Debris flows muddy or liquefied landslides are most destructive when they are caused by volcanic eruptions. A spectacular example of a massive debris flow resulted from the 1980 eruptions of Mount St. Helens in Washington State. Areas near the bases of many volcanoes in the Cascade Mountain Range of California, Oregon, and Washington are at risk from the same type of flows during future volcanic eruptions.
Wildfires can also lead to destructive debris flows. In July 1994, a severe wildfire swept Storm King Mountain in Colorado, denuding the slopes of vegetation. Heavy rains on the mountain in September resulted in numerous debris flows, one of which blocked Interstate 70 and threatened to dam the Colorado River.
The term landslide describes many types of downhill earth movements ranging from rapidly moving catastrophic rock avalanches and debris flows in mountainous regions to more slowly moving earth slides. Some landslides move slowly and cause damage gradually, whereas others move so rapidly that they can destroy property and take lives suddenly and unexpectedly. Gravity is generally the force driving landslide movement.
Factors that trigger landslide movement include heavy rainfall, erosion, poor construction practices, freezing and thawing, earthquake shaking, and volcanic eruptions. Landslides are typically associated with periods of heavy rainfall or rapid snowmelt and tend to worsen the effects of flooding. Areas burned by forest and brushfires are particularly susceptible to landslides. Debris flows sometimes referred to as mudslides, mudflows, lahars, or debris avalanches are common types of fast-moving landslides. These flows generally occur during periods of heavy rainfall or rapid snowmelt. They usually start on steep hillsides as shallow landslides that liquefy and accelerate to speeds that are typically about 10 miles (16 kilometers) per hour, but can exceed 35 miles (56 kilometers) per hour.
The consistencies of debris flows range from watery mud to thick, rocky mud that can carry such large items as boulders, trees, and cars. Debris flows from many different sources can combine in channels and when this happens, their destructive power can increase greatly as these flow downhill and through channels, growing in volume with the addition of water, sand, mud, boulders, trees, and other materials. When the flows reach flatter ground, the debris spreads over a broad area, sometimes accumulating in thick deposits that can wreak havoc in developed areas.
Landslides cause property damage, injury, and death and adversely affect a variety of resources. For example, water supplies, fisheries, sewage disposal systems, forests, dams, and roadways can be affected for years after a slide event.
The negative economic effects of landslides include the cost to repair structures, loss of property value, disruption of transportation routes, medical costs in the event of injury, and indirect costs, such as lost timber and fish stocks. Landslides can affect water availability, quantity, and quality. Geotechnical studies and engineering projects to assess and stabilize potentially dangerous sites can be costly.
Landslides generally happen where they have occurred in the past, and in identifiable hazard locations. Areas that are prone to landslides include existing old landslides, the bases of steep slopes, the bases of drainage channels, and developed hillsides where leach-field septic systems are used.
Areas that are typically considered safe from landslides include areas that have not moved in the past; relatively flat areas away from sudden changes in slope; and areas at the top of or along ridges, but set back from the edge of slopes.
People can reduce their personal risk by learning about potential local landslide hazards and taking steps to reduce those hazards. Landslides are usually isolated events occurring without public warning. If you live in a landslide-prone area, be alert, particularly during periods of heavy rainfall or snowmelt or after a wildfire. If you see signs of a landslide or suspect a landslide may occur, you yourself must make the decision to evacuate.
The best source of information in a landslide situation is a local radio or television.
If You Are at Risk from Landslides, You Should:
- Develop an evacuation plan. If your home could be damaged in a landslide, you should know where to go if you have to leave. Making plans at the last minute can be upsetting, create confusion, and waste precious time. Contact 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.
- Familiarize yourself with the land around you. Knowing the land can help you assess your risk.
- Watch the patterns of storm water drainage on slopes near your home and 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.
Noticing small changes could alert you to an increased threat of a landslide.
- Discuss landslides and debris flows with members of your household. Everyone should know what to do to stay safe if one occurs.
- Be aware that, generally, landslide insurance is not available; however, in some cases, debris flow damage may be covered by flood insurance policies from the National Flood Insurance Program
During a Severe Storm, if you are in an Area Susceptible to Landslides, You Should:
- Stay alert and awake. Many landslide fatalities occur when people are sleeping.
- Listen to local stations on a portable, battery-powered radio or television or to NOAA Weather Radio for warnings of heavy rainfall. Be aware that short bursts of heavy rain may be particularly dangerous, especially after longer periods of heavy rain and damp weather.
- Consider leaving if it is safe to do so. Remember that driving during a severe 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 can save your life.
- 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 a large landslide. 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. Act quickly. Save yourself, not your belongings.
- 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 a possible debris flow.
- Bring your companion animals indoors and maintain direct control of them. Be sure that your pet disaster kit is ready to go, along with your family disaster kit, should you need to evacuate.
- Consider a precautionary evacuation of large or numerous animals. If you think an evacuation might be advised or ordered and if you have large, unusual, or numerous animals, start evacuating them as soon as you are aware of impending danger. If you are using a horse or other trailer to evacuate your animals, move early rather than wait until it may be too late to maneuver a trailer through slow traffic. Road hazards may make this too dangerous for you and for them.
- If you are ordered to or decide to evacuate, take your animals with you. If it is not safe for you, it is not safe for your animals.
If you learn or suspect that, a landslide is occurring or about to occur in your area, you should:
- Contact your local fire, police, or public works department. Local officials are the people best able to assess the potential danger.
- Inform affected neighbors. Your neighbors may not be aware of the potential hazard. Advising them of a threat may help save lives. Help neighbors who may need assistance to evacuate.
- Leave. Getting out of the path of a landslide or debris flow is your best protection. Take your pets with you when you leave, provided you can do so without endangering yourself.
If a landslide occurs, you should:
- Quickly move out of the path of the landslide. Moving away from the path to a stable area will reduce your risk.
After a landslide, you should:
- Stay away from the slide area. There may be danger of additional slides.
- Check for injured and trapped persons and animals near the slide, without entering the slide area. Direct rescuers to their locations.
- Help people who require special assistance infants, elderly people, those without transportation, large families who may need additional help in an emergency situation, people with disabilities, and the people who care for them.
- Listen to local stations on a portable, battery-powered radio or television for the latest emergency information.
- Watch for flooding, which may occur after a landslide or debris flow. Floods sometimes follow landslides and debris flows.
- Look for and report broken utility lines to appropriate authorities.
Reporting potential hazards will get the utilities turned off as quickly as possible, preventing further hazard and injury.
- Check your homes foundation, chimney, and surrounding land for damage.
- Replant damaged ground as soon as possible because erosion caused by loss of ground cover can lead to flash flooding.