Bed Material Discharge Research Articles

One-dimensional numerical simulation of CFB combustion: Improvement of hydrodynamic models for accurate prediction of pressure and particle size distributions

This study aims to improve the accuracy of one-dimensional numerical simulations of circulating fluidized bed (CFB) boilers by evaluating models related to solid properties, such as attrition, fragmentation, cyclone efficiency, and bed material discharge. The study validates models for estimating solids holdup and particle size distribution (PSD) using experimental results from a 12 MWth CFB boiler. The study found that accurate prediction of solid axial holdup and local PSD requires proper consideration of attrition and fragmentation, and the simulation results show that attrition has a more significant effect on numerical analysis performance than fragmentation. By using appropriate values for sand attrition constant (5E-7), char attrition constant (6E-6), and coal:1.5, sand:1.2, and char(ash):1.2 fragmentation constants, the pressure distribution measured from experiments was best predicted. The simulation successfully predicted the particle PSD distribution at a specific location in the CFB boiler and pressure change by height, similar to the experimental results In addition, this study investigated the accuracy of 3D computational fluid dynamics (CFD) simulations using solid properties obtained through 1D simulation as initial conditions compared to those based on experimental conditions. The results revealed that 3D CFD simulations using the solid properties from 1D numerical analysis are more accurate, implying that 1D-3D co-simulation approach is a good strategy to increase the simulation performance.

Effect of surface texture and structure on the development of stable fluvial armors

Stable fluvial armors are found in river systems under conditions of partial sediment transport and limited sediment supply, a common occurrence in nature. Stable armoring is also readily recreated in experimental flumes. Initially, this bed stabilizing phenomenon was examined for different flow discharges and solely related to surface coarsening and bedload transport reduction. The models developed suggest a specific armor composition (i.e., texture) dependent on the parent bed material and formative discharge. Following developments in topographic remote sensing, recent research suggests that armor structure is an important control on bed stability and roughness. In this paper, replicated flume runs during which digital elevation models (DEMs) were collected from both exposed and flooded gravel beds are used to interpret armoring manifestations and to assess their replicability. A range of methodologies was used for the analysis, providing information on (i) surface grain size and orientation, (ii) bed-elevation distributions, (iii) the spatial coherence of the elevations at the grain-scale, (iv) surface slope and aspect, (v) grain imbrication and (vi) the spatial variability in DEM properties. The bed-surface topography was found to be more responsive than bed-material size to changes in flow strength. Our experimental results also provide convincing evidence that gravel-beds' response to water-work during parallel degradation is unique (i.e., replicable) given the formative parameters. Based on this finding, relationships between the armors' properties and formative parameters are proposed, and are supported by adding extensive data from previous research.

Form and stability of step‐pool channels: Research progress

Research examining the hydraulics, morphology, and stability of step‐pool mountain streams has blossomed in the last decade, resulting in more than a dozen dissertations. These, along with other research projects, have transformed our understanding of step‐pool channels. Contributions have been made toward understanding depositional step formation and destruction, scour downstream of steps, step‐pool hydraulics, and the effect of sediment transport on step stability. We propose that depositional steps exist in a jammed state whereby the boulders are structurally arranged within the channel and thereby stabilize it. Once a step has formed, a scour pool with a characteristic length and depth develops downstream, creating a zone where additional steps are unlikely to occur. Downstream of the scour hole, steps are more likely to occur as the high energy associated with the plunge pool has dissipated. Data suggest that the presence of cobbles or boulders limits pool scour as well as the degree to which well‐defined, channel‐spanning step‐pools form. We propose a state‐space for step‐pools in which conditions for a step to form include (1) the ratio between width and boulder diameter (the jamming ratio), (2) the ratio between applied shear stress and the stress needed to mobilize the bed (relative Shields number), and (3) the ratio between bed material supply and discharge (bed sediment concentration). Available data suggest this model is plausible. Emerging critical research questions are discussed.

Sediment transport equation assessment for selected rivers in Malaysia

This paper describes a total of 122 sediment data obtained from May 2000 until October 2002 at Kinta River Catchment in the river sediment collection and analysis project. Data collection including suspended load, bed load, bed material and flow discharge have been carried out at six study sites consisting of four rivers which are situated at Kinta River Catchment, namely Kinta River, Pari River, Raia River and Kampar River. The sediment transport equation assessments have been carried out using Yang, Engelund & Hansen, Ackers & White and Graf equations. The results of Yahaya (1999) and Ariffin (2004) studies for Kerayong River, Kulim River and Langat River catchment (224 sets of data) are also included in this present study.

Anabranching and maximum flow efficiency in Magela Creek, northern Australia

Anabranching is the prevailing river pattern found along alluvial tracts of the world's largest rivers. Hydraulic geometry and bed material discharge are compared between single channel and anabranching reaches up to 4 times bank‐full discharge in Magela Creek, northern Australia. The anabranching channels exhibit greater sediment transporting capacity per unit available stream power, i.e., maximum flow efficiency (MFE). Simple flume experiments corroborate our field results showing the flow efficiency gains associated with anabranching, and highlight the prospect of a dominant anabranch, which is found in many anabranching rivers. These results demonstrate that anabranching can constitute a stable river pattern in dynamic equilibrium under circumstances in which a continuous single channel would be unable to maintain sediment conveyance. We propose the existence of a flow efficiency continuum that embraces dynamic equilibrium and disequilibrium (vertically accreting) anabranching rivers.

Evaluation of sediment transport in the lower part of the Fitzroy River, Queensland

Comprehensive sediment sampling carried out by the Queensland Department of Natural Resources and Mines (DNRM) at the Laurel Banks site located about 76 km upstream of the Fitzroy River mouth during four flood events in the period 1994 -1999 allowed the evaluation of 14 different sediment transport formulae. The bed material load was accurately predicted by the formula of Bagnold and by the formula of Pacheco-Ceballos. Finally the authors have developed a sediment transport formula best suited to the Fitzroy River conditions. By applying acceptable formulae to hydrograph data for 35 years it was confirmed that approximately 20 million tons of sediment might be eroded from the catchment during the most severe flood events. The mean annual total sediment discharge has been computed to be more than 5 million tons, and the mean bed material discharge sums up to about 260,000 tons.

Closure to “Bed Material Discharge Prediction for Nonuniform Bed Sediments” by Fazle Karim

Closure to “Bed Material Discharge Prediction for Nonuniform Bed Sediments” by Fazle Karim

Causes of river bed degradation

River bed degradation can proceed downstream as well as upstream depending upon the cause of degradation. The causes of downstream progressing degradation are primarily related to changes in independent river channel variables, such as increase in water discharge, decrease in size of bed material, and decrease in bed material discharge. The causes of upstream progressing degradation are all related to an imposed increase in river slope which can occur as a result of natural river behavior or by man‐made changes. Study of various case histories indicates that river slopes are increased by lowering a base level, by decreasing the length of a river, or by removal of a control point. Case histories also show that downstream and upstream progressing degradation can act in combination along a river system: downstream progressing degradation along the main stream of a river system can initiate upstream progressing degradation on a tributary.

Universal Flow Diagrams for Mobile Boundary Channels

Using dimensional analysis and regression techniques a series of diagrams, involving five dimensionless flow parameters, showing the interrelationship of the discharge intensity, flow depth, mean bed material size and discharge, and channel slope for mobile boundary channels have been prepared. The universal flow diagrams (UF diagrams) cover 10 grain size ranges, from 0.1 to 40 mm, with each diagram based on an analysis of 1300 or more data items. Examples are given showing the use of the UF diagrams for design and analysis of channel and river problems.

Sediment Movement and Friction in Alluvial Streams

This study is aimed at developing a simple but reliable sediment discharge relation that could be free from the difficulties associated with the use of existing ones. It treats total bed material discharge instead of separate estimates of suspended and bed transport because most hydraulic problems associated with sediment transport can be more effectively solved by considering total sediment discharge, and because of the uncertainties in finding when or where the sediments are being transported as suspended, or load, especially in the presence of bed forms

Sediment Transport Measurements in Gravel River

A need for estimates of sediment transport rates in gravel-bed rivers prompted a field study of methods for measuring total sediment discharge. Measurements were carried out on the Elbow River at Bragg Creek, Alberta, during the summer flood periods of 1967, 1968, and 1969. Suspended load was measured directly by sampling. Bed-load was measured directly by sampling. Bed-load was measured by two types of sampler, herein designated the VUV (Hungarian type) and Basket type, respectively, and the results were compared to those obtained by measuring volumetrically the refilling rate of a large pit excavated in the river bed. This paper presents sediment-discharge rating curves and compares popular bed-load formulas, derived from flume measurements, to a natural gravel-bed river. A practical method is proposed for estimating bed-material discharge, utilizing field observations in conjunction with graphical correlations recently developed from an extensive analysis of flume data.

Sediment Dispersion in Flow with Moving Boundaries

The longitudinal dispersion equation for sediment in unsteady nonuniform flow with moving boundaries is formulated from the three-dimensional instantaneous diffusion equation for movement of sediment by means of the time- and space-averaging processes. The equation is then studied for its applicability and validity in the sediment problem. The analytical expressions of the longitudinal dispersion coefficient for sediment in flow with moving boundaries are obtained for both turbulent and laminar flow. The effect of moving boundaries in turbulent flow on the value of the longitudinal dispersion coefficient has theoretically been shown to be negligible, but may be significant at a place where a sand or water wave breaks, causing an infinite gradient of bed or flow profile. The longitudinal dispersion equation that may often be called the suspended-load equation is shown to play an important role in the mathematical modeling of an alluvial channel where the suspended bed-material discharge is greater than the contact-bed discharge.

Design of Stable Channels in Alluvial Materials

A method of designing sand-bed channels is suggested which considers bed roughness, resistance to flow, bed material discharge, and channel geometry. An alluvial channel may be designed when the discharge, fall diameter of the bed material, and the water temperature are known.

Interpretations of Sedimentary Structures by Flume Experiments: ABSTRACT

The various types of bed roughness that are formed by the interaction between the flowing water and a sand bed form distinctive but complex sedimentary structures. The resultant forms of bed roughness and sedimentary structures are related to many variables such as regime of flow, channel geometry, velocity, and the characteristics of the sediment. By studying the characteristics of the sedimentary structures, it is possible to determine quantitatively the magnitude of many of these variables. The characteristics of the sediment can be determined by analyzing samples from the sedimentary structures. The regimes of flow and forms of bed roughness can be identified from the cross-bedding of the sedimentary structures. The spacing and amplitude of these structures indicate v locity and depth of flow at the time of deposition. From relations describing channel geometry and with preceding information, channel width can be determined approximately. Last, knowing channel geometry, type of bed roughness, and the characteristics of the sediment, various sediment transport relations can be used to estimate the total bed-material discharge. In summary, utilizing knowledge of the relations between bed forms, bars, sedimentary structures, hydraulic variables, sediment variables, sediment discharge, and channel geometry, it is possible to determine quantitatively many of the hydraulic and hydrologic characteristics of the depositional environment that formed the sedimentary structures. End_of_Article - Last_Page 2036------------

Total Bed-Material Transport

A method of estimating the bed-material discharge of sand-bed flumes, canals, and rivers is presented. The method uses a group of curves developed from flume data that relate intensity of transport, intensity of shear, and the median fall diameter of the bed material. Also, these curves illustrate that the intensity of transport varies with regime of flow and form of bed roughness. Application of the method yields good results for sand-bed channels when the depth of flow is less than 4 ft.

Practical Computations of Bed-Material Discharge

At least five alternative measures of fluid forces or fluid velocities may have usually dominant effects on the discharge of bed-material in a sand-bed stream. Interrelationships among most of these measures vary widely with changes in the resistance to flow. Approximately defined average relationships between discharge of bed-material per foot of stream width and each of four usually dominant measures (mean velocity, stream power, and two forms of effective shear on the bed sediment) are shown graphically as possible bases for practical computations of bed-material discharge. Such usually secondary factors as depth of flow, particle size of the bed sediment, water temperature, and concentration of fine sediment affect each defined relationship, generally by different amounts.