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1.
在煤层气开发过程中,煤粉聚集及沉降会堵塞煤层气运移通道及导致卡泵、埋泵等事故。为了查明不同粒度煤粉的聚集及沉降特征,选取粒度>140目（<106 μm）、>70~140目（106~<212 μm）和>50~70目（212~<300 μm）3种粒度范围的煤粉,开展了在去离子水中煤粉的聚集及沉降实验,从煤粉聚集及沉降特征观察、悬浮液中煤粉含量及煤粉粒度分布探究不同粒度煤粉在去离子水悬浮液中的聚集及沉降特征。结果表明,随着静置时间的增加,各粒度煤粉悬浮液的颜色均不同程度地变浅,逐渐出现分层,其中,粒度>140目的煤粉悬浮液最先出现分层。煤粉粒度越小,煤粉悬浮液顶部漂浮的煤粉量越多;煤粉粒度越大,其下沉到煤粉悬浮液底部的煤粉量越多。不同粒度煤粉悬浮溶液中煤粉含量均随着静置时间的增加呈现不同程度的降低,在停止搅拌后3 min内,煤粉含量下降最快,粒度为>70~140目的煤粉悬浮液中煤粉含量最大。根据不同粒度煤粉悬浮液中煤粉粒度分布曲线,将煤粉聚集及沉降过程分为3个阶段:单峰变双峰阶段（煤粉快速上浮及沉降）、双峰变单峰阶段（煤粉快速聚集及沉降）和单峰阶段（煤粉缓慢沉降）。粒度>140目的煤粉在悬浮液中最先达到缓慢沉降阶段,粒度>70~140目的煤粉在悬浮液中最后到达缓慢沉降阶段。从煤粉的受力、扩展的DLVO理论及煤粉的有机分子结构方面探讨了煤粉聚集沉降的机理:煤样含有大量的脂肪烃和芳香烃等疏水性基团,疏水性强,润湿性低;随着煤粉粒度的减小,其比表面积显著增大,煤粉表面吸附大量空气,形成气膜;同时,煤粉颗粒间相互吸附聚集,内部形成很多微孔隙,导致粒度小的煤粉易聚集漂浮在悬浮液面上。实验得到的不同粒度煤粉的上浮、下沉及悬浮情况,为后期煤层气开发中煤粉管控措施提供依据。   相似文献   

2.
A computer-based numerical model of turbidity current flow and sedimentation is presented that integrates geological observations with basic equations for fluid and sediment motion. The model quantifies those aspects of turbidity currents that make them different from better-understood fluvial processes, including water mixing across the upper flow boundary and the interactions between the suspended-sediment concentration and the flow dynamics and sedimentation. The model includes three numerical components: (1) a layer-averaged three-equation flow model for tracing downslope flow evolution using continuity and momentum equations, (2) a sedimentation/fluidization model for tracing sediment-size fractionation in sedimenting multicomponent suspensions and (3) a concentration-viscosity model for quantifying the changes in resistance of such suspensions toward fluid and sediment motion. The model traces the evolution of a model turbidity current in terms the layer-averaged flow velocity, flow thickness, sediment concentration distribution, and the rate of sedimentation and sediment size fractionation. It generates synthetic turbidites with downslope variations in thickness and grain-size structuring at each point along the flow path. This study represents an effort to evaluate quantitatively the effects of basin geometry, sediment supply and sediment properties on the mechanics of turbidity current flow and sedimentation and on the geometry and grain size characteristics of the resulting deposits.  相似文献   

3.
A process-based, forward computer model of turbidity current flow and sedimentation, termed the TCFS model, has been developed to trace the downslope evolution of individual turbidity flows. Details of the model itself have been presented in a preceding paper. We here outline a series of tests of the TGFS model. The sensitivity tests of the TCFS model to general geological controls reveal the quantitative relationship between these controls and the behaviour of turbidity flows and the geometry and textural features of the resulting turbidites. Experimental turbidity currents on relatively steep slopes accelerate more rapidly and reach higher velocities than those on gentle slopes. Flows with larger initial volumes have higher initial velocities, travel further downslope, and form beds of greater thickness and downslope extent than smaller flows. Experimental high-concentration flows with suspended-sediment concentrations of 25% accelerate more rapidly and reach higher downslope velocities than dilute flows with 5% suspended sediment. The higher velocities and enhanced hindered-settling effects of the high-concentration flows lead to much greater transport distances and reduced vertical and lateral sediment size grading in the resulting turbidites. Beds formed by experimental high-concentration flows are massive or show coarse-tail grading whereas beds formed by low-concentration flows show distribution-grading. Experimental flows fed by coarse sediment sources tend to deposit the bulk of their suspended sediment loads on the proximal slope, resulting in more rapid flow deceleration and sedimentation than flows fed by silt-rich, fine-grained sediment sources. Turbidites formed by coarse-sediment flows tend to have a wedge-shaped geometry, with low downslope extent and high surface relief, whereas turbidites formed by fine-sediment flows tend to have a tabular geometry, with greater downslope extent and lower surface relief. A specific geological test of the TCFS model is based on studies of modern turbidity currents in Bute Inlet, British Columbia, Canada. With the input initial and boundary conditions estimated from Bute Inlet, the model predicts the downslope velocity evolution of turbidity currents comparable to those of modern and ancient turbidity flows measured in Bute Inlet. Model-calculated vertical and downslope grain-size properties of turbidites are similar to those exhibited by surface and cored Bute Inlet turbidites. Model flows tend to decelerate more rapidly than some stronger turbidity currents in the Bute Inlet system, and model beds tend to decrease in grain-size downslope more rapidly than observed bottom sediments. This is probably because the TCFS model flows lacked clay, which is abundant in Bute Inlet; they do not fully simulate turbulent mixing of suspended sediments; and they better represent the unsteady, depositional stage of turbidity-currents than the preceding stage of more-or-less steady-flow conditions. These tests demonstrate that the TCFS model provides a semi-quantitative method to study the growth patterns of submarine turbidite systems. It can serve as a predictive tool for analysing the facies architecture of ancient turbidite systems through simulating multi-depositional events by improving its erosion function, and the compatibility between its numerical components.  相似文献   

4.
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   

5.
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   

6.
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   

7.
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   

8.
The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post‐depositional consolidation and soft‐sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain‐support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non‐cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle‐support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain‐to‐grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle‐support mechanism depends upon flow conditions, particle concentration, grain‐size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse‐grain tail (or dense‐grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large‐scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensu 4 ), in which fluid turbulence is the main particle‐support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge‐like flows and quasi‐steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge‐like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi‐steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening‐up units capped by fining‐up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near‐steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co‐exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.  相似文献   

9.
ABSTRACT Mud‐rich sandstone beds in the Lower Cretaceous Britannia Formation, UK North Sea, were deposited by sediment flows transitional between debris flows and turbidity currents, termed slurry flows. Much of the mud in these flows was transported as sand‐ and silt‐sized grains that were approximately hydraulically equivalent to suspended quartz and feldspar. In the eastern Britannia Field, individual slurry beds are continuous over long distances, and abundant core makes it possible to document facies changes across the field. Most beds display regular areal grain‐size changes. In this study, fining trends, especially in the size of the largest grains, are used to estimate palaeoflow and palaeoslope directions. In the middle part of the Britannia Formation, stratigraphic zones 40 and 45, slurry flows moved from south‐west and south towards the north‐east and north. Most zone 45 beds lens out before reaching the northern edge of the field, apparently by wedging out against the northern basin slope. Zone 40 and 45 beds show downflow facies transitions from low‐mud‐content, dish‐structured and wispy‐laminated sandstone to high‐mud‐content banded units. In zone 50, at the top of the formation, flows moved from north to south or north‐west to south‐east, and their deposits show transitions from proximal mud‐rich banded and mixed slurried beds to more distal lower‐mud‐content banded and wispy‐laminated units. The contrasting facies trends in zones 40 and 45 and zone 50 may reflect differing grain‐size relationships between quartz and feldspar grains and mud particles in the depositing flows. In zones 40 and 45, quartz grains average 0·30–0·32 mm in diameter, ≈ 0·10 mm coarser than in zone 50. The medium‐grained quartz in zones 40 and 45 flows may have been slightly coarser than the associated mud grains, resulting in the preferential deposition of quartz in proximal areas and downslope enrichment of the flows in mud. In zone 50 flows, mud was probably slightly coarser than the associated fine‐grained quartz, resulting in early mud sedimentation and enrichment of the distal flows in fine‐grained quartz and feldspar. Mud particles in all flows may have had an effective grain size of ≈ 0·25 mm. Both mud content and suspended‐load fallout rate played key roles in the sedimentation of Britannia slurry flows and structuring of the resulting deposits. During deposition of zones 40 and 45, the area of the eastern Britannia Field in block 16/26 may have been a locally enclosed subbasin within which the depositing slurry flows were locally ponded. Slurry beds in the eastern Britannia Field are ‘lumpy’ sheet‐like bodies that show facies changes but little additional complexity. There is no thin‐bedded facies that might represent waning flows analogous to low‐density turbidity currents. The dominance of laminar, cohesion‐dominated shear layers during sedimentation prevented most bed erosion, and the deposystem lacked channel, levee and overbank facies that commonly make up turbidity current‐dominated systems. Britannia slurry flows, although turbulent and capable of size‐fractionating even fine‐grained sediments, left sand bodies with geometries and facies more like those deposited by poorly differentiated laminar debris flows.  相似文献   

10.
Proglacial Quaternary lacustrine sediments deposited along the Caribou River Valley, Yukon, Canada, formed in a lake impounded by glacial ice that was retreating downslope. Sedimentation in the lake was dominated by turbid sediment underflows generated from the upslope, previously deglaciated region. The base of the sedimentary succession indicates a gradual transition from sporadic low-density distal flows to higher density proximal flows. Continued sediment accumulation resulted in the construction of a subaqueous clay and silt bank. Sedimentation was dominated by deposition of suspension load clay carried by subseasonal bottom countercurrents induced by katabatic winds. This sedimentation pattern prevailed until the subaqueous bank was disturbed by mass movement. Removal of the sediment bank increased the depth of the nearshore area sufficiently to allow turbid underflows to dominate sedimentation once more. The changing sedimentation patterns reflect events in the areas away from the ice front, rather than changes in the activity of the impounding glacier. Similar successions could be developed in other glacial lakes impounded by glaciers which moved up topographical slopes, either pre-existing or generated by glacioisostatic depression.  相似文献   

11.
在长江河口潮滩、分流河道和水下三角洲共获得18个柱样,进行沉积学分析和210Pb测定,并对其中6根柱样进行137Cs测定。经研究发现,长江口外在水深25～30m,122°30′N,31°00′E附近存在一个泥质沉积中心,沉积速率达2.0～6.3cm/yr。另外,在潮滩和涨潮槽也获得较高沉积速率,其中南汇和横沙岛潮滩沉积速率(1.03～1.94cm/yr)高于崇明东滩(0.51～0.76cm/yr),涨潮槽沉积速率也达0.86cm/yr。此外,在石洞口、南汇、九段沙潮滩和三角洲前缘有部分柱样未获沉积速率,推测为沉积环境不稳定或沉积速率过快所致。  相似文献   

12.
长江口沉积物210Pb分布及沉积环境解释   总被引：19，自引：0，他引：19  
在长江河口潮滩、分流河道和水下三角洲共获得18个柱样,进行沉积学分析和210Pb测定,并对其中6根柱样进行137Cs测定。经研究发现,长江口外在水深25～30m,122°30′N,31°00′E附近存在一个泥质沉积中心,沉积速率达2.0～6.3cm/yr。另外,在潮滩和涨潮槽也获得较高沉积速率,其中南汇和横沙岛潮滩沉积速率(1.03～1.94cm/yr)高于崇明东滩(0.51～0.76cm/yr),涨潮槽沉积速率也达0.86cm/yr。此外,在石洞口、南汇、九段沙潮滩和三角洲前缘有部分柱样未获沉积速率,推测为沉积环境不稳定或沉积速率过快所致。  相似文献   

13.
Laboratory tests conducted in order to investigate some of the dynamic aspects of the formation of floored interstices are described. In essence, the experiments consist of pouring a suspension of fine-grained carbonate mud (powdered clams) onto a quantity of clam shells (fragments or whole ones) under water. Settling of the carbonate suspension leads to gravity differentation and internal sedimentation phenomena. The coarser fractions, settling the fastest, will accumulate in cavities provided by some of the hollow shells, or by hollows resulting from packing of the fragments, and may even accumulate on top of some of the shells. Inside individual shells floored interstices frequently exhibiting graded bedding, were formed. Sediment pyramids on top of some of the clam shells may also exhibit graded bedding, although not as frequently as internal sediments. Sedimentation pyramids are bordered by symmetrical sets of maximum-slope angles. Closely related to the phenomena taking place during settling from suspension, appeared to be the occurrence of narrow, more or less vertical channels of clear water streaming upwards through the settling suspension. Photographs record the textural changes taking place as the result of the introduction of mud suspensions into grain-supported sedimentary fabrics. Conclusions in regard with the palaeoenvironmental significance of the experiments conducted are given.  相似文献   

14.
Most aqueous sedimentary environments contain varying concentrations of fine‐grained, often clay‐rich, sediment that is transported in suspension and may modify the properties of the flow and underlying mobile bed. This paper presents results from a series of laboratory experiments examining the mean and turbulent properties of clay‐laden (kaolinite) flows, of various volumetric sediment concentrations between 0·046% and 12·7%, moving over a fixed, idealized current ripple. As the kaolinite concentration was raised, with flow velocity and depth constant, four flow types were observed to occur: (i) turbulent flow, in which flow separation is dominant in the leeside of the ripple; (ii) turbulence‐enhanced transitional flow, in which turbulence in the leeside separation zone region is enhanced; (iii) turbulence‐attenuated transitional flow, in which turbulence along the separation zone shear layer and in the free flow above it becomes damped, eventually leading to a reduction in the size of the separation zone wake region; and (iv) laminar plug flow, in which turbulence is damped and flow is almost stagnant in the lee of the ripple. Such modulation of turbulence by increasing clay concentrations suggests that many paradigms of flow and bedform dynamics, which have been based on extensive past work in clear water flows, require revision. The present results highlight a need to fully characterize the boundary conditions for turbulence modulation as a function of clay type and applied flow conditions, and the effects of such flows on fully mobile cohesionless beds.  相似文献   

15.
A Greek lignite fly ash was selected because of its hydraulic properties and was pulverized to produce a specific surface of 8300 cm2/g, and a gradation with D15, D50, and D85 equal to 1.3 m, 6 m and 20 m, respectively. The pulverized fly ash suspension properties were optimized by adding a suitable superplasticizer and a suitable accelerator at optimum dosages. The experimental evaluation of suspension properties included sedimentation tests, viscosity'measurements, rheological properties, setting times and development of strength with time. Groutability and effectiveness were evaluated by injecting selected suspensions into clean sands. Pulverization improves remarkably the behavior of fly ash suspensions. Suspensions with water to solids ratios of 1.5:1 and 2:1 by weight and containing additives, behave as Bingham fluids, have apparent viscosity lower than 70 cP, bleed capacity lower than 5%, initial setting time lower than 24 h, and can be injected into relatively coarse sands. These characteristics are comparable to those obtained for ordinary and microfine cement suspensions, showing that pulverized fly ash suspensions can be used for permeation grouting.  相似文献   

16.
Monitoring of estuarine condition depends on water quality parameters that have significant and interpretable ecological effects and can be assessed either in situ or via rapid laboratory techniques. Two commonly measured parameters are water column turbidity (NTU) and total suspended solids (TSS). Under certain conditions, either of these measures could represent a proxy for runoff and provide rapid, in situ measures to improve protection of the public by decreasing the time required to detect and monitor associated effects (e.g., reduced water clarity and eutrophication). The Neuse River Estuary (NRE) has experienced a decline in water quality due to anthropogenic inputs, including stormwater containing nutrient and particle loads. Water samples were collected from the NRE during both dry weather and storm events over 16 mo across the entire estuarine gradient. Particle size distributions, ratio of particulate organic carbon to nitrogen, chlorophylla (chla), TSS, and NTU were measured in each of these samples, with the data separated into regions based on salinity and depth of sample collection. Particle analyses were directed at identifying suspensions dominated by phytoplankton, runoff particles, or resuspended sediments. Particle size distributions for suspensions in Region I (all samples with salinity less than 2) varied little during sampling, resulting in a robust NTU-TSS relationship. This relationship confirmed the usefulness of turbidity as a measure of runoff and resuspension of recently deposited runoff in the upper NRE. Phytoplankton cells represented a majority of the particles in Region II (surface samples with salinity greater than 2), based on the close relationship between chla and total particle volume in these samples. Suspensions of large, nearly uniform diameter particles, which are likely aggregates and resuspended sediment, were observed in Region III (bottom samples with salinity greater than 2). Using these techniques as part of routine monitoring, particle suspension measures in different hydrographic regions of an estuary provide evidence useful for identifying the source and water quality consequences of particle suspensions (e.g., microbial contamination and algal blooms).  相似文献   

17.
To this day, deterministic physical models capable of explaining the evolution of grain-size distributions in the course of transport are still lacking. For this reason, various attributes of particle frequency distributions, in particular curve shapes and textural parameters, have for many decades been investigated for potential information about transport behaviour and size-sorting processes of sediments in numerous environments. Such approaches are essentially conceptual and hence rely heavily on the validity of the assumptions on which they are based. A factor which has to date been largely ignored in this context, is the fact that different methods of grain-size analysis (e. g. sieving, laser absorption and diffraction, settling velocity measurements), when applied to the same sample material, produce variable curve shapes, and hence incongruous textural data. This is illustrated by selected examples showing the differences between sieving and settling results, conversion of settling velocities into equivalent settling diameters (psi-phi-transformations), and the influences of particle shape, particle density, and water temperature. It is demonstrated that particle-size distributions are not only method-dependent but also dependent on the adopted post-processing procedure. As a result, only frequency curves generated by the same method and subsequently processed by identical computational procedures can be meaningfully compared. Furthermore, the computation of textural parameters from bi- or multimodal size distributions produces spurious results which are unrelated to the processes leading to the mixing of different size populations frequently observed in nature. In such cases, only the decomposition of such distributions into individual populations and the spatial comparison of such populations makes any sense. Because a physical explanation for the generation of size distributions is lacking, a particular curve shape of a grain-size population has no meaning on its own. Only a systematic comparison of progressively changing curve shapes (and associated textural parameters) of sediments collected on a closely spaced grid can yield data suitable for sediment trend analysis.  相似文献   

18.
Large‐scale experiments generating ground‐hugging multiphase flows were carried out with the aim of modelling the rate of sedimentation, of pyroclastic density currents. The current was initiated by the impact on the ground of a dense gas‐particle fountain issuing from a vertical conduit. On impact, a thick massive deposit was formed. The grain size of the massive deposit was almost identical to that of the mixture feeding the fountain, suggesting that similar layers formed at the impact of a natural volcanic fountain should be representative of the parent grain‐size distribution of the eruption. The flow evolved laterally into a turbulent suspension current that sedimented a thin, tractive layer. A good correlation was found between the ratio of transported/sedimented load and the normalized Rouse number of the turbulent current. A model of the sedimentation rate was developed, which shows a relationship between grain size and flow runout. A current fed with coarser particles has a higher sedimentation rate, a larger grain‐size selectivity and runs shorter than a current fed with finer particles. Application of the model to pyroclastic deposits of Vesuvius and Campi Flegrei of Southern Italy resulted in sedimentation rates falling inside the range of experiments and allowed definition of the duration of pyroclastic density currents which add important information on the hazard of such dangerous flows. The model could possibly be extended, in the future, to other geological density currents as, for example, turbidity currents.  相似文献   

19.
A computer code using sequential fragmentation/transport theory was used to deconvolute and characterize a large grain-size data set taken from the AD 79 Vesuvio deposits. The results allow us to interpret transport and deposition processes. Four principal morphological classes of grain-size spectra were recognized in the AD 79 deposits: 1 unimodal distributions with coarse modes and very good sorting; 2 polymodal distributions in which relative fractions of each subpopulation are considerably variable; 3 polymodal distributions, but with one mode greatly prevailing over the other ones; 4 flat spectra in which a large number of size classes show the same loading. Because different eruptive, transport and deposition conditions may have operated on pyroclasts which occur in the same bed, we have assigned grain-size subpopulations, with different modes to specific mechanisms of particle movement and sedimentation depending on the size range of the particles and the textures of the beds. The fragmentation/transport processes considered here occur either within dilute flows (as fall, traction, saltation and suspension loads) or in high-concentration flows (as a fluidized system or one with an extremely high sedimentation rate). Variation in strength and position of modes throughout the entire vertical section of AD 79 products illustrates changes in transport and deposition processes with time. Size spectra from Vesuvio quantitatively demonstrate contemporaneous deposition from fall and surge mechanisms as well as contributions from different levels of hydrovolcanic products. In contrast, vertical variations in size spectra within individual pyroclastic flow deposits suggest variation from high particle concentration near the base of the bed to more dilute depositional conditions towards the top. Lateral variations in size spectra for one marker horizon show how a local pyroclastic flow in a channel grades into a surge on the margins. This study supports the model of continuous modification in loadings of several discrete subpopulations during deposition from a single explosive cloud.  相似文献   

20.
Artifacts are commonly buried by approximately 50 cm of sediment at prehistoric archeological sites (early Archaic through Mississippian) on uplands of the Sandhills of the upper Coastal Plain of the southeastern United States. Bioturbation, eolian sedimentation, and colluviation are the primary processes that can explain artifact burial because of the upland position of the sites in an erosional landscape setting. Colluvial sedimentation is discounted at most of the sites because they occur on interstream divides and upper hillslope positions. Thus, the focus is on eolian sedimentation versus bioturbation as burial agents. Six sites in the midst of the Sandhills region along the corridor of South Carolina Highway 151 in Chesterfield County provide the data. The Sandhills consist primarily of Cretaceous and Tertiary marine, fluvial, and eolian sediments that are highly dissected and overlie crystalline rocks in the deep subsurface. Two of the sites are on high fluvial terrace remnants that predate 12 ka and serve as controls where bioturbation is the only reasonable burial process. Hillslope positions of the sites are on erosional elements of the landscape (crests, shoulder slopes, and upper backslopes) where sediment transfer operates (colluvial and overland flow), but where deposition is minimal. The sites occur on very sandy soils having a texture of loamy sand to sand. In some instances, a fine textured cover sand, which is about 1.5 m thick, overlies a clayey subsoil or Bt horizon. This cover sand has been interpreted by some as an eolian sand sheet that buries a second parent material and paleosol, but standard particle size and heavy mineral data indicate that it is simply a thick E horizon over a Bt horizon. Standard particle size fractionation at whole phi intervals, and particle size analysis of the heavy mineral fraction, indicate that eolian sedimentation is unlikely at five of the six sites. Heavy minerals were analyzed with respect to the sedimentological principle of hydraulic equivalence, which provides clear separation of eolian versus water-laid sediment. Results of particle size analysis suggest that the cover sands are water-laid (probably fluvial) at five of the six sites, which favors the bioturbation process of artifact burial. Heavy mineral analysis corroborates the standard particle size data, indicating that only one site, 38CT16, possibly is composed of eolian sediment. Soil profile development suggests that the age of the sediment at site 38CT16 probably is older than 12 ka and was in place prior to human occupation. Therefore, possible eolian sedimentation at that site is not relevant to artifact burial, which also suggests bioturbation is the primary process of artifact burial. Additional evidence favoring bioturbation as a vigorous artifact burial process in the Sandhills comes from the two sites on high elevation sandy fluvial terraces (38CT34, 38CT17) where artifacts are also buried. At these terraced sites bioturbation is the only possible burial process. Overall results suggest that bioturbation best explains the occurrence of buried artifacts and that eolian sedimentation processes are not readily apparent, and are not required, in explaining artifact burial. © 1998 John Wiley & Sons, Inc.  相似文献   

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