National Weather Service United States Department of Commerce

Bighorn Fire: Image credit Jeremy Michael

 

Bighorn Fire - Burn Scar Flash Flooding and Debris Flows

--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."

 

Wildfires have lasting effects on the landscape, both in the immediate area and potentially locations several miles away.  Locations downhill and downstream from burned areas are very susceptible to Flash Flooding and Debris Flows, especially near steep terrain. Rainfall that would normally be absorbed will run off extremely quickly after a wildfire, as burned soil can be as water repellent as pavement. As a result much less rainfall is required to produce a flash flood.  

 

What is a Debris Flow?

As water runs downhill through burned areas it can create major erosion and pick up large amounts of ash, sand, silt, rocks and burned vegetation. The force of the rushing water and debris can damage or destroy culverts, bridges, roadways, and buildings even miles away from the burned area.

 

Debris flow 1Debrs flow 2

 

How Can I Be Prepared?

In the event of moderate to heavy rainfall, do not wait for a flash flood warning in order to take steps to protect life and property. Thunderstorms that develop over the burned area may begin to produce flash flooding and debris flows before a warning can be issued. If you are in an area vulnerable to flooding and debris flows, plan in advance and move away from the area. There may be very little time to react once the storms and rain start.

 

How Much Rainfall Can Produce a Flash Flood After a Wildfire?

The time required for a flash flood to begin depends on how severe the fire was, how steep the terrain is, and the rate of precipitation. A general rule of thumb is that half an inch of rainfall in less than an hour is sufficient to cause Flash Flooding in a burn area, but it depends on the factors above. Light precipitation on steep terrain that was severely burned can cause flash flooding within minutes. 

Areas of less severely burned and flatter terrain can absorb more water leading to more time before flooding develops even in heavier precipitation. The important point is that areas burned by wildfire take much less rainfall to cause flash flooding than before a wildfire. In fact, Thunderstorms that develop over burn areas can produce Flash Flooding and Debris Flows nearly as fast as National Weather Service radar can detect the rainfall. If heavy rainfall is observed even for a very short time there is the potential for Flash Flooding and/or Debris Flows.

 

Informational Video (released July 2, 2020)

 

Precipitation Forecast

 

Additional Resources and Information