Flash flood warning issued for park fire scar

The National Weather Service issued a flash flood warning for the Park Fire burn scar Monday morning as a storm dumped heavy rains across Northern California.

“Heavy rainfall will result in widespread and severe flash flooding of creeks, creeks and ditches in the Tehama County area of ​​the Park Fire burn scar,” the weather service warned. “Severe debris flows are also to be expected on roads. Roads and driveways can be washed out in places.”

The warning is in effect until 1:15 p.m. Monday for Mineral, Paynes Creek, Manton, the Highway 36 corridor, Paynes Creek, Mill Creek, Deer Creek and Pine Creek.

Landslides, debris flows and rockfall-prone regions have been identified in the burn scar of the 429,000-acre park fire, according to a U.S. Forest Service emergency report for burned areas earlier this fall.

The weather service warned of heavy rains on burned areas that could trigger debris flows, a sludge mix of mud, rocks and vegetation. During a fire, vegetation and roots burn and water-repellent compounds in the soil evaporate, endangering the stability of the soil. After a fire, downpours can quickly trigger debris flows as water stirs up ash and floods downstream areas. The speed and force of debris flows make them dangerous and sometimes deadly.

The Park Fire ripped through steep ravines in the foothills of Butte and Tehama counties. Dozens of streams flow through steep canyon systems, which could put Chico and other downstream areas at risk of debris flows later this winter.

Less than three-tenths of an inch of rain could cause a debris flow in the park fire’s burn scar in 15 minutes, according to Victoria Stempniewicz, a geologist on the Forest Service report.

“Debris flows are typically not necessarily activated by long periods of rain, but sometimes just by short bursts of intense rain,” Stempniewicz said.

This 15-minute rainfall intensity of 0.31 inches occurs on average once per year in the Upper Mill Creek area on the northeast side of the fire, according to the National Oceanic and Atmospheric Administration.

Mill Creek flows through the community of Mineral, home to the Lassen Volcano National Park headquarters and many popular hiking trails. There are approximately 99 miles of trails within the Park Fire burn path, 33 miles of which are in a region of high ground burning severity associated with an increased risk of debris flows.

Stempniewicz said computer models do not project impacts from debris flows outside the burn scar rim, but said numerous other trails outside the scar near Mineral may be at risk from debris flows.

“We know there is a greater chance that Mill Creek (debris) will be transported further downstream because Mill Creek is very confined and burns hot,” Stempniewicz said.

Big Chico Creek, which flows through the city of Chico, is also downstream of a debris flow-prone area of ​​the Park Fire burn scar.

Many regions of low and moderate ground burning intensity were identified in the western area of ​​the fire, including near the ignition point in Upper Bidwell Park, as the fire burned rapidly through the grass. In areas with low intensity of soil burning, roots remain intact, allowing water to penetrate vegetation. The risk of flooding and debris flows increases significantly with medium and high severity of soil burning.

“It wasn’t until the heavy timber arrived in the highlands that it started to look really ugly,” said Eric Nicita, a soil scientist in the Forest Service report. “Where it burned hot, it was not in a very favorable location, on the slopes of Chico and Mill (creeks). That’s what really brings this into the public eye.”

Local thunderstorms can be enough to trigger debris flows. It may be difficult or even impossible to predict these isolated downpours more than a few minutes or hours in advance. The weather service previously issued a flash flood warning for the Park Fire burn scar in late August and mid-September. Additional flash flood watches and warnings are expected throughout the rainy season.

“It’s important to listen to weather service warnings,” Nicita said. “Even if there are two or three warnings and nothing happens, it is the warning you ignored that puts your safety at risk.”

Previous research found that erosion following wildfires increased exponentially across Northern California between 1984 and 2021.

This trend hasn’t been seen in Southern California, but debris flows can still be dangerous, if not deadly, in other parts of the state. In 2018, a debris flow in Montecino killed nearly two dozen people when more than half an inch of rain fell in five minutes.

Reach Anthony Edwards: [email protected], Reach Harsha Devulapalli: [email protected]