XXXIII Convegno Nazionale di Idraulica e Costruzioni Idrauliche Brescia, 10-15 settembre 2012 WAVE AND TURBULENT REYNOLDS STRESSES I N I RREGULAR SHOALI NG WAVES M. Ben Meftah 1 , F. De Serio 1 , M. Mossa 1 & A.F. Petrillo 1 (1) Water Engineering and Chemistry Department, Technical University of Bari, Italy, e-mail: m.ben.meftah@poliba.it,f.deserio@poliba.it, m.mossa@poliba.it, a.petrillo@poliba.t SOMMARIO The dynamics of regular breaking waves has been both widely and successfully investigated. In any case, many natural coastal processes are commonly due to irregular breaking waves, the behaviour of which requires thorough study. The present research aims to investigate the distributions of the wave and turbulent Reynolds shear stresses in a laboratory irregular wave, characterized by a narrow banded spectrum, which develops on a sloping sand bottom, in intermediate waters. Experiments focused on the wave shoaling region, in order to analyze the effects of breaking induced turbulence outside the surf zone, taking into account that turbulence is not limited to the breaking region but it spreads also outside the surf zone. The phase-averaging technique was used to separate the turbulent components from the steady ones. All the analysed values derive directly from real measurements and are not interpolated. Moreover, a 3D Acoustic Doppler Velocimeter was adopted to measure the wave velocity, consequently the longshore component of the velocity is also available to estimate the shear stresses. These experimental data were also used to test some literary numerical models and relevant results have been obtained, which confirm the outputs of the abovementioned models in the cases of non dissipative waves propagating above a flat bottom and dissipative waves propagating over a sloping bottom. 1 I NTRODUCTI ON Many coastal processes, such as undertow currents, sediment transport and action on maritime structures are principally due to irregular waves, a thorough understanding of which is required. Anyhow, velocity data under irregular breaking waves are almost non-existent (Ting, 2001). The aim of the present paper is to analyse the turbulent behaviour of a narrow banded irregular breaking wave, propagating over a sloping and erodible bottom, in intermediate waters up to the surf zone. Firstly an analysis of wave and turbulent Reynolds stresses is performed, particularly focusing on their order of magnitude, which is a point not yet clear. Secondly, the models by Zou et al. (2006) are taken into account and used to reproduce the wave Reynolds stresses. The comparison with the measured values allows the calibration of some model parameters and shows that the behaviour of these cross-correlations is different along the vertical, being differently affected by wave breaking and bottom roughness. Some interesting conclusions were derived. As known, estimating the turbulent components from instantaneous velocity data in irregular waves is challenging. In fact separating the wave-induced and turbulent