Backgrounder: Landslides and Mudflows
From the Federal Emergency Management Agency (FEMA) - October 23, 2004
EMERGENCY INFORMATION
- Acres of property may be damaged and buildings and homes destroyed
by landslides. Landslides can provoke associated dangers such as broken
electrical, water, gas, and sewage lines, and disrupt roadways and railways.
- Landslide warning signs include cracks opening on hillslopes, evidence
of slow, downhill movement of rock and soil; tilting of trees, poles,
or walls; or visible changes such as the formation of sags and bumps
in the slope.
- Landslide, mudflow, and debris-flow problems are often caused by land
mismanagement. Improper land-use practices on ground of questionable
stability, particularly in mountain, canyon, and coastal regions, can
create and accelerate serious landslide problems. Land-use zoning in
partnership with professional inspections and proper design can alleviate
many problems associated with landslides, mudflows, and debris flows.
DANGER ZONES
Landslides occur in every state of the union and its island territories.
California, West Virginia, Utah, Kentucky, Tennessee, Puerto Rico, Ohio,
and Washington have the most severe landslide problem.
WHAT IS A LANDSLIDE?
Landslides occur when masses of rock, earth, or debris move down a slope.
Landslides may be very small or very large, and can move at slow to very
high speeds. Many landslides have been occurring over the same terrain
since prehistoric times. They are activated by storms and fires and by
human modification of the land. New landslides occur as a result of rainstorms.
earthquakes, volcanic eruptions, and various human activities.
WHAT IS A MUDFLOW?
Mudflows (or debris flows) are rivers of rock, earth, and other debris
saturated with water. They develop when water rapidly accumulates in the
ground, such as during heavy rainfall or rapid snowmelt, changing the
earth into a flowing river of mud or "slurry." A slurry can flow rapidly
down slopes or through channels, and can strike with little or no warning
at avalanche speeds. A slurry can travel several miles from its source,
growing in size as it picks up trees, cars, and other materials along
the way.
HELP YOUR COMMUNITY GET READY
The media can raise awareness about landslide and mudflows by providing
important information to the community. Here are some suggestions:
- In an area prone to landslides, publish a special section with emergency
information on landslides and mudflows. Localize the information by
including the phone numbers of local emergency services offices, the
American Red Cross, and hospitals.
- Report on what city and county governments are doing to reduce the
possibility of landslides. Interview local officials about local land-use
zoning regulations.
- Interview local officials and major insurers regarding the National
Flood Insurance Program. Remind your community that mudflow is covered
by flood insurance policies from the National Flood Insurance Program.
- Work with local emergency services and American
Red Cross officials to prepare special reports for people with mobility
impairments on what to do if an evacuation is ordered.
DID YOU KNOW...
- The most expensive landslide in U.S. history occurred in Thistle,
Utah, in spring, 1983. It reached 1/2 miles from top to bottom and ranged
in width from 1,000 feet to about 1 mile. Total costs attributable to
the landslide exceeded $500 million.
- Landsliding in the United States is estimated to cause an annual loss
of about $1.5 billion and at least 25 fatalities.
- The Loma Prieta earthquake in October 1989 triggered thousands of
landslides throughout an area of 5,400 square miles. In addition to
causing at least tens of millions of dollars of damage to houses, other
structures, and utilities, landslides blocked many transportation routes,
greatly hampering rescue and relief efforts.
- Mudflows tend to flow in channels, but will often spread out over
a floodplain. They generally occur in places where they have occurred
before.
The information provided on this site is for informational purposes only.
Always consult your doctor for medical advice.
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