Size-sorting during suspension transportation—lognormality and other characteristics

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.     

Textural characteristics of the Krishna River sediments,India

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.     

The link between grain‐size components and depositional processes in a modern clastic lake

It is widely recognized that lake sediment grain‐size distributions tend to be polymodal and consist of two or more grain‐size components. However, for specific cases, the genesis of each component usually is poorly understood. In this study, the grain‐size components of the surface sediments of Hulun Lake, Inner Mongolia, were partitioned using the log‐normal distribution function method and the relationship between the identity of each grain‐size component and the hydraulic condition of the lake was investigated in order to relate the constituent components to specific depositional processes in the lake. The data indicate that the modern clastic sediments of Hulun Lake contain six distinct unimodal grain‐size distributions representing six 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 conditions 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, saltation medium sand and traction coarse sand. The percentage contribution of several grain‐size components interpreted as being indicative of nearshore environments is shown to be correlated negatively with water depth across the modern lake bed; this suggests that the proportion of these components in core data might be useful as a proxy for water depth. This possibility was tested using a sediment core from Hulun Lake where high percentages of the nearshore grain‐size components were found to be correlated with low regional precipitation reconstructed from the pollen profile of the same core. The coincidence of two independent proxies does not only demonstrate the validity of log‐normal distribution function in partitioning polymodal sediments but reveals the potential of lake sediment grain‐size components for the research of lake‐level fluctuations during the geological past.     

Interpretation of grain-size distributions from measured sediment data, Platte River, Nebraska

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.     

辽西义县组尖山沟层沉积环境粒度分析

尖山沟层沉积剖面粒度分析结果表明该层为湖相由浅变深、湖体扩张的两个相似的正粒序沉积旋回组成,与内陆河流密切相关、离岸不远的半深湖深湖沉积环境。碎屑颗粒以递变悬浮方式搬运,沉积物按粒度大小和密度梯度成层分布,在流动中由下至上粒度逐渐变细。由于涡流发育及作用,使沉积物中基本没有滚动颗粒。受火山作用的影响,尖山沟层粒度分析参数与特征表现出与正常沉积相不完全一致性,呈现出独特的粒度分布规律。     

长江三角洲ZK01孔末次盛冰期以来沉积物粒度特征及环境演化

根据长江三角洲ZK01孔的粒度资料,从粒度参数、斜体图、概率累积曲线和频率分布曲线等对末次盛冰期以来的沉积物粒度特征进行了详细分析,并结合沉积物的岩性、沉积构造和有孔虫的分布特点,探讨了该区末次盛冰期以来的沉积环境演化。分析表明,ZK01孔末次盛冰期以来,沉积物自下而上可分为5段:河床沉积物以砂和砾质砂为主,粒度概率累积曲线为以跳跃总体为主的两段式;河漫滩相为砂质和黏土互层,概率累积曲线呈以悬浮总体为主的具过渡的两段式,频率分布曲线为双峰或多峰式;河口湾沉积物的粒度参数变化显著,在C-M图上表现为粗细分异明显的两类,对应两种两段式累积曲线,频率分布曲线亦有单峰和双峰式两类;浅海相主要为淤泥质黏土,粒度参数较稳定,概率累积曲线为一段式,几乎均为悬浮总体;三角洲沉积物以细砂和粉砂为主,概率累积曲线为以跳跃总体为主的具过渡的两段式。研究区该时期依次经历了河床、河漫滩、河口湾、浅海和三角洲等5种沉积环境。     

贵州中坪地区上三叠统二桥组砂岩粒度分析及其沉积环境意义

通过对贵州中坪地区上三叠统二桥组16件砂岩样品进行粒度分析,结合二桥组砂岩地层所在的区域地质背景,研究砂岩形成时的沉积环境及水动力情况.对砂岩的粒度概率累计曲线、C-M图、粒度特征参数、结构参数散点图解等的分析显示,二桥组砂岩粒度概率累积曲线图具有较好的正态分布特征,频率直方图多呈单峰式;正态概率累计曲线主要为多跳一悬式、一跳一悬式、一跳一悬夹过渡式,主要由悬浮组分和跳跃组分组成,缺少滚动组分.中坪地区上三叠统二桥组从下部到上部处于一个由三角洲平原及前缘相向浅湖相过渡的沉积环境.     

Regulation of sand transport in the Colorado River by changes in the surface grain size of eddy sandbars over multi-year timescales

In settings where the transport of sand is partially or fully supply limited, changes in the upstream supply of sand are coupled to changes in the grain size of sand on the bed. In this manner, the transport of sand under the supply-limited case is ‘grain-size regulated’. Since the closure of Glen Canyon Dam in 1963, the downstream reach of the Colorado River in Marble and Grand Canyons has exhibited evidence of sand-supply limitation. Sand transport in the river is now approximately equally regulated by changes in the discharge of water and changes in the grain sizes of sand on the channel bed and eddy sandbars. Previous work has shown that changes in the grain size of sand on the bed of the channel (driven by changes in the upstream supply of sand owing to both tributary floods and high dam releases) are important in regulating sand transport over timescales of days to months. In this study, suspended-sand data are analysed in conjunction with bed grain-size data to determine whether changes in the grain size of sand on the bed of the channel or changes in the grain size of sand on the surface of eddy sandbars have been more important in regulating sand transport in the post-dam Colorado River over longer, multi-year timescales. The results of this study show that this combined theory- and field-based approach can be used to deduce which environments in a complicated setting are the most important environments for regulating sediment transport. In the case of the regulated Colorado River in Marble and Upper Grand Canyons, suspended-sand transport has been regulated mostly by changes in the surface grain size of eddy sandbars.     

Sedimentation by river-induced turbidity currents: field measurements and interpretation

Turbidity currents, initiated from spring runoffs of an influent river, were observed in the upper region of a reservoir in Hokkaido, Japan, by measuring water temperature, velocity and suspended-sediment concentration. Their profiles offer some physical parameters for the sedimentary conditions, assuming the turbidity currents to be quasi-uniform. The bottom sediment deposited by the turbidity currents was then collected by a portable core sampler. The bottom sediment consists of more than 90% silt and clay, and thus offers a hydraulically smooth bed for shear flow; a plane bed as a bed configuration was formed on the reservoir bed, probably because of the low shear velocity and small grain size of sediment. Using a graphic method with log-normal probability paper, the bottom sediment is divided into several overlapping log-normal subpopulations. Grain-size analysis indicates that the bottom sediment may be regarded as cohesionless; criteria for ‘complete deposition’ of transported grains can then be incorporated into the ‘extended Shields diagram’ giving the minimum shear stress to erode bottom sediment. Applying the new diagram to the grain size distribution of the bottom sediment, it is suggested that each of the log-normal subpopulations was deposited in each of four different ‘modes of deposition’, i.e. ‘traction’, ‘saltation (or intermittent suspension)’, ‘suspension’ and ‘suspension under equilibrium’. The last mode may be observed under a sedimentary condition where upward flux of suspended sediment by eddy diffusion is almost equal to its depositional flux due to gravity. The mean and critical grain sizes for bottom sediment and each of the corresponding subpopulations decrease consistently with an increase of Ψ=F_d² log₁₀Re (F_d is the densimetric Froude number and Re is the flow Reynolds number). Ψ correlates inversely with shear velocity, which bears a linear relationship to mean velocity. These results lead to the conclusion that relatively fine suspended sediment is deposited as a result of decreasing bottom friction with a relative decrease of turbulent energy.     

Methods of computation of suspended load from bed materials and flow parameters

Computation of the grain-size distribution of the suspended load above a sand bed must take into consideration: (1) sorting processes from the bed to the bed layer and (2) sorting between the bed layer and suspension. Grain-size distributions of the bed layers above sand beds of three different types have been computed in this work, both by the Einstein and the Gessler methods. Using these as references, suspended load distributions have been obtained in each case by the Rouse suspension equation. A new formula has also been developed in partial modification of Hunt's method for direct computation of bed load and suspended load from a bed's grain-size distribution and flow parameters. Comparison of the computed data with actual observations in laboratory flumes show that no one method is particularly superior to the others, but the present method is advantageous because it affords direct computation of the suspended load from a bed's grain-size distribution, without going through an intermediate stage (bed load). The possible sources of error in each of the methods have been discussed.     

Magnetic tracing of sediment dynamics of mudbanks off southwest coast of India

In this study, we utilized environmental magnetic in combination with sedimentological and hydrodynamic data to investigate the formative processes of mudbanks along southwest coast of India. We document the linkages between enrichment of silt-sized magnetic particles and formation processes of mudbanks along Alappuzha coast. A trend of increasing magnetite concentration and coarsening in magnetic grain size is observed at mudbank stations M2 and M3, while the mud-deficient station (M1) showed an opposite trend. A strong relationship between magnetic and physical grain size for all samples implies that the magnetic particle size and clastic grain size are largely adjunct. Analysis of rock magnetic and grain size data of surficial and suspended sediments from non-mudbank (M1) and two mudbank stations (M2, M3) reflect the differential sediment partitioning and transport regimes which controlled the formation of mudbanks along Alappuzha coast. Two plausible mechanisms responsible for the formation of mudbanks are identified: grain size-selective entrainment is the dominant process during pre-monsoon; weaker hydrodynamics (waves and bottom currents) favors accumulation of silt-sized (fine and coarse) magnetic and non-magnetic fractions resulting in the formation of magnetically low-enriched sediment bed of mixed grain sizes. At the onset of monsoon, wave-induced energetic bottom currents enhance the suspension of entire sediment bedload at stations M2, M3 to form fluid mud. Concurrently, mineral-density-based selective fractionation allows the settling of coarse silt-sized magnetic particles, while the fine magnetic silt-size particles accumulate forming thick fluid mud as a suspension load resulting in the formation of mudbanks. An observed increase in magnetic susceptibility and coarsening in magnetic grain size of surficial sediments at mudbank stations (M2, M3) during monsoon period supports the interpretation. Our findings are summarized in a conceptual model which can be very well applied to investigate sediment dynamics associated with mudbank formation in coastal and shelf sedimentary systems.     

滑坡滑带(面)土及堆积物的粒度多组分分布特征研究

文章利用高分辨率激光粒度仪MS2000对我国典型地区5种沉积类型滑坡的滑带(面)土和滑坡细粒堆积物的粒度多组分分布特征进行了系统、深入研究,总结了不同沉积类型滑坡滑带(面)土与滑坡堆积物的粒度多组分分布特征及差异性,其对于认识滑坡的成因机制具有重要的理论和实践意义。研究认为:(1)不同沉积类型的滑坡滑带(面)土和滑坡堆积物的粒度一般含有4个组分,其分别为风成的细、中、粗粒组分和水成的悬浮组分; (2)不同地区不同沉积类型的滑坡其滑带(面)土粒度与滑坡堆积物的粒度多组分具有不同的分布特征; (3)同一地区滑坡的滑带(面)土与滑坡堆积物的粒度多组分分布特征有较大差异,主要受控于滑坡本身的形成演化过程。     

江汉平原洪水沉积物的粒度特征及环境意义——以2005年汉江大洪水为例

2005年10月初汉江发生了自1983年以来最大的一次洪水。通过对江汉下游仙桃河段不同地貌单元洪水沉积物的系统取样和分析,发现2005年汉江洪水沉积的粒度特征具有以下特点：  1)不同河道地貌单元的沉积物粒级组分有所不同,高河漫滩的粒径相对偏细,主要以粉砂级组分为主（49.56.％）,粘土含量相对较高（约为4.871％）。低河漫滩和平水位边滩沉积物的粒径相对偏粗,主要以砂级组分为主（低河漫滩为82.96.％,边滩为69.72.％）,低河漫滩沉积物中粘土含量较少,只有1.058.％; 2)平水位边滩沉积物的粒径的统计平均值比高河漫滩沉积物值偏大,而标准差值的统计平均值却比高河漫滩和低河漫滩沉积物的偏小;   3) 3种地貌单元中,高河漫滩的偏度平均值最大,低河漫滩的偏度平均值最小,但低河漫滩偏度平均值的分布范围却较大;   4)低河漫滩沉积的粒度参数值曲线的空间波动起伏最为明显,而高河漫滩粒度参数值的空间变化不大;   5) 3种地貌单元的峰态系数的统计平均值均大于1.15（为窄态）,频率曲线表现为单峰,概率累积曲线都较平缓且均为2段式,反映了一般河流高水位沉积的基本特征。分析认为,粒度特征与河道不同地貌单元的微地貌形态和沉积结构是一致的,均是由洪水时不同地貌单元的水动力特征决定的。     

辽南大莲泡沉积物的沉积特征及古环境变化

辽南大莲泡是形成于冰后期的一个现已干涸的小型山间湖泊。对大莲泡沉积物粒度作一般分析，并用对应因子分析法作了计算分析，结果表明：大莲泡沉积物粒度分布主要受入湖的沟谷泥石流、河流和湖泊沉积作用控制。即下层以泥石流沉积为主；中层以湖泊沉积为主；上层以河流作用为主。据此并结合孢粉分析资料（主要以湿生沼生和旱生、盐生草本孢粉带更替为依据），初步恢复了大莲泡沉积环境的演变过程。全新世高海面在辽南地区是存在的，大莲泡与海相通的特殊位置及地层特征和孢粉特点揭示在全新世气候最适宜期海水在涨潮时曾作过短暂的停留，高海面在渤海沿岸出现位置不超过现代海面之上５ｍ。     

On the frequency distribution of turbidite thickness

The frequency distribution of turbidite thickness records information on flow hydrodynamics, initial sediment volumes and source migration and is an important component of petroleum reservoir models. However, the nature of this thickness distribution is currently uncertain, with log‐normal or negative‐exponential frequency distributions and power‐law cumulative frequency distributions having been proposed by different authors. A detailed analysis of the Miocene Marnoso Arenacea Formation of the Italian Apennines shows that turbidite bed thickness and sand‐interval thickness within each bed have a frequency distribution comprising the sum of a series of log‐normal frequency distributions. These strata were deposited predominantly in a basin‐plain setting, and bed amalgamation is relatively rare. Beds or sand intervals truncated by erosion were excluded from this analysis. Each log‐normal frequency distribution characterizes bed or sand‐interval thickness for a given basal grain‐size or basal Bouma division. Measurements from the Silurian Aberystwyth Grits in Wales, the Cretaceous Great Valley Sequence in California and the Permian Karoo Basin in South Africa show that this conclusion holds for sequences of disparate age and variable location. The median thickness of these log‐normal distributions is positively correlated with basal grain‐size. The power‐law exponent relating the basal grain‐size and median thickness is different for turbidites with a basal A or B division and those with only C, D and E divisions. These two types of turbidite have been termed ‘thin bedded’ and ‘thick bedded’ by previous workers. A change in the power‐law exponent is proposed to be related to: (i) a transition from viscous to inertial settling of sediment grains; and (ii) hindered settling at high sediment concentrations. The bimodal thickness distribution of ‘thin‐bedded’ and ‘thick‐bedded’ turbidites noted by previous workers is explained as the result of a change in the power‐law exponent. This analysis supports the view that A and B divisions were deposited from high‐concentration flow components and that distinct grain‐size modes undergo different depositional processes. Summation of log‐normal frequency distributions for thin‐ and thick‐bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power‐law trend. Thus, the occurrence of both log‐normal and segmented power‐law frequency distributions can be explained in a holistic fashion. Power‐law frequency distributions of turbidite thickness have previously been linked to power‐law distributions of earthquake magnitude or volumes of submarine slope failure. The log‐normal distribution for a given grain‐size class observed in this study suggests an alternative view, that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters, in addition to the settling velocity of the grain‐sizes present.     

杭州湾底质的粒度特征和泥沙来源

杭州湾底质的分布,与潮流、波浪作用以及泥沙来源有着密切的关系。作为强潮海湾的杭州湾,潮流作用是主要的动力因素。底质的粒径大小与水动力强度相适应。底质的类型以泥质粉砂和粉砂为主。底质的分布具有一定的规律性,从湾口向湾内,颗粒粒径由细到粗,分选则由差到好。杭州湾的泥沙来源,以海域来沙为主,其中主要是长江输出的泥沙,随涨潮流进入杭州湾。     

The variability of critical shear stress, friction angle, and grain protrusion in water-worked sediments

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.     

海啸波作用下泥沙运动——Ⅱ.床沙组成变化分析

在波浪水槽实验的基础上,对海啸波作用下的床沙组成变化规律开展研究。实验采用1/10~1/20的组合坡度,选取N波作为入射波。实验对波高进行采集,对波浪的上爬、回落和水跃过程进行拍摄记录,对每个波作用后的地形进行测量,并对初始和最终的床面顶层泥沙进行采样筛分。实验结果表明,N波作用下泥沙在离岸区水跃发生区域堆积,淤积沙坝泥沙粒径呈粗化趋势。同时采用规则波和非规则波进行对比,波浪作用后形成岸滩也为沙坝剖面,淤积沙坝泥沙粒径呈细化趋势,结果均遵循Çelikoğlu提出的泥沙运动的基本规律,细颗粒泥沙会在强烈的紊动作用下从床面中被筛选出来,并被搬运到低紊动地区,此过程造成了剧烈紊动区泥沙的粗化。     

Aerodynamic grain‐size distribution of blown sand

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.     

The grain/bed collision in sand transport by wind

In the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain/bed collision has been identified as having a significant role. The grain/bed collision has been studied by both physical experiments and numerical simulation. A principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system. Thus the study of the exact physical mechanism of the collision has to some extent been neglected. The transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension. Thus the precise physics of the grain/bed collision become highly significant with regard to the physics of dust release from a surface. This paper re-examines previous collision data and compares them with a simple collision model. This model proposed that the impinging grain strikes a single surface grain and rebounds: an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding. The collision data are shown to support the model: first, because the duration of the collisions observed on the high speed film supports a short contact time with the surface and, second, because the data fit well with the model.