Posted on 01/12/2005 10:47:17 PM PST by BurbankKarl
Assessing Landslide Hazard Over a 130-Year Period for La Conchita, California For the Association of Pacific Coast Geographers Annual Meeting September 12 and 15, 2001 Hosted by the UCSB Geography Department
by Jeffrey J. Hemphill Remote Sensing Research Unit Department of Geography University of California Santa Barbara
Abstract
A landslide consisting of 600,000 tons of mud and silt slid 600 feet down a cliff face and buried nine homes in La Conchita, California on March 4, 1995. The landslide history of this area was considered in the ensuing litigation, which centered on the ranch above the failed slope and whether or not a direct causal link between irrigation and the slope failure could be made. Residents of La Conchita attributed the cause of the landslide to excessive irrigation by the ranch owners. Important for this investigation were the analysis of the naturally unstable geological setting, the unique hydrologic environment of the tectonically elevated bluffs, and unusually high rainfall brought about by El Nino. Establishing the history of landslides in the area relied on historical zoning maps, written accounts of landslides, historic air photo interpretation, digital elevation models, geologic information, and hydrologic analysis.
Introduction
La Conchita is a small coastal town located in Ventura County, California next to the 101 Freeway, approximately 50 miles southeast of Santa Barbara, just down the coast from the town of Carpinteria near Rincon Point (See Figure 1, Location Map, at the end of the paper). The rugged and eroded appearance of the cliffs above the town has a marked difference to that of other coastal sections of the 101 Freeway. The town itself is covers 28 acres and is situated on a gently sloping elevated marine terrace that is about 20-100 feet above sea level and today has approximately 150 structures (SMC, 1996). The southwestern most part of the town is 19 feet above sea level, and the highest point in the town is 120 feet above sea level in the vicinity of the toe of the 1995 slide. The higher average elevation in this area is due to the build-up of mudflow debris that has accumulated along the base of the cliff. The La Conchita Ranch extends inland from the 500-600 foot high cliffs above the town for about a half a mile. The highest point on the ranch property is about 1,400 feet above sea level at the base of Red Mountain. The ranch consists of more than 680 acres, of which approximately 415 acres are avocado and citrus trees. These trees were planted some time after 1974 when the land use of the ranch property changed from grazing and dry farming to irrigated agriculture.
A noticeable feature when driving past La Conchita on the 101 Freeway is the Rincon Oil Field and the offshore drilling facility, called Rincon Island, that extends about half a mile out from Punta Gorda point. Oil and natural gas were discovered there in 1927 in Pliocene age strata about 3000 feet below sea level. (Sylvester, 1986) Around 1960, Rincon Island started operation, and to date more than 150 million barrels have been extracted. (Sylvester, 1986) The tectonic pressures exerted by active faults are responsible for the existence of the oil and natural gas near the surface in this area and in the Santa Barbara Channel. The Ventura Avenue Anticline and the Red Mountain Thrust Fault, as well as numerous smaller faults, have significantly influenced the topography of this area.
Landslide History
Beginning as far back as the late 1800s there are records of landslides in the La Conchita area. In essays written by a resident that are nearly a century old, there is reference to debris flows covering cultivated lands and damage to structures caused by mass wasting events. The following material summarizes important historical landslide events researched as part of the legal investigation by one Kristing Coddington and documented in a chronology spanning 1865 to 1958 (Coddington, 1998). In 1865 there was a wagon trail through this section of coastline that connected Santa Barbara and Ventura which a surveyor described as follows: “the character of this road was so changeable in consequence of the falling down of masses of earth from the cliffs, which in some places were 400 feet high, and from the washing of the earth by the waves, that the road for the transportation of goods was nearly worthless.” Southern Pacific Railroad laid tracks through La Conchita in 1887; two years later in 1889 sections of the tracks were buried by the first of two landslides. In December 1889 the Ventura Free Press reports that, “West of Ventura somewhere on the cliffs there is a dead engine stalled in the landslides…” The January 29, 1909 edition of the Santa Barbara Independent reported on the Punta Gorda slide, and, as a consequence of this slide, changes to the rail and the section of road running through La Conchita were made. The author of this article described the stability of the area surrounding La Conchita by saying: “The character of the soil of the mountain, which rises almost abruptly from the sea, is such that there can be no security from slides, such as the avalanche of dirt and rocks that last Saturday swept down on the road and buried a work train.” As a result of this slide, Southern Pacific Railroad had to excavate several railroad cars, and major plans for construction of a causeway and the reinforcement of the railroad began. In an effort to reduce the hazard imposed by the steep cliff face, Southern Pacific Railroad bulldozed flat the area adjacent to the railroad which facilitated the development of the area and the eventual construction of houses. In 1924 the La Conchita del Mar subdivision was established. It consisted of approximately 330 lots and another 47 lots in a row up against the base of the cliff (SMC, 1996). A zoning map drawn in the early 1930’s shows that most of the land in the town of La Conchita was agricultural and, according to the legend on the drawing, the lowest value parcels were actually composed of mudflow debris.
Geologic History
Geologic evidence suggests that the 1995 landslide, and other landslide events along this section of coastline, are a relatively frequent occurrence. The distinguished geologist, William C. Putnam, described the geology of this area in detail in a report authored for the Geologic Society of America in 1942. The report points out numerous signs that there have been significant changes to the topography of this area in geologically recent times. For example, the exposed strata visible on the cliff face and in areas on top of the bluffs contain shells of species that established themselves in warmer postglacial times (Putnam, 1942). Marine sediments, mollusk shells, and sandstone boulders with bore holes and barnacles can be found in various locations in this area (Putnam, 1942; Sylvester, 1988; Harden, 1986). There are eight or nine distinct scarps where the different uplifted wave cut platforms are visible near the surface (Sylvester, 88; Harden, 1987). The Punta Gorda marine terrace is about 1300 feet above sea level in the middle of the La Conchita ranch, and it is estimated to have been at or near sea level between 40,000 and 60,000 year ago. Located nearby is a fluvial terrace that was dated using charcoal deposits and determined to be as young as 2000 years old (Harden, 1987). Based on a detailed analysis of soil samples, it is estimated that uplifting rates are about 4.2 – 5 meters per 1000 years. In terms of tectonic uplifting, this is one of the fastest in the world (Sylvester, 1988).
Two drainages border the La Conchita ranch, the Padre San Juan and Javon Canyons, and both are deeply incised and have steep eroding walls. Putnam (1942) noted the young, but abnormally deep, well-developed drainage pattern in this area as being a result of tectonic uplifting. He also noted the presence of gravel benches in the canyon walls that developed during episodes of canyon cutting followed by filling in the recent geologic past. Putnam attributed the deep canyon cutting and filling episodes to two possible causes, climatic fluctuations that caused changes in the amount of winter runoff and tectonic uplifting. As far as landslides are concerned, Putnam (1942) states that, “Nearly every square foot of surface on the hill slopes underlain by upper Pico clay shale is in motion down slope or has moved in the very recent geologic past.” Given the geologic composition and the tectonic origins of cliffs above La Conchita, it is difficult to understand how development was allowed in the area at the base of the cliffs, given the high risk of landslides.
Geologic stability of subsurface soil layers and the slope of the hillside are important considerations for landslide risk assessment. Also important are the hydrologic environment and proximity to active faults. One influential feature in this area is the Red Mountain Thrust Fault that runs from the Ventura River to the east, around the Red Mountain Dome above the La Conchita Ranch, and off shore near Rincon Point up the coast to the west of La Conchita. This fault has exerted significant influence on the drainage pattern visible around the study area; differential erosion rates of resistant Miocene age strata over weak clay and shale has caused the canyons bordering the ranch above La Conchita to become deeply incised in some areas and restricted in others (Putnam 1942). The canyons are confined where they cross the resistant rock layers upturned by the Red Mountain Fault, and the canyons are deep where they traverse weaker clay and shale layers.
Discussion
Seventy-one homeowners sued La Conchita Ranch Co. in Bateman v. La Conchita Ranch Co. The premise of the case brought against the ranch owners was that irrigation was the cause of the 1995 landslide, because irrigation has the tendency to raise the water table and potentially cause weakness in hill slopes. In the area around the toe of the slide water emanates from beneath the cliffs on a seasonal cycle. This is a sign that the heterogeneous mix of slide debris that forms the base of the cliff is draining and the pore pressure within the toe of the slide is being maintained at a relatively constant level. In light of extenuating environmental and geological circumstances, irrigation cannot be assigned as the direct cause of the slide. The presence of excess groundwater from intense precipitation reduced the cohesion between the soil layers by rapidly increasing the pore pressure, which initiated the landslide. In January 1995 over 18 inches of rain fell, resulting in the wettest January ever recorded in this area, and one local rain gauge recorded 1.6 inches of rain in a one half hour period in Santa Barbara. This unusually high precipitation was also noted in a Santa Barbara News Press article titled “La Conchita residents lose $24 million landslide suit” written by Chuck Schultz on January 16, 1999. The article reports that home values in the area surrounding the toe of the slide, and in the vicinity, have decreased significantly as a result of the landslide hazard.
Ranch personnel reported the first observed visible evidence that the hillside was getting ready to break loose in 1994; a series of longitudinal cracks had developed in an asphalt road above the ravine where the landslide occurred (SMC, 1996). Devices installed in bore holes of various depths in the toe of the slide recorded slight movements and changes in ground water content within the toe of the slide until January afternoon in 1995 when the first segment of the landslide broke loose and accelerated down slope (SMC, 1996). This first segment did not reach the town, but within 20 minutes two more segments broke loose along the aforementioned cracks and accelerated down slope as a coherent mass of mud and debris. There were two other slide events leading up to the big one in 1995—one occurred in 1988 and another in 1991. These two smaller debris slides did not have sufficient mass to accelerate down slope and were stopped by the ranch road. After both of these smaller slides, the road was cleared and improved, but after the 1995 slide the road was gone.
Vegetation on hill slopes is commonly believed to help prevent landslides, but this is only partially true (Campbell, 1975). The cliffs above La Conchita are covered with a dense carpet of coastal sage shrub and some scattered trees. Pockets of phreatophytes and ivy grow in areas where there are natural springs. This vegetation cover may in fact increase the risk of landslides, because their characteristically thick humus layer increases the amount of rainfall that can infiltrate the surface by reducing the potential for direct runoff. Although vegetation cover does reduce grain-by-grain erosion caused by flowing water, a thick layer of decaying leaves can facilitate rapid infiltration and a sudden spike in pore-water pressure that could have reduced the cohesion in the soil comprising the sides of the baranca that constituted the source material of the 1995 slide. In a 1975 Geological Survey Professional Paper authored by Russel H. Campbell, the correlation between heavy precipitation events and landslides in southern California’s Santa Monica Mountains was explored. Aerial reconnaissance, official reports, and eye witness accounts were used to census landslides after two intense storm periods that occurred in November/December of 1965 and January/February of 1969, the later period being the focus of the report. The situation and location of soil slips and debris flows during this period were compared with rain gauge records, radar derived precipitation intensity maps, and soils maps. According to data on the timing of landslides and hourly precipitation totals gathered from various sources during these intense storm periods in 1965 and 1969, he found that intense rainfall, greater than 0.25 in/hr, was recorded at or near almost all of the major slide events recorded during the two major storm events. Special physical and environmental conditions in this area of Southern California, and other semiarid coastal mountainous climate zones, create an environment that is conducive to landslides of all types (Campbell, 1975).
A further consideration in assessing the La Conchita landslide is the fact that the ranch itself is isolated on a mesa, blocked essentially on all sides from any groundwater recharge. The only possible sources of groundwater are precipitation and irrigation. Although it is possible that the irrigation water could have percolated to a limited extent into the groundwater, it is unlikely to be the cause of the landslide. During winter, when there is an abundance of rainfall, irrigation is not necessary. Moisture from groundwater is essentially sucked out of the upper layers of the soil by evapotranspiration during the hot summer months when irrigation is necessary to sustain the more than 400 acres of Lemon and Avocado trees.
Excess water from precipitation that does infiltrate through to the water table drains along the path of least resistance and follows the force of gravity; the faults that exit through the cliff face provide this path of least resistance. These faults, and their tell tale scarps, are visible in the aerial photography. There is an active fault that runs directly through the cliff face where the slide mass broke loose, and this could have potentially been the point of weakness that caused the landslide. The groundwater table within the elevated mesa can maintain a relatively constant level so long as the water entering the uppermost surface can be balanced by groundwater percolation, transmission though the soil layers, and eventual drainage out of springs in the canyon walls or out of the cliff face. The rainfall that causes landslides must be of sufficient duration and strength to raise the field capacity of the soil, the point where under gravity infiltration will equal percolation (Campbell, 1975). At this point, the soil layer will become saturated with additional infiltration. With an abrupt increase in infiltration that exceeds percolation, the pore-pressure between soil particles decreases, and the upper layers of soil become saturated. As water replaces air between the soil granules, the resistance of soil layers to shear stress decreases and the mass of the saturated soil breaks loose. From the analysis of the timing of several slope failures and precipitation totals during the 1969 study period, Campbell states that a minimum total of 10.5 inches of rain with a minimum threshold intensity of 0.25 in/hr was sufficient to induce sliding in most cases. Campbell points out that, in several instances during the “80 year storm” that occurred in January of 1969, catastrophic damaging slides occurred well before the total rainfall reached 10 inches. This, he postulated, supports the hypothesis that intense rainfall contributes to slope failure more so than a large total resulting from sustained light rainfall.
Conclusions
The investigation of the 1995 La Conchita landslide utilized several sources of historic information to evaluate the accusation that excessive irrigation by the bluff top ranch owners caused the landslide. Establishing the landslide history of the area relied upon historical aerial photography dating back to 1927 and DEMs (Figure 2, end of paper) constructed from the contours of a topographic map plotted in 1869, 1947 and 1995. Visual analysis confirmed that numerous alterations to the cliffs of La Conchita have occurred over the past 130 years. Extremely intense precipitation brought about by El Nino was also a contributing factor. Geologically speaking, this area is unique. The uplifted marine terraces that comprise the cliffs from which the slide originated and the accumulated slide debris upon which La Conchita is built further the contention that this landslide was a natural occurrence. The argument that this landslide could not be attributed directly to excessive irrigation had “the greater convincing force” in the words of the judge in this case, and, based on testimony from experts and evidence from historical sources and measurements taken by geotechincal professionals, it was the ruling by the judge that this case can never again be brought against the ranch owners, based on the premise that the irrigation caused the landslide.
Not far from La Conchita is a "surfer" beach called Sea Cliff. Hidden from the freeway by trees and rocks. Once you get down, you can build a driftwood fire and cook hot dogs, drink beer and party up:) Jes' take your trash back up with you. It beats any well known beach you know. There! It's a secret no more:):)
Arnold was there yesterday -- what did he promise the residents in the way of support to rebuild their homes? If he did anything except point the way to the Red Cross shelters, the taxpayers of CA should make their views known, just as the scientists made theirs known 10 years ago.
...10 3 years ago
They dont need insurance to buy a home..I mean, its not even standard to get landslide insurance apperantly.
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