Wildfires can dramatically change the landscape around us, leaving behind burn scars that oftentimes go unnoticed until it’s too late. These burn scars are at high risk of debris flows and flash floods – both dangerous events that can happen with little warning, particularly after intense rainfall. The Idaho Office of Emergency Management (IOEM) encourages everyone to understand these risks is crucial for staying safe if you live, work, or travel near areas impacted by recent fires.

What is a Debris Flow?

According to the National Weather Service (NWS) in Boise, debris flows are a type of fast-moving, deadly landslide. They’re powerful mixtures of mud, rocks, boulders, trees, and even homes or vehicles. They sometimes are referred to as “mudslides” or “mudflows,” but debris flows are technically the most powerful and dangerous of the three. Debris flows are often triggered by intense rain following wildfires, as the rain quickly dislodges loose, unanchored soil and other debris from burned slopes.

Why Debris Flows and Flash Floods Occur in Burned Areas

Wildfires burn away vegetation and cause a chemical reaction in the soil, leading to a hardened, water-resistant layer. Known as “hydrophobic soil,” this layer prevents rain from soaking into the ground, causing it to run off quickly instead. Without vegetation to hold the soil in place, the runoff flows faster and gains power as it heads downhill, increasing the risk of flash flooding and debris flows.

1. Intense Rainfall: Debris flows often occur with intense bursts of rain—just half an inch in an hour can trigger one. These events don’t require prolonged rainfall, just a sudden, heavy downpour.
2. Rapid Erosion: After a fire, loose rocks, soil, and other debris are left without vegetation to stabilize them, which increases erosion and contributes to fast-moving flows.
3. Flash Flood Hazards: In burn areas, flash flooding can happen even in places that haven’t previously experienced flooding. Roads, trails, and streams may flood suddenly, creating hazards for motorists, pedestrians, and structures.

Why Are Debris Flows So Dangerous?

Debris flows and flash floods are fast and unpredictable. They can move faster than a person can run and even catch up to vehicles. They also don’t always follow existing channels; a debris flow might start in a stream and then veer into neighborhoods or roads with no prior history of flooding. It is important to monitor weather and take action at the first sign of potential danger.

Tips for Preparedness

1. Stay Informed and Alert

• Sign up for Alerts: Enroll in local emergency alert systems such as the Idaho State Alert and Warning System receive warnings about severe weather.
• Track the Weather: Use reliable sources, including NOAA Weather Radio, the NWS Boise website, and local sheriff’s offices and news outlets for updates.

Remember, storms can form rapidly, especially in mountainous areas, and intense rain over a burn scar can lead to flash flooding within minutes.

2. Recognize the Warning Signs

• Weather Patterns: Thunderstorms often build over mountains, and dark clouds indicate heavy rainfall, which may cause flooding downstream, even if the rain isn’t falling where you are.
• Changes in Landscape: Cracks in the ground, unusual sounds, or sudden rises in stream water levels can signal that a debris flow is starting.

3. Have a Preparedness Plan

• Know Your Evacuation Routes: Identify safe paths to higher ground, and create multiple evacuation routes in case roads become impassable. Always have a plan for where to go if evacuation is necessary.
• Prepare an Emergency Kit: When disaster strikes, being prepared can truly make all the difference. Having a well-stocked emergency kit can be crucial in how you and your household handle unexpected situations. Be sure to include items like:
  • Water
  • Food
  • Flashlights
  • First Aid Kits
  • Extra Clothes and Shoes
  • Battery Power or Hand Crack Radio
  • Extra Batteries

4. Know What to Do During a Flash Flood Warning

• Don’t Wait: If you receive a flash flood or debris flow warning, take immediate action to evacuate. These events can develop rapidly, so early action is essential.
• Avoid Flooded Areas: Never attempt to drive or walk through floodwaters. Just six inches of water can knock you over, and as little as a foot can carry a car. Always remember, “Turn Around, Don’t Drown.”

Flash flooding and debris flows are serious hazards in areas affected by wildfires. By staying informed, having an evacuation plan, and recognizing warning signs, you can protect yourself and your family from these unpredictable natural events. Stay resilient, stay safe, and remember that early preparation can make all the difference.