Graduate Thesis Or Dissertation
 

Wave runup : Physics, variability, and societal impacts

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8w32rb56c

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  • The U.S. Pacific Northwest is home to one of the most extreme wave climates in the world with waves of 10 m in height arriving to the coast approximately each year. With an average water temperature of 12℃, the beaches in the region are too cold to go on a swim without a wetsuit. Thus the majority of the recreation takes place in the sand, making the swash zone an important place of interaction between beachgoers and the Pacific Ocean. This dissertation focuses on improving our understanding of the physical mechanisms, distribution, characteristics, variability, and societal impacts of wave runup. The work presented here is undoubtedly influenced by the Pacific Northwest but the results are both widely applicable and relevant to a general audience. All beach accesses in Oregon are marked by signs indicating the many dangers that a visitor might face, prevalent in all beaches are sneaker wave warnings. Chapter 2 of this dissertation presents a compilation of all sneaker wave incidents reported in the media between 2005 and 2017 in Oregon and Northern California. A hindcast analysis of water levels is performed on each event to determine the general characteristics of impactful sneaker waves. For cases involving coastal structures it was found that the sneaker waves coincided with rapidly increasing wave overtopping probabilities due to rising tide levels. Long swells arriving from winter storms were found to be correlated with the majority of sneaker wave events. These events occurred for wave heights of all sizes and a wide range of beach slopes. For the majority of the cases where a threshold analysis could be performed, the water level was expected to exceed it. The results motivated the study of other properties of wave runup. One characteristic of the mildly sloping beaches of the PNW is the occurrence bore-bore capture (BBC), where multiple broken waves combine into a single bore before running up the beach face. Chapter 3 explores the relationship between BBC and wave runup. This effect is explored with a series of numerical experiments with varying complexity. The effect of BBC is isolated in experiments where a large wave interrupts an otherwise regular wave train. The anomalous wave interacts with the previous one due do amplitude dispersion and the resulting runup is measured. It is found that the runup level increases non linearly with respect to the runup that the large wave would have produced as part of a regular wave train. A subsequent experiment is performed where two regular wave trains are combined producing a group behavior. It was found that BBC can in uence the runup magnitude by up to 30%. The relative phase between the infragravity waves and the incident waves is a controlling factor for runup size. Finally, in the presence of realistic random sea-states, it was found that BBC is a sufficient but not necessary condition for large runups. The variability and rogue characteristics of runup are relevant for beach safety, erosion, and hazard assessments. Using variation of parameters and ensemble modeling techniques, sea states with maximum potential for rogue runups (with thresholds defined from a Rayleigh distribution) are identified and discussed in Chapter 4. Variability of runup magnitude, period, and velocity are also investigated. Beach states with maximum likelihood of unexpected runup occurrence and maximum variance are identified. The effect of beach morphology in maximum runup is explored by comparing the model results on planar and bilinear beaches. It was found that the maximum runup is larger on bilinear beaches and that the depth at which both slopes meet is a controlling factor on the runup amplification.
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