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: 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 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

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