Graduate Thesis Or Dissertation


Remote Sensing of Nearshore Hydrodynamic and Morphodynamic Processes Public Deposited

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  • The use of remote sensing techniques in coastal science and engineering has rapidly increased in the past few decades. This dissertation outlines new remote sensing tools using two remote sensing technologies (lidar and X-band marine radar) along with two nearshore hydrodynamic and morphodynamic analyses supported or motivated by these remote sensing observations. The thesis is organized in manuscript format and contains four manuscripts as individual chapters. The first and last chapters of the thesis serve as the Introduction and Conclusions to the central four chapters. In the first manuscript (Chapter 2), a recently-established, permanent coastal lidar station in Duck, NC is presented. This system marks the first continuously-scanning and fully-automated nearshore lidar collection and processing system, and the resulting hourly data products are provided to the coastal community in real-time. The manuscript describes the original work in developing the set-up, collection, and processing algorithms for the lidar system. The high spatial and temporal resolution of the resulting data allows for analyses of beach evolution over a range of time scales, from seconds to years. In Chapter 3, data from the nearshore lidar system is applied in conjunction with continuous hydrodynamic data from the same site to analyze beach cusp development and evolution over a 21-month period. Cusp fields were found to form and evolve rapidly, often on the scale of individual tidal cycles. These formation events occurred primarily during low energy, long period swell conditions with narrow-banded frequency spread and reflective beach conditions. Results show that the generation of cusp fields on the lower beach were often influenced by cusp fields on the upper beach, with the upper beach cusp system exerting some control over the location and spacing of lower beach cusps. However, the presence of an upper beach cusp system alone was not sufficient to induce cusp formation on the lower beach. Chapters 4 and 5 utilize X-band radar remote sensing for rip current analysis. Current fields are visible in X-band radar images due to the interactions between short surface waves and the underlying currents, which result in changes in the roughness of the water surface and thus changes in radar backscatter intensity. In the fourth chapter, a surface roughness model is developed and compared to observations from Duck, NC (same field site as the lidar system). The roughness model utilizes rip current fields simulated using the nearshore circulation model ROMS. The modeled changes in surface roughness are compared to spatially- and temporally-overlapping X-band radar images. Results show that current-induced changes in surface roughness quantified through the change in the mean square slope of the water surface at gravity-capillary wavelengths are an effective proxy for current-induced changes in radar backscatter intensity. In the fifth chapter, X-band radar observations of transient and bathymetric rip currents are used as motivation for a modeling study focused on the conditions leading to transient rip activity on non-uniform beaches. A phase-resolving Boussinesq model (funwaveC) is used to simulate rip dynamics in a range of wave conditions on both a uniform and an alongshore varying bathymetry. The underlying bathymetry was found to strongly influence the alongshore- and time-averaged kinetic energy and exchange velocity. However, the transient exchange velocity calculated from the temporally-demeaned velocity field was similar across both bathymetric types. Instead, the largest differences in the transient exchange velocity were observed between simulations with and without directional spreading in the incident wave field. Directional spreading was also found to play an important role in alongshore- and time-averaged total and transient enstrophy. Alongshore vorticity wavenumber spectra show that the underlying bathymetry plays a large role in controlling surf zone vorticity at large spatial scales, but had no impact at smaller spatial scales, which were instead controlled by directional spread.
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