We have proposed an image processing technique to measure simultaneously the water-surface and velocity vectors in open-channel flow. A boundary growing process is employed to extract the locations of water-surface using the gradient of luminance in the image and two threshold values of gradient. Simultaneously with the water-surface, the velocity field in the channel is measured using a particle image velocimetry.
This measuring technique is applied to the supercritical flow in the channel with a concave bed. Analysis of the instantaneously measured data of water depth and flow velocity has revealed that the fluctuation of water-surface and the velocity field interact each other. These results suggest that the present method is well applicable to rapid flows with the fluctuating water-surface.