Southern California 2009-2010 Rainstorms and Chance of Mudslides in Burn Areas

A fire of that intensity on the rugged foothills above La Canada, La Crescenta, Tujunga and along the Angeles Crest Highway had not burned for decades. Fires and mudslides are a part of expected life in the foothills and while the Station fire burned 160,000 acres in mostly uninhabited areas of the Angeles National Forest, local police and county emergency crews have warned residents about the dangers of mudslides after a rainfall.

The National Weather Service predictions of a 90% chance of showers Tuesday night, October 13 through Wednesday October 14 will have many homeowners waiting to see if any significant mudslides do occur then or in rainfalls to follow.

Mudflow probabilities
In a recent community meeting in the La Crescenta - Tujunga areas that about 300 people attended, the United States Geological Survey released a comprehensive report saying the "tributary basins that drain into Pacoima Canyon, Big Tujunga Canyon, Arroyo Seco, West Fork of the San Gabriel River, and Devils Canyon were identified as having probabilities of debris-flow occurrence greater than 80 percent."

The document is titled "Emergency Assessment of Postfire Debris-Flow Hazards for the 2009 Station Fire, San Gabriel Mountains, Southern California."

Statistical models are given in the 24-page report detailing the likelihood of rainstorms of different strengths:

"We have found that postfire debris flows in southern California can be triggered by short- duration, high-intensity thunderstorms as well as longer duration, lower intensity storms (Cannon and others, 2008). To identify the potential effects of both types of storms, we estimated the probability that a given basin will produce debris flows and a possible debris-flow volume at the basin outlet in response to both a 3-hour-duration thunderstorm and a 12-hour-duration storm."

The burn area from the Station fire is divided into 5 different zones with zone 1 being the area including homes and businesses in the foothill communities.

Online resource
The Los Angeles County Department of Public Works has complete information for residents and offers the chance to subscribe to mudflow alerts including information on properly using sandbags - or, if sand is not available - the use of local soil to fill bags.

While the Station fire was the largest, there is information for Palos Verdes with mudflow charts and the Sloan fire with mudflow charts.

Major mudslides that can harm homes may not occur, but having useful information for precaution can offer a plan of action and lead to peace of mind.