California Landslides Defy Easy Solutions

Relentless storms from a series of atmospheric rivers have saturated the steep mountains and bald hillsides scarred from wildfires along much of California's long coastline, causing hundreds of landslides this month. So far, the debris has mostly blocked roads and has not harmed communities, as in 2018 when mudslides roared through Montecito, killing 23 people and wiping out 130 homes. But more rain is in the forecast, increasing the threat. Experts say California learned important lessons from the Montecito slides and has more tools now. The recent storms are putting those efforts to the test as climate change produces more severe weather. Here's a look at what AP reports California is facing.

• Why is California prone to mudslides?: The state has relatively young mountains from a geology standpoint, meaning much of its steep terrain is still in motion and covered in loose rocks and soil that can be sloughed off easily, especially when the ground is wet, according to geologists. Almost all of the state has received rainfall totals 400% to 600% above average since Christmas. Since New Year's Eve, the California Department of Conservation's landslide mapping team has documented more than 300 landslides. The drought has made matters worse. Dan Shugar, associate professor of geoscience at the University of Calgary, said drought can have a counterintuitive effect when combined with so much rain. "You'd think if the ground is dry it should be able to absorb a lot of water, but when ground becomes too dry, the permeability of the ground actually decreases," he said. As water runs off the hardened soil, it can begin carrying soil and debris away, he said. And wildfires have left some hillsides with little to no vegetation to hold the soil in place.
• Which areas are most vulnerable?: Hillsides that have burned in the past two to three years with communities below them are most at risk, said Jeremy Lancaster of the state Department of Conservation. That includes areas that recently burned in Napa, Mariposa, and Monterey counties, he said. In 2018, the mudslides in Montecito occurred about a month after one of the largest fires in California's history tore through the same area, charring 280,000 acres. Lancaster warned that the threat of landslides will linger long after the rains have subsided as the water seeps 50 to 100 feet into the soil, dislodging things. "They can occur weeks later, if not months," he said.
• What can be done to protect communities?: Lancaster said California has dramatically increased its efforts to identify hotspots since the Montecito mudslides. His department continually updates its map so communities can make decisions, including whether to evacuate. The state is working on a system to better pinpoint how much rain might trigger a slide.
• What is the most effective defense?: One of the best ways to manage landslides is with debris basins—pits carved out of the landscape to catch material flowing downhill. But basins, which can require a lot of land, can disrupt the ecosystem and lead to beaches needing to be replenished by collecting sediment that flows out of the canyons, experts say. And they are costly. In addition, if old debris isn't removed, they can be overwhelmed by new mudslides. After the 2018 slides, the Los Angeles Times reported that debris basins above Montecito were undersized and hadn't been sufficiently emptied. The community raised millions to address the problem, said Patrick McElroy, a retired Santa Barbara fire chief who founded the nonprofit Project for Resilient Communities. The organization hired an engineering company to map the canyons and installed debris nets. So far, the nets are working. The best solution for the Montecito and Santa Barbara area is to have both nets and debris basins, said Larry Gurrola, the engineering geologist hired by the organization. "These things protect the community and save lives," he said.

(More California storms stories.)