Influence of Grain Size on Sediment Transport during Initial Stages of Horizontal Dam Break-Type Flows

The influence of grain size on sediment transport during the initial stages of dam break-type flows over horizontal mobile sediment beds was investigated through dimensional analysis and experiments. A new dimensional analysis reconciles the unbalanced dimensions in Bagnold's formulations describing the grain size influence on steady-flow sediment transport and indicates that the sediment transport is inversely proportional to the square root of the grain size, consistent with Bagnold's empirical correlation. Total transport rates were measured for quartz grains ranging from 0.22 to 2.65 mm in diameter. Regression between the dimensionless variables indicated that the mobility number collapses the sediment transport data better than the Shields number and indicated that the total sediment transport is approximately inversely proportional to the square root of the grain size and the velocity to the fourth power. The Meyer-Peter Muller transport model based on the Shields number requires transport coefficients for the finest sand that are significantly larger than the conventional values suggested in the literature, between 30 for k(s) = 2.5D(50) and 40 for k(s) = D-50. The values for other sediment sizes are closer to conventional values, within the usual range of 10-12 for k(s) = 2.5D(50), but somewhat higher at 16-22 for k(s) = D-50. The occurrence of sheet flow in a suspension regime appears possible, which may explain the significantly larger transport coefficients required for the finest grain size.