Risk of mudslides increases around fire zones in LA County; prepare for the worst case scenario, officials warn

With more rain on the way, officials warned Sunday of an increased risk of mudslides in Los Angeles County’s burned areas, with a 10-20% chance of significant flooding and debris flows that could destroy roads and homes in and around wildfire-ravaged areas .

“This is the worst-case scenario to prepare for,” said Kristan Lund, a meteorologist with the National Weather Service in Oxnard.

“We have a growing concern about the burn scars,” Lund said. The charred footprint of the Eaton fire, which blackened more than 14,000 acres, is the most troubling. “These flash floods and these debris flows can happen in and near or under these burn scars.”

A flood watch went into effect at 10.00 Sunday and was to continue until Monday at 4 p.m. for the burned areas of the Eaton fire in the Altadena and Pasadena areas; the Palisades and Franklin fires in the Pacific Palisades and Malibu areas; Hughes Fire around Castaic Lake; and the bridge fire in the San Gabriel Mountains west and southwest of Wrightwood. The flood watch started six hours earlier than originally expected as rain began to fall steadily around Los Angeles.

“The highest risk of debris flows would be after 4 p.m. Sunday,” the weather service said.

By 3 p.m., a handful of locations in western Los Angeles County had received a half-inch of rain, officials said. Downtown Los Angeles had received 0.32 inches of rain. There have also been reports of street flooding in the LA area due to blocked drains.

“We still have some showers coming up from the south,” said Carol Smith, a meteorologist with the National Weather Service. “More is coming.”

Meteorologist Joe Sirard said part of the Pacific Coast Highway was closed due to flooding in Topanga Canyon. A rain gauge recorded 0.74 inches on Topanga Canyon Boulevard at the edge of the fire line, he said.

In Ventura County, the Lake Casitas area had received 1.38 inches of rain Sunday afternoon.

If you’re not sure if you’re close enough to the burn scar, “assume you are,” Lund said. However, a town like Montebello — about 10 miles from the burned area of ​​the Eaton fire — is not considered close.

“If you’re close to the fire, you want to be on the prepared side,” Lund said. Steps people can take include avoiding being in or around the area from Sunday afternoon to Monday afternoon; use of sandbags to protect property; and for people who decide to stay, get supplies in case road access is blocked.

A “landslide” is a catch-all term that can describe any movement of rock, dirt or debris down a slope. A “debris flow” can happen when water flows quickly downhill and, in addition to mud, picks up rocks, branches, and sometimes massive boulders. This is also considered a type of shallow landslide, which can occur with potentially fatal force.

Landslides are a risk after wildfires, because the heat from the fire makes the ground water-repellent. When rainfall intensity is high—falling at more than half an inch per hour—water can begin to flow on the surface downhill instead of percolating underground and can begin to pick up rocks and debris.

“It really has to do with the track of the storm,” Lund said. “They have the greatest potential for significant debris flows. They are the most recent burn scars; they are close to communities or vulnerable infrastructure.”

The orientation of the terrain is also vulnerable in this particular storm. The burn scars are over south-facing hillsides, and moisture from the storm is being drawn in from the south, forecasters say.

Those factors could cause “somewhat heavier rainfall in those areas,” Lund said.

Expected rainfall amounts for the three-day storm continue to tick upwards. Through Monday, Covina could receive 1.32 inches of rain; downtown LA, 1.14 inches; Long Beach, 1.12 inches; Canoga Park, 1.05 inches; Santa Clarita, 1.04 inches; Fillmore, 1.02 inches; Redondo Beach, 0.95 of an inch; and Thousand Oaks, 0.87 inch.

In addition to the risk of debris flows, there is the potential for water surges over the ocean, as well as damaging winds and strong thunderstorms, Lund said.

And heavier rain can still occur even if you don’t see lightning or hear thunder. But if you hear or see a thunderstorm, “you’re probably going to have higher rainfall,” Lund said.

There is a 15% to 25% chance of thunderstorms developing over a swath of Southern California that includes the recently burned areas, meteorologist Smith said. Thunderstorms may bring a chance for rainfall amounts of half an inch to three-quarters of an inch per hour in isolated areas.

Smith said the Palisades fire area could see more than an inch of rain; The Eaton fire area can be up to 2 inches.

Light rain began to fall over the region on Saturday evening.

Read more: Flooding, landslides are risks as fire-ravaged Los Angeles braces for rain

The storm brings the first significant rain of the year. Much of Southern California is in “severe drought” with some areas in the southernmost part of the state in “extreme drought,” according to American Drought Monitor.

Read more: With the possibility of rain this weekend, Bass is working to back up burned areas

Officials advised residents in fire zones using sandbags to direct runoff and protect property, clear drainage paths, follow evacuation orders and stay off roads covered in debris. They also said residents should keep trash cans and vehicles off the street to allow rainwater to move freely and avoid contact with contaminated runoff.

“If officials say to avoid a certain area, please do so,” Smith said.

Sign up for Essential California for news, features and recommendations from the LA Times and beyond delivered to your inbox six days a week.

This story originally appeared in Los Angeles Times.