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 Landslides are the downslope movement of rock, soil, or related debris. Geologists use the term “mass movement” to describe a great variety of processes such as rock fall, creep, slump, mudflow, earth flow, debris flow, and debris avalanche. In most mass movement, water plays a pivotal role by assisting in the decomposition and loosening of rock, lubricating rock and soil surfaces to enhance the beginning of movement, adding weight to an incipient landslide, and imparting a buoyancy to the individual particles, which helps overcome the inertia to move. The composition of slides is also very important, and the proportions of rock, sand, clay, and water will dictate the initiation, speed, and areal extent of each slide.Although landslides are propelled by gravity, they can be triggered by other natural geologic disasters or human activity. Volcanic eruptions and earthquakes can initiate earth movement on a grand scale. A variety of debris flows called “lahars”—a mixture of volcanic ash and water— are specific to volcanic activity. Lahars are often the major hazard experienced in a volcanic episode. Although earthquakes can initiate debris flows, the major causes of landslides in the northwest are continuous rains that saturate soils. Landslides are frequently the direct consequence of human activity. Seemingly insignificant modifications of surface flow and drainage may induce landslides. In an urban setting, improper drainage most often induces disastrous sliding. The placement of buildings, to capture a spectacular view, on slide-prone coastal dunes, eroding headlands and sand spits, or at the edge of a receding shoreline, may lead to the loss of the structure. It has been noted that in Portland, population pressure has pushed construction into many areas and sites previously rejected as landslide-prone. Agricultural irrigation and forestry practices such as clear-cutting and stripping vegetation from naturally oversteepened slopes have been shown to be responsible for a spate of landslides. Highway construction on similar slope conditions awaits only the first good rain to provoke earth movement. In an urban setting, improper drainage most often induces disastrous sliding. Many unstable areas can be recognized. Tip-offs to incipient hazard-prone slopes include scarps, tilted and bent (“gun-stocked”) trees, wetlands and standing water, irregular and hummocky ground topography, and oversteepened slopes with a thick soil cover. The technology of spotting landslides by use of aerial photography has become so refined that NASA routinely recognizes and maps mass-movement features on several of the planets in our solar system as well as on our own moon. Learn more about landslide hazards in Oregon: Department of Land Conservation and Development landslide hazards website Special Paper 34 - Slope Failures in Oregon, GIS Inventory for three 1996/1997 storm events |
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