2005 La Conchita Landslide



Introduction
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Location of La Conchita, California

On the afternoon of January 15, 2005, La Conchita was trapped by a landslide. This landslide has all of the characteristics of a typical landslide, which will be explained later. The landslide caused Highway 101 to be shut down, with only one way in and out the residents had no other way to leave the city. The community of La Conchita sits on the coastline that is prone to landslides. La Conchita also sits on a history of landslides and old debris flow. (Cotton) Figure 1 shows the exact location of La Conchita on the bases of near by counties. It also sits on a geological unstable ground.The unstable ground is already a big problem for the community if there were any movement to the ground. It meant that the ground can give any second and cause a major landslide without any other factors added to it. There was an estimated 400,000 tons of debris. That particular landslide destroyed approximately thirty-to-forty homes while killing ten people. Destroying thirty-forty homes is not a great amount in a regular city but it is in that small community of about 160 homes.




Landslide of 1995

Ten years before 2005 there was another landslide that occurred in the same area.Figure 2 shows a map of the 1995 landslide,circled in blue, and the 2005 landslide, circle in yellow. On March 4, 1995, that landslide moved tons of dirt into the city and destroyed about nine homes in just a few minutes. This also had all of the characteristics of a typical landslide; It caused the center of the mass to move outward and downward, the zone of depletion is up slope and the zone of accumulation is down slope. There were no casualties because of the landslide. Six days later, a debris flow made its way down to the northern part of La Conchita and destroyed an additional five homes.

Background Information

The 2005 landslide occurred without any new material of soil but it was just a remobilization of the 1995 landslide. Figure 3 shows the terms of landslides and how a landslide occurs. This occurred in the southeastern portion of the 1995 landslide. The dimension of the land slide was about 350 meters long and 80-100 meters wide. During the week before the land slide, there were record breaking rainfall that fell into the city. Many people were predicting that the land slide was caused by the intense amount of rain that saturated the ground and made it move. On a seasonal average, rain from between October and January totals about 19.4 inches but during that year, From the end of December and beginning of January, during a two week period, rain fell about 14.9 inches. Little rain can help the ground keep its cohesion but if there is too much water it can cause many problems. It can add a lot of weight, it strengthens the clay material, causes subsurface erosion and increases pore pressure. By adding weight, included by the gravity pulling the ground down, there is more energy for the ground to give therefore causing a mudslide. Even though the rain occurred weeks before the landslide. It can take weeks maybe even months for the rainfall to have deep infiltration of the ground. Having deep infiltration can only cause the landslide to be even bigger. The deeper the rain goes into the subsurface, the more ground will release if and once there is a mudslide. Groundwater has a big role in causing landslides. (Thompson)
On top of the hills are orchards of avocados. They are underlain by the marine terrace deposit which consists of unconsolidated silt, sand and gravel. This community has a history of land slides even before the community was built. This is a small community so everyone knew each other. They left their car doors unlocked, house door unlocked, they watched each other kids and pets, and they had community BBQ’s in the summer time on a regular basis. This was a close knit community. When this occurred it was on a day where the community planned a BBQ on the beach. During that time many people were getting ready to go to the BBQ while some were packing up as well just in case of an emergency evacuation from the intense rain that occurred during the weeks before.
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La Conchita Landslide 2005

After the slide occurred, the residents were trapped because during that time the rain caused debris to fall into the 101 freeway. Which was the residents only way in and out by car. They were able to walk across it to go to the beach but unable to drive their cars.
After this incident, our governor gave an amount of money to the city in order to fund a study to learn how to better prepare for the landslide. The report was supposed to be finished and given to the city earlier this year. This report should help the city better prepare for the landslide and possibly even help predict when the next one will occur. Living on this kind of ground can be very dangerous. Even if hundreds of mudslides have come and gone, the unstable ground will always be there and if people are willing to take the risk to move into this uncertainty of hazardness, they can. This problem of mudslides in this community will probably not go away for a really long time if ever.

Conclusion

After two major landslides in the span of 10 years, the residents of La Conchita, California are quite familiar with the idea of living in such a dangerous area. Even though landslides are unable to detect, they are capable of being predicted. It is very important to monitor the movements of the mountains or hills. Officials should keep note of any cracks on the hillside. They should also monitor the rainfall each month during the raining season. If the rainfall surpasses the average amount, warnings should be put out to residents that there is a possibility of a landslide. The warning should last for about a month or two after the rainy season has gone by.


Figures

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Figure 2: Blue Shades 1995 Slide, Yellow Shades 2005 Slide


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Figure 3: How a Typical Landslide Happens.


Sources

Jibson, Randal W. "Landslide Hazards at La Conchita, California" http://pubs.usgs.gov/of/2005/1067/508of05-1067.html
http://domex.nps.edu/corp/files/govdocs1/159/159300.pdf
Thompson, Troy. "Groundwater and landslides" http://www.wgwa.org/articles/landslidesandgroundwater.pdf
http://adsabs.harvard.edu/abs/2005AGUFM.G11A1179S
Cotton, William R. "CATASTROPHIC DEBRIS FLOW FAILURE OF THE LA CONCHITA HILLSIDE: LESSONS REVISITED, VENTURA COUNTY, CALIFORNIA" http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85694.htm