The impact of a wildfire is not over once the flames are extinguished. Particularly when a fire burns on steep and variable terrain common in Colorado’s mountains and foothills, the risk of flash floods and debris flows in the surrounding area can be heightened for years.
It’s the case in Larimer and Boulder counties this week, where flash flood warnings were issued for the Alexander Mountain and Stone Canyon burn areas, respectively, on Wednesday. The threat continues with rain in the forecast through the end of the week.
Deadly flash floods hit the community of Glen Haven near the Cameron Peak burn area in 2022, nearly two years after the state’s largest wildfire in history burned more than 200,000 acres.
The phenomenon is caused by changes to the landscape caused by the fire. We enlisted the help of Peter Nelson, an associate professor in the Department of Civil and Environmental Engineering at Colorado State University, to explain those changes.
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Flash floods are part of life for those living near Cameron Peak Fire burn scar
“So when a landscape burns, the response of the landscape to rainfall changes dramatically,” Nelson said. “You can think of the ground as being kind of like a sponge. Under normal, unburned circumstances, the sponge – the ground – is able to soak in a lot of the rain that falls through infiltration. After a fire, the infiltration capacity of the landscape is seriously diminished.”
Organic materials at the surface like grass, brush and leaves are burned in the fire. When burned at high intensity, compounds form in the top layer of soil that cause it to repel water, according to the National Weather Service.
The scientific term for it is “hydrophobicity.” Nelson equated it to water beading up on the soil, similar to water on a duck's back.
When rain comes, it’s the people who live downstream or downhill from the burn areas who need to pay the most attention, Nelson said, as those areas are likely to see flooding caused by burn scar runoff.
What if the landscape is burned twice, by separate fires?
Parts of the northern border of the Alexander Mountain burn area actually overlap with portions of the southern edge of the Cameron Peak burn scar from four years ago.
Denver7’s Jason Gruenauer asked the question: Is it possible to have a sort of doubling of the effect on the landscape?
Nelson said we may learn the answer to that question in the wake of the Alexander Mountain Fire, as such an overlap is rare. He explained, though, that the landscape’s resistance to water depends on the intensity of the flames. In this case, he said a worsening of the flood risk on the twice-burned areas may be unlikely.
“The hotter the burn was – the more severe the fire was – the worse those hydrologic responses tend to be,” he said. “So if the area that burned in Cameron Peak in 2020 had a lower fuel load for this new fire, it may not have burned at quite as high severity this time.”
What can be done to decrease the flood risk?
Nelson is part of a group that has researched the impact of mulch drops after wildfires. Helicopters drop mulch over a burn area in a large-scale treatment that helps to reseed the forest and reduce erosion and debris flows. The practice was used after the 2020 fires.
But does it work? Nelson said his team is still collecting data on the effects of the mulch drops over the Cameron Peak and East Troublesome scars. He spoke about the early returns on the research.
“It depends where it's been placed and at what concentration, what sort of aerial coverage,” he explained. “So far, I think our takeaways from that research are that the sooner you can do it, the more likely it is to be successful. The greater amount of mulch that you can apply to a hill slip of concern, the more likely it is to be effective.”
Mulch drops were not yet an option ahead of rainy days near the Alexander Mountain burn area when we talked to Nelson on Thursday, as the fire wasn’t fully contained. He said, though, it’s likely an option that will be considered during recovery from the fire after full containment.
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