Abstract
The results of the experimental investigation of the coupling of two‐ and three‐dimensional instabilities of surface gravity waves with initial wave steepness of 0.09≤a0k0≤0.20 are reported. The essentially two‐dimensional instabilities (type I) are observed to cause sufficient wave train modulations to trigger the predominantly three‐dimensional instabilities (type II). Type II, in turn, limits the growth of the type I and leads to wave breaking and the directional spreading of the wave energy. Hence, type I and II instabilities interact strongly during the evolution of wave trains with moderate initial steepness.