Mass Movements. Smith Chapter 8 Mass Movements Downslope movement of large volumes of surface materials under gravity.

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