The project engages emergency planners, businesses, universities, government agencies, and others in preparing for major natural disasters. 1). On March 4, 1995, the hill behind La Conchita failed, moving tens of meters in minutes, and buried nine homes with no loss of life. [2] It lies on a narrow strip of land about 250 meters (820 feet) wide between the shoreline and abutting a 180-meter (590ft) high bluff. Study with Quizlet and memorize flashcards containing terms like A comprehensive term for any type of downslope movement of Earth materials is, What strategy has been suggested to stabilize the La Conchita, California, landslide?, In Yosemite National Park, strong bedrock influences the formation of what types of hillslopes? 3G). 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. These schematics illustrate the major types of landslide movement. Figure 2. The content on this page is copyrighted under a Creative Commons Attribution 4.0 International license. For those who aren't, La Conchita a small (population about 340 people), unincorporated seaside village located on the Pacific coast north of Los Angeles (Fig. The timing of the two landslides with respect to the triggering storms is of primary interest. A .gov website belongs to an official government organization in the United States. The landslide recurred on part of a previous landslide in 1995.La Conchita, California. 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. [2], Geologic evidence indicates that landslides have been occurring at and near La Conchita for many thousands of years up to the present, on a geologically frequent basis. 3H). In 1995, the community experienced a major slide, which damaged and destroyed a significant portion of the community. The depth was estimated at greater than 30 m (100 ft), and the volume was estimated at 1.3 million m3 (1.7 million yd3). Different landslide scenarios are more or less likely to occur as a result of different specific rainfall conditions, and no part of the community can be considered safe from landslides. 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. There are five basic categories of flows that differ from one another in fundamental ways. The largest single landslide in modern U. history whose origins were completely natural was the Gros Ventre slide in Wyoming . A summary of recent and past landslides and debris flows caused by rainfall in Southern 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 term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. In La Conchita, there was a landslide and earthflow in the spring of 1995 (see figure 1). These types of landslides take weeks, even months, to happen. An idealized slump-earth flow showing commonly used nomenclature for labeling the parts of a landslide. 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. 13. Share sensitive information only on official, secure websites. Casualties in the United States are primarily caused by rockfalls, rock slides, and debris flows. 1) (34.4 degrees north, 119.5 degrees west). In some instances, for example in many newspaper reports, mudflows and debris flows are commonly referred to as mudslides.. The developed part of the slope where the houses were impacted has a flatter slope, and so the flow probably slowed to no more than 5 m/s (15 ft/s) in the neighborhood. What was the type of landslide in recent 2005 La Conchita landslide? This cookie is set by GDPR Cookie Consent plugin. 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. Although rainfall intensities were not extreme, moderate to highintensity rainfall persisted for more than 2 weeks, and the landslide occurred at the culmination of this 15day highrainfall period (fig. Articles from Britannica Encyclopedias for elementary and high school students. Types of landslides Factors contribute to landslides Landslide in Utah. The historic slides are part of the larger Rincon Mountain slide, which started many thousands of years ago and will continue generating slides in the future.. Although landslides are primarily associated with mountainous regions, they can also occur in areas of generally low relief. Subsequently, residents formed the La Conchita Community Organization (LCCO) to coordinate with government officials to determine the best way to protect the community. When was the last landslide in La Conchita? Figure 3: Some major types of mass movements. The La Conchita landslide then occurred on March 4. There were 10 confirmed fatalities. Surface cracks in the upper part of the slope were noted as far back as the summer of 1994. It does not store any personal data. The landslide destroyed 13 houses and severely damaged 23 others. 13, 52 p. National Oceanic and Atmospheric Administration, 1995b, Climatological dataCalifornia, JanuaryMarch 1995: National Climatic Data Center, v. 99, no. Debris avalanche: This is a variety of very rapid to extremely rapid debris flow (fig. 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 town of La Conchita, California, experienced major landslides in 1995 and 2005. But the mud stopped before reaching the homes, said county Geologist Jim O'Tousa. 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. 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. This also is consistent with the shallower, fluid mode of failure: shallow, rapid debris flows most commonly occur during periods of prolonged, intense rainfall with little or no lag time (Campbell, 1975; Keefer and others, 1987; Jibson, 1989). A translational slide typically takes place along structural features, such as a bedding plane or the interface between resistant bedrock and weaker overlying material. The lower part of the slope is siltstone, sandstone, and mudstone of the Pliocene Pico Formation (O'Tousa, 1995). However, dry flows of granular material are also possible. The La Conchita landslides that occurred in 1995 and 2005 form only a small percent of a much larger landslide complex, according to the geologists. This apparently resulted from a highly hazardous situation involving a twophased landslide mechanism: (1) a saturated, highly fluid layer at depth on which the landslide mobilized that (2) carried a thick layer of drier, much more viscous material that effectively acted as a battering ram. The 1995 landslide was a deep, coherent slumpearth flow that deformed plastically and moved slowly enough that people could get out of its way. 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. 4 What type of mass wasting took place in La Conchita? What are various methods available for deploying a Windows application? The slope material liquefies and runs out, forming a bowl or depression at the head. This is a typical type of landslide. 5 and 6). This cookie is set by GDPR Cookie Consent plugin. The materials may move by falling, toppling, sliding, spreading, or flowing. Debris avalanche: This is a variety of very rapid to extremely rapid debris flow (fig. The most common types of landslides are described as follows and are illustrated in figure 3. A wall built after the 1995 landslide to keep minor landslide debris off the road was tilted forward and(or) overtopped in places by debris from the 2005 landslide (fig. I estimate from viewing the video that high on the slope, the landslide was moving perhaps 10 m/s (30 ft/s). The dominant mode of movement is lateral extension accompanied by shear or tensile fractures. This was not the first destructive landslide to damage this community, nor is it likely to be the last. landslide, also called landslip, the movement downslope of a mass of rock, debris, earth, or soil (soil being a mixture of earth and debris). What type of mass wasting took place in La Conchita? Fires that denude slopes of vegetation intensify the susceptibility of slopes to debris flows. The plaintiffs also sued the County of Ventura for damages, claiming that a wall that the County built at the base of the landslide caused or contributed to the landslide. The 2005 landslide involved few new materials and seems to have been a continuation of the original 1995 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. The 2005 landslide occurred at the end of a 15day period that produced record and nearrecord amounts of rainfall in many areas of southern California. In some instances, for example in many newspaper reports, mudflows and debris flows are commonly referred to as "mudslides.". and more. The landslide killed 10 people, and destroyed or damaged dozens of houses. The landslide slumped as a coherent mass of material. This mode of movement would most likely be relatively slow (compared to 2005) but still could pose serious hazards to property and, perhaps, life. A) La Conchita, California The community of La Conchita in Ventura County, California experiences frequent landslides. 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. There is a good web site where you can read and see pictures about the slide in La Conchita: By reading it I can tell you that the bluff above La Conchita has produced a variety of landslides over an extended period of time. The KCALTV video indicates that the landslide material mobilized simultaneously and nearly instantaneously into a highly fluid, rapidly moving debris flow. Omissions? Of primary interest to the general public and various Governmental entities is the current state of hazard at La Conchita. Figure 8 shows an aerial photograph of La Conchita taken in September 2004. Little or no newly failed material was involved in the landslide; rather, it consisted of a re-mobilization of the southeastern portion of the 1995 landslide deposit, involving about 200,000 m3 (250,000 yd3). 3I). Even modest rain storms during normal, non-El Nio years can trigger post-wildfire debris flows. U.S. Department of the Interior | U.S. Geological Survey 1B, p. S319S332. In a rotational slide the axis of rotation is roughly parallel to the contours of the slope. La Conchita lies on a narrow coastal strip about 250 m (800 ft) wide between the shoreline and a 180m (600ft) high bluff having a slope of about 35; above the top of the bluff is a gently rising terrace surface covered by avocado and citrus orchards (fig. 5 What was different about the 2005 slide at La Conchita compared to the 1995 slide in the same place? 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. Other classification systems incorporate additional variables, such as the rate of movement and the water, air, or ice content of the landslide material. 1995 and 2005, La Conchita, California: On March 4, 1995, a fast-moving earthflow damaged nine houses in the southern California coastal community of La Conchita. Reported landslides began in 1865. OF 05-1067, "La Conchita Ranch is sold for $2.5 million", "La Conchita Lawsuit Settled, But It's Not Over", "Settlement reached in La Conchita lawsuit", Assessing Landslide Hazard Over a 130-Year Period for La Conchita, California, https://en.wikipedia.org/w/index.php?title=La_Conchita_landslides&oldid=1134048066, 2005 natural disasters in the United States, Short description is different from Wikidata, Creative Commons Attribution-ShareAlike License 3.0, The 2005 La Conchita landslide is the setting of, This page was last edited on 16 January 2023, at 19:34. Historical accounts dating back to 1865 have reported landslides in the area around La Conchita as being a regular occurrence (Hemphill, 2001). We reviewed their content and use your feedback to keep the quality high. Eyewitness accounts indicate, however, that similar to 2005, dust was in the air and much of the deposit included relatively dry material. Coherent blocks of material subside into the weaker substrate, and the slow downslope movement frequently extends long distances as a result of the retrogressive extension from the zone of origin, such as an eroding riverbank or coastline. 3H). Heavy rains of January 1995 led to a March 4, 1995, landslide in the La Conchita Community of Ventura County (County), California. [7], There is no reason to believe that landslides will not continue to threaten or impact La Conchita. 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. LockA locked padlock 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. 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.
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