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Hans Burchard1,2, Joanna Staneva3, Götz Flöser4, Rolf Riethmüller4,
and Thomas Badewien5
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
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:
Online since 1999, 50000 logins, mostly from Canada & US
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.
GETM – General Estuarine Transport Model – www.getm.eu
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).
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