Long-term beach and dune evolution : Development and application of the CS-model

Abstract: Coastal flooding and erosion are worldwide problems that are further aggravated by rising sea levels and increasing population densities in coastal areas. In many of those areas, sandy beaches and dunes protect thehinterland from waves and extreme water levels during storms, while providing natural and recreational values. Sandy coasts are dynamic systems. Thus, risk assessments and coastal management strategies require knowledge about their long-term evolution. From coastal management and spatial planning perspectives, typically, the coastal evolution on timescales from decades up to centuries, and spatial scales in the order of kilometres are of interest. However, there is a lack of model tools integrating all the relevant processes forsimulation of beach and dune evolution at these spatial and temporal scales.In this thesis, the CS-model – a semi-empirical cross-shore sediment transport and beach and dune evolution model - is developed and tested. Included processes are dune erosion and overwash, beach-bar exchange, dune build-up through aeolian transport, and beach erosion and accretion due to gradients in longshore transport. Physics-based equations for dune erosion and overwash, beach-bar exchange, and sea level rise are have been developed and validated in previous studies. Methods to simulate the aeolian transport and morphological dune evolution are developed from established geomorphological concepts, which are translated into numerical formulations. Robust decision support should be based on probabilistic simulations for a range of future scenarios. Therefore, the CS-model is designed to be computationally efficient, through reduced complexity transport equations and a simplified schematization of the beach profile.The model is applied to two study sites, Ängelholm Beach in Sweden and the Kennemer Dunes in the Netherlands. In Ängelholm, the geomorphological concepts are tested against grain size and topographic data. Then, the model is calibrated and validated against a seven-year data set and the long-term beach and dune evolution from 2017–2100 is simulated for a range of sea level rise scenarios. At Kennemer Dunes, the model is applied to a 22-year data set, to study the relative importance of different transport processes for the long-termdune evolution. At Ängelholm Beach, dune erosion was found to be a dominant process for the long-term dune evolution, because the aeolian transport capacity was limited. At Kennemer Dunes, the gradients in longshore sediment transport were governing the dune volume evolution. The simulations also showed that the beach and shoreface nourishments were the most critical factors for dune growth along the long-term eroding stretches of the beach. Overall, the results are promising and suggest that the model could be a useful tool for long-term coastal risk assessment and evaluation of coastal management strategies.