Study on the Unsteady characteristics of Turbulence Structures of Flood Flows

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The field observation of flow structure, river bed configurations and sediment transport during aflood stage are very important. In the Hii River, the flow patterns on the flow surface during a flood stage were taken by video camera which was fixed on a top of a ladder fixed vertically at the downstream rail of the bridge, and almost simultaneously, the three-dimensional river-bed configurations were measured by a echo-sounder attached to a cross-type float which traveled downstream about 50 m at the mean velocity of flood flow. The measurements were made every 2 or 3 hours during a flood. 9 flood measurements were made from 1993 to 1998.The measurement of surface velocity distributions were made over the area of about 15m x 15m. Velocity vectors on the water surface were calculated by applying two-dimensional cross-correlation method to two images taken in a short span of time. The velocity distributions, the stream lines, the divergence distributions and the vorticity distributions were calculated by using the obtained velocity vectors.Based on the data obtained by the image processing technique, the characteristics of boils, vortex motions and parallel spiral flows with longitudinal axes and their time variation during a flood were examined, By using the data obtained by the measurements of river bed, the multiple structure of sand waves, the time variation of wave height and wave length of dunes, the velocity of sand waves and the sediment discharge by the movement of dunes were examined. Manning's roughness coefficient were calculated by using the obtained flow depth, mean velocity and water surface slope, and it is found that Manning's roughness coefficient was maximum when the wave height of dunes reached the highest level.