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What are landslides?
'Landslide' is a general term for a wide variety of downslope movements of earth materials that result in the perceptible downward and outward movement of soil, rock, and vegetation under the influence of gravity. The materials may move by falling, toppling, sliding, spreading, or flowing. Some landslides are rapid, occurring in seconds, whereas others may take hours, weeks, or even longer to develop. Although landslides usually occur on steep slopes, they also can occur in areas of low relief. Landslides can occur as ground failure of river bluffs, cut-and-fill failures that may accompany highway and building excavations, collapse of mine-waste piles, and slope failures associated with quarries and open-pit mines. Underwater landslides usually involve areas of low relief and small slope gradients in lakes and reservoirs or in offshore marine settings. What damages can landslides cause? Landslides are among the most widespread geologic hazard on Earth. Landslides cause billions of dollars in damages and thousands of deaths and injuries each year around the world. Landslides threaten lives and property in every State in the Nation, resulting in an estimated 25 to 50 deaths and damage exceeding $2 billion annually. Landslides and other forms of ground failure impact communities all across the Nation. Despite advances in science and technology, losses continue to result in human suffering, billions of dollars in property losses, and environmental degradation. As population increases and our society becomes ever more complex, the economic and societal costs of landslides and other ground failures will continue to rise. Washington is one of the most landslide-prone states in the country, with hundreds to thousands of events each year. The direct cost of landslide damage includes the repair of roads and property. Indirect costs, such as loss of property value and tax revenue, and environmental effects, such as the degradation of water quality, can exceed direct costs. What are some 'warning signs' of a landslide?
Causes of Landslides:
Landslide is a general term for a wide variety of downslope movements of earth materials that result in the perceptible downward and outward movement of soil, rock, and vegetation under the influence of gravity. The materials may move by falling, toppling, sliding, spreading, or flowing. Some landslides are rapid, occurring in seconds, whereas others may take hours, weeks, or even longer to develop. Although landslides usually occur on steep slopes, they also can occur in areas of low relief. Landslides can occur as ground failure of river bluffs, cut-and-fill failures that may accompany highway and building excavations, collapse of mine-waste piles, and slope failures associated with quarries and open-pit mines. Underwater landslides usually involve areas of low relief and small slope gradients in lakes and reservoirs or in offshore marine settings. Landslides can be triggered by both natural changes in the environment and human activities. Inherent weaknesses in the rock or soil often combine with one or more triggering events, such as heavy rain, snowmelt, and changes in groundwater level, or seismic or volcanic activity. Long term climate change may result in an increase in precipitation and ground saturation and a rise in groundwater level, reducing the shear strength and increasing the weight of the soil. Erosion may remove the toe and lateral slope support of potential landslides. Storms and sea level rise often exacerbate coastal erosion and landslides. Earthquakes and volcanoes often trigger landslides. Human activities triggering landslides are usually associated with construction and changes in slope and surface water and groundwater levels. Changes in irrigation, runoff and drainage can increase erosion and change groundwater levels and ground saturation. |
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How can landslide hazards be mitigated?
Vulnerability to landslide hazards is a function of location, type of human activity, use, and frequency of landslide events. The effects of landslides on people and structures can be lessened by total avoidance of landslide hazard areas or by restricting, prohibiting, or imposing conditions on hazard-zone activity. Local governments can reduce landslide effects through land-use policies and regulations. Individuals can reduce their exposure to hazards by educating themselves on the past hazard history of a site and by making inquiries to planning and engineering departments of local governments. They can also obtain the professional services of an engineering geologist, a geotechnical engineer, or a civil engineer, who can properly evaluate the hazard potential of a site, built or unbuilt. The hazard from landslides can be reduced by avoiding construction on steep slopes and existing landslides, or by stabilizing the slopes. Stability increases when ground water is prevented from rising in the landslide mass by:
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Landform susceptible to landslides:
The designation of a specific landform connotes both a genetic classification and a type of landscape. For example, a sand dune landform denotes deposits formed by wind movement and sorting, which form unconsolidated, smooth, flowing hills and ridges. An appreciation of the genetic aspects of landforms enables one to estimate their potential susceptibility for movement. The type of landscape of each landform provides a basis for separating the various landforms and thus recognizing those most prone to sliding. Landslides can occur in almost any landform If the conditions are right (e.g., steep slopes, high moisture level, no vegetative cover). Conversely, landslides may not occur on the most landslide-susceptible terrain if certain conditions are not present (e.g., clay shales on flat slopes with low moisture levels). Experience in observing and working with various landforms, however, has demonstrated that landslides are common in some landforms and rare in others. The subdivisions are based on topographic expression and, in the case of hilly terrains, also on drainage patterns. |
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Slope stability is also increased when a retaining structure and/ or the weight of a soil/rock berm are placed at the toe of the landslide or when mass is removed from the top of the slope. Over the past few decades, an array of techniques and practices has evolved to reduce and cope with losses from landslide hazards. Careful development can reduce losses by avoiding the hazards or by reducing the damage potential. Landslide risk can be reduced by five approaches used individually or in combination to reduce or eliminate losses.
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