Presentation Transcript

1. The Role of Low Impact Development in Flood Risk Management Planning

   Watershed Scale Application for Water Quality and Flood Control Lisa Hair, PE 2011 Flood Risk Management and USACE Silver Jackets Workshop
2. 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.
3. Stormwater ordinances Local drivers vary: water quality, flooding, beach and shellfish contamination
4. 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
5. Why Detention Does Not Work 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
6. LID Practices: Infiltration, evaportranspiration, harvest and use to restore the runoff volume of predevelopment conditions Does not replace treatment where needed

7. 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
8. Findings in Nashville Corps Mill Creek Study Continued urbanization would cause: Increased flooding, most noticeable in small storm events in Mill Creek Tributaries to run dry, an ecosystem problem Threat to endangered species Sedimentation and habitat alteration Photo Credit: The Conservation Fund
9. 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
10. 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
11. 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
12. Benefits of LID for Floodplain Management “Stormwater Magazine” Fall 2011 article by Daniel E. Medinaet. 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
13. Study Models Inundation Reduced Medina et al. 2011
14. 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
15. 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)
16. 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)
17. Not New Concept for USACE
18. PLG 52, Floodplain Management Plans
19. PLG 52, Floodplain Management Plans
20. 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
21. 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
22. 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
23. 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.
24. Conclusion Be aware of state or local partner’s stormwater management requirements, water quality issues, water supply needs, long-range 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
25. Questions?