John Lawrence


Coastal defence systems are implemented in many countries for the stability of coastlines and prevention of erosion and flooding. The maintenance of such schemes includes the use of 'soft' engineering techniques, which require accurate predictions of sediment transport and profile change. This thesis describes the development of a numerical model for coarse-grained crossshore transport for use in such schemes. The model combines a hydrodynamic model based on weakly non-linear Boussinesq equations, coupled to a sediment transport module and a morphology change module. Studies have been performed on the hydrodynamic and sediment models to assess the performance of the components for this purpose. The 1-D Boussinesq model has been validated with physical wave flume data. The model is shown to provide good predictions for shoaling and breaking waves near the coastline, and is also shown to provide good predictions for the properties of a reflected wave field. The model is then used to perfonn a study on the nature of wave shoaling and reflection with regard to the velocity field, and the development of the velocity skewness pattern is discussed. Recent sediment transport formulae have been reviewed, and a bed-load sediment transport model has been developed. A model for differential transport of different grain sized niaterial has also been introduced. Developed from a river sediment model, this is able to predict sorting of grain sizes over the cross-shore profile. Results of the combined model are shown for natural plane beaches, and for beaches coupled with sea walls. The model predicts reduced erosion patterns for irregular wave fields compared to regular waves, and for mixed sediment composition sea beds compared to homogeneous sea beds. These findings show agreement with features found in previous physical studies. A series of sensitivity studjes has also been performed with respect to hydrodynamic and sediment properties. The model shows a high degree of sensitivity for the profile changes to these parameters. The ability of the model to show predictions for an evolving beach profile subject to tidal water depth variation is also introduced.

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