Measurement and comparison of settling velocities of cohesive sediments from the German estuaries Weser and Ems

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Justus Patzke
Justus.Patzke@tuhh.de
www.tuhh.de/wb
Prof. Peter Fröhle
froehle@tuhh.de
www.tuhh.de/wb
Measurement and comparison of settling
velocities of cohesive sediments from the
German estuaries Weser and Ems
Dr. Edgar Nehlsen
nehlsen@tuhh.de
www.tuhh.de/wb
INTERCOH
18-23/09/2023
Markus Witt
Markus.Witt@tuhh.de
www.tuhh.de/wb
Dr. Roland Hesse
Roland.Hesse@baw.de
www.baw.de
Patzke, J.; Hesse, R.F.; Witt, M. ; Nehlsen, E.; Fröhle, P.
For An Improved Understanding Of Estuarine Sediment Transport –FAUST
The FAUST project addresses challenges in modelling sediment exchange at the water / soil interface by extracting natural soil samples
from German estuaries, collecting quasi-in-situ data and performing lab experiments. In this poster a novel methodology for determining
settling rates is discussed, measurement results from lab experiments are presented and parameterizations of settling models are realized.
(IRCE)
TAKE HOME MESSAGES
Acknowledgment
The authors would like to thank Federal Waterways Engineering and Research Institute (BAW) Germany, which funded the project. Samples are taken with the help of ships and employees from WSA Bremerhaven. We’d like to thank the Hamburg Port
Authority for offering ship capacities and ideas to develop a new type of core sampler and the TUHH-Institute of Multiphase Flows for lending the ultrasonic sensor. Last but not least we’d like to thank the students U. Apogo, K. Arkawazi & B. Bakhshi by helping
to collect settling velocity data in the lab.
FAUST investigates the exchange at the sediment-water interface:
1. Natural mud was collected from the fairway of the Ems and the
Weser estuary
2. A methodology using ultrasonic sensors was developed to
analyse settling velocities and their temporal and spacial
distribution
6. Results
•Deposition and accumulation of fine sediments
even in the fairway is an increasing challenge
for authorities of waterways
•Improved prediction of location and amounts of
deposited sediments is required
•Estuary-specific parameterization of bottom
exchange characteristics is required to improve
numerical models
1. Motivation 2. Concept
•Collect natural samples at sedimentation sites in the fairway
•Investigation of quasi-in-situ behavior (erosion, density,
grain size distribution, settling behavior)
•Development of a methodology to determine settling
velocities in the flocculation and hindered settling regimes
•Determine effective settling velocities for different SSC,
salinities & temperatures to study their influence
•Use settling data generated in the lab to fit appropriate
models
5. Methodology
Time evolution patterns of of settling
velocities for 9 settling columns with different
initial concentrations C0are shown in figure
3. The y-axis contains positive settling in
downward direction. As the blue colors fade,
settling dominates over turbulence.
Turbulence is introduced by the stirring
process. At t = 0 stirring stops and after some
time the turbulence decreases and the
effective settling velocity is derived. A
sensitivity analysis has shown that the time
lag is about 240s after stirring, see the black
boxes in figure 3. Then, over a period of 45s
velocities are recorded and averaged to an
effective settling velocity. A high density layer
is forming at the bottom where no
measurements can be taken. This layer is
colored in white.
3. Site & Sampling
Sediment cores from the
ETM within the fairway
of the estuaries Weser,
Elbe and Ems were
collected. For this
poster, data was
generated from samples
of two sites:
•Weser (W),
Nordenham (NH),
Weser-km57
•Ems (E), Weener
(W), Ems-km 8
A settling column approach has been developed to determine settling velocities
and to provide a deeper understanding of the settling behavior of cohesive
sediments from German estuaries as a function of SSC. Suspensions of defined
initial SSC are prepared and homogenized by stirring. Signal Processing’s 4Mhz
ultrasonic sensor is aligned and fixed in a downward vertical direction with
contact to the suspension’s surface, see figure 2. Initial measurements are taken
immediately after stirring, see Figure 3. In this way, a sensitivity analysis is
performed to define the time lag until settling and stirring induced turbulence are
4. Experimental Setup
Fig 2.: schematic of the
setup for ultra-sonic
measurements
Fig 1.: Investigated sites of project FAUST. Focus of this poster are settling
velocities of sediments from the Weser (at Nordenham) and Ems (at
Weener), Maps from ©ITZBund2023
3. Eff. settling velocities are derived as a function of SSC, avg.
max. vel. are Wsm,Weser ≈ 1,7 mm/s, Wsm,Ems ≈ 1,35 mm/s
4. Parameterization of appropriate expressions for Wsis applied
5. Sediment specific values for regime transition, gel point and
the settling velocity of an individual floc have been derived
Assumptions:
•Initially turbulence prevents
particles to settle freely and forcing
flocculation processes.
•Effective settling velocity is
determined when gravitational
settling becomes dominant.
The Methodology was applied to derive effective settling velocities for sediments from the
German estuaries Weser and Ems, see figure 5. Weser sediments exhibit higher velocities
in both regimes.
•Next, models by Raudkivi (1998) and Winterwerp (2007) are utilized for regime-
specific parameterization
•Additionally, Mehta‘s model (1996) is used for cross-regime simulation of settling
velocities
•Using max. velocities Wsmax,M from Mehta's model (1996) the transition concentration
between flocculation and hindered settling is determined to about 8g/L (W) & 7 g/L (E)
•The transition is also sought by dividing a measurement series in two parts and fit
Raudkivi and Winterwerp‘s model (dotted verticals) correspondingly. The transition
point is moved dynamically using a sliding window. By this, several realizations of the
model fits are realized (light gray and blue lines in fig. 6)
Poster
InterCOH 2019 Data
Lutoclines &
Density profiles
Poster
InterCOH 2021 Paper
Frontiers 2022
Fig 3.: Time evolution
of settling velocities
Fig 4.: Eff. Velocity profiles for different initial
concentrations
•Max. settl. Vel., Wsm,mes (measured) ≈1,35-1,7 mm/s & Wsmax,mod (Mehta) ≈1,2-1,5 mm/s
•Mehta: (Max.) deposition flux, Fs(m) ≈20-45 kg/m²s
•Winterwerp: Gelling point, Cgel ≈100-250 g/L & individual velocity Ws0 ≈1,0-2,75 mm/s
•Combination of models: Transitional conc. (floc vs. hindered settl.), Ctrans ≈7-8 g/L
in equilibrium. At this point free settling is
assumed and effective settling can be determined.
Figure 6 shows exemple results and model fits for Ems sediments with a salinity of 7 ppt
and a temperature of ~20°C. The Raudkivi (light grey), Winterwerp (light red) and Mehta
(black line) models are fitted to the data (black points). Thus, several critical parameters
are derived from the measured data and the model fits:
Fig 5.: Effective
settling velocities
for Weser and
Ems estuaries
Fig 6.: Model
fits for Ems
sediment
Figure 4 shows resulting velocity profiles for 9
different C0with an evaluation period of 240s
≤t≤285s. With increasing C0, the bottom
layer thickness increases from a few mm at
C0= 1.8 g/L to about 300mm at C0=90 g/L.
From the top of the column the vertical
velocity profiles exhibit an almost constant
gradient. Approaching the vicinity of the
interface, the velocities decrease to almost
zero mm/s within a range of about 50 mm.
This indicates the thickness of the transition
between flocculation settling and hindered
settling.