A landslide is when a mass of soil, rocks and other debris moves down a slope, powered by the force of gravity. Sometimes, this movement is so sudden and rapid that it causes devastating loss of life and structural damage. A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability.
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“ Landslide and Debris Flow as Natural Disasters ”
Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.
Landslides occur in all U.S. states and territories. In a landslide, masses of rock, earth, or debris move down a slope. Landslides may be small or large, slow or rapid. They are activated by storms, earthquakes, volcanic eruptions, fires, and human modification of land.
Landslide problems can be caused by land mismanagement, particularly in mountain, canyon, and coastal regions. Land-use zoning, professional inspections, and proper design can minimize many landslide, mudflow, and debris flow problems. Landslides are a serious geologic hazard that occurs in almost all 50 states. Every year in the United States, they cause significant damages and 25 to 50 deaths. Globally, landslides cause billions of dollars in damages and thousands of deaths and injuries each year.
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.
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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. 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.
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. Landslides occur when masses of rock, earth, or debris move down a slope.
Risks or Dangers from landslides
The immediate risk to human life from a landslide or mudslide is being caught in its path: sand, and thick mud especially, can cause suffocation, and people can be trapped or crushed by boulders or other debris, or by buildings collapsing under the weight of the flow.
Landslides can also disrupt power lines and water and sewerage pipes, potentially leading to electric shock and contaminated drinking-water. Roads and other transportation arteries may be blocked by debris, raising the risk for accidents and hampering access by rescue and medical services.
Landslides, mudflows and debris avalanches frequently accompany other natural hazards such as floods and earthquakes. The October 17, 1989 earthquake resulted in many areas of unstable land throughout the County which will be further impacted by winter storm conditions.
Areas are at Risk
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.
Tips on Surviving landslides
How to avoid them
Be in tune with your surroundings. If you’re travelling to a new area, swot up on it and find out about the potential risks (landslides or otherwise). Check out the topography: are there dodgy-looking slopes (steep or eroded) in the area? And especially: what is the weather doing? Intense rainstorms can be dangerous, especially if there’s been a preceding period of wet weather.
If you decide not to leave the area, then at least stay awake if you think there’s a chance of a landslide: many such disasters have occurred while their victims were asleep. Keep a portable, battery-powered radio with you to stay in touch with any safety announcements. Move up to a second story if possible, which might help to keep you above the level of the debris.
Listen for unusual sounds that might indicate moving debris, such as tree branches breaking, boulders knocking, or a faint rumbling that increases in volume.
A trickle of mud or soil may precede the main landslide. If you are near a stream or channel, beware of a sudden increase or decrease in water flow, or a change from clear to muddy water: this could mean landslide activity upstream. If you’re driving, remember that road embankments are prone to landslides. Also watch out for fallen rocks and mud.
Any of the above signs mean you may have only a few minutes (or even seconds) to get out of the path of the landslide.
Also, beware if the following occur for the first time:
Cracks in walls or the ground.
Doors or windows stick.
Outside walls or stairs lean away from the main building.
Underground utility lines break.
The ground bulges at the base of a slope.
Water breaks through the ground surface.
Fences, walls, utility poles or trees tilt.
Reinforce the foundation and walls of your home. Identify vulnerable areas of your home and add temporary shoring, bracing or shear wall supports where necessary.
Install flexible rather than stiff pipe fitting to avoid gas or water leaks in the event of a landslide or mudflow.
Mudflow is covered by flood insurance policies from the National Flood Insurance Program (NFIP). You can buy flood insurance even if you do not live in the flood plain. Keep your insurance coverage up to date.
Stockpile emergency building supplies such as ropes, buckets, large plastic bags, plywood, sandbags, plastic sheeting and lumber.
Maintain emergency supplies such as water, foods that require little cooking, a first aid kit, portable radio and flashlights.
In high risk areas, construct channels to direct the mudflows around your home or buildings.
If you are inside during a landslide and the building is not in peril of sliding down a hill, stay inside and get under a desk, table or other sturdy furniture.
If you are outside and cannot get into a sturdy building while rocks and debris tumble toward you, curl into a tight ball and protect your head.
Usually, you can survive a mudflow or debris avalanche only by avoiding it. If you are in a valley, get out as soon as possible once you hear rumbling from upstream or feel the ground tremble. These are signs that a mudflow may be coming your way.
If caught in a mudflow, try grabbing onto a large rock, tree or anything being carried along.
What you should do
Before the landslide
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.
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.
Contact your local emergency management office or American Red Cross chapter for information on local landslide and debris flow hazards.
Get a ground assessment of your property.
County or state geological experts, local planning department or departments of natural resources may have specific information on areas vulnerable to landslides. Consult an appropriate professional expert for advice on corrective measures you can take.
Minimize home hazards.
Plant ground cover on slopes and build retaining walls. In mudflow areas, build channels or deflection walls to direct the flow around buildings.
Remember: If you build walls to divert debris flows and the flow lands on a neighbor's property, you may be liable for damages. Explore a neighborhood or special district project.
Install flexible pipe fittings to avoid gas or water leaks. Flexible fittings are more resistant to breakage.
Familiarize yourself with your surrounding area.
Small changes in your local landscape could alert you to the potential of greater future threat.
Observe the patterns of storm-water drainage on slopes and especially the places where runoff water converge
Watch for any sign of land movement, such as small slides, flows, or progressively leaning trees, on the hillsides near your home.
Be particularly observant of your surrounding area before and during intense storms that could heighten the possibility of landslide or debris flow from heavy rains. Many debris flow fatalities occur when people are sleeping.
Talk to your insurance agent. Debris flow may be covered by flood insurance policies from the National Flood Insurance Program (NFIP).