Estimating ship-induced sediment transport in confined waters

Abstract

The maintenance of waterways is a challenging task for federal authorities. An expensive subtask is dredging and disposing of accumulated sediments. Dredging volumes are often estimated based on experience while the underlying physical transport processes causing sedimentations are not fully explored yet. For instance, in confined waterways moving ships have an influence on the turbidity by resuspending sediments which can then be transported by prevailing currents. Therefore, the German Federal Waterways Engineering and Research Institute (BAW) conducted an eight-day field campaign using a network of three probes which recorded parameters like turbidity, pressure, and flow velocities in the Kiel Canal (Schleswig-Holstein, Germany). The advantages of this canal as test site are the laboratory-like conditions with almost no natural flow and no influence of tides. Data assimilation and analysis of the field campaign have been performed at the Research Institute for Water and Environment at the University of Siegen as part of a research cooperation with the BAW to estimate the ship-induced proportion of the totally transported sediment volume in the Kiel Canal. Therefore the three high-frequency turbidity records from the canal bed were used to model the turbidity distribution in the canal cross-section. Linking the turbidity distributions with the measured flow velocities yields an estimation of the totally transported sediment volume during the field campaign. In a second step smoothing the turbidity and flow velocity time series and recalculating the estimation removes all ship influences from the transported volumes so that the difference in both volumes describes the ship-induced proportion of the totally transported sediment volume. As a result, a proportion of about 10% of the entirely transported sediments can be attributed to ship-induced resuspension. In reverse, the majority of the sediment transport originates from a slow but steady drainage flow in the Kiel Canal. The estimated results are plausible and in an expected dimension with regard to the annual dredging rates of the last years. The developed methods are currently applied to a new dataset from a second field campaign in the Elbe River. This system is, in contrast to the Kiel Canal, dominated by tidal dynamics. Preliminary results with a focus on the main differences between both campaigns will also be presented.

Full text

Estimating ship-induced sediment transport in confined waters
Marius Ulm (marius.ulm@uni-siegen.de)1, Sebastian Niehüser1, Arne Arns1, Jürgen Jensen1, Bernhard Kondziella2, Klemens Uliczka2
1University of Siegen, Research Institute for Water and Environment (fwu), Germany 2Federal Waterways Engineering and Research Institute (BAW), Office Hamburg, Germany
EGU2017-5070
Visit our project on
ResearchGate!
Background
§ Dredging and disposing of accumulated sediments from waterways are
expensive tasks for federal authorities.
§ Physical transport processes and the impact of passing ships on these
processes are not fully explored yet.
§ Moving ships have an influence on the turbidity by resuspending
sediments which can then be transported by prevailing currents, but:
§ How large is the ship-induced proportion of the totally transported
sediment volume in a waterway?
Take-home messages
§ A proportion of about 10% of the entirely transported sediment can
be attributed to ship-induced resuspension under laboratory-like
conditions in the Kiel Canal.
§ Especially tidal but also discharge flows in the Elbe river dominate
the transport regime. The ship-induced proportion of the totally
transported sediment volume is <2%.
Ä
Estimating the ship-induced proportion
1. Approximating a 2D turbidity distribution for each time step by
extrapolating discrete turbidity data recorded with the probes
(see Fig. 5). Validation using ADCP SSC profiles.
2. Estimating the transported volume by linking the 2D turbidity
distributions with corresponding flow velocities for each time step and
summing up all time steps (see Fig. 6).
→ Recorded data yield the transported sediment volume due to “natural”
currents (mainly dewatering) including the ship-induced volume (A).
3. Removing all ship-induced signals from turbidity and flow data using
smoothing techniques.
→ Smoothed data yield the “naturally” transported sediment volume (B).
The ship-induced volume results from the volume difference (A-B).
Elbe field campaign and preliminary results
§ Six probes at the canal bed (same setup as for the Kiel Canal campaign),
recording for 16 days (see Fig. 7).
§ Estimation of the ship-induced proportion of the totally transported
sediment volume similar to the Kiel Canal method.
→ Tidal currents are the major cause of sediment resuspension and
transportation. Preliminary result: the ship-induced proportion is <2%.
Fig. 5: Schematic illustration of the transport simulation with extrapolated
turbidity distributions for each time step.
References
§ Jensen, J., Ulm, M., Kelln, V., Niehüser, S., Arns, A. (2016): Schiffserzeugter Sedimenttransport an Seeschifffahrtsstraßen (SeST)
Abschlussbericht zum Arbeitspaket 2 – „Validierung entwickelter Ansätze“. Project report of the 2nd work package, unpublished. Research
Institute for Water and Environment, University of Siegen.
§ Jensen, J., Ulm, M., Niehüser, S., Arns, A. (2017): Schiffserzeugter Sedimenttransport an Seeschifffahrtsstraßen (SeST) Abschlussbericht zum
Arbeitspaket 3 – „Quantifizierung des SeST im Tideestuar“. Project report of the 3rd work package, unpublished. Research Institute for Water
and Environment, University of Siegen.
§ Niehüser, S., Ulm, M., Arns, A., Jensen, J., Kelln, V., Uliczka, K., Kondziella, B. (2016): Interaction between ship-induced stress and associated
characteristics of turbidity records. In: Proceedings of the 4th International Conference on Ship Manoeuvring in Shallow and Confined Water
(MASHCON), Hamburg, 23.-25.05.2016. doi:10.18451/978-3-939230-38-0_3
§ Uliczka, K., Kondziella, B. (2016): Ship-Induced Sediment Transport in Coastal Waterways (SeST). In: Proceedings of the 4th International
Conference on Ship Manoeuvring in Shallow and Confined Water (MASHCON), Hamburg, 23.-25.05.2016.
doi:10.18451/978-3-939230-38-0_1
erosion
sinking
turbulenceflocculation
deposition
suspended
sediment
concentration
SSC
resuspension
return flow
swell flow
propeller
flow
sediment transport
SSC
SSC
Fig. 1: Flow and turbidity regime before (a), during (b), and after (c) a
passing ship in confined waters.
(a) (b) (c)
vS
t1
t2
t3
… tn
Kiel Canal field campaign
§ Three probes at the canal
bed (turbidity, pressure,
3D flow velocity; see
example in Fig. 4),
recording for 8 days.
§ AIS recorder at a nearby
bridge (length, width,
draft, speed/curse over
ground, position, ship
identifier).
§ ADCP flow/SSC profiles
(ship-based).
NORTH
SEA
BALTIC
SEA
Elbe (tidal)
Elbe
Kiel Canal
Hamburg
Kiel
Bruns-
büttel
km 17.925
GERMANY
DENMARK
Fig. 2: Map of the Kiel Canal.
Fig. 3: Cross-section at km 17.925,
probe locations and
exemplary vessel.
Fig. 4: Ship-induced turbidity (a), water level depression (b), and
flow velocity (c) at one exemplary passage.
Fig. 6: Cumulated sediment transport, calculated with observed and
smoothed data using the full record (8 days).
Fig. 7: Field campaign cross-section in the Elbe river at km 646.8, probe
locations and exemplary vessel.
20 km
-18 m BMTL
Probes 4 3
(3x)
2 1
MTL (± 1.85 m)
← BMTL ≈ 1100 m →
Ä vx ≈ ± 1.3 m/s
(tidal current)
-12 m BMWL
Probes 1 2 3
MWL (≈ const.)
← BMWL ≈ 170 m →
Ä vx ≈ 0 m/s
(laboratory-like)
Ä vx ≈ +0.2 m/s
(during dewatering)
(c)(b)(a)
-2
0.1
0
-0.1
-0.2
-0.3
-0.4
Water level depression [m]
Turbidity [NTU]
Flow velocity in x-direction [m/s]
50
100
150
200
250
300
350
400
450
-0.4
-0.6
-0.2
0
0.2
0.4
0.6
0.8
1.0
1.2
-0.6 -0.8
0 2 4 6 -2 0 2 4 6 -2 0 2 4 6[min] [min] [min]
Sep 17 Sep 18 Sep 19 Sep 20 Sep 21 Sep 22 Sep 23 Sep 24 Sep 25
Cumulated sediment transport [metric tons]
0
100
200
300
400
500
600 Calculatory transport with turbidity and flow as observed (A)
Calculatory transport with smoothed data (B)
approx. 10%
ship-induced
sediment transport