Wildfire and Debris Flow: A Geologic Hazard

A two-story home damaged by a debris flow. The building is surrounded by displaced boulders and woody debris, and the exterior is caked with mud up to the bottom of the second floor. The caption below provides more information.
Above: A home damaged by the January 9, 2018 debris flow event in Montecito, California. The event was triggered by heavy rain on the Thomas Fire burn area above Montecito. This home is located near Buena Vista Creek. Note the mud line is asymmetric, indicating surge front depths were greater on the left side of the structure than on the right side.
Photo credit: Brian Swanson, CGS.

Scientific Background

Californians who live on or below hillsides—especially in areas impacted by recent wildfires—should be aware that the rainy season increases the possibility of potentially dangerous debris flows, a geologic hazard that is often identified in the news as mudflows or mudslides.

Four geological processes and landforms sensitive to wildfires: A. Rockfall. B. Post-fire debris flow. C. Alluvial fan. D. Floodplain.A debris flow is a fast-moving mass of material—slurries of water, rock, soil, vegetation, and even boulders and trees—that moves rapidly downhill. Debris flows after wildfire are typically more dangerous because they are fast-moving, triggered by short, intense periods of rainfall, and can cause both property damage and loss of life.

The January 2018 Montecito debris flows, in an area of Santa Barbara County impacted by the Thomas fire in December, caused at least 21 fatalities, damaged or destroyed more than 500 structures and seven bridges, and shut down Highway 101 for two weeks. California Geological Survey scientists estimated the Montecito debris flow as having speeds of 10-15 mph, being up to 25-30 feet deep, and capable of carrying boulders as large as a tow truck.

Watershed Emergency Response

Mission: California Watershed Emergency Response Teams (WERTs) help communities prepare after wildfire by rapidly documenting and communicating post-fire risks to life and property posed by debris flow, flood, and rock fall hazards. The WERT response is led by the California Department of Forestry and Fire Protection (CAL FIRE) and co-led by the California Geological Survey (Department of Conservation).

The WERT five-step process to risk reduction: 1. Determine soil burn severity. 2. Identify values at risk. 3. Identify, model, and classify hazards. 4. Develop emergency protective measure(s). 5. Communicate findings.WERT objectives are completed in a rapid step-wise manner to achieve the goal of risk reduction. A fundamental step in the WERT process is the identification and characterization of values-at-risk (VARs). VARs are the values or resources at risk of damage or loss by post-wildfire geologic and/or hydrologic hazards. The WERT process utilizes a qualitative approach to evaluate risk to these values, and relies on a combination of modeling and best professional judgment to guide relative risk determination and the development of emergency protection measures. The final step in risk reduction is to communicate the evaluation findings to local jurisdictions responsible for emergency planning and preparedness.

The decision to conduct a WERT response is made by CAL FIRE in coordination with local and federal agencies, and is also based on:

  • Fire size and intensity, and its location in relation to values-at-risk (VARs).
  • Proximity of intensely burned areas with steep slopes to housing developments.
  • Likelihood of debris flows based on topography, geology, climate, etc. impacting VARs.
  • Proximity of VARs to flood and debris flow prone areas affected by the fire.
  • Presence of transportation networks, water supply systems, campgrounds, etc. at potentially high risk.
  • Fire that includes a significant percentage of state responsibility areas.
Smoke column above the Lake Fire (2020). Photo credit: Austin Dave.
A large smoke column rises thousands of feet above chaparral-covered mountains.

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