Entraining Flow of a Concentrated Benthic Suspension

Abstract

The Prandtl mixing-length model of turbulent exchange of mass and momentum is applied to calculate the entrainment of overlying water into a layer of suspended fine sediment at a horizontal bed. In the field the flow and turbulence in such a concentrated benthic suspension (CBS) are driven by a streamwise pressure gradient resulting from the tide. However, in this report a proposed laboratory experiment is simulated in which flow and turbulence in the CBS are driven by a movable bottom screen which is started instantaneously from rest. The aims of the calculations are to show the feasibility of this laboratory experiment, to carry out a sensitivity analysis and, in future work, to compare with experimental results. Damping functions accounting for the reduction in turbulent exchange caused by density stratification are calibrated using results of entrainment experiments with two-fluid systems reported in the literature. The adopted modelof the rheological behaviour of the CBS is of the Herschel-Bulkley type. In addition, the effects of hindered settling and sidewall friction are included. Entrainment rates are found to be particularly sensitive to the speed of the screen and the excess weight of the sediment, whereas the rheological model has little influence.