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Landslides in the San Gabriel Mountains by Bethany Cutts & Nicole Lehrhoff
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The story of the Genofiles is not an uncommon one to those living in the San Gabriel Mountains. Sometimes serene, other times a petrifying wrestling match with nature. In this case, the Genofiles survived the match but who actually won is not so easy to assess. (This story can be located in John McPhee’s book The Control of Nature)
Landslides as defined by Harvey Blatt, are "mass-wastings that always occur on slopes, and consist of downslope movement of a mass of soil and bedrock." Blatt also points out that various landslides have caused some of history’s worst disasters all over the world. These slides, or flows, can be broken into a variety of categories: avalanches, debris slides, rockslides, snow slides, talus creeps, mudslides, earthflows, and mudflows. Most of these slides occur on slopes, in response to the natural pull of gravity. The specific type of slide is determined by the amount of water involved in the slide, and by the percentage of clay minerals in the unstable rock or sediment (Blatt 120). Ultimately, gravity and friction determine the extent of a landslide. On a hill slope, gravity pulls in two directions, perpendicular to the surface (gp) and along the slope (gt). When gp is greater than that of gt, the particle remains anchored in place. It is only when gt exceeds gp that the particle will move. Several environmental factors can alter the relationship of gp and gt.
Landslides are triggered by four major events: earthquakes, excessive precipitation, shoreline wave activity, and modern civilization. The shaking that earthquakes produce disables the friction that restrains particle movement thereby separating particles. Excessive precipitation produces a similar effect to that of earthquakes. Precipitation decreases the amount of friction between particles and creates pore pressure that can work to push the particle apart. This effect is compounded by the presence of a week ionic bond between clay particles, which is easily broken when wet. however the destabilizing occurs because the slide contains a large percent of clay minerals, the clay swells when it gets wet further reducing the friction. In addition the free flow of water forces grain apart. When the flat clay particles separate, they slide easily past each other.
Wave action on shorelines can take advantage of any of the destructive methods mentioned for the three other causes of landslides. Normal wave action can erode the foot of a hill or cliff and heavier waves can shake loose particles of saturate soils and cause slipping. Some of the most serious landslides are those that occur in areas that experience forest fire followed by periods of soaking rains because the soil is destabilized and dry. It is more vulnerable to slippage. Slides are caused by human activities in several ways. One being that the support at the toe of the slope is removed, this most often occurs when construction is being done at the base of the slope. Another way a slide can occur is building on top of slope; the slope may not be able to withstand the additional load. As long as humans continue to disturb nature for development of cities and economics, the more likely landslides will occur.
The amount of damage generated by a landslide depends on the location and magnitude of the slide. Wetter slides are more likely to effect a larger area before coming to a stop. Scientists and reporters use a damage intensity scale to quantify the amount of damage caused by landslides as they pertain to human populations. The scale ranges from 0, when no real damage occurs to 6, which completely decimates a society and is likely to cause many deaths. To prevent mass amounts of damage, researchers have named certain areas as having particularly high risk of a landslide. These areas include those with a history of landslides, in or near drainage hollows, at the base or top of an old or steep hill especially one that is highly developed.
In inhabited areas that are likely to experience a landslide, local and regional politicians, developers, and geologists have come together to reduce the damage caused by possible landslides. They take extensive measures to change the way water flows from mountain tops, bolt unstable rock material to sturdier underlying material, build retaining walls, and promote methods of retaining loose materials.
The Genofiles' is McPhee’s book experience what is known as a debris flow. Debris flows occur most commonly in stream valleys and resemble a texture similar to fresh concrete. They consist of water mixed with solid materials, most of which are a least as large as a sand particle, and can be as large as cars, and sometimes larger. Debris flows can at times ooze along and at other times move as fast as the fastest river rapids. The particular flow that McPhee described can be attributed to three of the triggers, previous earthquakes, heavy ran fall and human activities. The whole front of the San Gabriel Mountains had shattered and broken, by undercutting of streams, and local earthquakes. Their house build by a stream at the base of the San Gabriel Mountain range, filled up within six minutes, and at the end of the slide was buried under twelve feet of debris. Like many of their neighbors they did not feel threatened before the storm. They were confident in the man made concrete basins designed to catch the falling debris. It was later determined that the basin was improperly designed and had not been cleaned out. The Genofiles’ sued the Los Angeles County and were awarded three hundred and thirty seven thousand five hundred dollars.
Blatt, H. (1997). Our Geologic Environment. New Jersey: Prentice-Hall Inc.
McFee, J. (1989). The Control of Nature. Toronto: HarperCollins
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