What causes mudslides, and what can be done to lessen the danger?

Mudslides are dangerous torrents that are usually referred to by geologists and first responders as debris flows. They have been particularly lethal in parts of California where landscape, climate, weather and geology can combine to make them more likely.

California has worked to add tools to protect against mudslides, including pinpointing hot spots, creating basins and placing nets to capture falling debris before it hits homes.

Why is California prone to mudslides?

California 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.

Severe drought can be a problem, too. Experts say that when hard rains fall on extremely dry and hard-packed soil, it can rush downhill and pick up energy, carrying soil and debris away.

And wildfires — an increasing problem across the West — can leave hillsides with little to no vegetation to hold the soil in place.

What are the most vulnerable areas?

The areas most at risk are on or near hillsides that have burned in recent years, with little or no vegetation left to hold soil in place.

Burning vegetation and soil on a slope more than doubles the rate of water runoff, according to the California Department of Conservation, and there have been severe events over the years.

On Jan. 9, 2018, intense rain fell on a weeks-old wildfire burn scar in the mountains above Montecito on the Santa Barbara County coast, unleashing huge torrents that tore through the community, killing 23 people and destroying or damaging hundreds of homes.

In 1934 a storm over the Southern California mountains unleashed runoff so intense that 30 people were killed, more than 480 homes were destroyed.

And on Christmas Day in 2003, rain over fire-scarred mountains unleashed a debris flow that killed 16 people who had gathered at a church facility in a canyon.

What can be done to protect communities?

One of the best ways to manage landslides is with debris basins — pits carved out of the landscape to catch material flowing downhill that the U.S. Geological Survey says can reach speeds exceeding 35 mph (56 kph).

Often located at the mouths of canyons, the basins collect debris while allowing water to continue downstream, according to Los Angeles County Public Works. This prevents blockages of the storm drain system but requires the removal of sediment from the basins, something that can take days or months depending on their size.

But basins, which can require a lot of land, can also disrupt the natural ecosystem and lead to beaches needing to be replenished by collecting sediment that flows out of the canyons, according to experts. They’re also costly — Santa Barbara County spent $20 million on a new basin after 2018 — and must be emptied occasionally or they can be overwhelmed by new landslides or mudslides.

After the 2018 mudslides hit Montecito, the Los Angeles Times reported that debris basins above the community were undersized and hadn’t been sufficiently emptied. The community raised millions to address the problem, hiring an engineering company to map the canyons and install debris nets.

The California Department of Conservation runs a geological and landslide mapping team that seeks to identify hot spots, and continually updates its map so local communities can make decisions including potential evacuations.

Experts say agencies use a variety of tools to gauge the likelihood of landslides in a given area, including terrain maps and lidar – pulsed light from lasers to penetrate foliage to see the ground. Then they can watch for early warnings, such as changes over time in photos taken from the air, or from satellites, or in data from GPS monitoring stations, tilt meters and or other on-site instrumentation.

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