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1.
SUPRIYA SENGUPTA 《Sedimentology》1979,26(1):63-82
Grain-size frequency distributions of suspended loads at different flow velocities and over sand beds of four different grain-size patterns were studied in a laboratory flume. The proportion of bed material which went into suspension increased with decrease of grain-size in each case, but the modes of the suspended loads occurred in the size classes intermediate between the coarsest and the finest. With increase of flow velocity, as also with decrease of the bed's mean grain-size, the total amount of material in suspension markedly increased, mainly due to addition of particles to the medium size classes. The coarsest grains in the bed resisted erosion due to their weight, whereas the finest ones were either not available in sufficient quantities or resisted erosion due to their homogeneity. The finest of the erodible grains which were abundantly available in bed were therefore, lifted up in large quantities. This size sorting took place at or near the bed surface and was closely related to the process of bed form migration. Large accumulation of medium sized particles in suspension at high velocities led to lognormal grain-size distributions when the nature of the bed (source) material was suitable. At lower velocities, or over other types of bed materials, the phi (log)-probability plots of cumulative grain-size distributions of the suspended loads resolved into a number of straight lines. Mixtures of linear segments on phi-probability graphs therefore, need not necessarily indicate different modes of sediment transportation, as is commonly believed, but might reflect the conditions of flow and the nature of the source material. 相似文献
2.
SUPRIYA SENGUPTA 《Sedimentology》1975,22(2):257-273
Grain size distributions of the suspended loads above a bed of bimodal size distribution (size range 2-00-0.04 mm) were studied in a laboratory flume at water velocities varying from 42 to 160 cm/s. With increase of velocity the phi (logarithmic) size distribution of the suspended particles (at 5-20 cm above the bed) changed from a strongly skewed to a nearly symmetrical, unimodal form (nearly lognormal) through an intermediate bimodal stage. At low velocity the skewness of the distribution changed from positive to negative with increase of height. The experiments indicate that lognormality of‘weight frequency’ distribution of grain sizes is a transitional feature, attained through size sorting within a critical range of velocity and height above a sand bed of a given composition. The observed changes in the size distribution patterns were effected by a differential rate of increase in weight in the different size classes in suspension with increase of flow velocity. The phenomenon could be explained by the equation of relative suspension concentration which relates the relative concentration of a suspended particle of a particular diameter to the flow velocity of the turbulent fluid and the height of suspension above the bed. 相似文献
3.
The grain-size fractions in the bedload transported over the five heterogeneous sediment beds of different values of bed roughness were computed from the flume experiments. The existence of an entrapment factor associated with the sorting observed from the bed to active layer was modeled based on the modified critical shear stress to estimate the grain-size fractions in the transport layer under given hydraulic conditions. The efficiency of these models was tested with the observed data. Subsequently, the patterns of observed grain-size distributions in the transport layer were tested to identify the distributions developed in the active layer due to sorting using three probability density functions (pdf), such as, log-normal, log-hyperbolic and log-skew-Laplace. Tests indicated that a log-skew-Laplace distribution fitted best for 49%, a log-hyperbolic for 31%, and a log-normal for 20% out of forty-five bedload samples collected under unidirectional flow with changes in flow discharge and bed roughness. The results of this study would be useful to specify the grain-size distributions in the bedload formed under different hydrodynamic conditions in various sedimentary environments. 相似文献
4.
Breaks in the slope of log-probability plots of cumulative grain-size distributions of bed material are compared with frequency distributions of bedload and suspended sediment over a range of discharges at two stations on the Platte River in south-central Nebraska. The break between suspension and intermittent suspension as determined from the bed-material curve coincides with the upper limit of the grain-size overlap between bedload particles and suspended-sediment particles, whereas the break between intermittent suspension and traction corresponds to the grain size at the lower limit of overlap of bedload particles and suspended-sediment particles. Although grain-size distributions of bedload change little with discharge, the size of the coarsest grains in suspension increases with increasing discharge. Thus, the length of overlap of bedload and suspended-sediment distributions increases with increasing discharge. The limits of grain-size overlap of bedload and suspended-sediment distribution curves associated with near-flood discharges most closely approximate the breaks in the bed material grain-size distribution. 相似文献
5.
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
6.
Grain size distribution in suspension from bed materials 总被引：1，自引：0，他引：1
B. S. MAZUMDER 《Sedimentology》1994,41(2):271-277
Experimental results show that the grain size distribution of suspended material is related to flow parameters and grain size distribution in the bed. A theoretical model has been developed to compute the suspension grain size distribution on the basis of diffusion equations, taking into account the effect of hindered settling due to the increased concentration in suspension. Fluid velocity closest to the bed is estimated by using the concept of migration velocities of particles in the bed layer. Comparisons of data computed by the proposed method and data from actual observations show generally good agreement. 相似文献
7.
Grain sorting in gravel-bed streams and the choice of particle sizes for flow-competence evaluations 总被引：1，自引：0，他引：1
Flow-competence assessments of floods have been based on the largest particle sizes transported, and yield either the mean flow stress, mean velocity, or discharge per unit flow width. The use of extreme particle sizes has potential problems in that they may have been transported by debris flows rather than by the flood, it may be difficult to locate the largest particles within the flood deposits, and there are questions concerning how representative one or a few large particles might be of the transported sediments and therefore of the flood hydraulics. Such problems would be eliminated for the most part if competence evaluations are based on median grain sizes of transported sediments, or perhaps on some coarse percentile that is established by a reasonable number of grains. In order to examine such issues, the gravel-transport data of Milhous from Oak Creek, Oregon, and of Carling from Great Eggleshope Beck, England, have been analysed in terms of changing grain-size percentiles with varying flow stresses. A comparison between these two data sets is of added interest because the bed material in Oak Creek is segregated into well-developed pavement and subpavement layers, while such a layering of bed materials is largely absent in Great Eggleshope Beck. The analyses show that the trend of increasing sizes of the largest particles in the bedload samples (diameter Dm) with increasing flow stresses is consistent with similar dependencies based on sieve percentiles ranging from the medians (D50) to the 95th percentiles (D95). This indicates that the largest particles are an integral part of the overall distributions of bedload grain sizes, and respond to changing flow hydraulics along with the rest of the size distribution. In Oak Creek, the median grain size shows the largest change with increasing flow stresses, followed by D60, and so on to D95 which shows the smallest change. The variations in Dm continue this trend, and are similar to those for D95. This systematic variation of grain-size percentiles in Oak Creek is consistent with changes in the overall distributions which tend to be symmetrical and Gaussian for low discharges, but become skewed Rosin distributions for high discharges. In contrast, in Great Eggleshope Beck the several percentiles and Dm show the same rate of shift to coarser sizes as flow stresses increase. This results in part from differences in sampling techniques wherein the bedload samples from Great Eggleshope Beck represent a complete flood event, while shorterterm samples at a specific flow stage were obtained in Oak Creek. As a result of the integrated sampling in Great Eggleshope Beck, the bedload grain-size distributions are more complex, commonly with a bimodal pattern. However, after accounting for differences in sampling schemes in the two streams, contrasting patterns in changing grain-size distributions remain, and these are concluded to reflect grain sorting differences as the bedload grain-size distributions approach the distributions of the bed materials. It is surprising that if criteria commonly employed to demonstrate the equal mobility of different grain sizes are used in the comparison, then Great Eggleshope Beck is far closer to this condition in spite of its minimal development of a pavement. It is concluded that the respective shapes of the bed-material grain-size distributions, in particular their degrees of skewness, are more important to the observed sorting patterns than are the effects of a pavement layer regulating grain entrapment to produce an equal mobility of different grain sizes. Therefore, the comparison has established that flow-competence relationships will differ from one stream to another, depending on the pattern of grain sorting which is a function of the bedmaterial grain-size distribution. 相似文献
8.
Sediment production and transport in a proglacial stream: Hilda Glacier, Alberta, Canada 总被引：2，自引：0，他引：2
KATHLEEN M. HAMMER NORMAN D. SMITH 《Boreas: An International Journal of Quaternary Research》1983,12(2):91-106
Hilda Glacier, a small cirque glacier in the Canadian Rocky Mountains, yields two principal types of sediment: ablation till, deficient in fine material and produced by rockfalls and avalanches falling on to the glacier surface, and basal lodgement till, rich in fines and formed mainly by subglacial erosion. Recent recession from its Neoglacial maximum has exposed large areas of basal till with thin veneers of ablation till which, when combined with present subglacial and supraglacial debris, provide abundant material for erosion and transport by the mcltwatcr stream. Sediment transport measurements over two summers (1977–1978) showed that bed load and suspended load occur in approximately equal proportions and that dissolved loads are minor. Local source variations, especially bank slumps, are a major cause of scatter in sediment rating curves. Suspended-sediment concentrations are greater early in the melt season due to availability of loose sediment produced by freezing and thawing. Other contributors to scatter in suspended-sediment rating curves include rain showers and diurnal hysteretic effects. Although the distinction between bed load and suspended load is never sharp, available data suggest that the sand/ gravel grain-size boundary (-1ø) approximates the suspendcd-load/bed-load division for characteristic Hilda flows transporting gravel. This approximation, combined with till grain-size analyses, suspended-sediment measurements, and spatial distributions of till types, leads to the following computations of fluvial sediment sources: for suspended load - 6% supraglacial, 47% subglacial, 47% channel banks; for bed load - 46% supraglacial, 27% each subglacial and channel banks. Supraglacial debris provides only about one-fourth of all fluvial sediment, but nearly half of the bed load. 相似文献
9.
Subaqueous sediment gravity flow deposits: practical criteria for their field description and classification 总被引：4，自引：0，他引：4
GUIDO GHIBAUDO 《Sedimentology》1992,39(3):423-454
10.
JOSEPH J. LAMBIASE 《Sedimentology》1980,27(4):433-446
The Avon River estuary of Nova Scotia was studied with the intention of analysing the relations between grain-size distributions and hydraulics. The Avon is macrotidal; tidal ranges up to 15·6 m generate tidal currents up to 1·7 m s?1. Maximum current speed increases from the mouth (seaward end) to the head (shoreward end) of the estuary. Mean grain size decreases from the estuary mouth to the head. Thus, there is an inverse relationship between mean grain size and current speed. Consequently, textural parameters do not directly reflect hydraulic conditions. Graphical dissection of cumulative frequency curves into their component grain populations reveals a large coarse population at the estuary mouth that is absent at the head. There are several relationships between hydraulics and cumulative curves. Shields’ criterion predicts that all sediment in the system can be transported so that the large coarse population at the estuary mouth is not a lag. Local maximum shear velocity nearly equals the settling velocity of the grain size at the boundary of the coarse (C) and intermediate (A) grain populations. This has been previously interpreted to signifiy a transition from traction to intermittent suspension transport, and implies that the C population is a function of traction and that the A population is related to intermittent suspension (Middleton, 1976). Each grain population is transported at a different rate; suspended grains travel almost an order of magnitude faster than grains moved by traction according to Einstein's transport formula. Sediment transport paths in the estuary were determined from bedform migration directions and the computed net sediment transport per tidal cycle using Engelund and Hansen's formula. The areal distribution of the transport paths, combined with the differential transport rates of each grain population, produces hydraulic sorting. Hydraulic sorting causes coarse sediment to be excluded from the estuary head and creates the inverse relationship between current speed and mean grain size. 相似文献
11.
Grain-size characteristics of fine-grained unflocculated sediments II:'multi-round' distributions 总被引：2，自引：0，他引：2
The grain-size spectrum of sediment deposited by settling of a suspension with a ‘one-round’ grain size distribution is described by a power law in which the integer exponent characterizing the fine-grained limb is increased by one over that of the suspension. If this better-sorted sediment, in turn, is resuspended and settles, further sorting and steepening of the limb occurs. Each resuspension event or‘round’changes the distribution by a predictable amount. Equations describing this sorting process, based on the derivation of the one-round equation, are fitted to grain-size analyses of well-sorted sediment from a variety of locations to verify the model. A suite of sandy bottom sediment samples from the Bay of Fundy shows that the steepness of the fine-grained limbs of the sand fraction indeed increases in integral steps. 相似文献
12.
波罗的海西部达斯浅滩海底沉积物各粒级的空间分布由物源区向外有规律变化,反映出沉积水动力条件的控制作用,反过来也为根据沉积物各粒级的空间分布反演近海底优势水流方向和速度提供了依据。利用沉积水动力经验公式及分选系数梯度、Z统计量等设计了反演优势流向和流速的方法,并应用于该海区,估计了该区海底以上1m的近海底优势水流方向及优势流速。与有限的实测及模拟资料进行对比,估计的海流模式吻合较好,但流速的数值可能总体上偏高,需要用更多实测资料来校正和标定。 相似文献
13.
Supply limited sediment transport in a high-discharge event of the tropical Burdekin River, North Queensland, Australia 总被引：1，自引：0，他引：1
Kathryn J. Amos Jan Alexander Anthony Horn† Geoff D. Pocock‡ Chris R. Fielding§ 《Sedimentology》2004,51(1):145-162
Interactions between catchment variables and sediment transport processes in rivers are complex, and sediment transport behaviour during high‐flow events is not well documented. This paper presents an investigation into sediment transport processes in a short‐duration, high‐discharge event in the Burdekin River, a large sand‐ and gravel‐bed river in the monsoon‐ and cyclone‐influenced, semi‐arid tropics of north Queensland. The Burdekin's discharge is highly variable and strongly seasonal, with a recorded maximum of 40 400 m3 s?1. Sediment was sampled systematically across an 800 m wide, 12 m deep and straight reach using Helley‐Smith bedload and US P‐61 suspended sediment samplers over 16 days of a 29‐day discharge event in February and March 2000 (peak 11 155 m3 s?1). About 3·7 × 106 tonnes of suspended sediment and 3 × 105 tonnes of bedload are estimated to have been transported past the sample site during the flow event. The sediment load was predominantly supply limited. Wash load included clay, silt and very fine sand. The concentration of suspended bed material (including very coarse sand) varied with bedload transport rate, discharge and height above the bed. Bedload transport rate and changes in channel shape were greatest several days after peak discharge. Comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport, and that antecedent weather is an important control on suspended sediment concentration. Neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload (e.g. Ackers & White, 1973) nor observations of sediment transport characteristics in ephemeral streams (e.g. Reid & Frostick, 1987) are directly applicable to this river. 相似文献
14.
To evaluate the magnitude of variation in grain size distribution in the Krishna river, bed sediments and suspended sediments collected along the length of the river have been studied. There are both temporal and seasonal variation in the grain size distribution of suspended sediments. The statistical parameters show the change along the river in a non-linear fashion which may be due to human interference and due to different types of sediments contributed by tributaries to the Krishna river. The suspended sediments are mostly fine silt (4 to 16m), poorly sorted, showing coarse to fine skewed and are platyto leptokurtic. The bed sediments are mostly medium sand (350m) showing moderate to well sorted, coarse to fine skewed and are platy- to leptokurtic. The CM diagram of Krishna river bed sediments suggests that deposition takes place by (1) rolling (2) rolling and suspension and (3) graded suspension. The suspended sediments represent deposits of uniform suspension. 相似文献
15.
M. R. LEEDER 《Sedimentology》1983,30(4):485-491
ABSTRACT Bagnold's dynamic theory for sediment suspension requires that the immersed weight of suspended grains over unit bed area is supported by an upward-directed residual Reynolds stress, τyy , arising from asymmetrical shear turbulence. The present paper presents an analysis of previously published turbulence data which confirms the existence of this residual stress and indicates its generation in the lowermost part of the buffer layer of turbulent shear flows. The magnitude of τyy is estimated as about 0.3τyx . Calculations from experimental data on suspended fine sand transport over upper phase beds reveals that τyy , is in approximate equilibrium with the weight stress due to the suspended load. 相似文献
16.
TORE O. VORREN 《Boreas: An International Journal of Quaternary Research》1977,6(2):219-227
Vorren, T. O. 1977 06 01: Grain-size distribution and grain-size parameters of differcnt till type on Hardangervidda, south Norway.
Grain-size characteristics for different till types on Hardangervidda havc been studied: Basal tills are finer grained than ablation tills: phyllitic basal tills are relatively rich in gravel and clay: basal tills derived from a semiporphyritic granite are poor in clay and have a deficiency of very coarse sand. Of the Folk a Ward's (1957) grain-size distribution parameters, sorting and kurtosis seem to he the most diagnostic parameters for distinguishing between different types of till. A linear corrclation betwen sorting and the other grain-size parameters is indicated for the basal tills. 相似文献
Grain-size characteristics for different till types on Hardangervidda havc been studied: Basal tills are finer grained than ablation tills: phyllitic basal tills are relatively rich in gravel and clay: basal tills derived from a semiporphyritic granite are poor in clay and have a deficiency of very coarse sand. Of the Folk a Ward's (1957) grain-size distribution parameters, sorting and kurtosis seem to he the most diagnostic parameters for distinguishing between different types of till. A linear corrclation betwen sorting and the other grain-size parameters is indicated for the basal tills. 相似文献
17.
晚更新世以来台湾浅滩西部地层结构与古环境演化 总被引：1，自引：0，他引：1
根据对台湾浅滩西部地质浅钻的沉积相、粒度和AMS 14C年代分析及高分辨率浅地层剖面解释,本文研究了浅滩大型沙波区的地层结构与沉积环境演化.台湾浅滩西部保存了三期海相层和两期陆相层.下部的海相层(DU6)可能形成于氧同位素5.1期,以潮间带—潮下带浅海沉积为主;中间的海相层(DU4)可能形成于氧同位素3期,以滨岸—潮间带沉积为主;上部的海相层(DU2和DU1)为形成于末次冰消期以来的海侵砂和潮流砂.两期陆相层(DU5和DU3)为沟谷充填沉积,可能形成于氧同位素4和2期.冰后期海侵过程形成的中粗砂层构成了沙波的基底部分(DU2).沙波的主体部分(DU1)形成于全新世高海平面以来,主要由中粗砂和含砾中粗砂组成.沙波的粒度频率分布曲线主要众数组分-1~3 Φ,形态尖陡;概率累积曲线多为两段式,跃移组分含量高于80%,悬移组分不足20%,滚动组分缺失或很少.潮流砂(DU1)的平均粒径与分选系数呈良好的正相关,反映潮流的淘洗作用越强,沉积物粒径越粗、分选越好. 相似文献
18.
The Folk&Ward (F&W) and the log-hyperbolic methods are applied to a small - and easy to overlook - number of typical sand sized grain-size distributions from the Danish Wadden Sea. The sand originates from the same source, and the pattern of change in the grain-size distributions is, therefore, exclusively linked to dynamic sorting. In general, the F&W parameters reflect the observed grain-size trends far better than the corresponding log-hyperbolic parameters. The log-hyperbolic “typical log grain size”, ν, is sensitive to changes in skewness and cannot replace mean grain size, Mz, in grain-size trend analysis. The four log-hyperbolic parameters describing dispersion δ, τ− 1, κ− 1 and ζ are not able to give an unambiguous picture of spreading trends. The F&W sorting parameter, Sd, can therefore not be replaced by any of the log-hyperbolic parameters in grain-size trend analysis. In skewed grain-size distributions, there seems to be a general defect in the log-hyperbolic approximation of the best represented tail. This makes χ less sensitive to indicate the correct sign of slightly skewed grain-size distributions than Sk. However, when examining trends, the relative change between χ and Sk by and large seems to be the same. The log-hyperbolic peakedness parameter, ξ, is preferable to the F&W peakedness (kurtosis) parameter, Kg, because the latter is hypersensitive to small, insignificant variations in a grain-size distribution. By contrast, the concept of peakedness as defined by ξ, seems to relate more directly to sedimentary environmental conditions. In conclusion, the log-hyperbolic method has both advantages and disadvantages when compared with the F&W method. It is thus suggested that a sensible combination of the two methods could be advantageous to sediment trend analysis. 相似文献
19.
Grain-size distributions of fluvial, eolian and marine sediments were explicated decades ago. For lake sediments, however, there is still great uncertainty in explaining the genesis of grain-size components due to the inherent complexity of their polymodal distributions. In this study, the grain-size components of the surface sediments of Daihai Lake, Inner Mongolia, were partitioned using a lognormal distribution function and the relationship between the identity of each component and the specific sedimentary environment was investigated. The data indicate that the modern clastic sediments of Daihai Lake contain five distinct unimodal grain-size distributions representing five grain-size components. Each of the components retains its identity including modal size, manner of transportation and environment of deposition although the relative percentage varies with the hydraulic condition throughout the lake. These components are specified from fine to coarse modes as long-term suspension clay, offshore-suspension fine silt and medium-to-coarse silt, and nearshore-suspension fine sand and saltation medium sand. The percentage of the components interpreted as an indication of nearshore environments displays a negative correlation with water depth across the modern lakebed, suggesting a model for linking the nearshore components in sediment cores to the lake level status in the geological past. The model was applied to a sediment core from the lake where high percentages of the nearshore components in the core sediments were correlated with low regional precipitations reconstructed on the pollen profile of the same core. The coincidences between two independent proxies do not only demonstrate the validity of lognormal distribution function in partitioning polymodal sediments but also reveals the potential of the grain-size component–lake level status model for lake’s paleohydrological reconstruction. 相似文献
20.
DAN H. SHUGAR RAY KOSTASCHUK JAMES L. BEST† DANIEL R. PARSONS‡ STUART N. LANE§ OSCAR ORFEO¶ RICHARD J. HARDY§ 《Sedimentology》2010,57(1):252-272
The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate. 相似文献