Creep
slow, downslope movement of unconsolidated material and soft rocks.
plastic flow: clay rich material is liable. when a solid
behaves like a liquid. will happen on saturated thick
surface deposits on steeper slopes. can be affected by
pressure from overlying cap rocks or human
constructions.
heave: produced by freezing and thawing of water in
surface layers of clay/soil. expansion from freezing causes
bulging of surface parallel to slope. when thawing,
material drops back vertically leading to a net downslope
movement of the particles.
wetting and drying: causes clays to expand and contract, causing heave to occur in
same way and freeze thaw
permafrost areas; waterlogged summer conditions lead to accelerated creep
(solifluction), can involve viscous flow
responsible for convex-concave rolling landscapes (western Europe, temperate
area). terracettes caused by soil creep
Flows
(mudflow) rapid rock movement and weathered debris mixed with water. don’t
involve shearing, are turbulent structureless mixture of sediment and water
linked to following factors: steep slopes, narrow valleys, removal of vegetation and
construction projects, thick regolith, therefore common in tropics where deep
chemical weathering occurs, heavy rainfall to saturate ground, slope failure or slide
which may trigger flow, earthquakes/traffic vibrations; earthquakes can cause
liquefaction of saturated material.
conditions found on slopes of active volcanoes (lahars). loose volcanic ash
combines with run off from convectional rainstorms produced by eruptions.
devastating effects in LIC’s, especially when narrow valleys are densely populated.
Slides
single dramatic events when hillside section becomes unstable, shears away &
moves downhill.
shear stresses in slope exceed shear strength of soil/rock
slopes can be natural or result of human activity (road cuttings, embankments,
spoil heaps)
factors leading to slope failure: slope angle; steeper the slope, greater potential for
instability. geological structure; fractures (e.g. bedding planes) dipping out of slope
increase possibility of slippage. rock type; vertical cliff may be stable in some
rocks. clay slopes unstable on slope areas of less than 10 degrees. impermeable
rock layers trap water above trap, leaving highly lubricated layers. amount of water
present; water increase weight of soil/rock. pre-water decreases shear strength of
material. saturated materials very unstable. heavy rain/snow melt trigger slope
failures. removal of the toe of landslip; by excavation/natural erosion removes
support for the slope. loading of the head of the slope; construction projects can
cause slope to fall. Vibrations; explosions/earthquakes/heavy traffic temporarily
increases stress.
Falls
same features as landslides and caused by similar factors.
base undercutting is a common factor.
reduce horizontal pressure on cliff face, allows growth of vertical cracks which lead
to further falls.
significant after glaciation glaciers excavate deep valleys and support steep valley
sides.
after ice melts the sides less supported so liable to rock falls
produce scree that accumulates as cones/fans at base.
may join together to produce continuous slip or huge boulders.
some fragments may bounce significant distances.
slope angles rarely exceed 40 degrees
angle of scree slope depends on: size and shape of rock fragments and height of
cliff through which fragments have fallen
Vaiont dam disaster, Italy 1963
landslide resulted in 2600 deaths.
266m high dam constructed across Vaiont river to produce hydroelectricity.
site was a deep narrow valley chosen to store large water volumes.
slippage started when reservoir started to be filled so the slope was reinforced.
when it was filled completely, water seeped into the rock layer, increasing pore
water pressure and reducing cohesion.
9.10 heavy rain resulted in 270 million cubic m of rock sliding into reservoir at a
speed of 25m per secs.
created a wave 100m high which flowed over the dam and into valley below
Erosion processes on slopes
slope processes above are all mass movement types as all independent of running
water.
running water plays a part, especially when rainfall intensity exceeds infiltration
rate and overland flow occurs.
effects increased by human activity such as deforestation, over grazing, burning or
cultivation which leaves soil bare.
on gentle slopes water may run off as a uniform sheet, causing sheet erosion.
steep slopes the water becomes concentrated in channels leading to gully erosion.
ploughing down slope not across has a factor.
intermediate stage between the 2 produces fine channel networks known as rills.