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1.
We explore the link between channel‐bed texture and river basin concavity in equilibrium catchments using a numerical landscape evolution model. Theory from homogeneous sediment transport predicts that river basin concavity directly increases with bed sediment size. If the effective grain size on a river bed governs its concavity, then natural phenomena such as grain‐size sorting and channel armouring should be linked to concavity. We examine this hypothesis by allowing the bed sediment texture to evolve in a transport‐limited regime using a two grain‐size mixture of sand and gravel. Downstream ?ning through selective particle erosion is produced in equilibrium. As the channel‐bed texture adjusts downstream so does the local slope. Our model predicts that it is not the texture of the original sediment mixture that governs basin concavity. Rather, concavity is linked to the texture of the sorted surface layer. Two different textural regimes are produced in the experiments: a transitional regime where the mobility of sand and gravel changes with channel‐bed texture, and a sand‐dominated region where the mobility of sand and gravel is constant. The concavity of these regions varies depending on the median gravel‐ or sand‐grain size, erosion rate, and precipitation rate. The results highlight the importance of adjustments in both surface texture and slope in natural rivers in response to changes in ?uvial and sediment inputs throughout a drainage network. This adjustment can only be captured numerically using multiple grain sizes or empirical downstream ?ning rules. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
A tracer study performed on a 3 km long reach of the Danube River in Austria is presented. Forty artificial stones of three different sizes (intermediate b‐axis: 25 mm, 40 mm, 70 mm) were produced and a coded radio acoustic transmitter was implanted. The measurement system had to be improved to be applicable to large rivers with water depths up to 12 m. The positions of the stones were observed approximately once a week, depending on hydrology, over a period of at least one year by radio‐tracking from a boat, including a 15 year flood event. Transport paths and velocities, as well as the incipient motion of bedload transport, could be monitored for the first time on a large gravel‐bed river. The particle paths were found to be mostly bankline‐parallel, even though the stones passed a 30° river bend. The median of the transverse particle displacement was found to be 4% of the longitudinal displacement. Calculations considering both transverse slope and transverse flow velocities showed transverse transport to be 6·6% of the longitudinal transport indicating that marginal lateral transport is mainly influenced by morphology. A three‐dimensional (3D) numerical model using a stochastic particle tracing approach was validated with the data, indicating that the observed positions are well reproduced by the model. Within the observation period, 74% of all stones passed the reach. With more than 1000 detections, particle transport could be characterized by a mean travel velocity of about 10 m per day (variable for the different grain sizes); single tracer stones were transported up to 1000 m during a single flood event. Size‐selective behaviour could be shown and the incipient motion of the large 70 mm gravel was detected at lower discharges than predicted by commonly used uniform bedload transport formulae. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
3.
Most gravel‐bed streams exhibit a surface armour in which the median grain size of the surface particles is coarser than that of the subsurface particles. This armour has been interpreted to result when the supply of sediment is less than the ability of the stream to move sediment. While there may be certain sizes in the bed for which the supply is less than the ability of the stream to transport these sizes, for other sizes of particles the supply may match or even exceed the ability of the channel to transport these particles. These sizes of particles are called ‘supply‐limited’ and ‘hydraulically limited’ in their transport, respectively, and can be differentiated in dimensionless sediment transport rating curves by size fractions. The supply‐ and hydraulically limited sizes can be distinguished also by comparing the size of particles of the surface and subsurface. Those sizes that are supply‐limited are winnowed from the bed and are under‐represented in the surface layer. Progressive truncation of the surface and subsurface size distributions from the ?ne end and recalculation until the size distributions are similar (collapse), establishes the break between supply‐ and hydraulically limited sizes. At sites along 12 streams in Idaho ranging in drainage area from about 100 to 4900 km2, sediment transport rating curves by size class and surface and subsurface size distributions were examined. The break between sizes that were supply‐ and hydraulically limited as determined by examination of the transport rate and surface and subsurface size distributions was similar. The collapse size as described by its percentile in the cumulative size distribution averaged D36 of the surface and D73 of the subsurface. The discharge at which the collapse size began to move averaged 88 per cent of bankfull discharge. The collapse size decreased as bed load yield increased and increased with the degree of selective transport. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
4.
Evolution of bed material mobility and bedload grain size distributions under a range of discharges is rarely observed in braiding gravel-bed rivers. Yet, the changing of bedload grain size distributions with discharge is expected to be different from laterally-stable, threshold, channels on which most gravel bedload theory and observation are based. Here, simultaneous observations of flow, bedload transport rate, and morphological change were made in a physical model of a gravel-bed braided river to document the evolution of grain size distributions and bed mobility over three experimental event hydrographs. Bedload transport rate and grain size distributions were measured from bedload samples collected in sediment baskets. Morphological change was mapped with high-resolution (~1 mm precision) digital elevation models generated from close-range digital photogrammetry. Bedload transport rates were extremely low below a discharge equivalent to ~50% of the channel-forming discharge (dimensionless stream power ~70). Fractional transport rates and plots of grain size distributions indicate that the bed experienced partial mobility at low discharge when the coarsest grains on the bed were immobile, weak selective mobility at higher discharge, and occasionally near-equal mobility at peak channel-forming discharge. The transition to selective mobility and increased bedload transport rates coincided with the lower threshold for morphological change measured by the morphological active depth and active width. Below this threshold discharge, active depths were of the order of D90 and active widths were narrow (< 3% of wetted width). Above this discharge, both increased so that at channel-forming discharge, the active depth had a local maximum of 9D90 while active width was up to 20% of wetted width. The modelled rivers approached equal mobility when rates of morphological change were greatest. Therefore, changes in the morphological active layer with discharge are directly connected to the conditions of bed mobility, and strongly correlated with bedload transport rate. © 2018 John Wiley & Sons, Ltd. 相似文献
5.
Deposition and storage of fine‐grained (<62·5 μm) sediment in the hyporheic zone of gravel bed rivers frequently represents an important cause of aquatic habitat degradation. The particle size characteristics of such fine‐grained bed sediment (FGBS) exert an important control on its hydrodynamic properties and environmental impact. Traditionally, particle size analysis of FGBS in gravel bed rivers has focused on the absolute size distribution of the chemically dispersed mineral fraction. However, recent work has indicated that in common with fluvial suspended sediment, significant differences may exist between the absolute and the in situ, or effective, particle size composition of FGBS, as a result of the existence of aggregates, or composite particles. In the investigation reported in this paper, sealable bed traps that could be remotely opened to sample sediment deposited during specific storm runoff events and a laser back‐scatter probe were used to quantify the temporal and spatial variability of both the absolute and effective particle size composition of FGBS, and the associated suspended sediment from four gravel bed rivers in the Exe Basin, Devon, UK. The absolute particle size distributions of both the FGBS and suspended sediment evidenced c. >95%<62·5 μm sized primary particles and displayed a seasonal winter–summer fining, while the opposite trend was displayed by the effective particle size distribution of the FGBS and suspended sediment. The effective particle size distributions of both were typically highly aggregated, comprising up to 68%>62·5 μm sized particles. Spatial variation in the effective particle size and aggregation parameters was of secondary importance relative to temporal variation. The effective particle size distribution of the FGBS was consistently coarser and more aggregated than the associated suspended sediment and there was evidence of aggregate break‐up in samples of resuspended bed sediment. The implications of these findings for sediment transport modelling are considered. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
6.
Large grains matter: contrasting bed stability and morphodynamics during two nearly identical experiments 
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下载免费PDF全文 While the stabilizing function of large grains in step‐pool streams has long been recognized, the role they play in gravel‐bed streams is less clear. Most researchers have ignored the role of large grains in gravel‐bed streams, and have assumed that the median bed surface size controls the erodibility of alluvial boundaries. The experiments presented herein challenge this convention. Two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material. The two distributions had the same range of particle sizes, and nearly identical bulk d50 values (1.6 mm); however the d90 of experiment GSD1 was slightly finer (3.7 mm) than that for experiment GSD2 (3.9 mm). Transport rates during GSD1 were nearly four times greater than during GSD2 (even though the dimensionless shear stress was slightly lower), and the channel developed a sinuous pattern with well‐developed riffles, pools and bars. During GSD2 the initial rectangular channel remained virtually unchanged for the duration of the experiment. The relative stability of GSD2 seems to be associated with a slightly larger proportion of stable (large) grains on the bed surface: at the beginning of GSD1, 3.5% of the bed was immobile, while almost twice as much of it (6.1%) was immobile at the beginning of GSD2. The results demonstrate that the largest grains (not the median size) exert first‐order control on channel stability. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
7.
Differences in the transport rate and size of bedload exist for varying levels of flow in coarse‐grained channels. For gravel‐bed rivers, at least two phases of bedload transport, with notably differing qualities, have been described in the literature. Phase I consists primarily of sand and small gravel moving at relatively low rates over a stable channel surface. Transport rates during Phase II are considerably greater than Phase I and more coarse grains are moved, including material from both the channel surface and subsurface. Transition from Phase I to Phase II indicates initiation and transport of grains comprising the coarse surface layer common in steep mountain channels. While the existence of different phases of transport is generally acknowledged, the threshold between them is often poorly defined. We present the results of the application of a piecewise regression analysis to data on bedload transport collected at 12 gravel‐bed channels in Colorado and Wyoming, USA. The piecewise regression recognizes the existence of different linear relationships over different ranges of discharge. The inflection, where the fitted functions intersect, is interpreted as the point of transition from Phase I to Phase II transport; this is termed breakpoint. A comparison of grain sizes moved during the two phases shows that coarse gravel is rarely trapped in the samplers during Phase I transport, indicating negligible movement of grains in this size range. Gravel larger than about D16 of the channel surface is more consistently trapped during Phase II transport. The persistence of coarse gravel in bedload samples provides good evidence that conditions suitable for coarse grain transport have been reached, even though the size of the sediment approaches the size limits of the sampler (76 mm in all cases). A relative breakpoint (Rbr) was defined by the ratio between the discharge at the breakpoint and the 1·5‐year flow (a surrogate for bankfull discharge) expressed as a percentage. The median value of Rbr was about 80 percent, suggesting that Phase II begins at about 80 percent of the bankfull discharge, though the observed values of Rbr ranged from about 60 to 100 percent. Variation in this value appears to be independent of drainage area, median grain size, sorting of bed materials, and channel gradient, at least for the range of parameters measured in 12 gravel‐bed channels. Published in 2002 by John Wiley & Sons, Ltd. 相似文献
8.
Unsteady bedload transport was measured in two c. 5 m wide anabranches of a gravel‐bed braided stream draining the Haut Glacier d'Arolla, Switzerland, during the 1998 and 1999 melt seasons. Bedload was directly sampled using 152 mm square Helley–Smith type samplers deployed from a portable measuring bridge, and independent transport rate estimates for the coarser size fractions were obtained from the dispersion of magnetically tagged tracer pebbles. Bedload transport time series show pulsing behaviour under both marginal (1998) and partial (1999) transport regimes. There are generally weak correlations between transport rates and shear stresses determined from velocity data recorded at the measuring bridge. Characteristic parameters of the bedload grain‐size distributions (D50, D84) are weakly correlated with transport rates. Analysis of full bedload grain‐size distributions reveals greater structure, with a tendency for transport to become less size selective at higher transport rates. The bedload time series show autoregressive behaviour but are dif?cult to distinguish by this method. State–space plots, and associated measures of time‐series separation, reveal the structure of the time series more clearly. The measured pulses have distinctly different time‐series characteristics from those modelled using a one‐dimensional sediment routing model in which bed shear stress and grain size are varied randomly. These results suggest a mechanism of pulse generation based on irregular low‐amplitude bedforms, that may be generated in‐channel or may represent the advection of material supplied by bank erosion events. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
9.
Bedload transport is a complex phenomenon that is not well understood, especially for poorly sorted sediment and low transport rates, which is what is typically found in alpine gravel-bed rivers. In this paper, the interaction between bedload rate, bed stability and flow is investigated using flume experiments. Significant differences in bedload rates were observed for experiments conducted on beds formed with the same gravel material but presenting diverse arrangements and bedforms. Tests were performed under regimes of low transport rate, which are mainly controlled by gravel-bed roughness. Different scales of roughness were identified using the statistical characteristics of detailed bed elevation measurements: grain, structure and large bedform scales. The role played by these different roughness scales in bedload dynamics was examined. For quasi-flat beds, bed stability was quantified using a combination of bed surface criteria describing grain and structure scales. It was found that bed stability affects the bedload rate directly and not only through its influence on the flow or on the incipient motion. For beds with large bedforms, the analysis of bedload dynamics also showed the importance of accounting for effective bed shear stress distributions. An empirical bedload model for low transport regimes was suggested. Compared with previous formulae developed for alpine rivers, this model accounts for bed stability and distribution of effective bed shear stress. It significantly improves the understanding of gravel dynamics over complex beds such as arranged beds or those with large bedforms. However, further tests are needed to use the model outside the range of conditions of this study. © 2019 John Wiley & Sons, Ltd. 相似文献
10.
Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus) 下载免费PDF全文
下载免费PDF全文 The plants and animals that inhabit river channels may act as zoogeomorphic agents affecting the nature and rates of sediment recruitment, transport and deposition. The impact of benthic‐feeding fish, which disturb bed material sediments during their search for food, has received very little attention, even though benthic feeding species are widespread in rivers and may collectively expend significant amounts of energy foraging across the bed. An ex situ experiment was conducted to investigate the impact of a benthic feeding fish (Barbel Barbus barbus) on particle displacements, bed sediment structures, gravel entrainment and transport fluxes. In a laboratory flume changes in bed surface topography were measured and grain displacements examined when an imbricated, water‐worked bed of 5.6 to 16 mm gravels was exposed to feeding juvenile Barbel (on average, 0.195 m in length). Grain entrainment rates and bedload fluxes were measured under a moderate transport regime for substrates that had been exposed to feeding fish and control substrates which had not. On average, approximately 37% of the substrate, by area, was modified by foraging fish during a four‐hour treatment period, resulting in increased microtopographic roughness and reduced particle imbrication. Structural changes by fish corresponded with an average increase in bedload flux of 60% under entrainment flows, whilst on average the total number of grains transported during the entrainment phase was 82% higher from substrates that had been disturbed by Barbel. Together, these results indicate that by increasing surface microtopography and undoing the naturally stable structures produced by water working, foraging can increase the mobility of gravel‐bed materials. An interesting implication of this result is that by increasing the quantity of available, transportable sediment and lowering entrainment thresholds, benthic feeding might affect bedload fluxes in gravel‐bed rivers. The evidence presented here is sufficient to suggest that further investigation of this possibility is warranted. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
11.
Influence of bed clusters and size gradation on operational time distribution for non‐uniform bed‐load transport 下载免费PDF全文
下载免费PDF全文 The operational time distribution (OTD) defines the time for bed‐load sediment spent in motion, which is needed to characterize the random nature of sediment transport. This study explores the influence of bed clusters and size gradation on OTD for non‐uniform bed‐loads. First, both static and mobile bed armouring experiments were conducted in laboratorial flumes to monitor the transport of mixed sand/gravel sediments. Only in the mobile armouring experiment did apparent bed clusters develop, because of stable feeding and a longer transport period. Second, a generalized subordinated advection (GSA) model was applied to quantify the observed dynamics of tracer particles. Results show that for the static armour layer (without sediment feed), the best‐fit OTD assigns more weight to the large displacement of small particles, likely because of the size‐selective entrainment process. The capacity coefficient in the GSA model, which affects the width of the OTD, is space dependent only for small particles whose dynamics can be significantly affected by larger particles and whose distribution is more likely to be space dependent in a mixed sand and gravel system. However, the OTD for the mobile armour layer (with sediment recirculation) exhibited longer tails for larger particles. This is because the trailing edge of larger particles is more resistant to erosion, and their leading front may not be easily trapped by self‐organized bed clusters. The strong interaction between particle–bed may cause the capacity coefficient to be space‐dependent for bed‐load transport along mobile armour layers. Therefore, the combined laboratory experiments and stochastic model analysis show that the OTD may be affected more by particle–bed interactions (such as clusters) than by particle–particle interactions (e.g. hiding and exposing), and that the GSA model can quantify mixed‐size sand/gravel transport along river beds within either static or mobile armour layers. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
12.
Observations of bedload transport in a gravel bed river during high flow using fiber‐optic DTS methods 下载免费PDF全文
下载免费PDF全文 The question: ‘how does a streambed change over a minor flood?’ does not have a clear answer due to lack of measurement methods during high flows. We investigate bedload transport and disentrainment during a 1.5‐year flood by linking field measurements using fiber optic distributed temperature sensing (DTS) cable with sediment transport theory and an existing explicit analytical solution to predict depth of sediment deposition from amplitude and phase changes of the diurnal near‐bed pore‐water temperature. The method facilitates the study of gravel transport by using near‐bed temperature time series to estimate rates of sediment deposition continuously over the duration of a high flow event coinciding with bar formation. The observations indicate that all gravel and cobble particles present were transported along the riffle at a relatively low Shields Number for the median particle size, and were re‐deposited on the lee side of the bar at rates that varied over time during a constant flow. Approximately 1–6% of the bed was predicted to be mobile during the 1.5‐year flood, indicating that large inactive regions of the bed, particularly between riffles, persist between years despite field observations of narrow zones of local transport and bar growth on the order ~3–5 times the median particle size. In contrast, during a seven‐year flood approximately 8–55% of the bed was predicted to become mobile, indicating that the continuous along‐stream mobility required to mobilize coarse gravel through long pools and downstream to the next riffle is infrequent. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
13.
In January 1990 10 m3 of washed gravel (grain size 16–50 mm) were introduced in an impounded section of the River High-Rhine to test its potential as a spawning ground for salmonids. The process of infiltration of fine sediments into the gravel and subsequent clogging was monitored for three years. Levels of clogging of the gravel matrix were estimated and ranged from slight to moderately-heavy. The degree of clogging was lower in winter than in summer. Since no bedload transport was observed during the study period, flood events could wash out sediments from the top layer of the gravel bed only. An equilibrium diffusion technique using porewater samplers was employed to measure oxygen concentrations within the interstitial space during the spawning period ofThymallus thymallus (April),Salmo trutta fario andOncorhynchus mykiss (December till February). Oxygen concentration decreased with increasing depth and during the course of the study period. Oxygen concentrations measured as a reference in the interstitial of the confluence of the river Glatt were considerably lower than those of the new gravel bed. A wide range of O2 concentrations was found in winter 1991/92 and in spring 1992. This could be explained by the heterogeneous microstructure of the substrate. Successful embryonic and larval development of grayling in the gravel bed was observed in spring 1991 and 1992. However, no eggs or larvae of brown trout or rainbow trout were found. Habitat restoration projects for salmonids in impoundments of the High-Rhine are critically discussed. 相似文献
14.
Michael Bliss Singer 《地球表面变化过程与地形》2008,33(14):2277-2284
Grain‐size distributions of bed material sediment in large alluvial rivers are required in various scientific and management applications, but characterizing gravel beds in navigable rivers is hampered by difficulties in sediment extraction. The newly developed and preliminarily tested sampler reported here can extract sediment from a range of riverbeds. The 36 × 23 × 28 cm stainless steel toothed sampler is deployed from and dragged downstream by the weight of a jet boat, and it improves upon previous samplers that are unable to penetrate gravel bed surfaces, have small apertures, and/or cannot retain fine sediment. The presented sampler was used to extract 167 bed material sediment samples of up to 16 kg (dry weight) with an average sample size of ～6 kg from 67 cross‐sections spanning 160 river kilometres along the Sacramento River. It was also tested at three sites on a subaerial bar to compare surface, subsurface, and sampler distributions. Sampler penetration is ～5 cm. The device collects individual samples that satisfy the criterion for bed material sediment whereby the largest particle comprises no more than 5% of the total sample mass in gravel and sand beds, except where the degree of surface armouring is large (e.g. armor ratios >> 2) and where more than 10% of bed material sediment is composed of grains larger than 64 mm. When aggregated samples exceed 15 kg, all satisfy the criterion whereby the largest particle comprises no more than 1% of the total sample mass. Samples closely resemble surface size distributions, except where armouring is strong. The sampler should be subject to more rigorous field testing, but many of its current limitations are expected to become negligible with the advent a larger, heavier version of the sampling device. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
15.
Albert MOLINAS 《国际泥沙研究》2003,18(3)
A new method based on the Transport Capacity Fraction (TCF) concept is proposed to compute the fractional transport rates for nonuniform sediment mixtures in sand-bed channels. The TCF concept is derived from the understanding that the measurements and predictions of bed-material load are more accurate and reliable than the measurements and predictions of fractional loads. First the bed-material load is computed using an appropriate equation, then the fractional transport rates are determined by distributing the bed-material load into size groups through a transport capacity distribution function. For the computation of bed-material loads, the Ackers and White, Engelund and Hansen, and Yang equations are used in this study. Two new transport capacity distribution functions are developed for flows in sand-bed channels. The new expressions presented in this paper account for the sheltering and exposure effects that exist in mixtures. Comparisons with measured data show that the proposed method can signifi 相似文献
16.
《Continental Shelf Research》2007,27(10-11):1408-1421
Particle size distribution and size-specific settling velocity are critical parameters for understanding the transport of fine sediment on continental margins. In this study, observed floc size versus settling velocity, volume distributions of particles 2 μm–1 cm in diameter, and calculated effective densities for all particle sizes provided estimates of the mass distribution in suspension, which is used to apportion mass among component particles, microflocs, and macroflocs. Measurements were made during relatively quiescent environmental conditions. Observations of size distributions based on mass demonstrate an increase in the component particle fraction through time. The increase in the percentage of component particles in suspension had implications on water column properties, as small changes in the component particle fraction affected water column optical transmission in a way that was not as easily detected by changes in the volume concentration distribution or total mass concentration. Flocs larger than 133 μm in diameter only comprised one quarter to one third of the mass in suspension. This finding may explain why suspension bulk clearance rates are often an order of magnitude lower than those predicted by other methods. 相似文献
17.
We present herein clear field evidence for the persistence of a coarse surface layer in a gravel‐bed river during flows capable of transporting all grain sizes present on the channel bed. Detailed field measurements of channel topography and bed surface grain size were made in a gravel‐bed reach of the Colorado River prior to a flood in 2003. Runoff produced during the 2003 snowmelt was far above average, resulting in a sustained period of high flow with a peak discharge of 27 m3/s (170% of normal peak flow); all available grain sizes within the study reach were mobilized in this period of time. During the 2003 peak flow, the river avulsed immediately upstream of the study reach, thereby abandoning approximately one half kilometer of the former channel. The abandonment was rapid (probably within a few hours), leaving the bed texture essentially frozen in place at the peak of the flood. All locations sampled prior to the flood were resampled following the stream abandonment. In response to the high flow, the surface median grain size (D50s) coarsened slightly in the outer part of the bend while remaining nearly constant along the inner part of the bend, resulting in an overall increase from 18 to 21 mm for the study reach. Thus, the coarse bed surface texture persisted despite shear stresses throughout the bend that were well above the critical entrainment value. This may be explained because the response of the bed texture to increases in flow strength depends primarily upon the continued availability of the various grain size percentiles in the supply, which in this case was essentially unlimited for all sizes present in the channel. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
18.
Taking as an example six main rivers that drain the western flank of the Eastern Carpathians, a conceptual model has been developed, according to which fluvial bed sediment bimodality can be explained by the overlapping of two grain size distribution curves of different origins. Thus, for Carpathian tributaries of the Siret, coarse gravel joins an unimodal distribution presenting a right skewness with enhanced downstream fining. The source of the coarse material distributions is autohtonous (by abrasion and hydraulic sorting mechanisms). A second distribution with a sandy mode is, in general, skewed to the left. The source of the second distribution is allohtonous (the quantity of sand that reaches the river‐bed through the erosion of the hillslope basin terrains). The intersection of the two distributions occurs in the area of the 0·5–8 mm fractions, where, in fact, the right skewness (for gravel) and left skewness (for sand) histogram tails meet. This also explains the lack of particles in the 0·5–8 mm interval. For rivers where fine sediment sources are low, the 0·5–8 mm fractions have a higher proportion than the fractions under 1 mm. For the Siret River itself, bed sediment bimodality is greatly enhanced due to the fact that the second mode is more than 25% of the full sample. As opposed to its tributaries, the source of the first mode, of gravel, is allohtonous to the Siret river, generated by the massive input of coarse sediment through the Carpathian tributaries, while the second mode, of the sands, is local. In this case we can also observe that the two distributions of particles of different origins overlap in the 0·5–8 mm fraction domain, creating the illusion of ‘particle lack’ in the fluvial bed sediments. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
19.
The particle size distributions of fault gouge from the San Andreas, the San Gabriel, and the Lopez Canyon faults in Southern California were measured using sieving and Coulter-Counter techniques over a range of particle sizes from 2 m to 16 mm. The distributions were found to be power law (fractal) for the smaller fragments and log-normal by mass for sizes near and above the peak size. The apparent fractal dimensionD of the smaller particles in gouge samples from the San Andreas fault, the San Gabriel fault and the Lopez Canyon gouge were 2.4–3.6, 2.6–2.9 and 2.4–3.0, respectively. The averageD for the Lopez Canyon gouge was 2.7±0.2, which is in agreement with earlier studies of this gouge using planar 2-D sections. The fractal dimension of the finer fragments from all three faults is observed to be correlated with the peak fragment size, with finer gouges tending to have a largerD. A computer automaton is used to show that this observation may be explained as resulting from a fragmentation process which has a grinding limit at which particle reduction stops. 相似文献
20.
Expansive clay sediments are abundant in riverine and estuarine waters and bottom beds, and their particle size distributions (PSD) are important for the analysis of sediment transport. This paper presents an experimental study to evaluate, using a laser particle size analyzer under varying flow conditions, the intrinsic PSD of two expansive clays, a Ca- and a Na-montmorillonite and the influence of biological, chemical, and hydrodynamic factors on their flocculation and PSD. The considered biological factor consists of three extracellular polymeric substances of varying polarity, including xanthan gum, guar gum, and chitosan; the chemical factor is the salinity; and the hydrodynamic factor is the types of flow indicated by the Reynolds number and shear rate. The intrinsic PSD of both clays show a multimodal lognormal distribution with sizes ranging from 0.2 to 50 μm. All three biopolymers, xanthan gum, guar gum, and chitosan, can facilitate flocculation through long-range polymer bridging and short-range ion-dipole interaction, hydrogen bonding, and Coulomb force. The influence of salinity is different for the two clays: the particle size of the Na-montmorillonite increases with salinity, which is caused by flocculation resulting from the suppressed electrical double layer, while that of the Ca-montmorillonite is slightly reduced owing to the decreased basal spacing and cation exchange. For different hydrodynamic conditions, higher shear rate promotes the flocculation of Ca-montmorillonite, but breaks the Na-montmorillonite flocs. The significance of understanding the flocculation and PSD of expansive clays is also discussed in terms of sediment transport under different aquatic environments. 相似文献