Using an Integrated Approach to Characterize Alluvial Fan Flood Hazards – a Case Study on a Highly Managed and Developed Alluvial Fan Christy Leonard U.S

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Using an integrated approach to characterize alluvial fan flood hazards – A case study on a highly managed and developed alluvial fan Christy Leonard U.S. Army Corps of Engineers, Sacramento District, Geotechnical Branch, Geology Section [email protected]

BUILDING STRONG® FEMA’s 3-Stage Process

. Objective: 1) Define the area where alluvial fan flooding could occur.

BUILDING STRONG® FEMA’s 3-Stage Process

. Objective: 1) Define the area where alluvial fan flooding could occur. 2) Determine the type of alluvial fan flooding in terms of sediment transport processes.

BUILDING STRONG® FEMA’s 3-Stage Process

. Objective: 1) Define the area where alluvial fan flooding could occur. 2) Determine the type of alluvial fan flooding in terms of sediment transport processes. 3) Characterize the 1-percent-annual- exceedance flood in areas subject to alluvial fan flooding.

BUILDING STRONG® FEMA’s 3-Stage Process

Stage 1. Recognize and characterize alluvial fan landform.

Stage 2. Define areas of active erosion and deposition.

Stage 3. Define and characterize the 100- year flood.

BUILDING STRONG® FEMA’s 3-Stage Process

Stage 1. Recognize and characterize alluvial fan landform.

Stage 2. Define areas of active erosion and deposition.

Stage 3. Define and characterize the 100- year flood.

BUILDING STRONG® FEMA’s 3-Stage Process

Stage 1. Recognize and characterize alluvial fan landform.

Stage 2. Define areas of active erosion and deposition.

Stage 3. Define and characterize the 100- year flood.

BUILDING STRONG® Integrated Approach Fan Delineation

Topographic Geomorphic Data

Flow Lines NRCS Soil Map

Contour Lines

Slope

BUILDING STRONG® Integrated Approach Define Active/Inactive Areas

Topographic Geomorphic Data

Drainage Network Patterns Soil Development

Topographic Roughness Morphometric Variables

BUILDING STRONG® Flow Lines

Schwanghart, W., Kuhn, N. J. (2010)

BUILDING STRONG® Study Area . Located in Central Utah (~5,500 ft. elevation) . Semi-arid climate ► Majority of precipitation falls as snow in the Wasatch Range. ► Summer cloud burst storms. . Westward facing canyon feeds the alluvial fan. ► Maximum elevation of contributing watershed: 10,800 ft.

BUILDING STRONG® FEMA’s 3-Stage Process

Stage 1. Recognize and characterize alluvial fan landform.

Stage 2. Define areas of active erosion and deposition.

Stage 3. Define and characterize the 100- year flood.

BUILDING STRONG® Stage 1: Fan Delineation (Toe)

BUILDING STRONG® Stage 1: Fan Delineation (Lateral Bounds)

BUILDING STRONG® Stage 1: Fan Delineation

BUILDING STRONG® Stage 2: Define Active and Inactive Areas Stage 1. Recognize and characterize alluvial fan landform.

Stage 2. Define areas of active erosion and deposition.

Stage 3. Define and characterize the 100- year flood.

BUILDING STRONG® Stage 2: Define Recent Time

Geomorphic Timescales (1000s of years)?

Engineering Timescales (100s of years)?

BUILDING STRONG® Stage 2: Define Recent Time

BUILDING STRONG® Stage 2: Geomorphic Map Relative Age

Soil Development Drainage Network Topographic Patterns Roughness

BUILDING STRONG® Stage 2: Geomorphic Map - Soil

BUILDING STRONG® Stage 2: Geomorphic Map Drainage Network

Younger Older

BUILDING STRONG® Stage 2: Geomorphic Map Drainage Network

BUILDING STRONG® Stage 2: Geomorphic Map Topographic Roughness

BUILDING STRONG® Stage 2: Geomorphic Map