EXPERIMENTS ON THE ENTRAINMENT OF SEDIMENT INTO SUSPENSION BY A DENSE BOTTOM CURRENT

Experiments on the entrainment of bed sediment into suspension by density underflows are described. The density underflows were created by allowing saline water to move as a steady, continuous current under a body of essentially stagnant fresh water. The bed slope was set at 0.08. Two grades of fine crushed coal, with geometric mean sizes of 100 mum and 180 mum, were placed in a trough in the bed with a length of 2 m, over which the density currents passed. Measurements of profiles of flow velocity and concentration of suspended sediment allowed for a back calculation of boundary shear stress and the rate of sediment entrainment from the equations of momentum and mass balance. Two techniques for removing the effect of form drag from the boundary shear stress were explored. The resulting data were used to compare sediment entrainment rates due to density underflows with those observed in open channels. It appears possible to make rather modest adjustments in an existing predictive relation for sediment entrainment in open channels, so as to predict the case pertaining to density underflows as well.