effects of hydraulic roughness on surface textures of gravel bed rivers field studies of forest gravel bed rivers in northwestern washington and southeastern alaska demonstrate that bed surface grain size is responsive to hydraulic roughness caused by bank irregularities bars and wood debris we evaluate textural response by comparing reach average median grain size d50 to that predicted from the total bank full boundary shear stress t0 bf representing a hypothetical reference condition of low hydraulic roughness for a given t0 bf channels with progressively greater hydraulic roughness have systematically finer bed surfaces presumably due to reduced bed shear stress resulting in lower channel competence and diminished bed load transport capacity both of which promote textural fining in channels with significant hydraulic roughness observed values of d50 can be up to 90 smaller than those predicted from t 0 bf we find that wood debris plays an important role at our study sites not only providing hydraulic roughness but also influencing pool spacing frequency of textural patches and the amplitude and wavelength of bank and bar topography and their consequent roughness our observations also have biological implications we find that textural fining due to hydraulic roughness can create usable salmonid spawning gravels in channels that otherwise would be too coarse

long term slowdown of river tracer pebbles generic models and implications for interpreting short term tracer studies 1 we propose generic approaches to modeling the reduction in mobility that occurs as tracer pebbles seeded on the surface are mixed to less active locations e g beneath the bed or high on bars or advected downstream to reaches with different shear stress or bed grain size advection slowdown which may be negligible is quantified using a regression equation relating virtual velocity to tracer and reach properties mixing slowdown which is always present is simulated using a markov model of exchanges between three stores and assumptions about relative mean velocities of tracers in each store the model can be calibrated from a full search on one date and at least a surface search on another date calibration to six reaches of allt dubhaig scotland in which tracers were mapped after 2 and 8 years suggests mixing took 3 5 years to approach equilibrium in five reaches and was still proceeding in one simulated slowdown through mixing is far greater than that through advection and goes much of the way to explaining the 50 observed reduction in virtual velocity over 8 years compared to 2 years implications for the design and comparison of tracer pebble experiments are considered

topographic disturbance of subaqueous gravel substrates by signal crayfish pacifastacus leniusculus abstract the impact of signal crayfish pacifastacus leniusculus on the topography and fabric of six narrowly graded gravel substrates was investigated using repeat laser scanning of sediment surfaces in still water aquaria digital elevation models dems of the gravel surfaces were obtained before and after exposure to crayfish for five predetermined periods the impact on the substrate was quantified by establishing topographic and volumetric changes using dems of difference dod the presence of an individual medium sized crayfish for 24 h resulted in an average volume change in surface topography of 450 cm 3 over an area of 2400 cm 2 giving a sediment displacement of 1 7 kg m 2 d 1 the majority 78 of this volume change was associated with small scale 1 median grain diameter movements of surface grains this fabric adjustment altered grain orientations and friction angles crayfish also constructed pits and mounds that increased significantly the roughness of the gravel substrates and altered the protrusion of individual grains crayfish were able to move material up to 38 mm in diameter that had a submerged weight six times that of the individuals used in this study by modifying the arrangement of grains on the surface of fluvial substrates signal crayfish may counteract the low flow physical consolidation of gravel beds and reduce the entrainment stresses required to move river bed material the results of this study suggest that signal crayfish an internationally widespread invasive species may have substantial impacts on the physical environment of streams and rivers as well as on local benthic ecological communities

advance retreat and halt of abrupt gravel sand transitions in alluvial rivers downstream fining of bed sediment in alluvial rivers is usually gradual but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm i e a gravel sand transition gst or gravel front here we present an analytical model of gst migration that explicitly accounts for gravel and sand transport and deposition in the gravel reach sea level change subsidence and delta progradation the model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel wedge and predicts the circumstances required for the gravel front to advance retreat and halt predicted modern gst migration rates agree well with measured data at allt dubhaig and the fraser river and the model qualitatively captures the behavior of other documented gravel fronts the analysis shows that sea level change subsidence and delta progradation have a significant impact on the gst position in lowland rivers

specification of sediment maintenance flows for a large gravel bed river reservoir releases may be specified to flush interstitial fine sediment from gravel beds in the river downstream choice of an effective flow depends on trade offs among discharge flow duration and pool dredging as they determine rates of bed mobilization sand removal and gravel loss a basis for evaluating these trade offs is developed with an approximate method appropriate to the sparse data typically available sand and gravel transport are represented with rating curves approximate methods are introduced for estimating effective gravel entrainment subsurface sand supply and pool sediment trapping these are combined in a sand routing algorithm to evaluate flushing alternatives for the trinity river california a sediment maintenance flow of moderate size just sufficient to entrain the bed surface over the duration of the release limits gravel loss and maximizes sand trapping by pools larger discharges produce more fines removal but at the cost of greater gravel loss and reduced selective transport of fines dredged pools increase sand removal efficiency by providing multiple exits from the channel and minimize gravel loss if dredged sediment is screened and gravel returned to the river

experimental study of incipient motion in mixed size sediment transport rates of five sediments were measured in a laboratory flume three of these sediments had the same mean size the same size distribution shape and different values of grain size distribution standard deviation the critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate the sorting of the sediment mixture had little effect on the critical shear stress of individual fractions once the median size d50 of the mixture and a fraction s relative size di d50 are accounted for our data combined with previously published data show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction s relative grain size despite a broad variation in the available data of mixture sorting grain size distribution shape mean grain size and grain shape all fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions this result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment recirculating systems as well as for systems where the fractional transport rates are imposed on the system feed systems this equivalence in initial motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination

the equilibrium alluvial river under variable flow and its channel forming discharge when the water discharge sediment supply and base level vary around stable values an alluvial river evolves toward a mean equilibrium or graded state with small fluctuations around this mean state i e a dynamic or statistical equilibrium state here we present analytical relations describing the mean equilibrium geometry of an alluvial river under variable flow by linking channel slope width and bed surface texture the solution holds in river normal flow zones or outside both the hydrograph boundary layer and the backwater zone and accounts for grain size selective transport and particle abrasion we consider the variable flow rate as a series of continuously changing yet steady water discharges here termed an alternating steady discharge the analysis also provides a solution to the channel forming water discharge which is here defined as the steady water discharge that given the mean sediment supply provides the same equilibrium channel slope as the natural long term hydrograph the channel forming water discharge for the gravel load is larger than the one associated with the sand load the analysis illustrates how the load is distributed over the range of water discharge in the river normal flow zone which we term the normal flow load distribution the fact that the distribution of the imposed sediment supply spatially adapts to this normal flow load distribution is the origin of the hydrograph boundary layer the results quantify the findings by wolman and miller 1960 regarding the relevance of both magnitude and frequency of the flow rate with respect to channel geometry

mean size distribution of bed load on goodwin creek goodwin creek is a steep bed slope of study reach 0 003 gravel bed stream in the bluff line hills east of the mississippi river floodplain in the north central part of the state of mississippi records of the flow for a nine year period and 488 bed load transport samples collected during a four year period allow a comparison between weighted and nonweighted calculations of the size distribution of the bed load the mean size distribution of the bed load on goodwin creek was calculated using the size distribution of the bed load weighted by transport rate and flow frequency the size of the bed load was found to be slightly finer id n d5 d0 6 8 mm than the bed subsurface id n d5 d0 8 3 mm during the nine year period the weighting technique greatly reduced the bias nonweighted id n d5 d0 3 16 mm in the calculation of the mean size distribution

river channel slope flow resistance and gravel entrainment thresholds 1 river beds are traditionally assumed to become mobile at a fixed value of nondimensional shear stress but several flume and field studies have found that the critical value is higher in steep shallow flows explanations for this have been proposed in terms of the force balance on individual grains the trend can also be understood in bulk flow terms if total flow resistance has base and additional components the latter due to protruding immobile grains as well as any bedforms and the stress corresponding to additional resistance is not available for grain movement in threshold conditions a quantitative model based on these assumptions predicts that critical shields stress increases with slope critical stream power is near invariant with slope and each has a secondary dependence on bed sorting the proposed slope dependence is similar to what force balance models predict and consistent with flume data and most field data possible explanations are considered for the inability of this and other models to match the very low critical values of width averaged stress and power reported for some low gradient gravel bed rivers

grain size and topographical differences between static and mobile armour layers a series of laboratory flume experiments under conditions of sediment starvation zero sediment feeding and recirculation were conducted in order to identify the temporal evolution and surface properties of static and mobile armour layers the experiments were carried out in an 8 m long flume using a bimodal grain size mixture d50 6 2 mm and a range of shear stresses ranging from 4 0 to 8 6 n m 2 the results confirm that a static armour layer is coarser than a mobile one and that the grain size of a mobile armour layer is rather insensitive to changes in the imposed flow strength an analysis of laser scan bed surveys revealed the highly structured and imbricated nature of the static armour layer under these conditions the vertical roughness length scale of the bed diminished and it became topographically less complex at higher forming discharges the topography of mobile armour layers created by rising discharges differed they exhibited a greater roughness length scale and were less organized despite the fact that the grain size of the surface material maintained an approximately constant value during recirculation also the mobile armour tended to create larger cluster structures than static armour layers when formed by higher discharges these differences were mainly due to the transport of the coarser fraction of bed sediments which diminished to zero over the static armour because of being hidden within the bed whereas in the mobile armour the coarser particles protruded into the flow and were actively transported increasing the vertical roughness length scale overall the results show that an examination of the grain size characteristics of armour layers cannot be used to infer sediment mobility and bed roughness detailed elevation models of exposed surfaces of gravel bed rivers are required to provide critical insight on the sediment availability and sedimentation processes copyright 2011 john wiley sons ltd

what was the transport mode of large boulders in the campine plateau and the lower meuse valley during the mid pleistocene abstract the campine plateau in northeastern belgium a remnant of an alluvial fan deposited by the meuse river during the mid pleistocene is characterised by the presence of boulders with maximum dimensions of up to 2 m embedded in a gravel matrix these boulders originated in the ardennes region and are generally assumed to have been transported by ice rafting processes this paper investigates for the first time quantitatively the possibility of purely hydraulic transport of the boulders taking into account channel and flow characteristics in the boulder provenance area during the mid pleistocene empirical transport relations that describe incipient motion thresholds in nonuniform river beds as a function of the relative grain size or the ratio between the grain size of interest and the median grain size of the channel bed are applied in order to calculate critical water depths for transport of boulders of various sizes results indicate that hydraulic transport of boulders with intermediate diameters

interactions between ridges and plumes in the last 30 years geologists have made much progress on understanding slabs lithosphere and mid ocean ridges but mantle plumes remain enigmatic in their perspective montagner and ritsema 1 chart recent progress largely based on seismic anisotropy toward understanding how such plumes interact with mid ocean ridges existing data are insufficient however to determine whether mantle plumes act independently from plate tectonics the authors call for ocean bottom seismic stations to settle this question 1 http www sciencemag org cgi content full 294 5546 1472

distance of movement of coarse particles in gravel bed streams distributions of distance of bedload particle movement were examined in two gravel bed streams using several hundred magnetically tagged cobbles and pebbles the compound poisson model of einstein hubbell sayre and a simple gamma function model were compared with observed distributions of moved particles and of all particles both models fit the data reasonably well for small mean displacements but notable misfits occurred in an event with large mean displacement when mean particle travel distance approaches the scale of bar spacing trapping in the bars interrupts particle progress and the dispersion process the data remain very noisy so definitive discrimination of suitable models will require trials with more than 103 particles

large grains matter contrasting bed stability and morphodynamics during two nearly identical experiments while the stabilizing function of large grains in step pool streams has long been recognized the role they play in gravel bed streams is less clear most researchers have ignored the role of large grains in gravel bed streams and have assumed that the median bed surface size controls the erodibility of alluvial boundaries the experiments presented herein challenge this convention two experiments were conducted that demonstrate the significant morphodynamic implications of a slight change to the coarse tail of the bed material the two distributions had the same range of particle sizes and nearly identical bulk d50 values 1 6 mm however the d90 of experiment gsd1 was slightly finer 3 7 mm than that for experiment gsd2 3 9 mm transport rates during gsd1 were nearly four times greater than during gsd2 even though the dimensionless shear stress was slightly lower and the channel developed a sinuous pattern with well developed riffles pools and bars during gsd2 the initial rectangular channel remained virtually unchanged for the duration of the experiment the relative stability of gsd2 seems to be associated with a slightly larger proportion of stable large grains on the bed surface at the beginning of gsd1 3 5 of the bed was immobile while almost twice as much of it 6 1 was immobile at the beginning of gsd2 the results demonstrate that the largest grains not the median size exert first order control on channel stability copyright 2017 john wiley sons ltd

numerical simulation of downstream fining by selective transport in gravel bed rivers model development and illustration a numerical model has been developed for the routing of gravel sized sediment along a river channel which is free to adjust both its long profile and surface texture hydraulic calculations use a step backwater approach and sediment transport is predicted with the method of parker 1990a which uses a low degree of size selectivity exchange of sediment between the surface and subsurface is described using the modified exner equation of parker and sutherland 1990 the model is applied to an idealized channel based on the highly concave allt dubhaig scotland in which fining by particle wear is minor the rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to 102 years under the present flow regime of the river the evolution of the fining pattern during the model run and associated changes in sediment transport and bed aggradation are described it is concluded that strong profile concavity can force rapid downstream fining even though bed load transport is only slightly size selective this run of the model serves as a basis for testing of the sensitivity of downstream fining to alternative choices of parameter values and boundary conditions which are summarized here and will be described in a subsequent paper

coupling fluvial hydraulic models to predict gravel transport in spatially variable flows this study investigated spatial temporal variations of shear stress and bed load transport at three gravel bed river reaches of the williams fork river colorado a two dimensional flow model was used to compute spatial distributions of shear stress for four discharge levels between one third of bankfull qbf and qbf results indicate that mean values are highly variable among sites however the properties of the mean normalized distributions of are similar across sites for all flows the distributions of are then used with a transport function to compute bed load transport rates of individual grain size fractions probability distributions of the instantaneous unit width transport rates qb indicate that most of the bed load is transported through small portions of the bed with high the mean normalized probability distributions of qb are different among sites for all flows except at qbf when the distributions overlap we also find that the grain size distribution gsd of the bed load adjusts with discharge to resemble the grain size distribution of the subsurface at qbf we extend these results to 13 locations in the basin using the mean normalized distributions of shear stress and measured subsurface grain sizes to compute bed load transport rates at qbf we found a remarkably similar shape of the qb distribution among sites highlighting the basin wide balance between flow forces and gsd at qbf and the potential to predict sediment flux at the watershed scale

critical shear stress of bimodal sediment in sand gravel rivers a new model for the critical shear stress and the transport of graded sediment is presented the model is based on the size distribution of the bed surface and can be used to compute sediment transport rates in numerical simulations with an active layer model this model makes a distinction between unimodal and bimodal sediments it is assumed that all size fractions of unimodal sediments have the same critical shear stress while there is selective transport for the gravel fractions of bimodal sediments a recently published laboratory transport data set is used to calibrate our model

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

controls of strength and rate of downstream fining above a river base level we report sensitivity tests on a numerical model of the development of downstream fining by size selective transport and deposition above a base level in a gravelbed river with no lateral inputs of water or sediment strong fining requires deposition mainly to the surface otherwise its amount and streamwise distribution are not highly sensitive to any single factor but vary somewhat with hiding factor excess shear stress and bed sorting low initial concavity inhibits fining but different higher concavities generate fairly similar fining profiles lower sediment supply leads to a coarser bed with less fining but there is no simple relationship between fining and aggradation rate the rate of development of fining is sensitive to all these factors but is always rapid initially followed by a far slower approach towards final equilibrium surface grain size adjustment is potentially an important degree of freedom in river response to any environmental change or human impact which causes overloading

variables influencing water surface slopes in gravel bed streams at bankfull stage the relationship between water surface slope and 12 hydraulic variables was examined for a reach scale and a more local scale using data from 12 reaches measurements of width depth velocity slope and roughness elements were made on 12 reaches of gravel bed streams at bankfull stage all of the reaches have fairly well developed riffle pool sequences and are either straight or braided in channel pattern measurements were made at numerous locations within each reach so that the influence of hydraulic and resistance variables on water surface slope could be examined both within and among reaches results show that roughness elements and resistance coefficients correlate most highly with water surface slope bed particle size correlates well with water surface slope at both reach and local scales the magnitude of bed undulations correlates well with slope at the scale of a reach in the type of stream channels examined the important components of resistance in the channels are also the most important influences on water surface slope

morphologically based model of bed load transport capacity in a headwater stream 1 field measurements of channel and bed material properties are used to develop a network based model of bed load transport capacity in halfmoon creek a high gradient stream in the rocky mountains of colorado the formerly glaciated watershed contains self formed reaches that are generally disconnected from hillslopes and major sediment perturbations strung together by a sequence of bedrock and cascade channels measurements of channel geometry slope and bed sediment texture surface and substrate were taken in 27 reaches with gradients ranging from 0 6 to 6 to examine interactions among key variables affecting bed load transport the analysis shows that the grain size distribution of the substrate is relatively uniform throughout the basin however the distinction between surface and substrate texture varies systematically with channel gradient and shear stress reaches with high shear stress are characterized by coarse surface layers with a high threshold for transport limiting the mobility of the sediment supplied in reaches with lower gradient and shear stress the difference between surface and substrate texture is less increasing both the frequency of transport and the mobility of the bed load while it might appear that the steeper reaches of halfmoon creek have a high capacity for carrying fine medium gravel the relative mobility of these sizes is affected by the presence of very coarse sediment on the bed surface as a result modeled estimates of bed load transport rate indicate that average annual bed load sediment yields increase downstream nearly linearly with discharge suggesting a simple scaling of bed load transport capacity with discharge and drainage area

quantitative determination of the activity of within reach sediment storage in a small gravel bed river using transit time and response time abstract reach scale sediment storage is rarely quantified in sediment budget studies yet it has a considerable effect on the sediment delivery ratio at the basin scale and on the accuracy of morphological methods of bedload estimation at the reach scale deployment of magnetic tracer particles allows accurate characterisation of sediment fluxes in gravel bed rivers and provides the opportunity to quantify storage activity using reservoir theory activity was quantified at reach and sub reach scales in two reaches of a small gravel bed river and the possibility of quantifying the activity of smaller scale sediment stores is explored reach scale transit time functions were derived from the cumulative output of sediment against age or time the shape of the functions in both reaches was determined by flood incidence and magnitude hydraulics sediment tracer availability grain size and local morphology accurate transit time functions were difficult to determine due to reliance upon tracer output and the associated problems of tracer exhaustion over time and imperfect tracer recovery rates in addition if tracers of different ages are allowed to mix for example where upstream input is possible then the resultant transit time distributions age are not comparable with the timing of the hydraulic processes responsible unless the hydraulic conditions are constant these results suggest that transit time is difficult to determine from tracer studies in gravel bed rivers consequently a refinement of transit time the response time is introduced and is defined as the time after the initial input of tracer sediment when cumulative tracer output exceeds the amount of tracer sediment remaining in storage whilst still based upon tracer output the calculation of response time also utilises storage data and is expressed relative to time since the start of the study rather than age it is therefore directly comparable with the incidence of hydraulic conditions this provides a more informative measure of activity which is readily available from tracer data in gravel bed rivers and allows evaluation of the importance of flood incidence and magnitude grain size morphology and reach characteristics stability like transit time response time estimates are hindered by mixing of sediment of different ages at the sub reach scale however it does account for errors from less than 100 recovery and tracer exhaustion more effectively than transit time quantification of absolute and relative size effects in transport is also possible from response time

width adjustment in experimental gravel bed channels in response to overbank flows 1 we conducted a series of flume experiments to investigate the response of self formed gravel bed channels to floods of varying magnitude and duration floods were generated by increasing the discharge into a channel created in sand and gravel sized sediment with a median grain size of 2 mm flooding increased the shields stress along the channel perimeter causing bank erosion and rapid channel widening the sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed but was rather transported downstream a process likely facilitated by transient fining of the bed surface at the end of each experiment bank sediments were no longer in motion partial bed load transport characterized the flat bed portion of the channel and the shields stress approached a constant value of 0 056 about 1 2 times the critical shields stress for incipient motion furthermore the discharge was entirely accommodated by flow within the channel the creation of a stable channel entirely eliminated overbank flows we speculate that similar processes may occur in nature but only where bank sediments are non cohesive and where channel narrowing processes cannot counteract bank erosion during overbank flows we also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates suggesting that simple methods may be appropriate for predicting width increases in channels with non cohesive unvegetated banks even during overbank flows last we present a model for predicting the equilibrium width and depth of a stable gravel bed channel with a known channel forming shields stress

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

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

observations on sediment mobility in a large gravel bed river article this study investigates sediment mobility in a large gravel bed river tagliamento river northeastern italy field data were used to identify the morphological effectiveness of a range of flows floods with recurrence interval b1 to 3 5 year and for a detailed analysis of the partial transport condition the analyses were carried out on three cross sections where a number of areas representative of different morphological units main and secondary channels low and high bars islands were painted grain size of the painted area was measured using a photographic method monitoring of bed mobility during the study period supplied new photographs of the painted areas measurements of size and travel distance of the mobilized particles and estimate of flow depth after each flood event our analyses have shown that with dimensionless shear stress around 0 073 17 n m 2 the morphological effect over the painted areas was primarily partial transport while with stresses n0 1 25 n m 2 the bed experienced full mobility i e all the painted particles were transported or totally buried full sediment mobility was observed on lower morphological units i e main and secondary channels under floods with very high frequency rib1 y whereas on the low bars full mobility occurred under floods with a recurrence interval slightly n1 year instead fine deposition and partial transport conditions were observed on the higher bars even after the largest of the monitored floods ri 3 5 y the analysis of the partial transport condition relies on more than 3500 measurements of particle travel distance travel distances range from a few centimetres up to 22 m the transported particles were generally finer than the painted sediments and the analysis of the sediment transport ratio revealed that partial transport occurred under equal mobility conditions for particles finer than 32 mm and under size selective mobility for coarser fractions this appears to be corroborated for db32 mm by the insensitiveness of particle transport distance to particle sizes even if the exponent 1 of the relationship between the dimensionless shear stress and the maximum transported diameter would suggest that all fractions are transported under equal mobility conditions

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

sediment feed and recirculating flumes fundamental difference two standard modes of laboratory sediment transport flume operation are sediment feed and recirculating in the case of uniform sediment the final equilibriums achieved in the two modes are equivalent and independent of initial conditions even though the specified flow and sediment constraints differ when the sediment consists of a mixture of sizes however this symmetry breaks down in a sediment feed flume both the feed rate and size distribution are specified by the user and must be identical to those of the bed load at final equilibrium the final state thus becomes independent of initial conditions in a recirculating flume however the equilibrium size distribution of neither the bed load nor the bed surface material can be specified in advance as a result the final state depends on the initial conditions and can include the possibility of partial transport the equilibriums themselves are equivalent in that they obey the same laws of flow and sediment transport their ia priori n predictability however differs due to the difference in governing constraints when fractional transport is scaled by the bulk size distribution the two flume modes may produce very different transport relations even for the same sediment a consistent approach requires that the transport be scaled by the size distribution of the bed surface

self organization of river networks to threshold states numerical models in which grid cells self organize so that the shear stress in each cell equals a predetermined threshold value are remarkably successful in producing the fundamental structure of river networks such models are a reasonable approximation of networks characterized by near constant dimensionless shear stress bed shear stress7sol bed grain size with each cell s threshold value being proportional to median bed grain size previous work has shown that this is the case for channels cut into alluvium and that the characteristic narrow range of is different for gravel and sand bed channels channels in the italian apennines that are slowly incising into weak bedrock but which are covered by a veneer of gravel for most of the time are shown to be characterized by the same as alluvial gravel bed channels thus threshold models capture the fundamental behavior of many river networks even in areas of long term bedrock incision

channel bed mobility response to extreme sediment loading at mount pinatubo since the 1991 eruption of mount pinatubo the specific sediment yields from watersheds draining its slopes have been the highest ever recorded in spite of this overwhelming sediment load rivers inundated by pyroclastic flow deposits delivered almost half of the initial deposits to downslope alluvial and or debris fans by 1996 although most of this transport occurred by hyperconcentrated flows and debris flows lahars very high sediment transport rates also characterize low flow measured flow velocities and depths channel properties and the size of both mobile and stable clasts in the sacobia and pasig potrero rivers indicate median bed surface grain sizes of 2 to 8 mm grain size independent dimensionless critical shear stresses c of 0 016 to 0 041 and manning s n values of 0 017 to 0 024 values well below those previously reported for steep mountain channels the dramatic bed surface fining bed mobility and selective transport in these extremely sediment rich channels indicate that changes in grain size c values and bed roughness can increase transport capacity in response to high sediment supply our observations also suggest viewing the apparently contradictory concepts of an equal threshold of bed mobility and grain size dependent selective transport as end member concepts that apply to channels with low or intermittent and high or continuous sediment supply respectively

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

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

effects of sediment supply on surface textures of gravel bed rivers using previously published data from flume studies we test a new approach for quantifying the effects of sediment supply i e bed material supply on surface grain size of equilibrium gravel channels textural response to sediment supply is evaluated relative to a theoretical prediction of competent median grain size d9 50 we find that surface median grain size d50 varies inversely with sediment supply rate and systematically approaches the competent value d9 at low equilibrium transport rates furthermore equilibrium transport rate is a power function of the difference between applied and critical shear stresses and is therefore a power function of the difference between competent and observed median grain sizes d9 and d50 consequently we propose that the difference between predicted and observed median grain sizes can be used to determine sediment supply rate in equilibrium channels our analysis framework collapses data from different studies toward a single relationship between sediment supply rate and surface grain size while the approach appears promising we caution that it has been tested only on a limited set of laboratory data and a narrow range of channel conditions

sediment entrainment and depletion from patches of fine material in a gravel bed river characteristics at shear stresses ranging from 5 to 60 n m 2 experiments demonstrate that patches of fine sediment control both the intensity and duration of bed load under hydraulic conditions characteristic of the early stages of floods the experiments allow quantifying bed load at the entrainment threshold providing the first empirical evidence that marginal bed load transport can be attributed to the mobilization of sediments from patches bed load transport was recorded consistently once shear stress exceeded 5n m 2 the experiments produced low bed load rates 6 g sm depletion of material in the patches occurred rapidly with bed load rates and particle sizes decreasing after only 5 minutes combining flume and helley smith data for the study reach a breakpoint in the relation between shear stress and bed load rate was calculated to be around 30 n m 2 this represents the transition between bed load transport phases below the breakpoint transport occurs at a low rate and is composed predominantly of fine sediment from patches but above it much higher rates occur from across the reach as a whole hydraulic conditions at the threshold are those which occur during small frequent floods 25 bankfull flow equaled or exceeded 15 of time this indicates that sediment entrainment from patches of fine material is a frequent process and the threshold change between bed load phases occurs regularly

persistence of the surface texture of a gravel bed river during a large flood we present herein clear field evidence for the persistence of a coarse surface layer in a gravel bed river during flows capable of transporting all grain sizes present on the channel bed detailed field measurements of channel topography and bed surface grain size were made in a gravel bed reach of the colorado river prior to a flood in 2003 runoff produced during the 2003 snowmelt was far above average resulting in a sustained period of high flow with a peak discharge of 27 m3 s 170 of normal peak flow all available grain sizes within the study reach were mobilized in this period of time during the 2003 peak flow the river avulsed immediately upstream of the study reach thereby abandoning approximately one half kilometer of the former channel the abandonment was rapid probably within a few hours leaving the bed texture essentially frozen in place at the peak of the flood all locations sampled prior to the flood were resampled following the stream abandonment in response to the high flow the surface median grain size d50s coarsened slightly in the outer part of the bend while remaining nearly constant along the inner part of the bend resulting in an overall increase from 18 to 21 mm for the study reach thus the coarse bed surface texture persisted despite shear stresses throughout the bend that were well above the critical entrainment value this may be explained because the response of the bed texture to increases in flow strength depends primarily upon the continued availability of the various grain size percentiles in the supply which in this case was essentially unlimited for all sizes present in the channel copyright 2007 john wiley sons ltd

measurements in a braided river chute and lobe 2 sorting of bed load during entrainment transport and deposition grain size distributions of bed material bed load and bar head deposition were sampled during the evolution of a chute and lobe in the braided gravel bed sunwapta river alberta although bed shear stress and total bed load transport rate varied substantially within the reach and from day to day ferguson et al this issue the median diameter d50 of bed load was remarkably constant the bed initially fined from chute to bar head but became more uniform sediment deposited on the bar head also had a near constant d50 similar to that of the bed load truncated at the same lower size limit both load and deposition were finer on average than the bed suggesting overall selective transport despite little sign of local sorting in contrast to median diameters maximum sizes dmax of sediment in motion and deposited on the bar head varied substantially and showed a weak but significant dependence on shear stress inferring selective entrainment from this evidence alone would be dubious because dmax was also found to increase systematically with the mass of sediment sampled however two different methods of analysis of fractional transport rates throughout the size range also showed size selective entrainment and transport in the marginal transport conditions of this study size sorting appears to occur but only weakly and mainly in the coarser fractions

message in a stainless steel bottle thrown into deep geological time abstract in the 60th year after the placing of the corner stone under the polish polar station at hornsund ppsh spitsbergen svalbard norway a new capsule has been laid down in the vicinity of the ppsh it is made of stainless steel and includes five stainless steel containers each carrying a message on different themes in our lives the message is written in the language of objects each of them speaking for itself the capsule with containers has been buried in a 4 2 m deep hole drilled for scientific purposes the host rocks for the capsule are the precambrian gneisses representing the basement of the polar archipelago we estimate that the capsule now some 6 m a s l will appear again on the surface after ca 0 5 ma due to the combined effect of uplift and erosion we also believe that it will be found and the message understood contrary to other messages sent by mankind into space

analysis of bedload transport characteristics of idaho streams and rivers field data are essential in evaluating the adequacy of predictive equations for sediment transport each dataset based on the sediment transport rates and other relevant information gives an increased understanding and improved quantification of different factors influencing the sediment transport regime in the specific environment data collected for 33 sites on 31 mountain streams and rivers in central idaho have enabled the analysis of sediment transport characteristics in streams and rivers with different geological topographic morphological hydrological hydraulic and sedimentological characteristics all of these streams and rivers have armored poorly sorted bed material with the median particle size of surface layer coarser than the subsurface layer the fact that the largest particles in the bedload samples did not exceed the median particle size of the bed surface material indicates that the armor layer is stable for the observed flow discharges generally bankfull or less and in some cases two times higher than bankfull discharge the bedload transport is size selective the transport rates are generally low since sediment supply is less than the ability of flow to move the sediment for one range of flow discharges or the hydraulic ability of the stream is insufficient for entrainment of the coarse bed material detailed analyses of bedload transport rates bedload and bed material characteristics were performed for each site the obtained results and conclusions are used to identify different influences on bedload transport rates in analyzed gravel bed rivers copyright 2008 john wiley sons ltd

calculating bed load transport in steep boulder bed channels 1 steep rough channels occupy a large fraction of the total channel length in mountainous regions most sediment mobilized on hillslopes must pass through these streams before reaching lower gradient channels steep channels have wide grain size distributions that are composed of finer more mobile sediment and large rarely mobile grains the large grains can bear a significant portion of the total shear stress and thereby reduce the stress available to move the finer sediment conventional bed load transport equations often overpredict the sediment flux in steep channels by several orders of magnitude we hypothesize that sediment transport equations overpredict the sediment flux because they do not 1 account for the stress borne by rarely mobile grains 2 differentiate between highly and rarely mobile sediment and 3 account for the limited availability of mobile sediment here we modify a conventional bed load transport equation to include these three effects we use measurements of the flow bed properties and sediment flux in a small steep flume to test this equation we supply gravel at a constant rate through fields of regularly spaced immobile spheres and measure the bed coverage by gravel and sphere protrusion the percent of the sphere that protrudes above the gravel deposit for a given sphere spacing the proportion of the bed covered by gravel increases and the sphere protrusion decreases with greater sediment supply thus bed coverage and immobile grain protrusion may serve as proxies for sediment availability in steep rough streams unlike most transport equations that we tested our modified bed load equation predicts sediment fluxes to within an order of magnitude of the measured values our results demonstrate that accurately predicting bed load transport in steep rough streams may require accounting for the effects of local sediment availability coverage by mobile sediment and drag due to rarely mobile particles

bedform migration and bar evolution in a late quaternary monsoon influenced river gujarat western india abstract a study of a well exposed late quaternary calcrete conglomerate sheet from gujarat western india provides some insights on bedform migration and bar evolution in sub humid monsoon influenced rivers the architecture of the conglomerate sheet is defined by the presence of two elements channel ch and gravel bar gb this architectural simplicity is remarkable and may be typical of monsoon influenced rivers that show dominance of 3d dunes in the initial stage 1 the river was sand load dominated as seen in the presence of cross bedded sands at the base associated with this stage was a channel element ch1 at the northern extremity of the outcrop during stage 2 bank excavated blocks were redistributed in the channel which aided the appearance of gravel mesoforms within depressions on the channel bed and subsequent low stage sedimentation leading to the formation of channel element ch2 in stage 3 gravel dunes covered the entire channel bed and are separated from the preceding deposits by a channel lag falling stage dissection of the bar was accompanied by relocation of the flow along a smaller channel represented by channel element ch3 at the northern end of the outcrop and bar top dissection this history of bar evolution compares well with the central bar initiation and transverse bar braiding mechanisms described in flume studies except for change in bed material palaeocurrents during the termination of stage 3 showed typical patterns of convergence and divergence of bedforms in the southern extremity of the outcrop whereas the palaeocurrent directions are more consistent in the central regions this could be related to dune combination and splitting

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

experiments on the effect of hydrograph characteristics on vertical grain sorting in gravel bed rivers 1 desert ephemeral gravel bed streams typically have bed surfaces that are relatively unarmored compared to the substrate below while gravel bed streams in humid and snowmelt areas typically have well armored surfaces the degree of armoring can be characterized in terms of an armor ratio defined as the ratio of the surface median size to the substrate median size a set of field data shows desert ephemeral gravel bed streams with armor ratios ranging from 0 5 to 2 4 and with an average value of 1 2 the armor ratio of snowmelt fed gravel bed streams in the same set ranges from 2 to 7 with an average value of 3 4 the reason for this difference is sought in terms of differing hydrological characteristics and sediment supply regimes thirteen experiments were conducted to study the formation of armoring under a range of hydrological conditions the experiments have two limiting cases a relatively flat hydrograph that represents conditions produced by continuous snowmelt and a sharply peaked hydrograph that represents conditions associated with flash floods all constant hydrograph experiments developed a well armored structured surface while short asymmetrical hydrographs did not result in substantial vertical sorting all symmetrical hydrographs show some degree of sorting and the sorting tended to become more pronounced with longer duration sediment supply appears to be a first order control on bed surface armoring while the shape of the hydrograph plays a secondary role

the gravel sand transition along river channels abstract this paper discusses a hitherto neglected aspect of fluvial downstream fining the transition from a gravel bed to a sand bed from the few known examples and by additional study of new sites some general features of such transitions are given these transitions are generally rapid and involve a change in bed texture from unimodal gravels through bimodal sediments to exclusively sand material and are often associated with a break in slope we propose three possible causes local base level control an excess of sand supply and abrasion breakdown of fine gravel we further argue that such transitions occur in a variety of fluvial environments and are geographically more widespread than previously supposed and on this basis we suggest they may also aid in interpreting ancient sediment showing bimodality and rapid proximal to distal changes in grain size

patch dynamics and stability in steep rough streams 1 the beds of steep streams are typically composed of relatively immobile boulders and more mobile patches of gravel and cobbles little is known about how variability in flow and sediment flux affect the area thickness composition and grain mobility of sediment patches to better understand patch dynamics we measured flow sediment transport and bed properties in two steep channels patches close to the thalweg varied in area thickness and grain size whereas those outside the thalweg did not local variations in transport of several orders of magnitude occurred even on a patch with a spatially homogeneous grain size distribution during moderate flow events partial to selective transport dominated on the entire channel bed and all individual patches tracer particles moved freely between different patch classes e g fine and coarse patches exchanged particles and relatively fine sediment on all patch classes began motion at the same shear stress therefore the selective transport observed for the entire bed was not a result of the preferential transport of only fine patches but the high relative mobility of finer sediment on all patches our results suggest that local flow and sediment supply and not spatial grain size variations were the primary drivers of local bed load transport variability the use of reach averaged flow properties to understand local patch dynamics may not be applicable

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

a model of two phase bedload transport in an oregon coast range stream a descriptive model for bed material routing in small sand and gravel bedded channels with sequences of pools and armoured riffles was developed aspects of the model are demonstrated at flynn creek a 202 ha third order drainage in the oregon coast range assuming that channel geometry sediment transport competence and the availability of sediments for transport are all non uniform in the downstream direction the model described bedload transport as occurring in two phases phase i involves the transport of fine predominantly sand sized bed materials over stable gravel armoured riffles phase ii occurs at higher flows that can entrain riffle armour and transport riffle sediments in addition to phase i sediments phase i bedload transport was sampled during three moderate flow events a power function related phase i bedload transport to water discharge the relationship was consistent between storms and between two successive years phase ii bedload transport sampled during a 1 8 yr return period streamflow event in february 1979 proved non uniform in a downstream direction and unsteady over time at a given stream discharge transport rates closely correlated with the rapid scour and redeposition of the riffle at the bedload sampling cross section transport peaks corresponded to scour and transport decreased greatly during deposition the transport of large 12 5 mm diam bed material increased up to 12 fold during riffle scour

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

the causes of bedload pulses in a gravel channel the implications of bedload grain size distributions unsteady bedload transport was measured in two c 5 m wide anabranches of a gravel bed braided stream draining the haut glacier d arolla switzerland during the 1998 and 1999 melt seasons bedload was directly sampled using 152 mm square helley smith type samplers deployed from a portable measuring bridge and independent transport rate estimates for the coarser size fractions were obtained from the dispersion of magnetically tagged tracer pebbles bedload transport time series show pulsing behaviour under both marginal 1998 and partial 1999 transport regimes there are generally weak correlations between transport rates and shear stresses determined from velocity data recorded at the measuring bridge characteristic parameters of the bedload grain size distributions d50 d84 are weakly correlated with transport rates analysis of full bedload grain size distributions reveals greater structure with a tendency for transport to become less size selective at higher transport rates the bedload time series show autoregressive behaviour but are dif cult to distinguish by this method state space plots and associated measures of time series separation reveal the structure of the time series more clearly the measured pulses have distinctly different time series characteristics from those modelled using a one dimensional sediment routing model in which bed shear stress and grain size are varied randomly these results suggest a mechanism of pulse generation based on irregular low amplitude bedforms that may be generated in channel or may represent the advection of material supplied by bank erosion events copyright 2003 john wiley sons ltd

mobility of river tracer pebbles over different timescales 1 tracer pebbles are widely used to learn about gravel transport along rivers movement over short times and distances is dominated by factors controlling entrainment relative particle size and shear stress movement at longer scales also involves depositional factors burial and reexposure and exchange between channels bars and other depositional environments we mapped mixed size tracers in six reaches of a small scottish river after 2 and 8 years to investigate differences in relative and absolute mobility and infer the importance of burial and exchange patterns of relative mobility according to size and shear stress both within and between reaches did not change significantly some local bunching of tracers was apparent in both surveys with redistribution from pools into riffles and bars the main change was that virtual velocities were 50 lower and estimated gravel fluxes were also lower in the longer term this slowdown is attributed to vertical mixing giving decreased mobility as surface seeded tracers become buried long term storage in bars and other less active parts of the system and in this channel advection of tracers downstream onto a finer bed giving higher relative size

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

fluvial transport of sand and gravel mixtures with bimodal size distributions abstract initiation of motion and fractional transport rates of sediments with bimodal size distributions were measured in a laboratory channel and in a natural channel transport experiments were conducted with unimodal 100 sand and 100 gravel bed sediments and three bimodal mixtures of the two sediments 10 gravel 90 sand 25 gravel 75 sand and 45 gravel 55 sand in the bimodal sediment beds the critical shear stress for the initiation of motion showed very little change with grain size for the sand sizes but increased with grain size for the gravel sizes for low flow strengths only the sand sizes were in transport while for the highest flow strengths the bed load sediment size distribution approached that of the bed material

sediment dynamics across gravel sand transitions implications for river stability and floodplain recycling data collection was funded through a nerc gcrf building resilience grant ne p015905 1 to h d s and an epsrc gcrf institutional grant ep p510944 1 to m a sampling equipment was lent by ed tipper and bob hilton the rtk gps was loaned from the nerc geophysical equipment facility

stochastic prediction of sediment transport in sand gravel bed rivers classical deterministic bedload transport predictors are applied to sand gravel bed rivers the turbulent bed shear stress is modeled according to a probability distribution to obtain realistic bedload transport rates at incipient motion in extending the predictors to stochastic predictors for nonuniform sediment many parameters that represent near bed turbulence and the particle size distribution must be chosen the parameters that give realistic results are chosen by analyzing the results of a new experimental flume dataset with relatively large water depths choosing other combinations of parameters may give equal total bedload transport rates but at the cost of large errors in fractional transport rates attention is given to the hiding exposure phenomenon and a hindrance effect related to nonuni form sediment validation based on two independent field datasets shows that successful predictions of particle sizes near the threshold for motion are feasible using the stochastic approach while the deterministic approach gives successful predictions well above incipient motion

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

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

the complexity of gravel bed river topography examined with gradual wavelet reconstruction this study analyzes the nonlinear nature of gravel bed elevation data series obtained in a large scale flume facility for four different discharges including spatial transect series and temporal series at a fixed location the goals are to infer the degree of complexity to assess the scales and features in the signals that mostly contribute to this complexity and to discern the difference in the manner in which this complexity is expressed in time and space the asymmetry series magnitude of the first order derivative of the signal over a separation distance raised to the third power of the bed elevation data forms the basis of our nonlinearity analysis an important and novel dimension is the adoption of a recently introduced approach to surrogate data generation gradual wavelet reconstruction this produces partially linearized counterparts of the original series that preserve an increasing degree of the underlying nonlinear structure in the original data while randomizing the rest this allows us to discern the difference in the manner in which the nonlinear asymmetry is expressed in the temporal and spatial data series comparison in the real and phase space of the original and surrogate series is performed and the analysis reveals that the complexity of both spatial and temporal series increases with discharge and that the nature of nonlinearity is quantitatively and qualitatively different for the spatial series asymmetry is expressed at large scales and is shown to result from the organization of intermediate scale features in contrast asymmetry in the temporal series is a smaller scale phenomenon this is a consequence of the scale dependent propagating velocity of topographic features our results have implications for understanding the complexity of geomorphic processes as a function of the space and time scale considered the complexity over the bed form lagrangian time scale is different in nature to that over the flow velocity lagrangian time scale

when does bedload transport begin in steep boulder bed streams the paper presents the results of field measurements of critical conditions for bedload motion conducted in the rio cordon a steep boulder bed stream in the italian alps under conditions of high reynolds numbers and low relative submergence poorly explored before two methods have been used to determine threshold of motion the displacement of marked clasts and the flow competence approach which uses the largest grain size diameter transported by each flood event the high values of confirm the great relevance of non bedload effective shear stresses in step pool streams given by the additional form drag associated with this morphology relative submergence effects on the dimensionless critical shear stress have been quantified by considering the relative submergence ratio rh d84 and the major effect of relative size on the mobility of each particle in steep widely graded bed mixtures has been evaluated finally the dimensionless critical unit discharge has also been adopted in the regression equations as the critical hydraulic parameter because it may represent an easier parameter to use than the critical shear stress for steep rough mountain rivers copyright 2006 john wiley sons ltd

variation in the reference shields stress for bed load transport in gravel bed streams and rivers 1 the present study examines variations in the reference shear stress for bed load transport r using coupled measurements of flow and bed load transport in 45 gravel bed streams and rivers the study streams encompass a wide range in bank full discharge 1 2600 m3 s average channel gradient 0 0003 0 05 and median surface grain size 0 027 0 21 m a bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate w versus the reach average dimensionless shear stress the reference dimensionless shear stress r was then estimated by selecting the value of corresponding to a reference transport rate of w 0 002 the results indicate that the discharge corresponding to r averages 67 of the bank full discharge with the variation independent of reach scale morphologic and sediment properties however values of r increase systematically with average channel gradient ranging from 0 025 0 035 at sites with slopes of 0 001 0 006 to values greater than 0 10 at sites with slopes greater than 0 02 a corresponding relation for the bank full dimensionless shear stress bf formulated with data from 159 sites in north america and england mirrors the relation between r and channel gradient suggesting that the bank full channel geometry of gravel and cobble bedded streams is adjusted to a relatively constant excess shear stress bf r across a wide range of slopes

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

observations of flow and sediment entrainment on a large gravel bed river constant discharge reservoir releases on the trinity river california provide an unusual opportunity to unambiguously relate flow and gravel entrainment on a large gravel bed river bed shear stress 0 was estimated using local observations of depth averaged velocity gravel entrainment was measured using large tracer gravel installations lateral variability of 0 is large even for straight channels with simple trough like geometry no simple relation exists between local and cross section mean values of 0 fine grains less than 8 mm 20 30 of the bed material are transported at lower discharges than coarse grains scour to the base of the bed surface layer occurs at a dimensionless shear stress g 0 035 for g formed using local 0 and the median grain size of the gravel portion of the bed the dimensionless reference transport rate w 0 002 often used as a surrogate for the threshold of grain motion occurs at nearly the same g at smaller g entrainment and transport rates decrease rapidly becoming vanishingly small at g 0 031 even at very small gravel transport rates all sizes are transported although the coarsest sizes are in a state of partial transport in which only a portion of the exposed grains are entrained both entrainment and cumulative transport observations suggest that maximum scour depth for plane bed transport is slightly less than twice the surface layer thickness

prediction of sediment transport in step pool channels 1 in mountainous drainage networks sediment mobilized on hillslopes must first pass through steep streams before reaching lower gradient channels the bed of steep channels is typically composed of large relatively immobile boulders and finer more mobile gravel most sediment transport equations overpredict sediment flux in steep streams by several orders of magnitude because they do not account for the stress borne by immobile grains and the limited availability of the more mobile sediment we previously developed and tested in flume experiments a sediment transport equation that accounts for these two effects here we modify the parker 1990 bed load equation to include the resistance borne by steps and selective transport of the relatively mobile sediment using a range of hiding functions we test a number of resistance equations and hiding functions combined with our modified and the original parker equations against measured flow and sediment transport in three steep channels our modified sediment transport equation generally predicts the transported sediment volumes to within an order of magnitude of the measured values whereas the unmodified equations do not the most accurate sediment flux predictions were obtained from using our modified equation combined with a hiding function that calculates highly selective transport of the relatively mobile sediment furthermore this hiding function has a critical shields stress that is similar to those reported for lower gradient channels the effects of the immobile steps on flow and sediment transport are not adequately captured by simply increasing the critical shields stress to values reported in steep streams

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