Display omitted •Detailed air-water flow measurements were performed in a highly turbulent free-surface flow.•Three velocimetry approaches were applied, including intrusive and non-intrusive.•OF data were obtained from both side and top views.•Dual-tip phase-detection probe data were used to validate optical flow (OF) data.•The results highlighted both advantages and limitations of the three complementary metrologies. Self-aerated free-surface flow studies have a more recent history compared to classical fluid dynamics. Traditional velocimetry techniques are adversely affected by the presence of gas–liquid interfaces. In the present study, detailed air-water flow measurements were performed in a highly turbulent free-surface flow, and three velocimetry approaches were applied: (a) centreline dual-tip phase-detection needle probe measurements at all step edges downstream of the inception location of free-surface aeration; (b) Optical Flow (OF) data based upon ultra-high-speed video movies through the left sidewall; and (c) Optical Flow (OF) data based upon ultra-high-speed video movies overlooking the self-aerated flow, in a direction normal to the pseudo-bottom formed by the staircase profile. The study was conducted in a steep channel and three stepped invert geometries were tested. The results highlighted both advantages and limitations of the three complementary metrologies in free-surface flows with strong turbulence. The dual-tip phase-detection probe delivered reliable interfacial velocity data, but in the form of point measurements. The sideview OF technique provided a great level of details of the cavity recirculation and shear zone between mainstream and cavity, but the data were unreliable for void fractions>0.30 and only limited to the sidewall region. The top view OF technique characterised the surface velocity field across the entire chute width, highlighting the occurrence of three-dimensional air-water surface patterns. One observed limitation of the OF was the requirements of a high frame rate (i.e. 10,000 fps or more) and high-quality light source. Overall, these measurement techniques provided complementary results for a better understanding of the physical behaviour of highly turbulent multiphase flows on stepped channels.