Expanding knowledge concerning the close entanglement between subtidal and subaerial processes in coastal environments initiated the development of the open-source Windsurf modeling framework that enables us to simulate
multi-fraction sediment transport due to subtidal and subaerial processes simultaneously. The Windsurf framework couples separate model cores for subtidal morphodynamics related to waves and currents and storms and aeolian
sediment transport. The Windsurf framework bridges three gaps in our ability to model long-term coastal morphodynamics: differences in time scales, land/water boundary and differences in meshes.
The Windsurf framework is applied to the Sand Motor mega-nourishment. The Sand Motor is virtually permanentlyexposed to tides, waves and wind and is consequently highly dynamic. In order to understand the complex
morphological behavior of the Sand Motor, it is vital to take both subtidal and subaerial processes into account. The ultimate aim of this study is to identify governing processes in aeolian sediment transport estimates in coastal environments and improve the accuracy of long-term coastal morphodynamic modeling.
At the Sand Motor beach armoring occurs on the dry beach. In contrast to the dry beach, no armor layer can be established in the intertidal zone due to periodic flooding. Consequently, during low tide non-armored intertidal beaches are susceptible for wind erosion and, although moist, may provide a larger aeolian sediment supply than the vast dry beach areas. Hence, subtidal processes significantly influence the subaerial morphology and both need to be accounted for to understand the long-term aeolian morphodynamic behavior of the Sand Motor.

This study presents some recent developments in coastal morphological modeling focusing on flexible meshes, flexible coupling between models operating at different time scales, and a recently developed morphodynamic model for the intertidal and dry beach. This integrated modeling approach is applied to the Sand Engine mega nourishment in The Netherlands to illustrate the added-values of this integrated approach. A seamlessly coupled modeling system for Delft3D and AeoLiS has been developed and applied to compute the first years of evolution of the Sand Engine, both for the subaqueous and subaerial areas. The subaqueous bed level changes have been computed with the new Flexible Mesh version of Delft3D, resulting in comparable accuracy levels as to the standard Delft3D version. The integrated morphodynamic prediction of both subaqueous and subaerial reveals a qualitative behavior which is very similar to observations. Model results confirm that after the first year after construction the sand supply for aeolian transports is predominantly from the intertidal area. The AeoLiS model results indicate a significant intertidal erosion volume of about 230,000 m3 over the five year period, which is a not to be neglected volume, especially in multiyear or decadal predictions. Interestingly, the model results show that the spit, developed by the wave-related processes, is also subject to aeolian transports acting on the emerged spit during lower tides. The seamlessly coupled models are now able to combine the dry beach behaviour with subaqueous morphodynamic evolution, which is important in medium-term to decadal morphodynamic predictions but also relevant for designing such sandy solutions incorporating lakes, lagoons, and relief.