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Landslides: Causes-Effect and Reduction

by Sanjenbam Jugeshwor Singh
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The terms landslide or mudslide refer to the downward movement of large masses of rocks, soil, mud and organic debris. Areas with steep slopes, for example mountainous regions, are particularly susceptible to landslides. The term ‘’landslide’’ encompasses five modes of slope movement: falls, topples, slides, spread and flow. These are further sub-divided by the type of geologic materials (bedrock, debris or earth). Debris flows (commonly referred to as mudflows or mudslides) and rock falls are examples of common landslides types.
Almost every landslide has multiple causes. The primary cause of a landslide is the influence of gravity acting on weakened materials that make up a sloping area of land. Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope. Causes include factors that increase the effect of down-slope forces and the factors that contribute to low or reduced strength. Landslide can be initiated in slopes already on the verge of movement by rainfall, snowmelt, and change in water level, stream erosion, and changes in ground water, earthquakes, volcanic activity, disturbance by human activities or any combination of these factors. Earthquake shaking and other factors can also induce landslides underwater. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunami that damage coastal areas. While some landslides occur slowly over time (e.g. land movement on the order of few meters/yard per month), the most destructive ones happen suddenly after triggering event such as heavy rainfall or an earthquake. Landslide (29 midnight of June 2022)at Makhuam village in Noney District of Manipur could be such an example (due to heavy rain). Water cans trigger landslides and mudslides because it alters the pressure within the slope, which leads to slope instability. Consequently, the heavy water-laden slope materials (soil, rock etc.,) will succumb to the forces of gravity. Excessive water is thought to be one of the most common triggers for landslides. Other factors that weaken slope materials also contribute to the occurrence of landslides. These factors include both natural events such as geological weathering and erosion and human-related activities such deforestation and changes made to the flow of groundwater. Destruction of vegetation by droughts, fires and logging has been associated with increased risk for landslides. Landslides are classified according to the types of materials that falls and how that materials move down slope. For example, there are rock falls, mudslides and debris flow. Lahars are volcanic mud flow or debris flow that is capable of travelling at very fast speed down the slope of volcano.
The negative effects of landslides are: they damage/destroy ecosystems, harm agriculture, damage built infrastructure, lead to economic losses, deaths, social disruption and can lead to other hazards. When landslides occur, they can seriously damage or destroy ecosystems. Sometimes the effects can last for thousands of years. They can pollute streams and water bodies with sediment and debris. This invariably has severe repercussions on water quality and marine life. What’s more, these hazards can wipe out large tracts of forests, wildlife habitats and remove productive soils from slopes. In 1960, an earthquake triggered several landslides in Chile which destroyed more than 250 km2 of forest. Also, they can dam up rivers and streams. In doing so, water flow is restricted. Marine and terrestrial organisms that depend on the water flow may eventually die. Conversely, dams may flood the opposite side too. Outburst floods can introduce a tremendous amount of new sediment into streams. Or they can submerge and kill vegetation in the flooded region. One of the most damaging effects of landslides is the destruction of agricultural land. Debris spread over farms, cultivation and pastures, covering important agricultural land. They destroy seeds, plants, food stocks and grazing land. These lands can remain inaccessible for years ultimately impacting on farmers’ livelihoods. In Guatemala as an example, watersheds collapsed in 2005 due to landslides associated with Tropical Storm Stan. Consequently, farmers were severely affected by this event. Landslides can cause serious damage to the built infrastructure. They can destroy and/or damage houses, buildings and infrastructure close to hills and mountains. They block roads, railroads and shipping lanes. In 1980, a debris flow from Mount St Helens volcano filled the Columbia River with more than 34 million m3 of sediment. Cargo ships could not reach Oregon until the sediment was dredged. As it currently stands, economic losses due to these hazards have been increasing during the past decades. This is mostly linked to increasing development and investment in landslide-prone regions. Basically, landslide costs include both direct and indirect losses. Direct losses refer to costs of repair, replacement or maintenance of properties and assets affected by landslides. All other costs like loss of agricultural productivity, effects on water quality and reduced real estate values are indirect losses. The devastating 1983 Thistle landslide in Utah, United States, had direct and indirect economic losses on the order of $688 million.  
Landslides are responsible for a number of deaths, injury to people, and damage to housing, infrastructure and agricultural lands. In the United States alone, some 25-50 people are killed by landslides each year. In Italy, more than 10,000 people died in 840 landslides during the period 1279 to 1999. More often than not, the disasters happen without warning giving people no time to escape. Sometimes, such events can impact on the mental health of people also; children and youngsters especially at more at risk. Moreover, victims of landslides can develop crush injuries or crush syndrome after they have been rescued from the debris. Very often, landslides can trigger other hazards like tsunamis, volcanic eruptions, wildfires and earthquakes. One of the most famous landslides that initiated baffling tsunami waves is the 8000-year-old Storage submarine landslide off the coast of Norway. Tsunami waves flooded coastlines as far away as Greenland. Similarly, the 2007 landslide in Vancouver, Canada, resulted in the discharge of 3 million m3 of debris into the Chehalis Lake. Several hectares of shoreline forest were destroyed and tsunami waves pulled out trees from the shoreline to a height of 18 m.
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 groundwater is prevented from rising in the landslide mass by (1) covering the landslide with an impermeable membrane, (2) directing surface water away from the landslide, (3) draining groundwater away from the landslide, and (4) minimizing surface irrigation. 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.
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