Wildfire-Associated Debris Flows
   

Washingtonians who live on or below hillsides—especially in areas impacted by recent wildfires—should be aware that the rainy season and summer storms increase the chances of potentially dangerous debris flows. The Washington Geological Survey’s Wildfire-Associated Landslide Emergency Response Team (WALERT) conducts rapid debris flow hazard assessments in areas recently burned by wildfires. Click on the Reports icon below to see the available WALERT assessment reports.


What is a debris flow?
Debris flows after wildfires
Alluvial fans

What you should know and what you can do
What we do
Reports

What is a debris flow

A debris flow is a fast-moving mass of material—a slurry of water, rock, soil, vegetation, and even boulders and trees—that moves downhill by sliding, flowing and (or) falling. People who witness debris flows often compare the sound to a fast-moving freight train.

 

A wildfire-associated debris flow rushes down a hillslope, carrying soil, rocks, and vegetation.

Debris flows range from a few square yards to hundreds of acres in area, and from a few inches to 50 feet thick. Even smaller debris flows can be locally dangerous—imagine trying to walk through a three-inch deep mass of wet concrete moving at 30 mph.

Several conditions contribute to debris flows:

  • Steep slopes
  • Heavy rainfall
  • Wildfire
  • Weak or loose rock and soil
  • Earthquakes
  • Changes in surface or sub-surface runoff patterns
  • Improper construction and grading of roads

What is the difference between a mudslide and a debris flow?

Debris flows are often incorrectly identified as mudflows or mudslides in the news. A debris flow is far more powerful and dangerous than a mudslide or mudflow. It can move faster and farther, and is strong enough to carry enormous boulders and entire trees, not to mention cars, concrete Jersey barriers, and sandbags.

An abandoned truck burned during the 2014 Carlton Complex fire and later buried in debris from flash flooding and debris flows that occurred during the wildfire.

What is the difference between a large, slow-moving landslide and a debris flow?

Large, slow-moving landslides composed of rock and soil can cause extensive property damage but usually do not result in loss of life. Those impacted by a slow-moving landslide may have hours to months of warning. This is because it takes weeks, months, or years of water percolating through soil for there to be detectible landslide movement.

This is opposed to fast-moving landslides, such as debris flows, which are typically triggered by short, intense periods of rainfall. These landslides provide little or no warning and are more dangerous because of their speed. They can cause both property damage and loss of life.

Debris flows after wildfires

Areas impacted by wildfires are particularly prone to debris flows. The burning of vegetation (trees, shrubs, and ground cover) and forest floor duff can produce water-repellant soils, known as hydrophobic soil conditions. The formation of water repellant soils more than doubles the rate that water will flow into watercourses. Burned areas are also more prone to soil erosion due to this loss of the natural cover. The downhill flow of water can erode rills and gullies into a slope, and the lack of vegetation means raindrops can hit the soil directly, increasing their erosive power.

Rills and gullies eroded into a bare hillside by runoff from rainfall.

The increased post-wildfire runoff can cause flash floods, which can turn into debris flows. A flash flood is a rapid increase in flow along a stream channel that may allow the water to overflow channel banks and cause a flood. Typically there is very little time between the storm event upstream and the arrival of the flood downstream, often under an hour. If this flood contains rocks, trees, and other debris, it is termed a debris flow.


Wildfires increase the chances of potentially dangerous debris flows. Note that your area may still be at risk even if the immediate area has not been burned. A burned area far up on a hillside could trigger debris flows that would flow downhill into the valley below.

Alluvial Fans

Alluvial fans are especially likely to be impacted by debris flows because mountain creeks channel runoff. These broad, gently sloping, fan-shaped deposits are made of sediment and debris deposited when a stream emerges from steep mountain slopes onto a wide, flat valley. As the water flows out into the valley, the stream slows down, dropping its load of sediment and debris. Over tens to hundreds of years, this sediment builds up to form an alluvial fan. Debris flows can also add to alluvial fans, depositing large amounts of sediment and debris in a matter of hours.

Alluvial fans are broad, fan-shaped deposits that form where streams flow out onto flat valleys. Photo by Jeff Coe of the U.S. Geological Survey.

Alluvial fans are attractive places to build homes and other structures because they are raised above nearby river floodplains, providing panoramic views. However, alluvial fans are active depositional areas that evolve over time. Though fans may be dry most of the year, they may quickly become flooded during rain storms following a wildfire.

What you should know and what you can do

There are several important things to know about debris flows:

  • Debris flows move fast! If you wait to see if a debris flow is coming your way, it will be too late to leave safely. You cannot outrun a debris flow.
  • Debris flows can start in places they’ve never been before or return to previous areas; they might be smaller or larger the next time.
  • Debris flows can jump stream channels and plow through neighborhoods. When a debris flow is small, people can control it with walls, concrete Jersey barriers, and sandbags. When a debris flow is big enough, nothing can stop it.
  • Debris flows can be triggered by more rain falling on soil that is already waterlogged.
  • Don’t worry about every rain event, it takes an intense rain on a recently burned slope to trigger a debris flow (typically about half an inch per hour—like being in a thunderstorm). That said, just a few minutes of intense rain can start a debris flow. Also—and this is important—it’s the rain in the mountains that will start the debris flow, even if it's not raining, or only sprinkling, where you live.

What you can do

First and foremost, it is critical that residents heed evacuation warnings from local officials. Debris flows can destroy everything in their path. They can destroy homes and kill people. The only way to stay safe is to not be in their path. In the absence of an official notice, residents should pay attention to evolving conditions around their homes, especially if the area has been affected by wildfires. Here are some of the things you can do to be prepared for a debris flow:

  • Be ready for debris flows immediately after a wildfire, especially if you live on an alluvial fan. This danger can persist for up to five years after a wildfire.
  • Expect other flood dangers. When an area is flooded “Turn Around, Don’t Drown!®”. Never drive, walk, or bicycle through a flooded road or path. Even a few inches of water can hide currents that can sweep you away. Also, the water level can rise before you finish crossing. According to FEMA, a foot of water can float vehicles.
  • Pay attention to information provided by your local emergency managers.
  • Pay attention to official weather forecasts.
  • Sign up for flood and emergency alerts. The National Weather Service will issue a flash flood watch or warning for your area when rainfall is anticipated to be intense. For more information, visit www.floodsmart.gov/.
  • If you hear a debris flow coming it may be too late. Watch for emergency alerts and get out before an intense storm arrives.
  • If you must shelter in place, choose your spot in advance and stay alert. Make an emergency plan for you, your family, and your pets, and practice it.
  • Find the highest point nearby (such as a second story room or the roof) and be ready to get there at a moment’s notice. Listen and watch for rushing water, mud, and unusual sounds. Survivors describe sounds of cracking, breaking, roaring, or a freight train in advance of a debris flow.
  • Avoid sleeping in lower-floor bedrooms on the sides of houses that face slopes. Debris flows can bury people sleeping in lower-floor bedrooms adjacent to hazardous slopes.
  • Look for evidence that your home is built on an alluvial fan. This could include dry creek channels, large boulders, and depressions known as swales. A topographic map may also show a distinctive, broad fan shape.
  • Use caution and good judgment. Ultimately, you are responsible for your own safety and well-being.

Debris flows and floods can surge high up onto the sides of houses, potentially burying lower-floor bedrooms.

What we do

The Washington Geological Survey (WGS) Landslide Hazards Program assists communities impacted by wildfires by conducting rapid debris flow hazard assessments in areas recently burned by wildfires. This may include areas downstream that were not impacted by wildfire. Called the Wildfire-Associated Landslide Emergency Response Team (WALERT), our geologists visit areas that have been severely impacted by wildfire, and areas downstream of these areas, where conditions may pose a life/safety hazard.

Geologists assess soil conditions in an area severely burned by the 2017 Jolly Mountain Fire, near Teanaway Community Forest.

Our geologists use a number of tools to assess the potential for debris flows on recently burned areas. We use information about geology, soils, climate, and topography. We also use aerial imagery, satellite data, and hydrologic and debris flow models developed by agencies such as the U.S. Geological Survey. This information is coupled with aerial reconnaissance and on-the-ground observations of site-specific conditions to develop an understanding of vulnerable areas and risks.

Based upon this information, our geologists develop summary reports noting the areas where property and lives may be at risk of geologic hazards such as debris flows. This information is transferred to the appropriate agencies to inform emergency response plans and mitigation measures. These reports can be downloaded on the Reports page after a post-wildfire assessment has been completed.