Bed Material Size Distribution Research Articles

Steep Creek Risk Assessment for Pipeline Design : A Case Study From British Columbia, Canada

ABSTRACT Pipelines in mountainous terrain often cross alluvial fans formed by steep creek processes of debris flows and debris floods and are thus exposed to their associated hazards. The design of new pipeline infrastructure and maintenance of existing pipelines necessitates steep creek risk assessments and appropriate mitigation design. We present methodology for assessing steep creek risk along pipeline routes that evaluates the probability of such processes causing a pipeline loss of containment or disruption in service. The methodology consists of estimating event frequency, scour potential, and the vulnerability of the pipeline to break if impacted by boulders. The approach can be adapted to other landslide geohazards so that different geohazard locations can be evaluated with a common metric. Steep creek process frequency is estimated based on field observations and review of documented events, historical air photo records, and terrain mapping based on LiDAR-generated topography. Scour potential is estimated based on channel morphology, presence of bedrock, and grain size distribution of channel bed material. Vulnerability is estimated based on flow width and velocity and can be modified for different pipe diameters and wall thicknesses. Mitigation options for buried pipelines include those intended to decrease the likelihood of the pipeline being exposed and to increase the resiliency of the pipeline to boulder or organic debris impacts, if exposed. The methodology presented is embedded in risk-informed decision making where pipeline owners and regulators can define probability thresholds to pipeline exposure or rupture, and pipeline designers can demonstrate that proposed mitigation measures achieve these threshold criteria.

Riverbed armoring and sediment exchange process in a sand–gravel bed reach after the Three Gorges Project operation

Upstream damming greatly altered the flow and sediment regime entering downstream reaches in the Middle Yangtze River, and the bed material in a sand–gravel bed reach coarsened continuously, which had a significant influence on the sediment transport and bed evolution. In order to study the riverbed armoring, the sediment exchange process (SEP) among bed material, bed load and suspended load in a sand–gravel bed river is firstly clarified, and then, the three-state transition probability model (Markov chain) is proposed in this study, with the hiding-exposure effect of non-uniform sediment being considered. Finally, the equilibrium equation of sediment quantity in an active layer is presented to calculate the grain size distribution of bed material. In this model, the influences of flow and sediment conditions, riverbed erosion and deposition on the SEP are discussed. The results show that the composition of surface bed material at the Zhicheng station became obviously coarse, and the median grain size (d50) of surface bed material increased from 0.230 to 0.424 mm in 2003–2017, with an upward increasing trend. The proposed probabilistic model was validated against field measurements of bed material, and calculated results show reasonable agreement with the measured data at Zhicheng. Accordingly, the probabilistic model can be used to predict the riverbed armoring and to investigate the non-equilibrium transport of non-uniform sediment in a sand–gravel bed river.

Factors Influencing Fine Sediment on Stream Beds in the Midwestern United States.

Fine sediment (particles <2 mm in diameter) in stream beds has wide-ranging effects on hydraulics, geomorphology, and ecology and is a primary focus for stream quality management in many regions. We identify reach- and basin-scale factors associated with fine sediment in the beds of 83 stream reaches in the Midwestern United States using recursive partitioning of sand-bed and gravel-bed streams and a generalized linear model for the fraction of a stream bed covered by fine sediment. A water-surface gradient of 0.00075 is the best single determinant (80% correct classification) distinguishing sand-bed streams (lower gradient) from gravel-bed streams (higher gradient). In the higher gradient category, sand-bed streams generally had more variable monthly precipitation than gravel-bed streams. The fractional response model indicated that the proportion of a stream bed composed of fine sediment is related to high sediment supply and low transport capacity but also high gravel transport capacity. This result is consistent with both theory and observations that bed material can be transported indiscriminately with respect to particle size under high shear stress, which will drive the particle size distribution of bed material toward the distribution of supply. Management of fine sediment in Midwestern streams has been approached largely by focusing on sediment supply, which may be immutable in some places due to the landscape position or glacial history. Retention of coarse sediment is an alternative management approach to reduce the fraction of fine sediment in the beds of some Midwestern streams.

OBTAINING AND TESTING COARSE GRAIN SIZE DISTRIBUTIONS FROM PHOTOGRAPHIC GRID COUNTS

The grain size distribution (GSD) of surface bed material in alluvial rivers is of interest in many geological, ecological and engineering applications. In this work, which is a part of our broader Ph.D research in Vouraikos river on sediment transport, we are testing the possibility of obtaining such a distribution from photographs taken at the field (grid sampling), compared with the standard method which is the “pebble count”. We have also examined and quantified the errors resulting from radial distortion of the photographs. The motive of the work was to explore the opportunity of substituting field work for laboratory work. Results show that for this method to give reliable results, a better knowledge of the site-specific conditions is needed, which, in turn, requires more field work.

Experimental study on the attrition and size distribution of bed material in a recirculating fluidized bed

ABSTRACTAn experimental study was performed on the attrition of the bed materials in a recirculating fluidized bed (RCFB) using Indian standard (IS) grade I sand (size between 2.0 and 1.0 mm) at ambient conditions. Experiments were performed with superficial air velocity ranging from 7.13 to 9.16 m/s, bed inventory of 7–10 kg, and a spacing of 0.085 m between the jet top and draft tube bottom. The main objective of the study was to investigate the effects of operation time on the attrition and size distribution of sand particles in a RCFB. It was noticed that the prime mode of attrition of bed materials was abrasion, not fragmentation. Reduction in the downcomer bed height was observed with increasing operation time. It indicates that attrition was significant and fines were elutriated out with the fluidizing air. Furthermore, variations in the shape, size, and harmonic diameter of particles were studied with increasing operation time. It has been observed that the coefficient of uniformity and coefficient of curvature showed increasing patterns. It specifies that particles of different size ranges and fines were formed due to attrition of particles. At the end of the operation, it was found that a significant amount of fines was elutriated with fluidizing air from the reactor.

Three‐dimensional flow structure and patterns of bed shear stress in an evolving compound meander bend

AbstractCompound meander bends with multiple lobes of maximum curvature are common in actively evolving lowland rivers. Interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound bends is poorly understood. In this paper, acoustic Doppler current profiler (ADCP) measurements of the three‐dimensional (3D) flow velocities in a compound bend are examined to evaluate the influence of channel curvature and hydrologic variability on the structure of flow within the bend. Flow structure at various flow stages is related to changes in bed morphology over the study timeframe. Increases in local curvature within the upstream lobe of the bend reduce outer bank velocities at morphologically significant flows, creating a region that protects the bank from high momentum flow and high bed shear stresses. The dimensionless radius of curvature in the upstream lobe is one‐third less than that of the downstream lobe, with average bank erosion rates less than half of the erosion rates for the downstream lobe. Higher bank erosion rates within the downstream lobe correspond to the shift in a core of high velocity and bed shear stresses toward the outer bank as flow moves through the two lobes. These erosion patterns provide a mechanism for continued migration of the downstream lobe in the near future. Bed material size distributions within the bend correspond to spatial patterns of bed shear stress magnitudes, indicating that bed material sorting within the bend is governed by bed shear stress. Results suggest that patterns of flow, sediment entrainment, and planform evolution in compound meander bends are more complex than in simple meander bends. Moreover, interactions among local influences on the flow, such as woody debris, local topographic steering, and locally high curvature, tend to cause compound bends to evolve toward increasing planform complexity over time rather than stable configurations. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Study on the dynamic balance behaviors of bed material during the start-up process of a circulating fluidized bed boiler

The bed material size distribution and bed inventory are important to the speed, energy consumption, auto-control of the start-up process and solid concentration distribution of circulating fluidized bed (CFB) boiler. A dynamic model to simulate the dynamic balance of the particle size distribution (PSD) of bed material in the start-up process of a CFB boiler is developed. The model is applied to a 350MW CFB boiler to assess the dynamic changes of the PSDs of bed material under the influences of cyclone efficiency, initial bed material size distribution, coal quality and bed inventory. The results show that the cyclone efficiency plays important roles on the dynamic PSD and balanced PSD of bed material. The higher the cyclone efficiency, the finer the balanced bed material is. The fly ash particles and coarse particles among the initial material will escape the furnace in 0.5h and 3h respectively during the start-up process, and the circulating particles will achieve balance in ~4h. Coal quality affects the dynamic balance of the PSD of bed material significantly, the larger the mass fraction of circulating class particles over the coal ash, the shorter the start-up time is. The closer the initial material size distribution to the balanced material size distribution, the shorter the start-up time is. The less the bed inventory, the shorter the start-up time is.

Predicting bed load transport of sand and gravel on Goodwin Creek

Bed load transport rates are difficult to predict in channels with bed material composed of sand and gravel mixtures. The transport of bed load was measured on Goodwin Creek, and in a laboratory flume channel with a similar bed material size distribution. The range of bed load transport rates measured in the laboratory channel were similar to those measured in the channel of Goodwin Creek; however, the shear stresses calculated on Goodwin Creek were three times greater than in the laboratory channel for similar bed load transport rates. Much of this difference in shear stress was removed by applying the drag partitioning technique of Einstein, although significant differences between the two sets of data remain. Predictions of bed load transport rates using three previously published transport relations were good for most flows for the laboratory data. For the Goodwin Creek bed load data, predicted transport rates were close to measured ones for low flows but diverged by an order of magnitude or more for high shear stresses. Improved methods of shear stress partitioning are needed to improve the performance of bed load transport relations on streams of this type.

Modelling of sediment transport and bed deformation in rivers with continuous bends

A two dimensional (2D) RNG k–ε sediment model including the effects of secondary currents is developed to simulate the sediment transport and bed deformation in rivers with continuous bends. Non-uniform suspended and bedload sediment transports and the variation of effective bed material size distribution are included in the model. A semi-coupled scheme for sediment model is proposed, which can be used for simulating both the long- and short term sediment transport whenever riverbed changes. The model is applied to simulate the flow and sediment transport in the upper reach of the Yellow River which is a typical natural river reach with continuous bends. The river bed deformations caused by suspended and bedload sediments are investigated. Good agreement between the numerically simulated results and the field measurements is obtained, indicating that the model is capable of simulating the sediment transport and predicting the bed deformation of rivers having continuous bends with reasonable accuracy.

Effects of fluidized parameters on capture of heavy metal in various bed-material size distributions

Artificial waste was used to simulate agglomeration/defluidization in a fluidized bed and to examine the heavy-metal distribution in bottom ash for various bed-material particle-size distributions (narrow, flat, and Gaussian). Heavy-metal concentration in most particles increased with Na addition, and the largest (>1.000mm)- and smallest (<0.500mm)-particle-size fractions had the highest metal concentrations. Additionally, Cr had the highest retention rate within the bed particles, followed by Pb and Cd. Comparison of the heavy-metal concentrations and retention rates for different particle-size distributions showed that the total retention rates were higher for narrow and Gaussian distributions than for the flat distribution.

Effects of bed material size distribution, operating conditions and agglomeration phenomenon on heavy metal emission in fluidized bed combustion process

This study investigates the effects of the bed material size distribution, the operating conditions and the agglomeration/defluidization phenomenon on the heavy metal pollutant emissions in the combustion process. After defluidization, the emission concentration of heavy metals increased, because Na may form a low melting eutectic material that enhances bed material adherence. The emission of Cd increased when the feed simulated urban residues contained sodium; however, the presence of Na had no significant effect on the emission of Cr. Furthermore, the Cd emission concentration was low when the material had a Gaussian distribution; however, the decreases in the Cd emission when the bed material had narrow, binary or flat distributions were not significant. The heavy metal Cr showed the same trend. In addition to the operating parameters, the bed material size distribution may influence the heavy metal emissions during combustion processes.

Modelling dam-break flows over mobile beds using a 2D coupled approach

Dam-break flows usually propagate along rivers and floodplains, where the processes of fluid flow, sediment transport and bed evolution are closely linked. However, the majority of existing two-dimensional (2D) models used to simulate dam-break flows are only applicable to fixed beds. Details are given in this paper of the development of a 2D morphodynamic model for predicting dam-break flows over mobile beds. In this model, the common 2D shallow water equations are modified, so that the effects of sediment concentrations and bed evolution on the flood wave propagation can be considered. These equations are used together with the non-equilibrium transport equations for graded sediments and the equation of bed evolution. The governing equations are solved using a matrix method, thus the hydrodynamic, sediment transport and morphological processes can be jointly solved. The model employs an unstructured finite volume algorithm, with an approximate Riemann solver, based on the Roe-MUSCL scheme. A predictor–corrector scheme is used in time stepping, leading to a second-order accurate solution in both time and space. In addition, the model considers the adjustment process of bed material composition during the morphological evolution process. The model was first verified against results from existing numerical models and laboratory experiments. It was then used to simulate dam-break flows over a fixed bed and a mobile bed to examine the differences in the predicted flood wave speed and depth. The effects of bed material size distributions on the flood flow and bed evolution were also investigated. The results indicate that there is a great difference between the dam-break flow predictions made over a fixed bed and a mobile bed. At the initial stage of a dam-break flow, the rate of bed evolution could be comparable to that of water depth change. Therefore, it is often necessary to employ the turbid water governing equations using a coupled approach for simulating dam-break flows.

Optimising flow–sediment transport parameters for rivers

The unsteady flow–sediment transport equations for a real river–reservoir system contain unknown hydraulic, sediment and geometric parameters. Knowledge of these parameters and input data comprising initial and boundary conditions is required to simulate flow–sediment transport, water surface and river bed profiles. The sensitivity analysis and optimisation procedure, which involves determining these parameters by fitting a model with real river field data, is an important tool for finding the best-fit values of each parameter in a river–reservoir sediment transport model. Sensitivity analysis and optimisation methods have been found to be very useful when applied to sediment routeing problems in river–reservoir systems. This technique was employed for optimisation of some of the important parameters involved in graded sediment routeing and bed armouring processes using a non-linear coupled model (NCMG). The particular parameters studied in this investigation are degradation, bed armouring and grain size distribution of bed material in a river bed downstream of a dam. The effects of bed roughness, sediment parameters and thickness of the active layer on bed level changes are studied. A combined Gauss–Newton and modified Newton method are employed to calculate optimised parameters.

Channel response to gravel mining activities in mountain rivers

The removal of bed material from active river channels usually affects the bed profile of the streambed, causing progressive degradation upstream and downstream of the extraction site. These effects can extend for kilometers affecting hydraulic structures located in the vicinity of the river reach. In this paper, the geomorphic effects of gravel mining are reviewed and summarized. Some cases in Venezuelan streams are presented to illustrate the problem. To describe the processes of erosion and sedimentation in a gravel extraction pit, a recent developed mathematical model for the simulation of flow and sediment transport in gravel-cobble bed streams is applied to a hypothetical case of gravel mining in a river channel. A simple rectangular dredge pit is imposed as initial condition in the channel bed, and changes in bed elevations and grain size distribution of bed material are calculated by using the numerical model. The process of deposition within the pit, and the downstream and upstream migration of the erosion wave are well simulated by the model and closely resemble the phenomena observed in laboratory experiments. The response of the friction coefficient to the changes in flow and bed elevations shows the importance in modeling adequately flow resistance and sediment transport in gravel-cobble bed streams.

At-a-site variation and minimum flow resistance for mountain rivers

Existing flow resistance relationships for mountain rivers may be in error by typically 30%. In part this is because flow resistance is a complex phenomenon, not easy to quantify with the available data. However, a further reason is the reliance of past analyses on fitting relationships to data from multiple sites, hence emphasizing an averaging between-site variation. A more scientific approach is to derive separate at-a-site relationships and investigate how they can be collapsed into a single formula. Twenty-seven carefully selected field datasets from seven literature sources are therefore analysed empirically to show at-a-site variations in resistance, with the aim of improving the accuracy and applicability of mountain river flow resistance relationships. Considering only the bed grain roughness, the principal factors likely to determine flow resistance are identified as relative submergence d/ D 84 (where d is depth and D 84 is the bed material 84-percentile size), bed material size distribution and channel slope. The dependency of the resistance function (8/ f) 1/2 (where f is the Darcy-Weisbach resistance coefficient) on d/ D 84 is more accurately described by a power law than by a semi-logarithmic law, especially at high flows. From observations of coincident at-a-site datasets, two equations are derived, distinguished largely according to channel slope S. For S<0.8%, (8/ f) 1/2=3.84( d/ D 84) 0.547 ( r 2=0.986). For S>0.8% (8/ f) 1/2=3.10( d/ D 84) 0.93 ( r 2=0.959). These equations appear to define minimum values for flow resistance, probably for ideal conditions of bed grain roughness and uniform flow. Deviations from this ideal (because of form roughness, nonuniform flow or bank effects) will increase resistance in a manner which is not yet quantified. The equations therefore have a very specific applicability and are not generally applicable to all flows. The cause of the slope dependency remains unclear, nor is any dependency on bed material size distribution identified. Nevertheless the high correlation coefficients suggest an improved accuracy compared with other field-based relationships. The equations are derived for in-bank flows with d/ D 84<11 and slopes in the range 0.2–4%.

７１ＭＷｅ加圧流動層燃焼装置における石灰石による脱硫 Ｐａｒｔ１． 燃焼装置内における石灰石の摩耗と破砕

Attrition and fragmentation of limestone in a 71MWe pressurized flu-idized bed combustor were estimated. Formation of Ca-containing fine particles due to attrition rate was calculated from the material balance of calcium in fly-ash which was captured at the exit of combustor. The average rate of attrition (rate of reduction of radius) of limestone was estimated to be approximately 1μm/h.A population balance model was established to estimate the particle size distiribu-tion in the bed from the size distribution of fed limestone and attrition rate of bed materi-al in the bed. For some cases, the measured bed material size was far smaller than that of the estimated size distirbution. These results suggest that the fragmentation of lime-stone particle took place in the bed. The size distribution of limestone after fragmenta-tion was estimated so that the difference between estimated bed material size distribu-tion and measured size distribution should be minimized. The results indicates that the large limestone particles (greater than approximately 1mm) were broken into small parti-cles when they were fed into the combusotor.

THE SEDIMENT RUN OFF OF THE ISHIKARI RIVER

It is an important subject to evaluate sediment transport rate in river management works. Especially in a large scale river like Ishikari, establishment of a tool to predict the sediment transport at the estuary is becoming urgent subject from environmental view point. The propose of this paper is to study the effect of the past experienced flood to the grain size distribution of bed material and sediment load. For this propose, data of suspended load observation was collected and the suspended sediment observation was conducted in the Ishikari River. Using these data, and a numerical model, the relationship between suspended sediment load and the discharge history is discussed.

A STUDY ON SEDIMENT RUNOFF OF THE ISHIKARI RIVER

It is an important subject to evaluate sediment transport bubfet in river management works. In this paper sediment transport output from the Ishikari River is investigated as primitive example for this subject. Relationship between turbidity data and suspend sediment concentration data is investigated and a formula to evaluate wash load transport rate is proposed. Using the daily discharge data of the past 20 yeas, the annual sediment runoff from the Ishikari River mouth is calculated. The annual runoff of the bed material is also calculated by a numerical model for bed load and suspended load transport, bed deformation and grain size distribution of bed material. The results of wash load runoff and bed material load runoff are compared and it is found that wash load runoff can be explained as a part of bed material load.

Frequency distribution of river bed materials

ABSTRACTThe size distribution of river bed material plays an important role in the study of fluvial hydraulic processes. It is not always easy to sample the river bed material for the determination of its size distribution, but knowledge of the frequency distribution and of bed material sizes can be used to predict its size distribution. Therefore, data on bed material size distributions of 63 streams from 10 different countries have been analysed to determine the frequency distribution which can be best fitted to them. The log normal distribution cannot be fitted to the bed material mixtures when the entire range of sizes is considered. The method of power transformation is found to normalize these mixtures. Relationships have been proposed between the parameters of the normalization and the stream slope. These relations can be used to predict the size distribution of the river bed material.

The non-layering of gravel streambeds under ephemeral flood regimes

The two-layer format common to perennial streambeds, in which a relatively coarse armour overlies a finer subarmour, develops as a function of both the ingress and subsequent near-surface winnowing of interstitial material and the selective non-entrainment or slower transport velocity of coarse clasts. Ephemeral streams appear to lack such vertical layering or are characterized by weak layer development. Some of this may be due to the degree of mixing associated with the scour-and-fill process. However, continuous monitoring of bedload discharge in the Nahal Yatir in the northern Negev Desert reveals that sediment transport rates are extremely high so that the chance of armour layer development through selective non-entrainment is much reduced. Indeed, a comparison of the bedload and bed material size-distributions confirms a high degree of similarity and hints at equal mobility regardless of clast size. The monitoring programme also indicates that the bed becomes highly mobile at comparatively modest fluid shear, so that practically all floods are associated with high transport rates. Consequently, the winnowing that might be brought about by low transport-rate events does not occur. Even within a single event, winnowing is precluded by the rapid nature of flow recession that is so characteristic of flash-floods. The high degree of bed material mobility is attributable, in part, to the lack of strength that would otherwise be a corollary of armour development. However, it also highlights the divergent nature of the feedback loops that govern the relationship between flow and channel deposit in ephemeral and perennial systems.