Local Bed Research Articles

Local Structure Effects on Pressure Drop in Slender Fixed Beds of Spheres

AbstractFor slender fixed beds, the void fraction and flow properties are complex topics. Different factors can influence the local bed structure. For statistical analysis, 2800 fixed beds have been generated and the impact of friction factor and reactor‐to‐particle diameter ratio on the distribution has been shown. With particle‐resolved computational fluid dynamics, all local structure effects are taken into account for the flow simulations. Pressure drop measurements and simulations showed that these effects can lead to areas with low flow resistance, leading to overestimated pressure drop by typical correlations up to 85 %.

The effects of COVID‐19 on hip fracture management and mortality in a regional trauma centre

Older patients and those with comorbidities have worse outcomes from both hip fracture and COVID-19. Furthermore, testing positive for COVID-19 can increase mortality for any surgical procedure [1]. In anticipation of the initial COVID-19 surge, theatre staff were re-allocated in order to meet the expected increase in local critical care bed requirements, resulting in reduced capacity to deliver surgical services. We analysed the impact of the initial COVID-19 surge on the hip fracture population in a high-volume tertiary trauma centre (approximately 900 hip fracture surgical procedures per year) in the UK [2]. The study included a 2-month period following the onset of lockdown from 24 March 2020 until 23 May 2020, and we compared this with the same 2-month time period in 2015–2019. This included patients aged > 60 years who were admitted with a hip fracture. It did not include patients with high-energy injuries, pathological fractures and those who underwent surgery in other units before admission. Data were retrieved from the hospital’s inpatient fracture database, the Fracture Outcomes Research Department, which contributes to the National Hip Fracture Database [2]. The study was approved by the Belfast Trust’s Quality Improvement and Patient Safety Department. During the study period in 2020, 126 hip fracture patients were included, compared with an average of 115 patients per year in 2015–2019. This suggests that, despite nationwide advice to avoid unnecessary social contact and remain at home where possible, a cohort of patients continued to fall and sustain hip fractures. Looking specifically at the 2020 cohort (Table 1), 68.0% of patients were aged 80 or above, and 86.9% of those operated on were deemed ASA physical status 3 or worse. These patients clearly remain at significant risk of sustaining a fall and subsequent hip fracture, despite the COVID-19 lockdown restrictions. 2020 n = 126 2015–2019 n = 574 Aerosol generating procedures were avoided where possible in 2020, with an increased use of neuraxial over general anaesthesia demonstrated (p < 0.001) [3]. However, this result may be confounded due to a general trend towards neuraxial anaesthesia within the department in the last 5 years (Fig. 1). Time to theatre improved during the pandemic, despite a reduction in theatre capacity (time to theatre < 48 h was 64.2% in 2020 compared with 53.6% in 2015–2019, p = 0.03). The 30-day mortality in 2020 was also similar to that of 2015–2019 for both surgically and conservatively managed patients. This highlights the ability to maintain high standards of care despite the organisational changes made during surge planning. A total of 7.1% patients tested positive for COVID-19, all of whom were operated on (9/126 positive, 113/126 negative, 4/126 no result). The 30-day mortality rate of COVID-19-positive patients was 22.0%, compared with 2.7% in COVID-19-negative patients (p = 0.05) and corroborates broadly with the findings of a number of other publications [4, 5]. [Correction added 18 January 2021, after first online publication: the first sentence in this paragraph has been corrected in this version] Our results demonstrate the ability to maintain an effective hip fracture service with largely unchanged outcomes during the height of the COVID-19 pandemic. Strengths of the study included robust follow-up and the ability to compare our results with retrospective pre-pandemic data. There were substantially lower numbers of patients with COVID-19 in Northern Ireland when compared with the rest of the UK, which may limit the generalisability of the results; however, it is possible that these lower numbers may more closely represent future surges. Our study showed increased mortality in COVID-19-positive patients undergoing surgical management, which may raise ethical considerations for this cohort. The inclusion of only nine COVID-19-positive patients and the high rates of spinal anaesthesia may limit conclusions, with further research warranted to answer questions such as the timing of surgery in COVID-19-positive patients who require hip fracture surgery.

Acoustic scattering characteristics and inversions for suspended concentration and particle size above mixed sand and mud beds

The majority of reported field studies, using acoustic backscattering, for the measurement of nearbed suspended sediment processes, have been focussed on field sites with sand size fractions and unimodal size distributions. However, in many sedimentary environments, and particularly for estuaries and rivers, sands and muds coexist in the bed sediment substrate, forming a size regime that is often bimodal in nature. To examine the interaction of sound in these more complex sedimentary environments a numerical study is presented based on observations of sediment size distributions measured in the Dee estuary, UK. The work explores the interpretation of the backscatter signal from a mixed sediment composition in suspension, with mud-sand fractions varying with height above the bed. Consideration is given to the acoustical scattering properties and the inversion of the backscatter signal to extract information on the suspension. In common with most field deployments, the scenarios presented here use local bed sediments for the acoustic inversion of the backscattered signal. The results indicate that in general it is expected that particle size and concentration will diverge from what is actually in suspension, with the former being overestimated and the latter underestimated.

Combining SfM Photogrammetry and Terrestrial Laser Scanning to Assess Event-Scale Sediment Budgets along a Gravel-Bed Ephemeral Stream

Stream power represents the rate of energy expenditure along a stream reach and can be calculated using topographic data acquired via structure-from-motion (SfM) photogrammetry and terrestrial laser scanning (TLS). This study sought to quantitatively relate morphological adjustments in the Azohía Rambla, a gravel-bed ephemeral stream in southeastern Spain, to stream power (ω), critical power (ωc), and energy gradients (∂ω/∂s), along different reference channel reaches of 200 to 300 m in length. High-resolution digital terrain models (HRDTMs), combined with ortophotographs and point clouds from 2018, 2019, and 2020, and ground-based surveys, were used to estimate the spatial variability of morphological sediment budgets and to assess channel bed mobility during the study period at different spatial scales: reference channel reaches (RCRs), pilot bed survey areas (PBSAs), and representative geomorphic units (RGUs). The optimized complementary role of the SfM technique and terrestrial laser scanning allowed the generation of accurate and reliable HRDTMs, upon which a 1-D hydrodynamic model was calibrated and sediment budgets calculated. The resulting high-resolution maps allowed a spatially explicit analysis of stream power and transport efficiency in relation to volumes of erosion and deposition in the RCR and PBSA. In addition, net incision or downcutting and vertical sedimentary accretion were monitored for each flood event in relation to bedforms and hydraulic variables. Sediment sources and sinks and bed armoring processes showed different trends according to the critical energy and stream power gradient, which were verified from field observations. During flows exceeding bankfull discharges (between 18 and 24 m3 s−1 according to channel reach), significant variations in ∂ω/∂s values and ω/ωc ratios (e.g., −15 &lt; ∂ω/∂s &lt; 15 Wm−3; ω/ωc &gt; 2 for a peak discharge of 31 m3 s−1) were associated with a large amount of bedload mobilized upstream and vertical accretion along the middle reach (average rise height of 0.20 to 0.35 m for the same event). By contrast, more moderate peak flows (≤10 m3 s−1) only produced minor changes resulting in surface washing, selective transport, and local bed scouring.

Regional-scale abrupt Mid-Holocene ice sheet thinning in the western Ross Sea, Antarctica

Abstract Outlet glaciers drain the majority of ice flow in the Antarctic ice sheet. Theory and numerical models indicate that local bed topography can play a key role in modulating outlet glacier response to climate warming, potentially resulting in delayed, asynchronous, or enhanced retreat. However, the period of modern observations is too short to assess whether local or regional controls dominate ice sheet response on time scales that are critical for understanding ice sheet mass loss over this century and beyond. The recent geological past allows for insight into such centennial-scale ice sheet behavior. We present a cosmogenic surface-exposure chronology from Mawson Glacier, adjacent to a region of the Ross Sea that underwent dynamic marine-based ice sheet retreat following the Last Glacial Maximum. Our data record at least 220 m of abrupt ice thinning between 7.5 and 4.5 ka, followed by more gradual thinning until the last millennium. The timing, rates, and magnitudes of thinning at Mawson Glacier are remarkably similar to that documented 100 km to the south at Mackay Glacier. Together, both outlet glaciers demonstrate that abrupt deglaciation occurred across a broad region in the Mid-Holocene. This happened despite the complex bed topography of the western Ross Sea and implies an overarching external driver of retreat. When compared to regional sea-level and ocean-temperature changes, our data indicate that ocean warming most likely drove grounding-line retreat and ice drawdown, which then accelerated as a result of marine ice sheet instability.

Winding down the Chicxulub impact: The transition between impact and normal marine sedimentation near ground zero

The Chicxulub impact led to the formation of a ~ 200-km wide by ~1-km deep crater on México's Yucatán Peninsula. Over a period of hours after the impact the ocean re-entered and covered the impact basin beneath several hundred meters of water. A suite of impactites were deposited across the crater during crater formation, and by the resurge, tsunami and seiche events that followed. International Ocean Discovery Program/International Continental Scientific Drilling Program Expedition 364 drilled into the peak ring of the Chicxulub crater, and recovered ~130 m of impact deposits and a 75-cm thick, fine-grained, carbonate-rich “Transitional Unit”, above which normal marine sedimentation resumed. Here, we describe the results of analyses of the uppermost impact breccia (suevite) and the Transitional Unit, which suggests a gradual waning of energy recorded by this local K-Pg boundary sequence.The dominant depositional motif in the upper suevite and the Transitional Unit is of rapid sedimentation characterized by graded bedding, local cross bedding, and evidence of oscillatory currents. The lower Transitional Unit records the change from deposition of dominantly sand-sized to mainly silt to clay sized material with impact debris that decreases in both grain size and abundance upward. The middle part of the Transitional Unit is interrupted by a 20 cm thick soft sediment slump overlain by graded and oscillatory current cross-laminated beds. The uppermost Transitional Unit is also soft sediment deformed, contains trace fossils, and an increasing abundance of planktic foraminifer and calcareous nannoplankton survivors. The Transitional Unit, as with similar deposits in other marine target impact craters, records the final phases of impact-related sedimentation prior to resumption of normal marine conditions. Petrographic and stable isotopic analyses of carbon from organic matter provide insight into post-impact processes. δ13Corg values are between terrestrial and marine end members with fluctuations of 1–3‰.Timing of deposition of the Transitional Unit is complicated to ascertain. The repetitive normally graded laminae, both below and above the soft sediment deformed interval, record rapid deposition from currents driven by tsunami and seiches, processes that likely operated for weeks to potentially years post-impact due to subsequent continental margin collapse events. Highly siderophile element-enrichment at the top of the unit is likely from fine-grained ejecta that circulated in the atmosphere for several years prior to settling. The Transitional Unit is thus an exquisite record of the final phases of impact-related sedimentation related to one of the most consequential events in Earth history.

Assessment of fine sediment river bed stocks in seven Alpine catchments

While the finest fraction of suspension has long been considered to have limited interactions with the river bed, several recent studies based on flume and field observations raise questions about this hypothesis which is fundamental for suspended load modeling and river management. In this study, we report a large field campaign in which we quantify the river bed stocks of fine particles in 7 contrasted Alpine catchments. Using a simple protocol, we performed more than 300 riverbed measurements of the local surface and subsurface stocks. Results indicate that even when the river bed surface contains no fine particles, significant quantities can be found in the subsurface layer which is in most cases the layer having the higher stocks. We also observed that stocks highly depend on the facies considered suggesting that storage processes are strongly driven by the local hydraulics and river bed characteristics. By integrating these local stocks at the catchment scale, we estimated that they could represent more than 50% of the mean annual suspended load in catchments having large alluvial braided sections. On the opposite, these stocks could be as small as 1% in highly eroded head water catchments. This suggests that the bed of large alluvial Alpine rivers can be considered as a significant source of fine particles. These observations were confirmed by using a simplified vertical scouring model to estimate conditions for these stocks to be released in the flow. However, the use of this model suggests that other bed reworking processes (channel widening and migration) have to be considered. Finally, these observations suggest that interactions between particles transported as suspension and gravel beds are far from being negligible processes in catchments having large alluvial sections typically found in Alpine environments.

Numerical simulation of fast granular flow facing obstacles on steep terrains

The interaction between fast shallow granular flow and obstacles on steep terrain is an important aspect of granular mechanics and defending against geological hazards. In this study, we used a depth-averaged model for granular flow facing obstacles on steep terrains in a bed-fitted coordinate system where the obstacle system is treated as a local bed deviation term. A second-order Riemann-free scheme is extended to compute the depth-averaged model with a wetting–drying technique, which is verified by several granular flow cases, such as aluminum bar collapse and granular flow runout on a steep slope. Numerical simulations were performed for the case of granular flow facing a (i) single hemispherical obstacle and (ii) system of three hemispherical obstacles to produce a dynamical process and deposit profile, and show good agreement with experimental results. Granular flow facing a single obstacle on a concave plane produces a detached shock wave that moves upstream and a tailing rapid transition zone that moves down, which will merge to form a new shock for deposition. Granular flows facing a three-hemisphere obstacle system produce a tailing rapid transition zone that moves downstream and a downstream wavy shock that results from the interaction of three bow shocks in front of each obstacle. The downstream wavy shock moves upstream and merges with the upstream transition zone to form a new curved shock, which later relaxes to a deposit owing to bed friction. These findings provide some supplemental understandings of flow structures of fast granular flow facing obstacles.

Effect of binder and moisture content on compactibility and shear strength of river bed green sand mould

Green sand mold casting process is an ancient manufacturing process where the manufacturing cost is very low and can produce intricate shape casting for different industrial applications. In recent years, the sand casting foundries are shifted from western countries to India and other Asian countries due to easy availability of manpower and raw materials. However, in India silica sand availability is decreasing gradually due to high production rate of sand casting products and huge civil construction, leads to increase in the cost of production of sand casting products. The current investigation aims to study the effect of binder and moisture percentage on local river bed sand mould compactibility and shear strength. The sand mould cylindrical specimens are fabricated as per American Foundry Society (AFS) to determine shear strength of the mould. The result shows that the mould compactibility decreases and shear strength increases with the increase in binder percentage. The optimum sand mould compactibility 52% and shear strength 0.56 kg/cm2 are observed at 7% moisture and 14% bentonite. Furthermore, the compactibility and shear strength of river bed sand mould and commercially used pure silica sand mould are compared and found local river bed sand may be used as alternative silica sand in green sand mould casting.

Geochemical analyses suggest stratigraphic origin and late Miocene age of reworked vertebrate remains from Penanjong Beach in Brunei Darussalam (Borneo)

ABSTRACT We report on sporadic fossil vertebrates from Brunei Darussalam (Borneo). Most of these isolated remains are reworked and derive from Penanjong Beach known for former coastal cliffs used to be rich in marine molluscs. Previously, the only vertebrate remains reported were shark teeth. With new material, the fish fauna is now represented by six shark and a single ray taxa, while remains of three turtle families were discovered (Trionychidae, Cheloniidae, Geoemydidae). This fauna is compared to nearby upper Miocene fossiliferous beds from where remains of cheloniid and trionychid turtles were unearthed. To assess the origin of the reworked remains, rare earth element (REE) concentrations of the fossil bioapatite were compared between the reworked and the geographically closest in-situ Ambug Hill fauna. The obtained trend in REE variations are identical, revealing similar REE uptake mechanism and early diagenetic conditions. With the additional fact that the succession outcropping at Ambug Hill cuts the coastline imply that the majority of the reworked fossils derive from the local Miocene beds. This is supported by Sr-isotope ages from reworked calcite bivalve shells ranging from 9.74 to 6.62 Ma. These turtle remains thus represent the hitherto known first Neogene fossil tetrapods from Borneo.

Span shoulder migration in three-dimensional current-induced scour beneath submerged pipelines

New experiments involving the three-dimensional current-induced live-bed scour beneath submerged horizontal pipelines are presented, spanning larger Shields parameter and cylinder-to-sediment diameter ratio than previous studies. Specific emphasis is on gaining a better understanding of, and ability to predict, the span migration velocity during the initial and subsequent development of such a scour hole. Consistent with previous experimental observations, both a primary (faster) and secondary (slower) span migration are observed. Process visualization of suspended sediment patterns are in line with prior speculation that this transition coincides with reduced local bed shear stress amplifications as the scour hole both deepens and widens. Dimensional analysis and physical insight are combined, leading to a new rational model for predicting the span migration velocity in both live-bed and clear-water regimes, with predictions naturally coinciding at the limit of far field incipient motion conditions. In both regimes the data cluster as predicted, and fitted closed-form expressions are provided for predicting the span migration velocity. The rational approach likewise includes a new and simple criterion for the transition from primary to secondary migration in the live-bed regime. In the clear-water regime the model incorporates primary dependence on the ratio of the Shields parameter to its critical value, resolving apparent contradictions with a previous study which suggests that the depth-based Froude number is the most important governing parameter. The developed rational model can be used to quantitatively predict all known major features of the span migration velocity in the early stages of the three-dimensional (live-bed and clear-water) scour beneath submerged horizontal cylinders induced by perpendicular flow, and can hence be regarded as the first complete model for this evolution.

Re: A Multi-Institutional Analysis of the Effect of Positive Surgical Margins following Robot-Assisted Partial Nephrectomy on Oncologic Outcomes.

No AccessJournal of UrologyUrological Survey1 Nov 2020Re: A Multi-Institutional Analysis of the Effect of Positive Surgical Margins following Robot-Assisted Partial Nephrectomy on Oncologic Outcomes Jeffrey A. CadedduMD Jeffrey A. CadedduJeffrey A. Cadeddu More articles by this author View All Author Informationhttps://doi.org/10.1097/JU.0000000000001251.01AboutFull TextPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareFacebookLinked InTwitterEmail "Re: A Multi-Institutional Analysis of the Effect of Positive Surgical Margins following Robot-Assisted Partial Nephrectomy on Oncologic Outcomes." The Journal of Urology, 204(5), p. 1078 Suggested Reading : Local tumor bed recurrence following partial nephrectomy in patients with small renal masses. J Urol 2018; 199: 393. Link, Google Scholar © 2020 by American Urological Association Education and Research, Inc.FiguresReferencesRelatedDetails Volume 204Issue 5November 2020Page: 1078-1078 Advertisement Copyright & Permissions© 2020 by American Urological Association Education and Research, Inc.MetricsAuthor Information Jeffrey A. Cadeddu More articles by this author Expand All Advertisement PDF DownloadLoading ...

Variations in storm-induced bed level dynamics across intertidal flats

Hydrodynamic forces on intertidal flats vary over a range of temporal and spatial scales. These spatiotemporal inhomogeneities have implications for intertidal flat morphodynamics and ecology. We determine whether storm events are capable of altering the long-term morphological evolution of intertidal flats, and unravel the contributions of tidal flow, wind-driven flow, waves, and water depth on inhomogeneities in bed level dynamics (bed level changes over ~days) across these areas. We complement decades of bed level measurements on eight intertidal flats in two estuaries in the Netherlands with an extensive 1-month field campaign on one of those flats. Across this intertidal flat, the hydrodynamics and morphodynamics of a storm event were captured, including the post-storm recovery. We show that individual events can persistently alter the morphological evolution of intertidal flats; magnitudes of some bed level changes are even comparable to years of continuous evolution. The morphological impacts of events are largely controlled by the relative timing of the forcing processes, and not solely by their magnitudes. Spatiotemporal variations in bed level dynamics of intertidal flats are driven by a combination of: (1) the inhomogeneous distributions of the hydrodynamic forcing processes (including the under-explored role of the wind); and (2) the linear proportionality between bed level dynamics and the local bed slope.

Kinematics of Particles at Entrainment and Disentrainment

We address the issue of characterizing experimentally entrainment and disentrainment of sediment particles of cohesionless granular beds in turbulent open channel flows. Employing Particle Image Velocimetry, we identify episodes of entrainment and of disentrainment of bed particles by analysing the raw PIV images. We define a reference velocity for entrainment or disentrainment by space-averaging the flow field in the vicinity of the (entrained or disentrainned) particle and by time-averaging that space-average over a short duration encompassing the observed episode. All observations and measurements took place under generalized movement conditions and in non-controlled geometrical set-ups, resulting in unique databases of conditionally sampled turbulent flow kinematics associated with episodes of particle entrainment and of particle disentrainment. Exploring this database, the objective of this paper is to prove further insights on the dynamics of fluid-particle and particle-particle interactions at entrainment and disentrainment and to polemicize the use of a reference velocity to serve as a proxy for hydrodynamics actions responsible for entrainment or disentrainment. In particular, we quantify the reference velocity associated with entrainment and disentrainment episodes and discuss its potential to describe the observed motion vis-a-vis local bed micro-topography and the type of entrainment or disentrainment event. Entrainment may occur at a wide range of reference velocities, including smaller than mean (double-averaged) velocities. Anecdotal evidence was collected for some typologies of entrainment: (i) momentum transfer from flow to a single particle, (ii) momentum transfer from a perturbed local flow to a single particle, (iii) collective entrainment associated to momentum transfer between a moving and a resting particle and (iv) collective entrainment considered to be a dislodgment of several particles involving momentum transfer from other particles. In some of these cases, e.g., (ii) and (iii), the use of a reference velocity seems inadequate to characterize the entrainment episode. A word of caution about the use of entrainment models based on reference velocities is henceforth issued and contextualized. In the case of disentrainment, a reference velocity seems to constitute a better descriptor of the observed behaviour. The scatter in the observed values seems to express the contribution of bed micro-topography. All particles were found to experience frictional contacts with the resting bed surface particles, but some particles were stopped more abruptly due to the presence of an obstacle along their path. Most disentrainment of particles took place when the near-bed flow was featuring ejection events.

Modeling COVID-19 and Its Impacts on U.S. Immigration and CustomsEnforcement (ICE) Detention Facilities, 2020.

U.S. Immigration and Customs Enforcement (ICE) facilities house thousands of undocumented immigrants in environments discordant with the public health recommendations to reduce the transmission of 2019 novel coronavirus (COVID-19). Using ICE detainee population data obtained from the ICE Enforcement and Removal Operations (ERO) website as of March 2, 2020, we implemented a simple stochastic susceptible-exposed-infected-recovered model to estimate the rate of COVID-19 transmission within 111 ICE detention facilities and then examined impacts on regional hospital intensive care unit (ICU) capacity. Models considered three scenarios of transmission (optimistic, moderate, pessimistic) over 30-, 60-, and 90-day time horizons across a range of facility sizes. We found that 72% of individuals are expected to be infected by day 90 under the optimistic scenario (R0 = 2.5), while nearly 100% of individuals are expected to be infected by day 90 under a more pessimistic (R0 = 7) scenario. Although asynchronous outbreaks are more likely, day 90 estimates provide an approximation of total positive cases after all ICE facility outbreaks. We determined that, in the most optimistic scenario, coronavirus outbreaks among a minimum of 65 ICE facilities (59%) would overwhelm ICU beds within a 10-mile radius and outbreaks among a minimum of 8 ICE facilities (7%) would overwhelm local ICU beds within a 50-mile radius over a 90-day period, provided every ICU bed was made available for sick detainees. As policymakers seek to rapidly implement interventions that ensure the continued availability of life-saving medical resources across the USA, they may be overlooking the pressing need to slow the spread of COVID-19 infection in ICE’s detention facilities. Preventing the rapid spread necessitates intervention measures such as granting ICE detainees widespread release from an unsafe environment by returning them to the community.Electronic supplementary materialThe online version of this article (10.1007/s11524-020-00441-x) contains supplementary material, which is available to authorized users.

Development of Patient Evacuation Resource Classification System (PERC) Using Systems Engineering to Assist Hospital Evacuations in a Disaster.

The aim of this study was the development of a decision-support tool capable of identifying the most appropriate transportation and local area coalition bed space (resources) for patient evacuations during a crisis. Introduction of the new concept of dichotomous tables and how they are incorporated into the Patient Evacuation Resource Classification (PERC) system. The methods used were Systems Engineering fundamentals to conduct a requirements analysis of the process, develop a new construct guided by functional analysis, design an evidence-based model that accurately identifies patient resource requirements translatable to resource capabilities, and create a prototype for a proof of concept. Research resulted in the PERC system prototype. The PERC system prototype created by this research allows for system testing under a Solomon four-group design simulation with the participation of leading medical, emergency response, academic, consulting industry, and logistics professionals from the Washington, DC, metropolitan area.

Invited Commentary: Changing Landscape of Imaging in Recurrent Prostate Cancer.

Invited Commentary: Changing Landscape of Imaging in Recurrent Prostate Cancer.

Joint Stochastic Bedload Transport and Bed Elevation Model: Variance Regulation and Power Law Rests

We describe the joint dynamics of bedload transport and bed elevation changes with a stochastic population model, and we analyze (1) the dependence of bedload flux statistics on local bed elevations and (2) resting time distributions for sediment undergoing burial in the fluctuating sedimentary bed. The model involves entrainment and deposition in a control volume characterized by elevation‐dependent rates, and it exhibits a statistical regulation effect, whereby bed aggradation suppresses the variance of the bedload flux while degradation enhances it. This variance regulation effect is contingent on collective entrainment, whereby moving grains destabilize stationary grains in a positive feedback. When collective entrainment is turned off,bedload transport fluctuations become independent of the bed elevation. Return times from above in the bed elevation time series provide heavy‐tailed power law distributions of resting times with tail behavior characterized by the mean erosion rate and the active layer depth. These results imply bedload statistics measurements on relatively short timescales can be strongly biased by bed elevation changes when collective entrainment occurs, and they support the growing consensus that sediment burial generates heavy‐tailed sediment resting times that ultimately generate anomalous bedload diffusion.

Impact of local fluidized bed hydrodynamics on the distribution of liquid sprayed into the bed

In industrial Fluid Coking™ processes, wet agglomerates formed with coke particles flowing down from the spray regions to the stripper section cause fouling of the stripper sheds, which is a major operating problem. The objective of this work was to determine how to reduce agglomeration by increasing the superficial gas velocity, modifying the gas distributor configuration, or adding internals such as baffles. All these approaches were able to reduce agglomeration. Increasing the superficial gas velocity from 0.18 to 1 m/s reduced the proportion of the injected liquid that was trapped in agglomerates by 50%. Concentrating the gas bubble flow to the first half of the jet cavity with a baffle provided a further reduction in liquid trapped by 12%.

Performance of riffle structures on the stabilization of two successive knickpoints over a sandy bed

Two successive knickpoints with a 10% slope were constructed 1 m apart on a sandy bed in a rectangular flume with a longitudinal slope of 0.003. Bed erosion and knickpoint migration were studied experimentally for different discharges. The performance of two grade-control structures–Newbury rock riffles (NRR) and cross-vane riffles (CVR)–were studied experimentally for the stabilization of each knickpoint. Both of the structures were successful in controlling the bed erosion; however, the NRR operated relatively better than the CVR for they could concentrate the flow at the middle part of the channel to produce more regular contours with less local erosion and bed settlement. The experiments demonstrated that the construction of a control structure was not only effective in the stabilization of a knickpoint but also retarded the migration of its neighboring counterpart.