Extreme waves and modulational instability: wave flume experiments on irregular waves

We discuss the formation of large amplitude waves for sea states characterized by JONSWAP spectra with random phases. In this context we discuss experimental results performed in one of the largest wave tank facilities in the world. We present experimental evidence that the tail of the cumulative probability function of the wave heights for random waves strongly depends on the ratio between the wave steepness and the spectral bandwidth. When this ratio, called the Benjamin-Feir Index, is large the Rayleigh distribution clearly underestimates the occurrence of large amplitude waves. Our experimental results are also successfully compared with previously performed numerical simulations of the Dysthe equation.