A field experiment on the mechanics of irregular gravity waves

In the paper the experiment on the quasi-determinism theory of the wave groups, introduced by Boccotti (1998), is described for the field of progressive waves. The theory predicts that when an exceptionally high wave occurs in a fixed point at an instant t during a sea storm, with quasi-certainty it is possible to foresee what will happen in an area with sides of the order of some wavelengths around that point, before and after the instant t. In particular, the theory shows that an exceptionally high wave is formed due to the passage of a well defined three-dimensional wave group.The wave group has extremely interesting mechanics: it has a development stage during which the height of the central wave increases to a maximum, thanks to a progressive contraction of the wave front and a phase of decadence with opposite characteristics, during which the wave reduces itself and, as result, the height of the central wave progressively decreases. The single waves stir along the envelopment: they are born in the tail of the group, grow to a maximum when they arrive to occupy the central position of the group and then go to die in the head of group. To "capture" the wave groups and to notice their conformation, nine telescopic piles were built in the sea. Every pile substained two gauges for the measure of the instantaneous water level and the pressure fluctuactions under the water. The ultrasonic probes were located at a level of 1.30 m above the mean water level (M.W.L.). The transducers were set to a depth of 0.50 m under the W.W.L.. The set of columns should have represented a "trap" and the gauges used to verify the waves stired and changed as foreseen by the quasi-determinism theory. The gauges were connected to an electronic acquisition station located at the "Lido Comunale" of Reggio Calabria. The station was constitued by two PCs containing the digital acquisition boards. The data were acquired at 10 samples for second, with a maximum of 16 instruments connected simultaneously.The experiment confirmed that the general configuration of the extreme wave groups measured was consistent with the theoretical predictions in terms of the measured space-time autocovariance. During the development stage of a group, as the height of the central wave grows to a maximum, the width of the wave front reduces to a minimum. As an individual wave passes through the group, its wavelength decreases as the wave height increases towards the apex, after which the wavelength increases again as the wave moves towards the front of the group and abates.