The Sand Motor is an artificial sandy peninsula extruding from the Dutch coast about 1 kilometer into the North Sea (Stive et al., 2013). It is virtually permanently exposed 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. About 70% of the Sand Motor area is located above 2m+MSL and is therefore uniquely shaped by subaerial processes. These dry areas are hardly eroded due to the presence of a coarse sand armor layer that was naturally established over time. However, significant aeolian transport is observed originating from the intertidal beaches surrounding the Sand Motor. Due to periodic flooding no armor layer can be established in the intertidal zone. Consequently, subtidal processes significantly influence the subaerial morphology. An international collaboration 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 (XBeach; Roelvink, 2006) and subaerial morphodynamics (Coastal Dune Model; Duran and Moore, 2013) and multi-fraction aeolian sediment supply (AeoLiS; based on work by de Vries, 2015). Preliminary model results from a one-year one-dimensional simulation show a concentration of aeolian sediment supply from the intertidal beach area during calm conditions and an elevated aeolian activity shortly after a strong wind or surge event. The period of elevated transport may last for over a week resulting in the immediate initiation of recovery after a surge. Here we will present an application of the Windsurf modeling framework on the Sand Motor and a detailed description on how the interaction between subtidal and subaerial processes explain its complex morphological development.

The interface between the land and sea is complex. During high energy conditions large waves and elevated water levels can cause severe beach and dune erosion, whereas recovery takes place during extended periods of calm. Recent advances in numerical modeling have improved our ability to accurately model both storm-induced coastal hazards (e.g., XBeach; Roelvink et al., 2009), aeolian sediment transport in supply limited conditions (de Vries et. al. 2014), and dune eco-morphodynamics (e.g., Coastal Dune Model; Duran and Moore, 2013). However, process based numerical models incorporating the co-evolution of the coastal zone due to both subaqueous and subaerial processes to our knowledge, do not exist; hindering accurate forecasts of seasonal- to decadal-scale coastal evolution. Addressing this community need, a recent international collaboration has initiated the development of the open-source coupled numerical model Windsurf. Under the Windsurf framework, the coupled system resolves the most important subtidal and supratidal physical and ecological processes during both calm accretive conditions and high energy erosive periods. Here we present the coupled model’s ability to simulate short-term beach and dune building processes on daily to seasonal time scales. The welding of intertidal sandbars to the shoreline has long been recognized as an important mechanism for beach and dune building (e.g., Houser, 2009) as beaches are often supply limited. Field experiments on the Oregon coast indicate that discrete sandbar welding events can deliver as much as 20 m3/m of sand from the nearshore to the backshore via the intertidal, resulting in shoreline progradation, backshore aggradation, and dune growth. Here we compare these field observations to model simulations demonstrating Windsurf’s ability to simulate beach-dune exchanges in supply limited scenarios.