All You Need To Know About Mudslides

Mudslides can be extremely deadly and they can happen in all 50 states. That’s why it’s important to know how they work. Here’s everything you should know:

1. A mudslide is a specific kind of landslide. A landslide is when rock, earth or any other kind of debris moves down a slope. A mudslide is a type of fast-moving landslide made up of mud and debris. Both can occur any time during the year. They can strike without warning and that's what makes them so dangerous.

2. Mudslides usually happen after water saturates the ground on a slope very quickly, such as after a heavy rainfall. As a result, mudslides tend to occur during wetter seasons. On the Pacific coast of the United States, such as in California, they usually occur in winter or spring (December - April). They can happen up to an hour after a burst of intense rain stops. Earthquakes, volcanic eruptions, changes in groundwater levels, alternate freezing and thawing and the steepening of slopes due to erosion can also cause mudslides.

3. There are two main types of mudslides. The first type is a mudflow, which is just the movement of water and mud. This type is usually more shallow — about 15 feet deep or less. The second type is a debris flow. This is when water brings down mud and other debris like rocks, branches and even large boulders.

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4. Wildfires can make mudslides much worse. Vegetation helps anchor soil in place on hills, even during heavy storms. However, when this protective vegetation is burned off, hillsides become vulnerable to erosion and the slopes can come crashing down during mudslides. Heat, ash and oily residue from wildfires also make it harder for water to infiltrate the top layer of soil, making the ground almost water-repellent.

6. It’s difficult to predict when and where a mudslide might occur. This is because of different soil compositions and geographic characteristics. There are known areas of risk, but this is usually because these regions have been hit by mudslides before. The West Coast — notably California — is considered especially prone to mudslides because this region has earthquakes, rainfall and wildfires, which are all mudslide triggers. The mountains in California are also relatively tall, and the quick drop in elevation can make mudslides more likely.

7. Mudslides can be slow or fast moving, but they generally grow in momentum the further they flow. As the soil and mud slides, it accelerates downhill, gaining momentum. Debris flow downhill can reach speeds of 35 miles per hour.

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8. Mudslides can travel great distances. Multiple debris flows can start at different points high in canyons, then funnel into channels, merging and gaining volume which allows them to travel a long distance from the source. The places most at risk for mudslides are at canyon bottoms, stream channels, outlets of canyons and slopes excavated for buildings. If you live in those areas, be on the lookout for immediate signs of a pending mudslide, including tilting trees or sudden changes in rivers, especially after rain.

9. Mudslides can be deadly. In the United States, landslides and mudslides cause on average 25 to 50 deaths every year, according to the CDC.

10. The best way to protect yourself is to stay informed. Check with local authorities to see if a mudslide has ever happened where you live. Develop emergency and evacuation plans. Sign up for emergency alerts, especially if you live in a mudslide-prone area. During intense rainfall, watch for warnings so you can quickly leave the area. If a mudslide occurs, stay away from the affected area because additional flooding or slides can happen, even after the mudslide seems to be over. Never enter the path of the mudslide and report any broken utility lines.

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