Colorado meteorologist explains why the Poudre River was flooded

Tuesday’s Black Hollow Flood, which spilled into the Cache la Poudre River, destroyed five homes and killed one person while three more were missing, is unlikely to be the last flood to rock the river this summer.

National Weather Service Boulder meteorologist Paul Schlatter examined the floodplain and said what happened from a weather point of view was not an anomaly, but rather typical of the July thunderstorms in the Colorado mountains.

So he said residents and those seeking relaxation in Poudre Canyon should pay attention to the weather forecast this summer and fall. Even more so in the next two weeks because he said the forecast requires a continuous weather pattern with afternoon thunderstorm probabilities with monsoon moisture for northern Colorado.

“It’s not a question of if, but of when,” he said of another flood occurring in the drainage of the Poudre River. “In the conditions like they are up there (in the scorch of Cameron Peak Fire), it doesn’t take ‘a lot of rain to bring things down if it falls in the wrong place.’ ‘

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Schlatter said an inch of rain fell in 30 minutes on Tuesday afternoon from a slow-moving storm that crept over the burn scar in the Crown Point area at 5 mph.

He said that if the rain hadn’t fallen on the burn scar, it would have largely gone unnoticed.

Where it fell made the difference.

Black Hollow Creek drainage and the surrounding area are rated as severe when assessing the severity of the burns that occurred after the Cameron Peak Fire. The rating means that the soil is hydrophobic or repels water.

Schlatter explained that even typical rain like Tuesday’s on hydrophobic soils can lead to major flooding. The reason for this is that the water is not absorbed, so it flows downhill, picking up loose earth and ash as well as boulders and the charred remains of standing and lying trees.

Black Hollow’s drainage is steep and narrow, which, according to Schlatter, acted like a funnel for the collecting debris flow, which gained speed as it tumbled down the drainage and through the small Black Hollow neighborhood before flowing into the Poudre River.

“These debris flows can be 1.5 to 2 meters deep and at this size they have enough debris and power to remove everything, including houses,” said Schlatter. “We’re not sure if a debris dam has formed, but the severity of the burn scar made it so that no debris dam was necessary to generate that much force.

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It is possible that the same scenario could play out in Poudre Canyon as in Glenwood Canyon, where mudslides from the Grizzly Creek Fire have repeatedly closed Interstate 70. The Cameron Peak Fire scar has been under numerous flood warnings and warnings over the past month.

Tuesday’s flood closed Colorado Highway 14 from Tuesday evening until late Wednesday afternoon to allow the Colorado Department of Transportation crews to clear several small mudslides near Black Hollow.

Last week, crews began air mulching in the long-draw area of ​​the burn scar, which is nearly 209,000 acres and is 325 miles in circumference. Efforts by the City of Greeley and its partners will continue over the next several months in hopes of covering 12,000 acres in the Peterson and Barnes Meadow Reservoirs area.

Mulching helps reduce runoff from scorched areas, protect the canyon from flooding and reservoirs, and protect the river from sediment and debris flows.

In addition to the mulching project, the US Forest Service has planned road works along portions of Long Draw Road, which will include cleaning up ditches and culverts and adding roadbeds to the road.

Despite the mulching efforts, road works, and rubble and dangerous tree protection work in Larimer County and others that began this spring, a rain like Tuesday’s could undo much of their efforts.

“The probability is high that there will be another one,” said Schlatter. “It depends on how much rubble it brings with it.”

Flood current: Video shows the peak of the flooding of the Poudre River

Facts about flash floods

• # 1 weather-related killer in the US
• 6 inches of fast flowing flood water can blow your mind.
• 2 feet will levitate your car
• If you encounter a flash flood, look for a hill immediately
• Never try to run, swim, or drive in fast water.
• If you encounter high tide, stop, turn around, and find a safer route

Source: National Weather Service

Reporter Miles Blumhardt is looking for stories that affect your life. Be it news, outdoors, sports – whatever, he wants to report on it. Do you have a story idea? Contact him at [email protected] or on Twitter @MilesBlumhardt. Support his work and that of other Colorado journalists by purchasing a digital subscription today.