Journal of Fluid Mechanics

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The effects of flow stratification by non-cohesive sediment on transport in high-energy wave-driven flows

DANIEL C. CONLEYa1, SILVIA FALCHETTIa2a3, IRIS P. LOHMANNa4 and MAURIZIO BROCCHINIa5

a1 Marine Institute, University of Plymouth, Plymouth, UK

a2 NATO Undersea Research Centre, La Spezia, Italy

a3 Dipartimento di Ingegneria delle Costruzioni, dell'Ambiente e del Territorio, Università di Genova, 16145 Genova, Italy

a4 SimCorp A/S, Copenhagen, Denmark

a5 Istituto di Idraulica e Infrastrutture Viarie, Univesità Politecnica delle Marche, Ancona, Italy

Abstract

The two-way effects of the time-varying suppression of turbulence by gradients in suspended sediment concentration have been investigated using a modified form of the Generalized Ocean Turbulence Model (GOTM). Field measurements of fluid velocities and sediment concentrations collected under high-energy conditions (mobility number ≈ 900) have been simulated both including and neglecting the feedback between sediment and turbulence. The results show that, when present, this feedback increases the wave-coherent component of transport relative to the mean component of transport, which can even change the direction of transport. Comparisons between measured and simulated time series of near-bed sediment concentrations show great coherence (0.95 correlation) and it is argued that the differences in net transport rates may be partially explained by the use of a uniform grain size in the simulations. It is seen that the effects of sediment stratification scale with orbital velocity divided by sediment setting velocity, um/ws, for all grain sizes.

(Received July 10 2007)

(Revised May 02 2008)

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