Orthogonal waves and currents interacting with a fixed rippled bed

Wave and currents often are simultaneously present in coastal areas. In fact, currents originated by tides, by temperature or salinity gradients, by waves approaching the coast obliquely and so on, coexist with waves, giving rise to complex non-linear interactions. At the seabed the combined flow modifies the boundary layer structure depending on the bed roughness type. In particular, sandy bottoms are often modelled to form small scale sedimentary structures called ripples. This work presents some results of an experimental campaign which was carried out within the Hydralab+ WINGS project, aimed at investigating the effects of orthogonal waves and currents over a rippled bed. In this framework, velocity profiles along the water column and surface wave data were acquired. Close to the bed a couple of recirculating cells were identified along the ripple flanks, which are weakened by the current superposition with respect to the wave only case.

I. Introduction

Wave and currents often are simultaneously present in coastal areas, giving rise to a complex non-linear superposition that characterizes the mutual interaction of such forcings. As a consequence, several researchers in the last decades paid attention to understand the effects that take place when waves superpose to currents with an angle ranging from 0 to 90°. For instance, collinear waves and currents were investigated, among others, by [1]–[4]; in particular [2] showed that the ratio between the current and the wave orbital velocities strongly influenced the bed shear stresses depending on the nature of the bottom boundary layer. More recently [3] observed that the superposition of a steady current over an oscillatory flow reduced both the current directed velocity and the intensity of the vortex shedding.