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1.
The erodibility of a grain on a rough bed is controlled by, among other factors, its relative projection above the mean bed, its exposure relative to upstream grains, and its friction angle. Here we report direct measurements of friction angles, grain projection and exposure, and small-scale topographic structure on a variety of water-worked mixed-grain sediment surfaces. Using a simple analytical model of the force balance on individual grains, we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography. The friction angle, projection, and exposure of single grain sizes vary widely from point to point within a given bed surface; the variability within a single surface often exceeds the difference between the mean values of disparate surfaces. As a result, the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution, rather than a single value. On a given bed, the crtitical shear stress distributions of different grain sizes have similar lower bounds, but above their lower tails they diverge rapidly, with smaller grains having substantially higher median critical shear stresses. Large numbers of fines, trapp.ed within pockets on the bed or shielded by upstream grains, are effectively lost to the flow. Our calculations suggest that critical shear stress, as conventionally measured, is defined by the most erodible grains, entrained during transient shear stress excursions associated with the turbulent flow; this implies a physical basis for the indeterminacy of initial motion. These observations suggest that transport rate/shear stress relationships may be controlled, in part, by the increasing numbers of grains that become available for entrainment as mean shear stress increases. They also suggest that bed textures and grain size distributions may be controlled, within the constraints of an imposed shear stress and sediment supply regime, by the influence of each size fraction on the erodibility of other grain sizes present on the bed.  相似文献   

2.
Experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady/oscillatory flows. Derived critical shear stresses are compared with the mixed bed entrainment model of Wiberg & Smith (1987). As predicted by the model, coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively finer grains. Entrainment occurs generally at lower shear stresses than predicted by the model, especially under unidirectional flows. This may be the result of grains resting in unusually unstable positions during the experiments because the beds are ‘unworked’ at the start of the experiments. The model of Wiberg and Smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly defined. The pivoting angles of the beds used here are measured using a new technique designed specifically for comparison with the threshold data. The measured angles repeat the finding that the coarse grains are more mobile than the finer fractions of a mixture. The results are poorly described by the pivoting angle model presented by Wiberg & Smith (1987) and are better represented by a model of the form Φ = αDγ(Di/D50)β (after 21 ), where α, γ and β are empirical constants. The threshold model is found to be more effective using the improved pivoting relationship. The entrainment of grains is found to be easier beneath unidirectional flows than combined flows, in accordance with previous authors’ findings. A suggestion that this result is caused by a change in the erosion mechanism beneath wave flows is made. Wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive efficiency of the overlying current flow. The results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near-threshold flows.  相似文献   

3.
Vertical gradational structures develop as sand infiltrates into static gravel beds. Understanding the vertical distribution of interstitial sand deposits will improve predictions of ecological suitability and hyporheic hydrodynamics. A series of flume experiments was performed to investigate fine infiltration processes. Four sand distributions were introduced into flows over gravel beds. After each experiment, bed cores were extracted and analysed in vertical layers to examine the gradational trends with depth. Vertical trends of fine content were highly sensitive to the relative grain‐size distributions of the gravel bed and the introduced sand. For experiments with d15gravel/d85sand ratios 15·4 and larger unimpeded static percolation was observed, where sand filled the voids relatively uniformly from the bottom of the gravel layer to the top. Experiments with ratios 10·6 and smaller bridged. Sand clogged a thin layer of gravel pores near the bed surface, precluding subsequent infiltration. Interstitial sand deposits fined with depth of penetration for all experiments which was the result of three distinct but overlapping processes. (i) Granular sorting: As particles fell through the substrate, smaller material preferentially passed through the voids deeper into the gravel. (ii) Bed‐load sorting: Size segregation occurs in the wake of the leading bed form as smaller particles saltate further and settle first. (iii) Hydraulic sorting: Smaller sand was transported preferentially as suspended load filling the deep voids of the furthest flume positions downstream. Finally, when the experiments that formed a bridge layer were replicated with higher bed shear stresses, less interstitial sand deposition was observed. Higher shear stresses transported coarse particles downstream more efficiently causing bridge layers to form earlier and allowing less time for suspended load to settle into the deeper substrate pores before the pathways were closed.  相似文献   

4.
Aeolian sand entrainment, saltation and deposition are important and closely related near surface processes. Determining how grains are sorted by wind requires a detailed understanding of how aerodynamic sand transport processes vary within the saltating layer with height above the bed. Grain‐size distribution of sand throughout the saltation layer and, in particular, how the associated flux of different grain size changes with variation in wind velocity, remain unclear. In the present study, a blowdown wind tunnel with a 50 cm thick boundary layer was used to investigate saltating sand grains by analyzing the weight percentage and transport flux of different grain‐size fractions and the mean grain size at different wind velocities. It was found that mean grain size decreases with height above the sand bed before undergoing a reversal. The height of the reversal point ranges from 4 to 40 cm, and increases with wind velocity following a non‐linear relationship. The content of the finer fractions (very fine and fine sand) initially increases above the sand bed and then decreases slightly with height, whereas that of the coarser fractions (medium and coarse sand) exhibits the opposite trend. The content of coarser grains and the mean grain size of sand in the saltation layer increase with wind velocity, indicating erosional selectivity with respect to grains in multi‐sized sand beds; but this size selectivity decreases with increasing wind velocity. The vertical mass flux structure of fine sand and very fine sand does not obey a general exponential decay pattern under strong wind conditions; and the coarser the sand grain, the greater the decrease rate of their transport mass with height. The results of these experiments suggest that the grain‐size distribution of a saltating sand cloud is governed by both wind velocity and height within the near‐surface boundary layer.  相似文献   

5.
Open‐framework gravel (OFG) in river deposits is important because of its exceptionally high permeability, resulting from the lack of sediment in the pore spaces between the gravel grains. Fluvial OFG occurs as planar strata and cross strata of varying scale, and is interbedded with sand and sandy gravel. The origin of OFG has been related to: (1) proportion of sand available relative to gravel; (2) separation of sand from gravel during a specific flow stage and sediment transport rate (either high, falling or low); (3) separation of sand from gravel in bedforms superimposed on the backs of larger bedforms; (4) flow separation in the lee of dunes or unit bars. Laboratory flume experiments were undertaken to test and develop these theories for the origin of OFG. Bed sediment size distribution (sandy gravel with a mean diameter of 1·5 mm) was kept constant, but flow depth, flow velocity and aggradation rate were varied. Bedforms produced under these flow conditions were bedload sheets, dunes and unit bars. The fundamental cause of OFG is the sorting of sand from gravel associated with flow separation at the crest of bedforms, and further segregation of grain sizes during avalanching on the steep lee side. Sand in transport near the bed is deposited in the trough of the bedform, whereas bed‐load gravel avalanches down the leeside and overruns the sand in the trough. The effectiveness of this sorting mechanism increases as the height of the bedform increases. Infiltration of sand into the gravel framework is of minor importance in these experiments, and occurs mainly in bedform troughs. The geometry and proportion of OFG in fluvial deposits are influenced by variation in height of bedforms as they migrate, superposition of small bedforms on the backs of larger bedforms, aggradation rate, and changes in sediment supply. If the height of a bedform increases as it migrates downstream, so does the amount of OFG. Changes in the character of OFG on the lee‐side of unit bars depend on grain‐size sorting in the superimposed bedforms (dunes and bedload sheets). Thick deposits of cross‐stratified OFG require high bedforms (dunes, unit bars) and large amounts of aggradation. These conditions might be expected to occur during high falling stages in the deeper parts of river channels adjacent to compound‐bar tails and downstream of confluence scours. Increase in the amount of sand supplied relative to gravel reduces the development of OFG. Such increases in sand supply may be related to falling flow stage and/or upstream erosion of sandy deposits.  相似文献   

6.
Heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness. Distinct spatial zones in terms of grain size have been reported, for example sand ribbons and bedload sheets. The transition from areas of finer to coarser grained surface sediment is often abrupt. However, the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels. This paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistancé on the structure of the turbulent boundary layer. Measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river. Immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface, boundary shear stress and roughness length increase considerably. Downstream from this transition, velocity profiles become concave upwards. Downstream and upstream sections show significant differences in terms of near bed velocities (deceleration downstream of the transition), velocity gradient and turbulence intensity of the streamwise velocity component. Comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity (or shear stress) associated with the pressure drag produced by large and isolated obstacles.  相似文献   

7.
Reported here are results from new flume experiments examining deposition and entrainment of inert, silt‐sized particles (with spherical diameters in the range from 20 to 60 μm) to and from planar, impermeable and initially starved beds underlying channel flows. Bed surfaces comprised smooth or fixed sand‐size granular roughness and provided hydraulically smooth to transitionally rough boundaries. Results of these experiments were analysed with a simple model that describes the evolution of vertically averaged concentration of suspended sediment and accommodates the simultaneous delivery to and entrainment of grains from the bed. The rate of particle arrival to a bed diminishes linearly, and the rate of particle entrainment increases by the 5/2 power, as the value of the dimensionless Saffman parameter S = u*3/g’ν approaches a threshold value of order unity, where u is the conventional friction velocity of the turbulent channel flow, g’ is the acceleration due to gravity adjusted for the submerged buoyancy of individual particles and ν is the kinematic viscosity of the transporting fluid. This transport behaviour is consistent with the notion that non‐cohesive, silt‐sized particles can neither reach nor remain on an impermeable bed under flow conditions where mean lift imposed on stationary particles in the viscous sublayer equals or exceeds the submerged weight of individual particles. Within the size range of particles used in these experiments, particle size and the characteristic size of granular roughness, up to that of medium sand, did not affect rates of dimensionless arrival or entrainment to a significant degree. Instead, a new but consistent picture of fine‐particle transport is emerging. Silt‐sized material, at least, is subject to potentially significant interaction with the bed during intermittent suspension transport at intermediate flow speeds greater than the value required for initiation of transport (ca 20 cm sec?1) but less than the value (ca 50 cm sec?1) required by the Saffman criterion ensuring transport in fully passive suspension or, equivalently, ‘wash‐load’.  相似文献   

8.
砂石混合体由力学性质以及结构相差极大的材料组成,其组成的重塑地层易发生塌陷等问题,因此对砂石混合体力学特性的研究具有重要的工程意义。砾石形状是砂砾石力学特性研究的重要属性参数,但采用规则图形对砾石进行描述不能反映出其真实的力学性质,采用数字图像处理技术构建的砾石数据库能反映砾石真实形状并可对特定形状参数进行具体分析。由于砂石混合体的粒径分布较广,采用特征粒径等无法描述整体粒度分布,故本文结合分形理论构建砂石混合体的二重分形结构模型,通过粒度分维值反演出完整的级配分布曲线。考虑到砂石混合体离散型的特点,采用离散元软件进行直剪试验数值模拟并对细观结构进行分析,研究结果表明,砂石混合体一般具有2个粒度分维值:砂粒度分维值和砾石粒度分维值,砂、砾石粒度分维值越接近,抗剪强度和内摩擦角越大;当两者相等时,砂石混合体具有一重分维,此时均一性最好,抗剪强度和内摩擦角最大;轴向系数是形容砾石形状的一个重要参数,随着轴向系数的增加,砾石显示出明显的条状性,在直剪试验中抗转动能力增强、周围接触数量增加,导致抗剪强度和内摩擦角不断增加。  相似文献   

9.
Models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water. Of special interest is whether such process-based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods. A detailed derivation is undertaken, focusing first on the fluid flow and forces at the particle level. The resulting threshold equation for the particle-level velocity is then modified to yield the mean entrainment stress for the flow as a whole. This approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit, the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape. The resulting threshold equations contain a number of coefficients (e.g. drag and lift) whose values are poorly known, but can be constrained by requiring agreement with the Shields curve for the threshold of grains in uniform deposits. If pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models, smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment. However, if reasonable modifications of those coefficients are made for expected field conditions, then the models yield good agreement with the data. Sliding models, where sorting is due entirely to projection distances of the grains above the bed, yield somewhat poorer agreement with the field data; however, the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends. The existing measurements lack documentation of the mechanisms of grain movement, so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding. In spite of such uncertainties, the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence.  相似文献   

10.
Mcewan  Jefcoate  & Willetts 《Sedimentology》1999,46(3):407-416
A grain-scale model of fluvial bed load transport is described, with particular emphasis on the equilibrium between the saltating grains and the near bed flow, and its role in determining transport rate. The model calculates, explicitly, the modification of the velocity profile by the moving grains, together with the consequential reduction in surface fluid shear stress. As the surface fluid shear stress is reduced by the moving grains, so the entrainment rate decreases and the model reaches a steady state. The results provide insight into two important questions at a macroscopic level. First, they show that, in the absence of large static roughness, the dynamic roughness caused by the moving grains may be a significant contributor to flow resistance. Secondly, the model indicates the manner in which transport may be limited by a combination of the transport capacity of the flow and the availability of sediment for entrainment. Only in the case of high sediment availability does the fluid shear stress acting at the surface approach the critical entrainment value, reproducing the behaviour suggested by Bagnold (1956 ) and Owen (1964 ). This suggests that prediction formulae based on this assumption only describe the bed load transport system under particular conditions.  相似文献   

11.
Carling  Williams  Gölz  & Kelsey 《Sedimentology》2000,47(1):253-278
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12.
Li  Pu  Wang  Jiading  Hu  Kaiheng  Shen  Fei 《Landslides》2021,18(9):3041-3062

Channel morphology and bed sediment erodibility are two crucial factors that significantly affect debris flow entrainment processes. Current debris flow entrainment models mostly hypothesize the erodible beds are infinite with uniform slopes. In this study, a series of small-scale flume experiments were conducted to investigate the effects of bed longitudinal inflexion and sediment porosity on basal entrainment characteristics. Experimental observations revealed that sediment entrainment is negligible at early stages and accelerates rapidly as several erosion points appear. Continual evolution of flow-bed interfaces changes interactions between debris flows and bed sediments, rendering the interfacial shear action involved into a mixed shear and frontal collisional action. Lower bed sediment porosity will change the spatial arrangement and orientation of particle mixture, strengthen the interlocking and anti-slide forces of adjacent sediment particles, and promote the formation of particle clusters, all of which will increase bed sediment resistance to erosion. By examining the post-experimental bed morphology, the slope-cutting amounts and topographic reliefs are determined to positively correlate with longitudinal transition angles. These high topographic reliefs may indicate the propensity of triangular slab erosion, rather than strip-shaped slab erosion, in non-uniform channels with relatively steep erodible beds. Empirical formulas are obtained that denote the relationships among bed sediment strength, channel curvature radius, and sediment porosity through a multi-parameter regression analysis. This study may aid in clarifying the complex coupling effects of spatial variations in debris flow dynamics as well as sediment erodibility and bed morphology in non-uniform channels with abundant seismic loose material.

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13.
The dynamic interpretation of most current-structure sequences derives directly from experiments on the succession of bedforms produced by flows in flumes. The results of these and related studies have been used to construct stability field diagrams in which the fields of individual bedforms are usually expressed as a function of flow intensity (power, velocity, bed shear stress, etc.) and grain size. The data underlying existing stability-field diagrams were collected largely from the study of flows carrying coarse-grained sediment entrained through particle-by-particle bed erosion. Many flows, however, do not entrain sediment through simple bed erosion. Most turbidity currents originate by the development of turbulence in slumps, slides, and other slope failures. Such flows generally form with highly concentrated suspended loads and their bed-load layers derive sediment from the collapsing suspended-sediment clouds. Because the collapse properties of such clouds may be related as much to suspended particle concentration, size distribution, particle interactions, and other factors as to flow intensity, the stability fields of bedforms developed beneath such flows may differ in flow intensity-grain-size relationships from those beneath flows deriving sediment from bed erosion alone. Useful stability-field diagrams for turbidity currents must include suspended-load fallout rate as a third variable, independent of flow intensity and mean grain size. A preliminary stability-field diagram of this type indicates that Bouma Tabc sequences may theoretically form with essentially no velocity variation of the attendant flow. This type of analysis may have considerable relevance to the interpretation not only of turbidites but also of other deposits formed where bed-load layers are fed from above rather than below. These include shallow-shelf storm units deposited from highly concentrated flows and volcaniclastic layers formed where pyroclastic debris falls directly into moving water.  相似文献   

14.
Besides particle size, density and shape, the erodibility of a sediment bed depends also upon the exposure to prethreshold velocities in the overlying flow. Such flow effectively rearranges the grains (at and below the bed surface), causing them to become more resistant to subsequent erosion. The effects of the ‘stress history’, leading up to the critical condition for sediment movement, are investigated for unidirectional flows generated in a recirculating laboratory flume. The sediment beds investigated consisted of cohesionless quartz sand grains, with mean grain diameters of 0·194 mm (fine sand), 0·387 mm (medium sand) and 0·774 mm (coarse sand), with narrow particle-size distributions. The critical (threshold) shear velocity (target value) for the three beds was established, within 2·5 min of increasing the flow from zero velocity. The subsequent experiments were performed under prethreshold velocities at 70% (for 5, 10, 20, 40 and 80 min exposure duration), 80% (for 5, 10, 20, 40 and 80 min exposure duration), 90 and 95% (for 5, 10, 20, 40, 80 and 120 min exposure duration) of the target value. Following exposure to these different prethreshold conditions, the flow was increased then to reach actual critical conditions, within a period of 2·5 min. The critical condition for the initiation of sediment movement was established using visual observation (supplemented by video recordings), according to the Yalin criterion. The results show that if the exposure duration to prethreshold velocities remains constant, then the critical shear velocity increases with increasing prethreshold velocity. Likewise, if the prethreshold velocity remains constant, then the critical shear velocity increases with increasing exposure duration. In some circumstances, the critical shear velocity was found to increase by as much as 27%. An empirical formula is proposed to account for the exposure correction to be applied to the critical shear velocities of sand-sized sediment beds; this is prior to their inclusion into bedload transport formulae, for an improved prediction of the magnitude and nature of transport.  相似文献   

15.
The Lower Cretaceous Britannia Formation (North Sea) includes an assemblage of sandstone beds interpreted here to be the deposits of turbidity currents, debris flows and a spectrum of intermediate flow types termed slurry flows. The term ‘slurry flow’ is used here to refer to watery flows transitional between turbidity currents, in which particles are supported primarily by flow turbulence, and debris flows, in which particles are supported by flow strength. Thick, clean, dish‐structured sandstones and associated thin‐bedded sandstones showing Bouma Tb–e divisions were deposited by high‐ and low‐density turbidity currents respectively. Debris flow deposits are marked by deformed, intraformational mudstone and sandstone masses suspended within a sand‐rich mudstone matrix. Most Britannia slurry‐flow deposits contain 10–35% detrital mud matrix and are grain supported. Individual beds vary in thickness from a few centimetres to over 30 m. Seven sedimentary structure division types are recognized in slurry‐flow beds: (M1) current structured and massive divisions; (M2) banded units; (M3) wispy laminated sandstone; (M4) dish‐structured divisions; (M5) fine‐grained, microbanded to flat‐laminated units; (M6) foundered and mixed layers that were originally laminated to microbanded; and (M7) vertically water‐escape structured divisions. Water‐escape structures are abundant in slurry‐flow deposits, including a variety of vertical to subvertical pipe‐ and sheet‐like fluid‐escape conduits, dish structures and load structures. Structuring of Britannia slurry‐flow beds suggests that most flows began deposition as turbidity currents: fully turbulent flows characterized by turbulent grain suspension and, commonly, bed‐load transport and deposition (M1). Mud was apparently transported largely as hydrodynamically silt‐ to sand‐sized grains. As the flows waned, both mud and mineral grains settled, increasing near‐bed grain concentration and flow density. Low‐density mud grains settling into the denser near‐bed layers were trapped because of their reduced settling velocities, whereas denser quartz and feldspar continued settling to the bed. The result of this kinetic sieving was an increasing mud content and particle concentration in the near‐bed layers. Disaggregation of mud grains in the near‐bed zone as a result of intense shear and abrasion against rigid mineral grains caused a rapid increase in effective clay surface area and, hence, near‐bed cohesion, shear resistance and viscosity. Eventually, turbulence was suppressed in a layer immediately adjacent to the bed, which was transformed into a cohesion‐dominated viscous sublayer. The banding and lamination in M2 are thought to reflect the formation, evolution and deposition of such cohesion‐dominated sublayers. More rapid fallout from suspension in less muddy flows resulted in the development of thin, short‐lived viscous sublayers to form wispy laminated divisions (M3) and, in the least muddy flows with the highest suspended‐load fallout rates, direct suspension sedimentation formed dish‐structured M4 divisions. Markov chain analysis indicates that these divisions are stacked to form a range of bed types: (I) dish‐structured beds; (II) dish‐structured and wispy laminated beds; (III) banded, wispy laminated and/or dish‐structured beds; (IV) predominantly banded beds; and (V) thickly banded and mixed slurried beds. These different bed types form mainly in response to the varying mud contents of the depositing flows and the influence of mud on suspended‐load fallout rates. The Britannia sandstones provide a remarkable and perhaps unique window on the mechanics of sediment‐gravity flows transitional between turbidity currents and debris flows and the textures and structuring of their deposits.  相似文献   

16.
ABSTRACT Temporally and spatially averaged models of bedload transport are inadequate to describe the highly variable nature of particle motion at low transport stages. The primary sources of this variability are the resisting forces to downstream motion resulting from the geometrical relation (pocket friction angle) of a bed grain to the grains that it rests upon, variability of the near‐bed turbulent velocity field and the local modification of this velocity field by upstream, protruding grains. A model of bedload transport is presented that captures these sources of variability by directly integrating the equations of motion of each particle of a simulated mixed grain‐size sediment bed. Experimental data from the velocity field downstream and below the tops of upstream, protruding grains are presented. From these data, an empirical relation for the velocity modification resulting from upstream grains is provided to the bedload model. The temporal variability of near‐bed turbulence is provided by a measured near‐bed time series of velocity over a gravel bed. The distribution of pocket friction angles results as a consequence of directly calculating the initiation and cessation of motion of each particle as a result of the combination of fluid forcing and interaction with other particles. Calculations of bedload flux in a uniform boundary and simulated pocket friction angles agree favourably with previous studies.  相似文献   

17.
Fine particles may infiltrate through coarse alluvial beds and eventually saturate the subsurface pore space. It is essential to understand the conditions that lead to bed saturation, and to forecast the packing characteristics of saturated beds to assess the effect of excess fine sediment supply on a number of processes that occur in the stream–sediment boundary. To address this problem, in this study, a new method is introduced to predict the grain‐size distribution for the saturated condition, and the resulting porosity decrease, given the characteristics of the bed and the supplied sediments. The new method consists of the numerical aggregation of infilling fines in a finite bed volume, during which the bed properties change to affect further infilling. An existing semi‐empirical, particle packing model is implemented to identify these properties. It is shown that these types of models are adequate to describe regimes of natural sediment fabric quantitatively, and are thus useful tools in the analysis of sediment infiltration processes. Unlike previous developments to quantify saturated bed conditions, which assume that the supplied material is uniform and finer than the bed pore openings, the method developed herein considers poorly sorted fines, and can identify size fractions that are able to ingress into the bed due to being smaller than the particles that form the bed structure. Application of the new method to published experimental data showed that the final content of infiltrated fines is strongly sensitive to the initial bed packing density, highlighting the need to measure and understand open‐work gravel deposits. In addition, the new method was shown to be suitable for assessing the degree of bed saturation, when it was applied to a published data set of field samples.  相似文献   

18.
A discrete element method is applied to a three‐dimensional analysis related to sediment entrainment on a micro‐scale. Sediment entrainment is the process by which a fluid medium accelerates particles from rest and advects them upward until they are either transported as bedload or suspended by the flow. Modelling of the entrainment process is a critically important aspect for studies of erosion, pollutant resuspension and transport, and formation of bedforms in environmental flows. Previous discrete element method studies of sediment entrainment have assumed the flow within the particle bed to be negligible and have only allowed for the motion of the topmost particles. At the same time, micro‐scale experimental studies indicate that there is a small slip of the fluid flow at the top of the bed, indicating the presence of non‐vanishing fluid velocity within the topmost bed layers. The current study demonstrates that the onset of particle incipient motion, which immediately precedes particle entrainment, is highly sensitive to this small fluid flow within the topmost bed layers. Using an exponential decay profile for the inner‐bed fluid flow, the discrete element method calculations are repeated with different fluid penetration depths within the bed for several small particle Reynolds numbers. For cases with slip velocity corresponding to that observed in previous experiments with natural sediment, the predicted particle velocity is found to be a few percent of the fluid velocity at the top of the viscous wall layer, which is a reasonable range of velocities for observation of incipient particle motion. This method for prescribing the fluid flow within the particle bed allows for the current discrete element method to be extended in future studies to the analysis of sediment entrainment under the influence of events such as turbulent bursting. Additionally, predictions for the slip velocities and fluid flow profile within the bed suggest the need for further experimental studies to provide the data necessary for additional improvement of the discrete element method models.  相似文献   

19.
The Burdekin River is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge, in response to seasonal but erratic monsoonal rainfall. Floods of the Burdekin rise abruptly, reaching peak discharges of up to 40,000 m3 s-1 in less than 24 h; maintain peak flow for up to a few days, and recede exponentially. The geomorphology and deposits of these rivers reflect the extreme discharge fluctuations, and have not previously been described. A stretch of the upper Burdekin River comprising four bends and one straight reach was examined near the town of Charters Towers. The river bed is largely exposed for most of any year, with a small, misfit perennial channel carrying low stage flow. Major geomorphic elements of bends include point bars with ridge-and-swale topography, three distinct types of chute channels, avalanche slipfaces up to 5 m or more high around the downstream edges of bars, and on the outer part of one point bar an elevated, vegetated ridge. Straight reaches are flat or gently inclined, sand- and gravel-covered surfaces. Much of the river bed is covered by well sorted, in places gravelly, coarse to very coarse-grained sand with local accumulations of pebble to boulder gravel. Lower parts of the river bed are periodically draped by mud which is desiccated on exposure. Dunes and plane beds are the most commonly occurring bedforms, with local development of gravelly antidunes. Most bank tops and upper, vegetated bars are covered by silt and fine-grained sand. The river bed also hosts a low-diversity but locally high-abundance, flood-tolerant flora dominated by the paperbark tree Melaleuca argentea, which plays an important role in controlling the distribution of sediment. The gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major (bankfull or near bankfull) flows, which have a recurrence of about 18 years (based on 65 years hydrographic data). The initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact, whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent, non-bankfull discharge events.  相似文献   

20.
Collision data are presented from coloured high-speed films of three size fractions of sand grains saltating over a bed of the total grain population. Each fraction was colour tagged and the proportion of each size ejected by grains colliding with the surface was recorded on a number of films taken as the bed was progressively eroded. The results confirm earlier findings that V3/V1?0.5–0.6, Vn/V1?.08 and the rebound angle increases with decreasing grain size. Ejected grains are examined in relation to their size, the impactor size, ejection speed and angle and the number of ejecta per collision. In addition, changes in grain parameters are observed with time. For fine impactors, ejection speeds generally increase with a decrease in ejecta size, but the fine fraction does not follow this trend for the coarse and medium impactors. Ejection angles are typically between 40° and 60°, with coarse grains having shallower mean angles than fine ejecta. The number of ejections per collision increases with a decrease in particle size for each impactor size. The general tendency for coarse particles to be ejected at lower speeds and shallower angles than fine particles will lead to sorting of the grain sizes. There is poor correlation between the forward momentum loss of the saltating grams at collision and both the forward momentum of the ejected grains and the number of ejected grains. Much of the forward momentum of the saltating grains is transfered to creeping grains. The composition and geometry of the bed are considered to be important factors in the evolution of the saltation cloud.  相似文献   

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