why floods follow fires

Why Floods Often Follow Fires

FEMA, USGS, and the National Weather Service (NWS) all have posted stories about floods that often follow fires. But WHY do they?

This NWS graphic tells the story.

About the Danger

NWS cautions that wildfires can leave lasting effects on the landscape and create a heightened risk of flooding for years.

Locations 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 normally absorbed by the forest canopy and vegetative debris 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.”

National Weather Service

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 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.

Relationship to Drought

Droughts often create fuel for fires. Not long ago, SE Texas was in an extreme drought. Moderate rains recently may make it feel as though the drought is over, but in reality, we are still officially in a severe drought.

FEMA warns that it can take up to FIVE years after a fire for the landscape to restore itself enough to reduce flood risk.

Earlier this week, the AP reported a story about three Texans killed in flash flooding that followed a New Mexico wildfire.

Still skeptical? Check out this BBC story about rainfall triggering flood warnings FOUR years after wildfires devastated parts of New South Wales and Victoria in Australia.

All the more reason to be cautious when camping or burning brush.

Posted by Bob Rehak on 10/25/2023

2248 Days since Hurricane Harvey