An increase in water content, resulting from either natural causes or human activity, typically weakens sandy materials through the reduction of interparticle friction and weakens clays through the dissolution of interparticle cements, the hydration of clay minerals, and the elimination of interparticle (capillary) tension. A) La Conchita, California The community of La Conchita in Ventura County, California experiences frequent landslides. "[4], Coordinates: .mw-parser-output .geo-default,.mw-parser-output .geo-dms,.mw-parser-output .geo-dec{display:inline}.mw-parser-output .geo-nondefault,.mw-parser-output .geo-multi-punct{display:none}.mw-parser-output .longitude,.mw-parser-output .latitude{white-space:nowrap}342150.5N 1192645W / 34.364028N 119.44583W / 34.364028; -119.44583. 3J). The landslide killed 10 people, and destroyed or damaged dozens of houses. 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. Photo by R.L. Fast-moving, highly destructive debris flows triggered by intense rainfall are one of the most dangerous post-fire hazards. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. D. All of these are correct <--------. Little or no newly failed material was involved in the landslide; rather, it consisted of a remobilization of the southeastern portion of the 1995 landslide deposit, involving about 200,000 m3 (250,000 yd3) (James O'Tousa, RJR Engineering, personal commun., 2005). [2] The landslide occurred after a 15-day period of unprecedented records of rainfall in Southern California. 1B, p. S319S332. 7). A .gov website belongs to an official government organization in the United States. 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). The deep mode of failure in 1995 is consistent with this delay: deeper landslides commonly are triggered by deep infiltration of rainfall, which can take weeks or months to occur (for example, Morton and Campbell, 1989). Figure 1: Air photo of La Conchita, CA taken in 2002. La Conchita Landslide: Case History and Remedial Measures Currently, we have insufficient data and understanding of the failure mechanisms of this landslide to adequately answer these questions, but it is clear that the hazard from renewed landslide movement is considerable. A landslide can be defined as follows: [T]he movement of a mass of rock, debris, or earth down a slope. The movement of the same landslide mass in 1995 and 2005 by two very different mechanisms, and with markedly different results, is difficult to explain. 5 and 6). 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. What about the remaining 1995 deposit? Surface cracks in the upper part of the slope were noted as far back as the summer of 1994. 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. 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. What mitigation took place after the La Conchita slide? In La Conchita, there was a landslide and earthflow in the spring of 1995 (see figure 1). 12). and more. October 17, 2022. The landslides in the area around La Conchita have been a regular occurrence, according to the historical accounts dating back to 1865. The cookie is used to store the user consent for the cookies in the category "Analytics". 7). 10 years after fatal mudslide, tiny La Conchita accepts the risks In both 1889 and 1909, the Southern Pacific rail line running along the coast was inundated. The main mass that failed is on the southeastern margin of the larger ancient landslide that encompasses the entire front of the bluff (see fig. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. They can be deadly, however, and can cost billions of dollars to clean up. People were evacuated and the houses nearest the slide were completely destroyed. Once the smoke clears from a wildfire, the danger is not over!! Please refer to the appropriate style manual or other sources if you have any questions. 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. Lateral spreading in fine-grained materials on shallow slopes is usually progressive. 13, 60 p. National Oceanic and Atmospheric Administration, 1994b, Climatological dataCalifornia, OctoberDecember 1994: National Climatic Data Center, v. 98, nos. A slide happens when a section of soil or rock suddenly gives way and moves down a slope. 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. The 2005 La Conchita landslide occurred on January 10. 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. 2 When was the last landslide in La Conchita? The town is located on a narrow coastal plain, with a steep . Earthflow: Earthflows have a characteristic hourglass shape (fig. You also have the option to opt-out of these cookies. Figure 4 shows the rainfall distribution during the 19941995 rainy season. Week 14 Lecture on Landslides - Landslides When gravity - Studocu This was not the first destructive landslide to damage this community, nor is it likely to be the last. What type of mass wasting took place in La Conchita? 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. The flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. A week later, debris flow in the same location damaged five more houses. 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. La Conchita, coastal area of southern California. A summary of recent and past landslides and debris flows caused by rainfall in Southern California. Thus, it appears that the landslide mobilized on a saturated layer deep in the 1995 deposit but that much of the material above this saturated zone was dry or nearly so. Since that time, other slides have occurred, covering at times cultivated land, roadways, and the community itself.[4]. The failure starts suddenly in a small area and spreads rapidly. In the past two decades major landslides devastated the town of La Conchita, California. O'Tousa, James, 1995, La Conchita landslide, Ventura County, California: Association of Engineering Geologists AEG News, v. 38, no. How many people died in the landslide on Rincon Mountain? Various processes that weaken the shear strength of a slope may also activate landslides. Often referred to as "mudflows," debris flows are a type of landslide made up of a rapidly moving mixture of dirt, rocks, trees, and water (and sometimes ash) that start on a hillside and travel downvalley. In 2005, an additional landslide occurred at the end of a 15day period of near-record rainfall levels. Language links are at the top of the page across from the title. 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. Although many types of mass movements are included in the general term landslide, the more restrictive use of the term refers only to mass movements, where there is a distinct zone of weakness that separates the slide material from more stable underlying material. Falls are strongly influenced by gravity, mechanical weathering, and the presence of interstitial water. A complete prediction of the process would include assessments of where, when, and how big. Rotational slide: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide (fig. On January 10, 2005, a landslide struck the community of La Conchita in Ventura County, California, destroying or seriously damaging 36 houses and killing 10 people. La Conchita, CA. 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? Many readers will be familiar with the La Conchita landslide in California. The most important fluidizing agent is water, but trapped air is sometimes involved. The 2005 landslide occurred at the culmination of an extremely wet 2week period (fig. Below are other science projects associated with this project. Short-term stresses imposed by earthquakes and rainstorms can likewise contribute to the activation of landslides. The La Conchita landslide then occurred on March 4. 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). The latter landslide killed 10 people, and destroyed or damaged dozens of houses. Figure 8 shows an aerial photograph of La Conchita taken in September 2004. 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. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc. Types of landslides Factors contribute to landslides Landslide in Utah. This dynamic tectonic environment has created a spectacular landscape of rugged mountains and steep-walled valleys that compose much of the regions scenic beauty. Please select which sections you would like to print: Geographic Database Manager and Geographic Information Systems Analyst, Department of Geography, Royal Holloway, University of London, Egham, Surrey, U.K. Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. Historical accounts dating back to 1865 have reported landslides in the area around La Conchita as being a regular occurrence (Hemphill, 2001). In 1995, after an exceptionally wet winter, the landslide moved tens of meters (tens of yards) damaging nine houses. Back scarp of ancient landslide Scarp of small recent landslide 1995 landslide Figure 2: Close-up image of the 1995 landslide event at La Conchita, CA. The slide covered 300 feet of Vista Del Rincon (VDR) Drive. Deformation continued as the rainy season began, and by December 1994, several open cracks on the hillside were channeling surface runoff into the subsurface. PDF Falling Into the Pacific: California Landslides and Land Use Controls https://pubs.usgs.gov/fs/2004/3072/fs-2004-3072.html, Combination or two or more principal types of movement. 3D). A type of landslide in which the distribution of particle velocities resembles that of a viscous fluid is called a flow. Wave energy is also typically higher during the winter, and particularly during El Nio events, thereby exacerbating the potential for coastal erosion.
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