PosterPDF Available

Measuring detailed vertical density profiles in cohesive sediment layers - A comparison of techniques

Authors:

Abstract and Figures

This contribution outlines the conception of the first ship-based measurement campaign in the research project ELMOD which was conducted in the port area of Hamburg. It puts a focus on the in-situ measurements of vertical density- and SSC-profiles with different methods. Additionally, the agreement of the in-situ measured density profiles with the vertical density distribution of sediment cores collected during the same campaign is checked. The utilized devices and techniques deliver very consistent results. The bed-density profiles and especially their gradient are in good agreement for the subsamples of the extracted cores, Admodus USP pro, 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. Hach Solitax sc and peristaltic pump (+vacuum filtration) deliver comparable SSC-values in the water column. In the relatively thin bottom layer with rapidly rising sediment concentrations both techniques reach their limits.
Content may be subject to copyright.
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.
... In both campaigns, the corer penetrated about one meter into the bed. In Witt et al. (2023) it was shown that the device is capable of collecting naturally stratified bed samples, thus the properties (grain size distribution, loss on ignition) of the sediment used in this study can be seen as an average of the top one-meter layer of the bed. ...
... The structural density, which is defined as the density at the gelling point, where an interconnected matrix of solids has formed (see Winterwerp et al. (2021)), for sediment from location SE was determined as~1080 kg/m³ from previous settling column experiments, thus densities 1050 kg/m³ and 1075 kg/m³ are slightly below the structural density. The full range of tested densities covers the in-situ measured densities of the top 50 cm layer of the sediment bed at location SE (Witt et al., 2023). ...
Article
Full-text available
The quantification of the erodibility of cohesive sediments is fundamental for an advanced understanding of estuarine sediment transport processes. In this study, the surface erosion threshold τc for cohesive sediments collected from two sites in the area of the Port of Hamburg in the River Elbe is investigated in laboratory experiments. An improved closed microcosm system (C-GEMS) is used for the erosion experiments, which allows the accumulation of suspended sediment concentration (SSC) over an experimental run. A total of 34 erosion experiments has been conducted with homogenized samples and bulk densities between 1050 kg/m³ and 1250 kg/m³. The covered range of bulk densities is seen to represent the values commonly exhibited by freshly deposited cohesive sediments. Two approaches to derive τc based on the erosion rate (ε-method) and the SSC (SSC-method) were elaborated and compared. For both approaches, only one parameter has to be set in order to facilitate transferability to other devices. The results show a better performance of the SSC-method in terms of lower uncertainties, especially at the upper application limits of the utilized C-GEMS. The application of the SSC method yields values for τc between 0.037 N/m² and 0.305 N/m², continuously increasing with bulk density. Repetition tests proved the repeatability of the experimental procedure and utilized methods to derive τc . The derived data for τc is used to fit two mathematical models: i) a highly empirical model relating τc to dry bulk density and ii) a recently proposed model relating τc to the physical properties of the sediment-mixture. While the derived parameters for the first model vary widely for the two sampling sites, the fit-parameter for the latter model is virtually independent of the investigated site, suggesting the superiority of this approach.
ResearchGate has not been able to resolve any references for this publication.