In recent years, dune-in-front-of-dike projects have been carried out at several locations, e.g. at Raversijde and Oosteroever in Belgium and the Hondsbossche dunes in the Netherlands. In the near future, many coastal defence systems require reinforcement to adapt to rising sea levels, and often, natural values along the coasts may also be enhanced. Therefore, it is anticipated that this type of hybrid coastal protection – a mix of grey and green solutions – will become more common in the future. Contrary to grey defence structures, such as earth dikes and rock or concrete structures, dunes are dynamic features. Their level of flood protection depends on their morphological evolution due to aeolian and marine transport processes, vegetation dynamics, and anthropogenic impact. Numerical models are commonly used tools to assess the safety level of dunes and predict their future evolution. In addition to event timescales (storms), the decadal timescale is typically of interest from a coastal management perspective, especially when considering sea level rise. On this timescale, dune build-up through aeolian transport depends on the wind's transport capacity, and the availability of sediment of the appropriate size exposed to the wind is an important process. Sediment availability for aeolian transport is controlled by other sediment transport processes, such as dune erosion and longshore sediment transport, nourishments, and limiting factors, such as surface moisture and armour layers. Simulation of dune evolution at the decadal timescale requires an integrated model approach that accounts for the non-linear interactions between marine and aeolian transport processes in the longshore and cross-shore direction. Reduced complexity approaches are required when these models are applied to large temporal (decades) and spatial scales (kilometres). This study aims to predict medium to long-term dune evolution by developing a new coupled long- term beach and dune evolution model, coDaC (coupled Dunes and Coasts ). The new model combines a semi-empirical cross-shore transport model, the CS-model (Hallin et al. 2019a), with a longshore transport and coastline evolution model, Unibest CL+ (Figure 1). The coupled model is applied to simulate 22 years of morphological dune evolution along an 8 km-long coastal stretch at the Kennemer Dunes in the Netherlands. @en

In recent years, dune-in-front-of-dike projects have been carried out at several locations, e.g. at Raversijde and Oosteroever in Belgium and the Hondsbossche dunes in the Netherlands. In the near future, many coastal defence systems require reinforcement to adapt to rising sea levels, and often, natural values along the coasts may also be enhanced. Therefore, it is anticipated that this type of hybrid coastal protection – a mix of grey and green solutions – will become more common in the future. Contrary to grey defence structures, such as earth dikes and rock or concrete structures, dunes are dynamic features. Their level of flood protection depends on their morphological evolution due to aeolian and marine transport processes, vegetation dynamics, and anthropogenic impact. Numerical models are commonly used tools to assess the safety level of dunes and predict their future evolution. In addition to event timescales (storms), the decadal timescale is typically of interest from a coastal management perspective, especially when considering sea level rise. On this timescale, dune build-up through aeolian transport depends on the wind's transport capacity, and the availability of sediment of the appropriate size exposed to the wind is an important process. Sediment availability for aeolian transport is controlled by other sediment transport processes, such as dune erosion and longshore sediment transport, nourishments, and limiting factors, such as surface moisture and armour layers. Simulation of dune evolution at the decadal timescale requires an integrated model approach that accounts for the non-linear interactions between marine and aeolian transport processes in the longshore and cross-shore direction. Reduced complexity approaches are required when these models are applied to large temporal (decades) and spatial scales (kilometres). This study aims to predict medium to long-term dune evolution by developing a new coupled long- term beach and dune evolution model, coDaC (coupled Dunes and Coasts ). The new model combines a semi-empirical cross-shore transport model, the CS-model (Hallin et al. 2019a), with a longshore transport and coastline evolution model, Unibest CL+ (Figure 1). The coupled model is applied to simulate 22 years of morphological dune evolution along an 8 km-long coastal stretch at the Kennemer Dunes in the Netherlands. @en