Rain Hazard Assessment

A map published by the United States Geologic Survey shows potential for debris flow in the canyons that burned in the Woolsey Fire. 

The winter season is off to a wet start this year. Areas of Malibu experienced heavy rain over Thanksgiving, but with no major road closures or mudslides, the weekend brought clear skies once again. 

Shortly after 6 p.m. Tuesday, Dec. 3, the City of Malibu issued an alert warning of a winter storm that could bring thunderstorms to the Malibu coast later in the week. Rain was predicted at up to a quarter-inch of rainfall per hour. 

With rain, mudslides, flooding and the possibility of power outages may be expected. The city is encouraging residents to be cautious and aware for potential rockslides.

“Use lots of caution when driving through the canyon and keep an eye out for rocks that could be in the road,” City Public Safety Manager Susan Dueñas said in a phone interview with The Malibu Times on Tuesday.

Rain Storm

LASD Lieutenant Jennifer Seetoo (left) and Battalion Chief Anthony Williams of LA County Fire Division 7 (right), chat with a colleague while checking out the capacity of a recently-cleared debris basin in Malibu West during a break in the rain on Tuesday morning, Jan. 15. Malibu neighborhoods affected by the Woolsey Fire experienced intense mud and debris flow during last winter’s rainy season.

Dueñas said officials didn’t expect precipitation to be too heavy; however, the city has its regular storm plans in place and is aware of areas that are most vulnerable, such as the canyons.

Dueñas recommended residents to stay away from the beach due to the potential for lightning during thunderstorms.

In a flash flood warning, avoid walking or driving through flood waters and move immediately to higher ground. During power outages, dark intersections should be treated as four-way stops. Residents were also urged to keep vehicles’ gas tanks full in case of evacuation notices during rain events. Free sandbags are available at all Fire Stations in Malibu and Zuma Beach Lifeguard Headquarters for those who need to stormproof their homes. Residents are also encouraged to pack an emergency kit that includes prescription medication, important documents, extra warm clothing, cash and other necessities in case of an evacuation or power outage.

Excessive amounts of rainfall can happen throughout the year—but Malibu is now entering the wet season with more rainfall on the horizon.

Last year, heavy rain fell just weeks after the Woolsey Fire, resulting in excessive flooding in the western areas of Pacific Coast Highway, Malibu West and Malibu Park, among other neighborhoods. One year later, early rain this season has yet to cause similar landslides in the area; however, danger remains. According to NOAA, the National Oceanic and Atmospheric Administration, “Normally, vegetation absorbs rainfall, reducing runoff. However, wildfires leave the ground charred, barren, and unable to absorb water, creating conditions ripe for flash flooding and mudflow. Flood risk remains significantly higher until vegetation is restored—up to 5 years after a wildfire.”

The US Geological Survey recently published a map for the burn area of the Woolsey Fire, indicating potential debris flow areas. The estimates “are based upon a design storm with a peak 15-minute rainfall intensity of 24 millimeters per hour,” and, unsurprisingly, the areas in Malibu with the highest probability of debris flow during a heavy rain event area are located surrounding the canyons that burned in Woolsey: the area around Yerba Buena Road, Mulholland Highway (as it reaches Pacific Coast Highway), Trancas Canyon, Zuma Canyon and the northern sections of Malibu Canyon. 

This week’s wet weather is not predicted to cause mudflow—according to the city’s alert, there was a “slight chance of limited mud and debris flows near recent burn scars due to isolated thunderstorms.” Another storm is forecast Friday night, Dec. 6 into Saturday, Dec. 7.

(0) comments

Welcome to the discussion.

Keep it Clean. Please avoid obscene, vulgar, lewd, racist or sexually-oriented language.
PLEASE TURN OFF YOUR CAPS LOCK.
Don't Threaten. Threats of harming another person will not be tolerated.
Be Truthful. Don't knowingly lie about anyone or anything.
Be Nice. No racism, sexism or any sort of -ism that is degrading to another person.
Be Proactive. Use the 'Report' link on each comment to let us know of abusive posts.
Share with Us. We'd love to hear eyewitness accounts, the history behind an article.