But they can continue to travel over much gentler ground that slopes at only 10 degrees. How far a debris flow can travel depends on how much debris it carries compared to the volume of water. A debris flow often starts off as a translational slide, but the water and rubble mix as the slide moves downslope, forming a slurry that flows.
Debris flows often leave a trail of rubble in their wake, forming distinctive ridges or levees. Debris flows range in size from 1m to 10m across, and may carry up to several cubic metres of debris. One-off debris flows may occur on open hillsides, but repeated debris flows are more common below gullies.
Debris cones form where repeated debris flows build up. Many debris cones in the Highlands have periods of activity and quiet. To trigger them, suitable weather conditions are needed and enough mud and rock must have collected higher up in the gully floor.
Creep is the slow downslope movement of material under gravity. It generally occurs over large areas. Solifluction is a mixture of creep and flow, which forms distinctive sheets, terraces and lobes of debris and boulders.
Solifluction sheets and lobes are found on steeper slopes where the process has moved loosened boulders and soil downslope. Some smaller solifluction features are still active most winters. Such slumps and slides occur in very wet weather, when the near-surface soil and rock debris gets saturated with water, and slides and flows downslope. They can develop into more dangerous debris flows on steep ground, where the landslide may travel a long distance, churning up the debris into a slurry.
Cairngorms Landscapes. Managing Scotlands rocks and landforms to safeguard them for the future is an important part of conserving our natural heritage. Wave action can cut caves from any type of rock. Or acidic stream waters can dissolve carbonate rocks such as limestone to form either caves or karst. There are generally three types of creep: 1 seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature; 2 continuous, where shear stress continuously exceeds the strength of the material; and 3 progressive, where slopes are reaching the point of failure as other types of mass movements.
Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges fig. The dominant mode of movement is lateral extension accompanied by shear or tensile fractures. The failure is caused by liquefaction, the process whereby saturated, loose, cohesionless sediments usually sands and silts are transformed from a solid into a liquefied state.
Failure is usually triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced. When coherent material, either bedrock or soil, rests on materials that liquefy, the upper units may undergo fracturing and extension and may then subside, translate, rotate, disintegrate, or liquefy and flow. Lateral spreading in fine-grained materials on shallow slopes is usually progressive. The failure starts suddenly in a small area and spreads rapidly.
Often the initial failure is a slump, but in some materials movement occurs for no apparent reason. Combination of two or more of the above types is known as a complex landslide.
Geological causes a. Weak or sensitive materials b. Weathered materials c. Sheared, jointed, or fissured materials d. Adversely oriented discontinuity bedding, schistosity, fault, unconformity, contact, and so forth e. Morphological causes a. Tectonic or volcanic uplift b. Glacial rebound c. Fluvial, wave, or glacial erosion of slope toe or lateral margins d. Subterranean erosion solution, piping e. Deposition loading slope or its crest f. Vegetation removal by fire, drought g.
Thawing h. Freeze-and-thaw weathering i. 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. A good example of a rotational landslide is the Holbeck Hall landslide , in Scarborough, North Yorkshire.
Rotational slides occur on curved slip surfaces where the upper surface of the displaced material may tilt backwards toward the scarp. A translational or planar landslide is a downslope movement of material that occurs along a distinctive planar surface of weakness such as a fault, joint or bedding plane. Some of the largest and most damaging landslides on Earth are translational. These landslides occur at all scales and are not self-stabilising. 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?
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