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1.
Grain-size distributions of gravels transported as bedload in Oak Creek, Oregon, show systematic variations with changing flow discharges. At low discharges the gravel distributions are nearly symmetrical and Gaussian. As discharges increase, the distributions become more skewed and follow the ideal Rosin distribution. The patterns of variations are established by goodness-of-fit comparisons between the measured and theoretical distributions, and by Q-mode factor analysis. Two end members are obtained in the factor analysis, having (respectively) almost perfect Gaussian and Rosin distributions, and the percentages of the two end members within individual samples vary systematically with discharge. Transformation from Gaussian to Rosin distribution with increasing discharge may be explained by processes of selective entrainment of grains from a bed of mixed sizes. Samples of bed material in Oak Creek follow the Rosin distribution. At high discharges, the transported bedload approaches the grain sizes of that bed-material source and mimics its Rosin distribution. Random-selection processes must be more important to grain entrainment at lower discharges, so that the resulting Gaussian distributions of transported bedload reflect similar distributions of bed stresses exerted by the stream flow. The results from Oak Creek demonstrate that the competence of the flow is reflected in the entire distribution of transported gravel sizes. A sequence of layers of fluvial gravels, modern or ancient, might show systematic variations between coarse Rosin and finer-grained Gaussian distributions, and these could be used to infer frequencies of various discharges and to establish a relationship to the source sediment. With further study, analyses of changing bedload grain-size distributions and their transport rates will lead to a better understanding of downstream variations in grain sizes of bed sediments and how their distributions reflect the progressive development of textural maturity. 相似文献
2.
Multivariate statistics were used to characterize and test the effectiveness of grain-size frequencies as environmental discriminators. Sediment from the following two depositional systems along eastern Lake Michigan were studied: (1) a closed system with respect to available grain sizes (Little Sable Point), and (2) an open system (Sleeping Bear Point-Manitou Passage). Principal components analysis shows that grain-size distributions are composed of two or more subgroups that reflect surface creep bedload, mixed suspension bedload, and uniform suspension. Discriminant function and principal latent vector analyses of the Little Sable Point environments show that, when available sediment is limited with respect to grain size (0.5 φto 3.0 φ), similar size distributions can occur in environments supposedly characterized by different energy conditions. Sediment in the Sleeping Bear Point-Manitou Passage system is not restricted to available grain sizes and the environments discriminated very well (α < 0.001). The grain-size distributions are such that they reflect differences in energy conditions within the environments. It is apparent that the grain sizes available to a depositional system control to a great extent the effectiveness of environmental discrimination. 相似文献
3.
The grain-size fractions in the bedload transported over the five heterogeneous sediment beds of different values of bed roughness were computed from the flume experiments. The existence of an entrapment factor associated with the sorting observed from the bed to active layer was modeled based on the modified critical shear stress to estimate the grain-size fractions in the transport layer under given hydraulic conditions. The efficiency of these models was tested with the observed data. Subsequently, the patterns of observed grain-size distributions in the transport layer were tested to identify the distributions developed in the active layer due to sorting using three probability density functions (pdf), such as, log-normal, log-hyperbolic and log-skew-Laplace. Tests indicated that a log-skew-Laplace distribution fitted best for 49%, a log-hyperbolic for 31%, and a log-normal for 20% out of forty-five bedload samples collected under unidirectional flow with changes in flow discharge and bed roughness. The results of this study would be useful to specify the grain-size distributions in the bedload formed under different hydrodynamic conditions in various sedimentary environments. 相似文献
4.
PENG GAO 《Sedimentology》2012,59(6):1926-1935
A recently developed bedload equation (Abrahams & Gao, 2006) has the form ib = ωG3˙4, where ib is the immersed bedload transport rate, ω is the stream power per unit area, G = 1?θc/θ, θ is the dimensionless shear stress and θc is the associated threshold value for the incipient motion of bed grains. This equation has a parsimonious form and provides good predictions of transport rate in both the saltation and sheetflow regimes (i.e. flows with low and high θ values, respectively). In this study, the equation was validated using data independent of those used for developing it. The data represent bedload of identical sizes transported in various steady, uniform, fully rough and turbulent flows over plane, mobile beds. The equation predicted ib quite well over five orders of magnitude. This equation was further compared with six classic bedload equations and showed the best performance. Its theoretical significance was subsequently examined in two ways. First, based on collision theory, the parameter G was related to the ratio of grain‐to‐grain collisions to the total collisions including both grain‐to‐grain and grain‐to‐bed collisions, Pg by Pg = G2, suggesting that G characterizes the dynamic processes of bedload transport from the perspective of granular flow, which partly accounts for the good performance of the equation. Moreover, examining the ability of two common equations to predict bedload in gravel‐bed rivers revealed that G can also be used to simplify equations for predicting transport capacities in such rivers. Second, a simple dimensionless form of the equation was created by introducing B = ib/ω. The theoretical nature of the term B was subsequently revealed by comparing this equation with both the Bagnold model and two commonly used parameters representing dimensionless bedload transport rates. 相似文献
5.
Breaks in the slope of log-probability plots of cumulative grain-size distributions of bed material are compared with frequency distributions of bedload and suspended sediment over a range of discharges at two stations on the Platte River in south-central Nebraska. The break between suspension and intermittent suspension as determined from the bed-material curve coincides with the upper limit of the grain-size overlap between bedload particles and suspended-sediment particles, whereas the break between intermittent suspension and traction corresponds to the grain size at the lower limit of overlap of bedload particles and suspended-sediment particles. Although grain-size distributions of bedload change little with discharge, the size of the coarsest grains in suspension increases with increasing discharge. Thus, the length of overlap of bedload and suspended-sediment distributions increases with increasing discharge. The limits of grain-size overlap of bedload and suspended-sediment distribution curves associated with near-flood discharges most closely approximate the breaks in the bed material grain-size distribution. 相似文献
6.
Accumulation of bivalve recruits in the bottom convergence at the center of coastal eddies has been suggested as a possible mechanism resulting in locally abundant adult populations. We investigated transport of juvenile gem clams (Gemma gemma) in a headland wake to determine whether they accumulated, and where. Velocity measurements during three flood tides showed that a wake consistently formed, but that flow speeds were too slow to transport juvenile clams to the eddy center. Instead, the clams were deposited just inside the wake perimeter, where shear velocities decreased to levels below critical erosion velocities of the clams. This result demonstrated that accumulation in a coastal flow separation can occur even in the absence of a well-defined eddy or a strong bottom convergence. Juvenile gem clams were carried, probably as bedload, to regions in the wake dominated by sediments with similar grain sizes, rather than similar fall velocities, suggesting that bedload transport was particularly dependent on particle diameter in this flow regime. Adult gem clam populations tended to be locally abundant in regions receiving transported juveniles, but clam transport on any specific flood tide was not sufficient to fully predict the adult distributions. 相似文献
7.
The ability of mud aggregates to form depositional bedforms is of considerable sedimentological importance for explaining the geomorphology of the Channel Country of central Australia as well as for understanding the depositional environment of certain argillaceous fluvial sequences in the rock record. The sediment transport and bedform development of mud aggregates from the floodplain of Cooper Creek, central Australia, was examined in a laboratory flume over a range of flow conditions. The aggregates were found to be clay-rich (>60% clay), nonsaline (<0·02%), fine sand-sized (mean d50=0·13 mm), low density (2300 kg m?3) and water-stable. Three wetting rates were applied to the sediment in the laboratory prior to wet sieving to replicate various field conditions and results in three mean aggregate sizes. Immersion wetting (no tension) represents inundation of the sediment by overland flow and results in aggregates of 0·13 mm. Tension wetting at 20 and 50 mm corresponds to high- and low-intensity rainfall and results in mean d50 sizes of 0·75 and 0·70 mm, respectively. Immersion wetting is the most applicable wetting mode for hydraulic transport of aggregated sediment on the Cooper Creek floodplain. Considerable variability in sediment transport rates in the field could result from differences in pre-wetting of the aggregated sediment. The dominance of smectite in the clay mineralogy of the sediment is an important factor in the development of the aggregates; disaggregated sediment reaggregated in a laboratory after 2–3 wetting/drying cycles. In flume experiments, bedforms of aggregated mud ranging from lower-regime plane beds to upper-regime antidunes were observed. The aggregates moved predominantly as bedload with measured peak bedload concentrations being high compared with other flume studies. The highly mobile nature of this sediment in the field is due to the ready entrainment of low-density aggregates in the form of self-mulching vertisols across extensive floodplains. The occurrence of low-sinuosity braid-like channels on this extensive low-gradient semi-arid floodplain can be attributed to: (a) the passage of floodwaters across a floodplain with steeper gradients than adjacent more sinuous anastomosing channels; (b) the highly mobile nature of the low-density sediment aggregates; (c) the ability of the aggregates to be transported as bedload; and (d) their durable nature during transport. 相似文献
8.
Fine- to medium-grained sand transported as bedload moves in lanes parallel to the flow that are thought to be preserved as parting lineation. A series of six flume experiments was designed to discover the morphology and spacing of these lanes, here called sand streaks, as functions of local shear velocity, U* (9 × 10-3 to 4.8 × 10-2 m s-1), depth (5 × 10-2 and 9.5 × 10-2 m), mean grain diameter (150, 200, 290, 1380 μm), and sediment bedload concentration (0.0–0.39). Low U* flows produce predominantly straight, non-intersecting sand streaks, moderate U* flows produce sub-parallel and en échelon sand streaks, and moderate to high U* flows produce wavy sand streaks and secondary streaks with a spacing an order of magnitude larger. The wavy sand streaks are thought to be composed of sand grains in suspension close to the bed. An upper grain-size limit for the sand streak structure occurs at a grain size between 290 and 1380μm. The spacings of the fine-and medium-grained sand streaks, at low to moderate U* (0.9 × 10-2 to 3 × 10-2m s-1), are similar to those predicted for low-speed fluid streaks, although the fine-grained sand forms more closely-spaced streaks than the medium-grained sand. The spacings of sand streaks formed at moderate to high U* and at bedload concentrations greater than 0.15, are wider than those predicted for the low-speed fluid streaks. The wider spacing is thought to reflect a new type of flow immediately above the moving bed layer in which the formation of low-speed streaks is inhibited. This results from an increase in either grain concentration or grain size. The spacing of parting lineation, also wider than that predicted for low-speed streaks, may reflect this. 相似文献
9.
High-temperature creep behavior in Ni2GeO4 spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4
microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to
1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following
flow laws:
where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion
(Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels
although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are
normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation
creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size
reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis
of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less
than 2 μm.
Received: 18 June 2000 / Accepted: 3 April 2001 相似文献
10.
The potential consequences of bedload transport of postlarvae for patterns of distribution of marine invertebrates were explored
by developing a bedload transport model for juvenile bivalves in a small estuary in New Jersey, USA. A simple numerical model
of tidal current hydrodynamics was developed based on field measurements of shear stresses near the bottom. Burrowing behavior
of bivalves was incorporated into the model of bedload transport by using estimates of entrainment rates of Gemma gemma and Mya arenaria in a laboratory flume, and jump lengths of the bivalves were estimated by methods previously developed for noncohesive particles.
Based on the flood domination and strong gradient of shear stresses in the Navesink estuary, our model predicted that juvenile
bivalves would accumulate in the center of the estuary, traveling up to several kilometers over 30 days. Field distributions
of juvenile bivalves were consistent with the model predictions for other species of bivalves but not for G. gemma, for which field distributions of both <500- and >500-μm individuals were concentrated in the eastern end of the estuary.
Differences between the bedload model and G. gemma distributions suggest that spatial variation in burrowing behavior or biological interactions are playing an important role
in maintaining distribution patterns of this species in spite of high levels of bedload transport. This modeling approach
is applicable to other juvenile benthic invertebrates that disperse as bedload and is a useful model against which to compare
field observations of rates of transport and patterns of distribution and abundance. 相似文献
11.
Grain coarsening in contact metamorphic carbonates: effects of second-phase particles, fluid flow and thermal perturbations 总被引：3，自引：0，他引：3
Under contact metamorphic conditions, carbonate rocks in the direct vicinity of the Adamello pluton reflect a temperature‐induced grain coarsening. Despite this large‐scale trend, a considerable grain size scatter occurs on the outcrop‐scale indicating local influence of second‐order effects such as thermal perturbations, fluid flow and second‐phase particles. Second‐phase particles, whose sizes range from nano‐ to the micron‐scale, induce the most pronounced data scatter resulting in grain sizes too small by up to a factor of 10, compared with theoretical grain growth in a pure system. Such values are restricted to relatively impure samples consisting of up to 10 vol.% micron‐scale second‐phase particles, or to samples containing a large number of nano‐scale particles. The obtained data set suggests that the second phases induce a temperature‐controlled reduction on calcite grain growth. The mean calcite grain size can therefore be expressed in the form D = C2 eQ*/RT(dp/fp)m*, where C2 is a constant, Q* is an activation energy, T the temperature and m* the exponent of the ratio dp/fp, i.e. of the average size of the second phases divided by their volume fraction. However, more data are needed to obtain reliable values for C2 and Q*. Besides variations in the average grain size, the presence of second‐phase particles generates crystal size distribution (CSD) shapes characterized by lognormal distributions, which differ from the Gaussian‐type distributions of the pure samples. In contrast, fluid‐enhanced grain growth does not change the shape of the CSDs, but due to enhanced transport properties, the average grain sizes increase by a factor of 2 and the variance of the distribution increases. Stable δ18O and δ13C isotope ratios in fluid‐affected zones only deviate slightly from the host rock values, suggesting low fluid/rock ratios. Grain growth modelling indicates that the fluid‐induced grain size variations can develop within several ka. As inferred from a combination of thermal and grain growth modelling, dykes with widths of up to 1 m have only a restricted influence on grain size deviations smaller than a factor of 1.1. To summarize, considerable grain size variations of up to one order of magnitude can locally result from second‐order effects. Such effects require special attention when comparing experimentally derived grain growth kinetics with field studies. 相似文献
12.
Comparisons between acoustic measurements and predictions of the bedload transport of marine gravels
Continuous, detailed records of marine gravel transport have been obtained acoustically and compared with bedload transport rates (qb) predicted by five bedload transport equations using measurements of the near-bed turbulent current flow. When mean flow data are used in these equations, total qb estimates are similar to those measured. However, when instantaneous flow data are used, total qb is over-estimated by approximately one order-of-magnitude. Based on the acoustic measurements, an empirical equation has been obtained that gives accurate estimates of total qb over a tidal cycle and simulates well the intermittent characteristics of marine bedload transport. 相似文献
13.
A log‐normal spectral analysis of inorganic grain‐size distributions from a Canadian boreal lake core: Towards refining depositional process proxy data from high latitude lakes 
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下载免费PDF全文 Paul R. Gammon Lisa A. Neville R. Timothy Patterson Martine M. Savard Graeme T. Swindles 《Sedimentology》2017,64(3):609-630
Better methods for interpreting grain‐size spectra will enhance current understanding of past transport–depositional processes. A high‐resolution inorganic grain‐size dataset has been measured from a freeze core extracted from ‘Alberta Lake E’ a boreal fresh water lake 40 km east of the Athabasca Oil Sands in north‐eastern Alberta, Canada. The grain‐size spectra are remarkably consistent throughout the core, exhibiting a structure comprising six persistent grain‐size distributions below ca 250 μm, plus a rare medium‐sand distribution. Automated deconvolution of the grain‐size spectra produced poor results. Constraining the modes of two of the distributions produced deconvolution solutions that were statistically excellent and consistent with the structure of each spectrum. Statistical analysis of the ‘constrained’ solutions indicates that deconvolution successfully extracted independent grain‐size populations. Conversely, the multimodal spectra generate traditional measures (for example, mean grain size) that are inconsistent combinations of different individual populations and thus are poor proxies of transport–depositional processes. Alberta Lake E is situated in a boreal wetland landscape where sediment delivery is dominated by overland flow transport during spring melt. This context means that the Alberta Lake E grain‐size spectra can be interpreted to reflect: (i) a bedload component transported during short‐duration high discharge events that reflect the intensity of the melt; and (ii) a finer suspended load component representing material whose magnitude is controlled by the volume of the spring melt. Stratigraphically, bedload and suspended load populations demonstrate different short‐wavelength and long‐wavelength cyclicity, suggesting that spring melt is likely to be driven by cyclic external forcing factors. The links between the grain‐size spectra and spring melt have potential for generating proxy records that better capture the external controls over spring melt in boreal systems and the risks associated with these energetic hydrodynamics. This is exemplified by the coarsest Alberta Lake E distributions, which indicate that more intense spring‐melt dynamics occurred in pre‐historical times. 相似文献
14.
This study presents an investigation of image texture approaches for mapping sub‐pixel fluvial grain‐size features from airborne imagery, allowing for the rapid acquisition of surface sand and coarse fraction (>1·41 mm) grain‐size information. Imagery at 30 mm resolution was acquired over four gravel bars from the Fraser River (British Columbia, Canada). Combined first‐order and second‐order image texture approaches (windowed standard deviation filter and the grey level co‐occurrence matrix) were used. First‐order image texture, through the application of a standard deviation filter and subsequent thresholding was used to detect the presence of surface sand, with optimal accuracy achieved at 91 ± 1·9%. A wide‐ranging parameter space investigation was used to derive optimum parameters for the grey‐level co‐occurrence matrix. Subsequently first‐order and second‐order image textures were used in multiple linear regression to achieve good calibrations with several sub‐pixel grain‐size percentiles; relative error at 1·44%, 3·18%, 6·80% and 10·6% for D5, D16, D35 and D50, respectively. The larger percentiles of D84 and D95 had relative errors of 24·7% and 29·7%, respectively. The breakdown of calibration precision for larger percentiles is attributed to a ‘pixel averaging effect’. It is concluded that multispectral imagery is not required, because sufficient image texture information can be derived from standard colour imagery. Recommendations are suggested for the application of this method to other localities and data sets, thus reducing exhaustive parameter searches in future studies. 相似文献
15.
Victor R. Baker 《Quaternary Research》1974,4(1):94-112
Quaternary terraces and pediments along Ralston Creek and Clear Creek, near Golden, Colorado, are associated with Verdos, Slocum, Louviers, and Broadway Alluviums. Terrace deposits can be locally correlated on the basis of elevation and relict paleosols. The terrace sediments probably represent aggradation by braided streams flowing from glaciated drainage basins. Engineering hydraulic calculation procedures suggest that flood flows were 2–3 m deep on steep gradients (0.008–0.01). Discharges were as great as 1400 m3/sec, nearly an order of magnitude greater than modern flood discharges. The most useful paleohydraulic calculation techniques were found to be the dimensionless shear approach applied to stream competence and bedload function theory applied to stream capacity. 相似文献
16.
Sediment diffusion during overbank flows 总被引：5，自引：0，他引：5
JAMES E. PIZZUTO 《Sedimentology》1987,34(2):301-317
Distinctive overbank sediments deposited since European settlement on the floodplain of the Brandywine Creek, Pennsylvania, are used to calibrate and test a diffusion model of overbank deposition. The predictions of the model can be calibrated to reproduce the topography of the post-settlement lithosome with an average error of 7%. The model also correctly predicts the decrease in mean grain size away from the channel. The model greatly underestimates the ability of floodwaters to transport sand away from the channel. Apparently, sand is transported across the floodplain by bedload transport and by advective suspended sediment transport as well as by diffusion. If flow duration data for 1912–1981 and the present rating curve for the Brandywine Creek at Chadds Ford, Pennsylvania, are assumed to apply throughout the post-settlement period, the model may be used to estimate palaeohydraulic characteristics of post-settlement floods. Calculations indicate that 212 post-settlement floods covered the floodplain to an average depth of 1.6 m, transported an average excess suspended sediment concentration of 6200 ppm, and deposited an average thickness of 1.4 cm of sediment on levees next to the channel. 相似文献
17.
Log-probability plots of grain-size distribution from the Platte, North Platte, and South Platte rivers are composed of four or five straight line segments. The line segments are grouped, dividing each curve into three regions. These regions are interpreted as subpopulations moved by different transport mechanisms. Consideration of the criterion for suspension and calculation of shear velocities associated with dominant discharges support this interpretation. The grain size cumulative curves are similar to each other but distinct from curves of fluvial systems transporting only fine-grained material, the difference being the presence of a subpopulation of grains moved in traction transport. One of two possible relationships seems to exist between the grain-size distributions and flow conditions within the Platte River system. Estimated shear velocities derived by varying flow conditions within reasonable limits predict a range of grain sizes within which the break between the intermittent suspension and traction loads should occur. This break appears to be associated with intermediate shear velocities if truncated normal distributions are assumed; but if overlapping distributions are assumed, the ‘break'is associated with estimated maximum shear velocities. 相似文献
18.
Multipath diffusion in geochronology 总被引：5，自引：0，他引：5
James K. W. Lee 《Contributions to Mineralogy and Petrology》1995,120(1):60-82
Recent developments in microanalytical tools such as the ion and laser microprobe have revealed spatial distributions of radiogenic isotopes in minerals which cannot be explained by a simple volume diffusion mechanism. Although it is known that diffusion of a substance along extended defects (such as dislocations, exsolution lamellae, micropores, microfractures, fission tracks, etc.), which may serve as high-diffusivity pathways in a crystal, can significantly influence the bulk diffusivity of a mineral, this has largely been ignored in the field of geochronology. A general numerical model has been developed, which solves coupled multipath diffusion equations that describe the simultaneous diffusion of a solute species through both the crystal lattice (via volume diffusion) and high-diffusivity pathways (via short-circuit diffusion) under non-steady state conditions. Addition of a radioactive source term to the appropriate equations further allows for the modelling of integrated cooling ages and closure temperatures, and has direct pertinence to geochronological and thermochronologial studies. Three key criteria can be used to distinguish multipath diffusion mechanisms from volume diffusion mechanisms: (a) non-Fickian concentration profiles, (b) enhanced solute diffusivity with increasing mineral grain size, and (c) a lack of any correlation between closure temperatures (and cooling ages) and larger grain sizes. With multipath diffusion, the effective diffusion dimension a for certain minerals appears to remain on the order of the grain size, and the model can adequately explain observed increases in the bulk diffusion coefficient D
b
with a in the hydrothermal bomb data of previous Ar diffusion studies. Arrhenius diagrams of a multipath diffusion D
b
vs 1/T will consist of curves that have a kink in them, reflecting a continuous change in the relative importance of the different diffusion mechanisms with temperature. The most important consequence of multipath diffusion is that the overall bulk diffusion coefficient D
b
of a diffusing species can be enhanced significantly above its volume diffusion coefficient D
v
. As a result, integrated ages and effective closure temperatures (T
c
) can be much lower than those predicted assuming only a volume diffusion mechanism, to the extent that minerals normally characterized by low volume-diffusion T
c
may potentially have older integrated ages that minerals normally associated with higher volume-diffusion T
c
. 相似文献
19.
SIMON P. TODD 《Sedimentology》1989,36(4):513-530
Within high-density flood flows a prominent mechanism of gravel transport and deposition is by stream-driven, high-density traction carpet (with a rheology similar to grain flow). These gravel carpets are envisaged to form the basal portion of a bipartite high-density flood flow, decoupled from an overlying sand- and silt-laden turbulent flow. Several examples already documented in the literature are reviewed and an additional case from the Lower Old Red Sandstone of southwest Ireland is presented. Two mechanisms of traction carpet initiation are discussed: by rapid entrainment of gravel into suspension on rising stage, followed by settling into the gravel traction carpet at peak and falling stage; and by overconcentration of a ‘normal’, low-density bedload. Gravel entrainment, suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension. Theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high-density, sand-bearing turbulent flood flow above. This tangential shear stress is converted to dispersive pressure, which aids buoyancy and quasi-static grain-to-grain contacts in the support of the clasts within the gravel carpet. The carpet is thought to have a quasi-plastic rheology but behave much like a viscous fluid at high shear rates. Stream-driven gravelly traction carpets are expected to produce sheet-like units of clast- to matrix-supported conglomerate, characterized by a parallel or an a(p)a(i) clast fabric. These units may be ungraded, normally or inversely graded, depending on the rate of shear, the viscosity of the flow and the celerity of deposition. 相似文献
20.
The relationship between values of hydraulic conductivity determined from grain-size methods,K
gr, and those from pumping-test methods,K
pt, have been evaluated statistically using data from recent and older sandy materials. It is shown that both methods generally give the same values only in recent sediments that have not been subjected to significant diagenetic alteration and give variable results in older and diagenetically altered sediments. The ratioK
pt/Kgr appears to vary, probably in response to the degree of diagenetic alteration. It is further found that methods incorporating the effects of grain size (d1O or d50) and sorting could give betterK values than those incorporating the effect of grain-size only and, thus, suggest the joint inclusion of both parameters in the grain-size determination of permeability. 相似文献