The continuing threat of debris flows: ‘It’s coming down’

LA CRESCENTA — Deep incisions along the hills of Deukmejian Wilderness Park are a telltale sign of more devastating debris flows to come in the near future, a chief scientist for the U.S. Geological Survey said.

The agency is logging more debris flows at lower levels of rain due to the Station fire than any other burn area, said Lucile Jones of the U.S. Geological Survey’s Multi-Hazards Demonstration Project.

The deep rain carvings and loosened dirt higher up in the mountains have become part of a formula for disaster that geologists predicted last year after the Station fire devoured vegetation in the area, officials said.

Debris catch basins have been holding up, but they will likely not be able to withstand the amount of material that would come down after several hours of intense rain, Jones said.

“When you look up in the mountains right now, you could see how a lot of stuff has gotten moving,” Jones said. “You could see these incisions all along the mountainside.”

Material has already started to move, but has yet to make it down and out to basins, she said.

“There is huge amount of material that’s loose up in the mountains, and a sustained heavy rain could bring that all way down,” Jones said. “So we haven’t seen as bad it can be, by a long shot.”

The relationship between fire and debris flows was first defined in the Crescenta Valley after a series of disasters in the 1920s and ’30s, she said.

Fire in the 1920s brought floods the next winter, Jones said.

Another fire on Thanksgiving Day 1933 burned Mount Lukens in Tujunga, bringing with it the threat of floods. That area was pummeled with 12 inches of rain through New Year’s Eve 1934, resulting in massive debris flows with more than 600,000 cubic yards of material bearing down on La Crescenta and Montrose.

As a result of the large mudslides, the catch basins were built, Jones said.

La Crescenta has larger debris basins than La Cañada Flintridge because of the earlier fires and floods, so they’ve been holding up better, she said.

La Cañada will continue to experience more debris flows, as it did a couple of weeks ago on Ocean View Boulevard, she said.

“What’s up there is already loose,” Jones said. “The only way out of this is if we move into a drought and we just don’t get any intense rains for the next several years.”

Only the top few inches of soil in the Deukmejian mountains are getting wet, Jones said. But once the soil fills with water, it flows over the topsoil and starts debris flows. But unlike the 12 inches of rain that was needed to produce the massive mudflows in 1934, far less water will be needed because the Station fire was so intense, she added.

“It’s coming down,” Jones said.

Large amounts of mud and debris have already accumulated in Deukmejian Wilderness Park, said George Chapjian, director of Community Services & Parks Department.

But while the debris basin at Deukmejian has been holding up, some trails in the park have been washed out, Chapjian said.

Ground erosion on the west hills of Deukmejian, which takes in parts of the San Rafael Hills and the Verdugo and San Gabriel mountains, has also been logged, he said.

And a large amount of debris was loosened from the rear of the canyon during the more serious rainstorm on Feb. 6, said Steve Zurn, Glendale Public Works director.

Large boulders and rocks have also come down from the mountains. Crews are trying to keep those boulders contained in the channels and basins, he said.

Still, north Glendale has fared well during the storms due to the large capacity of the area’s debris basins, Zurn said.

“We are preparing for the worst but hoping for the best,” he said. “There is only so much we can do before we get to a point where we could have catastrophic mudflows.”

Jones advised residents who live in the Station fire burn area to follow weather reports and sign up for public safety alerts.