Wildfires have increased the threat of mudslides in California

The risk of mudslides has significantly increased in California regions affected by wildfires.

Mudslides are common in steep, mountainous areas located in California and are usually triggered by rainfalls. They may cause severe infrastructure damage and result in casualties. 

In addition, wildfires worsen the situation. During a storm, vegetation can act as a protective system for the ground, holding it in place. Therefore, slopes with no vegetation become more vulnerable to sliding. Moreover, the fire products make the top layer of the ground less permeable reducing its ability to absorb rainwater. Consequently, the water flows on the surface and entrains debris and rock fragments downhill. “When the rainfall hits it, it just runs right off. The rapid runoff from these bare hill slopes can quickly pick up sediment, and that can transform into a really nasty debris flow,” Jason Kean, a hydrologist from the U.S. Geological Survey (USGS), stated.

The regions affected by fires are more prone to landsliding for about 3-5 years which corresponds to the amount of time needed for the vegetation to be restored. 

On December 3, 2019, a warning about potential floods and mudslides was issued by the National Weather Service for regions in South California including those affected by the Palisades fire in Pacific Palisades and the Getty Fire area in Brentwood. The most common measures taken to prevent significant damage and protect residents include evacuation plans and installation of protective measures (mostly steel cable nets) in the base of slopes to stop large rock boulders.

Regarding the most hazardous type of landslides, Mr. Kean stated that there are slides that may occur suddenly on a shiny day. Such a failure occurs when rainwater accumulates into the ground where the bedrock lies deep beneath the surface building high pore-water pressures. The collapse may occur without warning while the slide may be more than 3 meters deep.

Mudslides have proven to be deadly in several cases in California. Back in 1934, La Crescenta Valley was struck by a slide and a flood after a fire hit a nearby forest. 400 buildings were destroyed and 45 people were reported dead. Recently, in 2018, 130 houses suffered critical damage and 23 people died after another mudslide hit Montecito, an area also previously affected by Thomas fire.