A Pocket Guide to Landslides: What They are and How to Stay Safe From Them

Landslides are a common occurrence throughout the Wasatch Mountains due to several different factors. Areas that are prone to landslides include: existing old landslide paths, on or at the base of slopes, in or at the base of minor drainage hollows, and at the base or top of a steep cut slope.¹

 

What Causes Landslides?

National Geographic defines a landslide as, “the movement of rock, earth, or debris down a sloped section of land. Landslides are caused by rain, earthquakes, volcanoes, or other factors that make the slope unstable.”²

Landslides typically occur due to three major causes- geology, morphology, and human activity. 

Geological causes of a landslide are typically due to weak or fractured rock, or different layers that have varying strengths and stiffness. Steep slopes, mountainous terrain, different rock types, and narrow canyons all contribute to the Central Wasatch’s susceptibility to landslide hazards.³

Steep, rocky slopes of the Wasatch Mountains. Photo taken by Madeline Pettit. 

 

Morphological causes are related to the structure of the land. Slopes that lose vegetation to fire or drought are more susceptible to erosion which can then lead to landslides. The current drought that has spanned 2021-2022 has placed a strain on the native vegetation in the Wasatch Mountains and decreases the amount of plant life holding soil in place, which causes the land to slide. Rockfall is one of the most common types of slope-failure in Utah and, “may be triggered by freeze/thaw action, rainfall, changes in groundwater conditions, weathering and erosion of the rock and/or surrounding material, and root growth. Talus cones, an accumulation of rock debris, and scree-covered slopes, slopes covered in small loose stones, are indicators of a high rockfall hazard”.

Talus cones at Lake Blanche. Photo taken by Brian Kissmer.

 

Human activity such as construction and agriculture are other causes of landslides. An area that is impacted by irrigation, deforestation, or excavation becomes weak and destabilized. In the Central Wasatch Mountains the presence of the mining, timber and livestock industries weakened areas and contributed to landslides in the past. Lately, human activities such as cuts for roads, clearing of slopes for development, and poorly constructed rockery walls are common causes of landslides in the Central Wasatch.

Landslide at Alta in 2010. Image from Utah Geological Survey. 

 

What do I do in the Case of a Landslide?

The U.S Geological Survey has an extensive database of information about landslides. In this article we wanted to highlight some of their warning signs of landslides and tips on what to do when one is occuring. Being aware of these will help us all stay safe as we recreate and live in and around the Central Wasatch. 

 

Some common warning signs of a landslide are:

  • Springs, seeps, or saturated ground in areas that have not typically been wet before.
  • New cracks or unusual bulges in the ground, street pavements or sidewalks.
  • Soil moving away from foundations.
  • Sunken or down-dropped road beds.
  • Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content).
  • Sudden decrease in creek water levels though rain is still falling or just recently stopped.
  • A faint rumbling sound that increases in volume is noticeable as the landslide nears.
  • Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.

Debris flow deposits at Mill D South Fork in 2021. Image from Utah Geological Survey.

 

What to do during a landslide:

  • Evacuate. Getting out of the path of a landslide or debris flow is your best protection.
  • Curl into a tight ball and protect your head if escape is not possible.
  • Stay alert and awake. Many debris-flow fatalities occur when people are sleeping. Be aware that intense, short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.
  • If you are in areas susceptible to landslides and debris flows, consider leaving if it is safe to do so. Remember that driving during an intense storm can be hazardous. If you remain at home, move to a second story if possible. Staying out of the path of a landslide or debris flow saves lives.
  • Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing or falling mud or debris may precede larger landslides. Moving debris can flow quickly and sometimes without warning.
  • If you are near a stream or channel, be alert for any sudden increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate landslide activity upstream, so be prepared to move quickly. 
  • Be especially alert when driving. Embankments along roadsides are particularly susceptible to landslides. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of possible debris flows.

 

Take a look at the Environmental Dashboard to learn more about landslides in the Central Wasatch Mountains and locations where they typically occur throughout our canyons.

 

Written by Madeline Pettit.

Endnotes:

  1. https://www.usgs.gov/programs/landslide-hazards/landslide-preparedness
  2. https://education.nationalgeographic.org/resource/landslide
  3. https://geology.utah.gov/hazards/landslides/
  4. https://geology.utah.gov/hazards/landslides/rockfalls/
  5. https://www.usgs.gov/programs/landslide-hazards

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