New storm moves through L.A. region, bringing flood, mudslide risks

Rain lashes a palm tree on Monday. ((Dania Maxwell / Los Angeles Times))

Officials warned of potential mudslides, debris flows and flooding this week as yet another storm hit a region already waterlogged by weeks of heavy rain.

A large swath of the Westside, South Bay, central Los Angeles and parts of southeast L.A. County were under flooding warning through at least 9 a.m. before the latest storm moves through.

As of Tuesday afternoon, downtown Los Angeles had received about 17.5 inches of rain since the water year began on Oct. 1 and around 12 inches just during February. That’s almost 3 inches more than the area’s average for a full year, and 8 inches more than it typically gets by this point in the water year.

“This is one of the wetter Februarys on record,” said Ryan Kittell, a meteorologist with the National Weather Service in Oxnard. “It’s been an incredibly wet month.”

Despite the record rainfall, this latest bout of rain hasn’t yet spurred any major mudslides in Southern California.

“In normal winters where we haven’t had as much rain, this type of rainfall doesn’t cause that much [of an] issue because the ground can still absorb water,” Kittell said. But once the ground becomes completely saturated, he said, flooding can occur “really easily” and the earth can start to move, either with mud or rockslides.

Forecasts continue to show more precipitation through Wednesday, with some of the heaviest rainfall, including a threat of thunderstorms, expected early Wednesday for Los Angeles County.

Throughout the day Tuesday, the system moved through Northern and Central California, causing flooding and bringing dangerous winds across the Bay Area.

With damage expected throughout much of the state, the governor’s Office of Emergency Services earlier this week activated its state operations center to better coordinate the storm response, positioning additional rescue personnel across 17 counties.

The storm is forecast to taper off Wednesday night. It’s expected to be dry at least until the weekend, when there’s a chance for another storm Saturday night, becoming more likely Sunday and Monday.

Next week’s storm could bring an additional half an inch of rain to the area.

The weather service has received reports of mud and debris flows and flooding in the Santa Monica Mountains and the Pacific Palisades area. Most of the reports have been in Santa Barbara County, where the rainfall totals have been higher.

“Usually when we start to get rainfall totals for the season above 10 inches, the areas that are really vulnerable to mudslides and landslides become especially vulnerable to those type of issues,” Kittell said.

Residents of Rancho Palos Verdes, where a slow-moving landslide complex has been accelerating due to continued heavy rainfall, are bracing for the aftermath of this week’s storm. Although the weather service reported that only about 2 inches of rain has fallen in the area during this storm, record-setting rainfall over the past several months has saturated the ground, causing the landslide area to shift more rapidly, according to City Manager Ara Mihranian.

“In some areas, [the land] is moving up to 10 feet a year,” he said. “That’s significant movement, and we’re seeing the damage that’s being sustained throughout the community. We have approximately 400 homes that are threatened by this landslide.”

Two homes were red-tagged this fall, and other residents have reported sinkholes, cracks in their walls and doorways that have split. The pavement on Palos Verdes Drive South, a major roadway through the community, is buckling, and the city’s iconic Wayfarers Chapel, just off that road, has closed due to increased land movement.

The Rancho Palos Verdes City Council on Tuesday voted to request that Gov. Newsom declare a state of emergency in the city, an effort that could help them fast-track a $33-million landslide remediation project to stabilize the area.

Most populated areas of L.A. County have received 1 to 3 inches of rain since Monday morning, while foothill and mountain regions have gotten between 3 and 6 inches, according to the weather service. As of 2 p.m. Tuesday, 1.67 inches had fallen in downtown Los Angeles, 4.22 inches in Bel-Air and 6.3 inches in Topanga Canyon.

A flood watch is in effect for all of L.A. County except the Antelope Valley through 10 a.m. Wednesday, according to the weather service. Rivers, streams and low-lying areas could see flooding. Freeways and areas with poor drainage could also become inundated with water. There’s an increased risk for mudslides in vulnerable areas.

There was a brief lull in the rain Tuesday morning, but the showers were expected to pick back up and then intensify Tuesday night through Wednesday morning. Some areas of L.A. County could see thunderstorms overnight, which may cause periods of heavy downpours, Kittell said.

On top of the current totals, the L.A. area could get an additional quarter of an inch to an inch of rain, with up to 2 inches in the foothills and mountains. There’s also a “very small but nonzero chance” of severe weather, including strong local winds, small accumulation of hail or even a brief tornado, Kittell said.

Storms in early February drenched much of the state in historic rainfall, knocking out power for hundreds of thousands of people, flooding neighborhoods and triggering mudslides. Several people in Northern California were killed in the onslaught.

Los Angeles County authorities issued an evacuation warning Monday for Santa Maria Road north of Topanga Canyon Boulevard through 9 a.m. Wednesday because of possible mud and debris flows.

Flooding also snarled traffic across the region. A large sinkhole closed down the Skirball-Mulholland northbound 405 Freeway offramp indefinitely while crews make repairs, according to the California Department of Transportation.

Portions of the 101 Freeway from Seaward Avenue to California Street in Ventura are closed on account of flooding and Highway 1 was closed from 6 p.m. to 7 a.m. in both directions from Sycamore Canyon Road to Las Posas Road in Ventura County because of erosion on the southbound shoulder. Highway 150 is also closed in both directions between Stonegate Road and Topa Lane because of slides and storm-related damage.

As of Tuesday afternoon, city officials had responded to mounting reports of damage from the storms: 126 reports of fallen trees and branches, 97 flooding incidents including blockages of storm drains and basins, 47 reports of debris and mudflows, and 520 power outages affecting Los Angeles Department of Water and Power customers.

In Orange County, rains caused further movement to the rocky slope that sits above the coastal railway line in San Clemente, prompting officials to halt overnight freight rail traffic, according to an update late Tuesday from Metrolink and the Orange County Transportation Authority. The escalating damage has closed that railway section for passenger trains since late January.

“Movement of the slope accelerated from about 1 to 2 inches before the rain to about 10 to 12 inches,” officials said Tuesday. Officials planned to reevaluate the situation Wednesday.

Amid this season of above-average rainfall, officials are concerned about the potential in the coming months for deep-seated landslides, which the Federal Emergency Management Agency describes as rooted in bedrock and capable of devastating infrastructure and homes across a large area. During repeated heavy storms, water can accumulate and eventually destabilize land, causing it to collapse.

Such landslides can be particularly destructive and can happen even on a dry day, long after the rain has ended, according to Matt Thomas, a research hydrologist with the U.S. Geological Survey’s landslide hazards program.

“It’s not uncommon in places like Southern California at the tail end of a rainy season when we get into late February, March, April that as the rains keep coming the deep-seated slides begin to appear or become more of a concern,” Thomas said.

This story originally appeared in Los Angeles Times.