9.2 Hazards resulting from mass movements Flashcards
How can mass movements be predicted?
- precipitation levels can be measured through rain gauge/precipitation radars, which show risk of mass movement that are triggered by heavy rainfall
- soil moisture content can be measured using time-domain reflectometer
- changes in surface of sloped detected using radar technology and other systems - inclinometer
- seismometers measure seismic waves through the ground - indicate risk of trigger
- using past data
What does slope failure depend on?
SHEAR STRENGTH: internal resistance to Stop sliding
SHEAR STRESS: forces trying to pull part of the slope down
factors contributing to increased shear stress
- removal of lateral support through undercutting or slope steepening (e.g wave action)
- removal of underlying support
- loading of the slope e.g by the weight of water/vegetation
- transient stresses e.g earthquakes or vibrations from vehicle
- lateral pressure, for example by water freezing in cracks
What factors contribute to reduced shear strength?
- Weathering effects – Disintegration of granular rocks or hydration of clay materials
- Changes in pore water pressure – e.g when slope becomes saturated. can also reduce cohesion by reducing frictional strength
- Changes in structure – e.g creation of small cracks in clays
- Organic effects – Burrowing of animals and decaying tree roots.
different kinds of mass movement
- landslides
- rotational slides
- mud flows
- rock fall
- soil creep
human factors which decrease slop stability
-
construction:
- change in topography = increase stress due to construction (weight of houses)
- groundwater affected (sewage systems), water erosion effect internal strength and slope can become saturated - mudslides -
tourism:
- erosion of slope - trampling, machinery
- leads to increase construction, water sewage problems?, increased erosion -
Agriculture:
- soil compacted by machinery/animals
- slopes vulnerable to soil erosion/reduced vegetation cover due to overgrazing = reduced shear strength/higher soil erosion -
deforestation:
- less tree roots = less shear strength
- less trees = less interception of precipitation = more infiltration = less taken up by tree roots = higher saturation of soil
- forest roads disrupt water flows -
mining:
- removes support from slope, brings large amount of waste often left on slopes = added stress
impacts of Aberfan disaster
Social:
- 116 children, 28 adults dead
- survivors guilt for many
- PTSD
- grief for parents of children
- ‘lost generation’
- anger towards council
Economic:
- destroyed Pantglas junior school
- destroyed 18 houses
causes of Aberfan disaster
Human
- man made tips (coal waste)
- coal waste kay on top of mountain spring so dangerously fluid
- 3 yrs before written to council about danger of tips but ignored (and petition) < evaluation
Physical:
- torrential rainfall turned slurry liquid
impacts of Vaiont dam disaster
Social:
- 2,056 fatalities
- many homes/villages destroyed, displacing numerous families
Economic:
- loss of infrastructure: cost of rebuilding substantial
- local economy effected: businesses disrupted/destroyed
Environmental:
- caused destruction of natural habitats, including forests and wildlife
- release of water and debris into the valley and river system led to water pollution/contamination
- biodiversity significantly disrupted (animals killed/habitats destroyed)
methods to combat mass movements
- pinning: help drain water, reducing pore water pressure = increased shear strength
- grading: reduction of slope gradient, reduces gravitational force, reduces shear stress and helps to redistribute load
- afforestation: roots hold together soil = increased shear strength, as well as absorb/intercept water, reducing pore water pressure
- netting: prevents debris/rock falling
hazards caused by mass movement
landslides and avalanches
factors which increase the risk of an avalanche
TYPE:
- loose avalanches: comprise of fresh snow, soon after snowfall
- slab avalanches: later when snow developed cohesion, much larger and cause more destruction
More likely when:
- slopes steeper than 30 degrees
- lot of new snow falls over short period
- winds lead to drifts
- old snow melts/refreezes - encouraging new snow to slide off
Avalanche case study: The European avalanches of 1999
- killed 18 people in Feb 1999 - worst in 100 years
Reasons for varying levels of damage: - area was thought to be fairly safe
- enormous avalanche wall to defend village of Taconnaz, but villages of *Montroc and Le Tour did not have these
- rescue work hampered by low temperatures (7 degrees), caused snow to compact and made digging almost impossible
- nothing could have been done to prevent avalanche and warnings had been given out day before
- villagers and tourists in ‘safe’ zone thought they were safe - buildings in Montroc were classified as being almost completely free of danger yet not the case
Italian mudslides of 1998 : Human fault
- killed nearly 300 people, up to a years rainfall had fallen in the two preceding weeks
- unstable area, due to active Vesuvius volcano
Human error:
- river sarno’s bed cemented over
- clay soils of surrounding mountains has been rendered dangerously loose by forest fires and deforestation
- houses built on hillsides identfied as landslide zones
- over 20% of houses in Sarno built without permission
- most built over 2 metre thick layer of lava formed by eruption of Vesuvius meaning heavy rain can make it liquid - 900 million tonnes of land washed away every year
