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Landslides |
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July 2004
Produced by the National Disaster Education Coalition, Washington, D.C.
Landslides
Why talk about landslides?
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.
What are landslides, and what causes them?
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 brush fires
are particularly susceptible to landslides.
Debris flows—“muddy” or “liquefied” landslides
sometimes referred to as mudslides, mudflows, lahars, or debris
avalanches —
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.
Debris 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 they 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.
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.
Learn if landslides, including debris flows, could occur in your area by contacting
local officials, your state geological survey or
department of natural resources, or the geology department of a state university.
Get information on specific locations that are
vulnerable to landslides. Request a professional referral for a detailed landslidevulnerability
analysis
of your property, and take
corrective measures if necessary.
How do landslides affect us?
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. Water availability,
quantity, and quality can be affected
by landslides. Geotechnical studies and
engineering projects
to assess and stabilize potentially dangerous sites can be costly.
How can I protect myself from landslide?
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.
What is the best source of information in a landslide situation?
The best source of information in a landslide situation is a local radio or television
station.
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 soilcovered 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 (NFIP) (www.fema.gov/nfip).
• For general preparedness, every household should create and practice a Family
Disaster Plan and assemble and maintain a
Disaster Supplies Kit. In addition, every household at risk from landslides should
take
landslide-specific precautions and plan
for and practice what to do if a landslide occurs.
What to Do During Severe Storms, Which Can Trigger Landslides
• 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. Do not attempt
to evacuate them at the last minute.
• 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.
What to Do if You Suspect Imminent Landslide Danger
• 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 bestmprotection.
Take your pets with you when you leave,
provided you can do so without endangering yourself.
What to Do During a Landslide
• Quickly move out of the path of the landslide. Moving away from the path to a
stable
area will reduce your risk.
What to Do After a Landslide
• 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 home’s 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.
Media and Community Education Ideas
Help the reporters to localize the information by providing them
with the local emergency telephone number for the fire, police,
and emergency
medical services departments (usually 9-1-1) and emergency numbers for the local
utilities
and hospitals. Also
provide the business telephone numbers for the
local emergency management office, local American Red Cross chapter, and state
geological survey or department of natural resources.
• If your area is prone to landslides, ask your local newspaper or radio or television
station
to:
-Do a series on the dangers of landslides and debris flows.
-Do a story featuring interviews with local officials about land use management,
zoning regulations, and building codes for landslide safety.
-Highlight the importance of staying alert to land and rainfall conditions.
-Run public service ads about how to protect lives and property in a landslide.
-Report on what city and county governments are doing to reduce the possibility
of landslides.
• Work with officials of the local fire, police, and emergency medical services
departments;
utilities; hospitals; emergency management
office; and American Red Cross chapter to prepare and disseminate guidelines
for
people with mobility impairments about what to do
if they have to evacuate.
• 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.
Facts and Fiction
Fiction: Landslides are caused by the earth collapsing into a hole or a void.
Fact: Landslides exhibit vertical and horizontal movement down a slope,
and most are triggered by heavy rain and snowmelt,
earthquake shaking, volcanic
eruptions,
and gravity.
Fiction: Landslides are caused by human activities such as logging,
road construction, and farming on steep slopes.
Fact: Although human activities may cause landslides on unstable
slopes, most landslides are caused by natural forces or events,
such as heavy
rain and snowmelt, earthquake shaking, volcanic eruptions, and gravity.
Fiction: Landslides occur only on the West Coast.
Fact: California and the Pacific Northwest experience numerous landslides;
however, landslides also occur in most states and territories
in the United States.
The
Appalachian Mountain region on the East Coast, Puerto Rico, and Hawaii are highly
susceptible
to landslides.
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