New data shows Pacific Palisades, Altadena face heightened mudslide risk as storm pounds fire zones
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With the most powerful rainstorm of the season set to soak Southern California on Thursday, the first assessment of mudslide danger since the January firestorms shows that both the Pacific Palisades and the Altadena burn areas face significantly heightened risk of debris flows in the coming days and months.
In the San Gabriel Valley, where the Eaton fire charred a deadly and devastating path — leveling more than 9,400 structures including many homes — a state report noted that there’s a high likelihood that heavy rain could generate large, damaging post-fire flooding and debris flows.
In West Los Angeles, where the Palisades fire began Jan. 7, there’s a greater risk of flooding and rockslides than debris flows. But the reports — issued by the California Department of Forestry and Fire Protection and the California Department of Conservation — say in both areas the fires have left wide swaths of land, including many neighborhoods, at risk for significant additional damage in the next several years.
The reports act as a blueprint for potential damage that could result from not only this week’s wet weather but also future storms.
“Do not risk your life,” Los Angeles County Supervisor Kathryn Barger said during a news conference Wednesday. “The foothill communities will be at increased risk ... so please don’t let your guard down.”
As she spoke, an atmospheric river was moving into the region, lightly showering the Southland. But the heaviest bands of rain are expected on Thursday and bring with them an increased risk of debris flows and flooding, particularly in recent burn areas, according to the National Weather Service.
When hills are healthy, vegetation anchors soil in place. But when that vegetation is burned off, hillsides become vulnerable to erosion, and slopes can come crashing down.
Over the three-day storm, the Palisades scar is expected to receive 3.35 inches of rain; the Eaton scar, 5.08 inches; and the Bridge scar, 5.27 inches. But it’s not only how much rain the region receives that can make things dicey, but also how fast it falls.
Experts say the risk of mud and debris sliding off burned hillsides rises once rain starts falling at a rate of half an inch per hour. Rain rates are expected to peak between half an inch and an inch per hour during the most severe part of the storm anticipated between 2 p.m. and 10 p.m. Thursday, according to the weather service.
Recently burned areas are at risk for landslides in heavy rains, with soil no longer anchored by healthy vegetation. Making matters worse is that the heat from fire makes it harder for soil to absorb water, which can leave hillsides vulnerable to erosion. This can result in slopes crashing down in a torrent of mud, rocks and dead branches, imperiling homes — or people — in their path.
According to data compiled by the U.S. Geological Survey, a rainstorm that delivers a peak 15-minute rainfall intensity of 1.57 inches per hour would have an 80% to 100% chance of generating debris flows across much of the Eaton fire area.
In the Palisades, a rainstorm that unleashes a peak 15-minute rainfall intensity of 0.94 inch per hour would have a 60% to 80% chance of resulting in debris flows along a swath of Tuna Canyon Park, Temescal Canyon and above the Pacific Palisades. Portions of hillsides above Malibu and in the Mandeville Canyon area are at even higher risk, according to USGS data.
“Flooding and debris flows could escalate very fast with very little warning,” said Ryan Kittell, a meteorologist with the National Weather Service in Oxnard.
The areas at highest risk for debris flow and flooding in the Eaton fire zone include Alzada Drive in Altadena, homes located near a channel above the Los Flores debris basin, East Loma Alta Drive in Altadena, homes along the Arcadia foothills, homes in Pasadena Glen that are located near the channel that runs through the community, homes located downstream of the Sierra Madre Dam and properties in the Sierra Madre foothills, according to the Cal Fire report.
In the Palisades fire footprint, Los Flores Canyon, Pena Canyon, Topanga Canyon, above a portion of Palisades Drive, Temescal Canyon and Rustic Canyon are at risk for debris flow hazards, according to the report. For homes situated near water sources flowing down from the burn areas, there’s an added risk of flooding.
Dark Canyon, Carbon Canyon, the southern section of Los Flores Canyon, Piedra Gorda Canyon, Tuna Canyon, the northern portion of Topanga Canyon, the area above Camino de Yataste, Santa Ynez Canyon, Rivas Canyon, Sullivan Canyon and the area above Palisades Drive and The Summit neighborhood are at moderate risk of debris flows, the report states.
The rain will be nothing like the relatively modest storms that have brought largely beneficial rain to Southern California the last two weeks. Forecasters suggest avoiding travel if possible on Thursday.
After blistering criticism that Southern California officials weren’t prepared ahead of the historic firestorms, Los Angeles County authorities say they have worked to reduce the risk of flooding impacts.
Crews have rushed to clear out 154 basins designed to catch mud, rocks and other debris from tumbling into neighborhoods. They have also installed thousands of sandbags, K-rails and compost filter socks — a type of berm — to keep dangerous debris from flowing into storm drains and out into the ocean.
In some areas, officials are planning to use roads to capture sediment, due to the amount of rubble and ash still in the burn scars, said L.A. County Public Works Director Mark Pestrella.
L.A. County officials said they expect that debris flows will hit some homes during the storm. Officials say they have already contacted residents whose homes are next to burned slopes to alert them to the risk. Evacuation orders in some communities, including Altadena and Malibu, could be on the horizon, officials said.
“We do, in this incident, have vulnerable structures,” Pestrella said. “If our forecast comes to fruition … there are specific vulnerable structures that are at high safety risk, meaning that they could see debris run into the back of their homes.”
County officials say the region’s debris basins have enough capacity to handle mud, vegetation and other things that may flow down the hills during the storm. While the debris basins help mitigate debris flow hazards, there’s a history of the structures being overwhelmed in the San Gabriel mountains, according to the state report.
During a storm in 1980 that followed the Pinecrest fire, homes were flooded in Altadena when 11 inches of rain fell over a 24-hour period, overwhelming the flood basin in Rubio Canyon. Capacity of that basin has since been increased.
In 2010, the Mullally Canyon debris basin overtopped during a storm after the Station fire, resulting in 40 homes being damaged.
While last week’s series of storms brought only mild rain to the area, officials said it was enough that the basin at the Eaton wash dam filled with debris. Crews have since removed 150,000 cubic yards of mud, vegetation and rocks from that basin to give it additional capacity for this storm, Pestrella said.
One of the worst debris flows in modern California history occurred when a river of mud and rock flowed through Montecito in January 2018, killing 23 people and destroying at least 130 homes. The area had been burned less than a month earlier in the Thomas fire. That blaze, one of California’s most destructive on record, burned 282,000 acres in Ventura and Santa Barbara counties and destroyed more than 1,000 structures.
Contributing to the severity of the Montecito debris flow was the failure of government officials to build bigger basins that could have made mudslides far less catastrophic, a Times investigation found. The report also found that Santa Barbara County failed to thoroughly empty the existing basins before the disaster, drastically reducing their capacity to trap debris.
“We have the capacity for the size of the storm we expect,” Pestrella said Wednesday, adding that crews have been working “24/7 for the last couple weeks trucking debris out of these areas.”
Times staff writer Grace Toohey contributed to this report.
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