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Accelerated Post-Fire Flood Analyses Hydraulics URS Group Inc. 2010 ASFPM National Flood Conference...
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Transcript of Accelerated Post-Fire Flood Analyses Hydraulics URS Group Inc. 2010 ASFPM National Flood Conference...
Accelerated Post-Fire Flood Analyses Hydraulics
URS Group Inc.2010 ASFPM National Flood Conference
May 19, 2010
Accelerated Post-Fire Flood Analyses Hydraulics2
Overview
Southern California Wildfires of October 2007
Destruction of buildings
- Residential
- Industrial & Commercial
Increased flood hazards in burn areas
Analyses prepared for FEMA’s Region IX Office by MAP IX-Mainland, a Joint Venture of
- URS
- Dewberry
- Schaaf & Wheeler
- Airborne 1
- TerraPoint
Accelerated Post-Fire Flood Analyses Hydraulics3
Overview
Graphic from FEMA, created by Michael Baker Jr., Inc.
Accelerated Post-Fire Flood Analyses Hydraulics4
Study Area
Southern California
- Orange, San Bernardino, and Ventura Counties
• Approximately 104.0 stream miles identified for Emergency or Priority study
Graphic from FEMA, created by Michael Baker Jr., Inc.
Accelerated Post-Fire Flood Analyses Hydraulics5
Study Area
Southern California
- Orange County affected FIRM panels
Graphic from FEMA, created by Michael Baker Jr., Inc.
Accelerated Post-Fire Flood Analyses Hydraulics6
Needs
Rebuilding effort
- Large-scale
- High urgency
Reduce risk for future property losses
- Existing flood zones invalid due to altered land conditions
- Develop revised flood zones before rebuilding
Pre- and Post- burn analyses
- Analyses for 1%-annual-chance storm event
- Analyses for 20%-annual-chance storm event
Accelerated Post-Fire Flood Analyses Hydraulics
Timeline
October 2007 – Wildfires occur,
Through November 19 – Planning and scoping phases are completed
November 20-28 – Engineering Analyses completed for Emergency & Priority Streams
November 28 – Floodplain boundary shapefiles completed for Emergency & Priority Streams
7
Tues Nov 20
WedNov 21
Thurs Nov 22
FriNov 23
SatNov 24
Sun Nov 25
Mon Nov 26
TuesNov 27
WedNov 28
ThursNov 29
Accelerated Post-Fire Flood Analyses Hydraulics8
Hydrology
Hydrologic analyses were not performed in the URS Gaithersburg office
Presented at the ASFPM 2008 conference
Key concepts
- Land cover change – vegetation
- Soil change – permeability
Accelerated Post-Fire Flood Analyses Hydraulics9
Hydraulics - Overview
Physical Changes
- Increased peak flows from hydrologic changes
• Creates higher water surface elevations (WSELs)
- Floodplain vegetation reduced
• Creates lower WSELs due to decreased roughness in overbank flow
- In-stream debris
• Creates higher WSELs due to increased roughness in channel flow
Accelerated Post-Fire Flood Analyses Hydraulics10
Hydraulics – Determination of overbank roughness
Burn severity
- Determined in planning phase
- High, medium, low burn
- GIS raster (grid) format
Accelerated Post-Fire Flood Analyses Hydraulics11
Hydraulics – Determination of overbank roughness
Pre-burn roughness coefficients
- Determined aerial imagery
Post-burn roughness coefficients
- Adjusted from pre-burn roughness values
- Decreased by degree of burn severity
- Burn severity defined locally, by cross-section
Table from the final California Post-Fire Flood Studies report to FEMA, by MAPIX-Mainland
Accelerated Post-Fire Flood Analyses Hydraulics12
Hydraulics – Debris effects
Post-burn channel roughness coefficients
- Pre-burn roughness coefficients increased to account for debris obstructing channel flow
- Amount of increase determined by burn severity
- Defined locally
Table from the final California Post-Fire Flood Studies report to FEMA, by MAPIX-Mainland
Accelerated Post-Fire Flood Analyses Hydraulics13
Hydraulics – Conclusions
Physical changes due to burn analysis
- Partial contribution to decrease in WSELs
- Partial contribution to increase in WSELs
Resulting order of magnitude of WSELs
• Pre-burn 20%-annual-chance event
• Post-burn 20%-annual-chance event
• Pre-burn 100%-annual-chance event
• Post-burn 100%-annual-chance event
Net contribution of post-burn effects to significantly increase WSELs and widen floodplains
Questions?