The proposed project responds to the Georisks part of the 1^st transnational call for proposals on Bilateral Wadden Sea Research by aiming at assessing the present state and sensitivity to future trends in net sediment fluxes in the entire Wadden Sea area. In the light of regional relative sea level rise due to global sea level rise, postglacial rebounce, polder drainage and gas exploitation in the Wadden Sea, a mean rise of the Wadden Sea bottom due to net sediment flux into the Wadden Sea is believed to be the only process counteracting future drowning of the intertidal areas in the Wadden Sea. This process has been largely balancing the sea level rise in recent centuries, and it is a major societal concern, if its efficiency will remain strong enough under possibly accelerated climate change. 

  The processes driving sediment accumulation in the Wadden Sea are highly complex. The susceptibility of Wadden Sea sediment fluxes to climate change depends on a variety of factors. The major physical factors are changed net precipitation patterns (with impacts on estuarine circulation) and changed wind patterns (with impacts on wave stirring and sediment transport). Other important mechanisms are changed functions and distributions of benthic filter feeders, also due to invasions of new species (this will be discussed with potential partner projects within the Biorisks call). Biogeochemical processes further modulate fluxes of cohesive sediments (aggregation and bio-stabilisation). Accompanying shifts in water turbidity will have ecosystem wide effects. We here propose to systematically investigate the susceptibility of sediment accumulation in the Wadden Sea to changed external forcing by merging available data, and knowledge (guiding hypotheses) and by developing a comprehensive physical, geological, ecological and biogeochemical model for the entire Wadden Sea area. The integrated model system will quantitatively reproduce the major processes driving sediment accumulation in the Wadden Sea. 

  Therefore, a consortium of seven institutes from The Netherlands, Germany and Denmark has been established. They will set up a Wadden Sea numerical model system carefully calibrated to observational data sets of sediment concentrations and other key parameters. The system consists of various models (e.g. GETM, Delft3D) which interact which each other on a modular basis, coupling abiotic processes to biotic processes. The consortium will bench-mark the models. The rise or decline of the Wadden Sea floor will be inferred from the sedimentation and erosion fluxes computed by the sediment model and will be compared to the existing direct observations of bed-level change. This system will be applied in high spatial resolution to at least one focal year from which the present-day accumulation of sediment fluxes will be estimated in spatial and event-based (storms/calm periods, net precipitation/evaporation) resolution. A canonical hierarchy of sediment transport modules will be compiled. The overall model system will be analysed for sensitivity on climate parameters such as net precipitation, storm patterns and temperature rise to project the present-day situation into the next century. Maps of net erosion and net accumulation will be drawn for the various different present and future situations, to show the intertidal areas with increased risk for drowning. 

  Four well-monitored focal Wadden Sea subregions which differ in their physical and biogeochemical characteristics have been defined to gain deeper insights into local dynamics and trends in sediment transport by the set-up of local models with higher resolution. These areas are the Marsdiep-Vlie-Terschelling system, the Jade Bay, the waters around the German island of Sylt and the Danish Wadden Sea. The strategic goals of the proposed research are to create (i) a bilateral frame that collects techniques for and results of longterm monitoring, and (ii) a common set-up of a modular Wadden Sea model. The model system should allow a deeper understanding of critical sedimentary processes at the system scale. It will also provide a common tool for interdisciplinary research addressing the Wadden Sea as a unique entity.