The energy level and its directional distribution are key observations for understanding the energy balance in the wind-wave spectrum between wind-wave generation, nonlinear interactions, and dissipation. Here, properties of gravity waves are investigated from a fixed platform in the Black Sea, equipped with a stereo video system that resolves waves with frequency f up to 1.4 Hz and wavelengths from 0.6 to 11 m. One representative record is analyzed, corresponding to young wind waves with a peak frequency f p 5 0.33 Hz and a wind speed of 13 m s 21 . These measurements allow for a separation of the linear waves from the bound second-order harmonics. These harmonics are negligible for frequencies f up to 3 times f p but account for most of the energy at higher frequencies. The full spectrum is well described by a combination of linear components and the second-order spectrum. In the range 2f p to 4f p , the full frequency spectrum decays like f 25, which means a steeper decay of the linear spectrum. The directional spectrum exhibits a very pronounced bimodal distribution, with two peaks on either side of the wind direction, separated by 1508 at 4f p . This large separation is associated with a significant amount of energy traveling in opposite directions and thus sources of underwater acoustic and seismic noise. The magnitude of these sources can be quantified by the overlap integral I(f ), which is found to increase sharply from less than 0.01 at f 5 2f p to 0.11 at f 5 4f p and possibly up to 0.2 at f 5 5f p , close to the 0.5p value proposed in previous studies.

The Weak Turbulence Theory is a statistical framework to describe a large ensemble of nonlinearly interacting waves. The archetypal example of such system is the ocean surface that is made of interacting surface gravity waves. Here we describe a laboratory experiment dedicated to probe the statistical properties of turbulent gravity waves. We setup an isotropic state of interacting gravity waves in the Coriolis facility (13 m diameter circular wave tank) by exciting waves at 1 Hz by wedge wavemakers. We implement a stereoscopic technique to obtain a measurement of the surface elevation that is resolved both in space and time. Fourier analysis shows that the laboratory spectra are systematically steeper than the theoretical predictions and than the field observations in the Black Sea by Leckler et al. JPO 2015. We identify a strong impact of surface dissipation on the scaling of the Fourier spectrum at the scales that are accessible in the experiments. We use bicoherence and tricoherence statistical tools in frequency and/or wavevector space to identify the active nonlinear coupling. These analyses are also performed on the field data by Leckler et al. for comparison with the laboratory data. 3-wave coupling are characterized and shown to involve mostly quasi resonances of waves with second or higher order harmonics. 4-wave coupling are not observed in the laboratory but are evidenced in the field data. We finally discuss temporal scale separation to explain our observations. * nicolas.mordant@univ-grenoble-alpes.fr 2