Predicting extreme wave run-up on beaches in complex coastal environments [A validation of the XBeach Numerical Model]

Low-lying coastal communities, worldwide, are constantly exposed to coastal hazards which threaten to curtail their development and overall ability to advance their economies. This is especially the case for small islands and developing states (SIDS) who can ill afford the consequences of these extreme events. Specifically, these threats include: beach and dune erosion or overwash; damage to built capital; flooding of the coastal area; and at its worst, the loss of human life. The extent of the impact may be correlated to the portion of wave energy that runs up the foreshore of the beach during these severe to extreme events. However, predicting this extreme wave run-up becomes increasingly more difficult with increased coastline complexity; that is to say, on reef-lined coasts or those characterised by a highly variable bathymetry. This research aimed to identify a suitable tool that may be adopted by local planners by which this parameter may be accurately predicted, towards reducing disaster risk and facilitating climate change adaptation. The XBeach numerical model was proposed and a thorough model validation was carried out. This validation exercise included both laboratory and field cases to provide a comprehensive assessment of the model's capability. The results confirm the ability of XBeach to predict both maximum and 2-percent exceedance runup with overall error and bias within 20%. Moreover, the model proved to accurately simulate not only the hydrodynamics associated with extreme wave run-up but also the morphological impact associated with the overwash storm regime. The findings presented here have wide-scale practical applicability: from the incorporation of run-up estimates into inundation hazards maps leading to more accurate vulnerability assessments; to the evaluation of the performance of numerical models in complex coastal environments. Additionally, the research provided insight into the hydrodynamics associated with fringing-reef environments; a topic that is currently receiving close attention from the academic community. Lastly, using a somewhat unconventional approach, the research demonstrated the ability of XBeach to serve as a large domain model to generate boundary conditions for a higher resolution nested model. This nesting procedure has not been widely applied before but may prove valuable as the standard wave and surge prediction models exclude the often dominant contribution of infragravity waves