A NUMERICAL-MODEL FOR SEDIMENTATION FROM HIGHLY-CONCENTRATED MULTI-SIZED SUSPENSIONS

The principal numerical approach to describing sedimentation from multi-component suspensions (Mirza and Richardson, 1979) has been applied only to systems containing two or three particle size populations, but is theoretically applicable to suspensions containing a wider range of particle sizes. In order to adapt this model to the simulation of sedimentation from natural high-density sediment-laden flows, we have conducted computational tests on the sedimentation of suspensions with up to ten particle size populations. The tests run smoothly with binary systems and sometimes ternary systems, but fail with systems containing more than three particle size populations. One cause of the discrepancy between theoretical predictions and computational tests arises from the method used to calculate the changing concentration of particles in each zone of a stratified settling suspension. In quantifying the changing sediment budget for each sedimentation zone, the equation set fails to include the apparent outflow of finer particle populations across the upper boundary of the zone. In the present study, we express the sediment budget of each particle population within each sedimentation zone as the net sediment flux, the algebraic sum of the apparent particle inflow from the zone's lower boundary and the apparent particle outflow across the zone's upper boundary. This revised model successfully predicts the evolution of multi-component suspensions containing up to ten particle size populations over the length of time required for complete sedimentation, up to 8 x 10(5) seconds. It can be used to predict the sediment sorting and the vertical textural variation of beds formed by the simulated sedimentation of a multi-component suspension. The model provides the basis for future computer simulation of sedimentation from highly concentrated sediment flows and for the prediction of downslope textural and structural variations of turbidites.