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Measuring detailed vertical density profiles in
cohesive sediment layers
- A comparison of techniques -
Hamburg University of Technology
Institute of River and Coastal Engineering
Denickestraße 22
21073 Hamburg
Germany
www.tuhh.de/wb
Markus Witt
T. +49 40 428 78-44 12
E. markus.witt@tuhh.de
Dr.-Ing. Edgar Nehlsen
T. +49 40 428 78-42 74
E. nehlsen@tuhh.de
Justus Patzke
T. +49 40 428 78-21 81
E. justus.patzke@tuhh.de
Prof. Dr.-Ing. Peter Fröhle
T. +49 40 428 78-46 00
E. froehle@tuhh.de
Introduction
Between 2013 and 2018 several hydrological and morphological
changes have been observed in the Tidal Elbe, particularly an
unusually high increase in tidal range, turbidity and
sedimentation rates. [1]
The joint project ELMOD is conducted to improve the ability of
numerical models to represent the complex underlying
processes and deepen the understanding of estuarine
sediment transport in general. Two goals of the first ship-based
measurement campaign in June 2023 in the Port of Hamburg
were i) to utilize and compare different techniques to capture
detailed and continuous vertical density/SSC profiles and ii) to
examine the representation of the natural density profile by the
extracted sediment cores.
Methodology
The measurements were carried out around flood slack water.
Two ships were employed for the campaign. Ship 1 recorded the
cross sections C1-C3 (see Fig.1) with ADCP and sediment echo
sounder at three points of time, collected sediment cores from
position P1 and P2 and measured density profiles with a
Rheotune. Ship 2 stayed in the vicinity of Ship 1 (distance ~30
m) and was equipped with a Solitax suspended matter probe,
Admodus density probe and a peristaltic pump.
Ship1
Sediment echosounder
-
Stema EBP10 System [2]
-
Transducer: Teledyne OTSSB200/24KHz [3]
-
Calibrated by Rheotune measurements
Subsamples of sediment
core
-
Coring device developed at TUHH [4]
-
Sediment core (1,2m x 0,2m) divided into
layers of ~ 4cm thickness
-
Density of subsamples measured with Anton
Paar DMA 35 and suspension balance
Stema
Systems Rheotune
-
Working on tuning fork principle
-
Output rate: 20 Hz
-
Accuracy: <1% of density (Newtonian Fluid)
[5]
- Additional output of Bingham yield stress and
viscosity
Ship 2
Admodus USP pro
-
Working on ultrasonic principle
-
Output rate: 50 Hz
-
Accuracy: ±0.005 g/cm³ [6]
Hach Solitax
sc
-
Turbidity/SSC probe
-
Working on combined infrared absorption
scattered light technique [7]
-
Output rate 0.2 Hz
-
Probe was held in Position for ~90 seconds in
depth-intervals of 1-2.5 m
-
Average SSC-values for depth-
intervals of 0.1
m have been calculated
Peristaltic pump
-
SSC of water samples derived via vacuum
filtration
-
Samples taken in depth-intervals of 1-2.5 m
Results
Figure 2 shows the center cross-section C2 recorded by the
sediment echosounder and gives a general spatial impression
of the encountered conditions. In the sample area the density
of the bed increases to ~1200 kg/m³ over the top layer of
~40cm. The results of all employed techniques are shown in
Figure 3 for the sample site P2. The SSC-values derived by
Solitax and peristaltic pump (and vacuum filtration) have been
converted to densities therefore.
The measured SSC-values for the latter techniques are shown
in Figure 4 over the whole water column.
Discussion
The utilized devices and techniques deliver very consistent
results. The bed-density profiles (Fig.3) and especially their
gradient are in good agreement for the subsamples, Admodus,
Rheotune (and sediment echosounder), which shows that the
extracted sediment cores reflect the natural density profiles to
a high degree. The slightly different elevations of the
associated densities are seen to be due to small changes in
Ship positions.
Solitax and peristaltic pump deliver comparable SSC-values in
the water column (Fig.4). In the relatively thin bottom layer with
rapidly rising sediment concentrations both techniques reach
their limits. A further test in conditions with larger layers of fluid
mud is desirable. Figure 4: Comparison of measured
SSC-profiles at sample site P2
Figure 1: Location of first measurement campaign in the Port of Hamburg. Bed
elevation data collected three days before campaign.
Germany
Hamburg
Elbe
North Sea
P1
P2
C1
C2C3
The references are listed on the attached sheet.
Table 1: Utilized techniques for the collection of density/SSC-profiles
Figure 2: Density horizons of Cross-section C2 recorded by sediment echo-
sounder. Sample sites P1 and P2 are marked. (Stacked lines plot)
Figure 3: Comparison of the results of utilized methods for sample site P2
(Circle: Measured from Ship 1; Triangle Ship2)
Witt, M.; Patzke, J.; Nehlsen, E.; Fröhle, P.