Residual circulation and freshwater retention within an event-driven system of intertidal basins

This study explores the spatiotemporal variability in the residual circulation and its dependence on external forces in an interconnected system of intertidal basins. We focus on the Dutch Wadden Sea (DWS), where winds play a major role in water movements and storms significantly affect its short-term characteristics. We make use of realistic three-dimensional high-resolution numerical simulations to model the hydrodynamics in the DWS for the years 2005–2015. First, the ‘empirical orthogonal function’ method is applied to analyze systematically the relative contributions of various forcing mechanisms on the residual (i.e., tidally averaged) volume flow through the inlets and the shallow watersheds delimiting the system and each tidal basin. Then, we compute the tidally averaged flushing frequency of fresh water over the entire period of analysis, and we study its variations at short and long time scales. It is found that over 98% of the residual flow's spatiotemporal variations can be explained by the first three EOF modes, which are highly correlated with the cubic power of the vectorial wind components' speed and the amount of fresh water discharged into the back-barrier system. This study reveals that incidental short-duration events (e.g., storms) occurring at time scales of hours to days, albeit episodic and highly variable, strongly influence the residual transport's inter-annual variability and its long-term typical value in multiple-inlet coastal systems.