Why fire-scorched California is now readying for mudslides

Why fire-scorched California is now readying for mudslides

November 30, 2018 0 By Nazmul Khan

Long-awaited rain is pummeling California, threatening dangerous mudslides in areas reeling from recent fires. Officials are currently warning people to evacuate parts of southern California where the Holy Fire burned more than 23,000 acres this summer. Further north, in burn scars where the deadly Camp Fire recently raged, the National Weather Service has issued flash flood warnings, too. Paradise, a town that was devastated by the Camp Fire, remains under mandatory evacuation orders as the rains pour down.

It’s a familiar sequence of disasters for Californians: after the record-setting fire season of 2017, waist-high mud flowed through Santa Barbara County at more than 35 miles per hourkilling at least 17 people in January. And it’s a cascade of catastrophes we may see again in the wake of this year’s fire season, now the worst on record. Slopes stripped bare and desiccated by wildfires are more prone to these post-fire debris flows, a kind of landslide where rushing water picks up the remnants of burnt homes, trees, dirt, rocks, and even cars.

For people living in risky areas, it will be key to listen to emergency managers, and follow evacuation orders. The way to survive a landslide is not to be in its way, says geophysicist and disaster researcher Mika McKinnon. “You keep it from happening in the first place, or you’re not there when it happens. That’s it.”

The Verge spoke with McKinnon about the dangers of rain on burnt ground.

This interview has been edited for clarity and brevity.

Now that we’ve got rain in California right on the heels of these wildfires, what sort of hazards are we looking at here?

Anytime we have fires, we end up with landslides during the next big rains. This happens for a whole bunch of different reasons. One is you have a whole new layer of debris, so you’ve got broken tree branches, you’ve got ash — really slick, fine particles that slide really easily — you’ve got destroyed homes, and if there hasn’t been time to clean it all up, that’s just debris waiting to get swept away by water. Just like the first rains mean you’re going to clear all sorts of pine needles and leaves out of your gutters, when you’re going through a fire area, all of that garbage is going to get swept up as debris flows.

Rains bring the risk of mudslides in the Camp Fire burn scar.
Image: Lauren Dauphin/NASA Earth Observatory

What’s the difference between a mudslide, landslide, or debris flow?

Landslide is the big, overarching term. The only term bigger than that is mass movement, the generic term for anything falling downhill. The most common ways to divide up landslides are by material, and by mechanism. The material is the stuff that is moving: is it rock, is it debris, mud, dirt, soil, earth, snow? And the mechanism is how the landslide moved: does it fall, topple, rotate, flow?

So a mudslide would be mud and water sliding down a slope, slipping, kind of a blob. A debris flow would be a bunch of debris and water flowing like a fluid as opposed to a debris avalanche, where you have less water and it moves more like a snow avalanche does. You can mix between the material and the mechanism to come up with a name for different types of landslides, but the word landslide covers all of those.

Why do fires increase the risk for landslides?

When a fire comes through, it burns a lot of material, and this creates debris. It also kills the trees, chars the ground and produces a lot of ash. Ash is very fine, and very slick, which makes it a perfect sliding surface. It lubricates geologic materials. Fire is also hot, and dries things out and chars them up, and when it dries out that chunk of ground, it can create a hard dry layer on top that’s difficult for water to get through. That water skims across the surface instead and picks up more debris, and then you get a debris flow.

What areas are particularly vulnerable to landslides, and how can we predict where they’re going to hit?

The steeper the slope, the more likely you are to have landslides. It also depends on the material. Hard rock is more stable and loose materials — sands, dirts, soils — are weaker. If there are a lot of plants, plants tend to hold things together with their roots. They cling and weave together the materials. If there are fewer plants, you don’t have that root stability and the slopes are less stable.

The water will flow into valleys, and ravines, and gullies; all of those places are going to be more likely to have landslides. If you’ve got all that water washing [away] debris, tree branches, boulders, and ash, sooner or later, it’s not just water — it’s now a debris flow, a very fluid landslide that’s a mixture of water and debris. They’re very, very fast moving.

A satellite image of the Woolsey Fire burn scar.

A satellite image of the Woolsey Fire burn scar.
Image: NASA Earth Observatory

How does a landslide start?

A funny quirk of terminology is that we say that a landslide fails when it starts moving. From the landslide’s point of view, it’s succeeding. But what that really means is the slope or the material has failed, it’s no longer stable, and it’s now collapsing and moving downhill. What we’re more likely to see after the fires are debris flows, and debris avalanches. Instead of having an entire piece of the slope fail, the water picks up more and more material. And the more material it gathers, the more it picks up. It’s a feedback loop.

And how does a landslide stop?

It’s just going to keep going until one of a few things happens. One, it runs out of energy, so it’s gone all the way down and kept running until it’s run out of kinetic energy, and comes to a stop. And that pretty much never happens in northern California because we’ve got too much stuff in the way. But sometimes that might happen in a desert.

The next thing that could happen: something separates the water from the debris. If someone went through and expected there’d be this type of landslide, and built a debris retention barrier — which is like a giant strainer for a landslide — it could trap all the debris, so it’s a lot less dangerous. But that means that you needed to expect that you were going to get debris flows, and have the foresight, and planning, and money to build structures to protect against it.

You could also get so much debris that it all starts clogging and jamming together and creates a debris dam, or a landslide dam — blocking the rest of the landslide. And now all the debris starts piling up behind it, making it bigger, and bigger, and bigger. The water gets trapped in behind, and creates a flood, so now you’ve got upriver flooding. It’s a really weak dam, and so it will erode quickly and break. And then all that water that built up during the flood upstream is now released in an outburst flood swishing downstream.

What’s the most dangerous part of a mudslide?

With most disasters we have survival tips. For an earthquake, it’s “duck, cover, and hold on.” Or for flooding, it’s “turn around and don’t drown.” We don’t have survival tips for landslides, at all — aside from just don’t be there. So what’s the most dangerous part of a landslide? Getting buried by the landslide. Even a rockfall can kill you. A fist-sized rock falling even 10 feet can have enough kinetic energy that if it hits you in the head, it will kill you.

If you’re beside the landslide, if you manage to be uphill at all, you’re going to have slightly better luck.

What can we expect in the future with landslides in the West?

The research and the funding is just not there. Even when we know there’s a problem, we don’t often have the resources to be able to mitigate that risk. And that’s something we can start talking about more, and start asking about more. Because we’re going to have more droughts, and we’re going to have more floods and we’re going to have more fires, and we’re going to have more severe storms. That’s just our future. And if we’re going to keep having them, we’re going to have more landslides, and maybe it’s about time for us to start looking at that seriously, and start being more proactive in mapping out potential landslide hazard areas, and changing what sorts of buildings are zoned for construction in places we know are at risk, and starting to engineer protective barriers to deflect landslides or to catch them or to separate the debris from the water so they’re less dangerous.

There are things we can do. If we have the political will and the monetary resources, we can reduce our risk from landslides.

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