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Turbidity currents are sediment-laden flows that travel over a sloping bed under a stagnant ambient fluid, driven by the density difference between the current and the ambient. Turbidity currents transport large amounts of carbon, nutrients and fresh water through oceans and play an important role in global geochemical cycling and seafloor ecosystems. Supercritical currents are observed in steeper slopes. Subcritical currents are observed in milder slopes, where the near-bed and interface layers are prevented from interacting across the velocity maximum. Past works show the existence of such a barrier to vertical momentum transfer is essential for the body of the subcritical current to extend over hundreds of kilometers in length without much increase in height. Here we observe the body of subcritical currents to have a three layer structure, where the turbulent near-bed layer and the non-turbulent interface layer are separated by an intermediate layer of negative turbulence production. We explain the mechanism by which this layer prevents the near-bed turbulent structures from penetrating into the interface layer by transferring energy back from turbulence to the mean flow., This study investigates the underlying physical mechanisms of turbidity currents travelling thousands of miles in confined submarine settings, rather than diffusing after short distance. Using high resolution simulations with up to a billion grid points helps to understand the evolving layered structure of a current.

Long-running gravity currents are flows that are submerged beneath a deep layer of quiescent fluid and they travel over long distances along inclined or horizontal surfaces. They are driven by the density difference between the current and the clear ambient fluid above. In this work we present results on highly resolved direct numerical simulations of turbid underflows that involve nearly 1 billion degrees of freedom. We assess the effect of bed slope on the flow statistics. We explore the turbulence dynamics of the interface in the classical sub- and supercritical regimes. We investigate the structure of interfacial turbulence and its relation to the turbulence statistic. A transcritical regime is identified where intermittent cascading interfacial instabilities appear. We investigate how departure from the self-sustaining equilibrium state may be the mechanism responsible for this cyclic evolution of the transcritical regime.

Shallow-water deltas remain enigmatic in terms of placing the observed facies within a coherent process-based depositional model. Here we report tank experiments on mouth-bar formation from shallow water pure and stratified jets that, combined with recent flume experiments on bedforms, suggest new interpretations of field observations from shallow-water delta outcrops. Our experiments imply that the height, geometry and bedforms of the mouth bars depend on the jet properties and grain size of the supplied sediment. Pure jets with very coarse-grained sediment formed a high and steep mouth bar that is characterised by steep angle-of-repose cross bedding with related avalanche processes (grain flows) on the lee side. The experiments with stratified jets imply that mouth-bar deposition and growth are dominated by supercritical density flows that evolve from the initial jets on the lee side of the growing mouth bar. In stratified jets with very coarse-grained sediment, deposition on the mouth-bar lee side was both from grain-flow avalanches and density flows. While deposition on the upper lee slope was dominated by grain flows, a concentric field of low relief, asymmetric, downflow-migrating bedforms evolved on the lower slope and beyond the mouth bar. In the stratified jet with medium-grained sediment a very low relief mouth bar formed within a concentric field of low, asymmetric, downflow-migrating bedforms covering the entire lee slope and the area beyond. Many previous field studies show that mouth bars deposited from dense stratified jets (hyperpycnal flows) are characterised by a distinct facies assemblage of coarse-grained cross-stratified or low-angle cross-stratified sandstone passing downslope into finer-grained plane-parallel, or “quasi-parallel” laminated sand and into climbing-ripple cross-laminated sandstone. Comparison to flume and tank experiments suggests that the proximal coarse-grained planar and trough cross-stratified sandstones could represent deposition by supercritical dunes that pass downslope into antidunes, characterised by sinusoidal stratification and/or low-angle cross stratification. The repeated vertical transition between antidune deposits and climbing-ripple cross-laminated sandstone may indicate the superposition of ripples onto antidunes in finer-grained sediments, indicating ripple formation under supercritical flow conditions. Similar bedforms/sedimentary structures have previously been interpreted as hummocky cross-stratification or swaley cross-stratification and attributed to combined flows in storm-dominated settings, which probably in some cases must be revised.

This study investigates the connection between hydraulic and sediment transport processes and channel and lobe geometry in supercritical submarine fans. The geometry of lobate deposits is typically explored as an interrelationship between different geometric measures, e.g., length versus width or thickness versus area, or based on confinement. However, the applicability of these relationships can be unclear due to their multi-scale nature and a lack of understanding of what processes control fan organization at different scales. Here we use a set of laboratory experiments and high-resolution seismic data from the Golo fan to distinguish three different hierarchical scales of nested lobate bodies with different characteristics and stacking (i.e., lobe element, lobe, and lobe complex) and compare characteristics of these mesoscale elements to equations describing 1D hydraulics along supercritical distributary channels. In particular, we are interested in defining the synoptic process and element in steep bedload-dominated supercritical fans that can most accurately be described as a function of hydraulic variables in the distributary channels. Based on the importance of hydraulic jumps in lobe-element formation in the experiments, we propose that the densimetric Froude number of the distributary channel exerts control on the geometry of the lobe element deposits associated with an individual avulsion cycle. Specifically, we hypothesize that higher Froude numbers will lead to lobate deposits that have larger ratios of lobe-element thickness to channel depth compared to those produced at lower Froude numbers. Both the laboratory and field data support this hypothesis. However, the conclusions are dependent on the correct hierarchical linkage between elements in the field and in experiments. In the analysis, the maximum lobe-element thickness was found to be well approximated by the sequent depth of the supercritical flow in the distributary channel. Therefore, this approximation yields a prediction of the lobe-element thickness based only on the hydraulic properties of the distributary channel without the need for any calibration or regression coefficients in the prediction.

We present results of time-evolving coupled direct numerical simulations between an erodible bed and an overlying pressure-driven, turbulent flow field. A total of 6 simulations are considered, the details of which are shown in Table 1. The numerical setup consists of a horizontally periodic open channel, and the simulations are run at a shear Reynolds number of Reτ = 180. The coupling between the spatially and temporally evolving sediment bed and the flow field is enforced through the explicit immersed boundary method (IBM) of Uhlmann [1]. The flow field is fully resolved and is obtained by integrating the conservation of mass and momentum equations using a pseudo spectral code [2]. On the other hand, the sediment bed is modelled via the Exner equation [3]. Details about the numerical approach are available in [4-5].

This paper presents results of a field study designed to examine the structure of flow over mobile and fixed bedforms in a natural stream and to compare the results with findings of previous laboratory studies within the framework of double time‐space averaging approach. Measurements of turbulence were obtained in a small river in Illinois, USA, over a fine spatial grid of sampling points above a mobile sandy bedform and its artificially moulded replica. Flow structure over the artificial bedform is similar to that observed in laboratory studies, but is markedly different from the flow structure over natural bedforms. These differences are most pronounced in the roughness sublayer, whereas flow in the logarithmic layer over natural and artificial sand waves is fairly similar and exhibits spatial uniformity. The double time‐space averaged distributions of turbulence statistics conform to the multilayer model of flow structure over bedforms. Mean velocity distributions indicate neither classical flow recirculation nor substantial reduction of velocities in the lee of bedform crests. However, vertical patterns of turbulence statistics over depth suggest that stacked wakes similar to those observed in laboratory studies exist above the bedforms. Thus, despite the absence of flow separation, wake development seems to be induced by the systematic influence of upstream bedforms on the vertical structure of turbulence. Copyright © 2006 John Wiley & Sons, Ltd.

Nonintrusive measurement of closed-conduit erodible-bed development was undertaken for 12 experiments of ranges of flow strengths and sediment (solids) sizes. Analogous to open-channel flows, wavelets on the sediment bed of a closed-conduit are instigated by discontinuities in the bed, with wavelet lengths λ for laminar and turbulent open-channel and closed-conduit flows given by λ=175d0.75, where λ and sediment size d are in millimeters. For closed-conduit flows, ripples, and dunes grow from these wavelets (at rates increasing with increasing flow strength, and utilizing the mechanisms of bed-form coalescence and throughpassing) to limiting lengths, heights, steepnesses, and bed friction factors that are approximately maintained or possibly decrease thereafter. Limitation of free-surface deformation results in increased rates of bed-wave development for closed-conduit flows in comparison to open-channel flows. Measured results indicate that equilibrium closed-conduit ripple and dune magnitudes can be pre...

The motion of sediment and development of bed forms in natural rivers still constitute intriguing phenomena that always challenge engineers and researchers involved in a variety of problems including river hydraulics and other geophysical and environmental flows. Resistance to flow in alluvial streams has been widely investigated. Since the pioneer work of Exner [12] and, later, Einstein and Barbarossa [8], the profound effects of movable sediments on the flow structure have been recognized as relevant in order to determine the nature of flow resistance (Kennedy [21]; Yalin [40]; Vanoni [36]; Garcia and Parker [14]). This paper presents a simple methodology, based on energy balance, to compute the components of the total shear stress (grain and form-drag) acting on a uniform, two-dimensional flow over fully developed dunes. The method considers mainly an analysis of spatially-averaged (over several dune wavelengths) shear stress distributions. Then, boundary-layer theory is used in order to find an equivalent turbulent-wall-shear flow for the actual spatially-averaged flow over dunes and to derive a characteristic roughness length for the logarithmic velocity distribution above the dunes. The corresponding roughness function that also appears in the logarithmic velocity profile is also investigated and related to dimensionless sediment transport parameters. Alluvial roughness is found to be a function of a single parameter which includes the total Shields stress, a particle Reynolds number, and the ratio between flow depth and sediment diameter.