Computational modelling of morphodynamic response of a macro-tidal beach to future climate variabilities

Global climate variabilities have the potential to impact many coastlines around the world, and can have detrimental effects on the stability of coastlines and their function as natural coastal defenses. This paper investigates the impacts of future extreme storms and sea level rise on morphodynamics of the macro-tidal Sefton Coast, UK, taking a process-based model as a tool to simulate a snap-shot of future beach change during storms. Sefton Coast represents one of the largest dune systems in the UK, where frontal dunes function as a natural barrier against extreme conditions. Future storm conditions were determined from global predictions of future waves at the end of the twenty first century. Future sea levels were determined based on climate change allowances set by the UK Environment Agency (EA), while future surge conditions were determined based on the guidelines also provided by EA. A nested numerical modelling approach, that combined a regional scale model with a local scale model with high resolution, is used. The modelling suite was first validated against measured waves, water levels and beach change, before using it to simulate morphodynamic change from numerous statistically significant 'future' storm conditions. Future storm-induced morphodynamic change of the Sefton coast was then compared with that from the present storms. The results reveal that this beach will experience significant climate change driven impacts where dune retreat during storms will be considerably higher in future. The results also reveal that due to the shape of the beach and its orientation to predominant wave approach direction, there will be a strong longshore variability of morphodynamic response to storms. It was also found that the water level in front of the dune (and hence surge and future sea level rise) is the most critical factor that determines beach erosion during a storm.