Heavy rains of January 1995 led to a March 4, 1995, landslide in the La Conchita Community of Ventura County (County), California. 3). An idealized slump-earth flow showing commonly used nomenclature for labeling the parts of a landslide. 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. Alaska and Hawaii also experience all types of landslides. This mode of movement would most likely be relatively slow (compared to 2005) but still could pose serious hazards to property and, perhaps, life. 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). 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). An official website of the United States government. Other hazards, such as flash floods and debris flows, now become the focus. These schematics illustrate the major types of landslide movement. Colorado, and northern New Mexico) are complex in rock type and climate, and so the landslides there are also complex. 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. 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. The dominant mode of movement is lateral extension accompanied by shear or tensile fractures. The arcuate bench at the top of the bluff is the head of a very large prehistoric landslide that affected the entire bluff. On March 10, a subsequent debris flow from a canyon to the northwest (see canyon on left part of figs. Of primary interest to the general public and various Governmental entities is the current state of hazard at La Conchita. 3B). In 1995, the community experienced a major slide, which damaged and destroyed a significant portion of the community. Although landslides are primarily associated with mountainous regions, they can also occur in areas of generally low relief. The landslide entered the La Conchita neighborhood destroying 13 houses and severely damaging 23 others. A lock ( The 2005 landslide pushed many of the houses off their foundations and into each other at the toe of the landslide (figs. The landslide occurred after a 15-day period of unprecedented records of rainfall in Southern California. Law enforcement officers and media representatives were in the area, which facilitated capturing the moving landslide on video. The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional". 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 top of the bluff was covered by avocado and citrus orchards. The 2005 landslide was a shallower remobilization of the very same material into a rapid, highly fluid debris flow that buried 10 . The 2005 landslide mobilized the south flank of a larger landslide that occurred in March 1995, and destroyed 9 homes. and more. Official websites use .gov The 2005 landslide occurred on part of a previous landslide that occurred in 1995. Worldwide, landslides occur and cause thousands of casualties and billions in monetary losses annually. A .gov website belongs to an official government organization in the United States. What type of slide is the La Conchita slide? The two major types of slides are rotational slides and translational slides. 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. 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. It is likely that the new rainfall did not enter the deposits left behind after the 1995 landslide, but slid off it taking surface debris and few new materials with it. 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. 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. If the overlying material moves as a single, little-deformed mass, it is called a block slide. 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. In southern California, at least 25 cm (10 in) of rainfall during the winter is needed to nearly saturate the ground. The two formations are in fault contact along the active Red Mountain Fault, which extends across the slope face. 3F). An idealized slump-earth flow showing commonly used nomenclature for labeling the parts of a landslide. 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. Future landslide activity could move into the same areas that recently have been damaged or could mobilize in other directions that could damage any or all of the developed area. 1) (34.4 degrees north, 119.5 degrees west). Figure 1. 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. These maps em, 12201 Sunrise Valley Drive Reston, VA 20192, Preliminary soil-slip susceptibility maps, southwestern California, Southern California LandslidesAn Overview, Map of Historic and Prehistoric Landslides in Pacific Palisades, Landslides in Santa Monica Mountains and Vicinity, Preliminary Soil-Slip Susceptibility Maps for Southwestern California, Map of Fatal or More than $1 Million Damage in California from 1906 to 1984, Landslide Hazards at La Conchita, California, Southern CaliforniaWildfires and Debris Flows, National Assessment of Shoreline Change Part 4: Historical Coastal Cliff Retreat along the California Coast, Emergency assessment of post-fire debris-flow hazards for the 2013 Springs Fire, Ventura County, California, Coastal and Marine Hazards and Resources Program, Pacific Coastal and Marine Science Center, National assessment of shoreline change, part 4: Historical coastal cliff retreat along the California coast, Southern California Wildfires and debris flows, Southern California landslides-an overview, Landslide hazards at La Conchita, California, Groundwater and Streamflow Information Program, Geology, Minerals, Energy, and Geophysics Science Center, Soil slips, debris flows, and rainstorms in the Santa Monica Mountains and vicinity, southern California. Expansion of man-made developments into fire-prone wildlands has created situations where wildfires can destroy lives and property, as can the flooding and debris flows that are common in the aftermath of the fires. Also, the video shows dust in the air as the landslide flowed downslope. Earlier that morning, debris flows from canyons northwest of La Conchita reached Highway 101. Three years after the 2005 landslide, the owners of an avocado ranch agreed to turn over all 700 acres (280ha) of their land and other assets to settle the suit. The bench at the top of the bluff is the head of a very large prehistoric landslide that affected the entire bluff. We also use third-party cookies that help us analyze and understand how you use this website. 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. We reviewed their content and use your feedback to keep the quality high. The landslides in the area around La Conchita have been a regular occurrence, according to the historical accounts dating back to 1865. The lower part of the slope is siltstone, sandstone, and mudstone of the Pliocene Pico Formation (O'Tousa, 1995). The following summary of the 1995 La Conchita landslide is extracted from O'Tousa (1995) and Anderson (Robert Anderson, RJR Engineering, 2005, personal commun.). 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. URL: https://pubs.usgs.gov/of/2005/1067/508of05-1067.html The 2005 landslide killed 10 people, and destroyed or damaged dozens of houses. 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. Debris flows include <50% fines. A landslide can be defined as follows: [T]he movement of a mass of rock, debris, or earth down a slope. When was the last landslide in La Conchita? Some materials with a loose, open sediment fabric will weaken if they are mechanically disturbed or flooded with water. 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). What was the main cause of La Conchita landslide in 2005? 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. [2] The dimensions of the slides were approximately 120 meters (390 feet) wide, 330 meters (1,080ft) long, and 30 meters (98ft) deep. These cookies will be stored in your browser only with your consent. 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. The most common types of landslides are described as follows and are illustrated in figure 3. 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. (Click on the image to view it as a large JPG file). TYPES OF LANDSLIDES 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. After covering the road with 7-8 feet of debris, the slide stabilized. 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. These hazards are mitigated mainly through precautionary means such as restricting populations from susceptible areas, limiting land use, installing early warning systems, and building structures such as piles and retaining walls. 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. There is no reason to believe this pattern of landsliding will stop. 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. 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. 3F). The events of Jan. 10, 2005, when 400,000 tons of mud slid down the bluff behind La Conchita, killing seven adults and three children, are frozen in the eerie tableau a reminder of the hidden dangers lurking beneath the surface. 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). In 2005, after a 15-day period of near-record rainfall, a larger area failed rapidly, remobilizing part of the 1995 slide. 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. The cookie is used to store the user consent for the cookies in the category "Performance". Figure 8 shows an aerial photograph of La Conchita taken in September 2004. The 11ha (28acre) community was first established in 1924 when subdivision created about 200 lots that mostly contain singlefamily residences. 7). Let us know if you have suggestions to improve this article (requires login). What was different about the 2005 slide at La Conchita compared to the 1995 slide in the same place? The 2005 landslide involved few new materials and seems to have been a continuation of the original 1995 landslide. La Conchita, coastal area of southern California. Actually there have been several sizes, types, and ages of landslides. Fires that denude slopes of vegetation intensify the susceptibility of slopes to debris flows. 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. 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? However, this still leaves some troubling questions unanswered. Sometimes more than one type of movement occurs within a single landslide, and analysis of each requires detailed interpretation of landforms and geological sections. These types of landslides take weeks, even months, to happen. As such, the maps offer a partial answer to one part of the three parts necessary to predict the soil-slip/debris-flow process. 3G). 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.

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