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Heavy rain across the state of California has caused landslides, including one that left several homes dangling from a cliffside in San Clemente. The state has seen an unusual amount of rain in certain areas lately, causing destruction and blocking roads.

Landslides are caused by disturbances in the natural stability of a slope. Areas where wildfires or faulty land development destroyed vegetation on slopes are at increased risk, especially during and after heavy rains.

(Aside: It’s not just a California event. When I’m in the Great Smokies, I, not uncommonly, come across scars on the land caused by a previous landslide.)

An avalanche is somewhat different from a landslide. An avalanche is comprised of a large mass of snow and ice sliding down a mountain side, but may bring rocks and trees along with it. It’s most often seen from December to April in the Northern Hemisphere. A landslide breaks up even more terrain of the mountain side itself and can occur at any time.

A mudslide, sometimes called a “debris flow,” is a landslide with a high amount of water content. Mudslides act like a river that, if the mud is thick, has the consistency of wet concrete. Rocks, trees, and other objects are moved by its irresistible force. A debris flow can cause homes to collapse, as it is doing in parts of California, and present the risk of traumatic injury to residents.

An event characterized by a very rapid flow of water and debris is sometimes called a “mud flow.”  A mud flow has a even higher water content than other slides due, at least partially, to a lot of rain in a short period of time. Periods of heavy rainfall or snow melt saturate the ground and cause instability in sloping areas.

A place in the mountains is great, but…

Humans contribute to this susceptibility with poor planning: Roads that are cut into hills and mountains make mudslides more likely. Riverside retreats at the base of a hill or mountain are also vulnerable. Things to consider for homes on slopes:

  • Steep grades, natural or man-made runoff conduits, and areas of erosion are more prone to mudslides. Most municipalities have a geological survey specialist. Contact one and have them assess your property for possible risks.
  • Flexible pipe fittings are less likely to develop gas or water leaks.
  • Retaining or deflection walls will help block likely mudslide channels and direct flow around building. Be aware, though, that big flows go where they want.
  • Avoid building in areas that have experienced mudslides in the past.
  • Plan out at least two evacuation routes out of the area.
  • Always have a battery-powered NOAA weather radio.
  • Have a medical kit with items designed to deal with traumatic injury.

Although the exact timing of a landslide is unpredictable, debris flow-prone areas will begin to show signs that trouble is on the way. They include:

  • Cracks develop in walls, flooring, paving, driveways, or foundations.
  • Outside structures (for example, stairs or pavers) begin to separate from buildings.
  • Doors and windows start becoming jammed.
  • Utility lines start breaking.
  • Fences, trees, and utility poles start tilting.
  • Water starts accumulating in strange places
  • Roads and embankments along slopes start breaking off at the edges.
  • Flat terrain starts to “bulge” or slant at the base of the slope.
  • Sudden increases or decreases in water levels in nearby creeks might indicate a debris flow upstream.
  • Trickles of flowing mud may precede a larger flow.
  • Some report that approaching mudslides make a deep rumbling sound.

Landslides are particularly deadly at night

If a storm occurs at night and you live in an at-risk area, stay awake and alert. People who are sleeping may be more likely to die in a landslide.

When a slide occurs, you may not have much time to act. It would have been wise to follow warnings on a weather radio as they are reported. If there’s time, warn your neighbors before you evacuate. Moving away from the path of the slide and to higher ground is your best chance to avoid injury.

If you choose to remain in place, get to the second story if you have one. Watch for and avoid downed power lines. As the slide passes through, get under a table and curl into a ball, protecting your head. If you’re trapped in the mud, survival rates go up if you can form an air pocket around you. Holding your arm in front of your face may help form a space and give you time and a measure of maneuverability.

If you had to, you can survive without water for 3 days and without food for much longer.  As with earthquakes, always have a means of communications, whether it’s a cell phone or even just a loud whistle you can blow.

After a landslide event:

  • Stay away from the slide area. Additional slides may be forthcoming.
  • Listen for the latest emergency information.
  • Watch for flooding, not an uncommon occurrence following landslides.
  • Check for injured and trapped persons near the slide. In normal times, avoid the main slide area and direct professional rescuers to likely locations.
  • Report broken utility lines and damaged roadways as soon as possible to those in charge.
  • Replant damaged ground with trees as soon as possible to restore the land; erosion caused by loss of ground cover can lead to future landslides.

If your property has been damaged by the event, consider asking trained professionals to perform a thorough inspection to determine whether further measures should be taken.

Joe Alton MD

Learn more about natural disasters and over 200 medical issues you might encounter in the aftermath in the Book Excellence Award-winning “The Survival Medicine Handbook: The Essential Guide For When Help Is NOT On The Way” and other books by Dr. Joe Alton and Nurse Practitioner Amy Alton. Also, check out their entire line of quality medical kits and individual supplies at You’ll be glad you did.



Hey, don’t forget to check out our entire line of quality medical kits and individual supplies at Also, our Book Excellence Award-winning 700-page SURVIVAL MEDICINE HANDBOOK: THE ESSENTIAL GUIDE FOR WHEN HELP IS NOT ON THE WAY is now available in black and white on Amazon and in color and color spiral-bound versions at

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