Shallow tidal system morphodynamics: a synthesis of forms and behaviours in the Wadden Sea for long-term management with understanding

Tidal basins such as the Wadden Sea exhibit perpetual sediment dynamics and morphodynamics at scales ranging between that of bedforms and creeks to channels and tidal flats, to that of the entire basin and its transitions to neighbouring basins and the embankment on land. The Wadden Sea is the largest tidal wetland on the planet with globally important ecosystems. Tidal flats and salt marshes increase coastal flood safety by storm wave damping. The combination of accelerating sea-level rise, historic land loss and reclamation with ongoing economic activities, including mining, dredging and other disturbances, puts future ecosystem integrity and coastal flood defence at risk. The ability to adjust management in order to adapt to changes depends on scientific and societal understanding the dynamics of sediment (sand and mud) on a timescale of years to centuries. As such, a qualitative, comprehensive description is urgently needed of sediment dynamics and morphodynamics, around which all the needs and issues revolve and that experts/scientists in governmental institutions and consulting can use to inform policymakers and area managers.Here we synthesize the available knowledge of patterns, dynamics and interactions between various forms on the basis of bathymetric data, aerial photography, background data and literature. This holistic systems synthesis is a co-creation with societal partners in the Netherlands, who also co-designed the project (https://wadsed.nl/) by specifying their knowledge questions, perspectives on long-term development and on governance of this system. As such, their intimate knowledge of the Dutch Wadden Sea is incorporated and seeming conflicts of perceived trends (drowning vs. infilling) were reframed as research questions by the academic scientists. We will present our new insights in sediment dynamics and morphodynamics, specifically focussing on sediment dynamics during storms, channel-bar interactions and tidal ‘divides’ which are conceptually bounding the individual tidal basins but turn out to be quite open for water and mud exchange. This culminates into a description of tidal basins as multi-scale complex open systems diagrams, with explicit recognition of what processes and boundary conditions are affected, and potentially manageable, by human interference.