Bed Material Size Research Articles

Field Study of Flow Resistance in Step-pool Streams (Case study of Dizin River)

A series of steps and pools are ubiquitous bed forms in mountain stream channels, occurring where gradients exceed 2% and materials are in the gravel to boulder size range. Flow resistance, reflected by roughness elements, appears to be an important control on bed load transport rates and mean flow velocity. To estimate flow resistance some morphological features and velocity were measured in the step-pool channel of Dizin River, located in Karaj River watershed in Iran. Topographic surveys and bed sediment sampling were made in low flow condition while three-dimensional velocity measurements were made in low, medium and high flow conditions. Gradient variations are in the range of 7% to 14%. As flow resistance is a function of geometric, bed material size, longitudinal slope and hydraulic radius, dimensional analysis was conducted to develop a non-dimensional relationship for flow resistance in step-pool reaches. Thereafter, it was calibrated for the measured data set of Dizin river and validated for Rio Cordon data set. Comparable results of validation with a river located in a different environment may suggest that flow resistance features in semi arid and humid streams may have similar effects on non-dimensional resistance coefficient

Measurement of dimensionless Chezy coefficient in step-pool reach (Case Study of Dizin River in Iran)

Step–pool reaches are common bed forms in mountainous streams exceeding 3% gradient and provide an important means of flow resistance in steep rivers. Step-pool channels are characterized by longitudinal steps formed by large boulders which are constituted into discrete channel-spanning accumulations that separate pools comprising finer grain. Flow resistance, reflected by roughness materials, appears to be an important control on bed load transport rates and mean flow velocity. To estimate flow resistance some morphological features and velocity were measured in the step-pool channel of Dizin River, located in Karaj River watershed in Iran. Topographic surveys and bed sediment sampling were made in low flow condition while three-dimensional velocity measurements were made in low, medium and high flow conditions. As flow resistance is a function of geometric, bed material size, longitudinal slope and hydraulic radius, dimensional analysis was conducted to derive a non-dimensional relationship for Chezy coefficient in step-pool reaches. Thereafter, it was calibrated for the measured data set of Dizin, Ammameh and Rio Cordon. Comparable results of calibration with a river located in a different environment may suggest that flow resistance features in semi arid and humid streams may have similar effects on non-dimensional Chezy coefficient.

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.

Experimental and numerical analysis of the scour profile downstream of flip bucket with change in bed material size

Flip bucket is one of the energy dissipaters which passes over flow through the dam and is used for injecting the flow to the plunge pool. Impact of the water jet with river leads bed elevation to change and it not only threatens the stability of dam, but also influences the river in downstream. Experimental studies include record of changes in bed elevation in plunge pool of the Kheirabad dam in research institution of the ministry of power in various hydraulic conditions. Then, numerical simulation was conducted by Flow3D and after calibration of simulation using experimental results, a model with 2.7% error was achieved and results of sediment scour model for material sizes ranging from 2 to 15 mm and with flow rates from 113 to 282 lit/s were evaluated and compared with that of literature. Then, width and length of the hole were calculated based on these diameters and flow rates.

The analysis of factors influencing the dynamics of lowland riverbed erosion downstream of the Jeziorsko reservoir dam (central Poland)

Increased riverbed erosion could recently be observed in close vicinity of hydrotechnical constructions in many lowland rivers. The immediate effect of erosion in this case is a very significant reduction of the riverbed level over a considerable distance, downstream of large reservoir dams. This process is influenced simultaneously by both hydraulics of river flow and geotechnical properties of soil eroded. The paper presents an analysis of parametric changes in the level of river bottom and grain size of bed material, and the relative density in the context of changes in hydrological conditions and diversity of engineering-geological conditions. As the result, a multiparametric model of erosional potential of riverbed sediments has been proposed, which can significantly help in proper planning of locations and construction of correction thresholds under given hydraulic conditions of the river.

Clogging of an Alpine streambed by silt-sized particles – Insights from laboratory and field experiments

Clogging of streambeds by suspended particles (SP) can cause environmental problems, as it can negatively influence, e.g., habitats for macrozoobenthos, fish reproduction and groundwater recharge. This especially applies in the case of silt-sized SP. Until now, most research has dealt with coarse SP and was carried out in laboratory systems. The aims of this study are to examine (1) whether physical clogging by silt-sized SP exhibits the same dynamics and patterns as by sand-sized SP, and (2) the comparability of results between laboratory and field experiments.We carried out vertical column experiments with sand-sized bed material and silt-sized SP, which are rich in mica minerals. In laboratory experiments, we investigated the degree of clogging quantified by the reduction of porosity and hydraulic conductivity and the maximum clogging depth as a function of size and shape of bed material, size of SP, pore water flow velocity, and concentration of calcium cations. The SP were collected from an Alpine sedimentation basin, where our field experiments were carried out. To investigate the clogging process in the field, we buried columns filled with sand-sized quartz in the stream bed.We found that the maximal bed-to-grain ratio where clogging still occurs is larger for silt-sized SP than for sand-sized SP. The observed clogging depths and the reduction of flow rate through the column from our laboratory experiments were comparable to those from the field. However, our field results showed that the extent of clogging strongly depends on the naturally-occurring hydrological dynamics. The field location was characterized by a more polydisperse suspension, a strongly fluctuating water regime, and high SP concentrations at times, leading to more heterogeneous and more pronounced clogging when compared to laboratory results.

The Effect of Side Screen Attached to Flat-board Debris-flow Breaker on Damage Reduction

이 논문에서는 바닥스크린의 토석류 피해저감 기능을 더욱 극대화하기 위하여 바닥스크린에 측면스크린을 부착하여 그 효과를 파악함으로써 측면스크린의 적정 순간격을 도출하고자 하였다. 그 결과, 바닥스크린에 측면스크린을 부착함으로써 작은 입경의 공급재료가 소량 측면스크린을 통과하였다. 그 대신에 바닥스크린의 상단면부에는 토석류의 선단부를 구성하던 큰 입경의 토석들이 측면스크린에 의해 규제된 유로폭 내에서 다량 포착된 후, 이 두터운 조립질층의 공극을 작은 입경의 계상재료가 메워 세립화되는 것으로 나타났다. 이러한 세립화는 결국 바닥스크린의 상단면부로부터 하단면부로의 토사 투과를 차단하여 결국 바닥스크린 하단면부에서의 퇴적량은 감소하였다. 한편, 이러한 바닥스크린의 기능 향상은 측면스크린의 순간격이 계상재료의 평균입경 및 최대입경에 일치할 경우 가 동일하게 나타났다. 그러나 스크린 부재의 순간격이 클수록 시공비 절감에 유리할 것으로 추정되며, 따라서 바닥스크린에 부착하는 측면스크린의 순간격은 계상재료의 최대입경과 동일하게 하는 것이 바람직할 것으로 판단된다.The objectives of this study were to evaluate effects of the flat-board debris-flow breaker (FDB) equipped with side screen and to derive an optimal slit aperture of the side screen for maximizing its positive effects. The results showed that only a few small-sized materials were passed through side screen, causing storage at both sides of FDB. Instead, a massive amount of large-sized materials was trapped on the upper surface of FDB regulated by side screen, and this coarse layer was filled with fine sediment. This fining may not arrow passage of even fine materials from upper to lower surface of FDB. Meanwhile, concerning the damage reduction by debris flows, there was no functional difference between the mean and maximum grain-size slit apertures. However, because a larger slit aperture of the screens has an advantage in reducing construction costs, the slit aperture of FDB should be matched to the maximum grain size of bed materials.

Grain-size variability of river-bed sediments in mountain areas (the Gemenea and Slătioara rivers, Eastern Carpathians)

Rivers in mountain areas have alluvial beds largely consisting of coarse alluvia (blocks, cobble, gravel) mobilized and transported downstream only during exceptional hydrological phenomena. Their distribution influences river-bed stability, sediment transport rates and high-water levels, because alluvial deposits define bed roughness. As a rule, especially in large rivers, the size of alluvia deposited on river-beds gradually decreases downstream (downstream fining), as the river transport capacity is reduced. This tendency can be interrupted by discontinuities, caused by the lateral input of alluvia or by anthropogenic interventions. The aim of this paper is to analyze the spatial distribution of the size of bed material along two small rivers, Gemenea and Slatioara, located in the northeastern part of the Eastern Carpathians, Romania. The phenomena of grain-size fining or coarsening of the material deposited along these river-beds are of low intensity. The high degree of connectivity between river-bed and hillslopes and scarps of meadow terraces has generally resulted in unimodal distributions centered on the coarse fraction (gravel, cobble and blocks, over 60% of the total), whereas the fine fraction (sands) is below 1%. A slight tendency of bimodality begins to emerge towards the lower course of the Gemenea river profile (sand increases to slightly over 5% of the total bed material). This occurs with meadow development and the reduction of the connectivity between river-bed and adjacent hillslopes. Hence the conclusion that lateral input and longitudinal sorting of particles are among the main causes of bed deposits bimodality in our study area

Experimental analysis and numerical simulation of bed elevation change in mountain rivers.

Studies of sediment transport problems in mountainous rivers with steep slopes are difficult due to rapid variations in flow regimes, abrupt changes in topography, etc. Sediment transport in mountainous rivers with steep slopes is a complicated subject because bed materials in mountainous rivers are often heterogeneous and contain a wide range of bed material sizes, such as gravel, cobbles, boulders, etc. This paper presents a numerical model that was developed to simulate the river morphology in mountainous rivers where the maximum bed material size is in the range of cobbles. The governing equations were discretized using a finite difference method. In addition, an empirical bed load formula was established to calculate the bed load transport rate. The flow and sediment transport modules were constructed in a decoupled manner. The developed model was tested to simulate the river morphology in an artificial channel and in the Asungjun River section of the mountainous Yangyang Namdae River (South Korea). The simulation results exhibited good agreement with field data.

The theory and practice of NOx emission control for circulating fluidized bed boilers based on the re-specification of the fluidization state

The NOx emission of circulating fluidized bed (CFB) boilers in China is more and more stringently controlled due to the updated national emission regulation (NER). Based upon the theory of CFB combustion and previous research results related to NOx formation mechanism, a technical route was proposed by means of re-specifying the fluidization state in CFB boilers, to achieve the ultra-low NOx emission without installing the deNOx equipment additionally. The main method is to increase the solid circulating rate and decrease the bed inventory as well as the mean size of bed material, which would influence the bed temperature and oxidizing/reducing atmosphere distribution, and thereby benefit for the reduction of NOx emission. Besides, the reasonable control of bed temperature and air supply should be considered. This technique has been demonstrated to be effective in three CFB units with different capacities (150t·h−1, 260t·h−1 and 560t·h−1) and burning different types of coal. No obvious decrease of combustion efficiency was found in the field tests. The technical route provides a feasible approach to further control the NOx emission for CFB boilers.

Anti-wear beam effects on water wall wear in a CFB boiler

Anti-wear beams installed on water walls of circulating fluidized bed (CFB) boilers are one method to protect against water wall wear. Beam effects from superficial gas velocity, solid suspension density, bed material size and beam size were investigated on water wall wear in a CFB test rig experimentally using a measurement system consisting of thin-film resistor sensors and high-precision resistance probers. Multiple sensors were set up around an anti-wear beam vertically to obtain the axial profile of the wear rate of water wall.Results show that installed anti-wear beams may reduce the wear rate of water wall. Along the water wall around the beam vertically, the maximum wear rate occurs at the upper surface of the beam while the minimum rate occurs at the bottom surface. In general, the wear rate of the water wall around an anti-wear beam increased with the superficial gas velocity, initial bed material (static bed height) and particle size, while the wear rate just below the beam was not notably influenced by those parameters. An optimum anti-beam width in the range of 6–9mm is recommended under the test condition.

Downstream channel adjustment in a low-relief, glacially conditioned watershed

River management practices are often informed by theoretical expectations of downstream channel adjustment, which may not be valid in low relief, glacially conditioned watersheds such as those in the lower North American Great Lakes region. Downstream trends in channel morphology and bed material size within a low-relief, glacially conditioned watershed are explored here and compared to a theoretical watershed model where slope and grain size are expected to decline exponentially. The observed channel morphology is then tested against a theoretical concept of reach-scale channel grade. The downstream hydraulic geometry relations wbf∝Qbf0.51S−0.02 and dbf∝Qbf0.32S−0.21 were found to best describe downstream changes in channel morphology and are consistent with some prior studies. Bed material size varies irregularly down the channel. Slope-controlled downstream fining trends are evident where inputs from glacial materials and tributaries are negligible but are masked by cobble/boulder lag deposits where the channel is cut into these glacial deposits. The asynchronous variability in slope and grain size produces downstream variations in graded and nongraded, understeepened conditions separated at τ*ex=0. Graded reaches exist where τ*ex>0, but an upper boundary with nongraded, oversteepened reaches is less clear. The results emphasize the geomorphic legacy of inherited slopes and sediment sources in dictating the modern downstream patterns of fluvial characteristics and morphologies in glacially conditioned, and similarly complex, watersheds.

THE EFFECT OF FLOW CONDITIONS AND GEOMETRIC PARAMETERS ON THE SCOUR VALUE DOWNSTREAM COMPOSITE STRUCTURES OF WEIR AND GATE

In this research, a study was conducted experimentally to investigate the scour hole dimensions downstream the combined structures which consist of weir and gate. Twelve models have been designed and every model is formed from composite weir consists of two geometric shapes and three types of gates which are rectangular, semi-circular and triangular in shape, where multi factors were studied to find out the effect of changing geometry for both weir and gate, discharge flowing in the flume and particle size of bed material on the dimensions of scour hole. The experiments was conducted in a laboratory channel was constructed from blocks and concrete with length of 18 m, 1 m width and 1 m depth. The laboratory models were installed after 7 m from the main gate which is controlling the passage of water from the main reservoir into the flume.At the beginning, the calibration process was conducted to identify the actual discharge values that pass in the flume, then experiments were conducted to calculate the discharge coefficient for each model, which represents one of the studied factors within the dimensional analysis of the variables to derive the empirical formulas to calculate the dimensions of scour hole.Then the experiments were conducted in order to derive formulas to investigate the depth and length of the scour hole which formed in the sand floor spreading as a layer of 30 cm in thickness for a distance 4 m downstream combined structure. Two samples of sand were used in the experiments with different median size of particles ( d50), the first of 0.7 mm and the second of 1 mm.Using the dimensional analysis by π theorem and IBM SPSS 21 program, Four nonlinear relationships were derived to calculate the dimensionless scour depth (SD / d50) and another four nonlinear relationships calculates the dimensionless length of scour (SL / d50)depending on the laboratory results for each of the relative discharge (Qr), Froude number in terms of mean size of particle of bed material (Frd), non-dimensional difference head between upstream and downstream of combined structure (HD / d50), dimensionless distance between the lower edge of the weir and the upper edge of the gate (y3 / d50), dimensionless head over the crest of compound weir (h/d50) and the discharge coefficient (Cd), where the resulted determination coefficients (R2) from these relationships were good.

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 & Sons, Ltd.

Stability and scour development of bed material on crossbar block ramps

Block ramps are ecologically oriented drop structures with adequate energy dissipation and partially moderate flow velocities. A special case is given with crossbar block ramps, where the upstream and downstream level difference is reduced by a series of basins. To prevent the total structure from failing, the stability of single boulders within the crossbars and the bed material in between must be guaranteed. The present paper addresses the stability of bed material and scour development for various flow regimes. Any bed material erosion may affect the stability of the crossbar boulders, which in turn can result in major damages of the ramp. Therefore new design approaches are developed to choose an appropriate bed material size and to avoid failures of crossbar block ramp structures.

Assessing geomorphic sensitivity in relation to river capacity for adjustment

River sensitivity describes the nature and rate of channel adjustments. An approach to analysis of geomorphic river sensitivity outlined in this paper relates potential sensitivity based on the expected capacity of adjustment for a river type to the recent history of channel adjustment. This approach was trialled to assess low, moderate and high geomorphic sensitivity for four different types of river (10 reaches in total) along the Lower Tongariro River, North Island, New Zealand. Building upon the River Styles framework, river types were differentiated based upon valley setting (width and confinement), channel planform, geomorphic unit assemblages and bed material size. From this, the behavioural regime and potential for adjustment (type and extent) were determined. Historical maps and aerial photographs were geo-rectified and the channel planform digitised to assess channel adjustments for each reach from 1928 to 2007. Floodplain width controlled by terraces, exerted a strong influence upon reach scale sensitivity for the partly-confined, wandering, cobble-bed river. Although forced boundaries occur infrequently, the width of the active channel zone is constrained. An unconfined braided river reach directly downstream of the terrace-confined section was the most geomorphically sensitive reach. The channel in this reach adjusted recurrently to sediment inputs that were flushed through more confined, better connected upstream reaches. A meandering, sand-bed river in downstream reaches has exhibited negligible rates of channel migration. However, channel narrowing in this reach and the associated delta indicate that the system is approaching a threshold condition, beyond which channel avulsion is likely to occur. As this would trigger more rapid migration, this reach is considered to be more geomorphically sensitive than analysis of its low migration rate alone would indicate. This demonstrates how sensitivity is fashioned both by the behavioural regime of a reach and flow/sediment input from upstream. The approach to assess geomorphic river sensitivity outlined here could support ‘room to move’ or ‘freedom space’ approaches to river management by relating likely channel adjustments for the type of river under consideration to the area of land that is required to contain ‘natural’ patterns and rates of geomorphic functionality.

A study of the scour–fill threshold based on Lane's equilibrium relation: The lower Yellow River

The scour–fill threshold in river channels is an expression of scour–fill equilibrium, which may be understood as the balance between the river's sediment carrying capability and the sediment load imposed by the drainage basin. In the present study, the above capability–load relation is quantified using Lane's relation, γQJ–QsD50, wherein Q is water discharge, J is channel slope, Qs is sediment transport rate, D50 is median size of bed material, and γ is the specific weight of water. On this basis, we have established scour–fill thresholds for the lower Yellow River based on data from 141 flood events. The results show that the QsD50–γQJ relation may well distinguish between the scour and fill flood events. The following equation has been established: SDRFE=0.00099(γQJ)0.90(QsD50)−0.60, wherein SDRFE is channel sediment delivery ratio. From the equation, the scour–fill threshold is established as: QsD50=0.00001(γQJ)1.5. Starting from this formula, four possible options are proposed for sediment management of the lower Yellow River.

Scour regime effects on the time scale of wave scour below submerged pipes

In this study, a non-dimensional functional relationship was constructed to characterize the time scale of the scour process below submerged pipelines exposed to waves. The data used in the study were obtained from an experimental set-up which mainly consists of a wave channel, a wave generator, a test pipe and several measurement devices. The values for Shields parameters and non-dimensional time scale were calculated for non-cohesive bed material with different grain sizes. The results from both the present work and previous studies, despite being slightly different, indicate that both Keulegan–Carpenter number and Shields parameters are effective in the variation of time scale. In consequence of the results, the non-dimensional time scale is found proportional with both Shields parameter and Keulegan–Carpenter number. Also modified Ursell parameter was evaluated instead of Keulegan–Carpenter number to predict the non-dimensional time scale. The study reveals that the grain size of bed material and the scour regime have effects on the scour process. New empirical relations for each scour regime, clear-water and live-bed, were suggested.

Hydrodynamics of a hybrid circulating fluidized bed reactor with a partitioned loop seal system

A circulating fluidized bed (CFB) with a hybrid design has been developed and optimized for steam hydrogasification. The hybrid CFB is composed of a bubbling fluidized bed (BFB) type combustor and a fast fluidized bed (FB) type gasifier. Char is burnt in the combustor and the generated heat is supplied to the gasifier along with the bed materials. Two different types of fluidized beds are connected to each other with a newly developed partitioned loop seal to avoid direct contact between two separate gas streams flowing in each fluidized bed. Gas mixing tests were carried out with Air and Argon in a cold model hybrid CFB to test the loop seal efficiency. Increase in solid inventory in the loop seal can improve the gas separation efficiency. It can be realized at higher gas velocity in fast bed and with higher solid inventory in the loop seal system. In addition, bed hydrodynamics was investigated with varying gas flow conditions and particle sizes in order to obtain a full understanding of changes of solid holdup in the FB. The solid holdup in the FB increased with increasing gas velocity in the BFB. Conversely, increase in gas velocity in the FB contributed to reducing the solid holdup in the FB. It was observed that changing the particle size of bed material does not have a big impact on hydrodynamic parameters.

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.