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How the harrowing Thomas Fire planted the seed for California’s deadly mudslides

At least 15 people are dead in Southern California, after mudslides and floods swept away buildings and inundated roads on Jan. 9. (Video: Melissa Macaya, Taylor Turner/The Washington Post, Photo: Ventura County Sheriff's Office/The Washington Post)

Southern California hit quota for natural disasters back in December. It didn’t need more to make this season one of the costliest and deadliest in recent memory. Then buckets of rain poured down on the freshly burned ground in Santa Barbara County early Tuesday morning and triggered massive mudslides that have taken more than a dozen lives so far.

All of the dead were recovered near Montecito, north of Los Angeles, and rescuers are still digging through the remains to find more of the missing. Photos of the scene show homes ripped away from their foundations, settled into waist-high mud and debris. The Santa Barbara County sheriff said the scene looks “like a World War I battlefield.”

There was no warning, officials said, which is almost always the case with mudslides. They come with no notice, except for the pounding rain.

But Tuesday morning’s torrential rain — nearly one inch in just 15 minutes, according to the National Weather Service — was only a catalyst for this disaster. The seed was planted in December during what became California’s largest wildfire in modern history.

It’s one of nature’s more tragic paradoxes; a terrible wildfire season, one that begs for cool days and torrential rain, often evolves into an equally grim winter with deadly flash flooding and mudslides.

Wildfires don’t just burn aboveground plants and structures; they physically alter the ground itself. So, while it’s true that the absence of trees, grass and brush in a wildfire scar will promote water movement and flash flooding, it’s not the biggest effect.

‘Like a WWI battlefield’: 15 dead, dozens missing as mudslides wipe away homes

Wildfires burn biological material, which contain carbon. Carbon is the fuel in a fire — much like the charcoal in your grill, or the carbon-rich hardwood in your fireplace. Biological material is not limited to the grass and flowers above ground, though. Very intense, hot fires can burn subsurface material as well.

Take a shovel and dig up a blade-full of soil. You’ll find a combination of topsoil, leaves, twigs and bugs. The mixture is rich in carbon and absorbs water, two reasons it’s so great for plants.

Intense wildfires plow right through that plant-friendly stuff. They burn all the carbon-rich things — leaves, twigs, bugs, etc. — and what’s left is a very dense layer of noncombustible materials such as clay and rock. The top layer of dirt loses porosity, texture and the ability to absorb and hold water. It burns up the bugs that the ecosystem requires to burrow through the ground and aerate the soil.

In short, intense wildfires transform rich soil into a dense, water-repellent surface. To make matters worse, biologists have found that when rain hits the charred landscape, it can become even more dense and more water repellent.

It’s a long-term effect, too, which means Santa Barbara County will be prone to flooding and mudslides for years to come, potentially. Researchers have seen the ramifications of wildfires last from a year to decades, depending on how hot the fire was burning.