INTERNAL VELOCITY FIELD OF DAM-BREAK FLOWS ON DRY BED AND ESTIMATION OF BED SHEAR STRESS USING SPACE-TIME AVERAGE

Abstract

 In this study, the internal velocity field of dam-break flows on a dry bed was investigated, to estimate bed shear stress using the space-time average. The wave number spectrum of velocity fields at the same height was calculated from the PIV results. The width for spatial average was decided based on the peak wave number to calculate the vertical profiles of mean horizontal velocity and Reynolds stress. The profiles indicated that layers under y = 2 mm strongly affected by the bottom friction. The bed shear stress calculated from the friction velocity was of the same order as that obtained from the Manning formula, but indicated larger fluctuations with time variation. Further, it was indicated that bed shear stress caused by friction velocity was related to the local water surface slope.