Swash Processes and Dune Erosion: Emphasis on Vessel-Generated Waves

University dissertation from MediaTryck

Abstract: In navigable waterways, owing to the relatively short fetches for wind wave generation, the free waves generated by moving vessels often produce the dominant wave action with respect to erosion of the shore. In response to the fairly recent introduction of high-speed ferries, combined with a planned expansion within the passenger- and car-carrying ship market, the need to model and predict coastline changes as a result of exposure to vessel-generated waves has become a topic of increased research. Although several studies have been and continue to be conducted, the research on the impact of these types of waves on the shore is still in its early stages. In response, this study was initiated with the objective to generate state-of-the-art technical guidance and modeling tools for quantifying cross-shore profile changes near and above the shoreline caused by vessel-generated waves. To fulfill the objective, the behavior of vessel-generated waves along the coast and their effect with regards to beach and dune erosion were investigated. Several independent, but inter-related studies were conducted for this thesis. One study presents a method to generate a set of practical tools, in the form of graphs. The graphs can be used to provide recommended maximum vessel speeds that will cause lower run-up than already naturally occurring by wind-generated waves. Based on observations of experimental vessel-generated wave trains, a characteristic wave height distribution, consisting of increasing and subsequently decreasing wave heights, was noted. On mild but reflective beaches significant interaction between subsequent waves in the swash zone was also observed. A numerical model, based on the ?ballistics theory? was modified and shown to reproduce the swash hydrodynamics well if interaction was accounted for. An analytical sediment transport model for the swash zone was also employed and calibrated to field data. Lastly, analytical models to predict dune erosion were developed, calibrated, and validated. The models describe erosion on a wave-by-wave basis, and are thought to be useful to estimate the impact from vessel-generated wave trains where the events are episodic and of relatively short duration. The total erosion is a function of the sediment properties, intensity, and frequency of the swash, where the frequency can be determined with knowledge of regularly scheduled vessel passages. In addition to erosion from the direct impact of waves, models to predict dune recession by mass failure were developed. The models are based on elementary engineering statics and soil mechanics. The models developed and/or calibrated for this study are believed to be applicable to both vessel- and wind-generated waves.

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