laboratory simulation of clast abrasion brewer paul lewin j 2002 laboratory simulation of clast abrasion earth surface processes and landforms 27 pp 145 164 rae2008

surface based transport model for mixed size sediment we present a transport model for mixed sand gravel sediments fractional transport rates are referenced to the size distribution of the bed surface rather than subsurface making the model completely explicit and capable of predicting transient conditions the model is developed using a new data set of 48 coupled observations of flow transport and bed surface grain size using five different sediments the model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions the model uses the full size distribution of the bed surface including sand and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models the model shares some common elements with two previous surface based transport models but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow transport rate and sediments

the downstream gradation of particle sizes in the squamish river british columbia the exponential downstream decline in particle size predicted by sternberg 1875 is examined within a highly active alpine fluvial environment the gravel bed squamish river in southwestern british columbia transect survey procedures for sampling within the coarse alluvial gravels are described in detail downstream trends for various particle size statistics plotted with distance from a major sediment source are considered according to two scenarios evaluation of overall trends indicates that the downstream expression derived by sternberg 1875 is not the most appropriate rather curves are better described by power functions this reflects a very rapid decline in particle size immediately downstream of the major sediment source secondly evaluation of downstream trends in relation to channel planform determines that exponential functions with different coefficients describe adequately relations within individual planform reaches there is insufficient evidence to suggest which of these two scenarios may be appropriate several lines of evidence suggest that the trends found are more the result of selective sediment transport phenomena than of abrasion a condition attributed to channel competence within a system dominated by sediment supply conditions

processes of selective grain transport and the formation of placers on beaches placers are found on many beaches of the oregon coast in the beach that is the focus of this study the principal placer minerals are ilmenite 22 0 epidote 20 0 garnet 18 7 augite 15 5 hypersthene 8 1 hornblende 5 8 and zircon 2 0 the ilmenite fraction actually being partly chromite a series of beach face sand samples was obtained along a profile when sand was being eroded from the upper beach and transported to offshore bars since the processes of grain selective sorting leading to placer formation were probably most active at such a time the landward most sample contained 96 heavy minerals being part of the placer this concentration systematically decreased offshore to a 6 heavy mineral content in the seaward most sample 54 m offshore determinations of individual heavy mineral concentrations revealed that although they all tended to become concentrated within the placer with light quartz and feldspar dominating the offshore transport the sorting processes were m

the variability of critical shear stress friction angle and grain protrusion in water worked sediments the erodibility of a grain on a rough bed is controlled by among other factors its relative projection above the mean bed its exposure relative to upstream grains and its friction angle here we report direct measurements of friction angles grain projection and exposure and small scale topographic structure on a variety of water worked mixed grain sediment surfaces using a simple analytical model of the force balance on individual grains we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography the friction angle projection and exposure of single grain sizes vary widely from point to point within a given bed surface the variability within a single surface often exceeds the difference between the mean values of disparate surfaces as a result the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution rather than a single value on a given bed the crtitical shear stress distributions of different grain sizes have similar lower bounds but above their lower tails they diverge rapidly with smaller grains having substantially higher median critical shear stresses large numbers of fines trapp ed within pockets on the bed or shielded by upstream grains are effectively lost to the flow our calculations suggest that critical shear stress as conventionally measured is defined by the most erodible grains entrained during transient shear stress excursions associated with the turbulent flow this implies a physical basis for the indeterminacy of initial motion these observations suggest that transport rate shear stress relationships may be controlled in part by the increasing numbers of grains that become available for entrainment as mean shear stress increases they also suggest that bed textures and grain size distributions may be controlled within the constraints of an imposed shear stress and sediment supply regime by the influence of each size fraction on the erodibility of other grain sizes present on the bed

probabilistic exner sediment continuity equation for mixtures with no active layer the exner equation of sediment continuity is the foundation of river morphodynamics generalization of this equation to mixtures of grain sizes has required the introduction of an active layer i e a buffer layer between the sediment moving in the water column and the immobile substrate below the active layer is defined to be a well mixed layer with no vertical structure that encompasses those grains available to exchange directly with the moving sediment the sediment in the substrate below exchanges with the active layer only as the bed aggrades or degrades the active layer concept is a useful one that has served the research community well for 3 decades however the division of the erodible bed into a discrete active layer and substrate must represent only an approximation of a more general formulation that contains no active layer and in which parameters pertaining to the entrainment from and deposition to the bed vary continuously with depth below the sediment water interface here the probability density function of bed elevation is used to derive a general exner equation of sediment continuity with no discrete layers the formulation is applicable to both sediment mixtures and tracers in uniform sediment although the treatment requires more information than that of the active layer approach it offers the prospect of a better understanding of how streams create a stratigraphic record of their activities through deposition

estimating critical stream power for bedload transport calculations in gravel bed rivers the physical basis of bagnold s equation for critical stream power is examined although the general approach is well founded the original equation can be criticised for 1 failing to distinguish between the grain size in transport and the grain size that represents bed roughness 2 requiring a knowledge of critical flow depth in addition to gross channel properties 3 failing to allow for hiding and protrusion effects on the mobility of mixed size stream beds more general equations are presented which overcome these limitations they require a knowledge of channel slope but not depth the analysis suggests that the presence of form resistance in addition to grain roughness does not affect the calculation of critical power critical power is expected to depend more sensitively on grain size between reaches than within reaches

complex variations in sediment transport at three large river bifurcations during discharge waves in the river rhine river bifurcations strongly control the distribution of water and sediment over a river system a good understanding of this distribution process is crucial for river management in this paper an extensive data set from three large bifurcations in the dutch rhine is presented containing data on bed load transport suspended bed sediment transport dune development and hydrodynamics the data show complex variations in sediment transport during discharge waves the objective of this paper is to examine and explain these measured variations in sediment transport it is found that bend sorting upstream of the bifurcations leads to supply limitation particularly in the downstream branch that originates in the outer bend of the main channel tidal water level variations lead to cyclical variations in the sediment distribution over the downstream branches lags in dune development cause complex hysteresis patterns in flow parameters and sediment transport all bifurcations show evidence of sediment waves which probably are intrinsic bifurcation phenomena the complex transport processes at the three bifurcations cause distinct discontinuities in the downstream fining trend of the river differences among the studied river bifurcations are mainly due to differences in sediment mobility shields value because the variations in sediment transport are complex and poorly correlated with the flow discharge prediction of the sediment distribution with existing relationships for one dimensional models is problematic

dating the cooloola coastal dunes of south eastern queensland australia abstract the cooloola sand dunes are part of a series of aeolian parabolic dunes that stretch along the east coast of australia they form a chronosequence showing increasing weathering soil formation and water erosion across six geomorphically recognized soil landscapes these landscapes were recognized from air photographs and further refined on the basis of some 150 auger holes across the dunes data about the structure and floristics of the vegetation were collected at the same time there is a significant body of literature about the cooloola dunes but there are two areas that have not been satisfactorily considered first the previous dating which gave inconsistent results has been superseded by single grain osl dates and second cooloola has not been considered in a regional context here we report the results of single grain optically stimulated luminescence osl analyses for 31 samples for 21 sites across the geomorphic landscapes the sites were selected near the apex of each dune as this represents the last depositional date and the least disturbed by sand movement we recognized 10 units across the dunes on the basis of the dates available the dune ages ranged from recent to 725 ka the largest dunes at some 320 m high were dated at around 140 ka and these dunes carry the maximum vegetation biomass of this dunefield beyond this vegetation maximum the dunes had lost their parabolic shape due to prolonged weathering and erosion and the vegetation is greatly reduced in height and biomass the depth to the soil b horizon gradually deepens with age to form giant podzols in the older dunes the depth to the b horizon was remarkably consistent within the geomorphic landscapes across the length and breadth of the dunes these studies place the cooloola dunes in a unique position as the chronosequence is intact the possible processes that lead to the formation of the dunes and the regional implications are discussed secondly it shows the buildup and decline of the dune ecosystems and acts as a model for all ecosystems forest succession models which place less emphasis on soil development miss this important process

size density sorting of sand size spheres during deposition from bedload transport and implications concerning hydraulic equivalence it is well known that sediment sorting according to size shape and density occurs but the exact mechanisms involved are poorly understood to assess the effects of size and density sand size spheres of two densities were transported and deposited under controlled flume conditions observations on the motion of discrete particles show that grains smaller than bed roughness grains move continuously and have the same transport velocities regardless of density for grains near and slightly larger than the roughness movement is intermittent and for a given size heavy particles move more slowly than lights for grains much larger than bed roughness grains movement is continuous over the rough surface and light and heavy grains have nearly the same transport velocities analyses of bulk sediment deposited from plane bed transport show that the size and proportion of heavies decreases and that of lights increases with distance transported for ripple bed transport however the size relations between associated light and heavy grains remains essentially unchanged with transport distance and the proportion of light and heavy grains is extremely variable these results suggest that size density sorting in plane bed transport is a function of the transportabilities identified in the discrete grain studies but that sorting in ripple bed transport is related to deposition on and recycling through the bed forms application of these findings to the concept of hydraulic equivalence implies that some indication of bed configuration may be necessary for the concept to be useful

grain size sediment transport regime and channel slope in alluvial rivers the general relationship between channel morphology and the grain size of sediment in the channel bed is an important but poorly known aspect of alluvial rivers an analysis of an equation for total sediment flux in the limits of suspension bedload and mixed modes of transport indicates distinct steady state regimes of channel morphology such regimes are readily seen in published data for modern alluvial rivers by way of a conventional shields plot or a plot of channel slope as a function of relative grain size d h and the ratio ws u where d and ws are respectively mean diameter and fall speed of bed sediments and h and u are respectively mean depth and friction velocity of the flow with slope and mode of transport in an alluvial river constrained by grain size and channel depth alone estimates of discharge and sediment flux follow directly introduction of the sediment flux relationship into conventional diffusion models for the evolution of an alluvial system provides nominal estimates of

bed material texture in low order streams on the queen charlotte islands british columbia low order channels comprise a large proportion of the links of every drainage basin and are often at the centre of land management concerns they exhibit hydrological and geomorphological characteristics atypical of higher order links this paper examines the nature and causes of variations in the bed material texture of two streams on the queen charlotte islands british columbia the extant functional exponential model is found to be inadequate for explaining observed changes in grain size parameters with distance downstream recurrent disruption of sediment transport by large organic debris jams and the sporadic contamination of the fluvial sediment population by colluvial inputs preclude the development of longitudinal structure rather grain size varies erratically over short distances a stochastic model best describes the observed variations and should be adopted as an alternative to the exponential model in low order links characteristic variances are controlled by the degree of hillslope channel coupling and the extent and characteristics of non alluvial storage mechanisms

suspended sediment and bed load problems of the upper rhine summary the regulation of the southern upper rhine by dams down to iffezheim has considerably changed the sediment balance of the stream in the impounded sections fine grained sediments are deposited narrowing the cross section and thus impeding high water discharge concluding from the high illite content of the suspended material a considerable part of the settling sediment is derived from the waste waters of the alsatian potassium mining in the freely flowing rhine downstream of iffezheim in turn erosion occurs although the degradation of the riverbed downstream of the iffezheim dam is prevented successfully by artificial addition of gravel this has no positive influence on the erosion reaches farther northwards this becomes apparent both in the petrographic composition of the bed load material as well as in its particle size distribution medium grained to coarse gravel with high carbonate rock content are already deposited upstream of speyer downstream of speyer sand and fine gravel rich in crystalline components are taken up from the river bottom this situation might probably be improved by appropriate addition of sand and fine gravel downstream of iffezheim

sediment supply limited bedforms in sand gravel bed rivers the stability of bedforms in mixtures of gravel and sand is not well understood two bedform types are characteristic flow parallel sand ribbons and flow transverse barchans flume experi ments and field data presented here show that gradual transitions exist from sand ribbons to barchans and from barchans to fully de veloped dunes barchans and sand ribbons occur when not enough transportable sediment is available for the formation of fully devel oped ripples or dunes the reason is that a part of the bed sediment is immobile e g with an armor layer which limits the sediment sup ply and thus the volume of sediment available for the formation of bedforms bedform stability diagrams are shown to be extendable to sedi ment supply limited bedforms in sand gravel sediment if the par ticle parameters of the diagrams are derived from the transported sediment instead of the bed sediment barchans and forms transi tional to fully developed dunes plot in the dune stability fields sand ribbons on the other hand plot in the ripple lower plane bed and dune fields in the case of sediment supply limitation bedforms are partly or completely related to the characteristics of the sediment supply from upstream the sediment underlying the bedforms may be a stable armor and the exchange of sediment between this armor and the bedforms may be small or non existent consequently bedform char acteristics in sand gravel mixtures in supply limited conditions often are not predictable from the local hydraulics and sediment charac teristics

experiments on downstream fining of gravel ii wide and sandy runs this is the second of two papers about experiments on downstream fining of gravel here we briefly summarize the complete series of runs described in the first paper where the influence of sediment feed rate was evaluated in a narrow channel we then present results for variation in two more conditions sand content and channel width the experiments were carried out in a long flume with an effective 40 m long test section with two different widths of 0 3 and 2 7 m in addition two sediment mixtures were used with different sand contents i e 33 and 55 with the remainder being gravel the following results were obtained the increase in channel width allowed the formation of transverse topography enhanced the development of sediment patches and increased the fining rate of gravel for id d5 d0 and finer percentiles increasing sand content rendered the long profile of the bed much more concave caused marked patch development and most notably increased the fining rate for all percentiles of gravel sized bed material

routing of heterogeneous sediments over movable bed model development a one dimensional numerical model of sediment routing is derived to simulate erosion transport and deposition of individual size density fractions in the bed material within a relatively straight nonbifurcating alluvial channel the reach of interest is subdivided into a number of longitudinal elements of varying width averaged properties during each time step flow depths and velocities in each element are determined from the gradually varied flow equation using the standard step method for backwater calculations the bedload transport rate of each size fraction is calculated from a modified bagnold equation implementing a novel approach that takes into consideration the effects of turbulent fluctuations in the bed shear stress critical shear stresses for entrainment of particles from the mixed bed are determined using relationships that treat grain protrusion and hiding the transport of particles in suspension is modeled using a convection diffusion sediment continuity equation either explicitly

causes of concavity in longitudinal profiles of rivers a smooth concave configuration represents the most common quasi equilibrium profile of alluvial rivers in the present paper a generalized unified set of models for the longitudinal profile of rivers is presented various independent effects that drive concavity in longitudinal rivers are considered and classified the driving mechanisms considered here are 1 horizontal wavelike progradation of the river profile 2 abrasion of bed particles 3 aggradation of the river profile balancing subsidence at a constant speed and 4 the effect of tributaries adding sediment and water to the main stem of the river for simplicity it is assumed that the long term supply of water and sediment is constant hence a single discharge that is bank full discharge is used as an approximate characterization of the hydrologic regime for each point along the river sediment sorting has been studied elsewhere and is excluded from this analysis the river is assumed to have uniform bed material although the mean size may vary downstream it is concluded that the quasi equilibrium longitudinal profile created by each of the first three driving mechanisms mentioned above is concave the degree of concavity varies however depending on a set of physical conditions that can be identified by a set of dimensionless numbers a comparison of these numbers reveals the relative importance of the driving mechanisms for concavity a condition for concavity driven by the fourth mechanism that is tributary input is delineated

stabilizing self organized structures in gravel bed stream channels field and experimental observations stable reticulate structures which we call stone cells have been observed in cobble gravel channel beds with low bed material transport rates experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the shields threshold by more than 2 times so that bed material transport is low they are constructed by particles moving from less stable positions into more stable configurations against each other intermediate developments include clusters and stone lines they reduce sediment transport by orders of magnitude and are evidently a major stability promoting mechanism in gravel channels the timescale for their development suggests that the boundaries of many gravel bed channels are not in equilibrium with recent competent flows but reflect the history of recent dominant flows

do gravel bed river size distributions record channel network structure 1 bed load sediment particles supplied to channels by hillslopes are reduced in size by abrasion during downstream transport the branching structure of the channel network creates a distribution of downstream travel distances to a given reach of river and thus may strongly influence the grain size distribution of the long term bed load flux through that reach here we investigate this hypothesis using mass conservation and the sternberg exponential decay equation for particle abrasion to predict bed material variability at multiple scales for both natural and artificial drainage networks we assume that over a sufficiently long timescale no net deposition occurs and that grains less than 2 mm are swept away in suspension we find that abrasion during fluvial transport has a surprisingly small effect on the bed load sediment grain size distribution for the simple case of spatially uniform supply of poorly sorted hillslope sediments this occurs because at any point in the channel network local resupply offsets the size reduction of material transported from upstream thus river bed material may essentially mirror the coarse component of the size distribution of hillslope sediment supply furthermore there is a predictable distance downstream at which the bed load grain size distribution reaches a steady state in the absence of net deposition due to selective transport large scale variability in bed material such as downstream fining must then be due primarily to spatial gradients in hillslope sediment production and transport characteristics a second key finding is that average bed load flux will tend to stabilize at a constant value independent of upstream drainage area once the rate of silt production by bed load abrasion per unit travel distance is equal to the rate of coarse sediment supply per unit channel length q bed load flux equilibrates over a distance that scales with the inverse of the fining coefficient in the abrasion rate law and can be approximated simply as q 3 thus the efficiency of particle abrasion sets a fundamental length scale shorter for weaker rocks and longer for harder rocks which controls the expression in the river bed of variability in sediment supply we explore the role of the abrasion length scale in modulating the influence of sediment supply variability in a number of channel network contexts including individual tributary junctions a sequence of tributary inputs along a main stem channel and variable basin shapes and network architecture as expressed by the width function these findings highlight the need for both data and theory that can be used to predict the grain size distributions supplied to channels by hillslopes

field evidence for rapid downstream fining of river gravels through selective transport doubts have been expressed about the ability of either abrasion or sorting to explain strong downstream fining of river gravels we describe pronounced fining over a short distance in a scottish river that has no human disturbance or lateral input of water and sediment measured abrasion rates are far too small to explain observed downstream fining but bed load trap measurements and the dispersion of magnetic tracer pebbles in six subreaches both show sorting during transport the observed downstream fining is also simulated well by a numerical sediment routing model using hydraulic and transport laws consistent with our field measurements the geomorphological cause of the strong fining is slope reduction above a local base level control in this common situation the development of downstream fining is part of the river9s tendency to minimize downstream variation in bed load transport rates and can proceed far more rapidly than the major aggradation otherwise required for equilibration

present day downcutting of the raba river channel western carpathians poland and its environmental effects abstract timing and causes of the present day channel downcutting of carpathian tributaries to the vistula river are exemplified by a case study of the raba river a gravel bed stream characterized by great variability of water stage and discharge up to 3 m of river incision has occurred since the beginning of the present century associated with an increase in stream power caused by river control works and reduction in bed material load linked to changes in basin management gravel extraction from the channel and modifications in flood flows have increased rates of incision over the last thirty years lowering of ground water levels on the valley floor increase in river bank susceptibility to erosion decrease or cessation of overbank sediment accretion and impoverishment of plant and animal communities of riverside biotopes are the principal environmental effects of channel downcutting

experimental study of abrasion and its role in producing downstream fining in gravel bed rivers abstract previous experimental studies on abrasion of gravel showed diminution rates much lower than those typical of gravel bed rivers this might be one reason many studies of downstream fining in rivers stress hydraulic sorting as the dominant fining mechanism previous experiments however did not replicate well the particle to particle impact between the bed load gravels and bed gravels in rivers on alluvial fans during floods the purpose of this study was to reexamine the effect of abrasion on downstream fining to determine if abrasion is responsible for longitudinal changes in gravel size in rivers on alluvial fans a rotating drum with three interior vanes was used to produce repeated collisions among test gravel particles in water the drum was only partially filled with water in order to simulate the impact velocity of particle collisions during a flood the test particles were slightly weathered andesite and chert taken from the bed of the watarase river japan experiments were conducted using three unisize grains l 7 0 128 mm to 6 5 91 mm m 5 5 45 mm to 5 0 32 mm s 4 0 16 mm to 3 5 11 mm and mixtures of two different sizes from the three size classes there are six principal results from the experiments 1 test gravel particles break frequently and decrease in weight rapidly 2 abrasion of chert produces mostly gravel while andesite produces mostly sand and silt 3 l size chert cobbles decrease in weight rapidly as a result of being broken into smaller pieces while andesite cobbles break so rarely that their weight decreases very slowly 4 s size andesite pebbles decrease in weight more rapidly than chert 5 abrasion depends strongly on the presence of different sizes because smaller fragments are crushed by larger gravel particles 6 diminution coefficients of andesite and chert obtained from the experiment are in the range of 10 3 10 1 km 1 diminution coefficients from this experiment are one to two orders of magnitude larger than those from previous experiments on abrasion this is mainly because of the larger impact velocities in this experiment diminution coefficients from the experiment are also consistent with those obtained from many japanese rivers on alluvial fans 10 2 10 1 km 1 this result suggests that downstream fining of river bed gravel on alluvial fans can be explained by abrasion

upstream sediment input effects on experimental dune trough scour in sediment mixtures 1 understanding causes of dune irregularity especially dune trough scour is important for the modeling of vertical sorting of sediment mixtures in morphological models of rivers with sediment mixtures sediment in dunes is generally sorted in a fining upward manner which affects the sediment transport composition depending on the scour depth distribution of the dunes why dunes become more irregular and develop deep scour holes in some conditions is only partially understood moreover there is a feedback between vertical sorting and dune irregularity in gravelly sands erosion resistant coarse layers may form that decrease or inhibit dune trough scour the causes of dune irregularity and the feedback by coarse sediment layers are explored in experiments and are demonstrated to be related partly to the upstream sediment boundary condition determined by the experimental setup sediment feeding or recirculating sediment recirculation flumes promote fining of the transported sediment and formation of a less or immobile gravel lag layer in the dune troughs sediment feed flumes may force the transported sediment at the flume entry to be equal to the bed sediment equal mobility condition and hence allow no lag layer formation experiments show that the dune trough scour in recirculation is less deep than in feed flumes and the vertical sorting and transport sediment composition are different the experiments also indicate that dune irregularity in feed flumes is related to dimensionless shear stress similarly as in uniform sediments interpretation of experiments of dune dynamics in sediment mixtures should therefore account for differences in the upstream sediment supply condition more in general it is hypothesized that the upstream sediment supply condition resembling either recirculation or feed flumes in rivers may affect dune and vertical sorting dynamics

defining phases of bedload transport using piecewise regression 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 data archive http dx doi org 10 2737 rds 2007 0003

a two fraction model for the transport of sand gravel mixtures 1 a mixed size sediment transport model using only two fractions sand and gravel offers practical advantages in terms of the required input and conceptual advantages by readily incorporating the nonlinear effects of sand content on transport rate a two fraction model is developed from flume and field transport observations sand and gravel transport rates are collapsed to a single transport function using a scaling parameter representing incipient motion for each fraction the incipient motion function is consistent with established values in the limit of pure sand and gravel beds and shows a sharp change with increasing sand content over the transition from a clast to matrix supported bed a two fraction model captures an important and previously unmodeled nonlinear effect of sand content on gravel transport rate and suggests important implications for a number of fluvial processes including the response of channels to sediment supply and the mechanism for abrupt sorting in gravel sand transitions

unequal mobility of gravel and sand in weakly bimodal river sediments bed load was trapped during flood events over a 20 month period at the lower end of the allt dubhaig a small river in scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile the channel bed near the trap is predominantly gravel with a secondary sand mode total transport in each event depended mainly on peak shear stress rather than duration over a threshold bed load was mainly sand in smaller events bimodal in intermediate events and mainly gravel in the biggest floods mean and maximum grain diameter both increased with peak shear stress but in different ways analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility the slight selectivity in gravel transport is consistent with previous field studies of near equilibrium unimodal beds and supports assumptions made in the numerical model of hoey and ferguson 1994 which successfully simulates the observed amount of downstream fining over the 2 5 km upstream of the bed load trap

downstream changes in the lithology and grain size of fluvial gravels the watarase river japan evidence of the role of abrasion in downstream fining two processes have been considered to explain downstream grain size reduction in gravel bed rivers hydraulic sorting by which finer grains are transported faster and farther than coarse grains and abrasion by which individual particles are diminished in size most studies on the cause of downstream fining in rivers emphasize hydraulic sorting as the dominant fining mechanism however broken boulders are often observed in the bed material of japanese rivers also grain size changes may be correlated with the difference in resistance to breakdown among lithologies the purpose of this study is to use the lithologic composition of bed material on the watarase river to demonstrate that some downstream fining cannot be explained by hydraulic sorting alone the study reach on the watarase river flows on a dissected alluvial fan which means that the river is no longer aggrading the study reach is influenced only slightly by input of gravel from tributaries bed gravel in the study reach consists of several lithologies andesite quartz porphyry sandstone hornfels and chert the grain size distribution and lithology of the river bed material were examined at nine sites there are two principal results 1 size distribution of the gravel is strongly related to lithology andesite boulders or large cobbles make up the framework sizes in the upstream part of the study reach while chert pebbles make up the framework sizes in the downstream part there are few andesite pebbles or chert boulders in the river bed 2 longitudinal changes in the composition of each size class show selective transportation by lithology in every gravel size because the mobility of gravel depends mainly on its size hydraulic sorting by lithology does not occur within the same size class these results indicate clearly that particle abrasion occurs in the watarase river and is responsible at least in part for the downstream decrease in particle size of bed material

initial motion and pivoting characteristics of sand particles in uniform and heterogeneous beds experiments and modelling experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady oscillatory flows derived critical shear stresses are compared with the mixed bed entrainment model of wiberg smith 1987 as predicted by the model coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively finer grains entrainment occurs generally at lower shear stresses than predicted by the model especially under unidirectional flows this may be the result of grains resting in unusually unstable positions during the experiments because the beds are unworked at the start of the experiments the model of wiberg and smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly defined the pivoting angles of the beds used here are measured using a new technique designed specifically for comparison with the threshold data the measured angles repeat the finding that the coarse grains are more mobile than the finer fractions of a mixture the results are poorly described by the pivoting angle model presented by wiberg smith 1987 and are better represented by a model of the form d di d50 after li komar 1986 where and are empirical constants the threshold model is found to be more effective using the improved pivoting relationship the entrainment of grains is found to be easier beneath unidirectional flows than combined flows in accordance with previous authors findings a suggestion that this result is caused by a change in the erosion mechanism beneath wave flows is made wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive efficiency of the overlying current flow the results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near threshold flows

downstream variation in grain size along an alpine river analysis of controls and processes downstream variation in bed material size was studied along a 115 km reach of the piave river eastern alps italy this river drains a region producing high sediment yields and has a braided pattern along half of its length during the last century the river system has been affected by several human impacts hydroelectric dams diversions gravel mining which have strongly altered flow regime and sediment dynamics the aim of the study is to analyse the changes of bed material size in the downstream direction and to explain these changes in the light of both natural and anthropogenic factors surface material was sampled at 35 sites using the grid by number method the collected data give a detailed picture of the downstream variation in sediment size which is characterised by a complex pattern rather than by a simple decreasing trend an explanation of this variation was found through identification of discontinuities significant steps in the downstream trend and factors which can determine changes in sediment texture both natural lateral sediment sources and anthropogenic factors e g barrages are significant in this river system and they provide an explanation for most of the observed discontinuities seven out of eight discontinuities few tributaries disrupt downstream fining while as expected the barrages produce important changes in sediment texture fining processes were investigated in the lower part of the study reach where the lateral sediment sources and the barrages have minor effects on the bed material exponential models provide acceptable explanations of the downstream variations and give fining coefficients equal to 0 027 and 0 028 km 1 respectively for d50 and d84 evidence suggests that both abrasion and sorting are important in determining the downstream fining observed but it is likely that sorting is the dominant process

equal mobility of gravel in streams the remains of the day 1 the concept of equal mobility of gravel transport is 2 decades old the weak form of the hypothesis states that in order to transport the coarse half of the mean annual gravel load through a river reach at a rate equal to the fine half the coarser material must be overrepresented on the bed surface giving rise to a mobile bed armor the degree of mobile bed armoring necessary to accomplish this diminishes with increasing bed shear stress the strong form of the hypothesis states that the grain size distribution of the gravel portion of the bed load should be similar to that of the substrate and finer than that of the surface layer here the hypothesis is subjected to a rigorous test in the laboratory under controlled conditions which are nevertheless chosen to be as representative as possible of actual field conditions the grain size distributions used in the experiment included both model gravel and model sand at the end of the experiment the entire surface layer and a significant depth of substrate were removed from an entire reach of the channel and sieved the results of the experiment confirmed the weak form of equal mobility the surface layer was significantly coarsened compared with the bed load even when the sand was excluded from the size distributions the strong form of equal mobility was also supported by the experiment in that the size distribution of the gravel substrate was nearly identical to the size distribution of the gravel portion of the load the gravel substrate was however slightly but measurably coarser than the gravel load a tendency that has been observed in the field in cases for which the strong form is not satisfied

overbank sand deposition in relation to transport volumes during large magnitude floods in the dutch sand bed rhine river system using aerial photographs and field measurements sandy overbank deposits formed by the large magnitude floods of 1993 94 and 1995 were quantified along two branches of the dutch rhine river system the waal 1993 94 and 1995 and the ijssel 1995 these deposits were laid down intermittently all along the length of these rivers on the top and landward slope of the natural levees and covered about 4 per cent of the embanked floodplain on the waal and about 1 per cent on the ijssel the overbanks and transport mechanism is basically convective by nature the spatial variability of overbank sedimentation points to the important role played by helicoidal currents in determining overbank deposition the presence of embankments and training works appears to influence the sand transport to and morphological development of the floodplains along the dutch rhine river system overbank deposition volumes about equal present estimates of sand transport during a large magnitude flood it appears that studies on sand transport in the dutch rhine carried out so far underestimate sand transport during floods 1998 john wiley sons ltd

selective grain entrainment by a current from a bed of mixed sizes a reanalysis the selective entrainment of grains by a current from a bed of mixed sizes and densities is important to grain sorting processes that lead to the formation of placers in sands and to armored beds in gravels existing field and laboratory measurements of selective entrainment depart systematically from the standard threshold curves such as that of shields which are based on experiments with nearly uniform grain sizes entrainment measurements from beds of mixed sizes form trends which obliquely cross the threshold curves for uniform grains the crossing point being roughly at the median diameter of the size distribution due to this crossover the coarser size fractions of the distribution require lower flow stresses for their entrainment than if they formed uniform beds while the finer size fractions require higher stresses than uniform beds with deposits having medians in the range of medium sand through gravel the larger the individual particle within the mixed sizes the greater the flow stress required for its entrainment in this case the smaller grains are preferentially entrained possibly leading to bed armoring in contrast if the median is in the fine to medium sand range the smallest grains in the bed of mixed sizes are the most difficult to entrain due to sheltering by the larger grains where grain density is also involved this sorting can lead to the formation of placers a variety of empirical relationships is considered to quantify the evaluation of selective entrainment from deposits of mixed sizes but no single formulation is satisfactory for all data sets

numerical modelling of floodplain hydraulics and suspended sediment transport and deposition a numerical model is presented for the prediction of floodplain inundation extents flow depths and velocities and patterns of suspended sediment dispersion and deposition the model employs a simplified and novel treatment of fluid flow and a conventional convection diffusion mass balance relationship for suspended sediment transport and deposition these relationships are solved over a finite difference grid with a relatively fine nodal spacing which enables a detailed representation of complex floodplain topography the model is applied to a 600 m reach of the river culm devon uk continuous records of river stage and suspended sediment concentration monitored at the upstream boundary of the study reach provide the data needed to operate the model these data are supplemented by measurements of the in situ settling characteristics of the suspended sediment load model predictions of overbank flow depths and velocities and patterns of suspended sediment concentrations and deposition amounts are presented predicted patterns of overbank deposition are described and compared with field measurements obtained with the aid of astroturf sedimentation traps grain size distributions obtained for samples of deposited sediment retrieved from the sedimentation traps are also examined and compared with model predictions the results of both the field measurement programme and modelling work are shown to have significant implications for the representation of floodplain topography and suspended sediment particle size and settling properties in numerical simulations of overbank processes 1998 john wiley sons ltd

critical discharge for entrainment of poorly sorted gravel entrainment criteria for gravel sized particles on river beds are usually defined in terms of shear stress but some workers have proposed that critical discharge relationships are more reliable and more convenient the critical shear stress in poorly sorted gravels is known from recent work to depend much more on relative than absolute grain size it is shown that this effect can be included in a semi theoretical equation for the critical unit discharge to move individual grain sizes in a poorly sorted bed comparisons with published data on critical discharges in a variety of rivers show that both the sensitivity of critical discharge to grain size and the absolute levels of critical discharge are predicted quite well however any prediction of critical discharge is sensitive to the assumptions made first about flow resistance and secondly about the critical stress to move average sized particles

incipient motion of sand gravel sediment mixtures experiments on the incipient motion of gravel and sand mixtures of 0 100 10 90 25 75 45 55 and 100 0 percent ratios of gravel and sand respectively were made in a laboratory flume a series of transport experiments was made for each of the five sediment beds and the critical shear stress for motion was calculated for 12 size ranges by interpolating or extrapolating the bed shear stress for a very small transport rate the sand in each of the five bed mixtures began to move at nearly the same bed shear stress all gravel sizes began to move at nearly the same bed shear stress in the 100 gravel experiments but in the sand gravel experiments the gravel showed an increase in critical shear stress with increasing size reasons for this change in the initiation of motion of the gravel from the 100 gravel to the sand gravel mixtures may result from the abundant sand sized sediment inhibiting the formation of a coarse bed surface layer in the sand gravel experiments

threshold for particle entrainment into suspension laboratory observations regarding the limit conditions for particle entrainment into suspension are presented a high speed video system was used to investigate conditions for the entrainment of sediment particles and glass beads lying over a smooth boundary as well as over a rough bed the results extend experimental conditions of previous studies towards finer particle sizes a criterion for the limit of entrainment into suspension is proposed which is a function of the ratio between the flow shear velocity and particle settling velocity observations indicate that particles totally immersed within the viscous sublayer can be entrained into suspension by the flow which contradicts the conclusions of previous researchers a theoretical analysis of the entrainment process within the viscous sublayer based on force balance considerations is used to show that this phenomenon is related to turbulent flow events of high instantaneous values of the reynolds stress in agreement with previous observations in the case of experiments with a rough bed a hiding effect was observed which tends to preclude the entrainment of particles finer than the roughness elements this implies that as the ratio between particle and roughness element sizes becomes smaller progressively higher bed shear stresses are required to entrain particles into suspension on the other hand an overexposure effect was also observed which indicates that a particle moving on a smooth bed is more prone to be entrained than the same particle moving on a bed formed by identical particles

the key role of fluvial dunes in transport and deposition of sand gravel mixtures a preliminary note abstract sediment transport and deposition in sand gravel bed rivers is shown to depend on dunes the sediment mixture is vertically sorted in avalanches at the lee side of the dunes part of the resulting upward fining sets of the largest dunes that occur during a discharge wave is preserved in the bed furthermore the sediment is entrained and deposited size selectively in the dune troughs which results in an additional upward fining accumulation of lag deposits this deposit is the source for sediment entrained during the next discharge wave which will depend on the relict vertical sorting and on the depth from which it is entrained the entrainment and deposition depth of the sediment depends on the dune trough level below the average bed level and therefore on the dune height thus subsequent discharge waves of decreasing magnitude will leave the upward fining cross bedded sets at depths below the bed surface related to the concurrent dune height this is demonstrated with flume experiments and with new vibrocores collected from the river waal the netherlands

channel sediment variability along a river a case study of the siret river romania the siret river has the largest drainage basin 42 274 km2 in romania it gathers all the rivers from the eastern part of the eastern carpathians a fact that causes marked asymmetry of the basin this study is principally concerned with changes in the form of the longitudinal profile and the grain size variability introduced by the carpathian tributaries channel sediment analyses considered the petrography granulometry and morphometry of the pebbles relating these to the river bed and floodplain geometry and to some properties of the drainage basin the following conclusions arise the siret river undergoes an intense regrading of its longitudinal profile with marked aggradation between transects 24 and 26 see figures 1 and 2 this reflects selective accumulation of coarse material due to the massive contribution of the carpathian tributaries this phenomenon has been continuous throughout the holocene resulting in the gravel sheet formation of the pericarpathian piedmont

grain size sorting in grainflows at the lee side of deltas the sorting of sediment mixtures at the lee slope of deltas at the angle of repose is studied with experiments in a narrow deep flume with subaqueous gilbert type deltas using varied flow conditions and different sediment mixtures sediment deposition and sorting on the lee slope of the delta is the result of i grains falling from suspension that is initiated at the top of the delta ii kinematic sieving on the lee slope iii grainflows in which protruding large grains are dragged downslope by subsequent grainflows the result is a fining upward vertical sorting in the delta systematic variations in the trend depend on the delta height the migration celerity of the delta front and the flow conditions above the delta top the dependence on delta height and migration celerity is explained by the sorting processes in the grainflows and the dependence on flow conditions above the delta top is explained by suspension of fine sediment and settling on the lee side and toe of the delta large differences in sorting trends were found between various sediment mixtures the relevance of these results with respect to sorting in dunes and bars in rivers and laboratory flumes is discussed and the elements for a future vertical sorting model are suggested

critical shear stress of natural sediments the critical shear stress of individual fractions ci in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture for strongly bimodal sediments ci increases with grain size an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes ci to deviate from size independence in the direction of unisize shields values a quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of ci is observed and to predict the variation of ci with grain size in bimodal sediments because properties of sediment grain size distributions other than bimodality appear to have little influence on ci the trends presented here for 14 unimodal and bimodal sediments may be quite general

longitudinal profiles in simple alluvial systems theoretical considerations suggest that exponential or quadratic functions are apt for describing the longitudinal profiles of aggrading alluvial systems that are unaffected by significant lateral inputs of water or sediment a new set of field data confirms this for individual sedimentary links along a wandering gravel bed river in british columbia canada each link is viewed as a simple alluvial system without major water or sediment inputs within which grain size typically fines downstream in a relatively systematic manner such homogeneous reaches are suitable for the investigation of simple profile form it is found that quadratic approximations are the most flexible descriptor for link longitudinal profiles overall the river forms an irregularly cuspate long profile structured by these fundamental length scale units both link gradients and link fining rate reflect size selective aggradation associated with the bounding lateral inputs

the missing dimension effects of lateral variation on 1 d calculations of fluvial bedload transport most calculations of bedload transport in rivers including those in numerical models of aggradation and degradation are 1 d all hydraulic and transport rate calculations are averaged over the channel width because bedload transport laws are nonlinear width averaged calculations will underestimate the true bedload flux if there is any local spatial variation in either the bed or the flow this paper analyses the effects on bedload transport capacity of spatial variation in applied and critical c shear stress separately and in combination a simple but versatile statistical model is used to represent variability in with allowance for differences between sand and gravel bed rivers and for below bankfull flow bedload flux is shown to increase greatly with the variance of especially in gravel bed rivers variability in c through bed patchiness may increase reduce or make little difference to bedload flux depending on the correlation between and c simple width averaging leads to severe underestimation of bedload transport in most conditions some alternatives are considered the findings have implications for sediment routing models srms but further research is needed to explore the issue fully

grain size along two gravel bed rivers statistical variation spatial pattern and sedimentary links a new set of field data facilitates a detailed analysis of variations in bed material grain size within two confluent gravel bed rivers in northeastern british columbia canada a preliminary assessment of grain size variability establishes a basis for examination of the spatial pattern of grain size change standard anova techniques are inappropriate because individual samples have unequal variances and are not normally distributed alternative tests for homoscedasticity and comparison of means are therefore utilized within site between sample variability is not significant the grain size distributions that were obtained at individual sites are therefore representative of the depositional environments that were sampled in both rivers mean grain size does vary significantly between sites and there is therefore a basis for examining the data for spatial patterns such as downstream fining textural variations along the two rivers studied here are complex and show negligible overall fining in over 100 km this is the consequence of a large number of tributary inputs and non alluvial sediment sources which are the legacy of late pleistocene glaciation the identification of lateral sources like these is fundamental for understanding textural changes within rivers the sedimentary link a channel reach between significant lateral sediment inputs provides a means of isolating fluvial maturation processes abrasion and sorting from contingent lateral inputs strong fining trends are apparent in most links and classification of grain size measurements according to their location within particular links greatly improves the statistical explanation of textural variation identification of sedimentary links provides a means of applying models of fluvial fining processes so isolation of link networks will aid the development of basin scale models of textural variation 1998 john wiley sons ltd

critical shear stress for incipient motion of sand gravel streambeds results of an experimental study of the incipient motion of streambeds composed of sand gravel sediment mixtures are reported and compared with the earlier findings for uniform sediments the experiments were conducted in an 8 m long by 0 30 m wide glass walled tilting flume and an 18 m long by 0 80 1 10 m wide trapezoidal concrete channel a reference transport method is used to define the beginning of bed material movement the experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size intergranular effects mixture standard deviation effect of the shape of grain size distribution absolute value of median size absolute size effect and bed slope effect of relative depth on overall flow resistance the shear stress at incipient motion of median sized grains in mixtures is found to be the same as for uniform sediment of this size the present findings are consistent with available flume and field data a technique for calculating the critical shear stress of different grain sizes in coarse uniform sediments and unimodal weakly bimodal sediment mixtures is proposed

river adjustment to changes in sediment load the effects of tin mining on the ringarooma river tasmania 1875 1984 the mining of alluvial tin in the ringarooma basin began in 1875 reached a peak in 1900 20 and had virtually ceased by 1982 during that time 40 million m3 of mining waste were supplied to the main river quickly replacing the natural bed material and requiring major adjustments to the channel based on estimates of sediment supply from more than 50 widely scattered mines and the frequency of flows capable of transporting the introduced load the river s transport history is reconstructed using a mass conservation model because of the lengthy time period 110 years and river distance 75 km involved the model cannot predict detailed change but it does reproduce the main pattern of sediment movement in which successive phases of aggradation and degradation progress downstream peak storage is predicted in that part of the river where braiding and anastomosis are best developed aggradation was most rapid in the upper reaches close to major supply points becoming slower and later with distance downstream channel width increased by up to 300 per cent where the valley floor was broad and braiding became relatively common bridges had frequently to be replaced while bed levels were still rising in lower reaches degradation began in upper ones notably after 1950 and by 1984 had progressed downriver over 30 km rates of incision reached 0 5 m yr 1 especially in the early 1970s when record high flows occurred as a result of degradation the bed material became gravelly through either reexposure of the original bed or lag concentration of coarser fractions also a narrower unbraided channel has developed the river is beginning to heal itself and upper reaches now have reasonably stable beds but at least another 50 years will be required for the river to cleanse its channel of mining debris

partial transport of a sand gravel sediment grains of a single size within a mixed size bed are entrained over a range of flows within this range some grains exposed on the bed surface are active entrained at least once over the duration of a transport event while the remaining surface grains are immobile a condition we define as partial transport we demonstrate the existence and domain of partial transport using observations of grain entrainment on time series of bed photographs of flume experiments with a widely sorted sand gravel mixture the active proportion of the bed surface increases with bed shear stress 0 at a given 0 90 of the active grains are entrained when the cumulative mass transported exceeds approximately 4 times the active mass on the bed mobilization of grains in a size fraction increases from 10 to 90 over a range of 0 of a factor of 2 the bounds of this range increase with grain size di so that at a given 0 sizes over a range of a factor of 4 are in a state of partial transport fractional transport rates are independent of di for fully mobilized fractions and decrease rapidly with di for partially mobile fractions partial transport is associated with substantial transport rates of finer fully mobile sizes limits both the rate and size distribution of grain exchange with the bed subsurface and may be the dominant transport condition in many gravel bed rivers

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

a worldwide correlation for exponential bed particle size variation in subaerial aqueous flows the particle size of the bed sediments in or on many natural streams alluvial fans laboratory flumes irrigation canals and mine waste deltas varies exponentially with distance along the stream a plot of the available worldwide exponential bed particle size diminution coefficient data against stream length is presented which shows that all the data lie within a single narrow band extending over virtually the whole range of stream lengths and bed sediment particle sizes found on earth this correlation applies to both natural and artificial flows with both sand and gravel beds irrespective of either the solids concentration or whether normal or reverse sorting occurs this strongly suggests that there are common mechanisms underlying the exponential diminution of bed particles in subaerial aqueous flows of all kinds thus existing models of sorting and abrasion applicable to some such flows may be applicable to others a comparison of exponential laboratory abrasion and field diminution coefficients suggests that abrasion is unlikely to be significant in gravel and sand bed streams shorter than about 10 km to 100 km and about 500 km respectively copyright c 1999 john wiley sons ltd

surface based fractional transport rates mobilization thresholds and partial transport of a sand gravel sediment twenty eight coupled observations of flow transport and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted bimodal grain size distribution these observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface the key to our observations is the use of a sediment in which each size fraction has been painted a different color which permits reliable repeatable and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface at a given flow the fractional transport rates may be divided into two parts a finer grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate and a coarser grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run we define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion of that fraction zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude a result strongly in contrast to many earlier observations but consistent with our observations of incipient motion in sediments with bimodal grain size distributions

downstream variation in stream power abstract stream power has a significant influence on many form and process attributes of the fluvial system using a similar approach to that suggested by d m lawler in 1992 and 1995 a model is developed to explore how stream power might vary in the downstream direction based on the assumption that the longitudinal profile of the river has an exponential form no particular discharge is necessarily specified but near bankfull flows are regarded as significant morphologically and one such discharge is the focus here total stream power per unit channel length qs is predicted to peak at an intermediate location the position of which depends on the ratio b where b and are the downstream rates of change of discharge and slope respectively if b is a relatively conservative quantity then the form of the longitudinal profile and in particular its degree of concavity will have the major influence on the position of maximum stream power specific stream power w is even more sensitive to and is expected to peak closer to the headwaters about half way between source and the location of the stream power maximum although this will depend on how channel width w varies with distance from source the model is tested along three connected streams of different size in the catchment of the river trent taking advantage of a relationship between mean annual flood and link magnitude which enables this near bankfull discharge to be estimated at any point within the network the model is verified in so far as stream power does attain a mid basin maximum but the observed and predicted locations of the maxima do not always correspond largely because of the distinctive variation in discharge and especially slope along individual pathways for example maximum power lies far downstream along the trent because of the large increase in mid basin discharge associated with a series of large closely spaced tributaries the importance of slope is borne out by the near headwater locations for specific power maxima which tend to be further upstream than predicted along all three rivers as a result of the steepness of upper reaches the variety of geomorphic settings produces deviations from expectation which has implications for the movement and storage of material in the fluvial system

selective sorting and abrasion of river gravel i theory most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fining is due to some combination of selective sorting by which finer grains are preferentially transported downstream and abrasion by which individual particles are reduced in size here a framework for the simultaneous treatment of both phenomena is developed the analysis is restricted to bed load transport of gravel only abrasion due to binary collisions between moving bed load and stationary bed particles is considered the formulation includes the possibility of differential basin uplift and subsidence due to tectonism and mixtures of rock types with differing coefficients of abrasion the formulation of selective sorting is based on the concept of hiding as applied to a surface based bed load transport relation abrasion is first treated in terms of a lagrangian formulation for individual grains and t

slope induced changes in channel character along a gravel bed stream the allt dubhaig scotland changes in channel character along a small river in the scottish highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0 02 to o ooo15 because of local baselevel control this decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed the channel pattern alters progressively rather than abruptly in the sequence 1 near braided 2 meandering with active point bar chutes 3 meandering with active outer bank talweg 4 stable equiwidth sinuous the changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical cdncepts than by purely empirical discriminant or trend equations measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload it is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions are typical of many streams in mountainous areas and can be understood in terms of slope induced changes in hydraulic properties

change in the mathematical function type describing the longitudinal profile of a river through an evolutionary process japanese rivers where a large amount of sediment accumulates along the middle reaches are today not at grade nevertheless most of their longitudinal profiles can be described by one of the mathematical functions proposed for graded rivers for these rivers the mathematical function should show an aspect of fluvial processes different from the explanations of grade theories the geomorphological significance of differences in the mathematical functions describing the longitudinal profiles of rivers is discussed here a dynamic change in the shape of longitudinal profile of a river accompanying the change in mathematical function type is proposed in relation to changes in fluvial processes and stage of evolution of a river most japanese rivers can be described by either exponential or power functions the fluvial processes of the rivers described by exponential functions are in the depositional state the front of depositional area of gravels increases in altitude and migrates downstream the rivers exp

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

bedload transport in gravel bed streams field data obtained from oak creek are used to study the bedload transport in gravelbed streams a similarity approach is used to delineate a functional relationship obtained for the bedload transp

on why gravel bed streams are paved bedload transport in poorly sorted gravel bed streams downstream of dams is considered bedload and typical bed material subpavement size distributions are observed to be similar it follows that the coarse half of the subpavement moves through a reach at a rate near that of the fine half since coarser grains are intrinsically less mobile than fine grains it follows that some mechanism must act to nearly equalize mobility it is hypothesized that the pavement seen in gravel bed streams at low flow is in fact in place during typical transport events capable of moving all available sizes this pavement can provide the equalizing mechanism by exposing proportionally more coarse grains to the flow field data are used to quantify this concept and to develop a predictive relation for river pavement the model indicates that pavement should be absent in most sand bed streams in agreement with observation

the nature and controls on downstream fining within sedimentary links abstract at scales of 100 to 102 km grain size change in gravelly alluvial systems is characterized by punctuated downstream fining significant lateral sediment inputs define a sequence of sedimentary links within which fluvial processes produce fining trends using a new set of surface grain size data eighteen such links along two canadian gravel bed rivers are examined negative exponential functions are found to be most appropriate for describing within link grain size changes development of a pragmatic means of predicting grain size is then pursued by searching for a means to relate model diminution coefficients to link attributes consideration of lithologically differentiated fining trends indicates that lithology plays an insignificant role in controlling fining rate examination of within link longitudinal profiles indicates a reasonable relation between fining rate and the rate of change of channel slope the predictive value of this relation is limited by the regional nature of the data upon which it is based and the relative difficulty of obtaining accurate gradient information however as previously noted for alluvial fans although for different reasons channel gradient is correlated with link length and a reasonable predictive tool is apparent in a relation between link length and diminution rate the validity of the derived relation cannot be tested for want of a suitable data set several lines of evidence indicate that abrasion is unimportant as a fining mechanism on these rivers and it is argued that this may be typical of sedimentary links fining rate must then be a function of the efficiency with which sorting processes operate and it is suggested that this efficiency is dependent on aggradation rates associated with the lateral sediment influxes at the head and terminus of individual links

which tributaries disrupt downstream fining along gravel bed rivers tributaries and other lateral sediment sources can have a significant impact on river bed sediment texture and in turn on channel form sufficiently voluminous or distinct sediment inputs redefine the mainstem grain size distribution punctuating downstream maturation and isolating a sequence of discrete sedimentary links within these links fining processes usually dominate such that models of sorting and abrasion when applied to individual links provide reasonable predictions of grain size change links represent the fundamental natural unit within which fining models can be tested developed and applied identification of significant lateral sources is therefore important yet beyond vague references to relative tributary size sediment load and sediment calibre no criteria exist for the a priori discrimination of such sources in this paper a procedure for identifying significant lateral tributary sources without the benefit of grain size information is outlined a high resolution characterisation of bed material texture along two canadian gravel bed rivers facilitated classification of all their perennial tributaries as either significant or insignificant three absolute tributary basin parameters and their relative counterparts chosen to reflect the likely controls on tributary significance are then used to develop a discriminant function which isolates a large proportion of significant tributaries while minimising incorrect classifications examination of consistently misclassified anomalous tributaries reveals the importance of lateral source spacing and of inconsistencies in the geomorphic history of the contributing basins in turn a general tributary categorisation procedure is suggested which includes a logistic regression model for attaching probability statements to individual classifications the generality of the discriminant and logistic functions cannot be assessed because of the lack of other suitable data sets

implication of relationships among suspended sediment size water discharge and suspended sediment concentration the yellow river basin china according to the yearly maximum suspended sediment concentrations ssc in the yellow river and its tributaries the rivers are divided into three types of more than 300 20 300 and less than 20 kg m 3 the middle yellow river basin is located in the transitional zone from subhumid to semiarid climates and covered by a thick loess mantle neighboring on the desert areas to the northwest the surface material of the loess plateau exhibits some marked areal differentiation in grain size and forms three zones covered by sandy loess typical loess and clayey loess from northwest to southeast controlled by these physico geographical conditions the grain size of river transported sediment shows some particular characteristics at small water discharge or ssc the grain size of suspended sediment abruptly decreases to a minimum with increasing water discharge or ssc at water discharge of more than ca 40 m 3 s or at ssc of more than ca 40 kg m 3 the grain size increases with water discharge or ssc during the low stage season the relatively clear baseflow may scour the coarse bed materials so the suspended sediment is relatively coarse in the rainy season rainstorm runoff washes out fine loess materials to the river making suspended sediment fine during relatively strong rainstorms there often occur hyperconcentrated flows at ssc of more than 300 kg m 3 the relatively coarse grains could then remain suspended in the mixture of water and fine suspended sediment c 2002 elsevier science b v all rights reserved

crustal exhumation during ongoing compression in the variscan maures tanneron massif france geological and thermo mechanical aspects abstract the maures tanneron massif mtm together with corsica and sardinia represent the southeastern branch of the huge european variscan belt a continuous evolution from continental collision to exhumation is described from ca 350 ma to ca 320 ma based on an extended compilation of available geological and geochronological data this geological compilation is complemented with thermo mechanical modelling that tests the conditions under which deep seated hp units can melt and exhume massively during continued convergence we simulate in two dimensions the development of gravitational instabilities in a partially melting crust depending on rheological layering and heat sources and accounting for temperature dependent elasto visco plastic rheologies in order to reproduce pt exhumation patterns in the mtm over the appropriate time scale 20 my and spatial extent 100 km a best fit was obtained with a convergence rate of 0 5 cm yr and moderate diffusive surface processes the contribution of radiogenic heating in the pre stacked felsic crustal units is crucial a lateral alternation with mafic units is also required in order to prevent lateral spreading of the orogen a low viscosity partially molten crust 1020 pa s thus rather felsic accompanies crustal exhumation in a decoupled mode from the deeper mantle a shallow asthenosphere below the orogen lab at 70 km depth e g shallow slab break off produces too warm and sudden exhumation as opposed to a competent mantle lithosphere 120 km depth lab that restrains it the mtm witnesses the typical competition between far field plate convergence and internal body forces and our study pleads for a progressive evolution of transpression towards perpendicular extension from ca 320 ma

sediment characteristics and transportation dynamics of the ganga river abstract understanding of river systems that have experienced various forcing mechanisms such as climate tectonics sea level fluctuations and their linkages is a major concern for fluvial scientists the 2525 km long ganga river derives its fluvial flux from northern part of the indian subcontinent and drops in the ganga brahmaputra delta and the bengal fan regions this paper presents a study of the ganga river sediments for their textural properties grainsize characteristics and transportation dynamics a suite of recently deposited sediments 189 bedload samples and 27 suspended load samples of the river and its tributaries was collected from 63 locations dry and wet sieve methods of grainsize analysis were performed and folk and ward s parameters were calculated transportation dynamics of the sediment load was assessed by means of channel hydrology flow sediment rating curves bedform mechanics grainsize images and cumulative curves textural properties of the bedload sediments of the ganga river tributaries originating from the himalaya orogenic belt the northern indian craton and the ganga alluvial plain regions are characterised by the predominance of fine to very fine sand medium to fine sand and very fine sand to clay respectively downstream textural variations in the bedload and suspended load sediments of the ganga river are therefore complex and are strongly influenced by lateral sediment inputs by the tributaries and channel slope at the base of the himalaya a very sharp gravel sand transition is present in which median grainsize of bedload sediments decreases from over 0 16 to 2 46 within a distance of 35 km downstream decline in mean grainsize of bedload sediments in the upper ganga river within the alluvial plain can be expressed by an exponential formula as mean grainsize in 0 0024 distance in kilometres from the himalayan front 1 29 it is a result of selective transport phenomena rather than of abrasion the condition attributed to channel competence with low gradient of 70 to 25 cm km coarsening of the bedload sediments in the middle segment of the river develops due to the coarse bedload sediments mean grainsize 1 8 2 2 supply from the tributaries of the northern indian craton region and increased unit stream power the ganga river sediments show a wide range of grainsize parameters bedload sediments mean grainsize 1 5 3 8 standard deviation 0 3 0 7 skewness 0 2 0 5 and kurtosis 0 8 2 4 suspended load sediments mean grainsize 2 9 5 0 standard deviation 0 6 1 6 skewness 0 6 0 4 and kurtosis 0 7 4 6 mean grainsize of the bedload sediments is 2 6 which is 1 3 units coarser than the suspended load sediments the cumulative grainsize curves of the bedload sediments show a distinct break in slope representing nearly 80 of the bedload sediment transported as graded suspension during the peak monsoon discharge most of the bedload sediments are transported as the discontinuous fine sandy graded suspension by the unidirectional unsteady shooting flow at a high transport rate about 8 106 tons d a model of sedimentation transportation at river channel is proposed the ganga river mobilises a total of 729 106 tons of sediments annually through a narrow zone within its river valley under the present hydrogeological conditions the river sedimentation is climatically controlled and is predicted to produce a 2000 km long 2 to 40 km wide and 25 to 50 m thick ribbon shaped well sorted symmetrically skewed fine sand body

the gravel sand transition in a disturbed catchment abstract more than 40 million cubic metres of mining waste were supplied to the ringarooma river between 1875 and 1984 leading to successive phases of aggradation and degradation the natural bed material is gravel but given the volume of introduced load and the fact that much of the input was less than 5 mm in diameter the size composition of the bed changed from gravel to sand during the phase of downstream progressive aggradation a very sharp gravel sand transition developed in which median grain size decreased from over 30 mm to under 3 mm in less than 500 m with upstream supplies of mining debris becoming depleted first degradation followed the same downstream progressive pattern as aggradation causing the transition to migrate downstream by 1984 the river could be regarded as a series of zones each characterized by a particular bed condition a natural cobble gravel bed unaffected by mining inputs 0 32 km pre disturbance bed re exposed by degradation over 35 40 years 32 53 km sandy substrate with a gravel armour produced by differential transport during degradation 53 65 km sand dominated but with developing surface patches of coarser material 65 75 km sandy bed reflecting the size composition of the original mining input 75 118 km although the gravel sand transition itself is sharp the transitional zone is lengthy 53 75 km as degradation continues the gravel sand transition is expected to progress downstream but it has remained in a stable position for 12 years indeed two major floods during the period released large quantities of sand from the sub armour layer and newly formed banks of mine tailings causing fining both above and below the transition surface grain size is an adjustable component in the transitional zone as the river strives to recover from a major anthropogenic disturbance

observations of downstream fining on the north fork toutle river near mount st helens washington we carried out a series of measurements to provide data for testing models of downstream fining in an actively braiding gravel bed river in particular the model developed in the previous paper we sampled a series of nine cross sections for bulk grain size distributions over a downstream distance of 5 4 km on the north fork toutle river near mount st helens washington in addition we sampled two of the nine sections for detailed analysis of surface grain size distributions in the cross stream direction by wolman counting within areas of distinct mean grain size patches our theory predicts that the rate of downstream fining depends on the distribution of deposition and a relative mobility function that depends on the characteristics of the grain size patches application of the theory to the study area shows that most of the observed downstream fining can be explained using patchiness as the dominant mechanism

downstream fining in large sand bed rivers abstract downstream fining of bed sediments is a well known phenomenon in gravel bed rivers but also occurs in large sand bed rivers the underlying processes however are not necessarily the same due to the difference in sediment mobility grain size distribution width and geographical setting downstream fining studies focusing on large sand bed rivers are scarce nevertheless in this paper the processes that affect downstream fining in gravel bed rivers are reviewed and it is evaluated to what extent they are relevant for large sand bed rivers furthermore several new downstream fining processes are discussed ending in a conceptual model of downstream fining in large sand bed rivers in deriving the conceptual model downstream fining processes are divided into three categories abrasion selective transport and sediment addition extraction abrasion rates in large sand bed rivers are small due to the durable lithologies the small grain size the high degree of rounding the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment only in the upstream part of large sand bed rivers abrasion rates may be somewhat higher selective transport rates in large sand bed rivers everywhere overrun abrasion rates but are small compared to selective transport rates in gravel bed rivers because the unimodal sediments in sand bed rivers exhibit stronger hiding exposure effects during threshold of motion conditions but especially because many sand bed rivers though not all are in state of fully mobilised transport making differences in threshold of motion between coarse and fine grains relatively unimportant selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting which are all processes that act on a relatively large scale within sand bed rivers the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency there are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand bed rivers note that in contrast to abrasion selective transport only produces a stable downstream fining pattern if the river profile is concave generally a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand bed rivers overbank sedimentation for instance may significantly decrease the downstream fining rate in large sand bed rivers because especially fine grains are removed from the bed dredging can also strongly affect the downstream fining trend depending on the volume and the grain size of the sediment involved meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances increasing the downstream fining rate river bifurcations can cause a discontinuity in the downstream fining pattern because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch all these addition and extraction processes not only have a direct effect on the grain size composition of the river bed but also indirectly influence downstream fining by their effect on the longitudinal bed profile other sediment addition and extraction processes such as tributary confluences non alluvial sediment sources and dumping are less common in large sand bed rivers but can locally have a great effect a full determination of the relative importance of all downstream fining processes in large sand bed rivers requires extensive field measurements the construction of detailed fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above

the gravel sand transition flume study of channel response to reduced slope this paper reports flume experiments with a bimodal gravel sand bed and sediment feed designed to investigate river channel response to declining slope in successive runs an initial near equilibrium channel was subjected to progressively bigger reductions in flume slope this simulated a series of sites along a concave profile approaching a local base level and undergoing a transition from gravel to sand bed it also models the effects of a rise in base level on a fixed site slope reduction led to immediate increase in depth and reduction in velocity and shear stress hence progressive aggradation deposition was size selective and led to the following sequence of bed texture change with increasing severity of slope reduction 1 sand in lee of gravel clusters 2 elongated sandy patches and thin streaks 3 wider sand ribbons covering gravel completely and developing ripples there was feedback from the gradual bed surface changes to hydraulic conditions near bed velocity increased while depth shear stress and roughness height decreased these feedbacks were more pronounced with a bigger reduction in slope these results support the view that the gravel sand transition along river channels with a local base level control is assisted by positive feedback between changes in bed surface texture and near bed flow

fine bed material in pools of natural gravel bed channels natural gravel bed channels commonly contain a fine mode of sand and fine gravel that fills voids of the bed framework of coarser gravel if the supply of fine bed material exceeds the storage capacity of framework voids excess fine material forms surficial patches which can be voluminous in pools during low flow data collected in 34 natural channels in northern california and southern oregon indicate the following 1 fine material on the bed surface can be readily winnowed and transported at high particle velocities much of it in intermittent suspension fine material can dominate the bed material load in gravel bed channels but its abundance on the bed surface is limited by its increasing mobility as hiding places among prominent particles are filled 2 fine material in pools is typically replaced many times per year 3 the proportion v of residual pool volume filled with fine bed material correlates with annual sediment yield in channels whose parent material produces abundant sandy sediment 4 temporal and spatial changes in v appear to correspond to variations in the balance between sediment inputs and water discharge these results suggest that v can be used to monitor and evaluate the supply of excess fine material in gravel bed channels and that samples of fine material in pools can characterize the fine mobile mode of bed material load the sediment load of a stream channel the amount supplied and transported over a period of time can be difficult to evaluate contributions from hillslopes can be quantified with erosion surveys and sediment budgeting but uncertainty in routing makes it difficult to assess the load at any point in a channel network and direct measurement of transport rates is notoriously difficult bed material typically furnishes the bulk of the annual bed load especially in more distal channels but the active volume that contributes to the load is highly variable and difficult to evaluate it may be easier to detect variations in load by examining the mobility of the bed surface of gravel bed channels bed material in gravel bed channels characteristically includes a wide range of particle size and the bed can become

predicting the migration rates of subaqueous dunes in this paper we develop a simple physically based method for predicting what fraction of sediment moving over the crests of dunes will bypass their lee faces the bypass fraction is found by calculating a characteristic excursion length for every grain size making up a particular train of dunes all particles with excursion lengths greater than the downstream span of the lee face of the average dune are assumed not to contribute to dune propagation bypass fractions based on distributions of excursion lengths account for the discrepancies between total sediment discharge and dune migration rate measured by stein 1965 and guy et al 1966 in laboratory flumes as well as by us in the north loup river of nebraska calculations and these data agree over sediment transporting conditions associated with the entire stability field for dunes composed of medium sand the two laboratory studies show that commonly 30 60 of all sand moving over fully developed dunes is not deposited on slip faces measurements from the north loup river reveal that of all sediment moving over the dune crests there roughly 45 is not being captured on lee faces even though 99 of sediment is transported within 2 cm of the bed the method developed herein successfully estimates measured values for the bypass fraction from 0 to 80 our analysis indicates that the division between grains that are deposited versus those that are bypassing falls within the range of sand sizes making up the suspended load at dune crests

particle size variations between bed load and bed material in natural gravel bed channels particle sizes of bed load and bed material that represent materials transported and stored over a period of years are used to investigate selective transport in 13 previously sampled natural gravel bed channels the ratio d of median particle size of bed material to the transport and frequency weighted mean of median bed load size decreases to unity with increasing drainage area bank full discharge dimensionless stream power and bed material sorting in channels with high values of d significant volumes of fine bed load are transported during discharges that are less than bank full which is commonly associated with general entrainment of the coarse pavement in many gravel bed channels this indicates transport of fine bed load over a more stable substrate of coarser bed material the apparent breakdown in equal mobility of the bed as a whole may be caused by areal segregation of poorly sorted bed material into superiorly sorted patches of varying mean size likely sources of selectively transported fine bed load include fine patches that have low entrainment thresholds and high virtual particle velocities a simple sediment budget applied to measurements from three channels indicates that velocities of material from fine patches in pools relative to velocities of average bed material are high in low order channels and decrease distally as more bed material that represents the bed as a whole is accessed for bed load by deeper annual scour

tracer pebble movement along a concave river profile virtual velocity in relation to grain size and shear stress over 1400 tracer pebbles 16 256 mm in diameter were tracked for 2 years in six reaches of allt dubhaig scotland a small gravel bed river along which shear stress and bed surface grain size decrease toward a local base level pebble movement was size selective both within and between reaches within reaches the decrease in mean travel distance with increasing grain size is strongest in the coarse tail of the size distribution particle shape has a minor secondary effect a nondimensional grain velocity averaged over the duration of competent flow is used to compare different size classes and reaches over 90 of its variance is explained by relative grain size and reach shields stress the pattern of size selectivity is consistent with single event tracer results elsewhere bedload trap data from our distal reach and the concept of partial mobility it provides a mechanism for strong downstream fining by selective transport and deposition along rivers in which stress declines toward base level the nondimensional prediction equation for grain velocity may be of use in other rivers but requires testing

downstream fining through selective particle sorting in an equilibrium drainage network the phenomenon of downstream fining has been attributed to both particle abrasion and selective particle sorting the latter is generally considered to play the dominant role within resistant lithologies it has been recognized that tributaries can disrupt fining patterns however few downstream fining studies have considered the entire fluvial network structure here we combine a theory for selective transport with a model of river basin evolution in order to simulate the dynamics of selective sorting throughout a drainage network previous numerical modeling studies of single thread or braided channels have treated downstream fining as a phenomenon driven by differential deposition rates we show however that in an eroding drainage network downstream fining emerges as a natural dynamic adjustment to variable water sediment and energy inputs even under conditions of uniform size distribution in sediment flux thus although selective deposition and abrasion clearly can and do play a role in some fluvial systems neither is necessary to produce downstream fining within a drainage network

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small