of Engineers: Nashville Mill Creek 2007
 Papio Creek Partnership Watershed Plan, Omaha, 2007; LID adoption multi-jurisdiction
 Braden and Johnson, University of Illinois, ASCE 2006; LID adoption in Kane and Lake
Counties
 Maricopa County, adopted for flood control since 1985
 Capital Region Watershed District, Minnesota, Arlington-Pascal Project 2003 saw cost
savings for LID flood control; ordinance
 Los Angeles’ Sun Valley Watershed, LID flood control project changed county approach
to stormwater management
 Asheville, North Carolina, Flood Management Task Force 2007, LID Ordinances
Findings in Nashville Corps Mill Creek Study
• Continued urbanization would cause:
o Increased flooding, most noticeable in small storm events in Mill Creek
o Tributaries to run dry, an ecosystem problem
• Threat to endangered species
• Sedimentation and habitat alteration
Photo Credit:
The Conservation Fund
Findings in Nashville Study
• Infiltration basins added 10 weeks of baseflow during
summer: If only half achieved, significant improvements
in water quality and habitat survival would occur
• LID reduces peaks flooding in small storms, maintains
needed baseflow, eliminates need for regional detention
(opposed locally)
• Pollutant loading not a part of study
• LID only solution that maintained stream base flow,
protecting endangered species
Examples of LID in Flood Management
and Flood Modeling
• Papio Creek Partnership: Omaha 2007 – Flooding and
WQ Benefit Modeled: LID Only solution to both flooding
and impairment
• Capitol Region Watershed District: MN 2004 to 2011 –
Flooding, WQ, Capital Cost Benefits Realized in 190 acre
watershed
• Sun Valley, Los Angeles - Flooding, WQ, amenities,
cost/benefit optimized 2004 – 2011
How Much Flooding Can be Reduced?
• Typical LID design retains 1.0 to 1.5” storm; could be much
higher if feasible
• Effect is large for small events, say 3” storm
• Effect is not noticeable at major storms
• BUT – 80% to 90% of annual rainfall is <1.5”
• 80% to 90% of pollution reduced with a side benefit of
reducing small storm flooding
• Typically requirements are for new development or
redevelopment; retrofits more costly
Benefits of LID for Floodplain Management
• “Stormwater Magazine” Fall
2011 article by Daniel E. Medina
et. al.
• Case study; LID & watershed
scale hydrology; Hydraulic
modeling; flood damage analysis
• Modeled 13.2 sq mi urbanized
watershed in Atlanta Georgia
• Looked at flood loss reduction
with 1.2” LID retrofits
throughout watershed
Study Models Inundation Reduced
Medina et al. 2011
Study does not
model water
quality benefits,
the primary
objective from EPA
perspective. Flood
loss reduction is a
side benefit of this
water quality
approach.
Medina et al, 2011
LID Can Help Meet Watershed
Water Quality Regulations
•
•
•
•
•
•
NPDES Antibacksliding provisions
TMDLs: Applies in Urban, Suburban, Exurban
Source Water Protection for Drinking Water
Endangered Species Act
NPDES Phase I and Phase II permit requirements
Shoreline bacteria contamination (North Carolina)
LID can help protect floodplain services including habitat
Excess runoff volume from impervious
area scours streams
- Floodplain/stream connectivity maintained
- Connected floodplain absorbs floodwaters
- More natural hydrology and habitat maintained
- FEMA Region X per NMFS ESA Biological Opinion
- ESA Habitat Conservation Plans (Etowah, GA)
Not New Concept for USACE
PLG 52, Floodplain Management Plans
PLG 52, Floodplain Management Plans
Watershed LID Consistent with
FEMA Flood Insurance Credits
• FEMA’s Community Rating System (CRS) has had credit for
stormwater retention ordinances in the past, key example
Maricopa County (Phoenix)
• The New Draft CRS scoring has more credits for stormwater
retention ordinances and LID
• The New Draft CRS Scoring has credits for flood control
projects only if future conditions (build-out) are studies
• Credit for natural areas as part of large-scale, contiguous
Green Infrastructure Plan
Consistent with Professional Floodplain
Management Community
• ASFPM, NAFSMA concepts: Holistic management
• Natural and Beneficial Floodplain Functions: Floodplain
Management—More than Flood Loss Reduction, White Paper
Adopted by ASFPM Board 2008
• No Adverse Impact Initiative by ASFPM, 2004
• “If current land development patterns, minimum floodplain
requirements, and standard stormwater practices remain, no doubt
flooding and flood losses will worsen.” ASFPM
Flooding from Development
Well-Established – Mitigation New
• Tulsa, Oklahoma early application of future build-out
planning, set-back development as a result (“Flood Wars”).
Now development has higher % impervious than was planned
in the 1980’s.
• Charlotte-Mecklenburg Stormwater Services also modeled
future build-out, set-back development as a result (increased
floodway 45%)
• What if your town is already built to water’s edge, you can’t
set-back development, but watershed continues to develop?
One answer: ordinances from no increased runoff from
development
Example Watershed Strategic Plan Goals
Comprehensive: Pollution, Flood, Drought, Water Supply, Geomorphology and
Habitat
Stormwater as a Resource: How to Manage it that way?
Identify Infiltration Opportunities – soil mapping (“Green Seams”)
Prioritize: Development , Retrofit Areas, Floodplain Preservation
Mimic pre-development hydrology where feasible
Runoff reduction by local ordinance, water quality treatment
Development /Redevelopment Issues (Where and How to develop)
- Preserved open space, parks
- Low impact development techniques – rural, suburban, urban
- Early involvement of regulatory agencies and stakeholders
- Smart Growth: walkable, livable cities
Stormwater management planning has a role in all of these, affecting water
quality degradation and increasing flood potential.
Conclusion
• Be aware of state or local partner’s stormwater management
requirements, water quality issues, water supply needs, longrange development plans in the watershed
• Encourage adoption of LID ordinances to prevent increases in
flood potential, maintain healthy stream baseflow, assure
regulatory compliance, earn FEMA NFIP CRS credit, comply
with Corps PLG 52.
• Include watershed LID alternatives when modeling flood
mitigation alternatives
Thank you for your time.
QUESTIONS?
Low Impact Development Center, Inc.
www.lowimpactdevelopment.org
301.982.5559