Risks Of Landslides Rise With Winter Storms

Thirty-six years ago, one of the worst natural disasters occurred in the state of Washington. The major volcanic eruption at Mount St. Helens, caused by an earthquake weakening the north face, killed 57 people and reduced hundreds of square miles to wasteland.

The eruption created the largest landslide in U.S. history and was the most significant of its kind since the 1915 eruption of Lassen Peak in Northern California.

In the United States, landslides and mudslides result in an average of 25 to 50 deaths each year. Causes of landslides include earthquakes, storms, volcanoes, fire and human mismanagement.

The 2014 Oso mudslide, also in Washington, killed 43 and covered approximately 1 square mile.

What are the chances something similar could occur again? What about in California?

“California has a history of large landslides accompanying some winter storms,” said Jonathan Stock, a landslide expert with the U.S. Geological Survey (USGS). “Although most failures during or shortly after storms are shallow soil failures, large landslides do present a threat.”

Geologists classify landslides according to what’s moving and how it’s moving downhill. Sometimes only the surface materials such as soil and loose rock will fail and slide, other times, the landslide can be much deeper seated, where bedrock is actually moving downhill.

Last October, three Southern California counties – Los Angeles, Kern and Santa Barbara – received as much as 1.81 inches of rain in a half-hour. The storm created a mudslide up to five feet deep and shut down 45 miles of Interstate 5.

More than 200 vehicles were trapped in mud six-feet deep on State Highway 58 in Kern County.

“Winter and spring are typically landslide season in California, although the type of slides and triggering events can vary,” USGS scientist and landslide expert Mark Reid said. “Some past strong El Niño seasons have led to extended rainy periods. Widespread debris flows can occur after the ground is already wet and a large storm delivers intense precipitation. Areas recently affected by wildfires are more sensitive, and can generate debris flows during smaller storms. Debris-flow inducing storm events may occur during El Niño winters or during normal winters.

“Deeper-seated landslides typically require extended wet periods to allow water to reach their deeper slip surfaces,” he added. “This may take weeks or months of precipitation. Such slides often have a delayed response and may even move in the spring during sunny weather. If an El Niño winter brings periods of extended precipitation, it can lead to movement of deep-seated landslides.”

BePreparedCalifornia, a public health centric emergency preparedness resource maintained by the Department of Public Health, created specific preparedness sites for natural disasters here.

“Folks should maintain situation awareness,” said Jason Baker of Department of Public Health. “Be proactive and prepared. It is much easier to be prepared than it is to be repaired.”

Landslides occur when masses of rock, earth or debris move down a slope, while mudslides, also known as debris flows or mudflows, are a common type of fast-moving landslide that tends to flow in channels, according to the California Department of Public Health.

“Where USGS is monitoring soils in landslide prone areas of the San Francisco Bay Area, soil moistures are high and like those that have preceded landsliding during large storms in past years,” Stock said. “It is likely that many landslide-prone steeplands of central and northern California are now susceptible to shallow landslides if intense storms occur.”

Debris flows, where the majority of the materials are coarse-grained such as fine sand to boulder size particles and non-cohesive, are most often triggered by intense rainfall following a period of less intense precipitation or by rapid snow melt.

“In essence, debris flows are often deadlier than slow-moving landslides,” said Reid. “This is because they travel quickly and can reach out onto flatter ground adjacent to steep slopes and mountain streams. Slower moving slides can cause extensive property damage, however. And a large landslide that collapses catastrophically can be deadly, although these occur less frequently.”

To be prepared for possible landslides or mudslides, consider these helpful suggestions from the Department of Public Health.

Before the storm:

  • Assume that steep slopes and areas burned by wildfires are vulnerable to landslides and mudslides.
  • Learn whether landslides or mudslides have occurred previously in specific areas by contacting local authorities, a county geologist or the county planning department, state geological surveys or departments of natural resources or university departments of geology.
  • Contact local authorities about emergency and evacuation plans.
  • Develop emergency and evacuation plans for family and businesses.
  • Develop an emergency communication plan in case family members are separated.
  • Consider leaving if area is vulnerable to landslides.

During the storm:

  • Listen to the radio or watch TV for warnings about intense rainfall or for information and instructions from local officials.
  • Be aware of any sudden increase or decrease in water level on a stream or creek that might indicate debris flow upstream. A trickle of flowing mud may precede a larger flow.
  • Look for tilted trees, telephone poles, fences or walls, and for new holes or bare spots on hillsides.
  • Listen for rumbling sounds that might indicate an approaching landslide or mudslide.
  • Be alert when driving. Roads may become blocked or closed due to collapsed pavement or debris.
  • If a landslide or mudslide is starting, quickly move away from the path of the slide. Getting out of the path of a mudslide is the best protection. Move to the nearest high ground in a direction away from the path. If rocks and debris are approaching, run for the nearest shelter and take cover.

After the storm:

  • Stay away from the site. Flooding or additional slides may occur after a landslide or mudslide.
  • Check for injured or trapped people near the affected area, if it is possible to do so without entering the path of the landslide or mudslide.
  • Listen to the radio or TV for emergency information.
  • Report broken utility lines to the appropriate authorities.
  • Consult a geotechnical expert for advice on reducing additional landslide problems and risks.

Additional storm preparedness can be found at Cal OES and Ready.gov.


Jonathan Gudel

Jonathan Gudel is a Public Information Officer for the California Governor's Office of Emergency Services (Cal OES). Since joining Cal OES, he has assisted in the response and recovery efforts of the Aliso Canyon Gas Leak, the state's historic drought, the Oroville Dam Emergency Spillway Incident, unprecedented winter storms in 2017, the October (Sonoma County) and December (Santa Barbara County) 2017 wildfires, and statewide wildfire siege in 2018 . Previously, he worked in the newspaper industry for 12 years.

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