2014, Oso Landslide, Washington: On March 22, 2014, a landslide of approximately 18 million tons (10 million yd 3) traveled at 64 kph (40 mph), extended for nearly a 1.6 km (1 m), and dammed the North Fork of … Earth materials can become detached from a steep slope without significant shearing, fall freely under gravity, and land on a surface from which they bounce and fall farther. This 1995 landslide at La Conchita, a coastal town in California, swept away a hillside road and destroyed a number of houses. Figure 1. In 1995, La Conchita in Ventura County, California, experienced a landslide called a Slump. Mudflow: A mudflow is an earthflow consisting of material that is wet enough to flow rapidly and that contains at least 50 percent sand-, silt-, and clay-sized particles. An idealized Schuster, U.S. Geological Survey. Debris flow: A debris flow is a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope (fig. These volcanic debris flows (also known as lahars) reach great distances, once they leave the flanks of the volcano, and can damage structures in flat areas surrounding the volcanoes. The materials may move by falling, toppling, sliding, spreading, or flowing. The landslide, which occurred about 130 km northwest of Los Angeles, California, mobilized over 40,000 cubic yards of wet debris into a large scale debris flow (commonly referred to in the media as a mudslide) that flowed into a residential community at the foot of the slope, killing 10 persons and damaging or destroying 36 residences. In addition, debris flows and mudflows usually occur in small, steep stream channels and often are mistaken for floods; in fact, these two events often occur simultaneously in the same area. The various types of landslides can be differentiated by the kinds of material involved and the mode of movement. In a rotational slide the axis of rotation is roughly parallel to the contours of the slope. This effect can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and water-level changes along coastlines, earth dams, and the banks of lakes, reservoirs, canals, and rivers. This causes backwater flooding and, if the dam fails, subsequent downstream flooding. Triggered by earthquake shock or torrential rain in mountainous relief with steep gradients, a huge volume of avalanching rock or debris (of up to millions of metric tons) can reach a velocity of more than 50 metres (160 feet) per second and leave a long trail of destruction. Landslide, also called landslip, the movement downslope of a mass of rock, debris, earth, or soil (soil being a mixture of earth and debris). There are generally three types of creep: (1) seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature; (2) continuous, where shear stress continuously exceeds the strength of the material; and (3) progressive, where slopes are reaching the point of failure as other types of mass movements. In particular, it is clear from Figure 2 that the slope is mantled with old landslide scars and deposits. Another factor that affects the shear strength of a slope-forming material is the spatial disposition of its constituent particles, referred to as the sediment fabric. A translational slide is sometimes called a mud slide when it occurs along gently sloping, discrete shear planes in fine-grained rocks (such as fissured clays) and the displaced mass is fluidized by an increase in pore water pressure. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. Landsliding and flooding are closely allied because both are related to precipitation, runoff, and the saturation of ground by water. Shear strength is dependent mainly on two factors: frictional strength, which is the resistance to movement between the slope material’s interacting constituent particles, and cohesive strength, which is the bonding between the particles. After the 1995 landslide, the town of La Conchita built a retaining wall at the foot of the hill. Rockslides and other types of slides involve the displacement of material along one or more discrete shearing surfaces. The most common types of landslides are described as follows and are illustrated in figure 3. La Conchita Landslide Hazard During the second week of January, the media spotlight focused on a deadly landslide in the small seaside town of La Conchita, California. This is a typical type of landslide. The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. Geological Survey Short-term stresses, such as those imposed by earthquakes and rainstorms, can likewise contribute to the activation of landslides. Volcanic lava may melt snow at a rapid rate, causing a deluge of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volcanoes, devastating anything in its path. Debris flows also commonly mobilize from other types of landslides that occur on steep slopes, are nearly saturated, and consist of a large proportion of silt- and sand-sized material. The sliding can extend downward and outward along a broadly planar surface (a translational slide), or it can be rotational along a concave-upward set of shear surfaces (a slump). Oversteepening of slope Removal of support at base of slope Weak slope materials Adding a load at the People were evacuated and the houses nearest theLandslides in the United States cause approximately $3.5 billion (year 2001 dollars) Often the initial failure is a slump, but in some materials movement occurs for no apparent reason. Figure 2. 3C). Falls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs (fig. The failure starts suddenly in a small area and spreads rapidly. The primary regions of landslide occurrence and potential are the coastal and mountainous areas of California, Oregon, and Washington, the States comprising the intermountain west, and the mountainous and hilly regions of the Eastern United States. The digital file given to me for my research was imported from AutoCad DXF format into ArcInfo. Detailed Description An oblique LIDAR image of La Conchita, Calif., reveals the community’s grim landslide history.
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