In a first step to investigate the behaviour of sediment plumes released from dredging vessels, an experimental facility has been built to release scaled fine sediment plumes in the presence of cross flow. High-frequency measurements of velocity components and sediment concentration are obtained using acoustic and optical backscatter instruments. The paths of the axis of the experimental buoyant plumes in cross-flow have been compared to integral laws by Fisher et al. (1979), showing relatively good agreement for plumes not influenced by the dredger’s hull. Plumes with low relative density difference and high crossflow to outflow velocity ratio deviate from the integral laws due to additional mixing induced by the hull boundary layer and wake

De Mulder, Tom

Decrop, Boudewijn

Sas, Marc

Toorman, Erik

Troch, Peter

English

Ghent University Academic Bibliography

Experimental investigation of negatively buoyant sediment plumes resulting from Dredging operations

Decrop, B.

De Mulder, T.

Troch, P.

Toorman, E.

Sas, M.

Open Marine Archive

Coast/ab12, 2012, Ghent, Belgium. Book of Abstracts of the Fourth International Conference 
on the Application of Physical Modelling to Port and Coastal Protection. ISBN 978 90 382 2008 6 
EXPERIMENTAL INVESTIGATION OF NEGATIVELY BUOYANT SEDIMENT 
PLUMES RESULTING FROM DREDGING OPERATIONS 
BOUDEWJJN DECROP (1.4), TOM DE MULDER (3), PETER TROCH (3), ERIK TOORMAN (2) 
& RENAAT DE SUTTER (3) 
1. Introduction 
O> Hydraulics Laboratory, Ghent University, 
Sint-Pietersnieuwstraat 41, Ghent, B-9000, Belgium. Boudewijn.Decrop@ugent.be 
(2! Professor, Hydraulics Laboratory, KULeuven 
Kasteelpark Arenberg 40, Heverlee, B-3001, Belgium. Erik.Toorman@bwk.kuleuven.be 
(3! Professor, Hydraulics Laboratory, Ghent University, 
Sint-Pietersnieuwstraat 41, Ghent, B-9000, Belgium. 
14> IMDC, 
Coveliersstraat 15, Antwerp, B-2600, Belgium. 
Dredging is an activity often related to economical expansion projects along the world's coastlines and has been a 
strongly increasing for some years. In view of the increased attention to environmental impacts of coastal and 
offshore dredging operations, research in this field is on the rise. 
In this context the effect of turbidity generated by dredging activities plays an important role. The sediments 
brought in suspension can generate effects ranging from reduced light penetration in the water column over 
burial of sea bed ecosystems to dispersion of toxic materials attached to polluted sediments (Dankers, 2002; 
Smith et al., 2008). The dispersion of benthic material can have other impacts including siltation of still water 
bodies such as bays, docks or harbours. 
Sediments in highly-concentrated suspensions are released from the often used Trailing Suction Hopper 
Dredgers (THSD) through an overflow of the hopper. Depending on sediment concentration and ambient current 
velocity (relative to the sailing vessel) the flow can either behave as a density-driven, negatively buoyant jet or as 
a plume mixing fast with the seawater and therefore losing its buoyancy. In the first case the dynamic plume 
sinks to the seabed while part of the sediment is mixed into the water column, in the second case almost all fine 
sediment is brought in suspension (Spearman et al., 2007). Both cases lead to a large, non-buoyant, passive far-
field plume with the potential to travel large distances due to advection with coastal or estuarine currents. 
2. Methods 
In a first step to investigate the behaviour of this type of density current, an experimental facility has been built 
to release scaled sediment plumes in the presence of cross flow (Figure 1). In a mixing tank fitted with a 
circulating pump a mixture of water and sediment is homogenised. Using a second pump, the mixture is fed to a 
constant head vessel located on top of the flume. The constant head vessel is designed to keep a constant water 
level by being overtopped and leading the excess mixture back to the mixing tank. The constant head vessel is 
connected to an opening in a polycarbonate plate situated at 4 cm below the water level in the flume. The 
constant level in the vessel, combined with an orifice in the pipe guarantees for a constant, known discharge of 
the mixture with controlled sediment concentration into the flume. Fine sediment (kaolin clay) with a narrow 
particle size distribution has been used in order to minimise the effect of settling and particle size distributions 
and thus simplify the interpretation. 
Figure 1. Image of a negatively buoyant sediment plume in cross flow. 
73 
High-frequency measurements of velocity components and sediment concentration are obtained using 
acoustic and optical backscatter equipment, still regarded as the best options to obtain information in highly 
turbid flows. The Acoustic Doppler Velocimeter (ADV) is used to provide data on mean and fluctuating velocity 
profiles by means of the Pulse Coherent Doppler technique. In this laboratory study, the instrument's acoustic 
backscatter intensity is calibrated to measure the time-averaged sediment concentrations in the plume as well as 
turbulent fluctuations of the concentration allowing for the determination of turbulent sediment and momentum 
fluxes (e.g. Shabbir and Taulbee, 2000). The optical backscatter signal of an Argus Surface Meter (ASM) has been 
calibrated for the response in kaolin clay suspensions and has been installed to obtain additional profiles of 
mean sediment concentration at a higher spatial resolution. 
A background of diffused white light provides for using images as a shadowgraphy measurement of the 
obscuration due to sediment allowing for the determination of the average elevation of the concentration 
maximum at any distance from the source up to about 2 m (32 to 60 pipe diameters, depending on the pipe). 
The influence of mass density of the released material, exit velocity, pipe diameter and background current 
velocity are investigated. A total of 34 different experiments have been executed with Richardson numbers 
ranging from 0.01 to 17 and background to outflow velocity ratios between 0 and 8.7. In this way, dynamically 
scaled conditions have been produced corresponding to the range of conditions occurring while dredging in 
marine and coastal environments. At this stage of the research, the influence of waves, air bubbles, propellers 
and intermittent discharges due to ship pitching are not taken into account. 
3. Results and conclusions 
In order to verify the measurement techniques a sediment plume has been released into still water, generating 
the well-known buoyant jet without cross-flow. Radial profiles of axial velocity and sediment concentration have 
been measured using the ADV and show good correspondence with the Gaussian profile described by self-
similar conditions (Fisher, 1979; Lee and Chu, 2003). Furthermore, PLIF data of turbulent sediment concentration 
fluctuations (Cnns) by Papanicolaou (1988) have been compared and show good agreement in terms of the axial 
maximum and radial profile of CnniCm, with Cm the maximum concentration. 
The paths of the axis of the experimental buoyant plumes in cross-flow have been compared to integral laws 
by Fisher et al. (1979), showing relatively good agreement, proving that a turbulent plume of fine sediments 
behaves very similar to a two-liquid plume as long as particle settling velocity is small compared to advective 
velocities. An exception are the cases with a plume formed by low density material and high background flow 
velocity having a higher path and are therefore influenced by the wake of the schematised polycarbonate ship 
hull where shear flow induces increased vertical mixing and therefore a location of the plume axis closer to the 
surface compared to integral laws. 
At relatively close distance to the exit pipe, the background flow is perpendicular to the plume. This effect 
produces the radially symmetric pipe-flow profile to deform into two counter-rotating vortices. With this 
structure, the plume adopts a larger surface area leading to higher mixing and dilution rates. 
The experiments offer an extensive dataset on the mixing in this type of plumes, useful for calibration of a 
numerical model. 
4. References 
Dankers, P.J.T., 2002. The behavior of fines released due to dredging. A literature review. Technical University of 
Delft, pp 59. 
Lee, J.H.W and Chu, V., 2003. Turbulent jets and Plumes: A Lagrangian approach. Kluwer, pp378. 
Papanicolaou, P.N. and List, E.J., 1988. Investigations of round vertical turbulent buoyant jets. J. Fluid Mech. Vol 
195, pp. 341-391. 
Shabbir, A and Taulbee, D.B., 2000. Experimental balances for the second moments for a buoyant plume and 
their implication on turbulence modelling. International Journal of Heat and Mass Transfer 43, 1777-1790. 
Smith, G.G., Weitz, N., $oltau, C., Viljoen, A., Luger, S. and Maartens, L., 2008. Fate of fine sediment from 
dredger-based mining in a wave-dominated environment at Chameis Bay, Namibia. Journal of Coastal 
Research, 24(1), 232-247. West Palm Beach (Florida), ISSN 0749-0208. 
Spearman,]., Bray, RN., Land, J., Burt, T.N., Mead, C.T. and Scott, D., 2007. Plume dispersion modeling using 
dynamic representation of trailer dredger source terms. In: Estuarine and Coastal Fine Sediment Dynamics, 
p417-448. Proceeding in Marine Science. Elsevier. 
5. Acknowledgements 
The author wishes to thank the "lnstituut voor aanmoediging van Innovatie door Wetenschap en Technologie" 
(IWT) and International Marine and Dredging Consultants (IMDC) for funding this research project. 
74 


Experimental investigation of negatively buoyant sediment plumes resulting from dredging operations

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Experimental investigation of negatively buoyant sediment plumes resulting from Dredging operations

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