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Aug 21, 2010
Mass Movement - Slumping
Rotational slip (or slumping)
is a form of mass movement (or 'mass wasting'). It is a form of slide (or subset of landslide), where all the material, both at the surface and below, move at the same speed along the shear plane as one block unit. Therefore - a subset of
2 layers of rock: porous (at the top), impermeable (below, marked by the shear plane)
shear planes are concave in shape
slumps therefore leave behind a spoon shaped hollow in the slope
occur in series (one after the other), creating a stepped effect [later filled in with regolith to smooth out slope and decrease slop gradient]
slope usually consists of porous rock e.g. chalk
How they are formed:
Slumps occur on weaker rocks, especially clay, and have a rotational movement along a curved slip plane. Clay absorbs water, becomes saturated, and exceed its liquid limit. It then flows along a slip plane. Frequently the base of a cliff is undercut and weaken by erosion, thereby reducing its strength.
Nagle 2000 page 52
rainwater percolates down through the porous outer layer of rock
it is stopped by the layer of impermeable rock at the shear plane
water lubricates shear plane, allowing porous rock on top to slide down
rainwater percolates down through porous outer layer of rock
accumulation of water underground as it is blocked by impermeable rock below - water saturation
increased mass (increased pull of gravity) coupled with lubricated plane causes entire block to slide downslope
Mupe Bay cliffs, Dorset
Base of cliff is eroded
Supporting material is removed
Block of material slides downslope
The resulting slide leaves behind a bare rock face at the top, called the scarp. There can be multiple scarps. The block extends out beyond the slope, the bottom of which is called the toe. The toe is a fan-shaped bulging mass of material.
Process repeats in series, extending out the slope toe at the bottom, and decreasing slope gradient.
Rotational slip commonly occurs in regolith layers and soft/permeable rock (chalk - Isle of Wight), but can also occur in harder/less permeable rock (clay [and soil])
It also typically occurs in areas of homogenous rock (similar properties in all directions i.e. throughout the volume of material)
Factors affecting speed of rotational slip:
gravitational pull (more saturated rock, more gravitational pull due to larger mass)
water (to lubricate rock) small amounts of water also have cohesive effects, while too much creates mudflows
forms of weathering (e.g. chemical weathering, which will weaken rock material)
(PUT IN HUMAN CAUSE e.g. deforestation, urbanising areas where there is risk of slumping )
Anni Watkins and Scott Hughes
Cartersville, Georgia, USA
Slumped chalk slopes at Mupe Bay, Dorset. This shows an alternative method of slumping - base of cliff is eroded, removing support material, causing slope failure
"This computer simulation depicts the movement of a deep-seated "slump" type landslide in San Mateo County. Beginning a few days after the 1997 New Year's storm, the slump opened a large fissure on the uphill scarp and created a bulge at the downhill toe. As movement continued at an average rate of a few feet per day, the uphill side dropped further, broke through a retaining wall, and created a deep depression. At the same time the toe slipped out across the road. Over 250,000 tons of rock and soil moved in this landslide."
Case Study - Honduras 1998
Rainfall in excess of 900mm in some places
500,000 landslides triggered
70% of roads damaged
El Berrinche Rotational Slip - Teguçigalpa city, Honduras
~6 million m3
Destroyed entire neighbourhood of Colonia Soto
Blocked the Rio Choluteca, creating a temporary urban lake - created health problems
Arrows indicate the path of the sliding material, leading to the Rio Choluteca
Nagle, D (2000), Advanced Geography [Oxford University Press]
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