Abstract
In estimating river flow discharges from water surface velocity distributions measured by non-contact methods such as radio wave velocity meters or image-based techniques, it is important to use an appropriate value of surface velocity coefficient that yields a depth-averaged velocity from a surface velocity. Conventionally, a value of 0.85 has been widely used in practical purposes but its distribution feature under various conditions has not been clarified yet. In order to investigate a fundamental aspect of the coefficient, measurement data by a tethered ADCP, a remote-controlled ADCP and surface velocities obtained by space-time image velocimetry(STIV) as well as a three-dimensional numerical simulation data were compared for the floods of the Uono River and the Niyodo River. It was made clear from measured and numerical data that the variance of the coefficient distribution tends to increase with decrease of the relative water depth and vice versa.
