As storms move into Colorado Thursday and Friday, there is rising concern for flash floods, particularly at the burn scars from 2020's massive wildfires.
Various flood alerts were in effect across the state Thursday. No alerts have been issued as of Friday morning.
The National Weather Service out of Boulder said Thursday's storms begin to develop around late morning, and by the afternoon and early evening, the storms slowly moved into the urban corridor with multiple rounds of thunderstorms. The storms weakened by late evening — around 10 p.m. — and will continue to do so into early Friday.
Some urban and low-lying areas around the Interstate 25 corridor could see flash floods Thursday and Friday as well.
The threat of flash flooding and debris flows (which are powerful mixtures of mud, rocks, boulders, entire trees in fast-moving landslide) is particularly high around the east slopes of burn scars from 2020’s wildfires, including the East Troublesome Fire, Cameron Peak Fire, Calwood Fire and Williams Fork Fire. A significant flash flood is possible in this area, NWS said, and the threat stays high through Friday before diminishing slightly Saturday and Sunday.
Slow moving storms with heavy rain are expected again today. The highest threat of flooding remains over the new burn areas in Larimer and Boulder counties. Elsewhere, the risk is a bit lower than yesterday, but there's still a threat of heavy rain and minor flooding. #cowx pic.twitter.com/KR6IAHVzPb
— NWS Boulder (@NWSBoulder) July 2, 2021
The highest risk for flooding at the burn scars, like Thursday, is 2 p.m. to 8 p.m. Friday.
Thomas Veblen, a geography professor at the University of Colorado in Boulder, spoke with Denver7 in February about the threat summer storms would bring to the burn scars from 2020's wildfires. He specializes in forest ecology and vegetation dynamics in relation to natural and anthropogenic disturbances.
In short, he explained that the chemicals in the soil at the burn scars are repositioned through the effects of evaporation.
"So, you get a layer, which is a water-repellent layer in the soil," he said. "And because it’s water repellent, it’s kind of like the same process as an avalanche — you’ve got the soil that gets saturated on top of that layer, and then it can just slough off."
To help prevent this, crews will use helicopters to drop mulch on the soil to protect from the intensity of the rainfall, he said.
"The main concern is the loss of a forest cover that would otherwise protect the ground surface from very intensive precipitation during summer thunderstorms," Veblen said in February. "And so yes, that’s probably the primary management concern in all these burned areas next year — next summer — when we have thunderstorms. There’s a high chance that we’re going to have severe erosion."
He said these concerns won't just last through 2021 — flooding at burn scars will remain an immediate concern for five or more years after the fire.
Tracie Harrison with FEMA Region 8 explained these flood risks in burn scars will remain significantly higher until vegetation is restored, and the watershed is stabilized. She was one of three speakers featured in a webinar hosted by FEMA Region 8, Colorado Division of Homeland Security and Emergency Management, and the Colorado Water Conservation Board in April.
"So, normally, the vegetation absorbs rainwater — it reduces runoff. However, the wildfire leaves the ground charred and unable to absorb this water from their fire," she said. "It creates conditions ideal for flash flooding, and these flooding events are generally more severe, as it takes far less rain to produce a flash flood or mudflow on these burn areas."
Large-scale wildfires, especially like the East Troublesome Fire and Cameron Peak Fire, can dramatically alter the landscape and create landscapes that greatly increase the risk of post-fire flooding downstream and downslope of the burn scars, she said. This can happen in places that are not traditionally prone to flooding.
This kind of flooding, which can be spurred by a minor rainstorm, can loosen boulders to roll downhill, tear trees out of the ground and destroy buildings and bridges, Harrison said.
In the same webinar, Doug Mahan with the Colorado Water Conservation Board and the Colorado National Flood Insurance program coordinator said just one inch of water can cause $25,000 of flood damage to a home. He encouraged residents downhill of a burn scar to look into purchasing flood insurance.
Another webinar host, Peter Reinhardt with FEMA Region 8, said a general rule of thumb is if you can look up slope and see a burn area from where you're standing, you're at risk of flooding.
If you encounter a flash flood, the NWS says to follow three simple steps to stay safe:
- Get to higher ground and stay out of low-lying areas that could collect water and debris
- Do not drive or walk into a flooded area. Just 6 inches of water can knock a person off their feet and there may be dangerous debris under the surface of murky water. Twelve inches of water an carry away a small car and between 18 and 24 inches of water can carry most large SUVs, vans and trucks.
- Stay informed with emergency alerts from your county, social media, radio and other outlets.
Watch the Denver7 live weather stream below or click here for live 24/7 updates on current weather conditions around the state.
Thursday warnings (all ended by Thursday evening)
On Thursday, a Flash Flood Watch, which means the conditions are favorable for flooding, extended to the Wyoming border to south of Colorado Springs and included the foothills, Front Range and South Park until midnight due to slow-moving thunderstorms and heavy rain, according to the NWS. A watch was also in place until 6 p.m. for Interstate 70 through Glenwood Canyon for the Grizzly Creek Fire burn scar — an area that has already seen multiple mudslides in the past week. A full closure of the interstate is possible through the weekend.
Several Flash Flood Warning, which means flooding is imminent or already occurring, were also issued:
- Larimer County, including the Cameron Peak Fire burn area
- Idaho Springs, Central City, and Black Hawk
- Northeastern Clear Creek County and south central Gilpin County
- South central Weld County
- Greeley area (3-4 inches fell in one hour Thursday afternoon)
- Huerfano County, including Spring Fire burn scar area (as of 1:34 p.m., rain was falling at a rate exceeding 3 inches per hour)
A Flood Advisory (issued when flooding is not expected to be bad enough to issue a warning) was in effect for Grand County until 2:30 p.m., and heavy precipitation was possible over the northwest portions of the East Troublesome Fire burn scar. Southwestern Weld County was also under a Flood Advisory until 2:45 p.m., and Adams County was under one until 2:30 p.m.