Why do landslides happen?

A landslide is made of mud and other earth materials that fall down a slope, usually after a period of heavy rain. When buildings or other infrastructure balance on soils that cannot effectively capture and react to precipitation, landslides may occur. The weight of the water and the steep incline push the mud and dirt downhill.

Landslides can happen quickly or slowly. Flash landslides, where heavy, watery dirt falls suddenly without warning, are the most dangerous.

landslide over roadway
A landslide consists of mud and other earth materials that fall down a slope, usually after a period of heavy rain. Pictured is a landslide in El Salvador. Credit: Canva Pro

There’s a lot of physics involved in the reasons behind landslides, but often they are avoidable with proper planning. The increasing rate of intense storms, of course, has made past calculations somewhat unpredictable now. Soil that once was able to absorb rainfall may now be ill equipped for sudden, heavy surges of water. Or fires such as those in California, could change the properties of the topsoil, making it less likely to absorb water.

Gravity also plays a major role in landslides. This natural force is always pulling for things to “fall downhill.” That’s why engineers look at the steepness of the slope and accommodate for that in their planning.

Unfortunately, another force, like erosion, can affect the stability of the slope, which continuously changes the physics of a slope. Imagine ocean waves or rivers cutting into the edge of a slope, pulling critical base material away. Like a giant game of Jenga, with the right factors, a landslide can happen.

erosion of a parking lot caused by ocean waves
Erosion of parking lot from ocean waves on the Oregon Coast. Credit: Canva Pro

Landslides can happen in all 50 states, but there are certain places where landslides are more prone to happen. For example, on the West Coast, states like California, Washington, and Oregon see more landslides than other states because of their hilly terrain. Lands altered by humans or wildfire are also more at risk for landslides.

If you are worried about a landslide because you live near steep slopes and hills, contact your local authorities to see if landslides or other debris flows have occurred in your area before. The Red Cross also has a checklist. Experts also recommend making an evacuation plan for your home or business.

sandy, grassy and eroded streambank and stream
Erosion of the lower parts of a streambank can cause the entire section to fall into the stream. In the same way, ocean waves hitting steep slopes on embankments can erode the base layer and cause a landslide. Credit: SV Fisk

If a landslide occurs without warning, make every effort to get out of your home or office building. Attempt to go to higher ground, and away from the path of debris flow. The higher up a hill you are, the less likely it will be that you sustain injuries from a landslide.

Adapted by Susan V. Fisk and April Ulery, from a blog that originally ran on May 15, 2015 by Larry Baldwin

Watch SSSA’s video Soils Support Buildings and Infrastructure, for more information.

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