Landslides and Mass Wasting
Compared to spatially extensive natural hazards like earthquakes, tornadoes, and hurricanes, landslides are relatively localized and usually impact a minimal number of persons. They can be deadly, however, and can cost billions of dollars to clean up. While landslides are not preventable, it is usually not difficult to predict where they will happen (although timing is very unpredictable). Unfortunately, humans often increase both the potential for landslides and their exposure to them by the choices they make about where to build and how to manage landscapes.
In 2005, a landslide in La Conchita, California killed 10 people and destroyed or damaged three dozen homes. The landslide occurred along a stretch of the southern California coast between Santa Barbara and Ventura that is known for landslides. Over a half-dozen landslides had occurred at La Conchita in the 20th century, the most recent in 1995, just ten years before the fatal slide.
In 2005, a landslide in La Conchita, California killed 10 people and destroyed or damaged three dozen homes. The landslide occurred along a stretch of the southern California coast between Santa Barbara and Ventura that is known for landslides. Over a half-dozen landslides had occurred at La Conchita in the 20th century, the most recent in 1995, just ten years before the fatal slide.
Elements of a Landslide Hazard
1) Topography
In the two pictures above of La Conchita, it is easy to see how the terrain contributes to the landslide hazard in this stretch of coastline. The cliffs seem to rise out of the ocean, and there is only a narrow strip of flat land between the Pacific and the hills. In fact, this area is highly prone to landslides, with previous major events in 1865, 1889, 1909, and 1995.
2) Geology
Steep topography is a contributing factor for landslide hazards, but so is the geology. Where cliffs are comprised of bedrock (like the basalt formations along the Snake and Columbia Rivers), there is little chance of a mud flow, but along the California coast, the terrain is loosely piled ocean sediments that rose up out of the sea only in the last few million years.
3) Precipitation
Where the rain is minimal or where it falls in small amounts year-round, there is less likelihood that water will completely saturate the soil and produce a mudflow. But in this part of California, annual rainfall often comes all at once in just a few storms. In 2005, the area received 15 inches of rain in just two weeks to equal its annual average.
4) Vegetation
If there is thick vegetation on a site, it will help to both soak up the precipitation, and the root systems will help hold the soil together. At La Conchita, there was not very much vegetation, and the native vegetation had been removed on top of the cliff above the town to make way for orchards.
5) Human factors
One of the contributing factors in the La Conchita slide was the road that had been cut into the hillside, visible in the pictures above. The road cuts the hillside into blocks, and removes the stabilizing toe slope of the upper block. Once that block began to slide, it liquified and took the rest of the hill below with it.
Efforts to Mitigate
After the 1995 landslide, the town of La Conchita built a retaining wall at the foot of the hill. This is the only mitigation effort the town undertook. After the 2005 landslide, victims successfully sued the companies that owned the orchards on top of the hill, claiming that the orchard owners were not draining their property properly.
An observer happened to catch the 2005 landslide on video -- it was less than a minute from start to finish, meaning there was no time for victims to get out.
In the two pictures above of La Conchita, it is easy to see how the terrain contributes to the landslide hazard in this stretch of coastline. The cliffs seem to rise out of the ocean, and there is only a narrow strip of flat land between the Pacific and the hills. In fact, this area is highly prone to landslides, with previous major events in 1865, 1889, 1909, and 1995.
2) Geology
Steep topography is a contributing factor for landslide hazards, but so is the geology. Where cliffs are comprised of bedrock (like the basalt formations along the Snake and Columbia Rivers), there is little chance of a mud flow, but along the California coast, the terrain is loosely piled ocean sediments that rose up out of the sea only in the last few million years.
3) Precipitation
Where the rain is minimal or where it falls in small amounts year-round, there is less likelihood that water will completely saturate the soil and produce a mudflow. But in this part of California, annual rainfall often comes all at once in just a few storms. In 2005, the area received 15 inches of rain in just two weeks to equal its annual average.
4) Vegetation
If there is thick vegetation on a site, it will help to both soak up the precipitation, and the root systems will help hold the soil together. At La Conchita, there was not very much vegetation, and the native vegetation had been removed on top of the cliff above the town to make way for orchards.
5) Human factors
One of the contributing factors in the La Conchita slide was the road that had been cut into the hillside, visible in the pictures above. The road cuts the hillside into blocks, and removes the stabilizing toe slope of the upper block. Once that block began to slide, it liquified and took the rest of the hill below with it.
Efforts to Mitigate
After the 1995 landslide, the town of La Conchita built a retaining wall at the foot of the hill. This is the only mitigation effort the town undertook. After the 2005 landslide, victims successfully sued the companies that owned the orchards on top of the hill, claiming that the orchard owners were not draining their property properly.
An observer happened to catch the 2005 landslide on video -- it was less than a minute from start to finish, meaning there was no time for victims to get out.
Think about the following questions:
1) Given the history of landslides in the Las Conchitas area and the geology, do you think this landslide could have been prevented?
2) Were the mitigation measures taken by the town and the homeowners in the area sufficient?
3) What additional mitigation measures could have been taken to prevent such a tragedy?
4) Who should be responsible for landslides impacting homes in places like Las Conchitas?
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