Mud slides and debris flows: What are they? 

• Mud Slide: A river of liquid and flowing mud on the surface of normally dry land areas, as when earth is carried by a current of water 
• Debris Flow: rapid movements of rock, earth, and other debris saturated with water, carried by gravity

Rain over burned areas presents a very high risk of mud slides and debris flows. Wildfires destabilize the soil and duff layers, making them available to join rainwater runoff. As water saturates the ground of a burned area, runoff pulls sediment and debris into a liquid state (the slide or flow). Heavy rains over burned areas easily generate flows. High severity burn scars may initiate flows with only small amounts of rain. Flows build velocity as they move downslope, picking up larger boulders, trees and debris. These events are extremely destructive and require planning to maintain firefighter safety.

How can you protect yourself from a mudslide/debris flow? 

Some ways that firefighters can avoid these natural disasters:

• Keep informed on fire weather conditions and forecasts (IAP forecast or Climate Prediction Center Hazard Outlook, link in references)
• Communicate changing conditions up and down your chain of command
• Stay out of creeks, canyons, and chimneys in or below the burned area during precipitation
  • Designate low-lying areas as high-risk and communicate them to personnel
• Identify all drainage crossings and bridges where access/egress may be cut off by flows
• Be alert for signs of a mudslide/debris flow (loud wind-like noises, trees falling down, rolling materials)
• Maintain awareness of locations where ingress and egress may be impacted
• Set trigger points to leave the burned area (rain observed in the burned area, radar indicated storms)
• Identify safety zones and staging areas where resources can safely wait until the risk has passed
• Never attempt to cross a flow or flash flood – “Turn around, don’t drown.”

Discussion Questions:

• What time of year and in what geographic areas can you expect heavy rain over wildfires?
• Have you ever been in a mudslide/near one? Explain what it looks like and sounds like to others in your group.
• Think back to an experience when rain caused or posed a threat of debris flows on a fire.Did you consider the above bullet points? What course of action did you take and what protocol did you follow?How was this information communicated?

References:

• “What are debris flows and why do they occur?”
• FEMA Mudflow Fact Sheet
• Climate Prediction Center, Probabilistic Hazards Outlook