WHAT IS A LANDSLIDE?
WHAT IS A LANDSLIDE?
WHAT IS A LANDSLIDE?
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 do we mean by the word "landslide"?
• Landslide is a general term used to describe
the down-slope movement of soil, rock and
organic materials under the influence of gravity.
It also describes the landform that results.
• As our cities, towns, roads and highways
steadily encroach onto steeper slopes and
mountainsides, landslide hazards become an
increasingly serious threat to life and property.
However, by understanding the answers to the
following questions, we may be able to lessen
the effects of landslides .
WHAT IS A LANDSLIDE?
• Landslides are a serious geologic hazard common to almost every
state in the United States. It is estimated that nationally they cause
up to $2 billion in damages and from 25 to 50 deaths annually.
Globally, landslides cause billions of dollars in damage and
thousands of deaths and injuries each year. Individuals can take
steps to reduce their personal risk. Know about the hazard potential
where you live, take steps to reduce your risk, and practice
preparedness plans.
• 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 the force driving landslide
movement. Factors that allow the force of gravity to overcome the
resistance of earth material to landslide movement include:
saturation by water, steepening of slopes by erosion or construction,
alternate freezing or thawing, earthquake shaking, and volcanic
eruptions.
• Landslides are typically associated with periods of heavy rainfall or
rapid snow melt and tend to worsen the effects of flooding that often
accompanies these events. In areas burned by forest and brush fires,
a lower threshold of precipitation may initiate landslides..
WHAT IS A LANDSLIDE?
• 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
intense rainfall or rapid snow melt. They usually start on steep
hillsides as shallow landslides that liquefy and accelerate to
speeds that are typically about 10 miles per hour, but can
exceed 35 miles per hour. The consistency of debris flows
ranges from watery mud to thick, rocky mud that can carry
large items such as boulders, trees, and cars. Debris flows
from many different sources can combine in channels, and
their destructive power may be greatly increased. They
continue flowing down hills 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.
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.
Landslide and mudflows
• Landslide and mudflows usually strike without
warning. The force of rocks, soil, or other
debris moving down a slope can devastate
anything in its path. Take the following steps
to be ready.
What causes landslides?
Landslide Types
• Landslides or slope movements can be
classified in many ways. There are many
attributes used as criteria for identification and
classification including: Rate of movement:
This
ranges
from
very
slow
creep
(millimetres/year)
to
extremely
rapid
(metres/second). Type of material: Landslides
are composed of bedrock, unconsolidated
sediment and/or organic debris. Nature of
movement: The moving debris can slide, slump,
flow or fall.
Landslide Types
Common landslide types are:
Slide: movement parallel to
planes of weakness and
occasionally parallel to slope .
Creep: gradual movement
of slope materials
Common landslide types are:
Slump: complex movement of
materials on a slope; includes
rotational slump .
Topple: the end-over-end
motion of rock down a slope
Common landslide types are:
Fall: material free falls .
Flow: viscous to fluid-like motion
of debris Flow: viscous to fluidlike motion of debris
Common landslide types are:
Torrent::a sporadic and sudden channelized discharge of water and debris .
The Vaiont Landslide in the Dolomites
(The Dolomites are a mountain range in Northern Italy)
The Vaiont Landslide in the Dolomites
(The Dolomites are a mountain range in Northern Italy)
This map shows the scale of the
landslide, which slopped the contents
of the reservoir over the dam and
killed more than 2000 people in
Longarone
and
the
adjoining
communities
What should you do before the landslide?
How to Protect Your Property ?
Get a ground assessment of your property.
• Your county geologist or county planning department may
have specific information on areas vulnerable to landsliding.
Consult a professional geotechnical expert for opinions and
advice on landslide problems and 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 flow and the flow lands
on a neighbor's property, you may be liable for damages.
What should you do before the landslide?
• Learn to recognize the landslide warning signs.
– Doors or windows stick or jam for the first time.
– New cracks appear in plaster, tile, brick, or foundations.
– Outside walls, walks, or stairs begin pulling away from the
building.
– Slowly developing, widening cracks appear on the ground
or on paved areas such as streets or driveways.
– Underground utility lines break.
– Bulging ground appears at the base of a slope.
– Water breaks through the ground surface in new locations.
– Fences, retaining walls, utility poles, or trees tilt or move.
– You hear a faint rumbling sound that increases in volume
as the landslide nears.The ground slopes downward in one
specific direction and may begin shifting in that direction
under your feet.
What should you do before the landslide?
• Make evacuation plans.
Plan at least two evacuation routes since roads may become
blocked or closed.
• Develop
an
emergency
communication
plan.
In case family members are separated from one another during
a landslide or mudflow this is (a real possibility during the day
when adults are at work and children are at school), have a
plan for getting back together.
• Ask an out-of-state relative or friend to serve as the "family
contact". After a disaster, it's often easier to call long distance.
Make sure everyone knows the name, address, and phone
number of the contact person.
• Insurance
• Mudflow is covered by flood insurance policies from the
National Flood Insurance Program. Flood insurance can be
purchased through a local insurance agency.
What should you do during the landslide?
• If inside a building:
– Stay inside.
– Take cover under a desk, table, or other piece of sturdy furniture.
• If outdoors:
– Try and get out of the path of the landslide or mudflow.
– Run to the nearest high ground in a direction away from the path.
– If rocks and other debris are approaching, run for the nearest
shelter such as a group of trees or a building.
– If escape is not possible, curl into a tight ball and protect your
head.
• Sinkholes
– A sinkhole occurs when groundwater dissolves a vulnerable land
surface such as limestone, causing the land surface to collapse
from a lack of support. In June 1993, a 100-foot wide, 25-foot
deep sinkhole formed under a hotel parking lot in Atlanta, killing
two people and engulfing numerous cars.
What should you do during the 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. Embankments along roadsides are
particularly susceptible to landslides. Watch the road for collapsed
pavement, mud, fallen rocks, and other indications of possible debris flows.
What should you do after the landslide?
• Stay away from the slide area. There may be danger of
additional slides.
• 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.
• 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.
What should you do after the landslide?
• 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 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.
• Seek the advice of 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.
What should you do after the landslide?
Landslide and mudflows
Mitigation
• Mitigation includes any activities that prevent an
emergency, reduce the chance of an emergency
happening, or lessen the damaging effects of
unavoidable
emergencies.
Investing
in
preventive mitigation steps now such as planting
ground cover (low growing plants) on slopes, or
installing flexible pipe fitting to avoid gas or
water leaks, will help reduce the impact of
landslides and mudflows in the future. For more
information on mitigation, contact your local
emergency management office.
How can we minimize the effects of landslides?
• The Role of the Geoscientist and Engineer:
Landslide risk can be reduced with engineering and
geoscience investigations that lead to improved
community bylaws and zoning. The British Columbia
Municipal Act contains legal requirements that deal
with landslides and other hazards. By geological
mapping, detecting slope hazards and determining the
likelihood of landslide occurrence, geoscientists can
assist engineers, developers, planners and building
inspectors in avoiding high-risk areas. Through this
process, structures such as homes, schools, hospitals,
power-lines, fire stations and roads can be safely
located away from potential landslide risk areas.
How can we minimize the effects of landslides?
• Your Role:
• Learn more about your local geology and the
potential geologic hazards in your area. Consult an
engineer or geoscientist with relevant experience in
landslide work for advice or information on specific
hazards and potentially hazardous locations. Avoid
activities that result in the undercutting of steep
banks, and avoid the construction of buildings near
the top or base of steep slopes. Do not place fill on
steep slopes. Request information and assistance
from your municipal or district planner or building
inspector prior to land purchase, prior to subdividing, and prior to construction applications.
How can we minimize the effects of landslides?
Property damage in the Qualicum Beach area after a landslide (photo
courtesy of the Ministry of Environment, Lands and Parks).
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 lost 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 do landslides affect us?
•
A debris torrent at Britannia Creek in
August of 1991 caused extensive
damage to property (photo courtesy of
K. Fletcher).
In 1991, a rock slide near Loggers
Creek closed the Sea-to-Sky
Highway for 12 days and cost $7
million for repairs and preventative
structures (photo courtesy of the
Ministry of Transportation and
Highways).
Landslide losses
The Vancouver to Squamish highway
has been affected by 14 major debris
torrents since 1906. Twelve lives have
been lost and 11 bridges, 4 homes and
numerous structures have been
damaged or destroyed. Not one of
these 14 slide events was larger than
20 000 cubic metres (photo courtesy
of the Ministry of Transportation and
Highways).
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.
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.
FEATURES THAT MAY INDICATE CATASTROPHIC
LANDSLIDE MOVEMENT
• What to do and look for during and
immediately after heavy rains ?
• What to Do Before Intense Storms
What to Do Before Intense Storms ?
• Become familiar with the land around you. Learn whether
landslides and debris flows have occurred in your area by
contacting local officials, state geological surveys or
departments of natural resources, and university departments
of geology. Knowing the land can help you assess your risk for
danger.
• 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. Watching small changes could alert
you to the potential of a greater landslide threat.
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, non-jointed 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.
Features that might be noticed prior to
major landsliding
•
•
•
•
•
•
•
•
•
•
•
•
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.
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.
For further information on landslides in
your area
• Contact your County Geologist--Some cities also
staff geologists.
• Contact your State Geologist or Engineer
• If a very detailed site analysis is desired, contract
with a private consulting company specializing in
earth movement Such companies would likely be
those specializing in geotechnical engineering,
structural engineering, or civil engineering. Your,
Local, state, or county geologist could possibly advise
you as to the best kind of professional to contact.
How The Public Can Help After A Disaster?
•
•
•
•
•
When disaster strikes, people everywhere want to help those in need. To
ensure that this compassion and generosity are put to good use, the media
can highlight these facts:
Financial aid is an immediate need of disaster victims. Financial
contributions should be made through a recognized voluntary organization
to help ensure that contributions are put to their intended use.
Before donating food or clothing, wait for instructions from local officials.
Immediately after a disaster, relief workers usually don't have time or
facilities to setup distribution channels, and too often these items go to
waste.
Volunteers should go through a recognized voluntary agency such as the
American Red Cross or Salvation Army. They know what is needed and
are prepared to deal with the need. Local emergency services officials also
coordinate volunteer efforts for helping in disasters.
Organizations and community groups wishing to donate items should first
contact local officials, the American Red Cross, or Salvation Army to find
out what is needed and where to send it. Be prepared to deliver the items to
one place, tell officials when you'll be there, and provide for transportation,
driver, and unloading.
LANDSLIDE AVOIDANCE
Landslide Deflection
• Rockfalls (above) and rockslides (below - this is a slide of rock
debris) are an ever present danger in areas of high relief. It such
terrain, it is impossible to regrade slopes to stable angles. The scale
of landsliding may be too great to do anything about its causes - we
can only try to reduce its effects.
• In the diagram above we have the major elements of a rockfall.
There is a source area, in this case a layer of jointed rock. The joints
open - why?
• Several mechanisms suggest themselves. Firstly, water - or ice - in
the joints can cause pressures. Secondly, if the rock warms in the
sun, the mass expands and joints open. Any dirt or gravel which
falls into the joints stops them closing when the rock next cools.
Thirdly, the stratum could be undermined. This could be because of
erosion. It could be that the underlying rock softens or weathers.
The
mass
then
collapses.
We call the accumulation of rockfall debris a scree or talus. Screes
form at F=1, and are thus incredibly easy to destabilise - but that is a
different problem .
Landslide Deflection
• One approach to the problem from rockfalls or debris slides is
to construct a concrete structure to deflect rockslide or rockfall
debris. They are widely employed in mountainous areas to
protect both roads and railways. I would like to see an example
protecting a canal! The debris is deflected over the route to be
protected. The roof is thick reinforced concrete, and it is made
with a slope to throw off the debris. Sometimes a layer of
crushed rock or gravel is left on the roof. This protects it
against impact from large blocks. An alternative, where impact
on the roof is a problem, is to use old tyres to absorb the
impact .....
• NB. The photos are higher resolution image than I have
displayed them. If you right-click on them, you get the chance
to view (or save) at higher resolution.
• The examples are all from Taiwan
In the town of Lamosano, threatened by the Tessina landslide, a
concrete flume has been built to carry the landslide through the village!
Water can be injected into the base of the landslide to keep it moving.
This is the flume.