Accelerated Post-Fire Flood Analyses Hydraulics

URS Group Inc.2010 ASFPM National Flood Conference

May 19, 2010

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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

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Overview

Graphic from FEMA, created by Michael Baker Jr., Inc.

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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.

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Study Area

Southern California

- Orange County affected FIRM panels

Graphic from FEMA, created by Michael Baker Jr., Inc.

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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

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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

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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

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Hydraulics – Determination of overbank roughness

Burn severity

- Determined in planning phase

- High, medium, low burn

- GIS raster (grid) format

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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

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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

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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?