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Shoreline response to jetty extension and natural forcing events at Siuslaw Bay, Oregon 1981-1990 Public Deposited

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  • This paper investigated observed morphologic changes in sand accretion and erosion due to sedimentary processes at the jetty entrance to the Siuslaw River near Florence, Oregon. Ten years of data on morphologic change in sand and sediment distribution, collected semiannually between 1981-1990, was used to analyze the nearshore sedimentation processes around the jetty to determine if observed morphologic changes are the result of the extension of the Siuslaw River jetties or to natural climatic forcing processes, such as the 1982-1983 EL Nino. During the ten years of recorded data an El Nino occurred in the winter of 1982-1983 and jetties were extended in 1984-85. The study area extends over five miles north and south of the jetties and from the dune crest to an offshore depth of twenty feet. The study methodology uses morphological, graphical and statistical assessment techniques. The objective of this study was to comprehensively analyze all of the survey data collected by the Army Corps of Engineers (COE) to determine if any modification to shoreline morphology has occured over the ten year study and if that change has a causal relationship with the jetty extension. It poses these questions: (1) are these changes large enough to be observable in volume comparisons, graphing comparisons and morpological comparisons? (2) Within the data can the jetty event be separated from the El Nino event? and (3) After separation of these two events what conclusion does the statistical analysis point towards? To answer this last question I posed a Null Hypothesis that 1981=1985=1989=1990 from which to base the analysis. The Alternate Hypothesis is that 1981≠1985≠1989≠1990. Statistical analysis of these volume changes was based on comparing natural variation within the areas north and south of the jetties; specifically, the natural variation between an inner and an outer cell. This approach allows an analysis of the character of volume variance over time without having to contend with the El Nino affect. This specific approach is termed an Analysis of Variance (ANOVA). Within this approach are several techniques. Because I have no expectations for any volume variances, the Tukey-Kraiuer test for difference was applied. This is a valid test given that my sample sizes are the same for all years analyzed, and that the means for the years do not vary significantly from one another. The years 1981, 1985, 1989, and 1990 were used in this analysis.
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