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introduction to mass wasting causes of mass wasting prevention of mass wasting types of mass wasting
FALLS (rock fall and rock avalanche)
SLIDES (rocks slide and slump)
FLOWS (rock avalanche, debris flow, earth flow, and creep)
La Conchita
In 1995, the small coastal California community of La Conchita began experiencing a combination of mass-wasting phenomena. It began with a large slump, shown in picture 1 (credit to R.L. Schuster, USGS). As impressive as this slide is from the air, it is even more imposing when viewed from the ground, in picture 2. From this perspective, it appears that the entire mountain is moving into La Conchita. The perspective of picture 3 is from the slump block looking downward into town and the Pacific Ocean. The slump and related earth flow were relatively slow-moving, and no one was killed by this event, although nine homes were destroyed. The earth flow was stopped by a strong retaining wall (pictures 4 and 5), giving La Conchita residents a false sense of security.
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In January of 2005, a series of storms soaked the slopes above La Conchita, adding mass to the unstable slopes and lubricating the sediment grains. The result was a catastrophic failure of a small portion of the slope above La Conchita. This event, labelled in news reports as a mudslide, began as a sudden rupture occurred within the slope, releasing a slump block which instantly crumbled into a fast-moving, fluid-rich debris flow which roared into the eastern part of La Conchita. Geologists estimate that a total of about 400,000 tons of earth moved during this event, which destroyed 13 homes and killed 10 people. The future of the entire community of La Conchita is now very much in doubt because of the ongoing threat of further mass-wasting events, and the prohibitive cost, estimated at $100 million dollars, to stablize the slope above La Conchita.
Below are series of photos taken soon after the 2005 La Conchita disaster. Pictures 1 through 4 are aerial photos of La Conchita, taken by Mr. Allen Krivanek, showing the path of the debris flow and the resulting destruction. Pictures 3 and especially 4 enable you to see what's left of the retaining wall constructed after the 1995 slump-earth flow event. It may have helped to shield some of La Conchita from the 2005 debris flow, most of which was deflected toward the right (east). A careful look at picture 2 suggests that the road that cuts a diagonal across the slope from left to right is the likely culprit for recent slumps above La Conchita: cutting a road into a slope over-steepens the slope and removes support from the slope mass above the road. See the enlarged "Features of a Slump" picture below for clarification of this simple but important concept.
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Pictures 5 through 7 below are Associated Press photographs which provide close-up, ground views of the La Conchita disaster. Picture 8, also an AP photograph, gives a different aerial view than the aerial photographs above, showing the perspective of the debris flow from the source.
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