Maps pinpoint risk of debris flows during heavy rain

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Should a cloudburst suddenly drop an inch of rain on Sonoma Valley, Kenwood-area residents should be on alert for a potentially damaging mudslide off the slopes of Hood Mountain.

That’s one finding in a series of debris flow risk assessments by the U.S. Geological Service in the wake of the last year’s Northern California wildfires.

Most of Sonoma Valley has a moderate risk of a debris flow as a result of conditions created by the Nuns fire, which burned 56,500 acres in Sonoma and Napa counties a year ago, while the Kenwood area has a high risk.

A debris flow involves a cascade of water mixed with soil, rocks, boulders and woody debris, sometimes with the consistency of wet concrete, pouring off a rain-soaked hillside. Wildfires kill trees and brush, destabilizing slopes and triggering flows that can move with alarming speed.

Hood Mountain is “a hot spot” for potential debris flows, said Jason Kean, a USGS research hydrologist who helped develop maps relating to all of the western states’ major wildfires since 2014.

The devastating Tubbs fire that burned 36,800 acres, meanwhile, left a moderate-risk zone from the Napa-Lake county line to the northern Santa Rosa and Mark West area, while the 36,500-acre Redwood Valley fire left a sizable high-risk zone above the west side of Potter Valley.

The maps are online at https://on.doi.gov/2sR9fYA.

They estimate the likelihood of a debris flow in wildfire burn areas following a storm that drops rain at the rate of an inch an hour for 15 minutes, Kean said.

The USGS is not aware of any debris flows in the wake of last year’s North Bay wildfires, he said.

Southern California’s horrific Montecito mudslide in January, with debris flows up to 15 feet high that killed 21 people in the aftermath of the Thomas fire, is evidence of the potential harm, Kean said.

The National Weather Service, using the USGS maps as a guide, will issue warnings whenever rain makes a debris flow a 50 percent probability in a certain area.

Residents close to a drainage that comes down from a burn area “really need to be on their toes,” Kean said.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com.

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