Hans Burchard 1,2 , Joanna Staneva 3 , Götz Flöser 4 , Rolf Riethmüller 4 ,

1. Impact of density gradients on net sediment transport into the Wadden Sea Hans Burchard1,2, Joanna Staneva3, Götz Flöser4, Rolf Riethmüller4, Thomas Badewien5, and Richard Hofmeister1 1. Baltic Sea Research Institute Warnemünde, 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 hans.burchard@io-warnemuende.de

2. 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.

3. 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.

4. Salinity difference HW-LW

5. Temperature difference HW-LW

6. Density difference HW-LW

7. 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

8. 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.

9. Hypothesis: This must have a dynamic impact on tidal flow and SPM transport, see the theory of Jay and Musiak (1994) below.

10. Flat bottom Elbe estuary simulation: Flood and ebb profiles of current velocity, salinity, eddy diffusivity and SPM concentration at 3 psu vertical mean Salinity. Burchard & Baumert 1998

11. Flat bottom Elbe estuary simulation: SPM concentration and salinity contours (2,4,6, … 30 psu) for an idealised Elbe simulation. Burchard & Baumert 1998

12. GOTM is a water column model with modules for state-of-the-art turbulence closure models biogeochemical models of various complexities

13. Testing with GOTM supports hypothesis: Bottom-surface salinity Along-tide salinity gradient prescribed Residual onshore near-bed current

14. Quantification of water column stability Tidal straining Vertical mixing

15. Quantification of water column stability Potential density anomaly flood ebb Balance of potential density anomaly

16. 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 • …

17. 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.

18. 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).

19. 3D simulations with GETM for the Sylt-Rømø bight Approach: Simulating a closed Wadden Sea basin (Sylt-Rømø 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.

20. The Sylt-Rømø bight

21. Bottom salinity at high and low water during periodically steady state.

22. Vertically averaged current velocity during full flood and full ebb.

23. Cross-sectional dynamics

24. Tidal periods # 46-55

25. Total water volume and SPM unit mass in budget boxes Case with density differences, tidal periods # 46-55

26. Total excess SPM mass in budget boxes Case with density differences, tidal periods # 46-55

27. Total water volume and SPM unit mass in budget boxes Case with no density differences, tidal periods # 46-55

28. Total excess SPM mass in budget boxes Case with no density differences, tidal periods # 46-55

29. Total excess SPM mass in budget boxes, slow settling Case with density differences, tidal periods # 46-55

30. Total excess SPM mass in budget boxes, slow settling Case with no density differences, tidal periods # 46-55

31. 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.

32. River Warnow mouth in Warnemünde