Remote measurements of waves and currents over complex bathymetry Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5h73pz25f

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  • Waves are the primary input of energy in the nearshore region, and together with the currents forced through the transfer of momentum that occurs during the wave breaking process they are the principal mechanism for sediment transport in the nearshore. The basic physics of waves and currents are thought to be well understood, and simple models for waves and current interacting with simple (alongshore-uniform bathymetry) generally agree with in situ measurements. However, alongshore variability is ubiquitous, and the predictability of waves and currents over complex bathymetry is an important research topic. Significant effort has been placed into understanding and predicting (modeling) the kinematics and dynamics of waves and currents, but the accuracy of these models are largely untested, due to the lack of appropriate wave and current measurements. This dissertation first details the development and application of an optical remote sensing technique to measure the shape of wave frequency-direction spectra over large scale complex bathymetry. The optical wave spectral estimation technique is based on the wave slope dependence of reflected skylight, and while not a metric of the absolute wave energy, the measurement technique is shown to agree with the frequency and directional distributions of in situ measurements. Simultaneously, the optical technique offers increased directional resolution. Maps of the wave direction field over a submarine canyon, using the optical technique, show dramatic changes in the wave divergence and convergence which coincides with details of the large scale bathymetry gradients. The last section of this dissertation analyzes 14 months of surf zone longshore currents measured from a two-dimensional array using a previously described optical remote sensing technique. Based on metrics developed in the study, the longshore currents are shown to be alongshore nonuniform for the majority (70%) of the observations, suggesting that a simple one-dimensional force balance circulation model is not generally applicable. Related tests of a point model only show moderate skill when predicting the peak longshore current (rms error 0.38 m/s). Though cases of two-dimensional circulation (i.e. rip currents) are shown to coincide with alongshore-variable bathymetry, a metric of the degree of alongshore bathymetry variability, γB, has poor skill of predicting the strength of spatial longshore current variability.
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