Landslides and Debris Flows
In the 1970s, landslides—all categories of gravity-related slope failures in earth materials—caused nearly 600 deaths per year worldwide. Annual landslide losses in the United States, Japan, Italy, and India have been estimated at $1 billion or more for each country.
Landslide costs include direct and indirect losses affecting both public and private property. Direct costs can be defined as the costs of replacement, repair, or maintenance of damaged property or installations. An example of direct costs resulting from a single major event is the $200-million loss attributed to the 21-million-m 3 landslide and debris flow at Thistle, Utah, in 1983. The slide severed three major transportation arteries—U.S. highways 6 and 89 and the main line of the Denver and Rio Grande Western Railroad—and the lake it impounded by damming the Spanish Fork River inundated the town of Thistle, resulting in the destruction of businesses, homes, and railway switching yards. The indirect costs involved the cutoff of eastbound coal shipments along the railroad line. In 1983 oil was expensive and coal was crucial for generating electricity. With supplies from the west severed, eastern coal normally exported to Europe had to be rerouted. European industry, in turn, had to adjust to lowered supply. Ultimately, the landslide affected the international balance of payments.
Destructive landslides have been noted in European and Asian records for over three millennia. The oldest recorded landslides occurred in Hunan Province in central China 3,700 years ago, when earthquake-induced landslides dammed the Yi and Lo rivers.
Since then, slope failures have caused untold numbers of casualties and huge economic losses. In many countries, expenses related to landslides are immense and apparently growing. In addition to killing people, slope failures destroy or damage residential and industrial developments as well as agricultural and forest lands, and they eventually degrade the quality of water in rivers and streams. Landslides are often associated with other events: freeze-thaw episodes, torrential rains, floods, earthquakes, or volcanic activity.
Despite improvements in recognition, prediction, mitigative measures, and warning systems, worldwide landslide losses—of lives and property—are increasing, and the trend is expected to continue into the twenty-first century. Some of the causes for this increase are continued deforestation, possible increased regional precipitation due to short-term changing climate patterns, and, most important, increased human population.
Demographic projections estimate that by 2025 the world's population will number more than 8 billion people. The urban population will increase to 5.1 billion—more than the total number of humans alive today. In the United States the land areas of the 142 cities with populations greater than 100,000 increased by 19 percent in the 15-year period from 1970 to 1985. By the year 2000, 363,000 km2 in the conterminous United States will have been paved or built on. This is an area about the size of the state of Montana.
Accommodation of this population pressure will call for large volumes of geological materials in the construction of buildings, transportation routes, mines and quarries, dams and reservoirs, canals, and communication systems. All of these activities can contribute to the increase of damaging slope failures. In other countries, particularly developing nations, the urbanization pattern is being repeated but often without adequate land planning, zoning, or engineering. Not only do development projects draw people, but the projects themselves as well as the people who settle the surrounding area often occupy just those hillside slopes that are susceptible to sliding. At present, there is no organized program to provide the geological studies that could prevent the worst scenarios posed by this threat.
To reduce landslide losses, research efforts should encompass more than investigations of physical processes in hazardous areas aimed at understanding the nature of slope movement. Earth scientists also need to perfect methods for identifying areas at risk and for mitigating contributory factors. These goals are attainable. Scientists can predict areas at risk and advise means to avoid or moderate danger, but much of the research needed has yet to be done.
For the past half century, geologists have relied primarily on aerial photography and field studies—ideally in combination—for identification of vulnerable slopes and recognition of landslides. In recent years, since multispectral satellite coverage has become available for much of the world, an additional tool is available that can provide images in black and white or color as well as spectral bands through red, green, and near-infrared wavelengths. The coverage, scale, and quality of multispectral imagery is expected to improve considerably within the next decades and provide valuable information that can lead to improved identification of landslide-prone locations.
The information gathered from satellite reconnaissance can contribute to the growing store made use of in geographic information systems, which are digital systems of mapping spatial distribution that can be applied to the preparation of landslide susceptibility maps. These modern data-handling systems facilitate both pattern recognition and model building. Patterns and models that suggest landslide susceptibility can be tested, revised and improved, and then tested again against large numbers of observations.
As a result of these gains in knowledge, progress has been made in determining appropriate types of landslide mitigation. The most traditional mitigation technique is avoidance: keep away from areas at risk. When occupation of a site warrants risk, engineered control structures may be required, including surface water diversion and subsurface water drains, the construction of restraining structures such as walls and buttresses, and devices such as rock bolts. The establishment and enforcement of site grading codes calling for appropriate slope stabilization instituted in 1952 by Los Angeles County have worked well. Cut-and-fill grading techniques involving the removal of material from the slope head, regrading of uneven slopes, and hillslope benching are all of proven value. Consideration of such factors has had a major effect on reducing landslide losses in the United States, Canada, the European nations, the former Soviet Union, Japan, China, and other countries. Landslide research today is focused not so much on locating where landslides are and what hazards they represent as on figuring out how to cope with the potential hazard that they represent. This is an area of close cooperation between solid-earth scientists and geotechnical engineers.
Mitigation has also benefited from substantial progress in the development of physical warning systems for impending landslides. Significant improvement will result as better instrumentation and communication systems are developed. Of particular importance will be continuing advances in computer technology and satellite communications. Hazard-interaction problems require a shift in perspective from the incrementalism of individual hazards to a broader systems approach. Earth scientists, engineers, land-use planners, and public officials are becoming aware of interactive natural hazards that occur simultaneously or in sequence and that produce cumulative effects that differ from those of their component hazards acting separately.
Research on the social aspects of such relationships in terms of warning systems and emergency services is necessary. At what point should people evacuate and abandon their little piece of the Earth?
Human intervention can reduce landslide risk by influencing some contributory causes. Projects that undermine slopes in marginal equilibrium or destabilize susceptible areas by quick drawdown of reservoirs can be avoided. Among projects that can lay the groundwork for disastrous landslides are road building, mining, fluid injection, and building construction that entails clearing vegetation. Planning and designing such projects with the local landslide potential in mind is absolutely essential. While these activities may not individually cause a landslide, they can increase the likelihood of slope failure as preconditions to which cloudbursts or earthquakes are added.
Wherever hillsides receive precipitation over days and weeks, the pore-water pressure can build in rock fractures and decrease bulk shear strength, which can then induce displacements under less force than would be needed to shear a drier material. A proven mitigation technique in such cases is for geologists to locate the water surface in fractured rocks and drain off destabilizing water by drilling horizontal wells.
Then there are the regional-scale contributory causes of increased landslide susceptibility such as deforestation. According to the World Resources Institute, approximately 109,000 km2 of tropical forest is being destroyed annually—an area the size of Ohio.
Removal of the forest cover increases flooding, erosion, and landslide activity. This deforestation is causing serious landslide problems in many developing countries.
The activities of humankind have commonly accelerated the transformations, catapulting natural systems over thresholds and producing immediate environmental threats. The geomorphological processes affected by humans cover a staggering range of scale. From local denudation caused by livestock overgrazing, a significant component of the process of regional desertification, humankind has evolved into a major geomorphological agent. Proper understanding of progressive geomorphological changes can forestall precipitous transformations and prevent the loss of landform stability.
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Comment by praful rao
Even though the article is from 1993, I found much of what is written, relevant for us living in the Darjeeling-Sikkim Himalayas today in 2009.
In the above article, the italics are mine
and for those interested the full e-book can be read here
http://books.nap.edu/openbook.php?record_id=1990&page=1
