Landslides occur in all U.S. states and territories and can be caused by a variety of factors including earthquakes, storms, volcanic eruptions, fire and by human modification of land. Landslides can occur quickly, often with little notice and the best way to prepare is to stay informed about changes in and around your home that could signal that a landslide is likely to occur.
A landslide is a downward movement of rock and soil debris that has become detached from the underlying slope. The material can move by falling, toppling, sliding, spreading and flowing. There are many landslide vulnerable areas with high-risk terrain. These include seismic sensitive areas, mountainous areas with high relief, moderate relief areas with land degradation, areas of thick loess and areas of high rainfall.
There are many possible causes of landslides these can either be geological, morphological or human-induced. A few of these include saturation of slope material (rainfall), seismic activity (earthquakes and volcanoes), undercutting of cliffs and banks by waves and rivers, removal of vegetation, and modification of slopes.
Mountainous areas throughout arctic and temperate regions which have slope angles between 25degrees and 60degrees are at risk. However, other conditions may affect the likelihood of an avalanche being triggered as already mentioned. The avalanche problem is more severe in Europe than North America due to the higher population densities in mountain ranges. Vibration is a physical trigger cause by thunder, a gunshot, by explosions or other loud noises such as shouting. Earthquakes can start avalanches, as well as noise from heavy machinery.
An avalanche is a mass of snow, often mixed with ice and debris which travels down mountain sides, destroying all in its path. There are three main types of avalanche: Powder, Slab and Wet.
Often start from a single point and accumulates snow as it moves down the slope forming a snowball effect. This type is most common following heavy snowfall of one inch per hour or more and often on a smooth surface such as after rain or frost. Without the cohesion with the snow layer underneath the snow is too heavy to settle. This type of avalanche can travel between 62 and 186 miles per hour.
Most common type of winter avalanche due to the build up fresh snow. A slab is a compact snow surface layer that can detach from a weaker snow layer underneath. The slab slips forward as a whole block or breaks into pieces.
Often occurs after a warm spell or during the spring thaw. Snow becomes heavier as it begins to turn into water. Occurs frequently and are generally small and generally easier to predict than the other types.
Before a Landslide
The following are things you can do to protect yourself, your family and your property from the effects of a landslide or debris flow:
- To begin preparing, you should build an emergency kit and make a family communications plan.
- Prepare for landslides by following proper land-use procedures - avoid building near steep slopes, close to mountain edges, near drainage ways or along natural erosion valleys.
- Become familiar with the land around you. Learn whether debris flows have occurred in your area by contacting local officials. Slopes where debris flows have occurred in the past are likely to experience them in the future.
- Get a ground assessment of your property.
- Consult a professional for advice on appropriate preventative measures for your home or business, such as flexible pipe fittings, which can better resist breakage.
- Protect your property by planting ground cover on slopes and building retaining walls.
- In mudflow areas, build channels or deflection walls to direct the flow around buildings. Be aware, however, if you build walls to divert debris flow and the flow lands on a neighbor's property, you may be liable for damages.
- If you are at risk from a landslide talk to your insurance agent. Debris flow may be covered by flood insurance policies from the National Flood Insurance Program (NFIP).
Recognize Landslide Warning Signs
- Changes occur in your landscape such as patterns of storm-water drainage on slopes (especially the places where runoff water converges) land movement, small slides, flows, or progressively leaning trees.
- Doors or windows stick or jam for the first time.
- New cracks appear in plaster, tile, brick, or foundations.
- Outside walls, walks, or stairs begin pulling away from the building.
- Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
- Underground utility lines break.
- Bulging ground appears at the base of a slope.
- Water breaks through the ground surface in new locations.
- Fences, retaining walls, utility poles, or trees tilt or move.
- A faint rumbling sound that increases in volume is noticeable as the landslide nears.
- The ground slopes downward in one direction and may begin shifting in that direction under your feet.
- Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
- Collapsed pavement, mud, fallen rocks, and other indications of possible debris flow can be seen when driving (embankments along roadsides are particularly susceptible to landslides).
During a Landslide
- During a severe storm, stay alert and awake. Many deaths from landslides occur while people are sleeping.
- Listen to local news stations on a battery-powered radio for warnings of heavy rainfall.
- Listen for unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together.
- Move away from the path of a landslide or debris flow as quickly as possible. The danger from a mudflow increases near stream channels and with prolonged heavy rains. Mudflows can move faster than you can walk or run. Look upstream before crossing a bridge and do not cross the bridge if a mudflow is approaching.
- Avoid river valleys and low-lying areas.
- If you are near a stream or channel, be alert for any sudden increase or decrease in water flow and notice whether the water changes from clear to muddy. Such changes may mean there is debris flow activity upstream so be prepared to move quickly.
- Curl into a tight ball and protect your head if escape is not possible.
After a Landslide
- Go to a designated public shelter if you have been told to evacuate or you feel it is unsafe to remain in your home.
- Stay away from the slide area. There may be danger of additional slides.
- Listen to local radio or television stations for the latest emergency information.
- Watch for flooding, which may occur after a landslide or debris flow. Floods sometimes follow landslides and debris flows because they may both be started by the same event.
- Check for injured and trapped persons near the slide, without entering the direct slide area. Direct rescuers to their locations.
- Look for and report broken utility lines and damaged roadways and railways to appropriate authorities. Reporting potential hazards will get the utilities turned off as quickly as possible, preventing further hazard and injury.
- Check the building foundation, chimney, and surrounding land for damage. Damage to foundations, chimneys, or surrounding land may help you assess the safety of the area.
- Replant damaged ground as soon as possible since erosion caused by loss of ground cover can lead to flash flooding and additional landslides in the near future.
- Seek advice from a geotechnical expert for evaluating landslide hazards or designing corrective techniques to reduce landslide risk. A professional will be able to advise you of the best ways to prevent or reduce landslide risk, without creating further hazard.
Find additional information on how to plan and prepare for a landslide or debris flow emergency and learn about available resources by visiting the following websites:
- U.S. Geological Survey Landslide Hazard Program
- Federal Emergency Management Agency
- American Red Cross
Listen to Local Officials
Learn about the emergency plans that have been established in your area by your state and local government . In any emergency, always listen to the instructions given by local emergency management officials.
Some good links:
- Pacific Disaster Center - The PDC provides easy access to hazard and risk information and analytical products for executive decision-makers, disaster managers, and community planners.
- 8 Steps to Reduce your Avalanche Risk
- Landslides and Mudflows - Federal Emergency Management Agency
- National Landslide Information Center
- Mitigation Reference Link Page - see DHS&EM' complete list of mitigation links
If you have questions about mitigation, e-mail Alaska's Hazard Mitigation Officer, Brent Nichols.