Observation of sediment suspension by breaking-wave-generated vortices using volumetric three-component velocimetry

Abstract

The distribution of suspended sediment in plunging regular waves was measured simultaneously with the three-dimensional (3D) turbulent velocity field on a 1 in 40 plane slope using a volumetric three-component velocimetry (V3V) system. White solid glass spheres (diameter 0.25 mm, specific gravity 2.5) were used as sediment particles. The V3V system was positioned to capture the impingement of the first or second splash-up vortices on the bottom. The measurement volume encompassed the lower 25% of the water column and a bottom area of length and width equal to approximately ½ the local water depth. Phase separation of glass spheres and fluid tracers in the V3V images was performed by varying the settings of four particle processing parameters in the commercial software INSIGHT™ V3V 4G. The measured fluid velocity fields were decomposed into a wave-induced component and a turbulence component by ensemble averaging 28 experiment runs conducted under the same incident wave condition. The two-phase flow measurements were used to delineate the process of sediment entrainment, to determine the turbulent flow properties around the suspended sediment, and to investigate the relationships between sediment pickup rate and the different flow parameters. The two dominant mechanisms of sediment suspension observed were the deflected flow created when breaking wave vortices impinged on the bottom and the up-wash induced by the transverse vortices. The suspension process was associated with large apparent shear stresses and positive vertical velocities greater than or equal to the sediment fall velocity. It was observed that sediment trapping by transverse vortices with horizontal axis and columnar vortices with vertical axis was an important mechanism for keeping sediment in suspension for transport by the organized wave-induced flow (mean flow). It was found that measured sediment pickup rates generally correlated better with total apparent shear stress than bed shear stress or turbulent kinetic energy. The experimental results suggest that sediment suspension by breaking-wave-generated vortices cannot be modeled by bed shear stress alone but will also require information on the vertical velocities induced by these vortices over the bed.