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[1] 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 κ − ɛ 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.

[1] This paper is devoted to a morphodynamic model of incision into a reservoir deposit driven by partial or total removal of the associated dam. The model considers the erosional processes upstream of the position of the former dam, rather then the deposition that occurs downstream. A theory is developed to predict the evolution of both the width and depth of the incisional channel that develops as erosion progresses. The theory is implemented in a numerical model, which is tested against and verified with flume experiments on sudden, complete removal of a dam. In these experiments a channel of a given initial width is allowed to freely incise into a noncohesive reservoir deposit after sudden dam removal. These experiments show a phenomenon that we refer to as “erosional narrowing.” That is, as a channel of a given initial width rapidly incises into the deposit, it can become narrower. As the rate of incision slows, this short period of rapid narrowing is followed by a longer period of widening. In the model the incisional channel is abstracted to a trapezoidal channel with well-defined bed and bank regions, both of which are allowed to erode. Balance between bed and bank erosion plays a key role in the morphodynamics of the channel. More specifically, rapid erosion of the bed can cause the channel to narrow even as bank erosion progresses. As the rate of bed erosion slows, bank erosion causes channel widening. This observed pattern is explained in the context of a theoretical model tested against the experiments.

[1] A one-dimensional mathematical model of mudflow dynamics is formulated assuming mud to behave rheologically as a Herschel-Bulkley fluid. Novel features of the present contribution regard the propagation of mudflows over a curvilinear bottom. A set of nonlinear partial differential equations is found to govern the unsteady, nonuniform flow of such fluids. Following the approach of Huang and Garcia (1998), a depth-integrated form of the continuity equation and shear-integrated and plug-integrated forms of the momentum equations are derived. A perturbation analysis is also carried out under the assumption of “slender” flow. The leading-order problem takes the form of a kinematic wave equation. One straightforward application of this simplified model is the evaluation of the maximum run-out extension of a mud which flows over a bed with a variable topography. This solution is obtained by solving the steady state problem. Dynamics information are obtained by numerically solving the hyperbolic conservative equation by means of a free oscillating numerical method. The effects of bottom curvature either on the equilibrium solution or mudflow dynamics are analyzed. Experiments are also performed releasing a given volume of kaoline and water mixture from a reservoir into both a constant slope and an upward concave channel. Comparisons between the numerical solution and the experimental data are generally satisfactory, though suggesting limitations typical of any approach that neglects convective accelerations and assumes gradually varied flow.

[1] The present paper reports on a laboratory investigation of the erosion of a deltaic front induced by the removal of a dam. We built a laboratory model of a dam, and observed both the sedimentation in the reservoir due to the downstream propagation of a delta front and the erosion of the delta front during dam removal, including measurement of channel morphology and flow field. The experiments provide a detailed view of a phenomenon that has not been described in detail to date: erosional narrowing. After the sudden removal of a dam, the flow incises into the reservoir deposit, first rapidly and then more slowly. During the initial period of rapid incision the width of the channel can in some circumstances undergo rapid and substantial narrowing, all the while incorporating sediment from its sidewalls. As the rate of incision slows, the channel first stops narrowing and then enters a phase of slow widening. This pattern of narrowing followed by widening tends to propagate upstream. The minimum channel width attained at a cross section, however, increases with upstream distance from the dam. While the period of erosional narrowing is very short, the incision is so intense that large amounts of sediment are delivered downstream in a short period of time. The process thus has practical implications in regard to the strategy of dam removal. Undistorted Froude similitude is used to scale the results up to field dimensions.