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Burn Zone Flood Potential Varies By Storm

Grace Hood

It may not seem like it, but flash flooding and wildfires go hand in hand. The burned areas that wildfires leave behind are devoid of natural ground cover that keeps the soil from eroding too quickly.

But not all areas that are burned are at high risk of flooding or landslides. Dr. John Pitlick is a hydrology professor at the University of Colorado, Boulder. He says multiple factors, including the rainfall intensity and slope of the terrain contributes to the possibility for flooding in any given area – meaning that each storm is unique.

"In many burned areas, especially severely burned areas, we get the formation of water repellent soil areas. We call them hydrophobic soil; they reduce the permeability of the soil."

“Part of it does depend on the ground cover, actually just a layer of needles and forest litter will do a lot to prevent erosion. But also the other thing that is key is the gradient, the steepness of the hill slope,” says Pitlick. “So it really depends on kind of a combination of rainfall intensity and native permeability of the soil. That can be altered by water repellent layers that can form after fires.”

Pitlick says it’s very difficult to computer model how a burned area will react to lots of rainfall and if there will be landslides or flash floods because the water repellant soil produced by wildfires are not uniform. 

“In many burned areas, especially severely burned areas, we get the formation of water repellent soil areas we call them hydrophobic soil, they reduce the permeability of the soil. The problem is they’re pretty discontinuous, they don’t occur everywhere around the landscape. So understanding how an area will respond to rainfall based on that is pretty difficult.”

Some mitigation strategies can be used, such as mulching with hay until the natural ground cover regrows. Pitlick says the potential for landslides or flooding significantly reduces after the first year. 

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