共查询到20条相似文献，搜索用时 15 毫秒
1.
YanYan Yang LianYou Liu XiaoYan Li PeiJun Shi GuoMing Zhang YiYing Xiong YanLi Lyu LanLan Guo Bo Liang MengDi Zhao JiaDong Dai XiYang Zuo XuJiao Han 《Sedimentology》2019,66(2):590-603
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
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Deposits of depletive high-density turbidity currents: a flume analogue of bed geometry, structure and texture 总被引：1，自引：0，他引：1
Flume experiments were performed to study the flow properties and depositional characteristics of high‐density turbidity currents that were depletive and quasi‐steady to waning for periods of several tens of seconds. Such currents may serve as an analogue for rapidly expanding flows at the mouth of submarine channels. The turbidity currents carried up to 35 vol.% of fine‐grained natural sand, very fine sand‐sized glass beads or coarse silt‐sized glass beads. Data analysis focused on: (1) depositional processes related to flow expansion; (2) geometry of sediment bodies generated by the depletive flows; (3) vertical and horizontal sequences of sedimentary structures within the sediment bodies; and (4) spatial trends in grain‐size distribution within the deposits. The experimental turbidity currents formed distinct fan‐shaped sediment bodies within a wide basin. Most fans consisted of a proximal channel‐levee system connected in the downstream direction to a lobe. This basic geometry was independent of flow density, flow velocity, flow volume and sediment type, in spite of the fact that the turbidity currents of relatively high density were different from those of relatively low density in that they exhibited two‐layer flow, with a low‐density turbulent layer moving on top of a dense layer with visibly suppressed large‐scale turbulence. Yet, the geometry of individual morphological elements appeared to relate closely to initial flow conditions and grain size of suspended sediment. Notably, the fans changed from circular to elongate, and lobe and levee thickness increased with increasing grain size and flow velocity. Erosion was confined to the proximal part of the leveed channel. Erosive capacity increased with increasing flow velocity, but appeared to be constant for turbidity currents of different grain size and similar density. Structureless sediment filled the channel during the waning stages of the turbidity currents laden with fine sand. The adjacent levee sands were laminated. The massive character of the channel fills is attributed to rapid settling of suspension load and associated suppression of tractional transport. Sediment bypassing prevailed in fan channels composed of very fine sand and coarse silt, because channel floors remained fully exposed until the end of the experiments. Lobe deposits, formed by the fine sand‐laden, high‐density turbidity currents, contained massive sand in the central part grading to plane parallel‐laminated sand towards the fringes. The depletive flows produced a radial decrease in mean grain size in the lobe deposits of all fans. Vertical trends in grain size comprised inverse‐to‐normal grading in the levees and in the thickest part of the lobes, and normal grading in the channel and fringes of the fine sandy fans. The inverse grading is attributed to a process involving headward‐directed transport of relatively fine‐grained and low‐concentrated fluid at the level of the velocity maximum of the turbidity current. The normal grading is inferred to denote the waning stage of turbidity‐current transport. 相似文献
3.
The sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects. The behaviour of various particulate systems involving sand has been compared with the widely used Richardson–Zaki expression. The general form of the expression is found to hold, in as much as remaining as a suitable means to describe the hindered settling of irregular particles. The sedimentation exponent n in the Richardson–Zaki expression is found to be significantly larger for natural sand grains than for regular particles. The hindered settling effect is therefore greater, leading to lower concentration gradients than expected. The effect becomes more pronounced with increasing particle irregularity. At concentrations around 0·4, the hindered settling velocity of fine and medium natural sands reduces to about 70% of the value predicted using existing empirical expressions for n. Using appropriate expressions for the fluidization velocity and the clear water settling velocity, a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes. 相似文献
4.
Laser vs. settling velocity differences in silt grainsize measurements: estimation of palaeocurrent vigour 总被引：1，自引：1，他引：1
The use of grain‐size distribution of muds for the reconstruction of past deep ocean currents is becoming established and applied in the palaeoceanographic community. The methods are also applicable to shallow marine and tidal flat muds with similar inferences concerning the energy of wave and current sorting being drawn. Fine sediment grain‐size distributions can be obtained using a variety of instruments based on fundamentally different theoretical principles. These machines may give varying, sometimes misleading, results giving divergent interpretations of flow speed history. The new evidence presented here, combined with earlier work, suggests that of the three most commonly used analytical methods for fine silts with clays, settling velocity (Sedigraph) should be the method and instrument of choice, with electrical resistance pulse counters (e.g. Coulter Counter) as a suitable alternative. The data also show that laser particle sizers should be avoided for palaeocurrent reconstructions because the measured size of platey minerals can be dominated by their large projected area. This causes them to be recorded as the same size as larger equant grains although they have much smaller settling velocity and were deposited in aggregates. This produces results with a weaker relationship to the dynamics of deposition. The central problem is thus that some coarse clay/fine silt is recorded as medium to coarse silt, the key size in the ‘sortable silt’ mean size method of inferring changes in flow speed. For coarse silts (e.g. loess) the laser gives more satisfactory results. 相似文献
5.
恒定渗流作用下泥石流起动过程冲刷试验分析 总被引：2，自引：0，他引：2
渗流是泥石流水动力条件主要来源之一,不同渗流流量具有不同的渗流力和冲刷力,从而可引起不同规模泥石流。通过开展室内水槽试验,利用测压管量测渗流过程中的孔隙水压力,并结合高清摄像技术从微观角度记录堆积土体内部细颗粒的运移、骨架颗粒的坍塌现象,以此分析研究土体渗透破坏、起动形成泥石流过程中的渗流和冲刷作用。在此基础上设定水槽坡度为7°,调节恒定渗流流量分别为120、170、265、320 ml/s,分析不同恒定渗流流量对固体堆积物失稳、泥石流起动过程中流态变化的影响。分析结果表明,在恒定渗流流量作用下,堆积土体内部细颗粒迁移、骨架颗粒坍塌造成土体颗粒重排列、孔隙水压力上升进而导致土体抗力降低是泥石流土体颗粒失稳、起动、冲刷的重要原因;随着渗流流量增加,流速迅速上升,土体内孔隙水压力逐步增大,骨架颗粒的失稳、移动主要受渗流及水流冲刷两方面共同作用,堆积土体颗粒的移动分别表现出缓慢小幅滑动后稳定、过渡型滑动和快速流滑现象。 相似文献
6.
Landslides and their hydraulic effects are studied by numerical and experimental means. A two-dimensional model based on Navier–Stokes equations has been developed considering the sediments and water as a mixture. A Bingham law written in effective stress and an erosion–diffusion law at the water–sediment interface have been introduced into the model. Laboratory experiments consisting in the sliding of a block, a coarse gravel and a fine sand have been carried out. The results are compared with the numerical model. These experiments show the importance of sediment rheology and interstitial pressure in slide velocity and water wave generation. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
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Changes of Precipitation Infiltration Recharge in the Circumstances of Coal Mining Subsidence in the Shen‐Dong Coal Field,China 总被引：3，自引：0，他引：3
Coal mining subsidence is a universal environmental-geological problem in mining areas. By selecting the Shen-Dong coal mining subsidence area as the research field, this paper studies the changes in precipitation infiltration recharge in the circumstances of coal mining subsidence by means of field geological investigation and laboratory simulation experiments, which is expected to provide a scientific basis for eco-environmental restoration in the mining area. The results indicate that at the unstable stage of subsidence, three types of subsidence in the Shen-Dong mining area have positive effects on the precipitation infiltration recharge, and the type of full-thickness bedrock subsidence has the greatest influence. In the stable stage of subsidence, the precipitation infiltration process after long-term drought and the moisture migration in the aeration zone undergo three different stages: evaporation-infiltration before precipitation, infiltration-upward infiltration-infiltration during precipitation and evaporation-infiltration after precipitation. During a heavy rainfall infiltration process, the wetting front movement in fine sand, coarse sand and dualistic structure of fine-coarse sand consists of two stages: the stage of wetting front movement during precipitation, in which the wetting front movement distance has linear relationship with the depth, and the stage of wetting front movement after precipitation, in which the wetting front movement distance has the power function relationship with the depth. The wetting front movement velocity is influenced by the rainfall amount and the lithology in the aeration zone. However, as the depth increases, the movement velocity will decay exponentially. 相似文献
10.
Entrainment of planktonic foraminifera: effect of bulk density 总被引：1，自引：0，他引：1
REINHARD OEHMIG 《Sedimentology》1993,40(5):869-877
Depositional hydrodynamics have been studied using settling rate distributions of Norwegian deep sea sediments (between Jan Mayen Island and the Vøring Plateau), together with Shields’ critical shear stress velocities. Planktonic foraminifera are the dominant sand sized component of these sediments. The bulk density of the foraminifera was calculated from their settling velocity, sieve size and shape. Density decreases from 2·39 g cm?3 at 0·05 mm diameter to 1·37 g cm?3 at 0·35 mm diameter. These density and size data were used to construct a threshold sediment movement curve. From the similarity in their Shield's critical shear-stress velocities and the observed correlation of foraminifera size with decreasing percentage of fine fraction, it is concluded that the two components, the sand size foraminifera and the quartz and carbonate silt, are transport-equivalent. 相似文献
11.
ROBERT W. DALRYMPLE R. JOHN KNIGHT BRIAN A. ZAITLIN GERARD V. MIDDLETON 《Sedimentology》1990,37(4):577-612
The 40-km-long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high-tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel-shaped geometry is developed, and by erosional or non-depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single-channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary. Longitudinal progradation will produce a 20-m-thick, upward-fining succession, the lower 1/2–2/3 of which will consist of cross-bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb-dominated portion of this unit will be finer grained than the flood-dominated part, and will contain trough crossbedding produced by 3-D megaripples; the flood-dominated areas, by contrast, will consist mainly of compound cross-bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb- and flood-dominated areas) will create packages of ebb cross-bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral-accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper-flow-regime parallel lamination. The succession will be capped by a 4-m-thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability. 相似文献
12.
《Sedimentology》2018,65(4):1322-1330
This study examines the role of quartz sand in the production of dust using mixtures of quartz sand from the Sahara and diatomite aggregates from the Bodélé Depression in Chad. An aeolian abrasion chamber is used to reproduce the physical processes of aeolian abrasion and test the hypothesis that the breakdown of saltating diatomite flakes as they collide in saltation, and with the surface, is the most prolific mechanism of dust production (auto‐abrasion). This hypothesis is tested against the competing hypothesis that a hard, higher‐density quartz sand impactor is required to abrade fine‐grained sediments to generate dust. The results show that dust can be produced by a mixture of saltating diatomite and quartz sand particles. However, quartz sand is not required for saltating aggregates to produce dust. Indeed, these results, which used a mixture of very coarse‐grained aggregate (1 to 2 mm diameter) with fine quartz sand, indicate that the addition of quartz sand can decrease dust production. For a very coarse aggregate (1 to 2 mm), a pure diatomite aggregate produced the most dust, although using a coarse‐grained aggregate (0·5 to 1·0 mm) with a mixture of 20% quartz and 80% aggregate was found to produce the most dust overall. The results of this study confirm the auto‐abrasion hypothesis for the breakdown of diatomite particles in the Bodélé Depression, which is the single biggest source of atmospheric mineral dust on Earth. 相似文献
13.
钙质砂是一种海洋生物成因的特殊岩土介质,其颗粒形态与基本性质较一般陆源砂有很大的不同。一方面通过钙质砂及石英砂的显微镜三维成像试验,对比研究了钙质砂的三维颗粒形态特征。发现相比石英砂,钙质砂颗粒棱角更显著,轮廓趋于扁平。另一方面,对两砂进行一维压缩试验,分析了各粒组含量对钙质砂压缩特性的影响规律,并利用三维形态分析从细观上解释了钙质砂的压缩宏观特性。发现相同级配、密实度下,钙质砂的压缩性比石英砂高约60%～160%（以av1-2表征）,且粗粒组（5～1 mm）含量对钙质砂的压缩性影响最为显著;当粗粒组含量<25%,中、细粒组质量比值M一定时,存在最劣粗粒含量使钙质砂压缩性最大,且该值随着M的增大而减小,并提出了一个经验公式。 相似文献
14.
Estimating the settling velocity of bioclastic sediment using common grain‐size analysis techniques 
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下载免费PDF全文 Michael V. W. Cuttler Ryan J. Lowe James L. Falter Daniel Buscombe 《Sedimentology》2017,64(4):987-1004
Most techniques for estimating settling velocities of natural particles have been developed for siliciclastic sediments. Therefore, to understand how these techniques apply to bioclastic environments, measured settling velocities of bioclastic sedimentary deposits sampled from a nearshore fringing reef in Western Australia were compared with settling velocities calculated using results from several common grain‐size analysis techniques (sieve, laser diffraction and image analysis) and established models. The effects of sediment density and shape were also examined using a range of density values and three different models of settling velocity. Sediment density was found to have a significant effect on calculated settling velocity, causing a range in normalized root‐mean‐square error of up to 28%, depending upon settling velocity model and grain‐size method. Accounting for particle shape reduced errors in predicted settling velocity by 3% to 6% and removed any velocity‐dependent bias, which is particularly important for the fastest settling fractions. When shape was accounted for and measured density was used, normalized root‐mean‐square errors were 4%, 10% and 18% for laser diffraction, sieve and image analysis, respectively. The results of this study show that established models of settling velocity that account for particle shape can be used to estimate settling velocity of irregularly shaped, sand‐sized bioclastic sediments from sieve, laser diffraction, or image analysis‐derived measures of grain size with a limited amount of error. Collectively, these findings will allow for grain‐size data measured with different methods to be accurately converted to settling velocity for comparison. This will facilitate greater understanding of the hydraulic properties of bioclastic sediment which can help to increase our general knowledge of sediment dynamics in these environments. 相似文献
15.
Tatsuya Oki Mikio Kobayashi Shuji Owada Daiki Matsuura 《International Journal of Mineral Processing》2007
An equal settling ratio is an important factor in estimating particle separation accuracy. However, this factor is often calculated using the settling velocity in stationary water, there are no examples of calculation of the equal settling ratio in an actual separator. This is difficult because particle movement in a separator is very complicated, and even simple periodic motions, such as the oscillation field used with many separators, are ignored in many cases. The authors have previously reported on the relation between the equal settling ratio and the oscillation frequency by analysis of particle movement in vertically oscillating water, using spherical particles of glass (average size 435 μm) and zirconia (202 μm) which have the same settling velocity in stationary water. In this study, the influence of particle diameter on the change in the settling velocity in oscillating water was experimentally investigated for three pairs of glass and zirconia particles having different sizes under 0.5 mm, which have the same settling velocity in stationary water. The settling velocities of different-sized particles decreased at different rates in oscillating water, indicating that the equal settling ratio is reduced by water oscillation. We conclude that water oscillation improves the accuracy of size separation for glass particles over 300 μm and zirconia particles over 150 μm when glass and zirconia particles are separated from each other with the difference of these settling velocities. 相似文献
16.
Landward fining from multiple sources in a sand sheet deposited by the 1929 Grand Banks tsunami, Newfoundland 总被引：1，自引：0，他引：1
A sandy deposit from the 1929 Grand Banks tsunami in Newfoundland contains sediment from two distinct sources, one from an inferred gravel shoreline close to the deposit, and one from a sandy dune some 200 m seaward of the deposit. The deposit ranges from 0 to 15 cm thick, and is composed of a bimodal mix of fine and coarse sand. We took approximately 100 core samples of this deposit in an attempt to characterize lateral grain size trends within the sand. Although the coarse fraction does fine with distance inland, the fine fraction does not change size over the study area, and the aggregate grain size changes in no systematic way.
We interpret this deposit to represent the mixture of material picked up at the bar with material picked up at the gravel shoreline. The bar material does not fine in part because it is already fairly well sorted, but also because it is far from its source. The shoreline material, on the other hand, is poorly sorted so that the tsunami took only those grains it was capable of moving, and deposited them near their source.
We estimated the size of the tsunami by determining the flow depth-flow velocity combinations required to advect sand from the bar to the back of the deposit, and by estimating the shear velocity required for motion of the largest grain we found during our survey. This modeling indicates an average flow depth of about 2.5–2.8 m over the area, at a flow velocity of 1.9–2.2 m/s. This estimate compares well with eyewitness accounts of a maximum flow depth of 7 m at the shoreline if our estimate represents an average over the whole study area. 相似文献
17.
应用了静态沉降实验方法中的移液管法,初步研究了水中常见阳离子对河流中细颗粒泥沙沉速的影响,这些阳离子包括Na+、Ca2+、Mg2+和Al3+等。实验结果表明,阳离子浓度对泥沙沉速的影响可以分为两个阶段:①随着阳离子浓度的增加泥沙沉速也增大;②当离子浓度大于某值时,离子浓度对泥沙沉速的影响不大。在同样离子浓度条件下,离子价态高者影响较大。同时分析了长江和黄河干流中下游河段的水化学特性;分析结果表明,在一般的河水水质的情况下,有必要考虑水中阳离子(主要是Ca2+、Mg2+)对细颗粒泥沙沉速的影响。 相似文献
18.
Laboratory experiments show that the propagation and sedimentation patterns of particle-laden gravity currents are strongly influenced by the size of suspended particles. The main series of experiments consisted of fixed-volume releases of dilute mixtures containing two sizes of silicon carbide particles (25 μm and 69 μm mean diameter) within a 6-m flume. Polydisperse experiments involved mixtures of five different particle sizes and variation of the amounts of the finest and coarsest particles. All variables apart from the initial relative proportions of particles were identical in the experiments. The effects of mixing different proportions of fine and coarse particles is markedly non-linear. Adding small amounts of fine sediment to a coarse-grained gravity current has a much larger influence on flow velocity, run-out distance and sedimentation patterns than adding a small amount of coarse sediment to a fine-grained gravity current. The experiments show that adding small amounts of fine particles to a coarse-grained current results in enhanced flow velocities because the fine sediment remains suspended and maintains an excess current density for a much longer time. Thus, the distance to which coarse particles are transported increases substantially as the proportion of fines in the flow is increased. Our experiments suggest that sandy turbidity currents containing suspended fines will be much more extensive than turbidity currents composed of clean sand. 相似文献
19.
JAMES P. M. SYVITSKI K. W. ASPREY D. A. CLATTENBURG GORDON D. HODGE† 《Sedimentology》1985,32(1):83-107
The dynamics of suspended particles within a fjord's estuarine circulation are investigated and the results compared with larger non-enclosed prodelta environments. In the upper prodelta, the seaward-flowing river plume flows over the ambient marine water depositing much of the initial riverine suspended load. Sedimentation is dominated by coarse silt and fine-grained sand particles with coarseness determined by the tidal and fluvial stage. Particles less than 10 μn have similar settling velocities regardless of size because they settle in flocs: the settling velocity at a water depth of 5 m is 30 m day-1 and increases with depth so that at 30 m the particles settle at 100 m day-1. For larger particles, the downward settling velocity enhancement due to flocculation decreases with increasing grain size. Hydraulic sorting allows the preferential settling of feldspar and quartz over mica. Particle dynamics in the lower prodelta are dependent on the character of the freshwater wedge that thins seaward of the upper prodelta. The vertical flux of particles is controlled by biogeochemical interactions such as pelletization of fine particles and flocculation (which occurs within rather than below the surface layer in contrast to the upper prodelta). The pellets are produced by indiscriminate filter feeding zooplankton. Across the lower prodelta the suspensate character, recognized in the composition of both flocs and pellets, changes from a dominance of mineral grains to that of autochthonous organic matter. The interaction of bacteria with the suspended particles increases with depth and seaward distance. At depth, the mucoid filaments form stable interconnecting webs. Particle concentration in the surface layer decreases at a rate proportional to the negative one-half and three-halves power of the distance seaward over the upper and lower prodelta, respectively. This relationship is hypothesized as being universal for large marine deltas dominated by buoyancy flow dynamics, regardless of the levels of initial riverine particle concentration or their composition. 相似文献
20.
Tahar Ikni Ahmed Benamar Mohamed Kadri Nasre-Dine Ahfir Hua-Qing Wang 《Comptes Rendus Geoscience》2013,345(9-10):392-400
The transport and filtration behaviour of fine particles (silt) in columns packed with sand was investigated under saturated conditions by using step-input injections. Three samples of different particle size distributions (coarse medium, fine medium and a mixture of both) were used in order to highlight the influence of the pore size distribution on particle retention and size selection of recovered particles. The main parameters of particle transport and deposition were derived from the adjustment of the experimental breakthrough curves by an analytical model. The higher particle retention occurs in the mixture medium, owing to its large pore size distribution, and the filtration coefficient decreases with increasing flow velocity. Particle size distribution of recovered particles shows a thorough size selection: (i) the first recovered particles are the coarser ones; (ii) the size of the recovered particles increases with increasing flow velocity and enlarger pore distribution of the medium. 相似文献