Graduate Thesis Or Dissertation

 

Controls on movement of selected landslides in the Coast Range and western Cascades, Oregon Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vq27zs12v

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  • The movement characteristics of five landslides are compared and interpreted based on records of approximately 10-years duration. Condon landslide in the Oregon Coast Range has consistently exhibited brief (1 - 8 days) movement episodes in wet winter months, separated by long periods of no movement. The translatory movement is probably controlled by the orientation and structure of the underlying sedimentary rocks. From 1981 to 1990, annual movement averaged 109 mm, and individual events varied from 1 to 187 mm. All major movement events (> 10 mm in 4-10 days) were precipitation-induced. A non-linear relationship exists between movement rates and Antecedent Precipitation Index, which has a daily recession coefficient of 0.87. The API threshold for movement initiation was estimated to be 160 mm, based on 16 documented major events between 1984 and 1990. Groundwater level at the landslide responded to precipitation very quickly, with lag time usually less than 3 days. Movement started on days when the groundwater level rose above 2.5 m below ground surface, and a non-linear relationship exists between daily movement rate and groundwater level. Based on available data, there appears to be no change in movement characteristics of Condon landslide after two-third of it was clearcut in 1987. Wilhelm landslide, located near Condon landslide, has a similar movement pattern, but smaller movement magnitude (averaged 34 mm per year, 1985-1990). The Mid-Santiam and Jude Creek landslides in the volcanic terrane of the western Cascade Range move at much faster rates, averaging 3.8 and 7.8 m per year from 1982 to 1990, respectively. Unlike the Condon and Wilhelm landslides, where individual movement events correspond with individual storms, these two western Cascades landslides exhibit prolonged movement. The Mid-Santiam landslide moves all year, and annual movement shows little variation over the year. The other studied landslides all have large intra- and interannual variation in movement rates, and movement generally stops in the summer dry period. The Lookout Creek landslide (average annual movement = 79 mm, 1981-1990) has slowed in the past four years, and has exhibited movement patterns similar to the storm-dominated Coast Range slides. Geology and climatic patterns are the two most important factors contributing to the observed differences in timing and style of movement in the landslides studied. Climatic patterns trigger movement events, and geology influences movement patterns through control on geotechnical properties of landslide materials. These factors can be used to classify landslide movement patterns on a regional scale.
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