Extreme wave groups in a wave flume: Controlled generation and breaking onset

Extreme waves in random seas are usually breaking or close to breaking. Understanding the kinematics and evolution of such waves is important for determining loads on offshore structures. Controlled repeatable generation of realistic breaking waves in wave flume experiments is a difficult but important task. It is rather easy to generate an arbitrary breaking wave, but to the authors' knowledge there is no methodology for accurate generation of a wave group with a pre-defined spectrum related to a modelled sea state with spilling breaking at a prescribed position. Such waves can be used to model extreme breaking waves in a random sea and their interaction with structures. This paper offers such a methodology. The key feature of the method is the application of an iterative focussing procedure to a linearised amplitude spectrum rather than to a full nonlinear spectrum. The linearised spectrum is obtained using a harmonics separation technique and the general derivation of the method is given for an arbitrary number of components. The procedure is applied to generate focussed wave groups with amplitudes increased in small steps until local crest breaking occurs. As a result, the highest non breaking waves and weakly breaking waves are generated for otherwise identical conditions. The methodology is applied for four different wave spectra of the same peak frequency: JONSWAP, Pierson-Moskowitz, wide and narrow band Gaussian. It is found that steepness of the limiting breaking wave depends strongly on the choice of wave group spectrum. The results demonstrate that neglecting spectral properties of design waves may lead to misrepresentation of their breaking behaviour.