[1] U‐tube measurements of instantaneous velocities, concentrations, and fluxes for a well‐sorted, medium‐sized sand in oscillatory sheet flow are analyzed. The experiments involved two velocity‐asymmetric flows, the same two flows with an opposing current of 0.4 m/s, and a mixed skewed‐asymmetric flow, all with a velocity amplitude of 1.2 m/s and flow period of 7 s. We find that the net positive transport rate beneath velocity‐ asymmetric oscillatory flow results from large, but opposing sand fluxes during the positive and negative flow phase. With an increase in velocity asymmetry and, in particular, velocity skewness, the difference in the magnitude of the fluxes in the two half cycles increases, leading to larger net transport rates. This trend is consistent with the observed increase in skewness of the oscillatory bed shear stress. Phase‐lag effects, whereby sand stirred during the negative flow phase has not settled by the time of the negative‐to‐positive flow reversal and is subsequently transported during the positive flow phase, are notable but of minor importance to the net transport rate compared to earlier experiments with finer sands. In the vertical, the oscillatory flux is positive above the no‐ flow bed. Within the sheet flow pick‐up layer, the oscillatory flux is negative and similar in magnitude to the positive flux induced by the residual flow. The 0.4 m/s opposing current causes more sand to be picked up during the negative than during the positive flow phase. Above the no‐flow bed the resulting negative oscillatory flux is comparable in magnitude to the current‐related flux.

/pdf/observations-of-velocities-sand-concentrations-and-fluxes-3295sm4dm7.pdf

Observations of velocities, sand concentrations, and fluxes under velocity-asymmetric oscillatory flows

Practical sand transport formula for non-breaking waves and currents

This vision paper discusses the advances made over the last three decades in coastal sand transport and morphodynamics, and the research needs for the coming decades. The prime focus of the paper is on the relationship between the transport of sand particles and fluid motions in the coastal environment based on laboratory and field experiments as well as mathematical modelling. The paper mainly focuses on two main issues: (1) better understanding of sediment transport processes in the coastal zone and (2) the development of improved practical engineering sand transport formulae and morphodynamic models.

Coastal sediment dynamics: recent advances and future research needs

RANS-based simulation of turbulent wave boundary layer and sheet-flow sediment transport processes

Recent large-scale wave flume experiments on sheet-flow sediment transport beneath Stokes waves show more onshore-directed sediment transport than earlier sheet-flow experiments in oscillating flow tunnels. For fine sand, this extends to a reversal from offshore- (tunnels) to onshore (flumes)-directed transport. A remarkable hydrodynamic mechanism present in flumes (with free water surface), but not in tunnels (rigid lid), is the generation of progressive wave streaming, an onshore wave boundary layer current. This article investigates whether this streaming is the full explanation of the observed differences in transport. In this article, we present a numerical model of wave-induced sand transport that includes the effects of the free surface on the bottom boundary layer. With these effects and turbulence damping by sediment included, our model yields good reproductions of the vertical profile of the horizontal (mean) velocities, as well as transport rates of both fine and medium sized sediment. Similar to the measurements, the model reveals the reversal of transport direction by free surface effects for fine sand. A numerical investigation of the relative importance of the various free surface effects shows that progressive wave streaming indeed contributes substantially to increased onshore transport rates. However, especially for fine sands, horizontal gradients in sediment advection in the horizontally nonuniform flow field also are found to contribute significantly. We therefore conclude that not only streaming, but also inhomogeneous sediment advection should be considered in formulas of wave-induced sediment transport applied in morphodynamic modeling. We propose a variable time-scale parameter to account for these effects.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012JF002427

Sand transport beneath waves: the role of progressive wave streaming and other free surface effects

Existing models for wave-related (cross-shore) sand transport are primarily based on data from oscillatory flow tunnel experiments. However, theory and former experiments indicate that flow differences between full scale surface waves and oscillatory flow tunnels may have a substantial effect on the net sand transport. In this paper, high resolution measurements of boundary layer flow characteristics, sheet-flow layer sediment concentrations and net sand transport rates under full scale surface waves are presented. These experiments were performed in a large wave flume (GWK) for different wave conditions with medium (D50 = 0.25 mm) and fine (D50 = 0.14 mm) sand. It is shown that, especially under sheet-flow conditions, small wave induced net currents are of large importance for the total sand transport rates under these conditions.

https://icce-ojs-tamu.tdl.org/icce/index.php/icce/article/download/1271/pdf_82

Boundary layer flow and sand transport under full scale surface waves

Existing models for wave-related (cross-shore) sand transport are primarily based on data from oscillatory flow tunnel experiments. However, theory and former experiments indicate that flow differences between full scale surface waves and oscillatory flow tunnels may have a substantial effect on the net sand transport. In this paper, high resolution measurements of both boundary layer flow characteristics and net sand transport rates under full scale surface waves are presented. These experiments were performed for different wave conditions with medium (D50 = 0.25 mm) and fine (D50 = 0.14 mm) sand. It is shown that, especially under sheet-flow conditions, small wave induced net currents are of large importance for sand transport rates.

Jolanthe J.L.M. Schretlen

Papers

Sand transport beneath waves: the role of progressive wave streaming and other free surface effects

Boundary layer flow and sand transport under full scale surface waves

Sand transport under full-scale surface waves