THE CAPES - WHAT HAPPENED?
RINNE, Richard W., 5 Centerpointe Drive, Suite 240, Lake Oswego, OR 97035, Richard.Rinne@hartcrowser.com.

The Capes is a 76-acre gated community located on the Oregon Coast in Tillamook County, developed from about 1992 through 2000. The site is underlain by a Pleistocene/Holocene dune complex thought to have developed over the past 40,000(+/-) years.

A landslide scarp is present along the coastal margin in the north-central part of the project. The slide block measures approximately 800 feet north to south and 300 feet east to west

During winter storms of 1995-1996, the toe of the dune began to erode and retreat toward the east. The 1997-1998 El Nino apparently caused the tidal outlet to Netarts Bay to move northward in front of the Capes Development, probably further accelerating the erosion process. In December 1997, a surface crack appeared in the mid-slope area of the headwall. In January 1998, the movement began to accelerate and damage became evident. By the last week in March 1998, the scarp was about 30 feet high. The height increase of the landslide scarp for most of January and February was estimated at 4 to 6 inches/day.

The observed slide movement is translational as opposed to a circular failure. This is based on the site and soil geometry and results of seven inclinometer installations throughout the slide mass. Both horizontal translation (block) and circular failure modes were analyzed.

The majority of the slide block ranges from El. 50 to 80 feet (El.=elevation above mean sea level). The slide block slopes southward with a depressed linear feature near the toe of the headwall slope. Cracks and small scarps induced by the slope movement are present throughout the slide block.

At the base of the slide block along the beach, a steep 20- to 30-foot-high dune face is present consisting of dune sand overlying exposed layers of sand, gravel, clay, and peat, with concentrated zones of brush and tree stumps. In the southern two-thirds of the exposure along the beach, the clays and organic layers are up to 19 feet thick. Along the northern third of the dune face, a clayey boulder talus deposit lies directly above the dark gray clay.

If there had not been expensive townhouse units adjacent to the slide scarp, would anyone have noticed, or cared?

Cordilleran Section - 98th Annual Meeting (May 1315, 2002)
Session No. 36
Engineering Geology Case Histories of Landslides
CH2M Hill Alumni Center: Ballroom 110C
8:10 AM-12:00 PM, Wednesday, May 15, 2002
 

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