Modeling long-term beach and dune evolution with interacting longshore and cross-shore processes

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In future decades, coasts will be exposed to increasing risks because of climate change and sea level rise, which poses an increased threat of coastal inundation, erosion and ecosystem loss. The natural variability of coasts can make it difficult to identify these impacts. Most beaches worldwide show evidence of recent erosion. Sea level rise is currently not necessarily the primary driver, but will become very relevant in the future. Understanding both the short-term and long-term development of a coast is considered essential for coastal managers, in order to maintain safety in the future. For a generic evaluation of the state of the coastal zone, it is also important that not only the coastline changes are examined, but also the behavior of the dunes. The interactions between transport mechanisms should be investigated in order to make a good prediction of the large-scale and long-term development of the coastal zone. Therefore, the adaptation of the dunes and the coastline to interacting cross-shore and longshore sediment transport processes must be studied. To get a comprehensive view on those interactions, a new coupled surfzone-dune model is developed and validated.
This coupled model consists of a combination of coastline model ''Unibest-CL+'' and coastal profile model ''the CS-model''. The coupled model has the capability to simulate beach and dune evolution at decadal to centennial timescales including the relevant sediment transport processes that impact the beach and dune evolution: dune erosion and overwash, aeolian dune build-up, beach erosion and accretion due to gradients in longshore sediment transport, adaptation to sea level rise and response to nourishments. The model is tested quantitatively based on the sediment balance, in a very simple and a more complicated academic case focused on the individual effects of all transport mechanisms included in the coupled model. All test cases in the simple case -- which consist of the smallest scale situation the coupled model is able to simulate -- demonstrate sediment conservation and therewith confirm that the implementation of the different transport equations and their interaction in the coupled model is correct. In the complicated test cases – covering a varying number of cross-shore profiles along a larger stretch of coastline – the accuracy of the predicted sediment fluxes and the subsequent coastline and dune volume changes have been determined. The accuracy of every individual transport mechanism included in the coupled model is found to be sufficient and the different sediment fluxes within the coupled model are balanced.
After model testing, the model is used to hindcast the 22-year long coastal evolution near IJmuiden. The dune evolution in this highly dynamic case is simulated reasonably. Furthermore, the coastline evolution is predicted in accordance with the data. Overall, the coupled model is found to quantitatively and qualitatively perform satisfactory. The model is capable of quantifying the long-term beach and dune evolution with interacting longshore and cross-shore processes. Though coupling the models and therewith simulating the longshore sediment transport gradients instead of deriving them from data induces model uncertainties, the application of the model on the IJmuiden case shows that it is possible to simulate the longshore sediment transport gradients using a schematized wave climate to predict the dune volume evolution sufficiently. Furthermore, the interaction between the cross-shore and longshore sediment transport processes enables the simulation of the redistribution of nourishments, improving the predictive ability of both model components with regard to the coastline evolution as well as the evolution of the dune volume. For future research it is recommended to use the coupled model to look into nourishment strategies and their effectiveness on the long-term, climate change scenarios and their effect on decadal to centennial scale dune and beach evolution and the relative importance of different transport mechanisms on the long-term beach and dune evolution.