Areas scorched during last year’s historic wildfire season could pose flash flooding risks through the summer as rain picks up speed along steep terrain in the burn scars, sweeping debris onto major roads.
While heavy rain is typical of the summer monsoon season, Colorado is seeing “an abnormal year of flooding events,” partly because of last year’s historic wildfire season and the increased area of scorched land, Assistant State Climatologist Becky Bolinger said.
So far this year, mudslides along burn scars have caused dozens of road shutdowns, including along Interstate 70 in Glenwood Canyon, resulting in severe delays in traffic or significant detours. And that’s likely to continue through the end of monsoon season due to the altered composition of the soil along burn scars, Bolinger said.
Instead of the rain soaking into the soil, Bolinger compared the rainfall along burn scars to rain hitting a car and immediately running off.
“Particularly in areas that have suffered from wildfires, specifically last year, what happens to the soils is that they almost repel that water,” she said. “Basically, the fire changes the composition of the soil so that water cannot get into the soil as it would in a normal situation.”
The flooding becomes more dangerous along steep terrain as rain slides runs the slope and picks up speed as it goes, Bolinger said. If the slope is next to a road, there’s exponentially more danger.
“With that slope, that is what gets your flash flooding as opposed to a regular flooding event,” she said.
Rain can erode steep terrain between 24 to 40 tons per acre each year during the first few years after a wildfire, according to the U.S. Forest Service.
In the past month, vehicle and recreational traffic through Glenwood Canyon has been shut down several times after mud washed from the Grizzly Creek burn scar onto roads and when the risk of flash flooding was high. Lanes in both directions closed again Thursday night, with no estimated time of reopening, after more rain swept massive debris piles debris along the road, the Colorado Department of Transportation said in a tweet.
“Part of it is is just luck — or bad luck — of where these fires happened, particularly the Grizzly Creek fire, which wasn’t a really huge fire, but its placement next to I-70 and in that steep terrain has really led to that one being one of the highest impact areas this summer for flooding,” Bolinger said.
The Grizzly Creek fire burned 32,000 acres in and above Glenwood Canyon last year, at the same time as East Troublesome and Cameron Peak fires became the state’s two largest blazes in the state’s history.
Mudslides were triggered along the Cameron Peak burn scar earlier this week after more than an inch of rain fell near Crown Point and washed debris into the Poudre River near Black Hollow Road, upstream from the village of Rustic.
One person died and several people were reported missing as of Friday after flash floods sent debris flowing into Poudre Canyon northwest of Fort Collins. Several homes were destroyed and buildings were damaged as trees, mud, rocks and structures washed into the river, causing debris to pile up six feet high in some places.
Parts of Colorado 125 and U.S. 40 in Grand County were closed for several hours after a mudslide fell along the East Troublesome burn scar Thursday.
But sometimes, there doesn’t need to be much rain for flooding to become a disaster.
“There’s not a perfect relationship between how much rain you get and the level of flooding. Even if we are not expecting a ton of rain, it could just be enough,” Bolinger said. “You want to make sure what you’re going into because even a little bit of rain can quickly change the situation where you are.”
Rain-flushed mudslides are not just in burn zones either. Heavy rains in Telluride and Avon this week buried roads and pedestrian trails in debris.
How long should we anticipate the risk of flash flooding?
Flash flooding, especially along burn scars, could continue through monsoon season, which typically runs through September, Bolinger said.
“When we’re watching the forecast, if you see that forecast thunderstorm activity over any of these burn scars, immediately that’s going to be a risk that there’s going to be another flooding event,” she said.
What areas face the largest risk of flash flooding this weekend?
Widespread thunderstorms are forecast across the high country Friday afternoon through the evening, according to the National Weather Service’s Boulder office.
A flash flood watch is in effect through 9 p.m. for several areas, including the burn scars from the East Troublesome, Cameron Peak, Grizzly Peak and Calwood fires, the agency said.
Between 1 to 1½ inches of rain is expected to fall per hour in some areas, according to the weather service.
Water levels and the speed that runoff is flowing can quickly change during a flood, the weather service warned. If caught in a flash flood, you should seek higher ground and monitor weather conditions through the local forecast on the radio or the news, the weather service said. People should not walk or drive into high water.
“It is good for people to know that if you are going to be driving into the mountains, you really need to know what are your natural hazard risks that you could be faced with,” Bolinger said. “Flash flooding is probably something you should be aware of every summer, but especially if you’ll be driving near a burn scar, it becomes an even bigger issue.”