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Public Advisory — Landslides
What to do: What to do if you suspect imminent landslide danger: What to do during a landslide: What to do after a landslide: |
| source: edited from Newsday News, Saturday, Janaury 8, 2005 |
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Landslides are rock, earth, or debris flows on slopes due to gravity. They can occur on any terrain given the right conditions of soil, moisture, and the angle of slope. Also known as mud flows, debris flows, earth failures, slope failures, etc., they can be triggered by rains, floods, earthquakes, and other natural causes, as well as human-made causes, such as grading, terrain cutting and filling, excessive development, etc. Although the term landslide is often used somewhat loosely to mean any fairly rapid movement of rocks and sediment downslope, it is actually more accurate to use the term mass wasting to refer to the wide variety of mass movement processes that wear away at the Earth’s surface.
There are three main factors that control the type and rate of mass wasting that might occur at the Earth’s surface: Slope gradient: The steeper the slope of the land, the more likely that mass wasting will occur. Slope consolidation: Sediments and fractured or poorly cemented rocks and sediments are weak, and more prone to mass wasting. Water: If slope materials are saturated with water, they may lose cohesion and flow easily. There are three basic types of mass wasting: Falls - rocks fall or bounce through the air Slides - rock and/or sediment slides along Earth’s surface Flows - sediment flows across Earth’s surface FALLS Because weathering is an ongoing process, steep mountain slopes are constantly wasting away, often in the form of rocks falling and bouncing down slopes. Such falls can be triggered by the growth of plants (and their roots), earthquakes, or by people hiking on the slope. Rock falls occur in just a matter of seconds, so they are difficult to observe. But, you can tell where rock falls occur on a mountain slope by looking for talus, a buildup.SLIDES Whenever a mass of slope material moves as a coherent block , we say that a slide has taken place. There are several types of slides, but one of the most common is a slump. A slump occurs when a portion of hillside moves downslope under the influence of gravity. A slump has a characteristic shape, with a scarp or cliff at the top of the slump, and a bulge of material (often called the toe of the slump) at the base of the slump. FLOWS We say that a flow has occurred if the material moving downslope is being transported as a very thick fluid (like a river of debris, rock, and/or soil), rather than as a coherent unit. Often, water is the primary transport agent for the flow. On hard, non-jointed bedrock that has not moved in the past If you suspect immediate landslide danger, contact the Office of Disaster Preparedness and Management (ODPM), your Regional Corporation, the Fire Services, or the police; inform affected neighbours; and leave the area quickly If you live in a hilly area, watch the patterns of storm-water drainage on slopes near your home, and note 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. During severe rainfall, or storms, stay alert and stay awake. Many landslide and debris flow fatalities occur when people are sleeping. Listen to a radio 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. 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. If you are near a river, be alert for any sudden increase or decrease in water flow. Such changes may indicate landslide activity upstream, so be prepared to move quickly. If you live in an area susceptible to landslides and debris flows, consider leaving if it is safe to do so. If you remain at home, move to a part of the house farthest away from the source of the landslide or debris flows, such as an upper floor, but keep an escape route open should it become necessary to leave the house. 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 landslides or debris flows. To safeguard your home against landslides: - Plant ground cover on slopes and build retaining walls. - In mudflow areas, build channels or deflection walls to direct the flow around buildings. Learn to recognise 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. - Bulging ground appears at the base of a slope. - 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. 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, have a plan for getting back together. Landslide Precautions DURING 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. AFTER - Stay away from the slide area. There may be danger of additional slides. - Check for injured and trapped persons near the slide area. Give first aid if trained. - Remember to help your neighbours who may require special assistance—infants, elderly people, and people with disabilities. - Listen to a battery-operated radio or television for the latest emergency information. - Remember that flooding may occur after a mudflow or a landslide. - Check for damaged power lines. Report any damage to the Trinidad and Tobago Electricity Commission (TTEC). - Check the building foundation and surrounding land for damage. - Replant damaged ground as soon as possible since erosion caused by loss of ground cover can lead to flash flooding. |
| source: edited from Newsday News, article entitles "Landslide alert", Thursday, August 19 2010 http://newsday.co.tt/features/0,126079.html |