Statistical analysis of the interaction between wind-waves and currents during early wave generation

We present the experimental analysis of the interaction between wind waves and currents, during the generation process, through laboratory experiments in a wind-waves-current tunnel. The objective is the quantification of the effects of a co-/counter-current on the main characteristics and statistical estimators of the wave field. Twenty-two experiments were performed with two different wind speed values and eleven different current speed values (including zero values, absence of current), with a ratio of current speed to group celerity uc=cg 2 [0:47; +0:30], measuring the instantaneous water level in different sections with ultrasonic probes. The collected data allow the characterization of the free surface statistics, the calculation of the phase and group celerity of the waves, the analysis of the grouping. The aim is the description of the complex interaction between different forcing terms effective during wave generation. We found that (i) spectral shape and evolution is extremely sensitive to tiny counter-currents, with a fast grows of a second peak, (ii) grouping and the statistics of the free surface reflect the action of the current; (iii) energy transfer and breaking are significantly affected by the currents. The results are new and original and represent a set of data for understanding the generation of waves by the wind in all conditions in which the currents are not negligible, for example in the surf zone, lagoons, estuaries, swamps, shallow lakes and shallow reservoirs.