Threat of mudslides returns to California after devastating fires. How do they work?

By Rong-gong Lin II, Joseph Serna, and Rosanna XIA

Mudslides have always been at the heart of the peril of living close to the mountains in California. All it can take is an intense amount of rain in a short amount of time to create damaging flows of mud and debris that can kill people and destroy buildings.

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Mudslides are a particular risk for people living close to mountains. California’s mountains are quite tall, and the elevation drops extremely rapidly as water drains to the sea. The situation has become more dangerous as humans came to inhabit these paths of destruction.

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When hills are verdant and healthy, vegetation can anchor the soil in place, even during heavy storms. But when the protective blanket of vegetation is burned off, hillsides become vulnerable to erosion, and slopes can come crashing down in a torrent of mud, rocks and dead branches like white-water rapids, destroying homes.

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The heat from the fire makes it harder for water to infiltrate the top soil layer, Kean said. Ash tends to clog the soil after fire, and oily substances can affect the ground’s ability to absorb water. Impact from the rain can also help seal the top layer of sediment.

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