Graduate Thesis Or Dissertation
 

Shoreline changes due to jetty construction on the Oregon Coast

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  • Patterns of beach erosion and accretion due to jetty construction are examined for the coast of Oregon. All jetty systems are included with the exception of those on the Columbia River, making a total of nine systems. All evidence indicates that these areas of the Oregon coast are experiencing a seasonal reversal in the sand drift, but with a zero or near zero net drift over a several years time span. Thus, shoreline changes resulting from jetty construction are not the usual examples of jetties blocking a net drift as found in southern California and elsewhere. In general, accretion of the shoreline took place adjacent to the jetties following their construction, both to the north and south. This accretion resulted mainly from the embayment formed between the jetty and the pre-jetty shoreline, the embayment becoming filled until the shoreline is straight and again in equilibrium with the waves such that there is a zero net sand drift. In some cases, as at the entrance to Yaquina Bay, the jetties are oblique to the trend of the shoreline and so produced a protected zone from the waves where accretion could occur. Sand for the accretion adjacent to the jetties was derived from beach erosion at greater distances from the jetties. The severity of the erosion depended on the total amount of sand required for the beach accretion to a new equilibrium, and the length of beach that was undergoing erosion. When only a short stretch of beach occurs to one side of the jetties, as at Bayocean Spit, then the resulting erosion was particularly severe, in that case leading to the breaching of the spit. A computer model is developed to simulate the shoreline changes that occurred following construction of the jetties on the Siuslaw River mouth. The model demonstrates deposition next to the jetty to fill the embayment created by the jetty, and erosion at greater distances from the jetty. The shoreline advances of the model agreed closely with the actual shoreline changes found in surveys following jetty construction.
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