In the present paper, we propose an image processing technique for simultaneously measuring instantaneous velocity vectors and the corresponding water-surface profile in an open-channel flow. This technique utilizes images in which both the tracer particles in the flowing water and the water-surface profile are visualized by a laser light sheet. A boundary-detecting algorithm, in which gradient vectors of the image luminance and two thresholds are used for extracting image boundaries, is employed in the image processing for identifying the air-water interface. Combining this algorithm with a particle image velocimetry (PIV) allows us to simultaneously measure the water-surface heights and the velocity vectors. This newly developed measuring technique is then used to a supercritical open-channel flow over a concaved bed for the purpose of examining its applicability to rapid flows with strong water-surface fluctuations. Instantaneously measured data sets of water-surface heights and flow velocity vectors provide a lot of information on the flow dynamics with water-surface fluctuations. Analyses of the mean velocity, mean water surface and turbulent quantities show that (1) in the upstream region of the concave, the velocity fluctuations generated by shear instability are dominant between the main-channel and the concave, (2) the mixing shear layer develops in the middle to downstream regions, and (3) the velocity fluctuations are well correlated to the organized motions as well as the water-surface fluctuations especially in the downstream region. The space-time correlation analysis suggests that there is a strong interaction between the water-surface fluctuation and the organized motion in the mixing shear layer. It is strongly confirm from these results that the present technique is very useful in examination of rapid flows with significantly fluctuating water-surface.