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UBC Theses and Dissertations

Onshore/offshore transport mechanisms Walsh, Bruce William


The onshore/offshore transport mechanisms are investigated. Careful and detailed observations of the surf zone are made to find any differences or similarities between the resulting flows caused by different breaking types. Emphasis is placed on the spilling and plunging breakers. Even though the physical breaking properties are significantly different, the time-averaged properties of the surf zone for each type of breaking wave are similar. Using this as a basis, a model of the beach is developed using a control volume that is bounded by the beach face, the time-averaged water level, and a vertical plane at the breaking point. The momentum acting on the control volume in the onshore and offshore horizontal directions is balanced. The model shows that the onshore/offshore sediment transport is primarily dependent upon the magnitude of the wave setup shoreward of the breaking point, and the permeability of the beach. Increasing the permeability causes a reduction in the offshore net shear stress acting along the beach face which results in an increasing slope. Using this simple model, the difference between a gravel and sand beach can be explained, the gravel beach being steeper. The model is used to calculated the offshore net shear stress for a plane impermeable beach in the laboratory. The calculation gives the right order of magnitude (<10 N/m2), but proves to be sensitive to small inaccuracies in the measurement of the setup.

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