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|The Oregon coast is the scene of ongoing landslides, which can be directly correlated with erosion by high winter waves and increased rainfall during the major storms of January and February. Storm surges have caused considerable coastal damage by eroding sand and cutting away at headlands, which leads to sliding.
Once again, human intervention has been responsible for altering beach processes and changing patterns of deposition and erosion. Considerable money and effort have been expended to halt coastal erosion, which in places carries away as much as two feet per year. Much of the problem can be attributed to a poor understanding of coastal processes. Sea walls and riprap, as well as housing on sand spits and headlands, quite often result in effects opposite those desired.
Examples in the historic record are numerous. A jetty constructed on the northside of Tillamook Bay restricted the flow of sand down the coast to Tillamook Spit, where the community of Bay Ocean had been built in 1910. A post office, indoor pool, hotel, bowling alley, and 59 homes had been placed on the unconsolidated sand, which began to disappear in less than a decade. Retreating at around 50 feet per year, the spit was breached in 1939, and the community destroyed by 1940.
A large landslide in Tillamook County blocked 600 feet of Highway 6 on April 4, 1991, with 500,000 cubic yards of rock and soil partly damming the Wilson River. This highway has been closed annually by mudslides or rock-falls, and the 1991 episode took nearly two months for debris removal at a cost exceeding $2 million. The slide occurred when soils on the steep slope became saturated from nine inches of winter rain in two days. Cracks appeared along the upper planar slide and permitted infiltration of runoff to saturate soils. The soil liquefied immediately before movement. When the event took place, the slide was being monitored, and an attempt was being made to drain the slide block.
A similar long-standing problem exists in Curry County, where a section of coastal Highway 101 is periodically closed by landslides. Sliding began in 1938, when the roadway was newly constructed, and is still ongoing. In response to heavy rainfall, a debris flow took place on March 23, 1993, and blocked the highway for two weeks until a bypass was constructed.
A number of measures had been tried over the years to control the slide, including drains and re-grading the surface, but none were effective. More recently, increased knowledge of landslide behavior prompted the installation of a horizontal drainage system, which was able to decrease dramatically the amount of movement during the stormy winter of 1996.
South of Tillamook Bay, steep cliffs at Cape Meares, Ecola State Park, and Newport have been subject to continuous wave erosion and landsliding. Cape Meares in Tillamook County was cut back 350 feet between 1930 and 1960, and in 1961 a mass of 125 acres at Ecola State Park in Clatsop County was carried downslope at a rate of three feet per day.
Over a two-week period, much of the debris entered the ocean. At Newport in Lincoln County, coastal sliding that began in the 1920s accelerated during the middle 1940s. Roadways, drain pipes, and 15 houses were moved seaward. At present, storm waves are carrying off the mass of debris, which will eventually disappear.
This dramatic sequence of photos from the resort community of Bayocean , near Cape Meares, illustrates the chronic effects of beach erosion.
Oregon Department of Geology and Mineral Industries
800 NE Oregon Street, Suite 965, Portland, OR 97232-2162
(971) 673-1555, FAX (971) 673-1562
email us at DOGAMI
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