Modelling Coarse-Grained Beach Profile Evolution
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Coarse-grained beaches are particularly prevalent in the UK, composed of accumulations of either gravel, or mixed sand and gravel sediments. The aim of the work presented in this thesis is to improve capabilities for predicting coarse-grained beach 2D profile development. In particular, the effects of infiltration and sediment sorting are considered. In this study, the public domain numerical model, XBeach (v12) is developed further. This model was initially developed for studying sandy environments especially for the case of dune erosion. Here, the model is modified to enhance its capability to predict beach profile change on coarse-grained beaches. Improvements include: use of Lagrangian interpretation of velocity in place of Eulerian for driving sediment movement; introduction of a new morphological module based upon Soulsby’s sediment transport equation for waves and currents; incorporation of Packwood’s infiltration approach in the unsaturated area of the swash region; and implementation of a multiple sediment fraction algorithm for sediment sorting of mixed sediments. These changes are suggested and justified in order to significantly improve the application of this model to gravel and mixed beaches, especially with regard to swash velocity asymmetry which is responsible for development of the steep accretionary phase steep berm above waterline and sediment sorting. A comparison between model simulation and large scale experiments is presented with particular regard to the tendency for onshore transport and profile steepening during calm conditions; offshore transport and profile flattening during storm conditions; and sediment sorting in the swash zone. Data used for this and the model calibration comes from the Large Wave Channel (GWK) of the Coastal Research Centre (FZK) in Hannover, Germany. The results are found to agree well with the measured experimental data on gravel beach profile evolution. This is due to the inclusion of infiltration in the model which weakens the backwash volume and velocity in a more satisfying manner than through the use of asymmetric swash friction and transport coefficient. The model also simulates sediment sorting of a mixed sediment beach. However, the profile comparisons were not satisfactory due to limitations of the numerical model such as the constant permeability rate used throughout the simulation and the non-conservation of the sediment volume in the laboratory data by an order of 50%. From the simulation, it was found that the fine sediment moves offshore and the coarser sediment moves onshore. This is because of infiltration weakens the backwash velocity; the coarser sediment moving onshore barely moves back offshore while the fine sediment remains in motion. This pattern agrees with the pattern obtained from sediment samples analysis in the experiment and provides an explanation for the existence of composite beaches. The model is also shown to be capable of switching from accretionary to erosive conditions as the wave conditions become more storm-like. Again, the model simulations were in a good agreement with the observations from the GWK dataset. Numerical model simulations on the effects of the tidal cycle on coarse-grained beach profile evolution were also carried out. This preliminary investigation showed that the model was able to predict the anticipated profile change associated with a coarse-grained beach under such wave and tidal forcing. Tidally forced accretion and erosion were compared with those predicted under similar beach sediments and wave conditions for constant water level. The main differences are that the affected area is wider and the berm is located on the upper beach during flood for both gravel and mixed beaches. Therefore, the model developed in this study can be seen to be a robust tool with which to investigate cross-shore beach profile change on coarse-grained beaches and sediment sorting on mixed beaches. Further work is also indicated.
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