Alluvial Fan Hazard Planning

What Is An Alluvial Fan?

Alluvial fans are fan-shaped deposits of sediment (rocks, wood, gravel, and mud) that form where steep mountain streams empty onto flat valley bottoms.  

What Are Alluvial Fan Hazards?

An alluvial fan is a sign that catastrophic floods, often laden with sediment and debris, have occurred in the area and will occur again. These floods and sediment-laden events can be deadly because they are violent, move rapidly (15-30 feet per second), have unpredictable paths, and happen with little to no warning. Although alluvial fan floods can happen at any time, their potential to cause severe damage increases with heavy rainfall. The most damaging alluvial fan floods happen when heavy rains fall on soil that is already saturated triggering landslides. A landslide may flow directly down the stream channel, or it may form a temporary dam, which can later fail sending a surge of water and sediment onto the valley bottom below.  

Alluvial fan debris flood damage on Canyon Creek, November 1990 (photo Bellingham Herald)

CanyonCr_flood damage resized

Alluvial Fan Hazard Planning and Risk Assessment

Whatcom County Public Works River and Flood Division works with Whatcom County Planning and Development Services and other partners to assess the risk of alluvial fan hazards to life and property in Whatcom County.  Risk assessments consider both the likelihood that a hazard, such as a flood or landslide, will happen and the consequences if it does.  Assessing risk for alluvial fans is challenging because it is difficult to determine the likelihood of an event, how large it will be, and how it will act on the alluvial fan.

Completed Alluvial Fan Risk Assessment Reports 

Note - These reports may take a long time to load due to their file size.

Whatcom County Planning and Development Services has more information on alluvial fan hazard areas and other types of geological hazards here.

Alluvial Fan Maps

There are over 100 alluvial fans in Whatcom County, not all of which are currently mapped.  Maps of identified alluvial fans are available through the Whatcom County Planning and Development Services critical areas page. Even small streams exiting the mountains onto a valley bottom can form an alluvial fan. Over the next few years, Whatcom County will be working with the Washington State Department of Natural Resources (WDNR) to prepare a more comprehensive map of alluvial fans and landslides using recently acquired lidar data. Lidar is a laser-based technology that allows the user to virtually remove surface vegetation to more clearly see and accurately map landforms. View current lidar images available through the WDNR.

Lidar map showing some of the larger alluvial fans in western Whatcom County

WC alluvial fans map resized sm

What Can You Do?

Whatcom County residents can take action to plan and prepare for alluvial fan hazards.

  • Plan - Consider alluvial fan hazards in your decision-making when buying a home or undeveloped property.
  • Prepare - Know what to expect and what to do in the event of an alluvial fan flood.
  • Pay Attention - Heavy rainfall is the most common cause of alluvial fan flood events.
  • Respond - Implement your plan when needed.
  • Recover - Flood insurance is available through FEMA; consider buying it when needed.  Visit our flood insurance page for more information.