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	Volume 5, Issue 2 Pg 107 - 218 (April 2009)
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Advances and Applications in Fluid Mechanics

Advances and Applications in Fluid Mechanics
Volume 5, Issue 2, Pages 107 - 153 (April 2009)

TIDALLY DRIVEN SEDIMENT TRANSPORT AND ITS IMPACT ON DREDGE LANES
J. N. Aldridge (UK) and S. G. Sajjadi (USA)

Abstract:
Tidal flows and suspended sediment transport associated with dredge tracks in dredging lanes, arising from aggregate extraction, is investigated. A computational fluid dynamics (CFD) model is used in order to simulate tidal flows and suspended sediment transport over bed features typical of aggregate extraction dredging. This model includes (a) pressure-gradient forcing, and (b) enables rough boundaries to be simulated. A sediment-transport module involving resuspension, transport and deposition is also developed, thereby allowing budgets of material to be computed. Model results have been compared with analytical solutions and previous CFD models over flat beds and favourable agreement is achieved. The present CFD model is shown to be capable of describing the velocity flows within complex areas of morphology such as found in aggregate extraction areas. These flows fields have been utilised within numerical suspended sediment models to predict re-suspension and deposition of sediment within the model domain. The results of the CFD simulations clearly indicate a mechanism for retention of sands by dredge tracks, arising from the reduced bed shear stress within the tracks compared to the surrounding higher flat regions. It is shown that this mechanism appear to be (i) relatively insensitive to track orientation with respect to the main tidal axis, (ii) is size selective, preferentially trapping coarser particles. The preliminary practical implications of this work are that suspended sediment transport associated with the dredge track is relatively insensitive to the orientation of the dredge tracks compared to the principal tidal axis. Some aspects of dredge track morphology are probably more significant, and in particular, isolated dredge tracks with a large depth/width ratio (i.e., deep and narrow) are more likely to accumulate sediment than tracks with a small depth/width ratio (i.e., shallow and wide).

Keywords and phrases: tidal flows, sediment transport, CFD, dredging.

Communicated by K. K. Azad

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