Utah's invisible flood risk

Alluvial Fans

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Alluvial fans lie at the base of canyon mouths. They are formed from precipitation runoff and snowmelt occurring upstream of the fan itself. As streams or runoff emerge into the valley, the direction of flow becomes unpredictable, spreading out and away from the canyon mouth. Any sediment or debris that was carried downstream is deposited as fan material. Structures in the flow path on the fan might be flooded, or struck with debris or mudflow.

Where are Utah's Fans?

The State of Utah’s Flood Hazard Atlas has published the beta version of an interactive alluvial fan hazard map. Areas marked in red show areas of potentially higher risk based on surface investigations indicating active fan development. Click here to go to the interactive map site.

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The video below discusses alluvial fans in Utah and how reduce risk from floods, mudflows, and debris flows.

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Increased Risk After Fire

If a fire burns the slopes of the watershed upstream from a fan, the downstream risk of mud and debris flows increases dramatically.  Burned ground cannot absorb rain and runoff as undisturbed soil and vegetation can. Instead water races downslope collecting soil and debris—even boulders—along the way, forming a debris-filled mixture that is deposited onto the fan. On active fans, the flowpath of the water/sediment/debris is unpredictable. Structures or properties may be affected.  Risk to property is highly elevated until vegetation and soils are restored—a process that may take five years or more

If your community is located below a burn scar less than 5 years old take the following measures to reduce the risk to lives and property. 

  • Contact the State of Utah's Flood After Fire response group. This group has resources to help stabilize soil and reduce runoff, and potential flood risk.
  • Plan and publicize evacuation routes for  populated areas
  • Encourage your residents and businesses to consider flood insurance.  
Alluvial Fans and FEMA Floodplain Maps

Information For

Alluvial Fans and FEMA floodplain maps

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The NFIP identifies alluvial fan hazards on Flood Insurance Rate Maps (FIRMs) as Zone AO and provides information on flood depths and flood velocities. Construction requirements for structures located in Zones AO include they be adequately anchored to prevent flotation, collapse, or lateral movement resulting from flooding and debris, elevation of the lowest floor (including basement) above the highest adjacent grade, use of flood damage-resistant materials below the lowest floor, properly installed flood openings in foundation enclosures, and providing adequate protection from high-velocities or debris loads.


Mitigation Best Practices

For Communities, Counties, and Tribal Lands with Alluvial Fans


For Property Owners on Fans