Effects of surface waves on turbulent currents

Paolo Blondeaux, Giovanna Vittori, Nicoletta Tambroni

Monday 29 june 2015

17:33 - 17:36h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering, Poster pitches

Parallel session: Poster pitch: 3C. Coastal Engineering

In many shallow coastal environments, the sediment transport is often due to the combined action of surface waves and steady currents. The oscillatory flow induced by wave propagation contributes substantially to the pick up of the sediment from the bottom. Then, even though a net flux of sediment can be induced also by the waves because of steady streaming effects and the possible asymmetry of the velocity oscillations, the largest contribution to the sediment transport is possibly induced by the currents, which steadily drag the sediment in their direction. Hence, it is crucial to understand the interaction between waves and currents not only close to the bottom but also far from it. The experimental investigations of wave-current interaction (e.g. Kemp & Simons (1982, 1983), Klopman (1994) and Umeyama (2005)) show a significant reduction of the current velocity near the free surface when the waves propagate in the direction of the current, while the current velocity increases near the free surface when the waves propagate in the opposite direction. In order to study theoretically wave-current interaction, the hydrodynamic problem can be formulated using either an Eulerian approach or a Lagrangian approach (Dingemans et al., 1996; Groeneweg & Klopman, 1998; Groeneweg & Battjes, 2003). Recently, the interaction between waves and current was studied by Huang & Mei (2003) using an Eulerian approach. However, their analysis is not simple to be applied, since it is based on a perturbation approach which splits the fluid domain in an inviscid core region and viscous boundary layers and relates the order of magnitude of some quantities (e.g. the shear velocities, the bottom roughness, ...) to the wave steepness. Moreover, the approach requires the determination of the steady vorticity component in the core region where the vorticity distribution depends on a balance between viscous and convective effects. More recently, the hydrodynamic problem was solved also by Olabarrieta et al. (2010) by means of a three-dimensional numerical approach. However, the numerical solution requires high computational resources. In the present analysis, to avoid the decomposition of the fluid domain into an inviscid, core region and viscous boundary layers, we propose an approach which provides the flow field generated by the interaction of waves and currents in the whole water column. We consider a coastal region characterized by a constant finite depth and analyse the flow field induced by the simultaneous presence of a steady current and a progressive surface wave propagating with or against the current. The depth averaged velocity of the steady current is assumed to be of the same order of magnitude as the amplitude of the velocity oscillations induced by the wave propagation and both the velocities are assumed to be much smaller than the wave celerity. Reynolds stresses are quantified by introducing an eddy viscosity which depends on the shear velocity and on the distance from the bottom and the free surface. Even though some criticisms can be raised against this simple turbulence model, when applied to oscillatory flows, a comparison of the results it provides with the laboratory measurements of Kemp & Simons (1982, 1983), Klopman (1994) and Umeyama (2005) shows that the model can provide a reliable description of the phenomenon