Oktober 25, 2010
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A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.
Landslides occur when the stability of a slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone. Natural causes of landslides include:
- groundwater (porewater) pressure acting to destabilize the slope
- Loss or absence of vertical vegetative structure, soil nutrients, and soil structure (e.g. after a wildfire)
- erosion of the toe of a slope by rivers or ocean waves
- weakening of a slope through saturation by snowmelt, glaciers melting, or heavy rains
- earthquakes adding loads to barely-stable slope
- earthquake-caused liquefaction destabilizing slopes
- volcanic eruptions
landslides are aggravated by human activities, Human causes include:deforestation, cultivation and construction, which destabilize the already fragile slopes
- vibrations from machinery or traffic
- earthwork which alters the shape of a slope, or which imposes new loads on an existing slope
- in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock
- Construction, agricultural or forestry activities (logging) which change the amount of water which infiltrates the soil.
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 Debris flow
Slope material that becomes saturated with water may develop into a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses and cars, thus blocking bridges and tributaries causing flooding along its path.
Debris flow is often mistaken for flash flood, but they are entirely different processes.
Muddy-debris flows in alpine areas cause severe damage to structures and infrastructure and often claim human lives. Muddy-debris flows can start as a result of slope-related factors and shallow landslides can dam stream beds, resulting in temporary water blockage. As the impoundments fail, a “domino effect” may be created, with a remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel. The solid-liquid mixture can reach densities of up to 2 tons/m³ and velocities of up to 14 m/s (Chiarle and Luino, 1998; Arattano, 2003). These processes normally cause the first severe road interruptions, due not only to deposits accumulated on the road (from several cubic metres to hundreds of cubic metres), but in some cases to the complete removal of bridges or roadways or railways crossing the stream channel. Damage usually derives from a common underestimation of mud-debris flows: in the alpine valleys, for example, bridges are frequently destroyed by the impact force of the flow because their span is usually calculated only for a water discharge. For a small basin in the Italian Alps (area = 1.76 km²) affected by a debris flow, Chiarle and Luino (1998) estimated a peak discharge of 750 m3/s for a section located in the middle stretch of the main channel. At the same cross section, the maximum foreseeable water discharge (by HEC-1), was 19 m³/s, a value about 40 times lower than that calculated for the debris flow that occurred.
 Earth flow
Earthflows are downslope, viscous flows of saturated, fine-grained materials, which move at any speed from slow to fast. Typically, they can move at speeds from 0.17 to 20 km/h. Though these are a lot like mudflows, overall they are slower moving and are covered with solid material carried along by flow from within. They are different from fluid flows in that they are more rapid. Clay, fine sand and silt, and fine-grained, pyroclastic material are all susceptible to earthflows. The velocity of the earthflow is all dependent on how much water content is in the flow itself: if there is more water content in the flow, the higher the velocity will be.
These flows usually begin when the pore pressures in a fine-grained mass increase until enough of the weight of the material is supported by pore water to significantly decrease the internal shearing strength of the material. This thereby creates a bulging lobe which advances with a slow, rolling motion. As these lobes spread out, drainage of the mass increases and the margins dry out, thereby lowering the overall velocity of the flow. This process causes the flow to thicken. The bulbous variety of earthflows are not that spectacular, but they are much more common than their rapid counterparts. They develop a sag at their heads and are usually derived from the slumping at the source.
Earthflows occur much more during periods of high precipitation, which saturates the ground and adds water to the slope content. Fissures develop during the movement of clay-like material creates the intrusion of water into the earthflows. Water then increases the pore-water pressure and reduces the shearing strength of the material.
 Debris avalanche
A debris avalanche is a type of slide characterized by the chaotic movement of rocks soil and debris mixed with water or ice (or both). They are usually triggered by the saturation of thickly vegetated slopes which results in an incoherent mixture of broken timber, smaller vegetation and other debris. Debris avalanches differ from debris slides because their movement is much more rapid. This is usually a result of lower cohesion or higher water content and commonly steeper slopes.
Debris slides generally begin with large blocks that slump at the head of the slide and then break apart as they move towards the toe. This process is much slower than that of a debris avalanche. In a debris avalanche this progressive failure is very rapid and the entire mass seems to somewhat liquefy as it moves down the slope. This is caused by the combination of the excessive saturation of the material, and very steep slopes. As the mass moves down the slope it generally follows stream channels leaving behind a V-shaped scar that spreads out downhill. This differs from the more U-shaped scar of a slump. Debris avalanches can also travel well past the foot of the slope due to their tremendous speed.
A sturzstrom is a rare, poorly understood type of landslide, typically with a long run-out. Often very large, these slides are unusually mobile, flowing very far over a low angle, flat, or even slightly uphill terrain.
 Shallow landslide
Landslide in which the sliding surface is located within the soil mantle or weathered bedrock (typically to a depth from few decimetres to some metres). They usually include debris slides, debris flow, and failures of road cut-slopes. Landslides occurring as single large blocks of rock moving slowly down slope are sometimes called block glides.
Shallow landslides can often happen in areas that have slopes with high permeable soils on top of low permeable bottom soils. The low permeable, bottom soils trap the water in the shallower, high permeable soils creating high water pressure in the top soils. As the top soils are filled with water and become heavy, slopes can become very unstable and slide over the low permeable bottom soils. Say there is a slope with silt and sand as its top soil and bedrock as its bottom soil. During an intense rainstorm, the bedrock will keep the rain trapped in the top soils of silt and sand. As the topsoil becomes saturated and heavy, it can start to slide over the bedrock and become a shallow landslide. R. H. Campbell did a study on shallow landslides on Santa Cruz Island California. He notes that if permeability decreases with depth, a perched water table may develop in soils at intense precipitation. When pore water pressures are sufficient to reduce effective normal stress to a critical level, failure occurs.
 Deep-seated landslide
Landslides in which the sliding surface is mostly deeply located below the maximum rooting depth of trees (typically to depths greater than ten meters). Deep-seated landslides usually involve deep regolith, weathered rock, and/or bedrock and include large slope failure associated with translational, rotational, or complex movement. These typically move slowly, only several meters per year, but occasionally move faster. They tend to be larger than shallow landslides and form along a plane of weakness such as a fault or bedding plane. They can be visually identified by concave scarps at the top and steep areas at the toe.
 Causing tsunamis
Landslides that occur undersea, or have impact into water, can generate tsunamis. Massive landslides can also generate megatsunamis, which are usually hundreds of meters high. In 1958, one such tsunami occurred in Lituya Bay in Alaska.
 Related phenomena
- An avalanche, similar in mechanism to a landslide, involves a large amount of ice, snow and rock falling quickly down the side of a mountain.
- A pyroclastic flow is caused by a collapsing cloud of hot ash, gas and rocks from a volcanic explosion that moves rapidly down an erupting volcano.
 Landslide prediction mapping
Landslide hazard analysis and mapping can provide useful information for catastrophic loss reduction, and assist in the development of guidelines for sustainable land use planning. The analysis is used to identify the factors that are related to landslides, estimate the relative contribution of factors causing slope failures, establish a relation between the factors and landslides, and to predict the landslide hazard in the future based on such a relationship . The factors that have been used for landslide hazard analysis can usually be grouped into geomorphology, geology, land use/land cover, and hydrogeology . Since many factors are considered for landslide hazard mapping, GIS is an appropriate tool because it has functions of collection, storage, manipulation, display, and analysis of large amounts of spatially referenced data which can be handled fast and effectively . Remote sensing techniques are also highly employed for landslide hazard assessment and analysis. Before and after aerial photographs and satellite imagery are used to gather landslide characteristics, like distribution and classification, and factors like slope, lithology, and land use/land cover to be used to help predict future events . Before and after imagery also helps to reveal how the landscape changed after an event, what may have triggered the landslide, and shows the process of regeneration and recovery .
Using satellite imagery in combination with GIS and on-the-ground studies, it is possible to generate maps of likely occurrences of future landslides . Such maps should show the locations of previous events as well as clearly indicate the probable locations of future events. In general, to predict landslides, one must assume that their occurrence is determined by certain geologic factors, and that future landslides will occur under the same conditions as past events . Therefore, it is necessary to establish a relationship between the geomorphologic conditions in which the past events took place and the expected future conditions .
Natural disasters are a dramatic example of people living in conflict with the environment. Early predictions and warnings are essential for the reduction of property damage and loss of life. Because landslides occur frequently and can represent some of the most destructive forces on earth, it is imperative to have a good understanding as to what causes them and how people can either help prevent them from occurring or simply avoid them when they do occur. Sustainable land management and development is an essential key to reducing the negative impacts felt by landslides.
GIS offers a superior method for landslide analysis because it allows one to capture, store, manipulate, analyze, and display large amounts of data quickly and effectively. Because so many variables are involved, it is important to be able to overlay the many layers of data to develop a full and accurate portrayal of what is taking place on the Earth’s surface. Researchers need to know which variables are the most important factors that trigger landslides in any given location. Using GIS, extremely detailed maps can be generated to show past events and likely future events which have the potential to save lives, property, and money.
 Prehistoric landslides
- Landslide which moved Heart Mountain to its current location, the largest ever discovered on land. In the 48 million years since the slide occurred, erosion has removed most of the portion of the slide.
- Flims Rockslide, ca. 13,000 km3 (8.1×109 mi), Switzerland, some 10000 years ago in post-glacial Pleistocene/Holocene, the largest so far described in the alps and on dry land that can be easily identified in a modestly eroded state.
- The landslide around 200BC which formed Lake Waikaremoana on the North Island of New Zealand, where a large block of the Ngamoko Range slid and dammed a gorge of Waikaretaheke River, forming a natural reservoir up to 248 metres deep.
- Cheekye Fan, British Columbia, Canada, ca. 25 km2 (9.7 sq mi), Late Pleistocene in age.
 Prehistoric submarine landslides
- The Storegga Slide, Norway, ca. 3,500 km3 (840 cu mi), ca. 8,000 years ago, a catastrophic impact on the contemporary coastal Mesolithic population
- The Agulhas slide, ca. 20,000 km3 (1.2×1010 mi), off South Africa, post-Pliocene in age, the largest so far described
- The Ruatoria Debris Avalanche, off North Island New Zealand, ca. 3,000 km³ in volume, 170,000 years ago .
 Historical landslides
 19th Century
- Rockslide Goldau, canton of Schwyz, Switzerland, on September 2, 1806
- Cliff landslip of the Undercliff near Lyme Regis, Dorset, England, on 24 December 1839
- Face collapse of The Barrier in southwestern British Columbia, Canada, 1855–1856
- The Cap Diamant Québec rockslide on September 19, 1889
 20th Century
- Frank Slide, Turtle Mountain, Alberta, Canada, on 29 April 1903
- Amalfi landslide in Salerno, Italy on March 1924.
- Gros Ventre landslide in Wyoming, United States, on June 23, 1925
- Mount Rokko mudslide by heavy rain in Kobe, Hyogo, Japan on July 1938.
- Rio Santa and Cordillera Blanca avalanche in Ancash Region, Peru on December 1941.
- Alcalá del Jucar landslide in Albacete, Spain on December 1945.
- Guwahati Landslide in Assam, India on September 1948.
- Khait landslide, Khait, Tajikistan, Soviet Union, on July 10, 1949
- Santa Elena landslide in Antioquia Department, Colombia on July 1954.
- Molina di Vietri and Ponte Romano landslide in Salerno, Italy on October 1954.
- The Riñihuazo landslide in Chile after the Great Chilean Earthquake, on 22 May 1960
- Ranrahirca landslide in Peru on January 1962.
- Monte Toc landslide (260 millions cubic metres) falling into the Vajont Dam basin in Italy, causing a megatsunami and about 2000 casualties, on October 9, 1963
- Hope Slide landslide (46 million cubic metres) near Hope, British Columbia on January 9, 1965.
- El Cobre landslide with El Soldado cooper mine damage in Atacama, Chile on February 1965.
- The 1966 Aberfan disaster
- Santa Teresa landslide in Rio State, Brazil on February 1967.
- Caraguatatuba landslide in State of São Paulo, Brazil on March 1967.
- Kure mudslide by Typhoon Billie in Hiroshima, Japan on July 1967.
- Hida River landslide with two charter buses plunge in Gero, Gifu, Japan on August 1968.
- Darjeeling landslide in West Bengal on October 1968.
- Amherst and Nelson landslide by Hurricane Camille in Virginia on August 1969.
- the May 31, 1970 slide from Cerro Huascaran that buried the town of Yungay.
- Chungar landslide by avalanche in Peru, on March 1971.
- Saint-Jean-Vianney, Quebec, Canada. Small village near Saguenay river destroyed in May 1971.
- Khinjan Pass landslide in Baghian, Afghanistan on July 1971.
- Tosayamada landslide in Shikoku, Japan on July 1972.
- Amakusa mudslide in Kumamoto, Kyūshū, Japan on July 1972.
- Moyomarca hill mudslide in Huancayo, Peru on April 1974.
- Quebradablanca avalanche with swept 33 vehicle in Boyacá, Colombia on June 1974.
- Zona de Armenta and Omoa landslide by Hurricane Fifi in Cortes Department, Honduras, on September 1974.
- Pahire Phedi landslide in Nepal on June 1976.
- Baliem Valley landslide by 1976 Papua earthquake in Irian Jaya, Indonesia on July 1976.
- Siheung and Anyang landslide in Gyeonggi, South Korea on July 1977.
- Tuve landslide in Gothenburg, Sweden on November 30, 1977.
- Nilgiri Hills landslide in Tamil Nadu, India on November 1978
- The 1979 Abbotsford landslip, Dunedin, New Zealand on August 8, 1979.
- Ayvazhaci avalanche in Kayseri Province, Turkey, on March 1980.
- Landslides associated with the Mount St. Helens eruption on May 18, 1980.
- Mount Semeru landslide by heavy rain in East Java, Indonesia on August 1981
- Nakajima landslide in Nagasaki, Kyūshū, Japan on July 1982
- Ataco mudslide in El Salvador on September 1982
- Dongxing landslide in Gansu, China, on March 1983
- Thistle, Utah on 14 April 1983
- Chunchi mudslide in Chimborazo, Ecuador, on April 1983
- Almora landslide in Uttar Pradesh, India on July 1983
- Dongchuan landslide in Yunnan, China on May 1984
- The Mameyes Disaster – Ponce, Puerto Rico on October 7, 1985
- Buyeo and Seocheon landslide by Typhoon Thelma in Chungchongnam-do, South Korea, on July 1987.
- Val Pola landslide during Valtellina disaster (1987) Italy
- El Limon mudslide in Aragua, Venezuela on September 1987.
- Villatina mudslide in Colombia on September 1987.
- Wuxi County landslide in Sichuan, China on September 1987.
- Macka landslide in Trabzon, Turkey on June 1988
- Darwang and Niskot landslide in Myagdi, Nepal on September 1988.
- Sharora landslide by 1989 Tajikistan earthquale in Hisor District, Tajikistan on January 1989.
- Tsablanca landslide in Georgia on April 1989.
- Bhaji landslide in Maharashtra, India on July 1989
- Calama mudslide in Atacama, Chile on June 1991.
- Zhaotong landslide by torrential rain, in Yunnan, China on September 1991
- Ninghai mudslide in Zhejiang, China on September 1992.
- Llipi Limitada landslide in Larecaja Province, Bolivia, on December 1992.
- Nambija Bajo mudslide in Zamora, Ecuador on May 1993.
- The Pantai Remis landslide in 1993 in an abandoned coastal tin mine in Malaysia, forming a new cove
- Kagoshima mudslide in Kyūshū, Japan on August 1993.
- Yuangyang mudslide in Yunnan, China on July 1994
- Khooni Nallah and Banihal tunnel avalanche in Jammu and Kashimir region, India on January 1995.
- Wakhan landslide in Badakhshan, Afghanistan on April 1995.
- Cheorwon landslide in Gangwon, South Korea on July 1996.
- Tamburco mudslide by torrential rain in Apurímac Region, Peru on February 1997.
- Thredbo landslide, Australia on 30 July 1997, destroyed hostel.
- Pithoragarh mudslide in Uttar Pradesh, India on August 1998
- Lishui landslide in Zhejiang, China on September 1999
- The Vargas tragedy, due to heavy rains in Vargas State, Venezuela, on December, 1999, causing tens of thousands of casualties.
- Aldercrest-Banyon landslide in Kelso, Washington, in 1998-1999, one of the worst urban landslides in U.S. history destroying 127 homes and causing $40 million in damages.
 21st Century
- Payatas, Manila garbage slide on 11 July 2000.
- Mianning landslide by torrential rain in Liangshan, Sichuan, China on July 2000
- Amboori landslide, in Kerala, 2001
- Danba mudslide in Sichuan, China on July 2003
- Zuojiaying landslide in Nayong, Guizhou, China on December 2004
- La Conchita mudslide in California, United States on January 10, 2005, killed 10 people and destroyed 18 homes
- Jaigaon mudslide in Maharashtra, India on July 2005
- Southern Leyte landslide in the Philippines on 17 February 2006
- Devil’s Slide, an ongoing landslide in San Mateo County, California
- Landslide in Sulawesi, Indonesia, June 2006.
- Liangshan mudslide in Sichuan, China on May 2007
- 2007 Chittagong mudslide, in Chittagong, Bangladesh, on June 11, 2007.
- 2008 Cairo landslide on September 6, 2008.
- Xiangfen County mudslide with unlicensed Tashan coal mine collapse in Shanxi, China on September 2008.
- Lincang mudslide in Yunnan, China killed 82 people on November 2008 }
- Wulong mudslide in Chongqing, China on July 2009
- Hofu mudslide in Yamaguchi, Japan on July 2009.
- Liuzhou, Guangxi Region, China – derailed train, killing 4 
- Shiaolin landslide by Typhoon Morakot in Kaohsiung County, Taiwan killing 439 people when the whole village was buried or swept away on August 9, 2009 
- Nile Valley Landslide, no injuries but destroyed some houses, obliterated a quarter mile of Washington State Route 410 and redirecting the Naches River 10 miles west of Naches, Washington on 11 October 2009.
- Bayambang and Alcalá landslide in Benguet, Philippines on October 2009.
- San Vicente and San Salvador mudslide by Hurricane Ida in El Salvador on November 2009.
- Angra dos Reis mudslide in Ilha Grande Island, Rio State, Brazil on