10.5: Landslide Types and Processes - Geosciences LibreTexts Shallow landslides can occur at any time during the winter, but are more likely happen when the ground is nearly saturated. A complete prediction of the process would include assessments of where, when, and how big. On January 10, 2005, the southeastern portion of the 1995 landslide deposit failed, resulting in shallow, rapid fluid flow, unlike the 1995 landslide. 10 years after fatal mudslide, tiny La Conchita accepts the risks Debris flows are commonly caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil or rock on steep slopes. The bluff above La Conchita has produced a variety of landslides over an extended period of time. Debris flows are commonly caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil or rock on steep slopes. La Conchita, coastal area of southern California. LockA locked padlock A classification system based on these parameters is shown in table 1. Page Last Modified: Sat Jan 12 21:11 EDT 2013. A lock ( A few things, however, can be said. This website uses cookies to improve your experience while you navigate through the website. Coarse particles such as sand grains have high frictional strength but low cohesive strength, whereas the opposite is true for clays, which are composed of fine particles. Surface cracks in the upper part of the slope were noted as far back as the summer of 1994. Short-term stresses, such as those imposed by earthquakes and rainstorms, can likewise contribute to the activation of landslides. PDF Falling Into the Pacific: California Landslides and Land Use Controls This was not the first destructive landslide to damage this community, nor is it likely to be the last. Morton, D.M., 1971, Seismically triggered landslides above San Fernando Valley: California Geology, v. 24, no. The 1995 slide was 120 m (400 ft) wide, 330 m (1100 ft) long, and covered approximately 4 ha (10 acres). In addition, large and small ravines that incise the bluff have produced debris flows recently and in the past. Figure 3: Some major types of mass movements. The timing of the two landslides with respect to the triggering storms is of primary interest. Since only about 15 percent of the 1995 deposit remobilized in 2005, could the remainder also mobilize into a rapid debris flow, or is it more likely to remobilize as a deep slump? Hemphill, J.J., 2001, Assessing landslide hazard over a 130year period for La Conchita, California, in Association of Pacific Coast Geographers Annual Meeting, Santa Barbara, Calif., September 1215 2001. Background rates of coastal cliff erosion are variable along the California coast (seeNational Assessment of Shoreline Change Part 4: Historical Coastal Cliff Retreat along the California Coast - Open File Report 2007-1133) and tied to the rock or soil strength of the cliffs among other factors, but these measurements of historic coastal cliff retreat provide indications of places most susceptible to coastal landslides. Figure 8 shows an aerial photograph of La Conchita taken in September 2004. Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features. Secure .gov websites use HTTPS We reviewed their content and use your feedback to keep the quality high. Lateral spreads are distinctive because they usually occur on very gentle slopes or flat terrain (fig. Landslide Types and Processes - USGS 3I). Do they actually do gymnastics in Make It or Break It? The material moves as a single mass along a slippery zone. The information in this publication provides an introductory primer on understanding basic scientific facts about landslidesthe different types of landslides, how they are initiated, and some basic information about how they can begin to be managed as a hazard. The KCALTV video indicates that the landslide material mobilized simultaneously and nearly instantaneously into a highly fluid, rapidly moving debris flow. Wofford, Michael, 2005, Ventura, California weather conditions. Official websites use .gov Photo by R.L. When coherent material, either bedrock or soil, rests on materials that liquefy, the upper units may undergo fracturing and extension and may then subside, translate, rotate, disintegrate, or liquefy and flow. This mode of movement would most likely be relatively slow (compared to 2005) but still could pose serious hazards to property and, perhaps, life. Know the Signs of a Deep-Seated Landslide Officials in Southern California say a coastal landslide closed a historic cultural center, shut down rail service in the area and forced the evacuation of nearby residences. Future landslide activity could occur in the areas of the 1995 and 2005 landslides or in other areas and could damage or destroy any or all of the developed area. Rock of both formations is very weakly cemented and has been regionally associated with extensive landslide activity (Morton, 1971; Harp and Jibson, 1995, 1996; Parise and Jibson, 2000). The deposit covered approximately 4 hectares (9.9 acres), and the volume was estimated to be approximately 1.3 million cubic meters (1.7 million cubic yards) of sediment. 7). This cookie is set by GDPR Cookie Consent plugin. The 1995 landslide had a horizontal length, vertical length, and depth of 120 x 330 x 30 m and covered approximately 10 acres with a volume of 1.3x106 m3. Early Warning System for Southern California. landslide, also called landslip, the movement downslope of a mass of rock, debris, earth, or soil (soil being a mixture of earth and debris). Mean seasonal rainfall at Ojai (20 km [12 mi] northeast of La Conchita) from October 1 through March 3 (the day before the landslide occurred) is 390 mm (15.37 in) (National Oceanic and Atmospheric Administration, 1994a, 1995a). The 2005 landslide involved few new materials and seems to have been a continuation of the original 1995 landslide. In March 2006, Governor of California, Arnold Schwarzenegger, allocated $667,000 for a scientific study to determine control measures to be taken to prevent future landslides. The tragedy came 10 years after the hill collapsed in 1995 when nine homes were lost. Landslides can also be activated by processes that weaken the shear strength of a slopes material. The dominant mode of movement is lateral extension accompanied by shear or tensile fractures. ) or https:// means youve safely connected to the .gov website. From December 27, 2004, through January 10, 2005, the nearby city of Ventura received 378 millimeters (14.9inches) of rainfall, only slightly less than its mean annual total of 390 millimeters (15.4inches). Where did the La Conchita . The most common types of landslides are described as follows and are illustrated in figure 3. 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. However, dry flows of granular material are also possible. 1995 La Conchita, California, landslide. Incipient movement of the upper part of the slide was reported as early as the summer of 1994, when surface cracks were observed in the upper part of the slope (O'Tousa, 1995). Days later, on March 10, a subsequent debris flow from a canyon to the northwest damaged five additional houses in the northwestern part of La Conchita. Inspection of the site within a few hours of the landslide indicated that much of the deposit consisted of fairly dry material (James O'Tousa, RJR Engineering, personal commun., 2005). There are five basic categories of flows that differ from one another in fundamental ways. The landslide destroyed 13 houses and severely damaged 23 others. A slide happens when a section of soil or rock suddenly gives way and moves down a slope. An idealized slump-earth flow showing commonly used nomenclature for labeling the parts of a landslide. In both 1889 and 1909, the Southern Pacific rail line running along the coast was inundated. Sometimes more than one type of movement occurs within a single landslide, and, because the temporal and spatial relationships of these movements are often complex, their analysis often requires detailed interpretation of both landforms and geological sections, or cores. After this point, a rain burst of 5-6 mm (0.2 to 0.25 in) in one hour has been observed to trigger abundant shallow landslides (seeLandslides in Santa Monica Mountains and Vicinity- Prof Paper 851). The costliest landslide in U. history occurred in the Thistle, Utah area in April 1983. The landslide slumped as a coherent mass of material. Landslides are described and classified in various ways depending upon the material (s) involved, mode of movement, rate of movement, and the water, air or ice content of the landslide material (see Figure 4). Near a Slope? Unfortunately, we currently lack the understanding to accurately forecast what might happen in each possible rainfall scenario. The difference between slides and flows is gradational, with variations in fluid content, mobility, and type of movement, and composite slide movement and flow movement are common. 7 What happened in the La Conchita landslide 2005? Week 14 Lecture on Landslides - Landslides When gravity - Studocu But opting out of some of these cookies may affect your browsing experience. What experience do you need to become a teacher? The materials may move by falling, toppling, sliding, spreading, or flowing. There were 10 confirmed fatalities. The landslide entered the La Conchita neighborhood destroying 13 houses and severely damaging 23 others. Contact between the flowing mass and the underlying material can be distinct, or the contact can be one of diffuse shear. 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. 8 What year did the landslide happen in Ventura CA? Reading: Landslide Types and Processes | Geology | | Course Hero 3F). October 17, 2022. Photo by Pam Irvine, CGS. The various types of landslides can be differentiated by the kinds of material involved and the mode of movement. What are various methods available for deploying a Windows application? Once the smoke clears from a wildfire, the danger is not over!! This is the only mitigation effort the town undertook. The cookies is used to store the user consent for the cookies in the category "Necessary". At the time of our visit (January 14, 2005) water was still issuing from the base of the main landslide scarp and was ponding at several locations on the 2005 deposit (fig. Even in the absence of additional significant rainfall this year (2005), the remainder of the 1995 landslide could still remobilize, most likely as a deep slumpearth flow similar to that in 1995. Various processes that weaken the shear strength of a slope may also activate landslides. Our editors will review what youve submitted and determine whether to revise the article. The bench at the top of the bluff is the head of a very large prehistoric landslide that affected the entire bluff. 4). The landslide area was approximately 350 m (1,150 ft) long and 80100 m (260330 ft) wide. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. Accessibility StatementFor more information contact us atinfo@libretexts.org. 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. In 199495, about twice as much rain761 mm (29.96 in)fell during that period (National Oceanic and Atmospheric Administration, 1994b, 1995b). Figure 1: Air photo of La Conchita, CA taken in 2002. Rockfall. The bluff above La Conchita has a slope of approximately 35 degrees and consists of poorly cemented marine sediments. A translational slide typically takes place along structural features, such as a bedding plane or the interface between resistant bedrock and weaker overlying material. Rotational slip. The La Conchita landslide in Ventura County is an example of a deep-seated landslide that has experienced both styles of movement (seeLandslide Hazards at La Conchita, California- Open-File Report 2015-1067). 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). Most of the excess rain fell in January, which had 623 mm (24.53 in) as compared to a normal rainfall of 108 mm (4.26 in). The U.S. Geological Survey, Multi Hazards Demonstration Project (MHDP) uses hazards science to improve resiliency of communities to natural disasters including earthquakes, tsunamis, wildfires, landslides, floods and coastal erosion. . A .gov website belongs to an official government organization in the United States. Like the northern part of the state, southern California is well known to be susceptible to landslides (seePreliminary soil-slip susceptibility maps, southwestern California- Open-File Report 2003-17). D. All of these are correct <--------. The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. C. Preferential weathering along fractures can cause blocks to become rounded. The landslides in the area around La Conchita have been a regular occurrence, according to the historical accounts dating back to 1865. They can easily overflow channels and severely damage houses, vehicles, or other structures. About USGS/ Science Topics/ Maps, Products & Publications/ Education/Publication: FAQ. Historical accounts and geologic evidence show that landsliding of a variety of types and scales has been occurring at and near La Conchita for many thousands of years, and on a relatively frequent basis, up until the present. Fires that denude slopes of vegetation intensify the susceptibility of slopes to debris flows. Falls of large volume can trap enough air to facilitate the very rapid flow of rock or debris, forming rock avalanches and debris avalanches, respectively. c) Slides: A slide is the downslope movement of a soil or rock mass occurring dominantly on the surface of rupture or relatively thin zones of intense shear strain. A large section of the hillside slid down and buried several homes after receiving almost 30 inches of rain that winter. A .gov website belongs to an official government organization in the United States. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges (fig. Debris-flow source areas are often associated with steep gullies, and debris-flow deposits are usually indicated by the presence of debris fans at the mouths of gullies. The movement near the head of the slide is largely downward, exposing a steep head scarp, and movement within the displaced mass takes place along internal slip planes, each tending to tilt backward. A spread is the complex lateral movement of relatively coherent earth materials resting on a weaker substrate that is subject to liquefaction or plastic flow. Hazards from these types of landslides can occur both at the bottom of cliffs (from burial) and at the tops of cliffs (from falling over). This is a typical type of landslide. Figure 1: Air photo of La Conchita, CA taken in 2002. The project engages emergency planners, businesses, universities, government agencies, and others in preparing for major natural disasters. The cookie is used to store the user consent for the cookies in the category "Performance". Debris avalanche: This is a variety of very rapid to extremely rapid debris flow (fig. What mitigation took place after the La Conchita slide? On January 10, 2005, a major landslide occurred in La Conchita. The term "landslide" encompasses events such as rock falls, topples, slides, spreads, and flows. Adversely oriented discontinuity (bedding, schistosity, fault, unconformity, contact, and so forth), Contrast in permeability and/or stiffness of materials, Fluvial, wave, or glacial erosion of slope toe or lateral margins. Course Hero is not sponsored or endorsed by any college or university. The upper part of the slope consists of interlayered siliceous shale, siltstone, and sandstone of the Middle to Upper Miocene Monterey Formation. At the beginning of the rainy season, in December 1994, several open cracks had opened on the hillside, and surface runoff was infiltrating into the subsurface. The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. 3). Often the initial failure is a slump, but in some materials movement occurs for no apparent reason. Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. Landslides in La Conchita - ArcGIS StoryMaps The 2005 landslide occurred at the culmination of an extremely wet 2week period (fig. The 1995 landslide was a deep, coherent slumpearth flow that deformed plastically and moved slowly enough that people could get out of its way. Heavy rains of January 1995 led to a March 4, 1995, landslide in the La Conchita Community of Ventura County (County), California. 1). They write new content and verify and edit content received from contributors. 3G). Saturated soil slumps down a curved surface. This information provides tools to help make PDF files accessible. Southern California lies astride a major tectonic plate boundary defined by the San Andreas Fault and numerous related faults that are spread across a broad region. The ZIP Code is 93001, and the community is inside area code 805 . La Conchita ( / l kntit / ( listen); Spanish for "The Little Shell") is a small unincorporated community in western Ventura County, California, on U.S. Route 101 just southeast of the Santa Barbara county line. Spreads occur as the result of liquefaction caused by water saturation or earthquake shock in such substrates as loess, a weakly cemented wind-lain silt. A summary of recent and past landslides and debris flows caused by rainfall in Southern California. There were 10 confirmed fatalities. During and just after storms, existing coastal landslides may become reactivated and seemingly stable coastal cliffs may erode and fail rapidly. The volume of the landslide was estimated to be approximately 200,000 cubic meters with a surface 350 meters (1,150 feet) long and 80100 meters (260330 ft) wide. [2] The dimensions of the slides were approximately 120 meters (390 feet) wide, 330 meters (1,080ft) long, and 30 meters (98ft) deep. 10 and 11). Articles from Britannica Encyclopedias for elementary and high school students. Colorado, and northern New Mexico) are complex in rock type and climate, and so the landslides there are also complex. An official website of the United States government. Click the image to open a larger version in a new window. In some instances, for example in many newspaper reports, mudflows and debris flows are commonly referred to as "mudslides.". Falls are strongly influenced by gravity, mechanical weathering, and the presence of interstitial water. This report describes my field observations and provides a description of the La Conchita area and its landslide history, a comparison of the 1995 and 2005 landslides, and a discussion of continuing landslide hazards in the La Conchita area. Lesson All About Landslides: Land on the Run - TeachEngineering.org Omissions? Explain why. 1012. In 1995, the community experienced a major slide, which damaged and destroyed a significant portion of the community. An idealized slump-earth flow showing commonly used nomenclature for labeling the parts of a landslide. These include oversteepening of the base of the slope, such as by natural erosion or excavation, and loading of the slope, such as by an inflow of water, a rise in the groundwater table, or the accumulation of debris on the slopes surface. What happened at La Conchita in 1995 and 2005? Legal. You'll get a detailed solution from a subject matter expert that helps you learn core concepts. A week later, debris flow in the same location damaged five more houses. In the past two decades major landslides devastated the town of La Conchita, California. Coastal cliff failuresmay also occur simply as a result of heightened precipitation as well wave action makes cliffs inherently unstable, and rainfall may be the ultimate trigger for failure, even during times with little to no wave action. The slope material liquefies and runs out, forming a bowl or depression at the head. The landslide recurred on part of a previous landslide in 1995.La Conchita, California. The volume of material moved by the landslide was 2.8 cubic kilometers (0.67 mi 3) . Landslides are most abundant in areas underlain by marine shale . An unstable hillside The rain caused debris from an older landslide to fall about 150 feet. Figure 1. Prudence would certainly dictate, however, that we anticipate renewed landslide activity during or after future periods of prolonged and(or) intense rainfall. The 2005 landslide killed 10 people, and destroyed or damaged dozens of houses. La Conchita, CA.
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