A landslide is the movement of rock, debris or earth down a slope. They result from the failure of the materials which make up the hill slope and are driven by the force of gravity. Landslides are known also as landslips, slumps or slope failure.
Some of the most common types of landslide in Afghanistan are earth slides, rock falls and debris flows. The movement of landslide material can vary from abrupt collapses to slow gradual slides and at rates which range from almost undetectable to extremely rapid. Sudden and rapid events are the most dangerous because of a lack of warning and the speed at which material can travel down the slope as well as the force of its resulting impact. Extremely slow landslides might move only millimeters or centimeters a year and can be active over many years. Although this type of landslide is not a threat to people they can cause considerable damage to property.
Landslides can be triggered by natural causes or by human activity. They range from a single boulder in a rock fall or topple to tens of millions of cubic meters of material in a debris flow.
They can also vary in their extent, with some occurring very locally and impacting a very small area or hill slope while others affect much larger regional areas. The distance travelled by landslide material can also differ significantly with slides travelling from a few centimeters to many kilometers depending on the volume of material, water content and gradient of the slope.
The basic types of landslide movement are:
This is generally characterized by a rapid to extremely rapid rate of movement with the descent of material characterized by a freefall period. Falls are commonly triggered by earthquakes or erosion processes.
This is characterized by the tilting of rock without collapse, or by the forward rotation of rocks about a pivot point. Topples have a rapid rate of movement and failure is generally influenced by the fracture pattern in rock. Material descends by abrupt falling, sliding, bouncing and rolling.
This is the most destructive and turbulent form of landslide. Flows have a high water content which causes the slope material to lose cohesion, turning it into a slurry. They are channeled by the landscape and move rapidly.
This is one of the most common forms of failure and can be subdivided into translational and rotational slides. Rotational slides are sometimes called slumps because they move with rotation. Translational slides have a planar, or two dimensional surface of rupture. Slides are most common when the toe of the slope is undercut. They have a moderate rate of movement and the coherence of material is retained, moving largely intact or in broken pieces.
This phenomenon is characterized by the gradual lateral displacement of large volumes of distributed material over very gentle or flat terrain. Failure is caused by liquefaction which is the process when saturated loose sediment with little or no cohesion such as sands or silts are transformed into a liquid-like state. This process is triggered by rapid ground motion most commonly during earthquakes.
What to do before a landslide.?
Minimizing the risks from landslides
Landslide risk can be minimized by various methods, including:
- With expert input and careful planning, communities can identify unstable slopes and restrict or control development in hazard zones.
- For communities that are already established, the municipal or provincial authorities must consider whether protective engineering measures or buy-outs and moving of people and buildings should be undertaken.
How to protect your home against landslides?
Although landslides usually occur without warning, understanding this natural hazard and following some sensible rules can help to protect your family and home.
- Learn about your local geology and the potential for landslides in your area.
- Avoid actions that would increase instability. For example, do not undercut a steep bank; do not build near the top or base of steep slopes; do not place fill on steep slopes; do not drain pools or otherwise increase water flow down steep slopes.
- Learn how to recognize signs of potential failure in your locality. Examples include slope cracks, slope bulges, unusual seepage of water on the slope, and small rock or sediment falls.
- Know who to notify if you recognize these signs (e.g. municipal emergency contact numbers and municipal engineers).
What to do during landslide.?
- Find cover in the section of the building that is furthest away from the approaching landslide.
- Take shelter under a strong table or bench.
- Hold on firmly and stay put until all movement has ceased.
- Move quickly away from its likely path, keeping clear of embankments, trees, power lines and poles.
- Stay away from the landslide. The slope may experience additional failures for hours to days afterwards.
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.