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Mass Movement - Management, Case Studies, Definitions

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Lizzie Ollerenshaw

on 9 June 2014

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Transcript of Mass Movement - Management, Case Studies, Definitions

Open Cast Gold Mining Bolivia - Inland
The felling of trees to grow maize and vegetables has exposed the soil and caused the top layers to be washed away by the rain, exposing the impermeable, infertile rock.
A landslide took place in Llipi in December 1992 when thousands of tonnes of soil fell as a result of heavy rainfall. Homes were destroyed and many people were buried, out of the 1200 people living there, few survived.
Cherry Hills
On the 3rd of August 1999 a landslide occurred in Cherry Hills, Philippines as a result of over 500mm of rain falling in 3 days, this broke a 35 year record, due to the oncoming Typhoon Olga. The unstable rocks and rubble combined with the high saturation levels from the heavy rainfall, fell down the 20% incline and destroyed many homes and roads.
The poor area had unsatisfactory building quality and so under the force of the falling earth many houses were completely destroyed. The trenches dug in between houses to allow water to drain away caused water to get into the foundations of the houses and so they subsided and collapsed. Quarrying and the removal of trees and soil in the area is also partly to blame for the disaster.
In the end, 59 people were killed and 32 injured, there were 379 houses destroyed and over 120 families affected.
Despite cracks appearing in the wall for up to 6 months before the disaster, the residents were given 4 hours warning to evacuate the area and the rescue teams lacked adequate equipment to deal with the landslide.

Inland mass movement can cause river erosion leading to slumping, Moffat, Scotland. Sometimes a mass movement will dam a river. A lake will form behind the dam. When the dam bursts, the lake will drain causing catastrophic flooding.
Rural Case Studies!
Rivers Case Study!
Urban/HIC Case Studies!
A coal mining town was devastated by a landslide that enveloped a primary school, killing 116 children and 28 adults. The steep sided valleys meant that the force of gravity had a large impact on the landslide. There was a river running underneath the pile of waste material that made it unstable, this as well as the large amounts of rainfall, saturated the debris and caused it to fall.
As a result of this landslide new legislations have been put in place to prevent similar disasters from happening.

Landslide in Aberfan, South Wales in 1966
Dorset Coast - 2013
A wide range of landslides, both large and small. The coast is unstable because there are numerous situations where porous strata (chalk and sandstone) lie over impermeable clays. The rock became saturated with water from the sea and is also exposed to the elements, wave erosion and freeze thaw weathering occur on coastlines.
Rain water sinks through the porous rocks but once it reaches the underlying clays it can sink no further. The water builds up along the junction between the rock layers and seeps out of the cliffs as a series of springs. After periods of prolonged rainfall, the build up of water increases the weight of the cliff top. Increased poor pressure reduces the friction and allows large sections of the cliff top to break away.
Coastal Case Study!
Mass movement is the movement of surface material caused by gravity.
Of course geological agents such as water, wind and ice all work with gravity to cause a leveling of land. Water adds weight to the soil; it fills pore spaces of slope material and it exerts pressure which tends to push apart individual grains. This decreases the resistance of the material to movement.
Landslide is a general term that is commonly broken down into the more specialized terms such as slump, rock slide, mudflow and earth flow.
Soil Creep
This is the slowest form of mass movement and is caused by particles of soil freezing and thawing and moving down a shallow slope or by the wind. Soil creep can also be increased by human activity such as walking. This is not hazardous to humans and therefore there are no strategies in place to manage it
A slump is a downward and outward movement of rock or unconsolidated material moving as a unit or series of units. Large blocks of material move suddenly downward and outward along a curved plane.

There are four basic types of landslide, categorised by the main method of movement downhill.

A landslide may be -

Landslides occur when the line of fracture occurs along a shear plane which is roughly parallel with the ground surface. They can consist of both soil and rock mineral.
If the hill is not too steep, vegetation may prevent the moving mass from breaking up.
Almost all landslides occur on natural hill slopes are slides
Rock Fall
This is a rapid movement that usually occurs on the steepest slopes, they are the most catastrophic type of landslide. Individual rock fragments or slabs of rock suddenly become detached and fall to the base of the slope. They may be detached by a gradual process such as freeze thaw weathering or by sudden and dramatic events such as tectonic activity. Angular debris collects at the base of the slope and form scree.
Rotations take place where the cliff top block subsides, it rotates along the slip plane within the cliff, resulting in the flat surface tipping back towards the cliff. This causes the material to topple. Topples are instances when blocks of rock pivot and fall away from a slope
These include avalanches, mudflows, debris flows, earth flow and lahars. Water, air and ice are often involved in enabling fluid like motion of the material. Mud flows occur when the water content in the rock causes it to become lubricated and travel downhill by gravity. Mud flows happen after periods of heavy rainfall on unconsolidated material. Lahars are associated with volcanoes and are mudflows containing lava and extremely heated rocks, these have a devastating effect on the area which they cover.

Landslides and rock falls occur on steeper slopes as a result of gravity, on shallow slopes processes like soil creep are the more common forms of mass movement
Unconsolidated material that is loosely held together is more likely to fall compared to consolidated material which is densely compacted and less likely to fall

Absorbent rocks such as chalk soak up water adding to their weight and mass, this addition of water can cause landslides and mudslides due to gravity
The climate of an area will affect the types of mass movement that occur there, for example heavy rain and melt water will add to the volume of the soil and cause slope failure whereas frost may cause rock falls. Increase in rain could also lead to more powerful rivers and an increase in erosional processes that lead to overhang and slumping into rivers.
Tectonic Activity
Some of the steepest slopes are found in tectonically active areas which result from the gradual uplift following plate collision. Earthquakes may also trigger slope failure.
Weathering affects the upper slopes particularly any bare rock outcrops. Mechanical weathering, freeze-thaw included, will lead to more angular bare rock surface whereas chemical weathering will produce more rounded slopes.
If a slope is forested or covered in bushes and grass it is less likely to be active. This is because it will protect a slope from the direct effects of rainfall and the roots will help bind particles together. However on unconsolidated cliffs, vegetation can have a negative effect as the roots can help to pull apart rocks.
This is the length of time that a slope has been exposed to weathering. Newly formed landscapes are steep and un-vegetated are actively weathered and eroded until they assume a shape that is in balance with their environment.
Landslides and rock falls occur on steeper slopes as a result of gravity, on shallow slopes processes like soil creep are the more common forms of mass movement
Land Use
Human factors such as mining and quarrying can lead to mass movement taking place, sink holes occur as a result of mining activity and explosives used during quarrying can cause slumping and landslides
Basal Excavation
Can take the form of river undercutting a slope or the sea cutting a notch in a cliff. Human activity such as road construction can have the same effect. This can lead to steepening slopes and making them unstable
The removal of trees can create more unstable cliffs and make mass movement more likely, this is because trees naturally intercept water before it is absorbed by the soil, the roots uptake water for respiration and evapotranspiration within the plant, when these things are removed the slope is newly exposed and vulnerable.

Past Exam Questions
To what extent is there a range of human responses to hazards associated with mass movement? 30 marks

‘Impacts of earth hazards owe more to physical factors than human factors,’ how far do you agree with this statement? 30 marks

Asses the extent to which risk from mass movement results more from human rather than physical factors. 30 marks

‘The main impacts arising from mass movement events are environmental rather than social.’ How far do you agree? 30 marks

This is the process of drilling and injecting fluid into the ground at a high pressure in order to fracture shale rocks to release natural gas inside. The build up of pressure within the earth has been associated with tectonic activity as it can trigger minor earthquakes
Adding Moisture To Slopes
Humans add water to soil by lawn watering and irrigation, swimming pools through leakage and draining and reservoir leakage. All of which lubricates the sediment and makes it more likely to fail. Water trenches between houses contributed to the Cherry Hills landslide.
Management Strategies
Landslide hazard zoning
- Remote-sensing techniques are used to identify unsuitable areas for development.
-Data is then put onto a map, identifying high risk areas.
-This data can then be used to advise building work and construction in these areas to be banned or moved else where.

Ground Stabilisation
Prevents liquefaction which occurs when an area of ground becomes liquid like, as a result of applied stress. This can be caused by tectonic activity.
The ground can be stabilised by the planting of vegetation on slopes so that the roots can bind together and reduce mass movement.
The use of engineering methods by using strong materials such as steel, concrete and stone can also help stabilise slopes.
Although the data is accurate and easy to use, advice is not always followed. For example, many poor established squatter settlements are situated on slide prone steeply sloped areas surrounding many Latin-American urban centres.

Netting/Wire caging surrounding unstable rocks
Netting of rock faces prevents rockfall hazards. This management is often used on steep sloped rocky cliff faces near roads and paths.
Removal of weak rocks
In some cases where rockfall is a continuous hazard, the removal of lose rocks from the cliff face is the management strategy used. This method however has hazard of it's own and may even trigger further rock fall and mass movement.
Drying the land
Drying of land methods reduce saturation and risks of landslides. By planting vegetation such as eucalyptus and bamboo onto slopes, it increase the rate of evapotranspiration without the risk of the plants being too heavy to stand and end up creating cracks in the soil. These plants however must be managed and trimmed to make sure they do not get too big and slide.
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