Post-Fire Debris Flow Facts

First responders and CGS staff at the 2017 debris flow in Montecito

Debris Flows, Mudflows, Mudslides, and Landslides

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.

A debris flow is a fast-moving mass of material -- slurries of water, rock, soil, vegetation, and even boulders and trees – that moves downhill by sliding, flowing and/or falling. 

Debris flows range from a few square yards to hundreds of acres in area, and from a few inches to 50 feet deep. Even smaller ones can be locally dangerous: Imagine trying to walk through a 3-inch deep mass of wet concrete moving at 30 mph.

Conditions that 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
Areas impacted by fires are particularly prone to debris flows. The burning of vegetation and soil on slopes more than doubles the rate that water will run off into watercourses.

Debris flow, mudflow, mudslide: why so many names?

Photo of post-fire debris flow in Montecito
Common sight in Montecito after January 2017 debris flows
They are different kinds of landslides. 

A debris flow is far more powerful and dangerous than a mudslide or mudflow. It can move faster and farther, and it’s strong enough to carry enormous boulders and entire trees, not to mention cars, k-rails, and sandbags. 

Many people use the different names interchangeably.  However, scientists use each name to talk about a particular kind of landslide. In fact, there are several dozen different kinds of landslides. Scientists have all these names because it helps in understanding the process. Each name gives information about how that landslide moves, where and when it occurs, and what kind of material it carries. When scientists hear the name “debris flow”, they know this means a fast-moving, destructive landslide that can kill people.

What is the difference between large, slow-moving landslides and debris flows?

Photo of collapsed homes in Bluebird Canyon during a slow moving landslide
Bluebird Canyon after a slow moving landslide
Large, slow-moving landslides composed of rock and soil can cause extensive property damage but usually do not result in loss of life. 

Generally, the movement of such large slides is slow. It takes weeks or months for precipitation to percolate through the soil and lubricate a landslide to start detectible movement.

Debris flows 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 2017 Montecito debris flows, in an area of Santa Barbara County impacted by the Thomas fire in December, caused at least 21 fatalities, destroyed more than 100 homes, damaged more than 300 other homes, and shut down Highway 101 for nearly 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.

What can residents do?

First and foremost, it is critical that residents heed evacuation warnings from local officials. Debris flows can destroy everything in their path. They 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, and be aware of the following: 

  • Be ready for debris flows for 2-5 years after a wildfire. 
    Don’t worry about every storm, it takes an intense rain (typically about ½ inch per hour – like being in a thunderstorm) on a recently burned slope to trigger a debris flow. 

  • Pay attention to official weather forecasts.
    Just a few minutes of intense rain can start a debris flow. The National Weather Service will issue a flash flood watch or warning for your area when rainfall is anticipated to be intense. 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. 

  • Don’t rely on what you’ve seen in past debris flows.
    Debris flows can hit new areas or return to previous areas; they might be smaller - or larger - the next time. Whatever happened before, the next time could be different.

  • Get out before the storm arrives.
    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.

  • If you must shelter in place, choose your spot in advance and stay alert.
    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.

  • Never underestimate a debris flow.
    Debris flows can start in places they’ve never been before. They can leave stream channels and plow through neighborhoods. When a debris flow is small, people can control it with walls, k-rails, sandbags.  When a debris flow is big enough, nothing can stop it.

  • Expect other flood dangers.
    Storms that can cause debris flows can also cause more common flooding dangers. 

  • 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. 

  • Debris flows can also occur in the absence of fire during heavy winters. 
    Be aware that the soil may be waterlogged and that more rain can trigger debris flows. 
  • Watch for new springs or seeps and excess surface erosion on slopes on and around your property.
    If there are nearby streams, do they appear muddier than normal?

  • 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.     

  • Sign-up for emergency alerts from your local emergency management agency.

More about what we do:

CGS geologist standing amid debris flow damage
Janis Hernandez of CGS stands under a boulder wedged into tree branches about 10 feet high by the Montecito debris flow
The California Geological Survey (CGS) is one of DOC's divisions. 

CGS creates a wide variety of landslide maps for different purposes. We provide geologic analysis, input and advice regarding post-fire debris flows and related hazards when requested by the Governor’s Office of Emergency Services (CalOES) and the California Department of Forestry and Fire Protection (CAL FIRE). 

Over the past 15 years, CGS has been called upon 40 times to provide input on post-fire geologic hazards.
  
Our geologists with expertise in slope stability and landslide hazards typically work hand-in-hand with other technical specialists such as hydrologists and engineers to assess post-fire impacts. We use a number of tools to assess the potential for post-fire debris flows, including geologic, soils, climatologic and topographic information; aerial imagery and satellite data; and hydrologic and debris flow models developed by agencies such as the US 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 to geologic hazards such as debris flow. This information is transferred via CalOES to the appropriate agencies to inform emergency response plans and mitigation measures.    

Special thanks to Dr. Suzanne Perry, Disaster Scientist, U.S. Geological Survey.