Flood waves resulting from dam breaks and flash floods have been responsible for numerous losses. In the present study, sudden flood releases were investigated down a large stepped waterway initially dry. A new experimental technique was developed to obtain instantaneous void fractions, bubble count rates and velocities. Unsteady air-water flow properties were recorded with arrays of conductivity probes. The results showed a strong aeration of the surge leading edge. Instantaneous velocity measurements indicated an unsteady turbulent boundary layer region with a potential flow above. In the unsteady boundary layer, void fraction and velocity data suggested a ratio of bubble diffusivity to eddy viscosity of about unity. Practically, the study provides new information on free-surface aeration in surging waters in channels and on beach slopes.
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