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Burn scars from wildfires increase the risk for flash flooding; here's why

Flash flooding is almost a daily concern during the monsoon season, which can last into September, especially in southern Colorado where the Spring Creek burn area is located.

KUSA — Burn scar areas are extremely dangerous for several years after a wildfire, with the greatest risk tied to flash flooding.

Burn Area Emergency Response teams are brought in after the fire to assess that danger and determine how bad the fire burned.

“We’re focusing on the effects below the surface of the soil,” said Eric Nicita, a soil scientist on the team assigned to the Spring Creek burn area.

The Spring Creek Fire, the state's third largest on record, started June 27 near Garland and burned more than 108,000 acres.

The root of the problem with burn scars is what's beneath about 2 inches of the burned, dusty soil.

Nicita showed a 9NEWS crew a water repellency test. Drops of water beading on the surface of the soil would not soak into the ground. It is a condition Nicita called hydrophobicity.

“Organic material burns up and leaves a waxy residue behind,” Nacitia said. That coating on the soil is the biggest concern moving forward.

“So when that thunderstorm comes, it’s not getting sucked up by the soil, it’s running down the hill slope,” Nicita said.

This opens the door to mudslides, debris flows and flash flooding.

“So, for the next 5 years, this community and others will have to be dealing with this issue over and over again,” said Brad Rust, the team leader for this Spring Creek BAER team based out of the El Dorado National Forest, near Sacramento, California.

These BAER teams are brought in after the fire is out, but many other dangers remain on the burn scar areas.

Flash flooding is almost a daily concern during the monsoon season, which can last into September, especially in southern Colorado where the Spring Creek burn area is located.

The burned trees, most of which are burned all the way down to their roots -- leaving just the soil to hold them up -- present a danger for several years. There are also many trees that are partially fallen and leaning on other trees. A strong breeze may bring them the rest of the way down.

Pits (like sand traps), where roots systems are completely incinerated, also increase the flash flooding risk.

It is an overall slow recovery for the soils in burn areas, even though some grasses are already starting to grow back in some burned areas of the Spring Creek burn scar.

The BAER team told 9NEWS that it usually takes about five years for soil to reach the point where it starts to absorb stormwater. Even then, flash flooding can be a concern for another five years after that.

They said it takes 15 to 30 years for the soil to completely recover to pre-fire conditions.

The team concluded that an unusually high amount of the Spring Creek Fire -- 24 percent of the 108,045 acres that burned -- fit into the high-burn severity category.

“This is one of the hotter fires that I have seen in terms of percentage of high-burn severity,” Nicita said.

Click/tap here to read the complete BAER team assessment of the Spring Creek fire.