Introduction Definitions Classifications Stability Large Landslides Causes Investigation Analysis Response Management Case Histories-NZ Case Histories-HK Subsidence Revision Quiz Assignment MU References

Landslide Hazard: Introduction

Face Gully, Pohangina
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Among natural hazards, land instability features in virtually every country of the world. It can result from heavy rains, melting snow and ice, earthquakes, volcanoes and human activity. The most notable event in recent history was the Reventador, Ecuador landslide of March 1987 which was caused by a magnitude 6.9 earthquake following a month of heavy rains (Table 1). It caused 1,000 deaths and ruptured 33 km of the trans-Ecuadorian oil pipeline. The damage to the pipeline delayed oil exports for almost half a year, reducing government income by nearly 35 percent.

Fortunately individual slope failures are rarely as catastrophic as some other natural hazards although there have been some significant disasters. 1962 saw a huge debris avalanche fall from the North peak of Nevados Huascaran in the Peruvian Andes destroying nine towns and taking 4,000 lives. Then, despite warnings of further instability, an even larger debris avalanche swept down from the same mountain in 1970 levelling the city of Yungay and leaving many thousands of people dead (Table 1). In Europe the Vaiont Reservoir slide of 1963 resulted in some 2,600 deaths as the flood-wave produced by the 240 million cubic metres slide mass overtopped the dam and destroyed five villages in the valley below.

Hong Kong's problem:
a proliferation of cut-slopes associated with major urban developments. Failure of cut-slopes accounts for the majority of landslip fatalities in Hong Kong.
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Worldwide the annual economic losses from landslides, subsidence and other ground failures exceed those from all other natural hazards combined. A report by the US National Research Council in 1985 put the annual cost of landsliding in the United States at US$1-2 billion. Annual losses in Italy have been estimated to exceed $1,140 million with damage to roads, railways, aqueducts and housing in one region, Calabria, amounting to more than US$200 million back in 1973.

The hazards posed by hillslope instability are clearly shown in Tables 1 and 2. Reducing these hazards requires a combination of scientific research, engineering design, landuse planning and hillslope management. There are four factors that are fundamental to stability analysis (Crozier 1984): (1) the frequency of landslide activity on a slope, (2) the magnitude of movement, (3) the rate of movement and (4) the type of movement.

These factors can be used to derive subsidiary criteria depending on social, economic and technical conditions including: (1) remedial measures which will stabilise the slope, (2) cost of stabilisation, (3) magnitude, type and cost of damages likely to be caused by slope failure and (4) appropriate landuse in relation to slope stability.

Table 1. Global occurrence of slope instability and catastrophic landslides.

Geographic location	Date	Cause	Impacts
Vaerdalen Norway	1893	Liquefaction of sensitive marine clay exposed by stream erosion	111 deaths; 22 farms destroyed
Kansu Province China	December 1920	Loess flows (dry) triggered by a major earthquake	100,000 - 200,000 deaths
Kure Japan	September 1945	Debris flows initiated by typhoon Makurazaki	1,154 deaths
Southwest of Tokyo Japan	September 1958	Extensive landsliding initiated by a typhoon	~ 1,100 deaths
Rupanco region, Chile	May 1960	Widespread landsliding triggered by a strong earthquake following heavy rain	210 deaths; destroyed many buildings, port facilities, transportation routes and agricultural land
Mt Huascaran, Peru	January 1962	Huge landslide triggered by an ice and rock avalanche	4,000 - 5,000 deaths; destroyed several small villages
Vaiont, Italy	October 1963	Long-term geologic failure coupled with high groundwater inputs triggered large landslide	~ 3,000 deaths
Anchorage, Alaska, USA	March 1964	Large failures in sensitive clays induced by 1964 Alaska earthquake	9 deaths; 215 houses destroyed; 157 commercial properties damaged
Seward and Valdez, Alaska, USA	March 1964	Large submarine landslides caused by 1964 Alaska earthquake	33 deaths; extensive damage to harbour and waterfront property
Rio de Janeiro, Brazil	January 1966	Rainfall initiated landslides (mainly on excavated and deforested land)	1,000 deaths
West of Rio de Janeiro, Brazil	January 1967	High intensity rainstorm triggered thousands of landslides	1,700 deaths; extensive damage to property
Sikkim & West Bengal, India	1968	Landslides	33,000 deaths
Mt Huascaran Peru	May 1970	Earthquake-triggered debris avalanche	18,000 deaths; destroyed town of Yungay
Hong Kong Island and Victoria, China	May - June 1972	Several large road failures after heavy rain	138 deaths
Mantaro River Valley, Peru	April 1974	Huge landslides (dammed river)	Landslide lake-breakout flood killed 450; many farms and roads destroyed
North and west of Guatemala City	February 1976	Landslides triggered by large earthquake	~240 deaths; 500 homes damaged
Pahire Phedi, Nepal	June 1976	Landslide	150 deaths
El Salvador	September 1982	Landslides and floods	500 deaths; 25,000 made homeless
Strava, Italy	July 1985	Failure of earth embankment	206 deaths
Nevado del Ruiz, Colombia	November 1985	Debris flow (lahar)	22,000 deaths; US$212,000 in property damage
Reventador, Ecuador	March, 1987	Landslide	1000 deaths; oil pipelines cut
Catak, Turkey	June 1988	Landslide	300 deaths; destroyed several houses and school
Papua New Guinea	March 1991	Landslides	~200 deaths; 500 homes destroyed

Learning Outcomes

The module covers four general areas:

On completing this module you should be able to appreciate the types of slope and ground movements that result in land instability and subsidence. You should also have a basic understanding of the reasons these movements occur, enabling you to communicate effectively with scientists, engineers and planners on matters of landslide hazard mitigation and development of disaster response programmes.

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