Bed Material Size Research Articles

Review and assessment of the theories of stable alluvial channel design

Deformable alluvial channels are known to adjust their geometry and slope to achieve stable conditions for a specified influx of water and sediment. Designing the stable alluvial channel has been a captivating topic for scientists and engineers around the globe for years. The work which was commenced by Kennedy in 1895 has been continued and various approaches have been given so far, some of which are quite interrelated and others emerged with different ideas. In this comprehensive study, some of the classic and widely accepted approaches published in the literature have been thoroughly reviewed and have been verified with available river regime data. The data set has been sub divided into three categories based on the median bed material size (sand, gravel and cobble or boulder), in order to examine the applicability of various methods available for the design of stable alluvial channels. Detailed discussion related to the properties of the intercept coefficients in power function theory is not available in published literature. In this study, the coefficients are first calibrated and then applied with the respective exponents in order to derive the hydraulic geometry. Further, the derived hydraulic geometry from various approaches is summarized and discussed with comparative view point. The analysis shows that prediction from recently developed model based on the principle of maximum entropy and minimum energy dissipation is better than other approaches for the entire range of data set. The same model has been further generalized by assuming a wide trapezoidal channel cross-section through which an improvement in the prediction has been observed.

Developing a national stream morphology data exchange: Needs, challenges, and opportunities

Stream morphology data, primarily consisting of channel and foodplain geometry and bed material size measurements, historically have had a wide range of applications and uses including culvert/ bridge design, rainfall‐ runoff modeling, food inundation mapping (e.g., U.S. Federal Emergency Management Agency food insurance studies), climate change studies, channel stability/sediment source investigations, navigation studies, habitat assessments, and landscape change research. The need for stream morphology data in the United States, and thus the quantity of data collected, has grown substantially over the past 2 decades because of the expanded interests of resource management agencies in watershed management and restoration. The quantity of stream morphology data collected has also increased because of state‐of‐the‐art technologies capable of rapidly collecting high‐resolution data over large areas with heretofore unprecedented precision. Despite increasing needs for and the expanding quantity of stream morphology data, neither common reporting standards nor a central data archive exist for storing and serving these often large and spatially complex data sets. We are proposing an open‐ access data exchange for archiving and disseminating stream morphology data.

Fluidized bed combustion and fragmentation of wet sewage sludge

Thermo-chemical conversion of sewage sludge in fluidized beds is an attractive path to reduce the amount of landfilled waste and to mitigate the environmental impact related to sewage sludge disposal. The present study is directed to the characterization of the devolatilization and combustion patterns of lumps of wet sewage sludge in a fluidized bed combustor, with a focus on particle segregation, morphological changes and comminution phenomena associated with in-bed drying, devolatilization and char burn-out. The experimental investigation was carried out with the aid of different and complementary experimental techniques. The phenomenology of devolatilization and char burn-out has been investigated in an optically accessible quartz bench-scale fluidized bed combustor. Self-segregation of fuel lumps to the top of the bed during devolatilization was recorded. Stratified combustion of the emitted volatiles was observed to take place above the bed surface in a flameless fashion. Morphological changes of wet sewage sludge after in-bed devolatilization and burn-out has been accomplished by SEM/EDX and image analysis. The formation of balloon-like particles consisting of a porous outer layer enclosing a big single cavity is reported. The physical and chemical features of ash and char particles have been characterized. Particle fragmentation phenomena have been characterized by varying the oxidative vs. non-oxidative nature of the fluidizing gas, the initial size of the fuel particle, the bed material size, the fluidization velocity. Particle fragmentation occurs mainly during drying and pyrolysis/devolatilization and is very limited during char burn-out. Fragmentation is reported to take place when the initial size of the lump of wet sludge exceeds a critical value which depends on the bed material size and excess gas superficial velocity. Particle fragmentation is strongly affected by the size of bed material and moderately by the excess gas superficial velocity.

Simulation of agglomeration/defluidization inhibition process in aluminum–sodium system by experimental and thermodynamic approaches

This study modifies a defluidization time prediction model under the influence of agglomeration inhibition provided by the addition of Al into a fluidized bed system. Operational parameters, such as the Al/Na ratio, temperature, gas velocity, and the particle size of bed materials, were considered by the prediction model developed by Lin et al., and thermodynamic consideration of the Al–Na–Si reaction system revealed the agglomeration mechanism. An exponential decay of fN was found with the increase of the Al/Na ratio, and the data analysis gave a regression formula of fN=0.0601+0.4983∗exp [−4.4183∗(Al/Na ratio)], with an R2 of 0.9972. The simulation data generally match well with the experimental results, particularly at lower operation temperatures and Geldart B group's particles, with acceptable average errors of −8.4% and −4.9%, respectively. However, relatively poor matches were found in the cases with different gas velocities, a higher operation temperature (900°C), and a larger particle size (Geldart D group) of bed materials.

만곡하천의 자갈하상재료 분포에 따른 한계수류력 평가

The distribution of gravel-bed materials in mountainous river is formed by the process of deposition and transportation of sediment responding to stream power of the latest flood that is over the certain scale. The particle size of bed material was surveyed in the longitudinal points of river and detail points of a specific meandering section and used to estimate the critical velocity and stream power. Yang`s critical unit stream power and Bagnold`s critical stream power for gravel-bed materials increased with the distance from downstream to upstream. Dimensionless shear stress based on the designed flood discharge in Shields diagram was evaluated that the gravel-bed materials in most survey points may be transported as form of bedload. The mean diameter in the meandering section was the biggest size in first water impingement point of inflow water from upstream and the second big size in second water impingement point by reflection flow. The mean diameters were relatively the small sizes in points right after water impingement. The range of mean critical velocity was 0.77~2.60 m/s and critical unit stream power was big greatly in first water impingement point. The distribution of critical stream power, range of 7~171 , was shown that variation in longitudinal section was more obvious than that of cross section and estimated that critical stream power may be affected greatly in first and second water impingement point.

Bankfull Regional Curves for the Inner and Outer Bluegrass Regions of Kentucky1

Brockman, Ruth R., Carmen T. Agouridis, Stephen R. Workman, Lindell E. Ormsbee, and Alex W. Fogle, 2012. Bankfull Regional Curves for the Inner and Outer Bluegrass Regions of Kentucky. Journal of the American Water Resources Association (JAWRA) 48(2): 391‐406. DOI: 10.1111/j.1752‐1688.2011.00621.xAbstract: Bankfull regional curves that relate channel dimensions and discharge to watershed drainage area are useful tools for assisting in the correct identification of bankfull elevation and in stream restoration and reconstruction. This study assessed 28 stable streams located in two physiographic regions of Kentucky: the Inner Bluegrass and the Outer Bluegrass. Bankfull channel dimensions, discharge, and return period as well as average channel slope, median bed material size, sinuosity, Rosgen stream classification, and percent impervious area were determined. Significant relationships were found between drainage area and the bankfull characteristics of cross‐sectional area, width, mean depth, and discharge for both the Inner Bluegrass and Outer Bluegrass regions (α = 0.05). It was also found that the percent impervious area in a watershed had minimal effect on bankfull dimensions, which is attributed to the well‐vegetated nature of the streambanks, cohesive streambank materials, and bedrock control. No significant differences between any of the Inner Bluegrass and Outer Bluegrass regional curves were found (α = 0.05). Comparisons were made between the Inner Bluegrass and Outer Bluegrass curves and others developed in karst‐influenced areas in the Eastern United States. Although few significant differences were found between the regional curves for bankfull discharge and width, a number of the curves differed with regards to bankfull cross‐sectional area and mean depth.

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.

Experimental research on agglomeration in straw-fired fluidized beds

Agglomeration is a major problem in straw combustion in a fluidized bed. This paper presents the results of rice straw combustion experiments carried out under different operating conditions in a bubble-fluidized bed (BFB). The influences of bed material size and type, feeding mode, temperature, and fluidization number on agglomeration were discussed, and the mode of alkali accumulation in the bed was analyzed. The results indicated that low bed temperature, short fuel in-bed residence time, high fluidization number, and small bed material particles are conducive to agglomeration prevention. Successful extended combustion in a small BFB and in a bench-scale circulating fluidized bed was carried out without agglomeration under selected parameters. This work suggests the possibility for combusting high-alkali straw using fluidization technology.

Experimental profiles of lateral mixing of feed particles in a three‐dimensional fluidized bed

AbstractSolids mixing data of high quality is one of the most crucial steps for quantitative studies, but it is a difficult task to obtain in a fluidized bed especially with a 3D configuration. Therefore a novel sampling technique is developed with bed collapse method, for measuring lateral mixing of feed particles in a 3D fluidized bed. The sampling tool is designed using a “bottom‐to‐top sampling” idea. Its development, configuration and measurement repetition are discussed in detail. The effects of mixing time, fluidizing gas velocity, and particle size of bed material on the tracer distribution are investigated. A quantitative comparison of lateral dispersion coefficient shows that our results agree fairly well with measurements and predictions of correlations for lab‐scale fluidized systems in previous studies. The presented 2D profiles of the lateral mixing can be used to validate fundamental solids mixing models or verifying convenient measurement techniques. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Heat transfer characteristics in a horizontal swirling fluidized bed

An innovative horizontal swirling fluidized bed (HSFB) with a rectangular baffle in the center of an air distributor and three layers of horizontal secondary air nozzles located at each corner of fluidized bed was developed. Experiments on heat transfer characteristics were conducted in a cold HSFB test model. Heat transfer coefficients between immersed tubes and bed materials in the HSBF were measured with the help of a fast response heat transfer probe. The influences of fluidization velocity, particle size of bed materials, measurement height, probe orientation, and secondary air injection, etc. on heat transfer coefficients were intensively investigated. Test results indicated that heat transfer coefficients increase with fluidization velocity, and reach their maximum values at 1.5–3 times of the minimum fluidization velocity. Heat transfer coefficients are variated along the circumference of the probe, and heat transfer coefficients on the leeward side of the probe are larger than that on the windward side of the probe. Heat transfer coefficients decrease with increasing of measurement height; heat transfer coefficients of the longitudinal probe are larger than that of the transverse probe. The proper secondary air injection and particle size of bed materials can generate a preferred hydrodynamics in the dense zone and enhance heat transfer in a HSFB.

Regime Equations for Natural Meandering Cobble- and Gravel-Bed Rivers

Data obtained from 48 stable reaches of upland rivers in the UK were stratified by stream type to develop regime equations specifically for natural meandering cobble- and gravel-bed rivers: C3 and C4 stream types, according to the Rosgen classification. Multiple regression models were applied to derive equations for reach-averaged values of bankfull width, mean depth, slope, meander arc length and sinuosity in bankfull discharge and associated bed-material load, the caliber of the bed material, bank vegetation density, and valley slope. The equations show that their cross-sectional dimensions are primarily determined by the bankfull discharge, bank vegetation, and bed-material size, whereas their profile and plan form are very strongly influenced by the valley gradient. Although bankfull bed-material load only appears to have a minor influence on channel morphology, its effect is implicit in the value of bankfull discharge because this corresponds to the flow that transports most of the bed-material load....

Predicting the equilibrium bed slope in natural streams using a stochastic model for incipient sediment motion

ABSTRACTIn this paper, a method to predict the equilibrium bed slope in natural streams based on the incipient motion criterion is proposed. The method is based on the criterion suggested by Gessler to simulate the grain size distribution of the armour coat using the concept of critical shear stress of a sediment mixture. In particular, a different expression of the probability for a single particle size to be part of the armour coat is firstly proposed; then, a simple two‐steps criterion is suggested to estimate the safety factor required by the proposed approach. The method is applied in three different Italian regions (Calabria, Basilicata, and Tuscany) and required several field campaigns involving the survey of 251 stream reaches upstream of grade‐control structures. The area including five Calabrian streams is firstly used to test the suitability of the two‐steps approach. In this area, considering the detailed information about the particle size of bed material, the effect of using a simplified grain size distribution for each torrent is also checked. The method is then applied within the two additional areas (Basilicata and Tuscany) in order to check the geographic influence on the prediction of the stable longitudinal profile. A comparison between measured and estimated values of the equilibrium bed slope showed a good agreement for the Calabrian streams, where the number of the investigated stream reaches is greater, and satisfactory results for the torrents located in Basilicata and Tuscany where the field data set is more limited. The overall results encourage the extension of the proposed method to additional field data and suggest the use of this approach by hydraulic designers in order to stabilize the longitudinal profile of natural streams. Copyright © 2011 John Wiley & Sons, Ltd.

Effect of Flow Pulses on Degradation Downstream of Hapcheon Dam, South Korea

The changes in channel geometry downstream of Hapcheon Dam, South Korea, are closely examined. Daily pulses of water from peak hydropower generation and from sudden sluice gate operations affect the 45-km reach of the Hwang River between the Hapcheon Reregulation Dam and the Nakdong River. From 1983 to 2003, the median bed-material size, d50 , increased from 1.0 to 5.7 mm, and the bed slope of the reach decreased from 94 to 85 cm/km. The vertical riverbed degradation averaged 2.6 m for a distance of 20 km below the reregulation dam. A simple analytical model is developed to predict the increase in sediment transport and the river bed adjustments from flow pulses in comparison with steady flow discharges. Numerical model simulations confirm the theoretical prediction that sediment transport rates from daily pulses are 21% higher than for steady flow discharges. Unsteady sediment transport simulations indicate that the channel bed degradation should extend mostly 20–25 km below the reregulation dam and shou...

Physical Basis for Quasi-Universal Relationships Describing Bankfull Hydraulic Geometry of Sand-Bed Rivers

Empirical data indicate that hydraulic geometry relationships for single-thread sand-bed rivers (i.e., rivers with median bed-material size between 0.062 and 0.50 mm) can be delineated such that bankfull width, bankfull depth, and channel slope are related in consistent ways to bankfull discharge. Such relationships ought to be the external expression of physical relationships intrinsic to sand-bed river dynamics. In this study, a back-calculation is performed to identify parameters (exponents and coefficients) for three relationships taken to be intrinsic to sand-bed rivers: (1) a generalized Manning-Strickler resistance relationship; (2) a relationship for channel-forming Shields number; and (3) a relationship for sand yield at bankfull flow. To back-calculate parameters for the physical relationships, first the hydraulic geometry relationships are expressed in suitable dimensionless form. Second, the physical relationships are expressed with coefficients and exponents that are analytically related to p...

Changes in hydraulic geometry of the Hwang River below the Hapcheon Re-regulation Dam, South Korea

The changes in hydraulic geometry of the Hwang River downstream of Hapcheon Dam have been investigated since 1982. The study reach is 45 km long from the Hapcheon Re-regulation Dam to the confluence with the Nakdong River, in South Korea. The geographic information system (GIS) analysis of a time series of aerial photographs taken in 1982, 1993 and 2004 showed that the non-vegetated active channel width decreased an average of 152 m (47% width reduction since 1982). The average median bed material size increased from 1.07 mm in 1983 to 5.72 mm in 2003, and the bed slope of the reach decreased from 94 to 85 cm/km from 1983 to 2003. The analysis of aerial photographs and field surveys shows that the 2004 channel width was asymptotically approaching the estimated equilibrium width from the Julien and Wargadalam (Julien, P.Y. and Wargadalam, J., 1995. Alluvial channel geometry: theory and applications. Journal of Hydraulic Engineering, 121 (4), 312–325) hydraulic geometry equations. An exponential model is proposed and is in good agreement with field measurements. The sediment transport model GSTAR-1D shows that the thalweg elevation will reach equilibrium around 2013–2015. The thalweg elevation simulated for 2013 is expected to remain almost identical to the predictions for 2023. The bed elevation changes should affect primarily the 20 km reach immediately downstream of the re-regulation dam.

Block Ramp Design for Efficient Energy Dissipation

Block ramp structure can be a part of a layout of microhydel scheme, depending upon the conditions of topography and geology at the site. Block ramps transmit the flow of stream to a lower level inducing kinetic energy dissipation. Block ramps consists of base material in the form of stones inducing roughness whereas stepped chutes are overflowing structures which consists of step configuration at the bottom of the flowing stream. As there is a common objective of energy dissipation of flow, the geometrical elements of block ramps and stepped chutes are considered to be comparable under similar hydraulic conditions of skimming flow. This paper analyzes the experimental results on both these structures. A mathematical model in dimensionless form is proposed herein, which correlates the representative bed material size of block ramp with the step height of stepped chute. This correlation is expected to facilitate the design of block ramps using the literature available on stepped chutes.

Mass loss and apparent kinetics of a thermally thick wood particle devolatilizing in a bubbling fluidized bed combustor

This work presents the results of experiments conducted to determine the mass loss characteristics of a cylindrical wood particle undergoing devolatilization under oxidation conditions in a bubbling fluidized bed combustor. Cylindrical wood particles having five different sizes ranging from 10 to 30 mm and aspect ratio ( l/ d = 1) have been used for the study. Experiments were conducted in a lab scale bubbling fluidized bed combustor having silica sand as the inert bed material and air as the fluidizing medium. Total devolatilization time and mass of wood/char at different stages of devolatilization have been measured. Studies have been carried out at three different bed temperatures ( T bed = 750, 850 and 950 °C), two inert bed material sizes (mean size d p = 375 and 550 μm) and two fluidizing velocities ( u = 5 u mf and u = 10 u mf). Devolatilization time is most influenced by the initial wood size and bed temperature. Most of the mass is lost during the first half of the devolatilization process. There was no clear influence of the fluidization velocity and bed particle size on the various parameters studied. The apparent kinetics estimated from the measured mass history show that the activation energy varied narrowly between 15 and 27 kJ/mol and the pre-exponential factor from 0.11 and 0.45 s −1 for the wood sizes considered.

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.

Study on The Angle of Repose of Nonuniform Sediment

A conception of exposure degree is used in this article to account for the hiding and exposure mechanism of nonuniform sediment transport. The force arms of particles on the bed slope are assumed to be a function of the exposure degree, which is stochastically related to the size and gradation of bed materials. Based on this conception, the formula for the angle of repose of nonuniform sediment is developed. The angle of repose of nonuniform sediment is experimentally investigated in rotation drum at the rolling motion of sediment particles. The nonuniform sediment is gotten from the mixture of two kinds of uniform sediment with different weight ratios and two experimental plans that one for changing the weight ratio of two kinds of uniform sediment from 1:9 to 9:1 and the other changing the uniform sediment size and weight ratio are used. The effect of the rotation speed and the filling degree on the angle of repose is investigated, too. Finally, the formula has been tested against a wide range of laboratory data, the prediction by the newly proposed formulas is very good.

등가조고를 이용한 자갈하천의 하상저항 산정

본 연구는 하상마찰의 척도인 등가조고를 이용하여 자갈하천의 하상저항을 산정하는 것이다. 이를 위하여 국내 8개 자갈하천에서 실측을 통하여 마찰계수 f를 산정하였고, 하상재료의 누가입경분포를 조사하였으며, 각 단면의 실측 마찰계수를 나타내기 위하여 대수 연직유속분포를 가정하여 계산된 평균 등가조고와 누가입경분포를 이용하여 계산된 구간 등가조고를 비교하였다. 더불어 등가조고를 사용하여 계산된 Manning 계수를 Strickler 유형의 기존 경험식에 의한 Manning 계수와 비교함으로써, 국내 자갈하천에서 기존 경험식을 이용한 하상저항 산정의 적용성을 검토하였다. 분석결과, 구간 등가조고가 대상구간의 하상특성을 대표할 수 있음을 확인하였고, 구간 등가조고로 계산된 Manning 계수가 실측 자료로부터 계산된 Manning 계수와 비교하여 RMS 오차가 0.003이하로 계산되어 실무 적용에 타당할 것으로 판된되었다. 또한 실제 하천에서 등가조고 <TEX>$k_s$</TEX>와 특정입경 <TEX>$D_i$</TEX>과의 비례계수 사이의 상관성이 적은 것으로 나타나, 하상 특정입경을 사용하여 Manning 계수를 산정하는 기존 경험식을 적용할 때 적절한 방정식의 선택에 어려움이 있을 것으로 판단된다. The objective of this study is to estimate bed-resistance in gravel-bed rivers using the equivalent roughness height(<TEX>$k_s$</TEX>). We calculated the friction factor(f) with the measured data from 8 domestic gravel-bed rivers and investigated the size distributions of the bed materials. The averaged <TEX>$k_s$</TEX> in each cross-section, which is determined under the hypothesis that the vertical velocity distribution follows the logarithmic law, is compared with the reach <TEX>$k_s$</TEX> which is calculated with the cumulative grain diameter distribution curve of bed materials. Moreover, the applicability of existing formulae, such as Strickler type equations, is examined by comparing with Manning's n value converted from the <TEX>$k_s$</TEX>. According to the results, the reach <TEX>$k_s$</TEX> proves to be a good indicator of representative characteristic of bed materials in a reach, and the Manning's n based on the reach <TEX>$k_s$</TEX> is appropriate for practical estimation of the bed-resistance, for RMS errors between calculated and measured Manning's n is less than 0.003. The correlation between the <TEX>$k_s$</TEX> and specified bed-material size(<TEX>$D_i$</TEX>) is very low, so it is difficult to select a proper one among the existing empirical equations.