Estuarine circulation feeds the
Wadden Sea
Götz Flöser and Rolf Riethmüller, Department KOK, Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Contact: floeser@hzg.de
Introduction
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The permanent suspended matter (SPM) cross-shore gradient between the
Wadden Sea and the German Bight (see satellite scene right) should lead to a
continuous outward transport of suspended matter towards the North Sea
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Nevertheless, the Wadden Sea is stable on the order of ~1000 years, so there
must be a counteracting inward transport to compensate that
Theoretical predictions
Across the shore, a permanent density gradient is observed (diagram right)
The density gradient leads to a difference in ebb and flood vertical current profiles
Theory predicts also differences in vertical salinity gradient and turbulence
parameters between ebb and flood
These predictions resemble the well-known estuarine circulation in river mouths
that lead to inward sediment transport
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Hypothesis and Prediction
Density gradient
with
-
Ebb
The
undisturbed
velocity
profile (solid line; - for ebb, +
for flood) is distorted by the
density gradient. The ebb
current is lower near the
seafloor and higher at the
surface than the flood current.
The higher shear leads to
higher turbulence during flood
and
thus
to
a
more
homogeneous current profile.
without
tidal straining
U
Flood
ENVISAT / MERIS scene from March 10, 2012
showing suspended matter concentrations in the
German Bight. This cross-shore gradient should
lead to an outward transport of suspended matter.
Differences of temperature T, salinity S and
water density  between the German Bight and
the Wadden Sea (Hörnum pole; 2002-2009)
Observation: Current Velocity
Current
and
SPMC
profiles
measured near Spiekeroog Island
yield an averaged inward directed
velocity near the seafloor and an
outward directed one near the
surface. Along with depth-increasing
SPMC this leads to an inward SPMC
transport.
+
ebb
The current profiles measured in the Hörnum Deep
(moored ADCP) and the Marsdiep (Ferry) are averaged
and fitted to a logarithmic function.
u  u* /  ln
z
b
The parameters describing the curvature are compared. In
this way, the difference between ebb and flood is
documented.
The analysis of velocity data from a moored ADCP in
Hörnum Deep and Marsdiep Ferry data shows that, in both
cases, there is a statistically significant correlation between
curvature of the current profiles and the density gradient,
expressed by the Horizontal Richardson number
flood
The hypothesis predicts that:
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Flood current profiles differ from ebb profiles
Salinity differences (bottom – surface) differ between flood and ebb
Turbulence is higher in flood current
The suspended matter transport is directed inward at the bottom and outward at
the top
Some of these predictions can be tested.
Observation: Turbulence
Summary
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flood
ebb
Estuarine circulation can be detected in the Wadden Sea.
Current velocities and turbulence parameters follow the predictions of the simple theoretical
model: the water column is more homogeneous during flood than during ebb, the friction
velocity and the dissipation of turbulent kinetic energy are higher.
This process could counteract the outward transport of suspended matter by turbulent
diffusion.
The strength of the estuarine circulation depends on the magnitude of the horizontal density
gradient. Precipitation and temperature changes may have an impact on the transported
suspended matter.
Publications
Turbulence parameters measured during a field campaign in April 2008 north of Sylt
Island. High tide was at 11:22 UTC. N2 is the buoyancy frequency, ε the dissipation
rate of turbulent kinetic energy, c the suspended matter concentration, and u*b the
friction velocity.
It is easily seen that, e.g., the friction velocity (bottom) is higher during flood and the
stability of the water column lower (upper panel), thus confirming the theory’s
prediction.
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Burchard, H., G. Flöser, J.V. Staneva, T.H. Badewien, R. Riethmüller, 2008: Impact of Density
Gradients on Net Sediment Transport into the Wadden Sea. J. Phys. Ocenogr. 38, 566-587
Flöser, G., H. Burchard, R. Riethmüller, 2011: Observational evidence for estuarine circulation
in the German Wadden Sea. Cont. Shelf Res. 31, 1633-1639
Becherer, J., H. Burchard, G. Flöser, V. Mohrholz, L. Umlauf, 2011: Evidence of tidal straining
in well-mixed channel flow from micro-structure observations. Geophys. Res. Lett. 38, L17611
Helmholtz-Zentrum Geesthacht • Max-Planck-Straße 1 • 21502 Geesthacht • Phone +49 (0)4152 87-0 • Fax +49 (0)4152 87-1403 • www.hzg.de
Contact: Dr. Götz Flöser