Hans Burchard1,2, Joanna Staneva3, Götz Flöser4, Rolf Riethmüller4,

Thomas Badewien5, and Richard Hofmeister1

1. Baltic Sea Research Institute Warnemünde, Germany2. Bolding & Burchard Hyrodynamics, Rostock, Germany

3. ICBM, University of Oldenburg, Germany4. GKSS Research Centre, Geesthacht, Germany

5. Institute of Physics, University of Oldenburg, Germany

hans.burchard@io-warnemuende.de

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 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:

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:

Residualonshorenear-bedcurrent

Along-tidesalinity gradientprescribed

Bottom-surface salinity

Quantification of water column stability

Tidal straining Vertical mixing

flood ebb

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 withhydrostatic 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 algebraicsecond-moment closures (from GOTM turbulence module).

Wetting and drying of intertidal flats is supported also inbaroclinic mode.

Present GETM characteristics ... numerics ...

Consistent explicit mode splitting into barotropic and baroclinic mode.

High-order positive-definite TVD advection schemes withdirectional split.

Choice of different schemes for internal pressure gradientcalculation.

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-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

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 Warnemünde