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Mass Movement (Case Study: Landslides in Hong Kong (In June 1966, rainfall…
Mass Movement
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Mudflows
These conditions may be found on the slopes of active volcanoes where the mudflows are termed lahars. In this case, loose, volcanic ash combines with run-off from convectional rainstorms produced by eruptions.
When Nevado del Ruiz erupted in Colombia in 1985, it covered the town of Armero at an estimated speed of over 50km/h (lahar).
On steeper slopes mudflows make take place where speeds increase to between 1 and 40km per hour, especially after heavy rainfall, adding both volume and weight to the soil. The heavy rain increases the pore water pressure which forces the particles into a rapidly flowing mass of material.
Landslides
Landslides take place at speeds between 1 and 100m per second on slopes that are often greater than 40 degrees and have a low water content. They are most active in areas of high relief and unstable slopes.
In rock slides, the effects of bedding and joint planes are important in allowing the rock to fragment and they also provide slide planes.
Slides can be extremely rapid processes. They occur when a complete mass of material detaches itself from a slope and slides downhill. The shear stresses in the slope exceed the sheer strength of the soil or rock.
Earthflows
Earthflows may take place at speeds of 1 to 15km per year when material is being transported on slopes on 5-15 degrees with a high water content. When the regolith becomes saturated, it begins to flow downhill.
The movement of material may produce short flow tracks and small bulging lobes or tongues, yet may not be fast enough to break the vegetation.
Rotational Slides
Where rotational movement occurs, a process sometimes referred to as slumping, a curved rupture surface is produced.
Rotational movement can occur in areas of homogenous rock, but is more likely where softer materials overly more resistant or impermeable rock.
Rockfalls
Rockfalls produce scree (talus) which accumulates as cones or fans at the base. They may eventually join together to produce a continuous slope or huge boulders.
Rockfalls occur on steep slopes (over 40 degrees), often almost vertically jointed and fractured rock faces where the cohesion between masses of rock is overcome and the shear strength of a material is exceeded. They often result after several freeze-thaw cycles or by repeated wetting and drying or other weathering processes.
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