During the period 1970-76, Siletz Spit on the mid-Oregon coast has suffered foredune erosion. This erosion is associated with high wave conditions along the coast, produced by intense storms in the North Pacific. During the winter of 1972-73 and during January through March 1976 the erosion was particularly severe. The pre-1975 erosion of the spit has been documented in a previous study. One of the main purposes of the present study is to document the 1976 erosion and to contrast it with the earlier episodes. It differed principally in that a small wash-over of the spit occurred in one area and beach drift logs were thrown up onto the dunes. Neither occurred during previous erosion episodes, even during the 25 December 1972 storm when wave breakers exceeded a significant wave height of 7 meters. This is because the 1972 storm occurred at a time of low water level, neap tide conditions. In contrast, the 18 February 1976 major storm occurred during spring tide conditions, causing the wash-over and log throwing, even though the waves were smaller than in December 1972. The amount of dune retreat was somewhat less in 1976 than in 1972-73, partly because of the shorter time period over which the 1976 erosion occurred and partly because of the use of protective riprap. The large waves causing the erosion at Siletz Spit are generated by storms in the North Pacific. The storm systems for December 1972, January 1973, and February 1976 were analyzed as to fetch distances, wind speeds and directions, movements of the fetches, and other factors important in the generation of waves. Waves hindcasted from these data were compared with measurements of the waves obtained from a seismometer system at the Marine Science Center in Newport, Oregon. This comparison demonstrated that these distant fetches were responsible for the waves causing erosion on Siletz Spit. Waves generated locally by coastal winds appear to be of negligible importance in the erosion. This is also shown by a comparison between the wave measurements and data on the coastal winds at Newport during the times of maximum erosion. Tides were investigated for their role in contributing to coastal erosion. High spring tides allow the storm waves to better reach the dunes or sea cliffs and so permit greater erosion. Neap tides, with their lower water levels, diminish the amount of erosion. Many of the differences in the December 1972 and February 1976 erosion episodes were due to the difference in tide levels. Storm surges were determined for Yaquina Bay, Oregon, by subtracting the predicted high-tide levels from the observed levels. It was concluded that significant storm surges did not occur at the times of maximum spit erosion and therefore did not contribute to the erosion processes. / Graduation date: 1977

Siletz Bay is a drowned river valley filled with Holocene alluvial and estuarine sediments and is separated from the ocean by a sand spit 3.8 km in length. Since the area was settled by white man in the 1890's, the bay has apparently experienced rapid siltation, due to increased farming and logging. This along with the damming of the Siletz River sloughs has altered circulation patterns in the bay. Deflection of the Siletz River flow by the prograding Drift Creek delta has caused 105 m of erosion since 1912 on the east side of Siletz Spit. The ocean side of the spit suffers periodic erosional episodes separated by periods of accretion and dune building. The most recent and publicized erosion occurred during the winter of 1972-73 when it was feared that the spit might be breached; one partially constructed house was lost and three others were saved only by timely riprapping. A sand mining operation may have aggravated the recent erosion by disrupting the sand budget, the balance of sand additions and losses from the beach. All of the foredune on the spit has been stabilized by dune grass and much of it has been riprapped. The long term effects of stabilization and riprapping are uncertain. / Graduation date: 1975