Two-dimensional turbidity current model

A two-dimensional numerical model is described for flow of and deposition from a turbidity current (low to high concentration) with non-cohesive sediment. The bulk fluid velocity (sediment+water) is calculated from a momentum equation that is derived from the Navier-Stokes equation using the hydrostatic and thin shear layer approximations. Sediment concentrations are calculated with advection-diffusion equations for each grain size and turbulence is taken into account by using the Mellor-Yamada level 2½ second order closure model. Particles have an effect on the flow through the fluid density and viscosity and through the turbulent mixing term. Apart from the stratification effect on turbulence, an extra dissipation term due to particle presence, is incorporated in the turbulence equations through a drag term that is derived from a multiphase flow approach. The equations are solved numerically using a finite volume method on a non-staggered grid with a predictor-corrector method (for testing the sediment model) and with a second-order BW method (for the turbidity current model).; Results from a simplified one-dimensional version of the flow model (for a vertical profile) are compared with data from open-channel flume experiments with high sediment concentration. Results of the turbidity current model are compared with measured grain size distributions in Bute Inlet (British Columbia, Canada) and velocities are compared with times of cable breaks of the 1929 Grand Banks (Newfoundland, Canada) turbidity current. The outcome of the model agrees well with the observations. The model is then applied to look at the flow structure of turbidity currents for several simple cases. Deposits of turbidity currents are described for a number of flows of variable input conditions. Finally results of a sensitivity analysis are shown. All results show that the flow structure of turbidity currents is determined mainly by the sediment distribution, which influences velocity as a result of density and which affects the turbulence through stratification.