Modeling turbidity currents with nonuniform sediment and reverse buoyancy

This study focuses on a numerical model of turbidity currents with reversing buoyancy, i.e., flows that are rendered heavier than the ambient water due to the presence of suspended sediment but that as sedimentation progresses become lighter than the ambient water due to a difference in temperature or salinity. The flows considered here may be either pulsed events or continuous flows. It is well known that these flows are subject to a buoyancy reversal. Flows issuing upstream with a sufficient load of suspended sediment are heavier than the ambient water and thus form a bottom underflow. As sediment deposits in the downstream direction, however, the flow gradually loses its density excess, and eventually reverses its buoyancy, detaching upward from the bed. The nature of the sediment deposit emplaced near and downstream of the point of lofting can thus differ significantly from that emplaced upstream. A novel aspect of the work reported here is a mechanistic description of the tendency for the current to sort in the downstream direction a sediment mixture containing a wide range of grain sizes. The numerical model, which is based in the kappa - epsilon closure for turbulence, is verified with a unique set of experimental data intended to model sediment sorting associated with turbidity currents created by explosive undersea eruptions. Both cases with nonreversing buoyancy and reversing buoyancy are considered. As such, the model not only provides a detailed description of flows with reversing buoyancy, but also provides a tool to aid sedimentologists in back-calculating the flow emplaced by turbidity currents from the downstream variation in the grain size distribution of the bed deposit.