9 research outputs found
Dynamics and Deposition of Sediment-Bearing Multi- Pulsed Flows and Geological Implication
Copyright © 2019, SEPM (Society for Sedimentary Geology) Previous studies on dilute, multi-pulsed, subaqueous saline flows have demonstrated that pulses will inevitably advect forwards to merge with the flow front. On the assumption that pulse merging occurs in natural-scale turbidity currents, it was suggested that multi-pulsed turbidites that display vertical cycles of coarsening and fining would transition laterally to single-pulsed, normally graded turbidites beyond the point of pulse merging. In this study, experiments of dilute, single- and multi-pulsed sediment-bearing flows (turbidity currents) are conducted to test the linkages between downstream flow evolution and associated deposit structure. Experimental data confirm that pulse merging occurs in laboratory-scale turbidity currents. However, only a weak correspondence was seen between longitudinal variations in the internal flow dynamics and the vertical structure of deposits; multi-pulsed deposits were documented, but transitioned to single-pulsed deposits before the pulse merging point. This early transition is attributed to rapid sedimentation-related depletion of the coarser-grained suspended fraction in the laboratory setting, whose absence may have prevented the distal development of multi-pulsed deposits; this factor complicates estimation of the transition point in natural-scale turbidite systems
Dynamics and deposition of sediment-bearing multi-pulsed flows and geological implication
Previous studies on dilute, multi-pulsed, subaqueous saline flows have demonstrated that pulses will inevitably advect forwards to merge with the flow front. On the assumption that pulse merging occurs in natural-scale turbidity currents, it was suggested that multi-pulsed turbidites that display vertical cycles of coarsening and fining would transition laterally to single-pulsed, normally graded turbidites beyond the point of pulse merging. In this study, experiments of dilute, single- and multi-pulsed sediment-bearing flows (turbidity currents) are conducted to test the linkages between downstream flow evolution and associated deposit structure. Experimental data confirm that pulse merging occurs in laboratory-scale turbidity currents. However, only a weak correspondence was seen between longitudinal variations in the internal flow dynamics and the vertical structure of deposits; multi-pulsed deposits were documented, but transitioned to single-pulsed deposits before the pulse merging point. This early transition is attributed to rapid sedimentation-related depletion of the coarser-grained suspended fraction in the laboratory setting, whose absence may have prevented the distal development of multi-pulsed deposits; this factor complicates estimation of the transition point in natural-scale turbidite systems
Unconfined gravity current interactions with orthogonal topography: Implications for combined‐flow processes and the depositional record
Turbidity current behaviour is affected by interactions with seafloor topography. Changes in flow dynamics will depend on the orientation and gradient of the topography, and the magnitude and rheology of the incoming flow. A better understanding of how unconfined turbidity currents interact with topography will improve interpretations of the stratigraphic record, and is addressed herein using three-dimensional flume tank experiments with unconfined saline density currents that enter a horizontal basin before interacting with a ramp orientated perpendicular to flow direction. The incoming flow parameters remained constant, whilst the slope angle was independently varied. On a 20° slope, superelevation of the flow and flow stripping of the upper, dilute region of the flow occurred high on the slope surface. This resulted in a strongly divergent flow and the generation of complex multidirectional flows (i.e. combined flows). The superelevation and extent of flow stripping decreased as the slope angle increased. At 30° and 40°, flow reflection and deflection, respectively, are the dominant flow process at the base of slope, with the reflected or deflected flow interacting with the parental flow, and generating combined flows. Thus, complicated patterns of flow direction and behaviour are documented even on encountering simple, planar topographies orientated perpendicular to flow direction. Combined flows in deep-water settings have been linked to the interaction of turbidity currents with topography and the formation of internal waves with a dominant oscillatory flow component. Here, combined flow occurs in the absence of an oscillatory component. A new process model for the formation and distribution of hummock-like bedforms in deep-marine systems is introduced. This bedform model is coupled to a new understanding of the mechanics of onlap styles (draping versus abrupt pinchout) to produce a spatial model of gravity-current interaction, and deposition, on slopes to support palaeogeographical reconstructions
Near-bed and surface flow division patterns in experimental river bifurcations
Understanding channel bifurcation mechanics is of great importance for predicting and managing multichannel river processes and avulsion in distributary river deltas. To date, research on river channel bifurcations has focused on factors determining the stability and evolution of bifurcations. It has recently been shown that, theoretically, the nonlinearity of the relation between sediment transport and flow discharge causes one of the two distributaries of a (slightly) asymmetrical bifurcation to grow and the other to shrink. The positive feedback introduced by this effect results in highly asymmetrical bifurcations. However, there is a lack of detailed insight into flow dynamics within river bifurcations, the consequent effect on bed load flux through bifurcating channels, and thus the impact on bifurcation stability over time. In this paper, three key parameters (discharge ratio, width-to-depth ratio, and bed roughness) were varied in order to examine the secondary flow field and its effect on flow partitioning, particularly near-bed and surface flow, at an experimental bifurcation. Discharge ratio was controlled by varying downstream water levels. Flow fields were quantified using both particle image velocimetry and ultrasonic Doppler velocity profiling. Results show that a bifurcation induces secondary flow cells upstream of the bifurcation. In the case of unequal discharge ratio, a strong increase in the secondary flow near the bed causes a larger volume of near-bed flow to enter the dominant channel compared to surface and depth-averaged flow. However, this effect diminishes with larger width-to-depth ratio and with increased bed roughness. The flow structure and division pattern will likely have a stabilizing effect on river channel bifurcations. The magnitude of this effect in relation to previously identified destabilizing effects is addressed by proposing an adjustment to a widely used empirical bed load nodal-point partition equation. Our finding implies that river bifurcations can be stable under a wider range of conditions than previously thought. Key Points Secondary flow in symmetrical bifurcations causes strong near-bed flow curvature A disproportional amount of near-bed flow enters the dominant downstream channel Flow curvature adds a stabilizing feedback on bifurcation evolution
Tissue Glucocorticoid Metabolism in Adrenal Insufficiency:A Prospective Study of Dual-release Hydrocortisone Therapy
Background: Patients with adrenal insufficiency (AI) require life-long glucocorticoid (GC) replacement therapy. Within tissues, cortisol (F) availability is under the control of the isozymes of 11β-hydroxysteroid dehydrogenase (11β-HSD). We hypothesize that corticosteroid metabolism is altered in patients with AI because of the nonphysiological pattern of current immediate release hydrocortisone (IR-HC) replacement therapy. The use of a once-daily dual-release hydrocortisone (DR-HC) preparation, (Plenadren®), offers a more physiological cortisol profile and may alter corticosteroid metabolism in vivo.Study Design and Methods: Prospective crossover study assessing the impact of 12 weeks of DR-HC on systemic GC metabolism (urinary steroid metabolome profiling), cortisol activation in the liver (cortisone acetate challenge test), and subcutaneous adipose tissue (microdialysis, biopsy for gene expression analysis) in 51 patients with AI (primary and secondary) in comparison to IR-HC treatment and age- and BMI-matched controls.Results: Patients with AI receiving IR-HC had a higher median 24-hour urinary excretion of cortisol compared with healthy controls (72.1 µg/24 hours [IQR 43.6-124.2] vs 51.9 µg/24 hours [35.5-72.3], P = .02), with lower global activity of 11β-HSD2 and higher 5-alpha reductase activity. Following the switch from IR-HC to DR-HC therapy, there was a significant reduction in urinary cortisol and total GC metabolite excretion, which was most significant in the evening. There was an increase in 11β-HSD2 activity. Hepatic 11β-HSD1 activity was not significantly altered after switching to DR-HC, but there was a significant reduction in the expression and activity of 11β-HSD1 in subcutaneous adipose tissue.Conclusion: Using comprehensive in vivo techniques, we have demonstrated abnormalities in corticosteroid metabolism in patients with primary and secondary AI receiving IR-HC. This dysregulation of pre-receptor glucocorticoid metabolism results in enhanced glucocorticoid activation in adipose tissue, which was ameliorated by treatment with DR-HC
Dynamics and deposition of sediment-bearing multi-pulsed flows and geological implication
An experimental study of discharge partitioning and flow structure at symmetrical bifurcations
Recent research has examined the factors controlling the geometrical
configuration of bifurcations, determined the range of stability
conditions for a number of bifurcation types and assessed the impact of
perturbations on bifurcation evolution. However, the flow division
process and the parameters that influence flow and sediment partitioning
are still poorly characterized. To identify and isolate these
parameters, three-dimensional velocities were measured at 11
cross-sections in a fixed-walled experimental bifurcation. Water surface
gradients were controlled, and systematically varied, using a weir in
each distributary.
As may be expected, the steepest distributary conveyed the most
discharge ( was dominant) while the mildest distributary conveyed the
least discharge ( was subordinate). A zone of water surface
super-elevation was co-located with the bifurcation in symmetric cases
or displaced into the subordinate branch in asymmetric cases. Downstream
of a relatively acute-angled bifurcation, primary velocity cores were
near to the water surface and against the inner banks, with near-bed
zones of lower primary velocity at the outer banks. Downstream of an
obtuse-angled bifurcation, velocity cores were initially at the outer
banks, with near-bed zones of lower velocities at the inner banks, but
patterns soon reverted to match the acute-angled case. A single
secondary flow cell was generated in each distributary, with water
flowing inwards at the water surface and outwards at the bed.
Circulation was relatively enhanced within the subordinate branch, which
may help explain why subordinate distributaries remain open, may play a
role in determining the size of commonly-observed topographic features,
and may thus exert some control on the stability of asymmetric
bifurcations. Further, because larger values of circulation result from
larger gradient disadvantages, the length of confluence-diffluence units
in braided rivers or between diffluences within delta distributary
networks may vary depending upon flow structures inherited from upstream
and whether, and how, they are fed by dominant or subordinate
distributaries. Copyright (C) 2011 John Wiley & Sons, Ltd
Global (latitudinal) variation in submarine channel sinuosity: REPLY
We thank Sylvester et al. (2013) for the opportunity to further consider the nature of, and controls on, sinuosity variation in submarine channels. However, in stating that we propose a Coriolis force (Fc) control on the correlation between peak sinuosity and latitude, and discounted slope and sediment supply, Sylvester et al. misquote us. We concluded “The most probable causative controls on [the] global distribution [of peak sinuosity with latitude] are the Coriolis force… and latitudinal variations in both the nature of flows and sediment type” (Peakall et al., 2012, p. 14). Slope was shown to be a very weak control on global channel sinuosity variation, though it is important in controlling sinuosity in individual channels (Peakall et al., 2012, our figures 1 and 2A)