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Flood After Fire

Burned Areas Have an Increased Risk of Flash Flooding and Debris Flows

 

Recent wildfires will have lasting effects on the landscape, and create a heightened risk of flooding for years to come. Locations that are downhill and downstream from burned areas are highly susceptible to Flash Flooding and Debris Flows, especially in and near steep terrain. In some areas, where the fire burned hot enough or long enough, soils develop a layer that actually repels water, like rain on pavement. Rainfall that would normally be absorbed by the forest canopy and loose tree litter and duff on the ground will instead quickly run off. Because of this, much less rainfall is required to produce a flash flood, and the potential for debris flows increases with the loss of plant material that holds the soil in place.A good rule of thumb is: "If you can look uphill from where you are and see a burnt-out area, you are at risk."

About Debris Flows

What is a debris flow?

Debris flows are fast-moving, deadly landslides. They are powerful mixtures of mud, rocks, boulders, entire trees - and sometimes, homes or vehicles. You'll often hear "debris flows" called "mudslides" or "mudflows". Many people use the terms interchangeably, but to scientists, each is a different kind of landslide and debris flows are the most powerful and dangerous of the three.

What causes a debris flow?

Debris flows occur most commonly during intense rain after wildfires. A debris flow doesn't need a long rain or a saturated slope. It can start on a dry slope after only a few minutes of intense rain.

"Intense” rain means a burst of rain at a fast rate, about half an inch in an hour. With debris flows, the rainfall rate matters more than total rainfall.

Why are debris flows so dangerous?

Debris flows are fast and unpredictable. They can travel faster than you can run - and they can catch up to your car! Also, no one can say precisely where a debris flow will start or where it will go. It may begin in a stream channel, then jump out and spread through a neighborhood. A debris flow may happen where others have occurred, or in a place that has never seen one before.
 

Plan Now to Be Prepared

 

Before the Flood

The best thing you can do is to have a plan to react to flash flooding and debris flows. Remember it will take much less rain than you are used to for flash flooding or debris flows, and they will happen very quickly. Plan your evacuation route, and if you have time, use it. Your best option may be to climb uphill to safety.

  • Monitor the weather, and have multiple ways to receive warnings. NOAA Weather Radio is an excellent option; make sure your radio has the Tone Alert feature to alert you when a warning is issued, and Specific Area Message Encoding (SAME) to filter alerts to just your county, and battery back-up power.
  • Sign up for emergency alerts with your county or local Emergency Management Agency
  • Make an emergency kit, plan evacuation routes, and keep important papers in a safe, waterproof place.
  • Purchase a Flood Insurance Policy - it takes 30 days to go into effect. Find more information on FEMA's FloodSmart.gov web page.
  • Review your current insurance policy and become familiar with what is covered and ensure the limits adequately protect their building and personal belongings.
  • Itemize and take pictures of possessions.
  • More Flood Safety information is available here.

During the Flood

Monitor the weather. During a storm that could cause a landslide, stay alert and awake. Many deaths from landslides occur while people are sleeping. Be ready to move if a warning is issued or if you see flooding or debris flows. Because the time between rainfall and flash flooding will be so short, you may not receive a warning. 

Watch environmental cues. Know where your water comes from. Are there storm clouds upstream of you? It doesn't have to be raining at your location. Watch creeks and streams, and dry washes. Are normally dry areas filling with water? Are water levels rising or fluctuating? Fluctuating water levels can indicate a debris blocking water and then releasing upstream.

Be aware that by the time you are sure a debris flow is coming, that will be too late to get away safely. Never cross a road with water or mud flowing. Never cross a bridge if you see a flow approaching. It can grow faster and larger too quickly for you to escape. If you do get stuck in the path of a landslide move uphill as quickly as possible.

Debris Flow Warning Signs:

  • Listen and watch for rushing water, mud, unusual sounds.
  • Unusual sounds, such as trees cracking or boulders knocking together, might indicate
  • moving debris.
  • A faint rumbling sound that increases in volume is noticeable as the landslide nears.
  • Movement of fences, retaining walls, utility poles, boulders, or trees.

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