They can be very rapid where discontinuities are steep. Translational landslides commonly trigger debris flows in Scotland, such as the Stob Coire Sgriodain landslide.
Translational slides occur on curved slip surfaces where the upper surface of the displaced material may tilt backwards toward the scarp. Flows are landslides that involve the movement of material down a slope in the form of a fluid.
Flows often leave behind a distinctive, upside-down funnel shaped deposit where the landslide material has stopped moving. There are different types of flows: mud, debris and rock rock avalanches. Two of the most common in the UK are mud flows and debris flows. Mud flows can be found on the south coast of England, often associated with larger complex landslides such as Stonebarrow Hill in Dorset.
Discovering Geology introduces a range of geoscience topics to school-age students and learners of all ages. The Earth beneath our feet is constantly shifting and moving, and violently with catastrophic and immediate results. Find out more about earth hazards. What is a landslide? Why do landslides happen?
How to classify a landslide. Landslides in the UK and around the world. How to classify a landslide Discovering Geology — Understanding landslides. Landslides are classified by their type of movement.
Figure 3. Landslides are exceptionally destructive. Homes may be destroyed as hillsides collapse. Landslides can even bury entire villages. Landslides may create lakes when the rocky material dams a stream. If a landslide flows into a lake or bay, they can trigger a tsunami figure 4.
Figure 4. The landslide into Lituya Bay, Alaska, created a m tsunami that knocked down trees at elevations higher than the Empire State Building light gray. Landslides often occur on steep slopes in dry or semi-arid climates. The California coastline, with its steep cliffs and years of drought punctuated by seasons of abundant rainfall, is prone to landslides. At-risk communities have developed landslide warning systems. Geological Survey use rain gauges to monitor soil moisture.
If soil becomes saturated, the weather service issues a warning. Hillside properties in the San Francisco Bay Area and elsewhere may be prone to damage from landslides.
Geologists are studying the warning signs and progress of local landslides to help reduce risks and give people adequate warnings of these looming threats.
You can learn more here. Figure 5. The white areas on green hillsides mark scars from numerous mudflows. Falls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs. Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling.
Falls are strongly influenced by gravity, mechanical weathering, and the presence of interstitial water. Added water creates natural hazards produced by gravity figure 5. Shrink-and-swell weathering. Human causes a. Excavation of slope or its toe b. Loading of slope or its crest c.
Drawdown of reservoirs d. Deforestation e. Irrigation f. Mining g. Artificial vibration h. Water leakage from utilities. Although there are multiple types of causes of landslides, the three that cause most of the damaging landslides around the world are these:.
Slope saturation by water is a primary cause of landslides. This effect can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and water-level changes along coastlines, earth dams, and the banks of lakes, reservoirs, canals, and rivers.
Landsliding and flooding are closely allied because both are related to precipitation, runoff, and the saturation of ground by water. In addition, debris flows and mudflows usually occur in small, steep stream channels and often are mistaken for floods; in fact, these two events often occur simultaneously in the same area.
Landslides can cause flooding by forming landslide dams that block valleys and stream channels, allowing large amounts of water to back up. This causes backwater flooding and, if the dam fails, subsequent downstream flooding. Also, solid landslide debris can "bulk" or add volume and density to otherwise normal streamflow or cause channel blockages and diversions creating flood conditions or localized erosion.
Many mountainous areas that are vulnerable to landslides have also experienced at least moderate rates of earthquake occurrence in recorded times. The occurrence of earthquakes in steep landslide-prone areas greatly increases the likelihood that landslides will occur, due to ground shaking alone or shaking-caused dilation of soil materials, which allows rapid infiltration of water.
The Great Alaska Earthquake caused widespread landsliding and other ground failure, which caused most of the monetary loss due to the earthquake. Other areas of the United States, such as California and the Puget Sound region in Washington, have experienced slides, lateral spreading, and other types of ground failure due to moderate to large earthquakes. Widespread rockfalls also are caused by loosening of rocks as a result of ground shaking.
Worldwide, landslides caused by earthquakes kill people and damage structures at higher rates than in the United States. Landslides due to volcanic activity are some of the most devastating types. Volcanic lava may melt snow at a rapid rate, causing a deluge of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volcanoes, devastating anything in its path.
These volcanic debris flows also known as lahars reach great distances, once they leave the flanks of the volcano, and can damage structures in flat areas surrounding the volcanoes. The eruption of Mount St. Helens, in Washington triggered a massive landslide on the north flank of the volcano, the largest landslide in recorded times. Vulnerability to landslide hazards is a function of location, type of human activity, use, and frequency of landslide events. The effects of landslides on people and structures can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity.
Local governments can reduce landslide effects through land-use policies and regulations. Individuals can reduce their exposure to hazards by educating themselves on the past hazard history of a site and by making inquiries to planning and engineering departments of local governments. They can also obtain the professional services of an engineering geologist, a geotechnical engineer, or a civil engineer, who can properly evaluate the hazard potential of a site, built or unbuilt.
The hazard from landslides can be reduced by avoiding construction on steep slopes and existing landslides, or by stabilizing the slopes. Stability increases when ground water is prevented from rising in the landslide mass by 1 covering the landslide with an impermeable membrane, 2 directing surface water away from the landslide, 3 draining ground water away from the landslide, and 4 minimizing surface irrigation. The U. Geological Survey Landslide Program has information, publications, and educational information on its Web site.
For general information about slides, debris flows, rock falls, or other types of landslides in your area, contact your city or county geology or planning office.
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