Part 1.4: Landslides

This chapter is designed to enhance your understanding of:

the forces that cause landslides

different types of landslides and their effects

methods to reduce the vulnerability of human settlements to landslides.

Introduction

Landslides are a major threat each year to human settlements and infrastructure. "Landslide" is a general term covering a wide variety of land forms and processes involving the movement of soil and rock downslope under the influence of gravity. Although they may take place in conjunction with earthquakes, floods and volcanoes, they are much more widespread than those hazards and over time cause more property loss than any other geological event.

LANDSLIDE HAZARD DATA SHEET

Major landslides of the 20th century

Year	Location	Cause	Death	Damage
1920	China	earthquake	100,000	NA
1938	Kobe, Japan	rainfall	450-600	130,000 homes
1949	Tadzhikistan	earthquake	12-20,000	destroyed 33 villages
1963	Italy	rockslide	2-3,000	destroyed 5 villages
1964	Alaska USA	earthquake	None	US$ 180 million
1980	California USA	rainfall	None	US$ 500 million
1985	Colombia	vol. eruption	22,000	US$ 230 million

Annual losses due to landslides and mudflows

United States	$1-1.5 billion (including indirect costs)
Italy	$1.1 billion
Japan	$1-1.5 billion
India	$1 billion

Source Schuster and Flemming, 1988, p.321-325, UNEP/UNESCO

Causal phenomena

Landslides occur as a result of changes, either sudden or gradual, in the composition, structure, hydrology or vegetation on a slope. These changes can be due to:

Vibrations from earthquakes, blasting, machinery, traffic and thunder. Some of the most disastrous landslides have been triggered by earthquakes.

Changes in water content caused by heavy rainfall and rises in ground water levels.

Removal of lateral support by erosion, previous slope failure, construction, excavation, deforestation, or loss of stabilizing vegetation.

Loading with weight of rain, hail, snow, accumulation of loose rock or volcanic material, stockpiles of rock, waste piles, and weight of buildings and vegetation.

Weathering and other physical or chemical action may decrease strength of rocks and soils over time.

Landslides in urban areas are often induced by human actions:

Interruption of water courses and change in the water table.

New construction involving "cut and fill" methods which disrupt slope stability.

Q. From the list given above, which causes of landslides are most significant to your community? What type of data can you use in answering this question?

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Figure

Oxfam. UNDRO NEWS, May/ June, 1984.

General characteristics

Landslides usually occur as secondary effects of heavy storms, earthquakes and volcanic eruptions. The materials that compose landslides are divided into two classes, bedrock or soil (earth and organic matter debris). A landslide may be classified by its type of movement:

Falls - A fall is a mass of rock or other material that moves downward by falling or bouncing through the air. These are most common along steep road or railroad embankments, steep escarpments, or steeply undercut cliffs especially in coastal areas. Large individual boulders can cause significant damage.

Slides - Resulting from shear failure (slippage) along one or several surfaces, the slide material may remain intact or may break up.

Topples - A topple is due to overturning forces that cause a rotation of the rock out of its original position. The rock section may have settled at a precarious angle, balancing itself on a pivotal point from which it tilts or rotates forward. A topple may not involve much movement and it does not necessarily trigger a rockfall or rock slide.

Lateral spreads - Large blocks of soil spread out horizontally by fracturing off the original base. Lateral spreads occur generally on gentle slopes, usually less than 6 percent, and typically spread 3m to 5m but may move from 30m to 50m where conditions are favorable. Lateral spreads usually break up internally and form numerous fissures and scarps. The process can be caused by liquefaction whereby saturated, loose sands or silts assume a liquefied state. It is usually triggered by ground shaking (as with an earthquake). During the 1964 Alaskan earthquake, more than 200 bridges were damaged or destroyed by lateral spreading of flood plain deposits near river channels.

Flows - Flows move like a viscous fluid, sometimes very rapidly, and can cover several miles. Water is not essential for flows to occur, however, most flows form after periods of heavy rainfall. A mudflow contains at least 50 percent sand, silt and clay particles. A lahar is a mudflow that originates on the slope of a volcano and may be triggered by rainfall, sudden melting of snow or glaciers, or water flowing from crater lakes. A debris flow is a slurry of soils, rocks and organic matter combined with air and water. Debris flows usually occur on steep gullies. Very slow, almost imperceptible flows of soil and bedrock are called creeps . Over long periods of time, creep may cause telephone poles or other objects to tilt downhill.

Figure 1.4.1 - Landslide types

Predictability

Landslide velocity varies from the extremely slow (<.06 meters/year) to extremely fast (>3meters/second), which might imply a similar variation in predictability. In absolute terms, however, it is very difficult to predict the actual occurrence of a landslide although situations of high risk - forecasted heavy rainfall or seismic activity combined with landslide susceptibility - may lead to estimation of a time frame and possible consequences.

Estimation of landslide hazard potential includes historical information on the geology, geomorphology (study of land forms), hydrology and vegetation of a specific area.

Geology - Two aspects of geology are important in assessing the stability of land and predicting landslides.

1) Lithology - the study of characteristics of rock such as the composition, texture or other features which influence its behavior. These attributes determine the strength, permeability, susceptibility to chemical and physical weathering and other factors which affect slope stability.

2) Structure of rocks and soils - Structural features that may affect stability include sequence and type of layering, lithographic changes, planes, joints, faults and folds.

Geomorphology - The most important geomorphological consideration in the prediction of landslides is the history of landslides in a given area. Other important factors are steepness of slope in relation to the strength of the slope-forming materials, and slope aspect, or the direction in which the slope faces and its curvature.

Hydrology and climatology - The source, movement, amount of water and water pressure must be studied. Climatic patterns combined with soil type may cause different types of landslides. For example, monsoons in tropical regions may cause large debris slides of soils, rocks and organic matter.

Vegetation - Plant cover on slopes may have a positive or negative stabilizing effect. Roots may decrease water runoff and increase soil cohesion, or conversely may widen fractures in rock surfaces and promote infiltration.

Factors contributing to vulnerability

Settlements built on steep slopes, weak soils, cliff tops, at the base of steep slopes, on alluvial outwash fans or at the mouth of streams emerging from mountain valleys are all vulnerable. Roads and other communication lines through mountain areas are in danger. In most types of landslides, damage may occur to buildings even if foundations have been strengthened. Infra-structural elements such as buried utilities or brittle pipes are vulnerable.

Vulnerable sites

Typical adverse effects

Physical damage

Anything on top of or in the path of a landslide will suffer severe damage or total destruction.

Anything on top of or in the path of a landslide will suffer severe damage or total destruction. In addition, rubble may damage lines of communication or block roadways. Water ways may be blocked creating a flood risk. Casualties may not be widespread, except in the case of massive movements due to major hazards such as earthquakes and volcanoes.

In addition to direct damage from a landslide, many indirect adverse effects occur. These include:

loss of productivity of agricultural or forest lands (if buried)

reduced real estate values in high risk areas and lost tax revenues from these devaluations

adverse effects on water quality in streams and irrigation facilities

secondary physical effects such as flooding.

Casualties

Fatalities have occurred due to slope failure where population pressure has prompted settlement in areas vulnerable to landslides. Casualties may result from collapse of buildings or burial by landslide debris. Worldwide, approximately 600 deaths per year occur, mainly in the Circum Pacific region. Estimates for loss of life in the United States is 25 lives per year, greater than the average loss from earthquakes. Catastrophic landslides have killed many thousands of persons, such as the debris slide on the slopes of Huascaran in Peru triggered by an earthquake in 1970, which killed over 18,000 people.

Possible risk reduction measures

Preparation of a landslide hazard map

The implementation of risk reduction measures must be preceded by locating areas prone to slope failures. The landslide hazard map permits planners to determine the level of risk and to make decisions regarding avoidance, prevention or mitigation of existing and future landslide hazards. Reasonably accurate techniques are available to planners for mapping slope hazard areas. These techniques rely on past history, topographic maps, bedrock data and aerial photographs. Various types of mapping formats may be used. The maps can be supplemented by additional data such as proximity to earthquake zones, local undercutting by rivers or impaired drainage.

In France, the ZERMOS (Zones Exposed to Risks of Movements of the Soil and Subsoil) plan produces landslide hazard maps at scales of 1:25,000 or larger which are used as tools for mitigation planning. The maps portray degrees of risk of various types of landslides, including activity, rate and potential consequences.

Figure 1.4.2 - Detail from map showing relative slope stability in part of west-central King County, Washington, U.S.A.

"Mitigating Natural Disasters: Phenomena, Effects and Options" UNDRO, Geneva, 1991.

Q. What information is needed for the preparation of a landslide hazard map?

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ANSWER Preparation of landslide hazard maps require information on: past landslide events, topography, bedrock data, and aerial photographs.

Land use regulation

The most effective way to reduce damage caused by landslides is to locate development on stable ground and to utilize landslide susceptible areas as open space or for low intensity activities such as park land or grazing.

The most effective way to reduce damage caused by landslides is to locate development on stable ground and to utilize landslide susceptible areas as open space or for low intensity activities such as park land or grazing. Land use controls can be enacted to prevent hazardous areas from being used for settlements or as sites for important structures. The controls may also involve relocation away from the hazardous area particularly if alternative sites exist. Restrictions may be placed on the type and amount of building that may take place in high risk areas. Activities that might activate a landslide should be restricted. Where the need for land is critical, expensive engineering solutions for stabilization may be justified.

Legislation

Governments may assume responsibility for damage repair expenses from landslides as well as efforts to prevent them. In Japan, landslide control activities were originally connected to conservation legislation for river improvement, erosion control and maintenance of agricultural and forest lands. In 1969, a comprehensive control program was legislated devoted exclusively to landslides which provides governmental assumption of expenses for recovery from natural disasters for which no individuals bear responsibilities.

Insurance

Insurance programs may reduce losses to landslides by spreading the expenses over a larger base and including standards for site selection and construction techniques. In New Zealand, a national insurance program assists individuals whose homes have been damaged by landslides or other natural hazards beyond their control. A special disaster fund is accumulated by surcharge to a fire insurance program.

Structural changes

Vulnerability for structures built in the path of landslides is nearly 100 percent.

Strengthening of existing buildings and infrastructure is not considered by most experts to be a viable option for mitigation of damage due to landslides, as vulnerability for structures built in the path of landslides is nearly 100 percent. The selection of mitigation options depends on:

The value of land or structures in relation to the cost of the protective measures.

The opportunities to enforce landuse regulations and the availability of alternative locations.

The number of people affected by the intervention.

The predicted amount of damage.

Improvements and protective measures may be added to sites, such as improvement of soil drainage (by addition of permeable materials) and slope modifications (reduction of slope angle prior to construction). Concrete retaining walls may stabilize possible sites. Large scale engineering works may also be considered.

Q. What are the basic mitigation measures applicable to landslide hazards?

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ANSWER Appropriate mitigation measures include : land use planning, legislative controls or assurances, insurance, and in some cases site engineering projects.

Q. Are any of these measures now in practice in your community?

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Specific preparedness measures

Community education

The most damaging landslides are often related to the activities of people. Construction of roads, housing, and other infrastructure frequently causes landslides. Thus the most effective preparedness measures should be taken before people occupy a vulnerable area. Public education programs will help people understand the causes and effects of landslides, identify unstable areas and avoid settling on them. Some areas may be stabilized prior to settlement or subjected to strict land use regulations. For areas already built upon, ground stabilizing procedures, such as terracing and treeplanting, may be of some use in reducing damage but losses will not be completely avoided.

Monitoring, warning and evacuating systems

Areas susceptible to landslides may be monitored to allow timely warning and evacuation. Monitoring methods include field observation and use of inclinometers, vibration meters, and electrical fences or tripwires. Immediate relay of information is essential in places where rockfalls or debris flows are likely to occur rapidly. In these cases, use of the media, sirens or other widely reaching information systems may be required. Monitoring and warning systems should place inhabitants on alert when heavy rains occur or if ground water levels rise.

Public education programs may involve descriptions of climatic conditions or hazards that provoke landslides and what actions to take when such conditions exist. Evacuation plans for high risk areas should be stablished and practiced particularly when the risk of landslide is interconnected with threat of seismic, volcanic or flooding activity.

Typical post-disaster assistance needs

Needs for the direct impact area of a landslide include search and rescue equipment and personnel, and possibly use of earth removal equipment. Emergency shelter may be required for those whose homes have been lost or damaged. Experts trained in landslide hazard evaluation should be consulted to determine whether slide conditions pose additional threat to rescuers or residents.

Secondary effects of landslides such as flooding may require additional assistance measures. If the landslide is related to an earthquake, volcano or a flood, assistance to the landslide-affected area will be part of the total disaster assistance effort.

CASE STUDY

The Valtellina Landslide and Flood Emergency, Northern Italy, 1987

Valtellina, in the province of Sondrio among the Rhaetian Alps of Northern Italy was the scene of a landslide and Hood emergency that lasted from 18 July until the beginning of September 1987. Fifty-three people were killed or remained unaccounted for, at least 110 were injured and 25,000 were evacuated from 40 municipalities with a combined population of 48,500. Damage and destruction have been valued at US$ 800 million in Valtellina. It took five months merely to reopen the valley to through traffic. Hooding of the River Adda provoked landslides and led to casualties, destruction and disruption from 18 July until 28 July, when major debris avalanches blocked the Adda and began impounding a large lake. During the following month, the authorities had to design a strategy that would reduce the risk of overtopping or breaching of the debris dam as the water level rose. This necessitated the evacuation of settlements for 50 km downstream of the lake and the release of water impounded behind a hydro-electric dam upstream in order to scour a relief channel through the debris barrier.

The primary cause of the landslides was the unstable slopes of the valley. A secondary cause of slope instability was poor environmental management, mainly involving deforestation, mismanagement of water resources and over development of settlements and roadways. These conditions set the stage for the disaster which began with 290 mm of rainfall between 18-20 July.

During the subsequent week the floodwater ebbed, but mass earth movements continued to damage roads and railways. On 21 July, a 1 km long fracture 900 m above the valley floor was identified as a landslide risk zone. On 28 July, the Val Pola landslide occurred. It slid as an avalanche of at least 10 million cubic meters of debris, accelerating to 250 km/hr. The movement lasted 31 seconds and caused tremors of 3.9 on the Richter scale.

The course of the River Adda was blocked by a barrier of saturated debris 9 km square in size, 2800 m long and 40 m deep. A decision was made to install in the debris a drainage tunnel 6 m in diameter plus an open spillway over which water would be pumped. However, the danger of the debris failing, causing downstream flash flooding was great. On 24 August an evacuation of 19,500 downstream residents was required. Conditions remained serious for several days but the debris did not give way and the temporary lake was eventually drained.

The case of Valtellina 1987 was one in which the emergency phase lasted for nearly seven weeks. During that period it was necessary to reconcile two needs. To ensure public safety complicance with evacuation orders was required. To transport equipment and supplies to the scene of the emergency, access to the risk zone had to be maintained.

References

Disaster Management Center, Natural Hazards: Causes and Effects, University of Wisconsin Board of Regents, 1986.

Erickson, Jon, Volcanoes and Earthquakes, TAB books, Blue Ridge Summit, PA., 1988.

Erley, Duncan and William J. Kockelman, Reducing Landslide Hazards: A Guide for Planners, American Planning Association, Chicago, 1981.

Facing Geologic and Hydrologic Hazards, U.S. Geological Survey Professional Paper 1240-B, U.S. Government Printing office, Washington, D.C., 1981.

Kockelman, William J., "Some Techniques for Reducing Landslides Hazards", Bulletin of the Association of Engineering Geologists, Vol. XXIII, No. 1,1986, p 29-52.

Marsh, William M., Environmental Analysis: for Land Use and Site Planning, McGraw-Hill, 1978.

OAS/DRDE, Natural Hazards Primer, Organization of American States, Washington, D.C., 1990.

Reducing Losses from Landsliding in the United States, National Academy Press. Washington, D.C., 1985.

Schuster, Robert L. and Robert W. Fleming, "Socioeconomic Significance of Landslides and Mudflows", U.S. Geological Survey, Denver, Colorado.

UNDRO, Mitigating Natural Disasters, Phenomena, Effects and Options, United Nations, New York, 1990.

UNDRO News, "After Armenia, Tragedy in Tadjikistan," Jan/Feb. 1989.