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

Impact of along-flow density gradientson sediment transport in estuaries (!),

the Wadden Sea (!?) and in lakes (?)

The meso-tidal Elbe

The meso-tidal ElbeEstuarine Turbidity Maximum (ETM) in tidal Elbe: SPM observations

Observations by Jens Kappenberg (GKSS)

ETM theory by Jay and Musiak (1994)

Other ETM mechanism: gravitational circulation

Flood and ebb profilesof current velocity,salinity, eddy diffusivity and SPM concentrationat 3 psu vertical meanSalinity.

Flat bottom Elbeestuary simulation:

Burchard & Baumert 1998

SPM concentration and

salinity contours

(2,4,6, … 30 psu) for an

idealised Elbe simulation.

Burchard & Baumert 1998

Flat bottom Elbeestuary simulation:

High

Low Flood

Ebb

Burchard et al. (2004)

Elbe simulation with GETM

Suspended matter

concentrations

are substantially

increased in the

Wadden Sea of the

German Bight.

Total suspended matter from MERIS/ENVISAT on August, 12, 2003.

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

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:

Residualonshorenear-bedcurrent

Along-tidesalinity gradientprescribed

Bottom-surface salinity

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.

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-sectionaldynamics

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 ?

2D lake bathymetry initial temperature profile

Experiment: 3 hours of constant wind forcing normal to the lake, and then 45 hours no wind. Internal seiche develops.

15 deg C isotherm

Time series 1 m above groundat x = 3.05 km

stably stratified

unstably stratified

stably stratified

unstably stratified

Vertical profiles at x=3.05 km

velocity temperature

eddy dffusivity Turbulent kinetic energyDoes this lead to near-bottom up-slope net transport ?

Conclusions

Density gradients along oscillating flows may generate strong flow asymmetries leading to near-bedresidual transports.

For tidal estuaries, this is proven to lead to ETMs.

For the Wadden Sea there are hints that thisleads 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 ofsolutes or suspended matter. Need for field studies !