Search

High-resolution bathymetry, seismic reflection, and piston core data from a submarine channel on the western Niger Delta slope demonstrate that thick, coarse-grained, amalgamated sands in the channel thalweg/axis transition to thin, fine-grained, bedded sands and muds in the channel margin. Radiocarbon ages indicate that axis and margin deposits are coeval. Core data show that bed thickness, grain size, and deposition rate strongly decrease with increasing height above channel thalweg and/or distance from channel centerline. A 5 times decrease in bed thickness and 1–2 ψ decrease in grain size are evident over a 20 m elevation change (approximately the elevation difference between axis and margin). A simplified in-channel sedimentation model that solves vertical concentration and velocity profiles of turbidity currents accurately reproduces the vertical trends in grain size and bed thickness shown in the core data set. The close match between data and model suggests that the vertical distribution of grain size and bed thickness shown in this study is widely applicable and can be used to predict grain size and facies variation in data-poor areas (e.g., subsurface cores). This study emphasizes that facies models for submarine channel deposits should recognize that grain-size and thickness trends within contemporaneous axis-margin packages require a change in elevation above the thalweg. The transition from thick-bedded, amalgamated, coarser-grained sands to thin-bedded, nonamalgamated, finer-grained successions is primarily a reflection of a change in elevation. Even a relatively small elevation change (e.g., 1 m) is enough to result in a significant change in grain size, bed thickness, and facies.

[1] Density underflows in general and turbidity currents in particular differ from rivers in that their governing equations do not allow a steady, streamwise uniform “normal” solution. This is due to the fact that density underflows entrain ambient fluid, thus creating a tendency for underflow discharge to increase downstream. Recently, however, a simplified configuration known as the “turbidity current with a roof” (TCR) has been proposed. The artifice of a roof allows for steady, uniform solutions for flows driven solely by gravity acting on suspended sediment. A recent application of direct numerical simulation (DNS) of the Navier-Stokes equations by Cantero et al. (2009) has revealed that increasing dimensionless sediment fall velocity increases flow stratification, resulting in a damping of the turbulence. When the dimensionless fall velocity is increased beyond a threshold value, near-bed turbulence collapses. Here we use the DNS results as a means of testing the ability of three Reynolds-averaged Navier-Stokes (RANS) models of turbulent flow to capture stratification effects in the TCR. Results showed that the Mellor-Yamada and quasi-equilibrium k-ϵ models are able to adequately capture the characteristics of the flow under conditions of relatively modest stratification, whereas the standard k-ϵ model is a relatively poor predictor of turbulence characteristics. As stratification strengthens, however, the deviation of all RANS models from the DNS results increases. All are incapable of predicting the collapse of near-bed turbulence predicted by DNS under conditions of strong stratification. This deficiency is likely due to the inability of RANS models to replace viscous dissipation of turbulent energy with transfer to internal waves under conditions of strong stratification. Within the limits of modest stratification, the quasi-equilibrium k-ϵ model is used to derive predictors of flow which can be incorporated into simpler, layer-averaged models of turbidity currents.

[1] In the present work, we use a three-dimensional numerical model to simulate turbidity currents in a large-scale submarine environment. The model solves the Reynolds Averaged Navier–Stokes equations, along with a two-equation turbulence closure model, the sediment conservation equations for multiple grain-size classes and the Exner equation of bed sediment conservation. Four different grain-size classes (30, 64, 125, and 250μm) are considered. The model is applied to a modern seafloor environment in the continental slope of the Niger Delta where bathymetric data and seven piston cores were collected in and around a sinuous channel. Detailed analyses show the grain-size distribution for different beds within each core. Since the flow events responsible for the formation of this modern depository are unknown, we perform simulations with different inflow conditions in order to match the grain-size data. The model realistically predicts the flow field and its evolution over the complex topography and shows the spatial distribution of different grain-size classes. A strong lateral flow from the inner to the outer bank is observed at the bends of the low-relief channel. From the computed deposition rate, we obtain the fraction of each grain-size class at the core locations and compareD50 and D90 predicted from the model with the core data. The discrete values predicted from the model fall within the range of D50 and D90observed in different beds in the cores. The results demonstrates that a 3-D numerical model can be a useful tool for understanding the distribution pattern, thickness, and grain size of the turbidity currents and their deposits at the field scale and can help constrain the variabilities of reservoir architecture.

[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] In this work we present direct numerical simulations (DNS) of sediment-laden channel flows. In contrast to previous studies, where the flow has been driven by a constant, uniform pressure gradient, our flows are driven by the excess density imposed by suspended sediment. This configuration provides a simplified model of a turbidity current and is thus called the turbidity current with a roof configuration. Our calculations elucidate with DNS for the first time several fascinating features of sediment-laden flows, which may be summarized as follows. First, the presence of sediment breaks the symmetry of the flow because of a tendency to self-stratify. More specifically, this self-stratification is manifested in terms of a Reynolds-averaged suspended sediment concentration that declines in the upward normal direction and a Reynolds-averaged velocity profile with a maximum that is below the channel centerline. Second, this self-stratification damps the turbulence, particularly near the bottom wall. Two regimes are observed, one in which the flow remains turbulent but the level of turbulence is reduced and another in which the flow relaminarizes in a region near the bottom wall, i.e., bed. Third, the analysis allows the determination of a criterion for the break between these two regimes in terms of an appropriately defined dimensionless settling velocity. The results provide guidance for the improvement of Reynolds-averaged closures for turbulent flow in regard to stratification effects. Although the analysis reported here is not performed at the scale of large oceanic turbidity currents, which have sufficiently large Reynolds numbers to be inaccessible via DNS at this time, the implication of flow relaminarization is of considerable importance. Even a swift oceanic turbidity current which at some point crosses the threshold into the regime of relaminarization may lose the capacity to reentrain sediment that settles on the bed and thus may quickly die as it loses its driving force.

[1] Intricately meandering channels of various scales constitute a major morphological feature of the submarine slope and fan systems. These channels act as conduits of density-driven gravity underflows and in turn are shaped by these underflows. The relationship between channel curvature and the dynamics of sediment-laden underflows commonly known as turbidity current has been an enigma, and recently, a subject of controversy. This contribution unravels the flow field of turbidity current at submarine channel bends captured from large scale laboratory experiments. The experimental results show that a mildly sloping channel bank greatly enhances the tilt of the turbidity current-ambient water interface, so much so that the current completely separates from the convex or the inside bank. We also show that irrespective of the shape of the channel cross section, two cells of helical flow appear in confined submarine bend flow. The near-bed cell has a circulation pattern similar to that observed in fluvial channels; the other cell has an opposite sense of rotation. If, on the other hand, a portion of the flow detaches from the body of the current and spills to the concave or outside overbank area, the upper circulation cell becomes suppressed by the resulting lateral convection.

[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.