Bedload transport, self acceleration, downstream sorting, and flow dynamics of turbidity currents

The four chapters presented in this dissertation are self contained, yet related to each other. The first chapter of this dissertation reports on the first set of experiments which focus on bedload transport of granular material by density underflows. These underflows include saline density flows, hybrid saline/turbidity currents and a pure turbidity current. The use of dissolved salt is a surrogate for suspended mud that is so fine that it does not settle out readily. Thus all the currents can be considered to be model turbidity currents. The data cover four bed conditions: plane bed, dunes, upstream-migrating antidunes and downstream-migrating antidunes. The bedload transport relation obtained from the data is very similar to those obtained for open channel flows, and in fact is fitted well by an existing relation determined for open channel flows. In the case of dunes and downstream-migrating antidunes, form drag falls in a range that is similar to that due to dunes in sand-bed rivers.The second chapter presents novel results of self-accelerating turbidity currents. This chapter documents experimental turbidity currents composed of lightweight plastic particles ranging from 20 mum to 200 mum with a specific density between 1.3 and 1.5. These particles were either non-cohesive, or slightly cohesive. The experiments were performed in a 15 m long flume with a bottom of slope of 0.05. Self-acceleration of the head of the flow was achieved in some of the tests reported here. Measurements of velocity and suspended sediment taken at different stages of head evolution document this self-acceleration. In addition, these measurements are in agreement with previous empirical studies relating head thickness, concentration, velocity, and water depth. Stratigraphie analysis of the deposit shows the key role bed material plays in determining whether a given turbidity current will or will not accelerate. This factor ties the dynamics of a self-accelerating current to the existence of deposits laid down by antecedent currents. The conditions of the present tests appear to fulfill previous autosuspension criteria relating flow velocity, particle settling velocity and bed slope. Densimetric Froude number similarity analysis is used to estimate equivalent parameters for field scale turbidity currents.The third chapter focuses on a numerical model of turbidity currents with reversing buoyancy, i.e. flows which are rendered heavier than the ambient water due to the presence of suspended sediment, but lighter than the ambient water due to a difference in temperature or salinity. 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 non-reversing 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.The last chapter is the first part of a joint paper about the formation of internal deltas, or sediment wedges, by turbidity currents in submarine minibasins.The experiments reported here are performed in configuration to test the concept of an internal delta. Dissolved salt is used as a surrogate for the finer phase, and lightweight plastic particles are used as a surrogate for the coarser phase. The plastic particles are carried mostly as bedload. Succession sustained flows are allowed to create a deposit behind a barrier that simulates the downstream end of a minibasin. The flows are introduced in the supercritical regime the downstream barrier forces an internal hydraulic jump to the subcritical regime. This chapter concentrates on (a) the flow pattern created by the barrier and (b) the evolution of this flow pattern as the bed evolved morphodynamically. The documentation of the internal delta itself is given in a companion paper. (Abstract shortened by UMI.)...