Accelerated Post-Fire Flood Analyses Hydraulics URS Group Inc ASFPM National Flood Conference May 19, 2010

Accelerated Post-Fire Flood Analyses Hydraulics 2 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 Hydraulics 3 Overview Graphic from FEMA, created by Michael Baker Jr., Inc.

Accelerated Post-Fire Flood Analyses Hydraulics 4 Study Area  Southern California -Orange, San Bernardino, and Ventura Counties Approximately stream miles identified for Emergency or Priority study Graphic from FEMA, created by Michael Baker Jr., Inc.

Accelerated Post-Fire Flood Analyses Hydraulics 5 Study Area  Southern California -Orange County affected FIRM panels Graphic from FEMA, created by Michael Baker Jr., Inc.

Accelerated Post-Fire Flood Analyses Hydraulics 6 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 – Engineering Analyses completed for Emergency & Priority Streams  November 28 – Floodplain boundary shapefiles completed for Emergency & Priority Streams 7 Tues Nov 20 Wed Nov 21 Wed Nov 21 Thurs Nov 22 Fri Nov 23 Fri Nov 23 Sat Nov 24 Sat Nov 24 Sun Nov 25 Mon Nov 26 Tues Nov 27 Tues Nov 27 Wed Nov 28 Wed Nov 28 Thurs Nov 29

Accelerated Post-Fire Flood Analyses Hydraulics 8 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 Hydraulics 9 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 Hydraulics 10 Hydraulics – Determination of overbank roughness  Burn severity -Determined in planning phase -High, medium, low burn -GIS raster (grid) format

Accelerated Post-Fire Flood Analyses Hydraulics 11 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 Hydraulics 12 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 Hydraulics 13 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

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