Waves plus currents at a right angle: The rippled bed case

Abstract

[1] The present paper deals with wave plus current flow over a fixed rippled bed. More precisely, modifications of the current profiles due to the superimposition of orthogonal cylindrical waves have been investigated experimentally. Since the experimental setup permitted only the wave dominated regime to be investigated (i.e., the regime where orbital velocity is larger than current velocity), also a numerical k-e turbulence closure model has been developed in order to study a wider range of parameters, thus including the current dominated regime (i.e., where current velocity is larger than wave orbital one). In both cases a different response with respect to the flat bed case has been found. Indeed, in the flat bed case laminar wave boundary layers in a wave dominated regime induce a decrease in bottom shear stresses, while the presence of a rippled bed behaves as a macroroughness, which causes the wave boundary layer to become turbulent and therefore the current velocity near the bottom to be smaller than the one in the case of current only, with a consequent increase in the current bottom roughness.

Waves plus currents crossing at a right angle: near-bed velocity statistics

“…The bedform dimensions were chosen in such a way as to be in agreement with the characteristics of ripples generated during the experimental campaign performed by Andersen and Faraci (2003), in the presence of orthogonally superimposed waves and currents characterized by the same ratio between current velocity and wave orbital velocity. In particular it was found that the chosen bed form characteristics are typical for waves with periods between 1 and 1.4 s and heights of about 0.1 m propagating over a depth of about 0.3 m. The detailed experimental data of the RB campaign have been reported in Faraci et al (2008).…”

“…Opposite the wavemaker a porous plane beach limits the reflection to about 10% of the incident wave. For a detailed description please refer to Faraci et al (2008) or Faraci, Foti, Marini, and Scandura (2012).…”

“…Mathiesen and Madsen (1996) compared the wave roughness with that of a combined flow, and found that in the case of combined flow the roughness is similar to that of a pure wave and a single roughness scale can be considered. Fredsøe et al (1999), by means of both experimental results and a κ − ω turbulence closure model, found an increase up to one order of magnitude in the bed roughness when waves are superimposed to a current, while Faraci et al (2008) found that a rippled bed behaves like a macro-roughness towards the current, causing the wave boundary layer to become turbulent. Madsen et al (2010) observed that when wave induced bedforms are at an angle with the current, the near bottom flow tends to veer parallel to the ripple axis; a dramatic increase of the roughness experienced by a rippleparallel current is also observed when near perpendicular waves are present, due to the non-linear interaction of the velocity components.…”

“…Waves plus currents over rippled beds have been investigated, among others, by Ranasoma and Sleath (1994), Mathiesen and Madsen (1996), Fredsøe, Andersen, and Sumer (1999), Faraci, Foti, and Musumeci (2008) and Madsen, Negara, Lim, and Cheong (2010). Ranasoma and Sleath (1994) performed LDA measurements of orthogonal wave-current flow over fixed ripples.…”

“…In order to understand the role played by the bed roughness, three different fixed bottom conditions are considered, namely a sandy bed, a gravel bed and a rippled bed. The present analysis is performed using an existing experimental dataset (Faraci et al, 2008;Musumeci et al, 2006), acquired by the authors but never analysed in this perspective. In particular, this study is focused on the influence of the waves on the current properties.…”