An example of high-velocity open channel flows is a supercritical flow past ail abrupt drop. In such a geometry, the basic air-water flow properties were measured, including distributions of void fraction and bubble count rate, and local air and water chord size distributions, at and downstream of the backward-facing step. The bubble count rate distributions were compared with a conceptual model of streamwise distribution of air and water chords which yields a quasi-parabolic relationship between bubble count rate and void fraction. The proposed model was an attempt to explain the experimental relationship between bubble count rate and void fraction, rather a meticulous breakdown of the complex air-water structure. (c) 2008 Elsevier Ltd. All rights reserved.