Bed Stability Research Articles

Large grains matter: contrasting bed stability and morphodynamics during two nearly identical experiments

AbstractWhile the stabilizing function of large grains in step‐pool streams has long been recognized, the role they play in gravel‐bed streams is less clear. Most researchers have ignored the role of large grains in gravel‐bed streams, and have assumed that the median bed surface size controls the erodibility of alluvial boundaries. The experiments presented herein challenge this convention. Two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material. The two distributions had the same range of particle sizes, and nearly identical bulk d50 values (1.6 mm); however the d90 of experiment GSD1 was slightly finer (3.7 mm) than that for experiment GSD2 (3.9 mm). Transport rates during GSD1 were nearly four times greater than during GSD2 (even though the dimensionless shear stress was slightly lower), and the channel developed a sinuous pattern with well‐developed riffles, pools and bars. During GSD2 the initial rectangular channel remained virtually unchanged for the duration of the experiment. The relative stability of GSD2 seems to be associated with a slightly larger proportion of stable (large) grains on the bed surface: at the beginning of GSD1, 3.5% of the bed was immobile, while almost twice as much of it (6.1%) was immobile at the beginning of GSD2. The results demonstrate that the largest grains (not the median size) exert first‐order control on channel stability. Copyright © 2017 John Wiley & Sons, Ltd.

URBAN STREAM RESTORATION AND APPLIED PRACTICES IN NORTHEAST ILLINOIS

INTRODUCTION: In-stream and watershed dynamics in urban and urbanizing areas have significant impacts on local property and infrastructure, as well as the quality of the stream itself including: water quality, habitat, physical characteristics, and biodiversity. As land development occurs, natural vegetation and exposed soils are converted to buildings, pavement and other impervious surfaces. This leads to increased runoff during storm events as well as decreasing the time that it takes that stormwater to reach streams, wetlands, and other stormwater storage and conveyance systems. These hydrologic changes in a watershed often occur at a rapid pace which results in rapid destabilization and degradation of streams and rivers. Rivers and streams are naturally dynamic systems. They naturally erode and reshape themselves based on changes to the watershed or the stream itself. Erosion and deposition are natural processes that have always been important components of stream systems and in and of themselves are not undesirable. When natural stream dynamics are rapidly accelerated, however, an entire series of negative impacts to the stream and the biological systems that are depended on the stream occur. Rapid destabilization of streams often leads to significant bank and bed erosion that negatively impact stream health and frequently leads to negative impact to property, buildings and structures, as well as public infrastructure. Past approaches to stream bank and bed stabilization often involved channelization, armoring, and other gray infrastructure techniques to protect public and private property in the effected reaches of streams and rivers without taking into account the overall stream system dynamics. Early stabilization efforts frequently led to other unintended consequences by accelerating the rate of bank and bed erosion in untreated reaches, inadvertent flooding, and other infrastructure impacts. The complex nature of stream dynamics and fluvial geomorphology when applied to urban stream systems and significantly modified watersheds require the need for detailed analysis of the morphology of the stream. Consideration of the complex factors and processes that make up fluvial morphology are critical when selecting practices or methods of stream restoration. Many agencies and cooperative partners work to accumulate and analyze case studies and detailed research in order to develop a method of evaluating and prescribing different stream restoration techniques based on the morphologic conditions in the stream reach (Lyn D.A., and Newton J.F., 2015). An accumulation of case studies, research, and scholarly work on stream restoration techniques and practices helps shape and inform designers across multiple agencies in order to effectively select and design restoration practices. Ultimately, in urban streams, the designer is working to establish a condition of dynamic equilibrium in the treated stream reach. Dynamic equilibrium is defined as a stream reach that is in balance with sediment transport, aggradation, degradation, and bank and bed erosion. When those characteristics are in balance based on the inputs of sediment within the watershed, the bed load and sediments the stream transports, and discharge rate and volume, then the stream is considered to be in a relatively stable state (FISRWG, 1998). The selection then of stream restoration and stabilization practices in urban areas is dependent on not only the reach being treated, but also on the overall watershed dynamics. In addition to the physics of the actual practices implemented, including resistance to shear stresses and velocity of the water flow within the stream channel being treated, the practices must also take into account the larger picture of stream dynamics including sediment delivery and transport, within the watershed and not just within the treated reach. Successful urban stream restoration and stabilization techniques mimic the structures found in more undisturbed systems through the utilization of similar materials in an engineered configuration. In many streams the use of a combination of hard and soft armorment and stabilization solutions including stone, woody debris materials, modern geosynthetic reinforcement devices and native vegetation to stabilize and naturalize stream channels, thereby provided enhanced habitat, better water quality, and protecting property and infrastructure.

Finite element analysis of a vibration test bed frame

The stability and reliability of a vibration test bed are significantly influenced by the mechanical property of its structure. In order to reveal the mechanical property of the structure and its effect on the strength and stiffness, a finite element model of the damper vibration test bed frame has been created, the loads and constraints have been imposed, and the static solution has been carried out, from which the stress states and the maximum deformation of the test bed frame have been obtained. Moreover, according to the determined stress distribution, the strength and stiffness of the test bed frame have been extracted. The simulation results show that the maximum deformation, which is 0.0717 mm, is within the safety range and meets the stiffness safety condition. The maximum stress is 59.6 MPa and meets also the strength security condition. Finally, the minimum section stress has been compared to the theoretical prediction, and the results agree very well.

The Reliability of the Locking Devices of the Switches. Role of the Ballast Bed

The dimensioning of the locking device of a switch is made essentially as a function of the applied loads during the switch rails movement or considering a false route. The phenomena related to the vibrations induced by the track are therefore normally not taken into account. Such vibrations are particularly accentuated by the ballast bed deconsolidation in the sleeper panel supporting the locking device. Unfortunately, there is no specific technical reference in order to control these aspects, even though the most frequent cause of locking device components failures is relied to the vertical oscillations induced by the trains, rather than to the effort to commute the switch rails. In this study, the results of a field investigation carried out at two levels of ballast tamping are reported and discussed. It is found that the ballast compaction plays a fundamental role for the component to be prone to the early attainment of critical mechanical conditions. Procedures for the monitoring of the stability and efficiency of the ballast bed to ensure the required reliability of the locking device are also envisaged.

Flood hazards at ford stream crossings on ephemeral channels (south‐east coast of Spain)

Most road‐stream crossings over ephemeral channels are vulnerable to extreme hydrologic events. Ford stream crossings (FSCs) are usually dangerous for the road traffic during periods of high flow, in particular under flash flood conditions. The present paper analyzes the flood hazards on the Mediterranean coast in the Region of Murcia (south‐east Spain), affecting this type of road‐stream crossing over dry channels, according to hydraulic variables and bedload transport rates estimated for discharges at bankfull and flood‐prone stages. Under such conditions, the safety of people and vehicles was obtained using numerical models, developed by previous researchers; in particular, water levels and flow velocities across ford reaches were compared with different trend curves between water depths and corresponding critical velocities for children and adults, and for various prototype vehicles. Specifically, two approaches to assess this type of hazards were proposed: a specific Hydraulic Hazard Index and an algorithm for estimating the flood hazard from criteria of bed stability and bedload transport capacity (Flood Hazard at Fords, FHF). In addition, different exposure levels were established, using a Flood Vulnerability Index, based on the FHF, the road category, and the annual average daily traffic. The FHF model gave the best results with regard to the magnitude of the damage observed in recent flash floods for flow stages similar to those simulated. According to the danger thresholds established for this index, half‐bankfull flows represent here a high risk: 27.3% of FSCs for mini‐cars and 18.2% for large cars. At bankfull, the FHF exhibits very high values for mini‐cars (77.3% of FSCs) and for large passenger vehicles (50% of FSCs), while at the floodprone stage, extreme FHF values are reached for all kinds of vehicles at most of the ford crossings.

Performance of an anaerobic membrane bioreactor in which granular sludge and dynamic filtration are integrated

To alleviate the fouling of a filter, simple substrates, dynamic filtration, and granular sludge were applied in an anaerobic membrane bioreactor (AnMBR). The results showed that under a transmembrane pressure < 20 kPa, the filter flux ranged between 15 and 20 l (m−2 h)−1 for a period of 30 days. The flux was higher than the typical flux of AnMBRs with conventional membranes and most current dynamic filters. In addition, the low cost of the filter avoided the need for a higher flux. Moreover, a stable granular sludge bed, which consumed all volatile fatty acids, was maintained. A compact fouling/filtration layer formed on the filter, which contributed to low effluent chemical oxygen demand concentrations and turbidity. In addition, substrate scarcity in the filtration zone resulted in the evolution of diverse bacteria on the filter.

Spatial and temporal dynamics of macrophyte cover in a large regulated river

The River Ebro basin is extensively dammed. Dams alter the geomorphological functioning of the river by altering its flow regime (e.g. reducing mean and maximum discharges), increasing bed stability (armouring) and decreasing turbidity (water clarity). These effects, together with an increase in nutrient concentrations and water temperature, have generated optimal conditions for the proliferation of aquatic macrophytes. In this paper, we analyse the temporal and spatial changes of macrophyte cover in the lowermost Ebro through a series of field campaigns carried out between 2009 and 2010. Special attention was paid to the spatial distribution of macrophytes in relation to flow hydraulics, channel geometry and bed sedimentology. Temporal changes in macrophyte cover were analysed in relation to the frequency and magnitude of both natural floods and flushing flows (artificial flow releases from dams with generally a magnitude that equates around a2-year flood in the river). Spatially, the proportion of macrophytes along the reaches showed a variable pattern, with a succession of areas with both high and low plant density, coinciding with the alternation of riffles and pools in the channel. The highest values of plant cover (>65%) occurred in riffles and in transition to riffle areas, while the lowest densities (1% or almost negligible) were observed in pools and transition to pool areas. Water depth and the grain-size distribution of the riverbed materials (i.e. D84), are found to be the main factors controlling the degree of plant cover in the lower Ebro. Temporally, the macrophyte proportion varied during the hydrological year, with a clear increment from late spring to early autumn (i.e. vegetation cover reached 40%, on average, of the channel surface). Macrophyte coverage decreases immediately following a flushing flow but in the long term, vegetation re-occupied the area again, even slightly increasing in some sections; overall, the mean percentage of macrophyte cover was 19% higher at the end of the study period, despite the numerous flow events occurred on the meantime. This increase enhanced riverbed stability, which in turn reduced the possibility for bed-material entrainment. This study empirically confirms the necessity of improving the management options applied in the lower Ebro with complementary measures to help maximise the efficiency of flow releases (for instance, subject the macrophytes to a severe hydrological stress by decreasing discharge before a given flushing flow, undertake localise mechanical removal of plants in areas where density is high, and increase the frequency of floods in winter time when macrophyte stands are weaker).

The role of grain-size ratio in the mobility of mixed granular beds

The main goal of the study was to understand the effects of grain-size distribution on the stability of beds in the sand-silt range, which is a critical subject for the understanding of geomorphological processes in aquatic environments. Although theoretical models can explain the mobilization of a mixed bed, there is a clear lack in knowledge regarding the stabilizing effect of non-cohesive fine material. To connect existing findings, we analysed bed stability in relation to grain-size distribution in laboratory experiments. Erosion experiments in an annular flume were conducted using beds of different size compositions of spherical glass beads, i.e. a) the grain-size ratio RD=D50,coarse/D50,fine (the relative size of coarse and fine grains; D50=39–367μm) and b) the amount of fines. Several glass-bead combinations with unimodal and bimodal grain-size distributions (RD=3.9, 5.8, and 9.4) and varying fine fractions (10–40% dry weight) were subjected to increasing flow speeds (0.01–0.19ms−1). Using acoustic Doppler velocimetry (ADV) and optical backscatter, the flow profile in the vicinity of the bed surface, the changes in bed morphology, and the suspended sediment concentration (SSC) were measured. A new method was developed to evaluate the bed-level changes detected by the ADV as a proxy for the bed mobility. We found different modes of bed mobility depending on the grain-size ratio. For low grain-size ratios, an increase in the fine fraction (to 40%) led to increased bed-level changes during the experiment and the mobilization of the mixed bed at the highest flow speed. For high ratios an increase in fine fraction (to 40%) led to a decrease of bed-level changes and the beds remained stable, i.e. no bed forms developed even at the highest flow speed. Therefore, increasing the amount of fine particles can lead to different modes of behaviour depending on the grain-size ratio. For a bimodal sediment bed with spherical grains under unidirectional flow conditions, the grain-size ratio can be used to estimate the bed mobility, i.e. to evaluate if a bed is likely to mobilize at flow speeds of up to 0.19ms−1. Hence, for the stability of a non-cohesive, mixed bed in the sand-silt range, the grain-size ratio plays a fundamental role, while the amount of fine grains is a secondary factor.

Magnetic stabilization of fluidized beds: Effect of magnetic field orientation

Fluidized beds of granular materials can be stabilized by interparticle attractive forces which confer the expanded bed an elastic modulus that stabilizes it against flow perturbations. Stabilization in a structure of enduring contacts is seen to occur naturally due to the universal van der Waals forces for ∼50μm particle size beds albeit in a quite reduced interval of gas velocities over the minimum fluidization velocity vmf. As shown in this work, a magnetic field may induce attractive forces between magnetizable particles thus extending the stable fluidization interval well beyond vmf. The structure of the magnetically stabilized bed is however markedly anisotropic since attractive magnetic forces are maximum along the direction of the externally imposed field which leads to the formation of chain particle aggregates. This paper shows experimental measurements on the magnetic yield stress, gas velocity at the transition to marginal stability and microstructure of magnetostabilized beds as affected by the direction of the magnetic field. Data shows that magnetic stabilization is optimized for co-flow fields as chain aggregates are preferentially orientated parallel to the magnetic field and along the direction of minimum drag. As the magnetic field is tilted, particle chains become tilted according to a balance between the magnetic attractive force between the particles and the vertical drag force, which reduces the magnetic yield stress and therefore shortens the interval of magnetic stabilization.

Effects of an emergent vegetation patch on channel reach bathymetry and stability during repeated unsteady flows

AbstractWhile research into the interaction between in‐channel vegetation, flow, and bed sediment has increased in recent years, there is still a need to understand how unsteady flows affect these processes, particularly in terms of channel bed adjustments. In this study, flume experiments tested two flood hydrograph sizes run over sand/gravel and sand/silt beds to evaluate reach scale impacts of a midchannel vegetation patch of variable stem density on channel bathymetry and stability. Alternating flood hydrographs with periods of low, steady flow created flow sequences reflective of an extended unsteady flow regime, thereby simulating time scales consisting of multiple flood events. Digital elevation models provided detailed measurements of channel change following each flood event to enable analysis over each unsteady flow sequence. The vegetation patch created characteristic channel bathymetries dependent on sediment mixture and patch density that in all cases resulted in a more variable bed structure than channels without a patch. Reach scale stability, quantified based on net volumetric bed change, only occurred with a sparse patch in the low flood sequence, corresponding with little variation in surface composition and structure. In most other cases, scour measured at the patch prevented stability at the reach scale, especially in the finer substrate. Overall, findings show that a channel may only adjust to a stable bathymetry upon addition of a midchannel vegetation patch within a limited range of flow regimes and patch stem densities, and that for the experimental conditions tested here, in‐stream patches generally did not enhance reach scale bed stability.

Particle and pore dynamics under column leaching of goethitic and saprolitic nickel laterite agglomerates

In an agglomerated particle heap leach process, the pore network structure within the agglomerates and the particle bed is a critical determinant of efficient lixiviant flow. During the leaching period a degree of slumping can occur, changing the structure of the inter-particle void spaces and the porosity of the particle bed. The ability to characterise the agglomerate pore network structure and measure its changes over the leach period is essential for the understanding and improvement of lixiviant flow and the ultimate performance of the heap. Three dimensional (3D) non-destructive X-ray micro-tomography (XMT) offers a great opportunity to provide detailed spatially resolved structural information of these multiphase particulate systems. The aims of this paper are to compare and contrast the dynamic pore network structure of saprolitic (SAP) and goethitic (G) Ni laterite mineral agglomerate beds undergoing acid leaching by periodic scanning with XMT, as well as to extract quantitative porosity information from this data and correlate it with/to other measures of bed stability, specifically mass loss and particle bed slump. The resulting XMT analyses were able to reliably discriminate between solid and gas phases, enabling the estimation of porosity from the segmented images which in turn agreed well with the measured mass loss and slump. The SAP particle bed has consistently higher porosity and leach stability compared to the G particle bed and this is consistent with the higher HL processability of SAP ores compared to G ores observed in other studies.

Effects of surrounding land use and water depth on seagrass dynamics relative to a catastrophic algal bloom.

Seagrasses are the foundation of many coastal ecosystems and are in global decline because of anthropogenic impacts. For the Indian River Lagoon (Florida, U.S.A.), we developed competing multistate statistical models to quantify how environmental factors (surrounding land use, water depth, and time [year]) influenced the variability of seagrass state dynamics from 2003 to 2014 while accounting for time-specific detection probabilities that quantified our ability to determine seagrass state at particular locations and times. We classified seagrass states (presence or absence) at 764 points with geographic information system maps for years when seagrass maps were available and with aerial photographs when seagrass maps were not available. We used 4 categories (all conservation, mostly conservation, mostly urban, urban) to describe surrounding land use within sections of lagoonal waters, usually demarcated by land features that constricted these waters. The best models predicted that surrounding land use, depth, and year would affect transition and detection probabilities. Sections of the lagoon bordered by urban areas had the least stable seagrass beds and lowest detection probabilities, especially after a catastrophic seagrass die-off linked to an algal bloom. Sections of the lagoon bordered by conservation lands had the most stable seagrass beds, which supports watershed conservation efforts. Our results show that a multistate approach can empirically estimate state-transition probabilities as functions of environmental factors while accounting for state-dependent differences in seagrass detection probabilities as part of the overall statistical inference procedure.

Block Ramps in Curved Rivers: Morphology Analysis and Prototype Data Supported Design Criteria for Mild Bed Slopes

Eco-friendly river restoration structures are a valid solution for river training projects. Among this structure typology, block ramps have been successfully tested to solve problems related to river sediment control, bed stabilization and energy dissipation. Despite the conspicuous literature dealing with block ramps design in straight rivers, there are no studies analysing the erosive process occurring in the stilling basin downstream of a block ramp in a curved river bend. Therefore, this paper represents the first systematic analysis of their behaviour and of the resulting downstream equilibrium morphology in such geometric configuration. A dedicated model was built to simulate a wide range of hydraulic conditions. Experimental data analysis allowed describing the erosion dynamics occurring in the stilling basin and, at the same time, to derive a useful design relationship by which it is possible to estimate the maximum scour depth. Furthermore, the model results were successfully validated by using field measurements collected in the Porebianka River (Poland). Both field data and laboratory experimental results allowed furnishing a comprehensive description of the scour phenomenon. The proposed relationship represents the first trustable and valid tool for hydraulic design of such structure typology in curved rivers. Copyright © 2016 John Wiley & Sons, Ltd.

Recurrent neural network modeling applied to expanded bed adsorption chromatography of chitosanases produced by Paenibacillus ehimensis

Nonlinear autoregressive networks with external input (NARX) and multilayer perceptron (MLP) has been used to predict the activity and protein content for flow-through, washing and elution steps during expanded bed adsorption chromatography of chitosanases produced by Paenibacillus ehimensis. Bed expansion as well as the influence of particulate-containing feedstock study showed a stable bed operation without significant impact on the yield as well as purification factor when the cells were fed to the column. Also, NARXs showed a better performance to predict the chitosanolytic activity as well as total protein than MLPs.

In situ investigation of the effects of current velocity on sedimentary mussel bed stability

The intertidal sediments of the Wadden Sea is an environment where organisms must endure significant physical stresses. Organisms like the blue mussel (Mytilus edulis), face the difficulty of maintaining their position in an unstable substrate. Previous studies on mussel beds in sediment showed that the substrates mussels use for attachment can differ across a bed. Increased adhesion to shell debris in the sediment resulted in what appeared to be a reinforced bed edge. To quantify the erosion resilience in different substrate use zones of an intertidal mussel bed, three distinct zones in a mussel bed were exposed, in situ, to artificially generated current velocities. Velocities ranged from high (0.7ms−1) to very high (1.43ms−1), covering the upper extremes of current velocities measured on mussel beds and those well beyond normal conditions. Silt content of the sediment was decreased 20% towards the bed edge. Results showed that the resilience of the mussel bed to erosion differed significantly between zones. The zone along the bed edge showed the greatest resilience to erosion remaining intact at current velocities of 1.43ms−1. This was also the zone with the greatest mussel cover (90%) and highest density. Erosion resilience was lower in the zones closer to the bed center. Erosion of mussels was always precluded by erosion of the sediment underlying the mussel bed (undercutting). In the zone with an intermediate mussel cover (77%), undercutting already occurred at a current velocity of 0.7ms−1 and was twice as pronounced as in the site with the lowest mussel cover (53%). In all zones, mussel dislodgement did not occur below current velocities of 1.16ms−1. In zones towards the bed center, complete erosion of the bed occurred at 1.43ms−1. Results suggest that a 10% lower mussel coverage increased undercutting by 50%. The reinforced waveward bed edge proved to be capable of resisting erosive forces likely only exceeded during extreme storms. The presence of such a resilient barrier shields the rest of the bed from erosion allowing mussels in the sheltered zones to invest fewer resources in adhesion. Results show the significance of mussel coverage and adhesion on bed resilience. Any predictive modeling or artificial restoration efforts should account for and facilitate the development of such bed edges to improve a beds chance of survival.

Poly(N-vinylcarbazole-co-divinylbenzene) monolith microextraction coupled to liquid chromatography–high resolution Orbitrap mass spectrometry to analyse benzodiazepines in beer and urine

A poly(N-vinylcarbazole-co-divinylbenzene) (NVC/DVB) monolithic column was successfully prepared in a silanized capillary for microextraction of six benzodiazepines (BZPs) in complex samples. The polymerization was optimized primarily by changing the ratio of porogen content. The optimized polymer monolith had a homogeneous and continuous column bed, good permeability and mechanical stability. Poly(NVC/DVB) had good affinity to BZPs because of its through-pore structure and strong π-π stacking interactions derived from the rich benzene functional groups in the polymer. Potential factors that affect the extraction efficiency were studied in detail. In the optimized method, 3mL of sample (pH 2) was extracted with the polymer monolithic column and eluted with 80μL of methanol, and the eluent was analysed with ultra-high-performance liquid chromatography and quadrupole high resolution Orbitrap mass spectrometry (UHPLC/Q-Orbitrap MS). Under these conditions, the developed method gave the linear range of 0.005–0.5ng/mL for most of the six BZPs, whereas 0.05–0.5ng/mL for nitrazepam, 0.01–0.5ng/mL for lorazepam, respectively, with coefficients of determination (R2)≥0.9991. The detection limits (LODs) were 1.08–6.04ng/L, and the quantification limits (LOQs) were 3.60–20.1ng/L. The method repeatability was investigated in terms of intra- and inter-day precision, which were indicated by relative standard deviations (RSDs) of ≤8.3% and ≤9.9%, respectively. Finally, the proposed method was successfully applied to detect BZPs in beer and human urine samples. The percentages of extraction recovery were 80.4–94.2% for beer and 79.6–95.2% for urine.

Floating-fixed-bed-gasification: From Vision to Reality

Over the last years, numerous gasification concepts have been developed to a reliable and commercially available state leading to a strongly increasing numbers of built plants. One of these describes a novel way in the thermochemical gasification of biomass by using a unique reactor design and operational mode – the floating-fixed-bed reactor. Comparable gasification concepts show bottlenecks in downsizing possibilities and tar concentration in product gas from fluidized bed gasification as well as problems of channeling or compaction of the fuel bed in fixed-bed systems. The idea of the floating-fixed-bed reactor is to build up a stable bulk bed, which is floating on the inlet gas stream from pyrolysis and oxidation. Without uncontrolled particle movement in the reduction reactor and only relative movement of the fuel particles within the bed, relatively long gas residence could be provided, resulting in low tar concentrations in the product gas. The technology has proven the potential to bypass issues of comparable gasification technologies and verified the advantages of the innovative reactor design in several long-term test runs on pilot plants and also commercial plants. Beside continuous improvement of the gasification technology, the current development aims for upscale possibilities and increased fuel flexibility.

Scour Downstream of Grade Control Structures under the Influence of Upward Seepage

The installation of free falling jet grade control structures has become a popular choice for river bed stabilization. However, the formation and development of scour downstream of the structure may lead to failure of the structure itself. The current approaches to scour depth prediction are generally based on studies conducted with the absence of upward seepage. In the present study, the effects of upward seepage on the scour depth were investigated. A total of 78 tests without and with the application of upward seepage were carried out using three different sediment sizes, three different tailwater depths, four different flow discharges, and four different upward seepage flow discharge rates. In some tests, the three-dimensional components of the flow velocity within the scour hole were measured for both the cases with and without upward seepage. The scour depth measured for the no-seepage results compared well with the most accurate relationship found in the literature. It was found that generally the upward seepage reduced the downward velocity components near the bed, which led to a decrease in the maximum scour depth. A maximum scour depth reduction of 49% was found for a minimum tailwater depth, small sediment size, and high flow discharge. A decay of the downward velocity vector within the jet impingement was found due to the upward seepage flow velocity. The well known equation of D’Agostino and Ferro was modified to account for the effect of upward seepage, which satisfactorily predicted the experimental scour depth, with a reasonable average error of 10.7%.

Variations of bed elevations due to turbulence around submerged cylinder in sand beds

This paper presents the spatio-temporal variations in bed elevations and the near-bed turbulence statistics over the deformed bed generated around the submerged cylindrical piers embedded vertically on loose sediment bed at a constant flow discharge. Experiments were carried out in a laboratory flume for three blockage ratios in the range of 0.04–0.06 using three different sizes of submerged cylinders individually placed vertically at the centerline of the flume. Clear-water experimental conditions were maintained over the smooth sediment bed surface with a constant flow discharge (\(Q = 0.015\,{\rm m}^3/{\rm sec}\)), thereby giving three different cylinder Reynolds numbers \(Re_{D_c} = \frac{U_mD_c}{\nu }\) (=10200, 12750, 15300) away from the cylinder locations, where \(U_m\) is the maximum mean velocity, \(D_c\) is the cylinder diameter and \(\nu\) is the kinematic viscosity of fluid. Instantaneous sand bed elevations around the cylinders were recorded using a SeaTek 5MHz ultrasonic ranging system of net 24 transducers to estimate bed form migration, and the near-bed velocity data at transducer locations over the stable deformed bed around the pier-like structures were collected using down-looking three-dimensional (3D) Micro-acoustic Doppler velocimeter to estimate the bottom Reynolds shear stresses and the contributions of bursting events to the dominant shear stress component. The flow perturbation generated due to relatively lower flow blockage ratio favored to achieve the stable bed condition more rapidly than the others, and larger upstream scour-depth and deformed areas were noticed for greater flow blockage ratio due to larger cylinder diameter. For larger blockage ratio in the upstream of scour-hole near the bed, occurrences of probabilities of both boundary-ward interactions (Q1 and Q3) were the dominant; whereas in the downstream of the scoured region, occurrences of probabilities of second and third quadrant events (Q2 and Q4) were dominant. On the other hand, for the lower blockage ratio, quadrant (Q2) was dominant over Q4 in the downstream of scour-hole, and in the upstream of scour-hole, quadrant Q4 was the dominant.

Physical modeling of oil charging in tight reservoirs: A case study of Permian Lucaogou Formation in Jimsar Sag, Junggar Basin, NW China

Abstract Modeling experiments of oil charging were conducted to find out patterns and affecting factors of oil migration and seepage in tight reservoirs, and analyze oil migration and accumulation and low limit conditions of tight oil accumulation using core samples from tight reservoir beds of the Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin. Crude oil charging in tight reservoir beds has two pressure gradient points (start-up pressure gradient and critical pressure gradient, and has two features: low velocity non-Darcy seepage, quasi-linear seepage). During crude oil charging in tight reservoir beds in the Lucaogou Formation, the process of oil saturation increase can be divided into three types: saltation increase, quick increase and stable increase. Samples of quick increase type reached the highest oil saturation, the second place is the stable increase type, and saltation increase type is the last. Oil saturation increase is controlled by the combined effect of porosity, permeability, oil viscosity and displacement pressure gradient. These factors interact and complement one another. By establishing template for oil accumulation in tight reservoir beds, it can be seen that only when pressure gradient breaks through the critical pressure gradient and the oil flow is quasi-linear, can oil saturation reaches the lower limit value (30%) in tight reservoir beds. It is hard for stable tight reservoir beds to become tight firstly and be charged with oil and gas later; while for conventional reservoir beds, after oil and gas charging, the formation compaction, cementation, and secondary mineral outgrowth may be the reasons for the formation of tight oil accumulation with high oil saturation.