Hans Burchard 1,2, Joanna Staneva 3, Gtz Flser 4, Rolf Riethmller 4, Thomas Badewien 5, and Richard Hofmeister 1 1. Baltic Sea Research Institute Warnemnde, Germany 2. Bolding & Burchard Hyrodynamics, Rostock, Germany 3. ICBM, University of Oldenburg, Germany 4. GKSS Research Centre, Geesthacht, Germany 5. Institute of Physics, University of Oldenburg, Germany Impact of density gradients on net sediment transport into the Wadden Sea Observation 1: Suspended matter concentrations are substantially increased in the Wadden Sea of the German Bight. Total suspended matter from MERIS/ENVISAT on August, 12, 2003. The areal view shows locations of five automatic monitoring poles in the Wadden Sea of the German Bight, operated by GKSS and University of Oldenburg. They record several parameters in the water column, such as temperature and salinity. Salinity difference HW-LW Temperature difference HW-LW Density difference HW-LW Observation 2: In winter, salinity is significantly (1-2 psu) higher during high water than during low water. In summer, temperature is significantly (1-2 deg) lower during high water than during low water. Spiekeroog data Conclusion 1: The Wadden Sea water is generally less dense than the open sea water. Thus, the presence of a horizontal density gradient has to be assumed for most of the time. Hypothesis: This must have a dynamic impact on tidal flow and SPM transport, see the theory of Jay and Musiak (1994) below. Flood and ebb profiles of current velocity, salinity, eddy diffusivity and SPM concentration at 3 psu vertical mean Salinity. Flat bottom Elbe estuary simulation: Burchard & Baumert 1998 SPM concentration and salinity contours (2,4,6, 30 psu) for an idealised Elbe simulation. Burchard & Baumert 1998 Flat bottom Elbe estuary simulation: GOTM is a water column model with modules for state-of-the-art turbulence closure models biogeochemical models of various complexities Testing with GOTM supports hypothesis: Residual onshore near-bed current Along-tide salinity gradient prescribed Bottom-surface salinity Quantification of water column stability Tidal strainingVertical mixing floodebb Potential density anomaly Balance of potential density anomaly Quantification of water column stability GETM is a 3D numerical model for estuarine, coastal and shelf sea hydrodynamics with applications to the Tidal Elbe Wadden Sea Limfjord Lake of Geneva, Western Baltic Sea, North Sea Baltic Sea system Present GETM characteristics... physics... Solves three-dimensional primitive equations with hydrostatic and Boussinesq approximations. Based on general vertical coordinates. Options for Cartesian, spherical and curvilinear coordinates. Fully baroclinic with tracer equations for salinity, temperature, suspended matter and ecosystem (from GOTM bio module). Two-equation turbulence closure models with algebraic second-moment closures (from GOTM turbulence module). Wetting and drying of intertidal flats is supported also in baroclinic mode. Present GETM characteristics... numerics... Consistent explicit mode splitting into barotropic and baroclinic mode. High-order positive-definite TVD advection schemes with directional split. Choice of different schemes for internal pressure gradient calculation. Consistent treatment of zero-velocity bottom boundary condition for momentum. Positive-definite conservative schemes for ecosystem processes (in GOTM bio module). 3D simulations with GETM for the Sylt-Rm bight Approach: Simulating a closed Wadden Sea basin (Sylt-Rm bight) with small freshwater-runoff and net precipitation. Spin up model with variable and with constant density until periodic steady state. Then initialise both scenarios with const. SPM concentration. Quantify SPM content of fixed budget boxes. The Sylt-Rm bight Bottom salinity at high and low water during periodically steady state. Vertically averaged current velocity during full flood and full ebb. Cross-sectional dynamics Tidal periods # 46-55 Total water volume and SPM unit mass in budget boxes Case with density differences, tidal periods # 46-55 Total excess SPM mass in budget boxes Case with density differences, tidal periods # 46-55 Total water volume and SPM unit mass in budget boxes Case with no density differences, tidal periods # 46-55 Total excess SPM mass in budget boxes Case with no density differences, tidal periods # 46-55 Total excess SPM mass in budget boxes, slow settling Case with density differences, tidal periods # 46-55 Total excess SPM mass in budget boxes, slow settling Case with no density differences, tidal periods # 46-55 Conclusions: The hypothesis is strongly supported. Other mechanisms than density differences which are also reproduced by the model system (such as settling lag and barotropic tidal asymmetries) do not play a major role in this scenario. Now, targeted field studied are needed for further confirming the hypothesis. River Warnow mouth in Warnemnde