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1.
 High-grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature or even solidus temperature. Corresponding pyroclastic-flow particles range from plastic to partially liquid and are able to aggregate or coalesce. This contrasts with particles in pyroclastic flows producing unwelded ignimbrite, which are capable of elastic grain interactions. The low aspect ratio and great areal extent of high-grade ignimbrites requires transport in a particulate state either by (a) high-concentration mass flow facilitated by fluidizing gas reducing internal friction, or by (b) expanded turbulent flow of low but downward increasing concentration. This paper presents experiments designed to investigate the effects of plastic to liquid particles on these two contrasting transport mechanisms. Gas fluidization experiments using polyethyleneglycole (PEG) powders heated above minimum sintering (Tms) and melting (Tm) temperatures cover a wide range of fluidization velocities (Umf>Ua>0.6·Ut) but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash, where particle volume concentration outside the bubbles is high (≈10–1). When the powders reach a critical temperature Tm≥T≥Tms, defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized beds as well as in experiments using mixtures of high- and low-Tm (≥30 wt.%) PEG powders, when T≥Tms of the lower-Tm powder. This indicates that extended particulate transport at T≥Tms is not possible at such high particle concentrations. In the turbulent flow experiments, liquid sprays of molten PEG or water, vertically injected into a high-Re (>104) horizontal air flow, form a low-concentration (10–5 to 10–4) turbulent suspension current. Proximal formation of partially coalesced aggregates, which settle faster than individual particles, causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid-particle sedimentation from turbulent suspensions. As particles become finer downstream and coalescence efficiency decreases in response to cooling, more distally formed aggregates become too small and rare to modify sedimentation-rate decay from that of suspension flows containing solid particles. The key difference between the two transport systems is particle concentration, C. Since particle collision rate Rcoll∝C2, collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency (1≥Ecoal≥0) is larger than 10-3. Low-concentration suspensions, on the other hand, require much higher values of Ecoal for significant aggregation to occur. Dilute pyroclastic flows will have higher particle volume fractions (≈10–3) than the experimental currents, but then viscous pyroclasts should have lower coalescence efficiencies than PEG droplets. Experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high-grade ignimbrite sheets. Received: 28 August 1996 / Accepted: 3 December 1997  相似文献   

2.
The settling of solid particles in a fluid is an important process that needs to be considered in many fields of research. For example, the interactions among particles and between particles and the surrounding fluid are important topics in studying suspended sediment transport and water clarification. In this paper, the settling processes and interactions of twin spherical particles released side by side were experimentally studied. The Reynolds number varied in the range of 1-300, which is wit...  相似文献   

3.
The purpose of this paper is to examine the nature of particle saltation and movement over the beds of fixed roughness from flume experiments. A series of experiments are carried out to study the saltation of individual sand particles of different sizes over rough beds under different flow conditions. A 3‐D acoustic Doppler velocimeter is used to record the fluid velocity components; subsequently, under different flow conditions, the images of released sand particles are recorded using high‐speed video imaging technique. Systematic analysis is made with regard to the forces acting on the grains and the variation of their magnitudes along the saltation trajectories of the grains. Relations between the saltation parameters, flow intensity and bed roughness are developed. The distributions of the angle of orientations during a single saltation follows almost a Gaussian distribution. The shape of the Gaussian distribution depends on the particle size and bed roughness. Particle collisions with rough beds and the resulting coefficients of restitution are also discussed. A theoretical framework is developed to compute the mean particle velocity considering the spin in the energy balance equation. Results of the detailed analysis using the imaging technique are much better than in previously reported studies. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

4.
Laboratory flume experiments were carried out to evaluate the effect of particle density on bedload transport of sand‐sized particles and the effect of a suspended load of clay particles (kaolinite) on bedload transport of sand‐sized particles in rill flow conditions. Three materials in the range 400–600 µm were selected to simulate bedload transport of primary particles and aggregates: sand (2650 kg/m3), crushed brick (2450 kg/m3) and anthracite (1300–1700 kg/m3). In the two first experiments, two different methods were applied to determine bedload transport capacity of coarse particles for various conditions of flow discharge (from 2 to 15 L/min) and slope (2.2, 3 and 4%). In the third experiment, clear water was replaced with kaolinite–water mixture and bedload transport capacity of crushed brick particles was determined for a 4% slope and different concentrations of kaolinite (0, 7, 41 and 84 g/L). The results showed that bedload transport increased significantly with the decrease in particle density but the effect of particle density on transport rates was much less important than flow discharge. Velocity measurements of clear flow, flow mixed with coarse particles and coarse particles confirmed the existence of a differentiation between suspended load and bedload. In these experimental conditions, suspended load of kaolinite did not affect bedload rates of crushed brick particles. Three transport capacity formulae were tested against observed bedload rates. A calibration of the Foster formula revealed that the shear stress exponent should be greater than 1.5. The Low and the Govers unit stream power (USP) equations were then evaluated. The Low equation was preferred for the prediction of bedload rates of primary particles but it was not recommended in the case of aggregates of low density because of the limited experimental conditions applied to derive this equation. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

5.
Z. Shi  J. M. R. Hughes 《水文研究》2002,16(16):3279-3289
The microflow environments of aquatic plants with reference to Myriophyllum and Hydrilla are simulated in a laboratory flume. A Nix Streamflow microflow meter was used to measure the mean velocity profiles of flow at different densities of plants, flow ranges and measurement positions. Each mean velocity profile consists of three hydrodynamic regimes (i.e. within‐canopy zone, above‐canopy zone and a transitional zone between them), which indicate the presence of two benthic boundary layers (internal and external ones). Out of 38 measured mean velocity profiles, most do not fit a logarithmic relationship. The following hydrodynamic parameters are used in characterizing the flow regimes: local shear velocity (u*), roughness length (zo), canopy roughness Reynolds number (Re*), bed shear stress (τo) and laminar sublayer (σ). Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

6.
There is little information on the performance of vegetative filter strips (VFS) in filtering high‐concentration sediment from subcritical overland flow. Flume experiments on simulated grass strips were conducted using combinations of three slope gradients (3°, 9° and 15°), five 1‐m‐wide slope positions (from upslope to downslope), two flow rates (60 and 20 L min‐1 m‐1) and sediment concentrations of 100–300 kg m‐3 under simulated rainfall and non‐rainfall conditions. The results showed that sediment deposition efficiency increased with VFS width as a power function. Rainfall significantly reduced sediment deposited within VFS. Higher sediment concentration corresponded to a larger sediment deposition load but reduced deposition efficiency. Flow rate had a negative effect on deposition efficiency but no effect on deposition load. Sediments were more easily deposited at the upper slope position than downslope, and the upper slope position had a higher percentage of coarse sediments. The deposited sediment had significantly greater median diameters (D50) than the inflow sediment. A greater proportion of coarse sediments larger than 25 µm in diameter were deposited, and particles smaller than 1 µm and of 10–25 µm had a better deposition performance than particles of 1–10 µm. Rainfall reduced the deposited sediment D50 at a slope gradient of 3° and had no significant influence on it at 9° or 15°. A higher sediment concentration led to a smaller D50 of the deposited sediment. Rainfall had no significant effect on overland flow velocity. Both the deposited sediment load and D50 decreased with increasing flow velocity, and flow velocity was the most sensitive factor impacting sediment deposition. The results from this study should be useful to control sediment flowing into rivers in areas with serious soil erosion. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

7.
Aeolian sand transport is a complicated process that is affected by many factors (e.g. wind velocity, sand particle size, surface microtopography). Under different experimental conditions, erosion processes will therefore produce different results. In this study, we conducted a series of wind tunnel experiments across a range of wind velocities capable of entraining sand particles (8.0, 10.0, 12.0, and 14.0 m s-1) to study the dynamic changes of the shear velocity, aerodynamic roughness length, and sand transport. We found that the shear velocity and aerodynamic roughness length are not constant; rather, they change dynamically over time, and the rules that describe their changes depend on the free-stream air velocity. For wind tunnel experiments without feeding sand into the airflow, the sand bed elevation decreases with increasing erosion time, and this change significantly affected the values of shear velocity and aerodynamic roughness length. A Gaussian distribution function described the relationships between the sand transport rate (qT) and the duration of wind erosion (T). It is therefore necessary for modelers to consider both deflation of the bed and the time scale used when calculating sand transport or erosion rates. © 2018 John Wiley & Sons, Ltd.  相似文献   

8.
Yield strength is an important property of particle–fluid suspensions. In basaltic lavas that crystallize during flow emplacement, the onset of yield strength may result in threshold transitions in flow behavior and flow surface morphology. However, yield strength–crystallinity relations are poorly known, particularly in geologic suspensions, where difficulties of experimental and field measurements have limited data acquisition in the subliquidus temperature range. Here we describe two complementary experimental approaches designed to examine the effect of particle shape on the low-shear yield strength of subliquidus basalts. The first involves melting cubes of holocrystalline basalt samples with different initial textures to determine the temperature (crystallinity) at which these samples lose their cubic form. These experiments provide information on the minimum crystal volume fractions (0.20<φ<0.35) required to maintain the structual integrity of the cube. The second set of experiments uses suspensions of corn syrup and neutrally buoyant particles to isolate the effect of particle shape on yield strength development. From these experiments, we conclude that the shape is important in determining the volume fraction range over which suspensions exhibit a finite yield strength. As anisotropic particles may orient during flow, the effect of particle shape will be controlled by the orientation distribution of the constituent particles. We find that the so-called ‘excluded volume’ can be used to relate results of experiments on anisotropic particles to those of suspensions of spherical particles. Recent measurements of yield strength onset in basaltic melts at crystal volume fractions near 0.25 are consistent with our observations that crystal frameworks develop at low to moderate crystal volume fractions when crystals are anisotropic (e.g. plagioclase). We further suggest that conditions leading to yield strength onset at low crystallinities include rapid cooling (increased crystal anisotropy), heterogeneous nucleation (which promotes extensive crystal clustering and large cluster anisotropy) and static conditions (random crystal orientations).  相似文献   

9.
Abstract

Laboratory flume experiments were undertaken to measure the vertical profiles of mean flow velocity for three different flow discharges and four different stem densities of Hydrilla verticillata. The data were used to calculate three parameters, namely Manning's roughness coefficient, the Reynolds number and the Froude number. In addition, empirical equations were obtained for the vertical distribution of measured flow velocity within the transitional zone and above the plant canopy. The results show that: (a) the vertical distribution of measured flow velocity exhibits three zone profiles; (b) Manning's roughness coefficient decreases with increasing depth-averaged flow velocity; (c) the relationship between Manning's roughness coefficient and the depth-averaged flow velocity is within the smooth left inverse curve; (d) Manning's roughness coefficient significantly changes with increasing density of Hydrilla; (e) the Froude number is independent of the density of Hydrilla; and (f) both the Reynolds number and the Froude number increase with increasing depth-averaged flow velocity.

Citation Shi, J.Z., Li, Y.-H., Hughes, J.M.R., and Zhao, M., 2013. Hydrological characteristics of vegetated river flows: a laboratory flume study. Hydrological Sciences Journal, 58 (5), 1047–1058.

Editor Z.W. Kundzewicz  相似文献   

10.
Sediment transport has been extensively studied. There is still a need to learn more about the mechanisms that make bed particles move, which is caused by turbulent flow in the low transport stages(above the motion threshold and below continuous transport). This work is focused on the use of an advanced tool to obtain a better perception of sediment transport dynamical methods: an instrumented particle equipped with a micro-electromechanical systems(MEMS) sensor. Particle transport experiments w...  相似文献   

11.
Two end-member types of pyroclastic density current are commonly recognized: pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows. We provide scaling relations that unify these end-members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the Stokes number (ST), the stability factor (ΣT) and the dense-dilute condition (DD). We recognize five types of particle behaviors within a fluid eddy as a function of ST and ΣT: (1) particles sediment from the eddy, (2) particles are preferentially settled out during the downward motion of the eddy, but can be carried during its upward motion, (3) particles concentrate on the periphery of the eddy, (4) particles settling can be delayed or ‘fast-tracked’ as a function of the eddy spatial distribution, and (5) particles remain homogeneously distributed within the eddy. We extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current. This stratification may favor particle interactions in the basal part of the flow and DD determines whether the flow is dense or dilute. Using only intrinsic characteristics of the current, our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents.  相似文献   

12.
Summary Working on the hypothesis that atmospheric ice-forming nuclei are largely of terrestrial origin, the nucleating ability of various types of soil particles and mineral dusts has been investigated. Of the thirty substances tested, twenty-one, mainly silicate minerals of the clay and mica groups, were found to produce ice crystals in supercooled clouds and also on supercooled soap films at temperatures of – 18° C, or above, and of these, ten were active above – 12° C. The most abundant of these is kaolinite with a threshold temperature of – 9° C. Ten natural substances, again mainly silicates, were found to become more efficient ice nuclei having once been involved in ice-crystal formation, i.e. they could be pre-activated or «trained». Thus, ice crystals grown on kaolinite nuclei, which are initially active at –9° C, when evaporated and warmed to near 0° C in a dry atmosphere, leave behind nuclei which are thereafter effective at – 4° C. Particles of montmorillonite, another important constituent of some clays, and which are initially inactive even at –25° C, may be pre-activated to serve as ice nuclei at temperatures as high as –10° C. It is suggested that although such particles can initially form ice crystals only at cirrus levels, when the ice crystals evaporate they will leave behind some «trained» nuclei which may later seed lower clouds at temperatures only a few degrees below 0° C. On this hypothesis, the fact that efficient nuclei are occasionally more abundant at higher levels would not necessarily imply that they originate from outer space. Indeed, in view of our tests on products of stony meteorites, produced both by grinding and vaporization, which show them to be ineffective at temperatures above – 17° C, it seems likely that atmospheric ice nuclei are produced mainly at the earth's surface, the clay minerals, particularly kaolinite, being a major source.Although a good deal of work has been carried out in different laboratories on the ice-nucleating ability of a wide variety of inorganic compounds, there has been little agreement in the results. Careful tests carried out in our laboratory have revealed a number of reasons for this. Spurious results may be obtained because of the presence, in the air or the chemicals, of small traces of silver or free iodine, leading to the formation of silver iodide: if all such trace impurities are removed, many of the substances that have been claimed to provide efficient ice nuclei are found to be quite ineffective. It is dangerous to infer that all twinkling particles in a water cloud are ice crystals since particles of some seeding agents glitter even at positive temperatures. The threshold temperature of a nucleant will depend upon the criterion adopted for the onset of nucleation, i.e. upon the fraction of the total number of particles of seeding agent which are activated; this, in turn, will depend upon the fraction of particles which happen to possess suitable crystallographic faces for nucleation. Much may also depend upon the manner in which the test is performed. Since some nucleating materials produce ice crystals only after a delay of 30 seconds or more, they may appear to be ineffective if tested in the transient cloud of an expansion chamber but highly effective if allowed to remain in an ice-supersaturated atmosphere for a minute or more. Again, we have found that the efficiency of some nuclei is governed by the supersaturation as well as the temperature of the environment, and the supersaturation regimes in expansion, diffusion, and mixing-cloud chamber may be widely different. Highly soluble particles, although able to act as «sublimation» nuclei in atmospheres super-saturated relative to ice but sub-saturated relative to water, on entering a water cloud go quickly into solution and lose their nucleating ability.Inorganic substances which definitely nucleate a supercooled water cloud in a mixing-cloud chamber at temperatures of –15° C and above are: AgI (–4° C), PbI2 (–6° C), CuS (–6° C), Ag2S (–8° C), Ag2O (–9° C), HgI2 (–8° C), V2O5 (–14° C), Cu2I2 (–15° C), the figures in brackets indicating the threshold temperatures at which about one particle in 104 becomes active as an ice nucleus. Cadmium iodide (–12° C), ammonium fluoride (–9° C) and iodine (–14° C) are examples of salts which will act as sublimation nuclei in an ice-supersaturated atmosphere and will nucleate a supercooled soap film, but which are ineffective in a water cloud because of their solubility.Although the most efficient nucleating agents tend to be hexagonal in structure, there are some striking exceptions e.g. Ag2S, Ag2O, HgI2, but in most cases, we have been able to find a low-index crystal surface on which the ice lattice could grow with a misfit of only a few per cent.In an attempt to investigate the nucleation mechanism in more detail, we have studied the growth of ice on single crystals of various nucleating agents. Perfect orientation of ice crystals has so far been observed on the basal faces of silver iodide, lead iodide, cupric sulphide, cadmium iodide, and freshly-cleaved mica, on the (001) plane of iodine, and on the (010) plane of mercuric iodide.  相似文献   

13.
A numerical model of deep, uniform, oscillatory, rough-turbulent boundary-layer flow is described. The model is based upon the governing horizontal momentum equation and a closure scheme involving the turbulent-energy equation and various turbulence-scaling laws. Finite difference solutions of these equations are obtained for a range of values of the ‘relative roughness’ (A0/ks), whereA0 is the excursion amplitude of the water particles in the free-stream flow andks is the ‘equivalent bed roughness’. Typical vertical profiles of horizontal velocity, turbulence energy and eddy viscosity, and time-series of the bed shear stress are presented. The model results are then used to determine the wave drag coefficient, boundary-layer thickness and phase lead of the bed shear stress over the free-stream velocity, each as a function ofA0/ks. These results are shown to be in generally good agreement with previous experimental and theoretical results. Finally, the model is used to test for the existence of a universal velocity distribution for uniform oscillatory (sinusoidal) rough-turbulent flow. The ‘law of the wall’ and the ‘defect law’ proposed by Jonsson (1980, Ocean Engineering, 7, 109–152) are well supported by the model, and the existence of a logarithmic ‘velocity overlap layer’ in which both of these laws are valid is demonstrated forAo/ks30.  相似文献   

14.
Stream–subsurface exchange strongly influences the transport of contaminants, fine particles, and other ecologically relevant substances in streams. We used a recirculating laboratory flume (220 cm long and 20 cm wide) to study the effects of particle size, overlying velocity, and biofilm formation on stream–subsurface exchange of particles. Sodium chloride was used as a non‐reactive dissolved tracer and 1‐ and 5‐µm fluorescent microspheres were used as particulate tracers. Surface–subsurface exchange was observed with a clean sand bed and a bed colonized by an autotrophic–heterotrophic biofilm under two different overlying velocities, 0·9 and 5 cm s?1. Hydrodynamic interactions between the overlying flow and sand bed resulted in a reduction of solute and particle concentrations in the water column, and a corresponding accumulation of particles in both the sediments and in the biofilm. Increasing overlying velocity and particle size resulted in faster removal from the overlying water due to enhanced mass transfer to the bed. The presence of the biofilm did not affect solute exchange under any flow condition tested. The presence of the biofilm significantly increased the deposition of particles under an overlying velocity of 5 cm s?1, and produced a small but statistically insignificant increase at 0·9 cm?1. The particles preferentially deposited within the biofilm matrix relative to the underlying sand. These results demonstrate that hydrodynamic transport conditions, particle size, and biofilm formation play a key role in the transport of suspended particles, such as inorganic sediments, particulate organic matter, and pathogenic microorganisms in freshwater ecosystems, and should be taken into consideration when predicting the fate and transport of particles and contaminants in the environment. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

15.
Using the 160‐m‐long flume at Tsukuba University we undertook an experiment to provide a first estimate of the virtual velocity of sand in the size range 0.5–2.0 mm. For the flow velocity used in our experiment this sediment‐size range would conventionally be regarded as suspended sediment. The virtual velocity was found to be 37–41% of the flow velocity. Paradoxically, virtual velocity decreases as particle size decreases. Such a lower virtual velocity of finer sediment is not inconceivable. First, trapping of the sediment appears to be a function of bed roughness, and there is a probable relationship between bed roughness and trapping efficiency for particles of different sizes. Second, finer particles are more likely to find sheltered positions on a rough bed and thus experience lower mobility, relative to the more exposed coarser grains, as observed for bedload transport. Third, the virtual velocity of particles undergoing bedload transport has been found, in some instances, to be lower for finer clasts. We combine our data with previous studies of virtual velocity of bedload to develop, for the first time, a hypothesis for a holistic analysis of sediment movement in rivers. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

16.
We examined particle size distributions of suspended particulate matter (SPM); physical and environmental influences on the observed distributions; and relationships between particle size and geochemical partitioning of metals, over the fall and winter period in a small urban river (Don River, Toronto, Ontario, Canada). For this dataset, the majority of particles (80%) in suspension were less than 10 µm in size. In addition, while total SPM concentrations showed a positive trend with increasing discharge (Q); the proportions of particles found within given size classes were independent of both SPM concentration and Q. Temperature was the only measured environmental variable related to the particle concentrations within size classes. As water temperature increased, the concentration of particles in the smallest size class (1–4 µm) decreased, while the concentration of silt and/or algae sized particles (10–50 µm) increased. Increasing water temperatures may promote bacterial attachment to particles and their subsequent flocculation into larger sized particles. Decreasing concentrations of leachable (most labile) Cd, Zn and Mn were associated with increasing concentrations of the largest particles (70–150 µm) in suspension. In contrast, higher reducible (oxides) associated concentrations of Cd, Zn and Mn occurred with increasing concentrations of smaller particles (1–10 µm) in suspension. Both of these trends are speculated to reflect the importance of particle surface area for metal sorption reactions.  相似文献   

17.
Airborne correlation spectrometry (COSPEC) was used to measure the rate of SO2 emission at White Island on three dates, i.e., November 1983, 1230 ± 300 t/d; November 1984, 320 ± 120 t/d; and January 1985, 350 ± 150 t/d (t = metric tons). The lower emission rates are likely to reflect the long-term emission rates, whereas the November 1983 rate probably reflects conditions prior to the eruption of December 1983. The particle flux in the White Island plume, as determined with a quartz crystal microbalance/cascade in November 1983, was 1.3 t/d, unusually low for volcanic plumes. The observed plume particles, as shown from scanning electron microscopy, include halite, native sulfur, and silicates and are broadly similar to other volcanic plumes.Gas analyses from high-temperature volcanic fumaroles collected from June 1982 through November 1984 werde used together with the COSPEC data to estimate the flux of other gas species from White Island. The rates estimated are indicative of the long-term volcanic emission, i.e., 8000–9000 t/d H2O, 900–1000 t/d CO2, 70–80 t/d HCl, 1.5–2 t/d HF, and about 0.2 t/d NH3. The long-term thermal power output at White Island is estimated at about 400 MW.  相似文献   

18.
Shear flow instability arising from the velocity shear between the inner and the outer central plasma sheet regions is studied by treating the plasma as compressible. Based on the linearized MHD equations, dispersion relations for the surface wave modes occurring at the boundary of the inner central plasma sheet (ICPS) and the outer central plasma sheet (OCPS) are derived. The growth rates and the eigenmode frequencies are obtained numerically. Three data sets consisting of parameters relevant to the earth’s magnetotail are considered. The plasma sheet region is found to be stable for constant plasma flows unless MA>9.6, where MA is the Alfvén Mach number in the ICPS. However, for a continuously varying flow velocity profile in the ICPS, the instability is excited for MA\geq1.4. The excited modes have oscillation periods of 2–10 min and 1.5–6 s, and typical transverse wavelengths of 30–100 RE and 0.5–6 RE for data sets 1 and 2 (i.e., case of no neutral sheet) respectively. For the data set 3, which corresponds to a neutral sheet at the center of the plasma sheet, the excited oscillations have periods of 2 s-1 min with transverse wavelengths of 0.02–1 RE.  相似文献   

19.
Grain-size distributions of suspended load over a sand-gravel bed at two different flow velocities were studied in a laboratory flume.The experiments had been performed to study the influence of flow velocity and suspension height on grain-size distribution in suspension over a sand-gravel bed.The experimental findings show that with an increase of flow velocity,the grain-size distribution of suspended load changed from a skewed form to a bimodal one at higher suspension heights.This study focuses on the determination of the parameter β_n which is the ratio of the sediment diffusion coefficient to the momentum diffusion coefficient of n th grain-size.A new relationship has been proposed involving β_n,the normalizing settling velocity of sediment particles and suspension height,which is applicable for widest range of normalizing settling velocity available in literature so far.A similar parameter β for calculating total suspension concentration is also developed.The classical Rouse equation is modified with β_n and β and used to compute grain-size distribution and total concentration in suspension,respectively.The computed values have shown good agreement with the measured values of experimental data.  相似文献   

20.
A self-contained, field-portable recirculating seawater flume was designed and constructed in order to measure in situ the erodibility of cohesive estuarine muds. The flume can be easily deployed by one person and is designed for subaerial use only. Bed shear stresses up to 0.6 Nm−2 can be generated by the flume. Rates of sediment erosion are assessed in terms of the mass of particulate material eroded with time. Flow calibrations yield a drag coefficient (CD) of 4.5 × 10−3 which enables single mid-depth velocity measurements to be related directly to the imposed bed stress. Water motions within the flume are complex, but secondary (radial) circulations are not considered sufficiently competent to dominate sediment erosion. Stratification effects due to high levels of suspended sediment ( 1.5 gl−1) are negligible. However, the drag-reducing properties of the sediment suspension are uncertain.  相似文献   

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