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29 Cards in this Set
- Front
- Back
Mass movement |
Large-scale movement of the Earth’s surface that is not accompanied by a moving agent such as a river, glacier or waves Move under the force of gravity On different time and spatial scales More common when heavy precipitation |
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Sheer strength and sheer stress |
Shear strength - the internal resistance of a body to movement Shear stress - the force acting on a body that causes movement of the body down slope |
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Slope failure causes
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An increase in shear stress - forces to pull a mass down slope A decrease in shear strength - resistance to movement Critical threshold when stress outweighs strength |
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Factors holding slopes in place |
Friction and weight of particles Cohesive forces e.g. clay Vegetation roots bind the soil Human structures e.g. nets |
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Physical factors that increase shear stress |
Removal of support from undercutting or steepening - erosion from rivers and glaciers, wave action, previous mass movements Sudden shocks - earthquakes Loading of the slope weight - vegetation, water, sediment |
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Human factors that increase shear stress |
Adding weight to slopes - buildings and walking Steepening slopes - cuttings, quarries, adding waste |
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Physical factors that decrease shear strength |
Weathering - disintegration of rocks, hydrated of clay Drainage - wet areas on the slope saturation Rock type - tilt, impermeable rocks Animals - burrowing and walking |
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Human factors that decrease shear strength |
Removing vegetation - less roots to hold slope Drainage - adding water to the ground to saturate it |
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Water |
Many movements happen when increased water Weaken the slope by increasing shear stress Adds weight and the slope becomes saturated Water reduces the cohesion of particles by saturation Water decreases the amount of friction so decreases shear strength Increased volume of water Heavy or prolonged rain Rising water tables |
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Types of mass movement |
By speed - fast or slow By types of movement - slide or creep or flow By type of material - rock or soil or ice By water content - wet or dry |
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Slow movements |
Creep Slow and continual process Soil and low water content Results in terracettes Solifluction Slow downhill flow Saturated soil so high water content Periglacial environments Solifluction lobes |
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Flows |
Mudflow Soil or weak rock becomes saturated and begins to move downhill Can be slow but mainly rapid After heavy rainfall Volcanic hazards - melted snow and ice can combine with ash to form lahars |
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Difference between slides and flows |
Flows undergo internal derangement Slides move together and are not affected by internal derangement |
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Rapid movements |
Landslides Sudden movement of material downslope due to gravity Range of materials and water contents In contact with the ground and most are along a bedding plane with impermeable rock below so water lubricates the slide Rock falls Spontaneous debris movement on steep slopes From extreme physical or chemical weathering Travel through the air From scree slope below Slumps Rotational slides Rock rotates around a slip plane in the arc of a circle Plane is concave in shape Most common in clay where becomes saturated and slips |
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Slip plane locations |
Junction of two layers At a fault line Where there is a joint Along a bedding plane At the point where shear stress becomes greater than shear strength |
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Environmental impacts of mass movements |
Relief - reduces slope angle, fills in valleys, adds bulges Drainage - may dam or divert rivers Vegetation - trees lean or fall Soil - collects at the base of the slope |
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Social impacts of mass movements |
Buildings collapse Disasters e.g. Aberfan Loss of life |
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Economic impacts of mass movements |
Transport - road and rail destroyed, disruption and cost of repair Power and phone lines lean or fall - disruption and cost of repair Loss of farmland Damage to structures - buildings and bridges Quarrying and mining disasters |
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Short-term management of mass movements |
Plant vegetation to bind the slope and dry it out Stop walking on the slope to reduce stress |
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Long-term management of mass movements |
Reduce pressure on top of the cliff - limit building Reduce processes undercutting the cliff - revetements, sea wall Change slope angle - regrade it to gentler slope Reduce moisture on the slope - drains |
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CS - Aberfan disaster |
21st October 1966 A coal waste tip slide down a mountainside into the mining village of Aberfan in South Wales |
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Aberfan causes |
The spoil heap was located above a spring and at a steep angle No management of the tip Little vegetation in the area to bind to the waste Prolonged heavy rainfall 100,000 cubic metres of waste travelling up to 30km/hr Couldn’t alarm them as phone cable was stolen |
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Aberfan impacts |
147 killed - 116 school children Destroyed the primary school called Pantglas Junior school and 20 houses Loss of a generation Huge psychological impact Cost of clean up |
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Aberfan short-term responses |
Emergency rescue services 100s of people helped in the search for people but no one rescued alive after 11am and took 1 week to find all the bodies |
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Aberfan long-term responses |
Other tips checked and slope angles reduced 1969 Mine and Quarry Act passed to control the siting of tips Over £20 million in donations Expensive clean-up of £2 million No prosecutions |
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CS - Holbeck Hall |
3-5th June 1993 Landslip along the upper section of the boulder clay at the 60m cliffs at Start Bay, Scarborough Caused the destruction of 4 star Holbeck Hall Hotel Constructed 1883 One of the most rapidly retreating areas in Britain Succession of droughts made the area unstable - the boulder clay had become dry and cracked in previous years and then saturated by the rains in early spring and summer - 140mm of rain in the 2 months before The saturated clay became unstable and slumped along the slip plane causing an earth flow at the base of the slump Cut the cliff by 70m Cracks appeared 6 weeks before the main failure and garden had a minor movement so closed the cliff paths below the hotel Affected tourism in the area as the remains were demolished, owners of the hotel lost money and land Expensive painting lost from inside the hotel |
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CS - Venezuelan mudslides |
15-16th Dec 1999 Worst disaster in the country for 200 years Avalanche of rocks and mud began to pour down the 2000m high Mount Avila burying large parts of the 300km stretch of Central Coast The rains triggered a series of mudslides, landslides and flash floods Took the lives of 10,000 to 50,000 people in the narrow strip of land from the mountains to the Caribbean sea Worst hit was state of Vargas - countless mountainside slum dwellings were buried or swept out to sea Mudslides between 8m - 10m deep |
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Venezuelan mudslides - physical causes |
Northern venezuela has very high mountains over 2000 m just 10km from the sea Seaward facing slopes are very steep - rapid runoff and high energy streams Impermeable rocks which have been deeply weathered Precipitation was 40-50% higher than normal |
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Venezuelan mudslides - human causes |
Severe lack of preparation or education Corrupt politicians and planners allowed shanty towns to grow on steep valleys and near the coast and capital of Caracas Widespread deforestation - increased runoff and erosion |