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1.
Wen−Cheng Liu 《Environmental Geology》2005,47(4):535-546
Settling velocities of suspended cohesive sediment in estuaries vary over a range of several orders in magnitude. Variations in the suspended sediment concentration are often considered as the principal cause. Turbulence and the suspended sediment concentration, as well as other factors such as salinity, dissolved organic substances, flocculation ability, and the rate of floc growth affect setting velocities. A laterally–averaged finite difference model for hydrodynamics and cohesive sediment transport is developed and applied in the Tanshui River estuary, Taiwan. The model has been calibrated and verified with water surface elevation, longitudinal velocity, salinity, and cohesive sediment measured. The overall performance of the model is in qualitative agreement with the available data. The model is used to investigate the influence of settling velocity on cohesive sediment transport dynamics. The simulation indicates that the turbidity maximum zone is near Kuan–Du. When settling velocities increase the surface cohesive sediment concentration at Kuan–Du station trends to decrease and bottom cohesive sediment concentration increases. Both surface and bottom cohesive sediment concentrations decrease at Taipei Bridge and Pa–Ling Bridge. This implies that suspended sediment advected seaward and deposited. There is consequently a net seaward flux of suspended sediment near surface, and a net landward flux near the bed. 相似文献
2.
Juergen Schieber Zhiyang Li Zalmai Yawar Xiaomeng Cao Thomas Ashley Ryan Wilson 《Sedimentology》2023,70(1):121-144
Understanding how mud moves and deposits is essential for conceptualizing the dynamic nature of surface environments and their ancient counterparts. Experimental study has largely been pursued by civil engineers, using kaolinite as an active ingredient. Yet, applying their data to the physical comprehension of mudstone sedimentology is hampered by multiple flume configurations between labs, and data sets tailored to specific engineering needs. The need for a better grasp of underlying processes is acute, given recent flume studies that show that moving suspensions form large bedload floccules, migrating floccule ripples and bed accretion under currents capable of moving sand grains. To advance mudstone sedimentology, integrated study of suspended sediment concentration, salinity and bed shear stress on the deposition of floccules is crucial. Described here is a set of tightly controlled experiments that explored suspended sediment concentrations from 70 to 900 mg/l, freshwater, brackish and marine salinities, flow velocities in the 5 to 50 cm/s range (equivalent to 0.01 to 0.58 Pa bed shear), measured the size of in-flow and bedload floccules, and the critical velocity of sedimentation that marks the onset of sustained bedload accumulation. The critical velocity of sedimentation of kaolinite clays is in the 26 to 28 cm/s flow velocity range (0.22 to 0.25 Pa), appears insensitive to a wide range of suspended sediment concentrations and salinities, and coincides with the formation of sand-size bedload floccules. Further decrease of flow velocity/bed shear stress is accompanied by a steady increase in the size of bedload floccules. Large bedload floccules appear to form in the high-shear basal part of the flow, a phenomenon requiring further investigation. Better understanding of the mechanisms that facilitate mud deposition from moving suspensions is critical for more realistic assessments of the depositional conditions of mud and mudstones, as well as for refining predictive models for the flux of fine-grained sediments across the Earth's surface. 相似文献
3.
Field measurements of the vertical structure of near-bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time-dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms. Data were recorded from both a nonbarred, marine shoreface and a barred lacustrine shoreface, under both shoaling and breaking waves (significant heights of 0·25–1·50m; peak periods of 3 and 8 s) and in water depths of 0·5–5·0 m. Sediment concentrations are positively correlated with increasing elevation above the bed, but lagged in time. The time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near-bed oscillatory velocity. Both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation. Sediment concentrations appear to respond more slowly to the incident wind-wave forcing with distance away from the bed as a result of two factors: (1) the sequential increase in concentration induced by a succession of large waves in a group; and (ii) the relative increase in finer sediments with smaller settling velocities. Bedforms interact with the near-bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion, and, therefore, the vertical structure of the suspended sediment concentration at a range of time scales. The near-bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests. Concentration gradients in the presence of vortex ripples are large, as are the correlation between concentrations measured at different elevations within the fluid. 相似文献
4.
Dynamics of the turbidity maximum zone in a micro-tidal estuary: Hawkesbury River, Australia 总被引：3，自引：0，他引：3
Bed sediment, velocity and turbidity data are presented from a large (145 km long), generally well-mixed, micro-tidal estuary in south-eastern Australia. The percentage of mud in the bed sediments reaches a maximum in a relatively narrow zone centred ≈30–40 km from the estuary mouth. Regular tidal resuspension of these bed sediments produces a turbidity maximum (TM) zone in the same location. The maximum recorded depth-averaged turbidity was 90 FTU and the maximum near-bed turbidity was 228 FTU. These values correspond to suspended particulate matter (SPM) concentrations of roughly 86 and 219 mg l?1, respectively. Neither of the two existing theories that describe the development and location of the TM zone in the extensively studied meso- and macro-tidal estuaries of northern Europe (namely, gravitational circulation and tidal asymmetry) provide a complete explanation for the location of the TM zone in the Hawkesbury River. Two important factors distinguish the Hawkesbury from these other estuaries: (1) the fresh water discharge rate and supply of sediment to the estuary head is very low for most of the time, and (2) suspension concentrations derived from tidal stirring of the bed sediments are comparatively low. The first factor means that sediment delivery to the estuary is largely restricted to short-lived, large-magnitude, fluvial flood events. During these events the estuary becomes partially mixed and it is hypothesized that the resulting gravitational circulation focuses mud deposition at the flood-determined salt intrusion limit (some 35 km seaward of the typical salt intrusion limit). The second factor means that easily entrained high concentration suspensions (or fluid muds), typical of meso- and macro-tidal estuaries, are absent. Maintenance of the TM zone during low-flow periods is due to an erosion-lag process, together with a local divergence in tidal velocity residuals, which prevent the TM zone from becoming diffused along the estuary axis. 相似文献
5.
Sediment trapping and transport in the ACE Basin,South Carolina 总被引：1，自引：0，他引：1
A study took place during May 1998 and May 1999 to examine the processes controlling localized accumulation of fine-grained sediments in the lower Ashepoo River. This region, referred to as the Mud Reach, is an area of muddy bottom sediments bounded by fine sands. The Mud Reach is located downstream of the landward extent of the salt intrusion where an estuarine turbidity maximum commonly occurs. Tidal time-series measurements made in the Mud Reach during May 1998, when river discharge was at a 10-yr high, showed high concentrations of suspended sediment (0.05–1 g I−1) during maximum tidal current velocity with concentrations in the bottom 30 cm exceeding 70 g I−1 (fluid mud). A correlation between salinity stratification and increased suspended sediment concentration suggests that inhibited vertical mixing enhances the settling of flocculated sediments to the bed. Measurements made during May 1999 show a two-order-of-magnitude decrease in the concentration of near-bed sediments. A decrease in river discharge during the 1999 observation period of more than 100 m3 s−1 suggests that changes in the hydrography and in the supply of sediments to the system both may be important factors in the trapping of fine-grained sediments in the region. The source of sediments is likely from muddy deposits in the Fenwick Cut, a man-made section of the Atlantic Intracoastal Waterway about 2 km north of the Mud Reach that connects the Ashepoo and Edisto Rivers. The Fenwick Cut appears to be an effective area for trapping sediments where shoaling has increased by 130% in the last decade. Current measurements show that flow velocities decrease through the Cut, likely allowing for the settling of suspended particles that form the thick deposits of unconsolidated mud observed during both years. 相似文献
6.
海洋环境沉积物输运研究进展 总被引：6，自引：0，他引：6
海洋环境中沉积物的输运涉及复杂的过程和机制。20世纪后半叶发展起来的悬沙输运数学模型已经成为海洋沉积动力学的一个有力的研究工具。悬沙输运数学模型的有效运行需要正确的数值解法和模型中所含参数的确定,包括悬沙沉降速度、扩散系数、底床糙度和切应力,以及底边界上的沉降-再悬浮通量。由于复杂的水动力条件、屏蔽效应以及海底生物扰动等因素的作用,海洋环境推移质输运的经验、半经验公式具有一定的局限性。因此,充分考虑以上各种因素是正确预测海洋环境中推移质输运的关键。海洋环境沉积物输运理论的进一步发展需要着重进行各种过程和机制的研究,而这项工作依赖于高精度、高分辨率现场观测仪器的发展和更先进的颗粒态物质运动理论的建立。 相似文献
7.
Large symmetric and asymmetric dunes occur in the Fraser River, Canada. Symmetric dunes have stoss and lee sides of similar length, stoss and lee slope angles <8°, and rounded crests. Asymmetric dunes have superimposed small dunes on stoss sides, sharp crests, stoss sides longer than lee sides, stoss side slopes <3° and straight lee side slopes up to 19°. There is no evidence for lee side flow separation, although intermittent separated flow is possible, especially over asymmetric dunes. Dune symmetry and crest rounding of symmetric dunes are associated with high sediment transport rates. High near-bed velocity and bed load transport near dune crests result in crest rounding. Long, low-angle lee sides are produced by deposition of suspended sediment in dune troughs. Asymmetric dunes appear to be transitional features between large symmetric dunes and smaller dunes adjusted to lower flow velocity and sediment transport conditions. Small dunes on stoss sides reduce near-bed flow velocity and bed load transport, causing a sharper dune crest. Reduced deposition of suspended sediment in troughs results in a short, steep lee slope. Dunes in the Fraser River fall into upper plane bed or antidune stability fields on flume-based bedform phase diagrams. These diagrams are probably not applicable to large dunes in deep natural flows and care must be taken in modelling procedures that use phase diagram relations to predict bed configuration in such flows. 相似文献
8.
Lawrence P. Sanford 《Estuaries and Coasts》1994,17(1):148-165
Moored instruments were used to make observations of near bottom currents, waves, temperature, salinity, and turbidity at shallow (3.5 m and 5.5 m depth) dredged sediment disposal sites in upper Chesapeake Bay during the winters of 1990 and 1991 to investigate time-varying characteristics of resuspension processes over extended periods. Resulting time series data show the variability of two components of the suspended sediment concentration field. Background suspended sediment concentrations varied inversely with salinity and in direct relation to Susquehanna River flow. Muddy bottom sediments were also resuspended locally by both tidal currents and wind-wave forcing, resulting in short-term increases and decreases in suspended concentration, with higher peak concentrations near the bottom. In both years, episodes of wave-forced resuspension dominated tidal resuspension on an individual event basis, exceeding most tidal resuspension peaks by a factor of 3 to 5. The winds that generated the waves responsible for the observed resuspension events were not optimal for wave generation, however. Application of a simple wind-wave model showed that much greater wave-forced resuspension than that observed might be generated under the proper conditions. The consolidated sediments investigated in 1990 were less susceptible to both tidal and wave-forced resuspension than the recently deposited sediments investigated in 1991. There was also some indication that wave-forced resuspension increased erodibility of the bottom sediments on a short-term basis. Wave-forced resuspension is implicated as an important part of sediment transport processes in much of Chesapeake Bay. Its role in deeper, narrower, and more tidally energetic estuaries is not as clear, and should be investigated on a case-by-case basis. 相似文献
9.
A three-dimensional, time-dependent hydrodynamic and suspended sediment transport model was performed and applied to the Danshuei
River estuarine system and adjacent coastal sea in northern Taiwan. The model was validated with observed time-series salinity
in 2001, and with salinity and suspended sediment distributions in 2002. The predicted results quantitatively agreed with
the measured data. A local turbidity maximum was found in the bottom water of the Kuan-Du station. The validated model then
was conducted with no salinity gradient, no sediment supply from the sediment bed, wind stress, and different freshwater discharges
from upstream boundaries to comprehend the influences on suspended sediment dynamics in the Danshuei River estuarine system.
The results reveal that concentrations of the turbidity maximum simulated without salinity gradient are higher than those
of the turbidity maximum simulated with salinity gradient at the Kuan-Du station. Without bottom resuspension process, the
estuarine turbidity maximum zone at the Kuan-Du station vanishes. This suggests that bottom sediment resuspension is a very
important sediment source to the formation of estuarine turbidity maximum. The wind stress with northeast and southwest directions
may contribute to decrease the suspended sediment concentration. When the freshwater discharges increase at the upstream boundaries,
the limits of salt intrusion pushes downriver toward river mouth. Suspended sediment concentrations increase at the upriver
reaches in the Danshuei River to Tahan Stream, while decrease at Kuan-Du station. 相似文献
10.
Boundary layer dynamics and drag reduction in flows of high cohesive sediment suspensions 总被引：3，自引：0，他引：3
Drag reduction has been observed in suspension flows of low clay concentrations in previous studies. Here, velocity profiles and bed shear stresses, expressed as shear velocities, are measured using epoxy-coated hot-film sensors to evaluate drag reduction and controlling factors in suspension flows of high clay concentrations (4 and 8 g l–1 ). The directly measured shear velocity in the viscous sublayer is found to be reduced by as much as 70% relative to the profile-derived shear velocity in the logarithmic layer. Drag reduction is found to increase with increasing clay concentration and decreasing flow strength. Density profile data indicate that the suspension flows were not stratified, and examinations of particle size distributions suggest that flocculation was not significant in causing the observed drag reduction. Measurements of the velocity profiles and of the shear velocity in the viscous sublayer indicate significant thickening of the inner wall layer and show turbulence damping in the viscous sublayer. These effects become stronger for higher concentrations and lower flow strength, suggesting that they are responsible for drag reduction in flows of clay suspension. Empirical relationships have been derived that can be used to predict the magnitude of drag reduction and the reduced shear stress in mud suspensions for both laboratory and field cohesive sediment transport studies. 相似文献
11.
Data are presented from several experiments in the freshwater-saltwater interface (FSI) region of the Tamar Estuary. Longitudinal surveys of salinity and suspended particulate matter (SPM) at high water showed that the location of the FSI could be predicted in terms of a power-law regression with freshwater runoff. Longitudinal transects also were surveyed over periods of several hours. The FSI was observed to advect into the region on the flood with strong vertical mixing. After high water, stratification became intense as fresher water ebbed in the surface layers. The near-bed water in the stratified region began to ebb between 2 h and 3 h before low water. A model of the vertical structure of longitudinal currents showed that the enhanced stratification on the ebb, coupled with the longitudinal density gradient, partly produced this long period of slack, near-bed currents following high water. A strong turbidity maximum (TM) occurred during spring tides and was located slightly up-estuary of the FSI at high water. Longitudinal transects during a period of low freshwater runoff and large neap tide showed that at the start of the flood the TM was associated with the FSI region. As the FSI advected up-estuary on the flood there was considerable resuspension of sediment at the FSI. Some of this SPM moved with the FSI and reached the limit of saline intrusion, where it formed a slowly-eroding TM as particles settled during the long, high-water slack period. As the near-bed currents increased on the ebb and the FSI moved down-estuary, strong vertical mixing and resuspension of recently deposited sediment occurred in the unstratified water behind the FSI and the associated TM advected down-estuary. Additional effects were present with stronger tides and increased runoff. 相似文献
12.
波流共同作用下潮滩剖面沉积物和地貌分异规律——以长江口崇明东滩为例 总被引：1，自引：0，他引：1
通过对崇明东滩两个海滩剖面、表层沉积物和悬沙粒度以及同步水沙资料的分析,探讨波流共同作用下表层沉积物和地貌的分异规律。受波流共同作用的影响,表层沉积物中值粒径由破波带向两侧逐渐变细,分选由破波带向两侧逐渐变差,偏度由极正偏变为正偏,峭度由很窄尖变为宽平和中等峭度。由破波带向岸方向,流速逐渐减小,含沙量逐渐增加。悬沙和表层沉积物粒度特征的对比分析表明,潮间带上部的悬沙主要来源于破波带泥沙的再悬浮。破波带内泥沙以“波浪掀沙”引起的分选运移为主,而破波带两侧的泥沙以潮流对破波带水体的“平流输移”为主。以潮汐水位和高精度海滩剖面数据对崇明东滩微地貌类型按高程进行了新的划分。 相似文献
13.
14.
PETER J. TALLING DOUGLAS G. MASSON ESTHER J. SUMNER GIUSEPPE MALGESINI 《Sedimentology》2012,59(7):1937-2003
Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run‐out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain‐back for long distances into basinal lows. Deposition of ungraded mud (TE‐3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE‐2) and finely laminated mud (TE‐1) most probably result from floc settling at lower mud concentrations. Grain‐size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar‐laminated (TD) and ripple cross‐laminated (TC) non‐cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain‐size break beneath the ripple cross‐laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar‐laminated sand can be deposited by low‐amplitude bed waves in dilute flow (TB‐1), but it is most likely to be deposited mainly by high‐concentration near‐bed layers beneath high‐density flows (TB‐2). More widely spaced planar lamination (TB‐3) occurs beneath massive clean sand (TA), and is also formed by high‐density turbidity currents. High‐density turbidite deposits (TA, TB‐2 and TB‐3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low‐density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain‐size texture. Clean‐sand debrites can extend for several tens of kilometres before pinching out abruptly. Up‐current transitions suggest that clean‐sand debris flows sometimes form via transformation from high‐density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low‐strength cohesive debris flows produce extensive deposits restricted to distal areas. These low‐strength debris flows may contain clasts and travel long distances (DM‐2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM‐1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows. 相似文献
15.
16.
Preliminary results are reported from an experimental study of the interaction between turbulence, sediment transport and bedform dynamics over the transition from dunes to upper stage plane beds. Over the transition, typical dunes changed to humpback dunes (mean velocity 0–8 ms-1, depth 01 m, mean grain size 0.3 mm) to nominally plane beds with low relief bed waves up to a few mm high. All bedforms had a mean length of 0.7–0.8 m. Hot film anemometry and flow visualization clearly show that horizontal and vertical turbulent motions in dune troughs decrease progressively through the transition while horizontal turbulence intensities increase near the bed on dune backs through to a plane bed. Average bedload and suspended load concentrations increase progressively over the transition, and the near-bed transport rate immediately downstream of flow reattachment increases markedly relative to that near dune crests. This relative increase in sediment transport near reattachment appears to be due to suppression of upward directed turbulence by increased sediment concentration, such that velocity close to the bed can increase more quickly downstream of reattachment. Low-relief bedwaves on upper-stage plane beds are ubiquitous and give rise to laterally extensive, mm-thick planar laminae; however, within such laminae are laminae of more limited lateral extent and thickness, related to the turbulent bursting process over the downstream depositional surface of the bedwaves. 相似文献
17.
Data from a two-year period of monthly slackwater surveys reveal that in addition to the classical estuary turbidity maximum (ETM), another peak of bottom total suspended sediment (TSS) concentration, or a so-called secondary turbidity maximum (STM), often exists in the middle part of the York River estuary, Virginia. This STM, observed in most (but not all) of the slackwater surveys, moves back and forth in the region of about 20 to 40 km from the York River mouth where the mud percentage of bottom sediment is very high. The distribution of the potential energy anomaly, which was calculated using salinity data, indicates that the STM usually resides in the transition zone between the upstream well mixed and the downstream more stratified water columns. An analysis using the conservation equation of suspended sediment concentration in the water column reveals that four processes may contribute to the formation of the STM: convergence of bottom residual flow, tidal asymmetry, inhibition of turbulent diffusion by stratification, and bottom resuspension. The along-channel variations of the strength of bottom residual flow, the effect of tidal asymmetry, and the stratification patterns are probably due to the geometric features of the York River estuary. 相似文献
18.
In the classical view of fine sediment transport and deposition in streams, particles are expected to be removed from flowing water simply by direct sedimentation onto the streambed. However, recent research has demonstrated that fine sediments can propagate into pore spaces in the streambed due to hyporheic exchange and be removed by a combination of physical and chemical processes. This behaviour can significantly alter fine sediment size distributions during in-stream sediment transport because the physical transport of fine particles and their attachment to bed sediment grains are both a function of the particle size. Herein, we present model simulations for deposition of suspended sediments with a bimodal size distribution. We also applied this approach to analyse the results of laboratory flume observations of suspended sediment deposition. Results from model simulations and flume experiments clearly show that the rate of particle deposition increases with increasing particle size. Thus, the larger particles are preferentially removed from mixtures and there is a fining of the mixed suspensions over time. Both particle deposition mechanisms, i.e. particle sedimentation and filtration, contribute to the fining of the mixed fine particle suspensions over time, and their effects are clearly demonstrated using the fundamental process-based model. These results clearly demonstrate the effects of stream-subsurface exchange on the temporal evolution of the suspended fine sediment size distribution in downstream transport. 相似文献
19.
LAWRENCE A. AMY PETER J. TALLING VICTORIA O. EDMONDS ESTHER J. SUMNER ANNE LESUEUR 《Sedimentology》2006,53(6):1411-1434
The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass‐ballotini sand of particle size 35·5 μm < d < 250 μm and volume fractions, φs, up to 0·6 and cohesive kaolinite clay of particle size d < 35·5 μm and volume fractions, φm, up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at φs ～ 0·2 and entirely inhibited at φs ≥ 0·6. In noncohesive and cohesive mixtures with low sand concentrations (φs < 0·2), particle segregation was initially suppressed at φm ～ 0·07 and entirely suppressed at φm ≥ 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear‐flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The ‘abrupt’ change in settling regimes between regime I and V, over a relatively small change in mud concentration (<5% by volume), favours the development of either mud‐poor, graded sandy deposits or mud‐rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean ‘turbidite’ or muddy ‘debrite’ sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal ‘linked’ debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ≤ φs ≤ 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near‐bed sediment concentrations. 相似文献
20.
基于水、沙两相的分相测量试验结果,分析了床面附近泥沙颗粒的脉动和力学特性,指出床面附近的泥沙运动有着特殊的力学机制:颗粒相具有较强的非湍流脉动,其产生的脉动应力对颗粒的运动起着重要作用。论述了沙粒在水流中从推移运动到扬起悬浮的物理过程,讨论了过去一些理论中存在的问题和不足,概括了泥沙颗粒在水流中从床面扬起的基本模式,运用两相流理论分析了沙粒在水流中扬起的动力学机理。根据颗粒运动的垂向动量平衡原理,对泥沙颗粒的垂向浓度分布规律作了新的分析解释。证实了除浓度梯度之外,颗粒相的垂向脉动强度梯度也是泥沙扩散的重要扩散势,进一步揭示了悬移质浓度垂线分布存在两种类型的内在机理。 相似文献