With rain in the forecast, the National Weather Service in Boise is warning of the possibility of flash flooding and mudslides in the 294 square miles burned by the Pioneer Fire.
A low pressure system could bring up to a half inch of rain Thursday to parts of the Boise National Forest that were burned by the Pioneer Fire. While that could slow the still-burning blaze down, it could also bring flash flooding.
Troy Lindquist is a Service Hydrologist with the National Weather Service in Boise. In a video released by the Weather Service, he went to the Pioneer Fire and scraped off the top layer of ash. Below, he found soil that was water repellent, thanks to the heat of the fire. He poured a little water on that spot.
“And you can see that water ran off immediately, it beaded up nicely, ran down the slope quite quickly and even when we move down to a flat area, it’s just sitting there on top of that bare soil,” says Lindquist.
The Weather Service says during a fire, organic material like trees and plants can burn at a high intensity. Water repellent compounds are vaporized and they condense on cooler soil layers below the burning vegetation. That creates a layer of water-repellent soil below the ash layer. Water cannot penetrate the water-repellent layer. The water runs off the soil, like pavement, taking the ash layer, and anything else on top, with it. That’s where mudslides and debris flows come from after a fire.
NWS says flash flooding, debris and mud flow danger is “enhanced” with the coming rain over parts of the Pioneer Fire burn scar.
Watch the water slide down the water-repellent soil in the NWS video below:
— NWS Boise (@NWSBoise) September 20, 2016
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