Tidally- and wind-driven residual circulation at the multiple-inlet system East Frisian Wadden Sea

• Identification of residual circulation cells among communicating sub-basins and the North Sea.
• Interacting physical mechanisms responsible for residual fluxes at tidal inlets and tidal divides are distinguished.
• The importance of net inflows at the lateral margins of the East Frisian Wadden Sea was previously underestimated.
• Tide-driven net inflows across tidal divides are up to 24% of the basin's tidal prism.
• Relevance to suspended sediment budgets and ecosystem studies is discussed.

The residual circulation in lagoon-type multiple-inlet systems is primarily driven by the non-linear interaction of the offshore tides, wind-induced currents and the morphology of communicating sub-basins. Residual discharges at tidal inlets and across tidal divides of a chain of back-barrier basins are calculated with a process-based hydrodynamic model in the East Frisian Wadden Sea, southern North Sea. Non-linear mechanisms of dominant water fluxes between the sub-basins and the coastal sea are deciphered with respect to tidal and meteorological drivers. The orientation of residual pathways of annually-integrated water fluxes is consistent and similar for variable model scenarios, i.e. meteorological forcing, an imposed sea level rise or different bed friction coefficients, which permits the identification of dominant residual circulation cells in the system. It is further shown that the multi-basin system imports a substantial surplus of water from the western and eastern boundaries of the East Frisian Wadden Sea having an impact on the mass budget of the interconnected basins that has been underestimated so far in hydrological, sedimentological and ecosystem studies. The implications for the accumulation of fine-grained sediments, flushing capacities (residence times) and nutrient cycles in the Wadden Sea area are discussed.