Natural headland sand bypassing; towards identifying and modelling the mechanisms and processes

Natural headland sand bypassing: Towards identifying and modelling the mechanisms and processes contributes to the understanding of the mechanisms and processes of sand bypassing in artificial and non-artificial coastal environments through a numerical modelling study. Sand bypassing processes in general are a relevant but poorly understood topic. This study attempts to link the theory and physics of sand bypassing processes which is significantly important in definition of coastal sedimentary budget and consequently in coastal management. The main questions are how can we model sand bypassing processes and if the modelled sand bypassing processes represent the actual sand bypassing processes. In this study, it is shown that a process-based numerical model can be used to simulate the processes of sand bypassing around groyne and headland structures for both short (several days) and long term (a year) simulations. Result comparisons were made among analytical models, empirical models and field data. In general, the process-based model can produce reasonable results. The results of a hypothetical process-based model for a single groyne case indicated that the shoreline evolution patterns and sand bypassing rates are in agreement with the results of analytical models. The shoreline patterns behind the groyne structure are well predicted by the process-based model and represented the reality. Results of the shoreline pattern behind the groyne obtained from the model with the inclusion of wave-groups is better than the results of the model without the wave-groups. The main components that influence the longshore sand transport, i.e wave heights, wave angles, and sediment grain sizes, contribute to the sand bypassing processes and different sand transport rates at a groyne structure. The results of a hypothetical model for a case of two separated groynes showed that the morphodynamic characteristics of embayed beaches can be predicted by the process-based model. Results obtained from an empirical model are consistent with the results of the process based numerical model. Surf zone rip currents, particularly headland rips, are responsible for the sand transporting mechanism outside the surf zone area. Findings from the field survey analyses at a small nourished embayed beach which is located on the east coast of Malaysia revealed that wave climate seasonality leads to alternating behaviour patterns of the searshore beach profiles. The results of the cross-shore profile variations along the beach showed the classical onshore-offshore movement patterns of sand transport. Beach rotation, as a result of seasonal wave directional changes promoted sand leakage at one end of the embayment. Additionally, results from the sand volume analyses at the southern area outside the nourishment zone positively showed the contribution of sand into this region and thus verified the potential growth of sand southwards around the south headland. For the southern Gold Coast case study, a process-based model of a permanent sand bypassing system was introduced by utilising the concept of a sand bank discharge operation. Prior to morphological model investigation, models were successfully calibrated and validated and model results were compared against the available field data measurements. The results obtained from the morphological models revealed that additional supply of sand from the sand discharge operation contributed to the development of a sandspit. The sand bypassing process began when the sand started to move around a headland as sandwaves. The sandwaves manifested themselves, moving slowly in front of the beach, and creating an elongated sandspit. The sandspit grew bigger and bypassed another headland before finally attaching itself to the neighbouring beach. Results from the qualitative assessment showed that the morphological beach behaviour captured by the process based model represented the actual morphological beach behaviour as in reality. The modelled sand bypassing process in this study is identical to the conceptual model of headland sand bypassing. A final conclusion is that the combination of seasonal wave climates, in particular wave directions, and the sand bank discharge operations determined the succession of the permanent sand bypassing system. In summary, through numerical modelling this study has added to the understanding of coastal processes and the role of geological controls in governing sand bypassing processes and embayed beach morphodynamics. The morphological model developed in this study is useful to increase understanding of the natural sand distribution patterns due to combination of engineering efforts and natural coastal processes.