The instability of wave trains propagating over an onlique current: a laboratory experiment in a directional wave basin

Extreme waves represent a serious threat for marine structures and operations. Numerical and theoretical work has already demonstrated that the modulational instability plays a relevant role in the formation of extreme waves (Janssen 2003, Onorato et al. 2006, Onorato et al. 2001). However, strong deviations from Gaussian statistics can only be expected if waves are rather long crested i.e. the spectral energy is concentrated on a narrow range of directions (Onorato et al. 2002, Socquet-Juglard et al. 2005, Onorato et al. 2009). For more realistic short crested seas (i.e. broad directional distributions), the effect of modulational instability becomes less prominent and, as a result, the occurrence of extreme waves does not exceed predictions from second-order theory (e.g. Socquet_juglard et al. 2005). This transition between strongly to weakly non-Gaussian behavior is determined by a balance between nonlinearity (which promotes non-Gaussian behavior) and directionality (which suppresses non-Gaussian behavior). Thus, if there are circumstances when the nonlinearity is locally enhanced, we can expect that non-Gaussian behavior would persist also at broader directional spreads. In this respect, when waves propagates against an ambient current, wave steepness, and hence nonlinearity, increases as a consequence of the shortening of the wavelength, making nonlinear processes, such as the modulational instability mechanism, more likely. A number of laboratory experiments have been carried out to verify the behaviour of regular and irregular waves when opposing a strong current. Most experimental results until now have been obtained in wave flumes, where only one-dimensional propagation can be addressed. For the present study, we have accessed one of the largest directional wave tank in the world to address the more general two dimensional problem, where a multi directional wave field propagates obliquely over a uniform current in partial opposition. The aim is to explore the role of increasing wave steepness due to wavecurrent interaction on the modulational instability mechanism and the formation of large amplitude waves.