Cross-stream Transport Research Articles

Spatiotemporal bedload transport patterns over two-dimensional bedforms

Abstract. Despite a rich history of studies investigating fluid dynamics over bedforms and dunes in rivers, the spatiotemporal patterns of sub-bedform bedload transport remain poorly understood. Previous experiments assessing the effects of flow separation on downstream fluid turbulent structures and bedload transport suggest that localized, intermittent, high-magnitude transport events (i.e., permeable splat events) play an important role in both downstream and cross-stream bedload transport near flow reattachment. Here, we report results from flume experiments that assess the combined effects of flow separation–reattachment and flow re-acceleration over fixed two-dimensional bedforms (1.7 cm high; 30 cm long). A high-speed camera observed bedload transport along the entirety of the bedform at 250 frames per second. Grain trajectories, grain velocities, and grain transport directions were acquired from bedload images using semiautomated particle-tracking techniques. Downstream and vertical fluid velocities were measured 3 mm above the bed using laser Doppler velocimetry (LDV) at 15 distances along the bedform profile. Mean downstream fluid velocity increases nonlinearly with increasing distance along the bedform. However, observed bedload transport increases linearly with increasing distance along the bedform, except at the crest of the bedform, where both mean downstream fluid velocity and bedload transport decrease substantially. Bedload transport time series and manual particle-tracking data show a zone of high-magnitude, cross-stream transport near flow reattachment, suggesting that permeable splat events play an essential role in the region downstream of flow reattachment.

Stability of the Malvinas Current.

Deterministic and probabilistic tools from nonlinear dynamics are used to assess enduring near-surface Lagrangian aspects of the Malvinas Current. The deterministic tools are applied to a multiyear record of velocities derived from satellite altimetry data, revealing a resilient cross-stream transport barrier. This is composed of shearless-parabolic Lagrangian coherent structures (LCSs), which, extracted over sliding time windows along the multiyear altimetry-derived velocity record, lie in near-coincidental position. The probabilistic tools are applied on a large collection of historical satellite-tracked drifter trajectories, revealing weakly communicating flow regions as basins of attraction for long-time asymptotic almost-invariant sets on either side of the altimetry-derived barrier. Shearless-parabolic LCSs are detected for the first time from altimetry data, and their significance is supported on satellite-derived ocean color data, which reveal shapes that quite closely resemble the peculiar V shapes, dubbed "chevrons," that have recently confirmed the presence of similar LCSs in the atmosphere of Jupiter. Finally, using available in situ velocity and hydrographic data, sufficient and necessary conditions for nonlinear symmetric stability are found to be satisfied, suggesting a duality between Lagrangian and Eulerian stability for the Malvinas Current.

A novel mixing mechanism in sinuous seafloor channels: Implications for submarine channel evolution

Previous experimental studies of density currents in sinuous seafloor channels have almost exclusively studied hydrodynamics either by considering time independent, instantaneous, flow measurements or by compiling time-averaged flow measurements. Here we present a novel study of the time dependent dynamics of a density driven flow in a sinuous channel fed by a source of constant discharge. The experiments show that whilst source conditions may be temporally steady, flow conditions are temporally unsteady with timescales of flow variation driven by flow interaction with channel topography. Temporal variations reveal that both downstream and cross-stream flows vary significantly from time average observations and predictions, across scales larger than those predicted for turbulence in equivalent straight channels. Large-scale variations are shown to increase the average production of turbulence across the height of the flow, providing a new mechanism for enhanced mixing of sediment within gravity currents. Further such large-scale variations in flow conditions are recorded in the change in orientation of near-bed secondary flow, providing a plausible mechanism to reduce the cross-stream transport of bedload material and explain the ultimate stabilisation of sinuous seafloor channel systems.

Barotropic and baroclinic contributions to along-stream and across-stream transport in the Antarctic Circumpolar Current

The Southern Ocean's ability to store and transport heat and tracers as well as to dissipate momentum and energy are intimately related to the vertical structure of the Antarctic Circumpolar Current (ACC). Here the partition between barotropic and baroclinic flow in the time-mean ACC is investigated in a Southern Ocean state estimate. The zonal geostrophic transport is predominantly baroclinic, with at most 25% of the transport at any longitude carried by the barotropic component. Following surface streamlines, changes in vertical shear and near-bottom velocity are large, and result in changes in the local partition of barotropic/baroclinic vertically integrated transport from 10/90% in the center of the basins, to 50/50% near complex topography. The velocity at depth is not aligned with the surface velocity. This nonequivalent barotropic flow supports significant cross-stream transports. Barotropic and baroclinic mass transport across the ACC is, on average, in opposite directions, with the net barotropic cross-stream transport being poleward and the net baroclinic equatorward. The sum partially cancels out, leaving a net geostrophic poleward transport across the different fronts between −5 and −20 Sv. Temperature is also transported across the fronts by the nonequivalent barotropic part of the ACC, with maximum values across the northern ACC fronts equivalent to −0.2 PW. The sign and magnitude of these transports are not sensitive to the choice of stream-coordinate. These cross-stream volume and temperature transports are variable in space, and dependent on the interactions between deep flow and bathymetry, thus difficult to infer from surface and hydrographic observations alone.

Effect of point bar development on the local force balance governing flow in a simple, meandering gravel bed river

The patterns of depth, velocity, and shear stress that direct a river's morphologic evolution are governed by a balance of forces. Analyzing these forces, associated with pressure gradients, boundary friction, channel curvature, and along- and across-stream changes in fluid momentum driven by bed topography, can yield insight regarding the establishment and maintenance of stable channel forms. This study examined how components of the local force balance changed as a meandering channel evolved from a simple, flat-bedded initial condition to a more complex bar-pool morphology. A numerical flow model, constrained by measurements of velocity and water surface elevation, characterized the flow field for four time periods bracketing two floods. For each time increment, runs were performed for discharges up to bankfull, and individual force balance components were computed from model output. Formation and growth of point bars enhanced topographic steering effects, which were of similar magnitude to the pressure gradient and centrifugal forces. Convective accelerations induced by the bar reduced the cross-stream pressure gradient, intensified flow toward the outer bank, and routed sediment around the upstream end of the bar. Adjustments in the flow field thus served to balance streamwise transport along the inner bank onto the bar and cross-stream transport into the pool. Even in the early stages of bar development, topographically driven spatial gradients in velocity played a significant role in the force balance at flows up to bankfull, altering the orientation of the shear stress and sediment transport to drive bar growth. Copyright 2011 by the American Geophysical Union.

The role of mesoscale processes controlling biological variability in the Black Sea coastal waters: inferences from SeaWIFS-derived surface chlorophyll field

Several different time series of chlorophyll images from the 1997–2000 Seaviewing Wide Field-of-view Sensor data set have been analysed to gain a perspective on the dynamical and biological variability in the Black Sea, particularly on its northwestern shelf region and along the Anatolian coastal zone. The images are interpreted in terms of documenting the close link between biological production and physical dynamics of the boundary system, and of emphasizing the role of eddy processes on controlling mesoscale chlorophyll distributions. It is shown that western coastal waters of the Black Sea are characterized by permanently high chlorophyll concentrations of more than 4.0 mg m −3, often subject to considerable dynamical transformations, and modulated by mesoscale structures including southward elongated filament-like features extending up to 100 km offshore with a lifetime of up to a month. Filaments, meanders, offshore jets and other forms of mesoscale and sub-mesoscale structures appear as a common signature of the system along the Anatolian coast. They play an important role in the cross-stream transport of biota and chemical constituents, and thus supporting productivity within the interior parts of the basin. The images reveal phytoplankton blooms lasting several months in all three autumn seasons. During autumn 1999 to winter 2000, the phytoplankton biomass declines in December, and then increases to form another bloom in February–March 2000, albeit somewhat weaker. For the previous 2 years (1997, 1998), the autumn bloom is extended towards the first half of the winter but the late winter–early spring bloom is absent.

The Effect of Stage on Flow and Components of the Local Force Balance

Flow fields and water and bed surface topography were measured at two different stages as flow shoals over a submerged mid-channel bar in a straight reach downstream of a bend in Solfatara Creek, Wyoming. The data allow calculation and comparison of the magnitude of the component terms in the downstream and cross-stream force balance at the different stages. At the lower stage, corresponding to a discharge that is 30 per cent of the bankfull discharge, the convective acceleration terms in the equations describing the force balance are important, particularly the terms associated with the cross-stream transport of momentum. These terms are large because of the large accelerations and cross-stream flow forced by the shallow flow over the bar. At the higher stage, corresponding to a discharge that is 45 per cent of the bankfull discharge, flow is more directly downstream and cross-stream velocity is generally less in most of the channel. Downstream flow velocities at the higher stage are larger, but the acceleration is more gradual. Consequently, the convective accelerations at the higher stage tend to be less important than at the lower stage. Results from the two different stages suggest that some of the difference in conclusions reached by various workers on the significance of the various terms in the governing equations may be associated with the relative depth of flow. © 1997 John Wiley & Sons, Ltd.

Free-Stream Turbulence and Concave Curvature Effects on Heated, Transitional Boundary Layers

An experimental investigation of transition in concave-curved boundary layers at two free-stream turbulence levels (0.6 and 8.6 percent) was performed. For the lower free-stream turbulence intensity case, Go¨rtler vortices were observed in both laminar and turbulent flows using liquid crystal visualization and spanwise velocity and temperature traverses. Transition is thought to occur via a vortex breakdown mode. The vortex locations were invariant with time but were nonuniform across the span in both the laminar and turbulent flows. The upwash regions between two vortices were more unstable than were the downwash regions, containing higher levels of u’ and u’ v’, and lower skin friction coefficients and shape factors. Turbulent Prandtl numbers, measured using a triple-wire probe, were near unity for all post-transitional profiles, indicating no gross violation of Reynolds analogy. No streamwise vortices were observed in the higher turbulence intensity case. This may be due to the high eddy viscosity, which reduces the turbulent Go¨rtler number to subcritical values, thus eliminating the vortices, or due to an unsteadiness of the vortex structure that could not be observed by the techniques used. Based upon these results, predictions that assume two-dimensional modeling of the flow over a concave wall with high free-stream turbulence levels, as on the pressure surface of a turbine blade, seem to be adequate—there is no time-average, three-dimensional structure to be resolved. High levels of free-stream turbulence superimposed on a free-stream velocity gradient (which occurs within curved channels) cause a cross-stream transport of momentum within the flow outside the boundary layer. The total pressure within this region can rise above the value measured at the inlet to the test section.

Movement of small spheres in a plane mixing layer.

Experiments were performed in a two-phase, two-freestream plane mixing layer to study the behaviour of heavy spheres in an intermittent shear flow. Particles with three different diameters were employed in these experiments. Comparisons are presented between data on streamwise and transverse components of particle and gas velocities. These data, as well as flow visualization images, suggest that the ratio of the particle time-constant τp and the large-scale eddy turnover time plays an important role in the cross-stream transport of a heavy dispersed phase. When the τp/τ was approximately 0 (1), heavy particles began to move with gas flow. There exists movement of heavy spheres back toward the particle feedstream. The transverse component of large particle turbulent velocity, v', in the interior of the layer was observed to be significantly smaller than the corresponding streamwise component, u'. / reaches an asymptotic value, 0.2, around the high-speed side of the mixing layer.

The distribution and abundance of copepods in relation to the physical structure of the Gulf Stream

The horizontal and vertical distributions of selected copepod species across the Gulf Stream in September 1982 were investigated along a transect east of Cape Hatteras where the Stream turns offshore. Copepod species distributions across the Stream, and the extent of their diel migrations, can be grouped into several distinct patterns associated with different environmental habitats within the Stream, some of which change depth across the Stream along sloping isopycnals. Copepod behavior, such as probable temperature and depth preferences and diel vertical migration, interacts with the complex physical structure of the Stream to determine at what depths in the water column a species is found at different cross-stream locations. The different environmental habitats are also characterized by different current velocities and directions, and therefore the species groups experience different degrees of downstream and cross-stream transport. Direct cross-stream transport, in addition to mesoscale events such as ring formation, is a likely mechanism connecting some populations of Slope Water and Sargasso Sea zooplankton.