Geology Cafe

Landslide Hazards—An Awareness Guide

Table of Contents

Landslides and Debris Flows - Hazards and Preparedness

Landslide hazards occur in many places around the world and include fast-moving debris flows, rock fall, and slow-moving landslides. Each year, these hazards cost billions of dollars and cause numerous fatalities and injuries. Use information and links on this page to become more aware of the hazards posed by landslides and how to avoid them.

What to do if you suspect imminent landslide danger:

  1. Evacuate.
  2. Contact your local fire, police, or public works department.
  3. Inform affected neighbors.

Debris Flows and Other Rapidly Moving Landslides

Debris flows start on steep slopes—slopes steep enough to make walking difficult. Once started, however, debris flows can even travel over gently sloping ground. The most hazardous areas are canyon bottoms, stream channels, areas near the outlets of canyons, and slopes excavated for buildings and roads.

What Can You Do If You Live Near Steep Hills?

Prior to Intense Storms:

  1. Become familiar with the land around you. Learn whether debris flows have occurred in your area by contacting local officials, State geological surveys, or departments of natural resources, and university departments of geology. Slopes where debris flows have occurred in the past are likely to experience them in the future.
  2. Support your local government in efforts to develop and enforce land-use and building ordinances that regulate construction in areas susceptible to landslides and debris flows. Buildings should be located away from steep slopes, streams and rivers, intermittent-stream channels, and the mouths of mountain channels.
  3. Watch the patterns of storm-water drainage on slopes near your home, and note especially the places where runoff water converges, increasing flow over soil-covered slopes. Watch the hillsides around your home for any signs of land movement, such as small landslides or debris flows or progressively tilting trees.
  4. Contact your local authorities to learn about the emergency-response and evacuation plans for your area and develop your own emergency plans for your family and business.

During Intense Storms:

  1. Stay alert and stay awake! Many debris-flow fatalities occur when people are sleeping. Listen to a radio for warnings of intense rainfall. Be aware that intense short bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall and damp weather.
  2. 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 is hazardous.
  3. 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 flows. 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 debris flow activity upstream, so be prepared to move quickly. Don't delay! Save yourself, not your belongings.
  4. 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.

Slow-moving Landslides—What to do and look for during and after heavy rains

Areas that are generally prone to landslides:

  • On existing old landslides
  • On or at the base of slopes
  • In or at the base of minor drainage hollows
  • At the base or top of an old fill slope
  • At the base or top of a steep cut slope
  • Developed hillsides where leach field septic systems are used

Areas that are typically considered safe from landslides:

  • On hard, nonjointed (unfractured) bedrock that has not moved in the past
  • On relatively flat-lying areas away from sudden changes in slope angle
  • At the top or along the nose of ridges, set back from the tops of slopes

Features that might be noticed prior to major landsliding:

  • Springs, seeps, or saturated ground in areas that have not typically been wet
  • New cracks or unusual bulges in the ground, street pavements, or sidewalks
  • Soil moving away from foundations
  • Ancillary structures such as decks and patios tilting and (or) moving relative to the main house
  • Tilting or cracking of concrete floors and foundations
  • Broken water lines and other underground utilities
  • Leaning telephone poles, trees, retaining walls, or fences
  • Offset fence lines
  • 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
  • Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb

FACT SHEET (a 2 page .pdf files produced by the US Dept. of Health & Human Services)

Landslides and Mudslides

What landslides and debris flows are

Landslides occur when masses of rock, earth, or debris move down a slope. Debris flows , also known as mudslides , are a common type of fast-moving landslide that tends to flow in channels.

What causes landslides and debris flows
Landslides are caused by disturbances in the natural stability of a slope. They can accompany heavy rains or follow droughts, earthquakes, or volcanic eruptions. Mudslides develop when water rapidly accumulates in the ground and results in a surge of water-saturated rock, earth, and debris. Mudslides usually start on steep slopes and can be activated by natural disasters. Areas where wildfires or human modification of the land have destroyed vegetation on slopes are particularly vulnerable to landslides during and after heavy rains.

Health threats from landslides and debris flows
In the United States, landslides and debris flows result in 25 to 50 deaths each year. The health hazards associated with landslides and mudflows include:

  • Rapidly moving water and debris that can lead to trauma;
  • Broken electrical, water, gas, and sewage lines that can result in injury or illness; and
  • Disrupted roadways and railways that can endanger motorists and disrupt transport and access to health care.

What areas are at risk
Some areas are more likely to experience landslides or mudflows, including:

  • Areas where wildfires or human modification of the land have destroyed vegetation;
  • Areas where landslides have occurred before;
  • Steep slopes and areas at the bottom of slopes or canyons;
  • Slopes that have been altered for construction of buildings and roads;
  • Channels along a stream or river; and
  • Areas where surface runoff is directed.

What you can do to protect yourself

Before intense storms and rainfall

  • Assume that steep slopes and areas burned by wildfires are vulnerable to landslides and debris flows.
  • Learn whether landslides or debris flows have occurred previously in your area 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 your family and business.
  • Develop an emergency communication plan in case family members are separated.
  • If you live in an area vulnerable to landslides, consider leaving it.

During intense storms and rainfall

  • 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 mudflow.
  • Be alert when driving. Roads may become blocked or closed due to collapsed pavement or debris.
  • If landslide or debris flow danger is imminent, quickly move away from the path of the slide. Getting out of the path of a debris flow is your 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 (if possible, under a desk, table, or other piece of sturdy furniture).

After a landslide or debris flow

  • Stay away from the site. Flooding or additional slides may occur after a landslide or mudflow.
  • 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 mudflow.
  • Listen to the radio or TV for emergency information.
  • Report broken utility lines to the appropriate authorities.
  • Consult a geotechnical expert (a registered professional engineer with soils engineering expertise) for advice on reducing additional landslide problems and risks. Local authorities should be able to tell you how to contact a geotechnical expert.

Where you can get additional information on landslide and mudflow hazards
The Federal Emergency Management Agency (FEMA) has a fact sheet containing information on landslides and mudflows in the United States, as well as recommendations on how to prepare and behave during and after a landslide.

The U.S. Geological Survey (USGS) has a fact sheet containing information on landslide hazards and characteristics.

The American Red Cross (ARC) has a Web site containing information on landslides and mudslides.

The United States Geological Survey (USGS) landslide news Web site provides up-to-date information on recent and past landslide events.

For information about reducing losses from landslides at the national, state, and local levels, in both public and private sectors, review the following document: Spiker EC, Gori PL. National landslide hazards mitigation strategy -- a framework for loss reduction . U.S. Geological Survey Circular 1244 - Online Version 1.0, 2003.