The threshold of movement of a, sand bottom under random waves is analysed through an experimental investigation carried out in a wave tank by adopting proper wave groups. The results obtained show that the regular spectral waves associated with the sea, states corresponding to the groups are able to interpret, the phenomenon. Thus the mathematical models available for regular waves can be used to characterise the threshold of sand motion under random waves. Further experiments, already scheduled, will help to confirm the present results.
Knowledge of the response of a non cohesive bottom subjected to wave action is of considerable importance for coastal engineering as regards preservation and protection of coasts. The problem has been dealt with, theoretically and experimentally, by different authors, bearing in mind that theoretical research generally considers regular waves whereas experimental research takes into account both regular and irregular waves, that is to say random waves. Since the pioneer work of Bagnold (1946), research into regular waves has reached good levels of accuracy (Sleath, 1984; Blondeaux, 1990; Vittori and Blondeaux, 1990; Daprà, 1991; Kos"yan and Kochering, 1992; Kawata, Shirai and Tsuchiya, 1992) whereas research into random waves, only more recently undertaken, has certainly led to interesting results (Nishi, Sate and Nakamura, 1990; Ito, Murakami and Ito, 1995) but not yet definitive. Among the studies referred to regular waves, mention is made of the investigation undertaken by Rubatta (1964, 1965) on the threshold of movement which is able to provoke the global motion of a non cohesive bottom (threshold of global movement).