The sunny Southwest U.S., including Southern California, was hit recently by tropical cyclone Hilary, a weather event that gave some desert areas a year’s worth of rain in 24 hours. Have a dry climate, get wildfires. Wish for rain, get flash floods and mudslides.
I probably should write more about mudslides. We live part-time in Gatlinburg, Tennessee, with a mountain home overlooking the town and the Great Smoky Mountains National Park. As such, we live on a slope. How much of a slope? Let’s just say you’d need a pretty long ladder to access the backyard from our deck.
A mudslide, sometimes called a “debris flow”, is a landslide with a high-water content. Mudslides act like a river that, if the mud is thick, has the consistency of wet concrete. Mud, rocks, trees, and other large objects are carried along and can cause homes to collapse or slide off their foundations and a huge amount of traumatic injury to residents. In the U.S., 25-50 deaths occur on average due to this natural disaster.
Another type of landslide is a “mudflow “, which is characterized by a very rapid flow of water and debris. A mudflow is more “liquefied” due, at least partially, to a lot of rain added to loose soil in a short period of time.
Loose soil and a lot of rain in a short period of time is a recipe for a mudslide
Mudslides occur for a variety of reasons: Periods of heavy rainfall or snowmelt saturate the ground and cause instability in sloping areas. Areas prone to earthquakes, hurricanes, wildfires, and other natural disasters are especially susceptible. Indeed, in the case of the Los Angeles area, a magnitude 5.5 earthquake occurred at the same time as the storm.
Areas in the West that have hard compacted soil as a result of wildfires have difficulty absorbing large amounts of rain, especially if it falls in a short period of time. As such, water that couldn’t get through hard earth quickly forms flash floods that cascade down slopes, picking up soil and debris to become a mudslide.
CAN YOU PREVENT A MUDSLIDE?
Humans contribute to the risk of mudslides by planning poorly: Roads cut into hills and mountains and scenic mountain homes make mudslides more likely. River retreats at the base of a hill or mountain (in the “holler”, as we say in Tennessee) are also vulnerable.
Once you’ve built that home on a hillside, there’s a limited number of preventative measures that can be undertaken. It’s a different story, however, if you’re just now planning out that dream home:
- Beware of steep slopes, natural or man-made runoff conduits, or eroded areas.
- Have the county Geological Survey specialist assess your property for possible mudslide risk.
- Consider flexible pipe fittings (installed by pros) less prone to gas or water leaks.
- Consider building a retaining wall in likely mudslide channels.
- Avoid areas that have experienced mudslides in the past.
- Plan out an evacuation route.
- Have a battery-powered NOAA weather radio.
- Have a medical kit with items to deal with both traumatic injury and water sterilization.
WARNING SIGNS OF A HOME AT RISK
Watch for cracks developing in walls, floors, paving, and foundations
Sometimes, pressure from an unstable earth may give you a hint that trouble is on the way and give you time to evacuate. Mudslide-prone areas will begin to show signs of strain:
- Cracks develop in walls, flooring, paving, driveways, or foundations.
- Outside structures (for example, stairs) begin to separate from buildings.
- Doors and windows become difficult to open or close.
- 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.
- The terrain starts to “bulge” or slant at the base of the slope.
DURING THE EVENT
- Turn on the NOAA radio and listen to warnings as they are reported.
- Warn your neighbors!
- If a mudslide is imminent, get out of Dodge if at all possible, with the understanding that roads may be washed out. Weather alerts should be heeded.
- Stay away from known mudslide areas; new mudslides may still occur.
In some mudslides, as in Southern California, things happen very quickly and you don’t have time to evacuate:
- If you stay home, 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. As in avalanches, put your arms in front of your face to gain breathing room.
- It’s a good idea to carry a cell phone with you at all times in case you are trapped in the house.
Mudslides, like wildfires, leave scars on the land but are part and parcel of living with Mother Nature. Plan before you build, know the danger signs, and hit the road. Leave as early as possible in the face of an imminent threat.
Joe Alton MD
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