Nearshore wave-current interaction: hydrodynamic effects of rough and sloping beds.

Anthropic pressure on the coastal environment has increased sensibly in the last decades. Human activities and interventions on the coastline may trigger significant modifications of the natural physical processes, such as the coastal sediment transport, which may lead to important erosive phenomena. In recent years, the interest in the understanding of the large and small scale mechanisms that take place in the near-shore has grown significantly, from both a scientific and an engineering standpoint. Among those processes, particular attention has been given to the study of waves and currents, which are usually simultaneously present in coastal waters and interact with each other at an orthogonal or near-orthogonal angle. Their hydrodynamic interaction is rather complex and plays a major role in the sediment transport phenomenon. Although immense progress has been achieved in this area of research, some critical open questions still remain. For instance, most of the existing studies focuses mainly on the influence of waves on the current mean flow. Extensive studies on the combined flow turbulent field, which has a fundamental role in the current bottom friction, are indeed rather limited. Moreover, interaction of currents with nonlinear waves has been investigated mostly considering currents and waves propagating in the same direction, which is an unrepresentative scenario in the case of longshore currents. In the present work, an investigation on the hydrodynamics of wave-current orthogonal combined flow has been carried out. The work focuses on the effects of the oscillatory flow superposed on the current steady boundary layer, and on how the oscillatory flow affects the current velocity distribution. Two laboratory experimental campaigns of wave-current orthogonal interaction have been performed. The first one (called WINGS campaign) has been carried out in a shallow water basin at DHI Water and Environment (Hørsholm, Denmark), in order to investigate the orthogonal combined flow in the presence of different roughness beds. Tests of current only, wave only and combined flow have been carried out. Two currents have been generated (with nominal current velocity U = 0.140 m/s and 0.210 m/s) and a series of regular wave conditions have been performed (with wave height H = 0.05 - 0.18 m, and wave period T = 1.0 - 2.0 s). Two types of bed configurations have been used: fixed sand (d50 = 0.0012 m) and fixed gravel (d50 = 0.025 m). The second laboratory campaign (called ACCLIVE campaign) has been carried out in a wave basin at the Hydraulics Laboratory of the University of Catania (Italy). The combined flow has been generated over a gentle sloping 1:25 fixed bottom, in order to investigate the current velocity profile interacting with shoaling waves at a right angle. Three currents (u = 0.060, 0.110 and 0.140 m/s) and a regular wave condition (H = 0.085 m, T = 1.0 s) have been generated. Experiments in the presence of a lone current and combined flow have been performed. For both laboratory datasets, wave surface elevation and flow velocity measurements have been carried out. Velocity measurements have been performed both inside and outside of the current boundary layer. Mean flow has been investigated by computing time- and space-averaged velocity profiles. Friction velocity and equivalent roughness have been inferred from the velocity profiles by best fit technique, in order to quantify the shear stress experienced by the current mean flow. Tests in the presence of only current, only waves and combined flow have been performed. Instantaneous velocities have been Reynolds-averaged in order to obtain turbulent fluctuations time series and compute turbulence related quantities, such as turbulence intensities and Reynolds stresses. The mean current velocity profiles have been also compared with a selection of analytical models in order to assess their validity for the case of wave-current orthogonal flow for the considered wave and current condition ranges. Moreover, a series of CFD simulations have been carried out to investigate wave-current interaction, and highlight the limits of the numerical models. Two setups have been developed, a one-dimensional one and a three-dimensional one. Both models solve for Reynolds-Averaged Navier-Stokes equations for incompressible fluids, coupled with a k-omega Shear Stress Transport model to achieve turbulence closure. Cyclic boundary conditions coupled with body forces generated by sources in the momentum equations have been employed to generate the flow dynamics. The 1D model solves the bottom roughness through the use of both smooth and rough wall functions, whereas the 3D setup features a reconstruction of the real gravel bed used within the WINGS campaign. Current only, wave only and waves plus current tests have been conducted for the 1D model, whereas a preliminary current only flow for 3D model has been carried out. The models have been validated by means of the WINGS mean velocity profiles over sand and gravel bed. The analysis of the mean flow revealed a complex interaction of the waves and currents combined flow. Depending on the relative strength of the current with respect to the waves, the superposition of the oscillatory flow may determine an increase or a decrease of the bottom friction experienced by the current. Such a behavior is also strictly related to the bed physical roughness. Analysis of the turbulence intensities and Reynolds stresses seems to confirm the results of the mean flow investigation. Moreover, the application of the quadrant analysis provides an insight on the dynamics of ejections and sweeps in the presence of superposed waves. In the presence of shoaling waves, the effects on the current determined by the increase of wave orbital velocity are counteracted by the increment of the current Reynolds number and current boundary layer turbulence. Moreover, phase-averaged velocity analysis reveals an oscillating behavior of the velocities in the current direction determined by the presence of waves. The current oscillatory motion is characterized by a phase-shift, whose behavior is investigated as the waves shoal. Results of the CFD simulations show that the 1D model is able to reproduce correctly smooth and rough bed mean velocities in the presence of a lone current. However, in the presence of a superposed wave field, the use of wall functions in the bottom boundary condition induces the predicted bed shear stresses to deviate, which determines an overall underestimation of the velocity profile.