Study: Expect debris flows

LA CRESCENTA — Hundreds of residents packed the Rosemont Middle School cafeteria Tuesday night to get the latest details on how officials were preparing the barren hillsides for a potentially wet winter, only to hear that destructive debris flows were likely.

The U.S. Geological Survey also released a study Tuesday that determined even normal rainstorms this winter would likely produce large debris flows in the hills and canyons above La Crescenta and La Cañada Flintridge.

High-intensity and short-duration storms could even trigger mudflows, USGS Research Geologist Susan Cannon said.

But in a year when weather forecasters are predicting an El Niño weather pattern, the agency warned in its report Tuesday that larger, prolonged storms could send destructive debris flows down the canyons to Foothill Boulevard.

The message to foothill residents, many of whom fled the large Station fire in August: Prepare for the worst.

“Expect to leave several times this winter, and also next winter,” Cannon said. “Don’t get tired of evacuations.”

The La Crescenta area has nine debris catch basins — two of which are scheduled to be cleared out by Oct. 15, said Christopher Stone, principal engineer of the Los Angeles County Department of Public Works’ water resources division.

The capacities of another two debris basins will be expanded, he said.

Engineers have been visiting homes in the danger zones, advising 80 residences in the Briggs Terrace and Pine Glen Road areas to prepare for storm damage.

“We believe storm season starts Oct. 15,” Stone said, adding that residents should begin preparations now.

County crews were scheduled to start installing large concrete barriers, known as K-rails, today in the Pine Glen Road area to divert potential debris flows to catch basins. Briggs Terrace would also likely see the concrete additions.

“Again, this K-rail, the whole intent of it is to keep the mud in the streets,” Stone said.

The K-rails would be placed in gutters along streets and must stay in place for three to five years because the threat of mudslides will continue, he said.

“We feel you need the protection for the three to five years until the watershed has time to recover, the plants to start to grow, the roots to start holding back the soil, then we could look at the watershed and see when it’s reasonable to start taking these K-rails out,” Stone told the audience.