California Fire Survivors Brace for Debris-Filled Mudslides

WIRED | 11/20/2018 | Matt Simon
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California continues to take stock of its unprecedented conflagrations—between the Camp Fire in the north, which on its own is the deadliest and most destructive fire in California history, and the Woolsey Fire in the south, 82 lives have been lost so far, with hundreds more still missing.

As firefighters keep battling the blazes, a new menace is emerging: debris flows. The rains are coming to California, as soon as Tuesday night, and the fires have primed the soil for another disaster that can claim yet more lives.

Town - Montecito - Southern - California - Lesson

The coastal town of Montecito, in Southern California, learned this lesson less than a year ago. Last December, the massive Thomas Fire burned through the hills, right up to the edges of town. Thanks to the community’s comprehensive fire preparedness plan, only 7 homes were lost. But a month later, the scorched hills outside of town let loose a storm-driven debris flow that enveloped the town, killing 21.

The problem is a fundamental change to the soil itself. When a wildfire burns vegetation, it releases organic compounds. “These compounds are heated in the fire, volatilize, and then re-condense on soil particles,” says Brian Ebel, a hydrologist with the USGS.

Soil - Water - Compounds - Fear - Water

This soil normally absorbs water, but the organic compounds turn it hydrophobic, literally meaning “fear of water.” It is not an understatement: You can pour a bottle of water on this burned soil and it’ll bead right off, as if the dirt were polished marble. The water will pool up and sit there, and it can take more than an hour for it to penetrate fire-scorched soil, says Ebel.

Problem number two: vegetation anchors the soil. When fire burns it away, the soil loosens. “You can think of vegetation as a protective blanket,” says Jason Kean, a hydrologist at the USGS. Without it, sediment has nothing to hold...
(Excerpt) Read more at: WIRED
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