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. / Graduation date: 1992
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37271 |
| Date | 21 August 1991 |
| Creators | Wong, Bernard Bong-lap |
| Contributors | Swanson, Frederick J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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