Mass Movements
Mass Movements
• Smith Chapter 8
Mass Movements
• Downslope movement of large volumes of surface materials under gravity
Mass Movements
• Vary in speed– rapid movements can kill– slower movements can be costly
• soil creep, subsidence
Mass Movements
• Vary in materials– Landslides
• Rockfalls
• Mudflows
– Avalanches
Mass Movements
• Triggered– weather– earthquake, volcano– vibration
Mass Movements
• Risk increasing as land-hunger forces development of unstable & steep slopes
Germi, Iran
Mass Movements
• Early 1970s: produced around 600 deaths/year
• Late 1980s: several thousand annually
• Most deaths in Third World– USA: annual death toll 25-30
Mass Movements
• Annual losses in the billions US$ each year– especially in cities
• US$1-3 bn/year in the USA– Appalachian, Rocky Mtns, Pacific
• US$500 million in LA area from landslides in an el Nino year
Mass Movements
• Most deaths on Pacific Rim– associated with seismic activity– high population densities– heavy typhoon rainfall– rapid economic development
Mass Movements
• Increased by shanty-town settlement– on steeper, less-stable slopes
Caracas, Venezuela
• 1950s: fewer than 1 urban landslide/year
• City expands rapidly, esp. in 1970s
• Shanties spread out onto hills
• 1980s: 35-40 urban landslides/year
High landslide hazard: Guatemala City
La Paz, Bolivia
Avalanches
• Common on snow-covered slopes steeper than 20 degrees, less than 60 degrees
• Move at 80m/s (288 km/h)
• Settlement needs to avoid the runout tracks
Avalanches
• Frequent but usually harmless– USA 7,000-10,000 avalanches/year, only 1%
cause harm
• Threat to unwary travellers, badly-sited settlements
• Risks increased by alpine tourism development
Lake Tahoe CA
Avalanches
• More avalanche deaths in Europe– higher population density– Switzerland 25-30 deaths/year
Intense Landslide Risk
• High mountains– Areas of seismic shaking– High relief
• Soils made erodible by deforestation
• Thick deposits of fine-grained loosely-packed material
• Areas of high rainfall
Ganges delta
Unstable slopes, Nepal
Rohtang pass, India
Auguas Calientes, Peru
Rail line serving Maccu Piccu
Huascaran Peru
• Monument to the dead of Yungay
Yungay school bus
Frank Slide AB, 1903
• Rockfall caused by natural weathering of anticlinal rocks of Turtle Mountain
• Triggered by spring freeze-thaw
• Undercut by mining
• Destroyed Frank townsite
• Killed 70
Frank slide AB
Hope slide BC
Soil Mechanics
• Slopes fail when shear strength of slope is exceeded– excessive loading– cohesion failure of slope materials
Soil Mechanics
• Different configurations of slide– rotational– translational
Lake Merced 1967
Lake Merced 1967
Ground subsidence
Debris flows
• Slope materials become fluidised– saturated– follows natural drainage channels– great destructive force
• Very common in the wet tropics
China: Guangxi
Brazil 1991: 15 killed
Bhutan after monsoon
NZ west coast
NZ landslide hazard
How to trigger a landslide
• Make slope steeper– undercut it– add material
• Add weight to the slope
• Add water to the slope (very effective)
• Remove vegetation
• Provide vibrations and shocks
Aberfan 1966
• Coal-mine waste dump collapses on Welsh village
• Destroyed several homes and primary school during morning assembly
• Killed 140+, – including 111 children
Aberfan 1966
• Hazard noted in 1920s
• Mine dump not engineered
• Receiving wet slurry in 1966
• Village and primary school in harm’s way
• School principal had complained of the dangers, her protests were ignored
Reducing landslide risks
• Careful settlement siting
• Slope stabilisation
• Slope drainage modification
• Slope vegetation
• Revetments, slope freezing (temporary)
Reducing avalanche risks
• Trigger avalanches at safe times– smaller avalanches– with explosives, artillery
• Defensive structures
• Careful settlement siting
Avalanche defences