text from usgs, modified and supplemented by byu-idaho faculty
TRANSCRIPT
Text from USGS, modified and supplemented by BYU-Idaho faculty
Portugese Bend landslide complex, Palos Verdes peninsula, southern California (BYUI)
Learning Objectives Landslides are common problems across the country and in many parts of the world. Next to flooding, they are the most likely natural hazard you may have to deal with in your lifetime. Your goals for studying this chapter are:
• Understand where landslides occur.
• Understand the warning signs of landslides.
• Understand what to do in the event of a landslide.
• Understand the basic types of landslides and related
terminology.
• Understand the geologic and man-made causes of
landslides.
• Understand landslide mitigation measures and how they
are best applied.
(Utah Geological Survey)
Where do Landslides Occur? (USGS)
Landslides occur in every state and U.S. territory. The Appalachian Mountains, the Rocky Mountains and the Pacific
Coastal Ranges and some parts of Alaska and Hawaii have severe landslide problems. Any area composed of very weak
or fractured materials resting on a steep slope can and will likely experience landslides.
Although the physical cause of many landslides cannot be removed, geologic investigations, good engineering practices,
and effective enforcement of land-use management regulations can reduce landslide hazards.
USGS scientists continue to produce landslide susceptibility maps for many areas in the United States. In every state,
USGS scientists monitor streamflow, noting changes in sediment load carried by rivers and streams that may result from
landslides. Hydrologists with expertise in debris and mud flows are studying these
Why Study Landslides?
Landslides are a serious geologic hazard common to
almost every State in the United States. It is estimated
that in the United States, they cause in excess of $1
billion in damages and from about 25 to 50 deaths each
year. Globally, landslides cause hundreds of billions of
dollars in damages and hundreds of thousands of deaths and
injuries each year.
The National Landslide Information Center, developed by
the U.S. Geological Survey, is a multifunctional enterprise
dedicated to collection and distribution of all forms of
information related to landslides. It is designed to serve
landslide researchers, geotechnical practitioners engaged in
landslide stabilization, and all other people and entities
concerned in any way with landslide hazards analysis and
mitigation. Palos Verdes peninsula, southern California (BYUI)
Go to USGS http://landslides.usgs.gov/state_local/nationalmap/ for the interactive version of this map.
What is a Landslide?
The term landslide (or mass wasting, or mass movement) includes a wide range of ground movement such as rockfalls,
rock avalanches, slumps, mud flows, and debris flows. Although gravity acting on an over-steepened slope is the primary
reason for a landslide, there are other contributing factors:
-erosion by rivers, glaciers, or ocean waves create over-steepened slopes
-rock and soil slopes are weakened through saturation by snowmelt or heavy rains
-earthquakes create stresses that make weak slopes fail
-volcanic eruptions produce loose ash deposits and lahars
-excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made
structures may stress weak slopes to failure and other structures
Landslide Preparedness
Landslide Warning Signs
• Springs, seeps, or saturated ground in areas that have not typically been wet before.
• New cracks or unusual bulges in the ground, street pavements or sidewalks.
• Soil moving away from foundations.
• Ancillary structures such as decks and patios tilting and/or moving relative to the main
house.
• Tilting or cracking of concrete floors and foundations.
• Broken water lines and other underground utilities.
• Leaning telephone poles, trees, retaining walls or fences.
• Offset fence lines.
• Sunken or down-dropped road beds.
• Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content).
• Sudden decrease in creek water levels though rain is still falling or just recently stopped.
• Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb.
• A faint rumbling sound that increases in volume is noticeable as the landslide nears.
• Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
(USGS)
Areas that are generally prone to landslide
hazards
• On existing old landslides.
• On or at the base of slopes.
• In or at the base of minor drainage hollows.
• At the base or top of an old fill slope.
• At the base or top of a steep cut slope.
• Developed hillsides where leach field septic systems are used.
Areas that are typically considered safe from landslides
• On hard, unfractured bedrock that has not moved in the past.
• On relatively flat-lying areas away from sudden changes in slope angle.
• At the top or along the nose of ridges, set back from the tops of slopes.
El Salvador (USGS)
What To Do Before a Landslide
• Do not build near steep slopes, close to mountain edges, near drainage
ways, or natural erosion valleys.
• Get a ground assessment of your property.
• Contact local officials, state geological surveys or departments of
natural resources, and university departments of geology. Landslides
occur where they have before, and in identifiable hazard locations. Ask
for information on landslides in your area, specific information on
areas vulnerable to landslides, and request a professional referral for a
very detailed site analysis of your property, and corrective measures
you can take, if necessary.
• Watch the patterns of storm-water drainage near your home, and note
the places where runoff water converges, increasing flow in channels.
These are areas to avoid during a storm.
• Learn about the emergency-response and evacuation plans for your area. Develop your own emergency plan for your family or business.
• Minimize home hazards:
-Have flexible pipe fittings installed to avoid gas or water leaks, as flexible fittings are more resistant to breakage (only the gas company
or professionals should install gas fittings).
-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
flow and the flow lands on a neighbor's property, you may be liable for damages.
(USGS)
What To Do During a Landslide
• Stay alert and awake. Many debris-flow fatalities occur when people
are sleeping. Listen to a NOAA Weather Radio or portable, battery-
powered radio or television for warnings of intense rainfall. Be
aware that intense, short bursts of rain may be particularly
dangerous, especially after longer periods of heavy rainfall and
damp weather.
• If you are in areas susceptible to landslides and debris flows,
consider leaving if it is safe to do so. Remember that driving during
an intense 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 saves lives.
• 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 larger landslides.
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. Don't delay! Save yourself, not your
belongings.
• Be especially alert when driving. Bridges may be washed out, and
culverts overtopped. Do not cross flooding streams!! Turn Around,
Don't Drown®!. Embankments along roadsides are particularly
susceptible to landslides. Watch the road for collapsed pavement,
mud, fallen rocks, and other indications of possible debris flows.
• Be aware that strong shaking from earthquakes can induce or
intensify the effects of landslides. (USGS)
(USGS)
What to Do if You Suspect Imminent Landslide Danger
• Contact your local fire, police, or public works department. Local officials are the best persons able to assess
potential danger.
• Inform affected neighbors. Your neighbors may not be aware of potential hazards. Advising them of a potential
threat may help save lives. Help neighbors who may need assistance to evacuate.
• Evacuate. Getting out of the path of a landslide or debris flow is your best protection.
• Curl into a tight ball and protect your head if escape is not possible.
(USGS)
What To Do After a Landslide
• 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.
• Help a neighbor who may require special assistance - infants, elderly people,
and people with disabilities. Elderly people and people with disabilities may
require additional assistance. People who care for them or who have large
families may need additional assistance in emergency situations.
• 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.
(USGS)