Impact of alongflow density gradients on sediment transport

Impact of along-flow density gradients on sediment transport in estuaries (!), the Wadden Sea (!? ) and in lakes (? ) Hans Burchard Baltic Sea Research Institute Warnemünde, Germany hans. burchard@io-warnemuende. de Collaboration: Thomas Badewien, Karsten Bolding, Götz Flöser, Rolf Riethmüller, Joanna Staneva, Lars Umlauf

The meso-tidal Elbe

Estuarine Turbidity Maximum (ETM) in tidal Elbe: SPM observations The meso-tidal Elbe Observations by Jens Kappenberg (GKSS)

ETM theory by Jay and Musiak (1994) Other ETM mechanism: gravitational circulation

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

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

Elbe simulation with GETM Low Flood High Ebb Burchard et al. (2004)

Wadden Sea study (Burchard et al. , JPO, in print) 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 the 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

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

3 D 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.

The Sylt-Rømø 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

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

… and in lakes ? 2 D lake bathymetry initial temperature profile 15 deg C isotherm Experiment: 3 hours of constant wind forcing normal to the lake, and then 45 hours no wind. Internal seiche develops.

Time series 1 m above ground at x = 3. 05 km stably stratified unstably stratified

velocity temperature Vertical profiles at x=3. 05 km dffusivity Turbulent kinetic energy Does this lead to eddy near-bottom up-slope net transport ? stably stratified unstably stratified

Conclusions Density gradients along oscillating flows may generate strong flow asymmetries leading to near-bed residual transports. For tidal estuaries, this is proven to lead to ETMs. For the Wadden Sea there are hints that this leads to net sediment accumulation. Need for field studies ! For lakes, we hypothesise that in certain cases, this should also lead to up-slope residual transports of solutes or suspended matter. Need for field studies !