Dynamics of Wave Breaking at a Coastal Sea Wall
Antoine, Arthur L.
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Structural designs barely consider the dynamic scenario of a well-developed impinging wave hitting the structure. The usual area of focus is on static and stability factors (e.g. drag, inertia, resistive forces related to weight, buoyancy, sliding etc). Even the "Factor of Safety" which is regularly used in designs to account for unknown and/or unforeseen situations which might occur implies a degree of uncertainty about the dynamic scenario of breaking waves in the coastal environment. In the present study the hydrodynamics of a coastal structure-turbulent bore interaction was studied by examination (two-dimensional) of the singular case of a plunging breaking wave forming a well developed turbulent bore which impacted on a model sea wall structure. The turbulent bore impact event was found to display similar characteristics to the impact event of other wave shapes, in particular that of a plunging breaker. Examination of the impact event confirmed the conversion of nearly all horizontal velocity to vertical velocity during the "flip through" event. In accordance with theoretical expectations the location of maximum pressure was found to occur just below the still water level (SWL). Resulting pressure data in the present study consisted of two blunt spikes as opposed to the "church-roof" (high spike) shape seen in other results. The shape of the pressure data was attributed to the following: firstly, to the initial impact of the protruding jet of the breaking wave which causes the first maxima, secondly, to the sensor encountering the bulk of the entrapped air hence causing the drop in pressure between the blunt spikes and lastly, to the inherent hydrostatic pressure combined with the compression of the entrapped air bubbles, by the subsequent forward motion of the water within the wave, which causes the second maxima. The point of maximum pressure was found to always be within the second maxima. Observation of the turbulent bore-structure interaction showed that the consequential maximum pressure was a direct result of the compression of entrapped air by the weight of the water in the wave as it continued forward onto the structure combined with the inherent hydrostatic pressure of the wave. The project was conducted in an attempt to contribute to the vast knowledge of coastal structure-wave interactions and to add to the understanding of the physics and characteristics of breaking waves. Whilst numerous studies and experiments have been carried out on the phenomenon of breaking waves by previous researchers the current project highlights the advent of new equipment and technological advances in existing methods.