The 2005 landslide mobilized the south flank of a larger landslide that occurred in March 1995, and destroyed 9 homes. In this report, empirical models are used to predict the probability and magnitude of debris-flow occurrence in response to a 10-year rainstorm for the 2013 Springs fire in Ventura County, California. Landslide Hazards at La Conchita, California - USGS 1). 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). On March 10, a subsequent debris flow from a canyon to the northwest (see canyon on left part of figs. The timing of the two landslides with respect to the triggering storms is of primary interest. 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. 2 and 3) damaged five additional houses in the northwestern part of La Conchita. 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? 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. 3). From December 27, 2004 through January 10, 2005, Ventura received 378 mm (14.9 in) of rainfall, only slightly less than its mean annual total of 390 mm (15.4 in) (Wofford, 2005; National Oceanic and Atmospheric Administration, 1994a, 1995a). Deep-seated landslides can be triggered by deep infiltration of rainfall, which can take weeks or months to occur. Know the Signs of a Deep-Seated Landslide Types of mass movement Flows are a mixture of water, rock and sediment. 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. 1995 La Conchita, California, landslide. Maps displaying where shallow landslides are most likely to occur are based on observation of previous landslide activity, topographic slope, and information on the bedrock material (seePreliminary soil-slip susceptibility maps, southwestern California-Open-File Report 2003-17). Figure 1. In 1995, the community experienced a major slide, which damaged and destroyed a significant portion of the community. The already loosened sediment from the 1995 landslide made the 2005 event a lot more catastrophic. The landslide recurred on part of a previous landslide in 1995.La Conchita, California. Unfortunately, we currently lack the understanding to accurately forecast what might happen in each possible rainfall scenario. 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. 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. 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. Shear stresses can be built up within a slope by a number of processes. Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors. On January 10, 2005, the southeastern portion of the 1995 landslide deposit failed, resulting in shallow, rapid fluid flow, unlike the 1995 landslide. This is the only mitigation effort the town undertook. Actually there have been several sizes, types, and ages of landslides. Rotational slip. There are four different types of mass movement: 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 . Combination of two or more of the above types is known as a complex landslide. Landslides occur when gravitational and other types of shear stresses within a slope exceed the shear strength (resistance to shearing) of the materials that form the slope. 7). Earlier that morning, debris flows from canyons northwest of La Conchita reached Highway 101. 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 flow itself is elongate and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and under saturated conditions. The neighborhood of La Conchita has suffered from large landslides in the recent past that have wiped out homes and even killed people. In 199495, about twice as much rain761 mm (29.96 in)fell during that period (National Oceanic and Atmospheric Administration, 1994b, 1995b). The landslide scenarios sketched above potentially could impact any part of the La Conchita community. The ZIP Code is 93001, and the community is inside area code 805 . This slower rate also is suggested by eyewitnesses who stated that some residents were able to outrun the advancing flow, which would not have been possible at the higher upslope velocity. 1 What type of slide is the La Conchita slide? These tools convert Adobe PDF documents into HTML or ASCII text, which then can be read by a number of common screenreading programs that synthesize text as audible speech. 1). During and just after storms, existing coastal landslides may become reactivated and seemingly stable coastal cliffs may erode and fail rapidly. Fires that denude slopes of vegetation intensify the susceptibility of slopes to debris flows. This cookie is set by GDPR Cookie Consent plugin. Types of landslides Factors contribute to landslides Landslide in Utah. 7 What happened in the La Conchita landslide 2005? The bluff above La Conchita has produced a variety of landslides over an extended period of time. The landslide entered the La Conchita neighborhood destroying 13 houses and severely damaging 23 others. What type of slide is the La Conchita slide? - Heimduo La Conchita, CA. On January 10, 2005, the southeastern portion of the 1995 landslide deposit failed, resulting in shallow, rapid fluid flow, unlike the 1995 landslide. Note the numerous landslides that have taken place in the area, including the large slide in 1995. The 1995 slide was 120 m (400 ft) wide, 330 m (1100 ft) long, and covered approximately 4 ha (10 acres). La Conchita (1995) La Conchita landslide, Ventura County. Do they actually do gymnastics in Make It or Break It? On March 10, a subsequent debris flow from a canyon to the northwest (see canyon on left part of figs. 2 and 3) damaged five additional houses in the northwestern part of La Conchita. February 1995 produced only about onethird of the normal rainfall, but a modest storm on March 23 produced 21 mm (0.81 in) of rain. The term "landslide" encompasses events such as rock falls, topples, slides, spreads, and flows. Note the numerous landslides that have taken place in the area, including the large slide in 1995. 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. Document Accessibility: Adobe Systems Incorporated has information about PDFs and the visually impaired. Share sensitive information only on official, secure websites. But the mud stopped before reaching the homes, said county Geologist Jim O'Tousa. A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass (fig. In 2005, after a 15-day period of near-record rainfall, a larger area failed rapidly, remobilizing part of the 1995 slide. 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. . The cookie is used to store the user consent for the cookies in the category "Other. This cookie is set by GDPR Cookie Consent plugin. The volume of material moved by the landslide was 2.8 cubic kilometers (0.67 mi 3) . 13, 52 p. National Oceanic and Atmospheric Administration, 1995b, Climatological dataCalifornia, JanuaryMarch 1995: National Climatic Data Center, v. 99, no. 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. But opting out of some of these cookies may affect your browsing experience. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Such a failure scenario, involving a significant amount of dry material that fully mobilized on a saturated layer, indicates that most of the rain that fell on the surface of the 1995 deposit did not infiltrate but drained off the surface. This was a wholesale remobilization of a significant portion of the 1995 deposit. Over time, upslope ponding of water by such back-tilted blocks can enlarge the area of instability, so that a stable condition is reached only when the slope is reduced to a very low gradient. Landslides are generally classified by type of movement (slides, flows, spreads, topples, or falls) and type of material (rock, debris, or earth). The 2005 landslide was a shallower remobilization of the very same material into a rapid, highly fluid debris flow that buried 10 people. 2) On figure 2, label the following: main scarp, head of slide, body of slide, toe of slide. On January 10, 2005, a major landslide occurred in La Conchita. 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. There are five basic categories of flows that differ from one another in fundamental ways. A lock ( 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. Questions or Assistance: USGS Web Contact The cookies is used to store the user consent for the cookies in the category "Necessary". This mode of movement would most likely be relatively slow (compared to 2005) but still could pose serious hazards to property and, perhaps, life. In low-relief areas, landslides occur as cut-and-fill failures (roadway and building excavations), river bluff failures, lateral spreading landslides, collapse of mine-waste piles (especially coal), and a wide variety of slope failures associated with quarries and open-pit mines. Get a Britannica Premium subscription and gain access to exclusive content. Analytical cookies are used to understand how visitors interact with the website. 2 When was the last landslide in La Conchita? Share sensitive information only on official, secure websites.. Environmental & Historic Preservation Guidance, Real Estate, Lending or Insurance Professionals, State, Local, Tribal or Territorial Governments, Preparedness Activities, Research & Webinars, Voluntary & Community-Based Organizations, Environmental Planning & Historic Preservation, National Business Emergency Operations Center, Ventura County Public Works - Transportation Department. 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 rising groundwater level within the 1995 deposit would thus have resulted from deeper recharge from rainfall infiltration upslope. The landslide, a complex slumpearth flow, destroyed or severely damaged nine houses. The failure starts suddenly in a small area and spreads rapidly. There were 10 confirmed fatalities, unfortunately. In 1995, the community experienced a major slide, which damaged and destroyed a significant portion of the community. Chapter 12 Flashcards | Quizlet 3 What was the main cause of La Conchita landslide in 2005? Analysis of the 1995 and 2005 La Conchita, CA Landslides - NASA/ADS 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. What kind of mitigation was done after the La Conchita landslide? 9). Figure 2. I estimate from viewing the video that high on the slope, the landslide was moving perhaps 10 m/s (30 ft/s). A) La Conchita, California The community of La Conchita in Ventura County, California experiences frequent landslides. In 2008, family members of those killed and or who suffered loss of property in the 2005 La Conchita Landslide filed a lawsuit against the La Conchita Ranch Co., located at the top of slope. 13. What was different about the 2005 slide at La Conchita compared to the 1995 slide in the same place? Omissions? The 11ha (28acre) community was first established in 1924 when subdivision created about 200 lots that mostly contain singlefamily residences. Translational slide: In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting (fig. The USGS has conducted hazard assessments forpost-wildfire debris flowsfor four recent fires in southern CA, as well as numerous fires across the Western U.S. including central and northern California. 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. However, this still leaves some troubling questions unanswered. [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. The failure is caused by liquefaction, the process whereby saturated, loose, cohesionless sediments (usually sands and silts) are transformed from a solid into a liquefied state. landslide, also called landslip, the movement downslope of a mass of rock, debris, earth, or soil (soil being a mixture of earth and debris). 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. Falls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs (fig. 1) (34.4 degrees north, 119.5 degrees west). 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). How and why the same material failed twice in 10 years by fundamentally different mechanisms certainly will be the object of future research, and it is much too complex to analyze in detail at this time. Eyewitnesses also reported seeing material failing from the main scarp and lateral margins of the landslide as a result of removal of lateral support. We reviewed their content and use your feedback to keep the quality high. 10.5: Landslide Types and Processes is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by LibreTexts. 3G). 10 and 11). What happened at La Conchita in 1995 and 2005? Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges (fig. 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 1995 landslide was a deep, coherent slumpearth flow that deformed plastically and moved slowly enough that people could get out of its way. The storm of March 23 also may have played a role in triggering initial movement of the hillside, which already was approaching instability due to longerterm seasonal rises in groundwater levels. Debris avalanche: This is a variety of very rapid to extremely rapid debris flow (fig. Explain why. Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. 5 What was different about the 2005 slide at La Conchita compared to the 1995 slide in the same place? After the 1995 landslide, the town of La Conchita built a retaining wall at the foot of the hill. 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. Subsequently, residents formed the La Conchita Community Organization (LCCO) to coordinate with government officials to determine the best way to protect the community. The 2005 landslide was a shallower remobilization of the very same material into a rapid, highly fluid debris flow that buried 10 people. 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. Wave energy is also typically higher during the winter, and particularly during El Nio events, thereby exacerbating the potential for coastal erosion. Figure 8 shows an aerial photograph of La Conchita taken in September 2004. 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. 8.4: Examples of Landslides - Geosciences LibreTexts The slope material liquefies and runs out, forming a bowl or depression at the head. The 2005 La Conchita landslide occurred on January 10. If significant additional rainfall occurs, either this year or in future years, several landslide scenarios are possible: (a)deep movement of the 1995 deposit, as described above, (b) mobilization of the 1995 (and possibly the 2005) deposit into a rapid debris flow such as occurred on January 10, 2005, (c) triggering of subsidiary landslides from parts of the 1995 and 2005 deposits or scarps, (d) triggering of slumps and (or) earth flows on adjacent hillsides, and (e) triggering of rapid debris flows from various nearby slopes, particularly in ravines. 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. Geol 1014 Chapter 10 Flashcards | Quizlet Wildland fires are inevitable in the western United States. 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. 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). Photo by Pam Irvine, CGS. Failure is usually triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced. Historical accounts dating back to 1865 have reported landslides in the area around La Conchita as being a regular occurrence (Hemphill, 2001). Landslides are generally classified by type of movement (slides, flows, spreads, topples, or falls) and type of material (rock, debris, or earth). [3] Historical accounts support this. (Click on the image to view it as a large JPG file). In 1995, after an exceptionally wet winter, the landslide moved tens of meters (tens of yards) damaging nine houses. 3A). Previous work at the USGS has identified both the areas of southwestern California most susceptible to shallow landslides and the rainfall conditions required to trigger slope failures. A .gov website belongs to an official government organization in the United States. The catastrophic movement of the 2005 landslide damaged or destroyed 36 houses and killed 10 people. British Geological Survey - What is a landslide? Fires that denude slopes of vegetation intensify the susceptibility of slopes to debris flows. Figure 4. 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. This website uses cookies to improve your experience while you navigate through the website. 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.
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