9.2 Hazards resulting from mass movements Flashcards
(20 cards)
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
SWAVEL:
- slope angle: steeper slopes = stronger influence of gravity
- water saturation: weight or water and reduced soil cohesion
- Addition of weight: rainfall/snow or human acitivites
- Vibration: earthquakes
- Erosion: removal of support = less stable
- lateral pressure: causes internal stress along fractures or bedding planes
What factors contribute to reduced shear strength?
- Water saturation (reduces friction)
- Weathering (breaks down rock cohesion)
- Removal of vegetation (roots add strength)
- Weak rock structures (joints, bedding planes)
- Earthquakes (disrupt bonding)
- Clay-rich soils (weaken when wet)
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:
- 2,043 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
- regrading: reduction of slope gradient, reduces gravitational force, reduces shear stress and helps to redistribute load by moving material from the top to the bottom
- afforestation: roots hold together soil = increased shear strength, as well as absorb/intercept water, reducing pore water pressure
- netting: prevents debris/rock falling providing more stability
- restricting human activity: best way especially for LICs as cheapest: but sometimes not an option?
hazards caused by mass movement
landslides and avalanches
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
causes of Vaiont dam disaster
- 1963
- huge slab of rock, 200 metres wide, slid down a valley side into the Vaiont Reservoir
- created a huge wave, 100 metres high, that overtopped the dam, not breaking it and surged into valley below flooding villages
prediction and monitoring in Hong Kong
Geotechnical Engineering office**: 700 workers manage and improve safety systems
- use of slope monitoring systems = no major deaths since 1982
hazard mapping in hong kong
Hazard mapping and slope registration: Catalogue of Slopes: records updated, maintained and disclosed for 57,000 manmade slopes
- Helps planners and constructors assess risk and install precautions
- reduced fatalities by over 90% in 1970s
use of Preparedness/Public awareness in Hong Kong
- Fines: HK$50,000 and 1 year in prison if private owners do not comply with slope safety protocols
- regular community drills/school awareness: over 80% in high risk areas know what to do when warning
use of preventative methods in hong kong
Prevention: Drainage systems: help to remove excess water from heavy rainfall that leads to slips. Some confusion exists over responsibility, so some poorly maintained.
- Manmade slopes: main method of slope stabilisation. over 4,500 high-risk Slopes graded to reduce risk, drains built to intercept/direct water away.
-Impermeable hard covers added to protect slopes from infiltration and erosive effects of water.
different management strategies for mass movements
prediction
monitoring
hazard mapping
preparedness
prevention
modifying loss - responses
avalanches - conditions they occur
- steep slopes over 22 degrees
- north-facing: lack of sun inhibits stabilisation
different types of avalanches
Type of breakaway:
single point = loose snow
large area leaving wall = slab
Position of sliding surface:
whole snow cover included = full depth
some top strata only = surface
Water content: dry or wet
Form of avalanche: channelled in cross section or open
