bed material transport estimated from the virtual velocity of sediment this study evaluates the possibility of determining bed material transport using the virtual rate of travel of individual particles dimensions of the active layer of the streambed and porosity and density of streambed material magnetically tagged stones and scour indicators were employed in carnation creek british columbia to quantify transport rates observations cover flows up to 36 m3 s 1 0 081 transport rates ranging from 0 090 to 9 7 kg s 1 0 12 13 2 m3 h 1 display a relatively sensitive trend with maximum stream power as expected error analysis indicates that uncertainty in virtual velocity covers the majority of sample variance an evaluation of the two measurement techniques used to delineate active layer dimensions magnetically tagged stones and scour indicators indicates that they yield comparable depths widths and transport rates over the range of flows observed issues for further study are discussed 1998 john wiley sons ltd

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

a transferable method for the automated grain sizing of river gravels 1 the spatial and temporal resolution of surface grain size characterization is constrained by the limitations of traditional measurement techniques in this paper we present an extremely rapid image processing based procedure for the measurement of exposed fluvial gravels and other coarse grained sediments defining the steps required to minimize the errors in the derived grain size distribution this procedure differs significantly from those used previously it is based around a robust object detection algorithm that produces excellent results on images exhibiting a wide range of sedimentary conditions crucially without any user intervention or site specific parameterization the procedure is tested using a data set comprising 39 images from three rivers with contrasting grain lithology shape roundness and packing configuration and representing a very wide range of textures it is shown to perform more consistently than the best existing automated method achieving a precision equivalent to that obtainable by wolman sampling but taking between one sixth and one twentieth of the time the error in area by number grain size distribution percentiles is typically less than 0 05

scour chain employment in gravel bed rivers abstract the dynamics of river beds can be assessed fromscour and fill data obtained from scour chains chains have not been used extensively in gravel bed rivers due to a variety of employment difficulties chain insertion anchoring and relocation can be determined with minimal manpower by utilising a set of tested employment methodologies these are decribed in sufficient detail to allow fluvial geomorphologists ecologists and river engineers to more commonly utilise chains in studies of river stability and river response

particle clusters in gravel bed rivers an experimental morphological approach to bed material transport and stability concepts structured gravel river beds clearly exert a major influence on bed stability indexing structural stability by field measurements of bed strength neglects the processes operating to entrain and transport bed material in different parts of each structure this study takes a morphological approach to interpreting the critical processes using particle tracing to determine the movement of individual cluster particles over a range of flood event magnitudes and durations the experiment was carried out on the river south tyne uk it uses flow hydrographs measured nearby and also benefits from previous studies of historical development channel morphology and sediment transport at the same site more than 30 clusters were monitored over a seven month period during which clusters occupied 7 16 per cent of the bed threshold flows delimiting three apparently contrasting bed sediment process regimes for cluster particles are tentatively set at 100 m3 s 1 and 183 m3 s 1 durations of flow at these levels are critical for cluster development rather than flow peak values wake particles are transported most easily flow straightening in the wandering channel planform reduces the stability of clusters since mechanical strength is markedly reduced by this change of direction the overall area covered by clusters between significant transport events varies little implying a dynamic equilibrium condition copyright 2005 john wiley sons ltd

size and distance of travel of unconstrained clasts on a streambed we have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers distance of movement was scaled using the mean distance of movement of the median size group of the surface material size was scaled using the median b axis diameter of the subsurface material the scaled data are fitted with a nonlinear function clasts larger than the median of surface material exhibit a steep decline of mobility with an upper limit for mobility near di d50sub 10 smaller materials exhibit a less sensitive relation we suppose that trapping of small clasts affects their travel distance these results indicate that the movement of free surface stones is size preferred a subordinate analysis of free constrained and buried particles reveals that bed surface structure acts to reduce differential movement

the relationship between particle travel distance and channel morphology results from physical models of braided rivers channel form and sediment transport are closely linked in alluvial rivers and as such the development of a conceptual framework for the downstream controls on particle mobility and likely deposition sites has immense value in terms of the way we understand and predictively model rivers despite the development of conceptual models which frame flood scale particle transport distance termed path length as a function of channel bar locations an understanding of the controls on such path lengths in braided rivers remains especially elusive in large part due to the difficulty in explicitly linking morphology and particle transport distances in the field here we utilize a series of laboratory flume experiments to link path length distances with channel morphology our morphologic characterization is based on ultrahigh resolution digital elevation models and bar classifications derived from structure from motion topography while we simultaneously capture particle path lengths using fluorescent tracer particles over the course of five physical model simulations our findings underscore the importance of channel bars in acting as deposition sites for particles in transport 81 of recovered tracers were found in association with compound point lateral or diagonal bars bar heads 29 and bar margins 41 were the most common bar related deposition surfaces for recovered tracers peaks in particle deposition frequency corresponding to channel bars were often noted on path length distributions from tracer data most tracers were deposited in areas that had experienced shallow z 0 002 m deposition average path length distance 2 5 m was closely related to average confluence diffluence spacing 2 3 m across all runs the transferability of this understanding to braided streams has important implications for the development of simplified morphodynamic models which seek to predict braided channel evolution across multiflood timescales

equal mobility bed load transport in a small step pool channel in the ouachita mountains equal mobility transport emt of bed load is more evident than size selective transport during near bankfull flow events in a small step pool channel in the ouachita mountains of central arkansas bed load transport modes were studied by simulating five separate runoff events with peak discharges between 0 25 and 1 34 m3 s 1 0 to 1 6 year recurrence intervals in a natural channel using controlled releases from a storage tank emt occurrence was investigated using four different bed load relationships suggested by previous research with each of these approaches the relationship of a given bed load characteristic dmax distribution percentile displacement distance and skewness to some independent factor c and grain size was assessed to determine which transport mode was evident regression models derived using combinations of these four relationships with different datasets provide seven separate tests five of the seven tests indicate that emt occurred or was predominant several reasons may explain the apparent contradictory results but the confounding effects of changes in the structural arrangements of bed material prior to or during the events seem particularly important

modelling bedload yield in braided gravel bed rivers abstract this paper outlines an approach for estimating the annual bedload yield of a braided channel this procedure is based on the extension of theory of flow and sediment transport in braided rivers recently presented by paola 1996 the revised approach accounts explicitly for the relationship between increasing discharge and varying channel hydraulics and is suitable for use in obtaining bedload transport rate estimates over a range of discharges integration of such estimates using flow duration data allows annual bedload yield to be determined model parameterisation is achieved using topographic survey data for the waimakariri river new zealand comparison of modelled bedload yield with values estimated from repeated topographic surveys indicates that the model is able to accurately predict both the medium term c 30 years mean annual bedload yield of the waimakariri at crossbank the section 17 8 km upstream of the river mouth and also short term fluctuations in bedload yield associated with varying annual flow statistics streamwise patterns of volumetric erosion and deposition determined for a 45 km length of the waimakariri using the model are also in broad agreement with trends identified in topographic survey data for the period 1961 1997 however significant deviations between modelled and surveyed volumes of cut and fill are evident at some locations comparison of model performance with conventional applications of bedload transport equations which tend to underestimate transport rates for braided channels suggests that the approach presented here may represent a significant improvement this is the case because the model quantifies the relationship between braid intensity and spatial variability in flow hydraulics at a cross section output from the model suggests that braided rivers may transport a significant proportion of their annual bedload at lower discharges than those indicated by earlier theoretical approaches results also provide quantitative support for the argument that increased intensity of braiding may promote higher rates of bedload transport in gravel bed rivers

bed material transport estimate in large gravel bed rivers using the virtual velocity approach this paper reports on a first attempt of using the virtual velocity approach to assess sediment mobility and transport in two wide and complex gravel bed rivers of northern italy displacement length and virtual velocity of spray painted tracers were measured in the field also the thickness of the sediment active layer during floods was measured using scour chains and post flood morphological changes as documented by repeated survey of channel cross sections the effects of eight and seven floods were studied on the tagliamento and brenta rivers where 259 and 277 spray painted areas were surveyed respectively in the tagliamento river 36 of the spray painted areas experienced partial transport whereas in the brenta river this accounted for 20 whereas full removal gravel deposition was observed on 37 and 26 of these areas on the tagliamento and brenta rivers respectively the mean displacement length of particles the thickness of the active layer and the extent of partial transport are well correlated with the dimensionless shear stress the virtual velocity approach allowed calculation of bed material transport over a wide range of flood magnitudes annual coarse sediment transport was calculated up to 150 for the tagliamento and 30 103 m3 yr 1 for the brenta the outcomes of this work highlight the relevance of partial transport condition as it could represent more than 70 of the total bed material transported during low magnitude floods and up to 40 for near bankfull events results confirm that bed material load tends to be overestimated by traditional formulas copyright 2016 john wiley sons ltd

bedload layer thickness and disturbance depth in gravel bed streams a field investigation in ten gravel bed stream reaches determined that substrate disturbance depth associated with a moving bedload layer was a small multiple of the bed surface d90 disturbance depth during plane bed transport of coarse heterogeneous mixtures appeared similar in magnitude to particle exchange depth and moving layer thickness maximum disturbance depth was distributed approximately uniformly over the most active areas of the streambed when local scour and fill were negligible the distribution upper bound was the smaller of approximately 1 5 times the competent grain size or twice the surface d90 and was invariant with flow strength once the largest grains present were mobilized disturbance depth did not scale with grain sizes smaller than d50 when larger grains were mobilized thicker traction carpets were not predicted to occur because much larger shear stresses then observed naturally were needed to mobilize two or more layers of the bed simultaneously bedload transport rate in coa

gravel transport in the braided waimakariri river mechanisms measurements and predictions abstract yields of grabel bed load in the braided lower waimakariri river in new zealand are investigated by several methods and for different timescales short term several months medium term several years and long term 1967 1983 short term estimates of gravel yield were made for five such periods in 1986 1987 by a morphological method bed morphology in a selected reach was mapped before and after flood events the gross volume of scour in that time period measured and a mean transport rate through the reach computed on the basis of the step distance of gravel movement from scour zones to areas of accretion the findings indicate that a considerable fraction of the annual gravel load is derived from scour of braid banks with the gravel being shunted downstream from one zone of temporary storage to another computation of bed load yields during the same period by the meyer peter and muller mpm equation revealed excellent agreement with the morphological estimate comparison of the mpm prediction based on flow duration data for 1967 1983 also showed excellent agreement with the long term rate assessed by stadia survey of cross sections in the reach of accumulation downstream of the study reach predictions using the mpm formula for the medium term however while acceptable showed greater differences from the surveyed rates evidence indicates that this is related to highly dynamic bed conditions in which the availability of more mobile finer grained gravel in the reach changes from year to year as slugs of such sediment move intermittently downstream thus while prediction of a short term yield based on bed conditions at the time of study and a long term yield based on temporal mean conditions as inferred from bed conditions averaged along the river are realistic goals the accurate prediction of successive short term or medium term yields seems to be impossible unless detailed analysis of bed conditions especially grain size is undertaken in each time period a major task

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

accounting for uncertainty in cumulative sediment transport using bayesian statistics abstract that sediment transport estimates have large uncertainty is widely acknowledged when these estimates are used as the basis for a subsequent analysis such as cumulative sediment loads or budgets treatment of uncertainty requires careful consideration the propagation of uncertainty is a problem that has been studied in many other scientific disciplines in recent years bayesian statistical methods have been successfully used to this end in hydrology ecology climate science and other disciplines where uncertainty plays a major role their applications in sediment transport however have been few previous work demonstrated how deterministic sediment transport equations can be brought into a probabilistic framework using bayesian methods in this paper we extend this basic model and apply it to sediment transport observations collected on the snake river in wyoming usa these data were used previously to develop a 50 year sediment budget below jackson lake dam we revisit this example to demonstrate how viewing sediment transport probabilistically can help better characterize the propagation of uncertainty in the calculation of cumulative sediment transport we present the development of probabilistic sediment rating curves that rely on deterministic sediment transport equations and then show how these can be used to compute the distribution of sediment input and output for each year from 1958 to 2007 the bayesian approach described provides a robust way to quantify uncertainty and then propagate it through to subsequent analyses results show that transport uncertainty is quantified naturally in the bayesian approach making it unnecessary for modelers to assume some specified error rate e g 5 when developing estimates of cumulative transport further we demonstrate that a bayesian approach better constrains uncertainty and allows sediment deficit and surplus to be examined in terms of quantified risk

three dimensional flow structure around small scale bedforms in a simulated gravel bed environment pebble clusters are common small scale morphological features in gravel bed rivers occupying as much as 10 per cent of the bed surface important links exist between the presence of pebble clusters and the development of flow structures these links are poorly understood at the three dimensional level particularly neglected has been the effect of clusters on the lateral flow characteristics a laboratory study was conducted using a hydraulic flume within which simulated pebble clusters were superimposed onto a plane bed of gravel material high resolution three dimensional flow data were collected above the bed at two different flow depths using an acoustic doppler velocimeter the results present evidence of the importance of lateral flow in the development of turbulent flow structure narrow regions of high lateral and downstream turbulence intensity exist to both sides of clusters and in a three dimensional separation zone in their lee this may indicate the presence of horseshoe type vortical structures analogous to those identified in less hydraulically rough environments however it is likely that these structures are more complicated given the mutual interference of the surrounding medium the lateral flow was also identified as a key component in the upwelling identified by other authors in the lee of pebble clusters the results of the vertical flow analysis confirm the hypothesis that six regions with distinct vertical flow characteristics exist above clusters flow acceleration up the stoss side of the cluster recirculation behind the cluster in the wake region vortex shedding from the pebble crest and shear layer flow reattachment downstream of the cluster upwelling of flow downstream of the point of reattachment and recovery of flow copyright 2001 john wiley sons ltd

virtual rate and mean distance of travel of individual clasts in gravel bed channels travel distances in gravel bed rivers of tagged particles of various sizes are related to excess stream power estimated for peak discharge mean distance of movement irrespective of grain size is weakly correlated with stream power especially near the threshold of movement there may be several reasons for the weak correlation including variable effects of bed structure varying magnitudes of sediment mobilizing events and sampling problems grain size itself is of marginal significance the virtual rate of travel calculated using total time for which the flow is larger than that needed to initiate clast movement also bears a weak relation to the excess stream power over the period better results are obtained by relating the virtual rate of travel to the first peak of the flow event only this implies that the initial seeding of the tagged particles dominates the observations nonetheless an underlying general relation is indicated by the data which are derived from a wide range of flow regime types

an exchange flow between the okhotsk sea and the north pacific driven by the east kamchatka current 1 a new mechanism for driving the water mass exchange between the okhotsk sea and the north pacific is presented this exchange flow originates from the east kamchatka current ekc a western boundary current of the subpolar gyre and occurs through the two deepest straits of the kuril island chain the kruzenshtern and bussol straits an inflow toward the okhotsk sea occurs at the northern kruzenshtern strait and an outflow toward the north pacific occurs at the southern bussol strait by using the kelvin s circulation theorem around the island between the two straits we show that the transport of the exchange flow entering the okhotsk sea is determined such that the frictional stresses around the island exerted by the bifurcated ekc integrate to zero this forcing mechanism is different from the dynamical framework of the widely used island rule both an analytical analysis and 1 5 layer model experiments demonstrate that the strait width lateral viscosity and island geometry are controlling parameters for the exchange flow transport because they affect the magnitude and length scales of the frictional stresses inertia of the ekc decreases the exchange flow by enhancing the frictional stress along the northern coast of the kuril island model experiments with realistic topography further reveal that while the steep continental slopes have minor impact on the exchange flow transport the subsurface peninsula located east of the kuril island works to decrease the exchange flow by altering the length scale of the frictional stresses and enabling the ekc to flow past the island

evaluating short term morphological changes in a gravel bed braided river using terrestrial laser scanner abstract braided rivers are dynamic and complex environments shaped by the balance of the flow and sediment regimes and by the influence of the riparian vegetation and disturbances such as floods in particular the balance between sediment supply and transport capacity can determine the morphological evolution of a river for instance aggrading and widening trends are distinctive of reaches where sediment supply is higher than transport capacity in contrast incising and narrowing tendencies are dominant the aim of the present study is to analyze the short term morphological dynamics and the processes of erosion and sediment deposition along a small reach of a relatively unimpacted gravel bed braided river tagliamento river northeast italy using a terrestrial laser scanner tls the study area is around 23 ha and has been surveyed before after two periods with relevant flood events two of which were higher than the bankfull level and occurred between september 2010 and september 2011 the very high point clouds density allowed us to derive three high resolution digital elevation models dems with 0 125 0 125 m pixel size scan data cloud merging was achieved with an overall high degree of accuracy and resolution subcentimeter topographic data were more accurate for exposed surfaces than those collected in wet areas a detailed net of dgps control points allowed us to verify the high quality of the dems derived from the surveys rmse of about 5 cm two dems of difference dod were computed revealing different and consistent episodes of erosion and deposition within the analyzed area and changes in morphology of channel and bars could also be detected such as bar edge accretion and bank erosion demonstrating a strong dynamicity of the tagliamento river moreover a very detailed estimation of the surface roughness in the study area has been carried out permitting a large scale analysis of the roughness values distribution the results of the analysis on the tls collected data show that along a river with a high natural character i e tagliamento river the dynamic processes are also common during low magnitude events

coupling channel evolution monitoring and rfid tracking in a large wandering gravel bed river insights into sediment routing on geomorphic continuity through a riffle pool sequence bedload transport and bedform mobility in large gravel bed rivers are not easily monitored especially during floods large reaches present difficulties in bed access during flows for flow measurements because of these logistical issues the current knowledge about bedload transport processes and bedform mobility lacks field based information while this missing information would precisely match river management needs the lack of information linking channel evolution and particle displacements is even more striking in wandering reaches the durance river is a large wandering gravel bed river catchment area 14 280 km2 mean width 240 m located in the southern french alps and highly impacted by flow diversion and gravel mining in order to improve current understanding of the link between sediment transport processes and river bed morphodynamics we set up a sediment particle survey in the channel using radio frequency identification rfid tracking and topographic surveys gps rtk and scour chains for a 4 year recurrence interval flood by combining topographic changes before and after a flood intraflood erosion deposition patterns from scour chains differential routing of tracer particles and spatial distribution of bed shear stress through a complex reach this paper aims to define the critical shear stress for significant sediment mobility in this setting gravel tracking highlights displacement patterns in agreement with bar downstream migration and transport of particles across the riffle within this single flood event because no velocity measurements were possible during flood a telemac three dimensional model helped interpret particle displacements by estimating spatial distribution of shear stresses and flow directions at peak flow although rfid tracking in a large wandering gravel bed river does have some technical limitations burial recovery process time consuming it provides useful information on sediment routing through a riffle pool sequence

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

continuous measurement of sediment transport in the erlenbach stream using piezoelectric bedload impact sensors we report on bedload transport observations using piezoelectric bedload impact sensors pbis an indirect method of estimating the volume of bedload transport of coarse sediment the pbis device registers vibrations produced by bedload particle diameter 20 mm and records the signal as a sum of the number of impulses per time sediment transport at the erlenbach stream has been continuously monitored with a pbis array starting in 1986 the sensor array spans the width of an entire cross section and is mounted flush with the surface of a check dam immediately upstream of a sediment retention basin we compare pbis data with long term sedimentation records obtained from repeated surveys of material stored in the sediment retention basin with artificial sediment input under controlled conditions in the field and also with laboratory experiments the rate of bedload transport is proportional to the number of impacts on the sensor per unit time the reliability of the calibration relationship increases with the length of the observation period e g for higher numbers of impacts and larger bedload volumes sediment volumes for individual flood events estimated with the pbis method are in agreement with volumes estimated using an independent empirical method based on the effective runoff volume of water the peak water discharge and the critical discharge for the onset of sediment transport copyright 2007 john wiley sons ltd

entrainment displacement and transport of tracer gravels the mass and size distribution of grain entrainment per unit bed area may be measured by replacing a volume of the bed with tracer gravels and observing the mass difference before and after a transport event this measure of spatial entrainment is relevant to any process involving size selective exchange of sediment between transport and bed and may be directly used in calculations of sediment transport rate using an elementary relation for fractional transport components presented here this relation provides a basis for evaluating tracer data collected by different methods and may be used to provide physical insight regarding the expected behaviour of tracer grains the variation with grain size of total displacement length l ti depends on the degree of mobilization of the individual fractions on the bed surface l ti is independent of d i for smaller fully mobile sizes and decreases rapidly with d i for larger fractions in a state of partial transport in which a portion of the surface grains remain immobile through the flow event the boundary between fully and partially mobile grain sizes increases with flow strength these inferences are supported by values of l ti calculated from flume experiments and provide a physical explanation for a summary relation between l ti and d i based on field data 1997 john wiley sons ltd

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

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

on characterization of the imbrication of armored gravel surfaces abstract imbrication of static armor surfaces is investigated based on laser scanned dems collected from both flume experiments and the field two approaches are employed to study the imbrication including inclination index and aspect and slope as method the inclination index is found to be a sensitive and valid method for indicating the existence of imbrication application of this method however requires high resolution surfaces a positive relationship is obtained between the inclination index and effective grain size which can be used to deduce the historical armoring shear stress the as method based on the statistics of local gradients describes the imbrication in a more straightforward way however this method is based on the statistics of orientations of grids rather than individual grains an improved as method is proposed in this study based on orientations of grains with the help of automated grain segmentation method the improved as method makes this method more reasonable and increases the applicable range

assessment of morphological changes induced by flow and flood pulses in a gravel bed braided river the tagliamento river italy large gravel bed braided rivers show a highly dynamic evolution where even moderate flow pulses can produce significant morphological adjustment an estimation of network variations is needed by river authorities as well as by ecologists for better managing river ecosystems however few field investigations have been performed to measure the effect of unsteady discharges in braided rivers in this paper we report on the morphological changes induced by six different floods in a 1 km long reach of the tagliamento river italy topographic surveys were performed after each significant flood and an automatic digital camera continuously monitored the planimetric configuration repeated measurements of cross section profiles allowed the quantification of the transported sediments volume and in particular provided a basis to estimate the percentage of the floodplain subject to morphological evolution data analysis showed a good relationship between peak flow level active width and mean bed level changes similarly lateral channel shifting is likely to increase for larger floods monitoring of a range of discharges up to bankfull conditions revealed a fundamental difference between flow pulses events on the average occurring one or more times per year and flood pulses with a return period larger than two years the morphological evolution in the former case is mainly limited to a few active branches and is locally driven by bank erosion at the bend apex on the other hand larger floods induce a complete reworking of the network configuration where bifurcations and confluences play a crucial role single morphological units like branches nodes bars are no longer recognizable after this type of event braided rivers are therefore characterized by the occurrence of two different spatial and temporal scales that force the ecosystem dynamics

bedload entrainment in low gradient paraglacial coastal rivers of maine u s a implications for habitat restoration abstract the rivers of coastal maine flow through mainstem lakes and long low gradient reaches that break the continuum of bedload transport expected in nonparaglacial landscapes stream erosion of glacial deposits supplies coarse sediment to these systems the land use history includes intensive timber harvest and associated dam construction which may have altered the frequency of substrate mobilizing events these watersheds are vital habitat for the last remaining wild anadromous atlantic salmon in the united states future adjustments in channel morphology and habitat quality via natural stream processes or restoration projects depend on erosion transport and deposition of coarse sediment these factors motivate our study of competence at four sites in the sheepscot and narraguagus watersheds three of the four sites behaved roughly similarly with particle entrainment during intervals that include winter ice and spring flood conditions and relatively minor bed mobilization during moderate floods in the summer and fall with a recurrence interval of 2 3 years the fourth site on the sheepscot river mainstem exhibits more vigorous entrainment of marked particles and more complex three dimensional channel morphology this contrast is partially due to local geomorphic conditions that favor high shear stresses particularly relatively steep gradient but also likely to nourishment of the bedload saltation system by recruitment from an eroding glacial deposit upstream our results suggest that the frequency and magnitude of bedload transport are reach specific depending on factors including local channel geometry upstream sediment supply and transport and formation of anchor ice this presents a challenge for stream practitioners in this region different reaches may require contrasting management strategies our results underscore the importance of understanding channel processes at a given site and assessing conditions upstream and downstream as a prerequisite for conducting habitat restoration projects

bedload transport measurements at the erlenbach stream with geophones and automated basket samplers in the erlenbach stream a pre alpine steep channel in switzerland sediment transport has been monitored for more than 25 years near the confluence with the main valley river stream flow is monitored and sediment is collected in a retention basin with a capacity of about 2000 m3 the basin is surveyed at regular intervals and after large flood events in addition sediment transport has been continuously monitored with piezoelectric bedload impact and geophone sensors since 1986 in 2008 2009 the measuring system in the erlenbach stream was enhanced by installing an automatic system to obtain bedload samples movable metal baskets are mounted on a rail at the downstream wall of the large check dam above the retention basin and they can be moved automatically into the flow to take bedload transport samples the wire mesh of the baskets has a spacing of 10 mm to sample all sediment particles coarser than this size which is about the limiting grain size detected by the geophones the upgraded measuring system permits to obtain bedload samples over short sampling periods and to measure the grain size distribution of the transported material and its variation over time and with discharge the analysis of calibration relationships for the geophone measuring system confirms findings from very similar measurements which were performed until 1999 with piezoelectric bedload impact sensors there is a linear relationship between impulse counts and bedload mass passing over the sensors findings from flume experiments are used to discuss the most important factors which affect the calibration of the geophone signal the bedload transport rates as measured by the moving baskets are among the highest measured in natural streams with values of the order of several kilograms per meter per second copyright 2012 john wiley sons ltd

partial transport in a natural gravel bed channel 1 partial transport is documented in the gravel bed channel of carnation creek using magnetically tagged stones for four flood peaks the active proportion of surface grains was used to map streambed areas into distinct units of three different levels of grain entrainment in partially mobile regions of the bed the active proportion of surface grains declines with grain size as flow increases areas of partial transport grow at the expense of inactive areas and fully active areas replace areas with partial mobility approximately 25 50 of the bed remained in a state of partial mobility during a flood with a 2 year return period indicating that inactive regions of the bed surface typically persist from year to year during a flood with a 7 year return period surface grain entrainment was nearly complete indicating that full mobilization of surface grains is not a frequent event

channel adjustments bedload transport and sediment sources in a gravel bed river brenta river italy this paper deals with the brenta river the alps italy a gravel bed river strongly affected by human impact the aims of this work are a to reconstruct the channel adjustments that have occurred during the last decades and b to analyse the present condition of the river in terms of bedload transport and sediment sources the study reach which is 23 km long was investigated using historical maps aerial photographs 12 monumented cross sections laser altimetry data and field survey bedload transport rates were estimated by a morphological approach which allows the construction of a sediment budget dramatic channel adjustments have occurred in response to an alteration of sediment fluxes caused by different human interventions gravel mining dams torrent control works major channel adjustments specifically incision up to 8 9 m and narrowing took place between the 1950s and the 1980s on the other hand widening has become the most important process and incision has continued only in the upper part of the study reach in more recent times about the last 20 25 years the estimate of bedload transport rates for the period 1984 1997 was carried out starting from the erosion and deposition volumes of the single sub reaches such an estimate has shown that a a high spatial variability of gravel transport rates exists from 0 up to 73 200 85 400 m3 year and b most of the bedload is supplied by local sources bank erosion is the main source of sediments for bedload transport and its contribution can be estimated as from 9 up to 20 times higher than the upstream contribution finally from a practical point of view results from this study should be taken into account in addressing maintenance sediment management and restoration along a dynamic and unstable river such as the brenta copyright 2007 john wiley sons ltd

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

bedload transport measurements with impact plate geophones comparison of sensor calibration in different gravel bed streams indirect bedload transport measurements have been made with the swiss plate geophone system in five gravel bed mountain streams these geophone sensors record the motion of bedload particles transported over a steel plate mounted flush with the channel bed to calibrate the geophone system direct bedload transport measurements were undertaken simultaneously at the erlenbach in switzerland a moving basket sampler was used at the fischbach and ruetz streams in austria a helley smith type bedload sampler provided the calibration measurements a bunte type bedload trap was used at the rofenache stream in austria at the nahal eshtemoa in israel reid type slot bedload samplers were used to characterize the response of the geophone signal to bedload particles impacting on the plate geophone summary values were calculated from the raw signal and stored at one second intervals the number of impulses i e the number of peaks above a pre defined threshold value of the geophone output signal correlated well with field measured gravel transport loads and was found to be a robust parameter the relations of impulses to gravel transport loads were generally near linear but the steepness of the calibration relations differed from site to site by comparing the calibration measurements from the different field sites and utilizing insights gained during preliminary flume experiments it has been possible to identify the main factors that are responsible for site specific differences in the calibration coefficient the analysis of these calibration measurements indicates that the geophone signal also contains some information about the grain size distribution of bedload copyright 2013 john wiley sons ltd

vertical mixing of gravel over a long flood series vertical sediment exchange is a fundamental component of bedload transport in gravel bed channels this paper describes the characteristic depth of exchange achieved over a long flood series analysis is based on 11 recoveries of magnetically tagged gravels deployed in carnation creek canada completed between 1990 and 2008 vertical grain exchange mixes gravels throughout the streambed relatively rapidly within one to eight floods the mean burial depth approaches two times the surface layer thickness quantified by the 90th percentile of the size distribution finer gravels are mixed more rapidly into the bed than coarser gravels both active and passive grain exchanges throughout most of the bed produce the overall vertical distribution of marked grains gravel exchanges exhibit fairly consistent patterns once tracers are well mixed by large floods results highlight the role of flood sequence in determining exchange depths support the notion of an upper limit to exchange and underscore the importance of passive grain exchange copyright 2011 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

measuring gravel transport and dispersion in a mountain river using passive radio tracers random walk models of fluvial sediment transport recognize that grains move intermittently with short duration steps separated by rests that are comparatively long these models are built upon the probability distributions of the step length and the resting time motivated by these models tracer experiments have attempted to measure directly the steps and rests of sediment grains in natural streams this paper describes results from a large tracer experiment designed to test stochastic transport models we used passive integrated transponder pit tags to label 893 coarse gravel clasts and placed them in halfmoon creek a small alpine stream near leadville colorado usa the pit tags allow us to locate and identify tracers without picking them up or digging them out of the streambed they also enable us to find a very high percentage of our rocks 98 after three years and 96 after the fourth year we use the annual tracer displacement to test two stochastic transport models the einstein hubbell sayre ehs model and the yang sayre gamma exponential model gem we find that the gem is a better fit to the observations particularly for slower moving tracers and suggest that the strength of the gem is that the gamma distribution of step lengths approximates a compound poisson distribution published in 2012 this article is a us government work and is in the public domain in the usa

breakup and reestablishment of the armour layer in a large gravel bed river below dams the lower ebro changes in armour layer during floods under supply limited conditions are little known this paper describes the breakup and the reestablishment of the bed armour layer in the regulated gravel bed ebro river during a flooding period the study was conducted over a 28 km study reach from 2002 to 2004 the surface subsurface and bed load grain size distribution constitute the bases for the analysis of bed armouring dynamics the results indicate that the magnitude of floods controlled the degree of armouring of the river bed the initial mean armouring ratio was 2 3 with maximum values reaching 4 4 floods in the winter of 2002 2003 q8 caused the breakup of the armour layer in several sections this resulted in the erratic bed load pattern observed during the december 2002 flushing flow and in the increase in bed load transport during successive events most grain size classes were entrained and transported causing river bed incision the mean armouring ratio decreased to 1 9 in contrast during low magnitude floods in 2003 2004 q2 the coarsest fractions 64 mm did not take part in the bed load while finer particles were winnowed thus surface deposits coarsened as a result the armour layer was reestablished i e the mean armouring ratio increased to 2 3 and the supply of subsurface sediment decreased the supply and transport of bed material appear to be in balance in the river reach immediately below the dam in contrast the transport of medium and finer size classes in the downstream reaches was higher than their supply from upstream a phenomenon that progressively reduced their availability in the river bed surface hence the armour layer reworking

evolutionary trajectory of channel morphology and controlling factors in a large gravel bed river abstract interpretation of channel changes is not always straightforward since it requires a detailed reconstruction of the evolutionary trajectory of channel morphology a quantitative analysis of controlling factors and finally identification of links between evolutionary trajectories and controlling factors the aim of this paper is to explain channel adjustments and controlling factors in the tagliamento river a large gravel bed river in northeastern italy traditional methods for studying historical channel changes i e use of aerial photos topographic data were employed but numerical modelling turned out to be very useful for the quantification of bedload transport and for supporting interpretation of past changes and likely future channel evolution river channel underwent three main phases of adjustment over the last 200 years the first two phases from the end of the nineteenth century to the early 1990s were characterized by narrowing channel width decreased from 1250 to 540 m and incision about 1 m and the third phase from the 1990s to present day by widening from 540 to 600 m and slight aggradation about 0 2 m combining evolutionary trajectories of channel morphology and analysis of controlling factors we argue that the long term channel evolution of the tagliamento river was driven mainly by human intervention at the reach scale i e sediment mining and channelization changes in sediment supply in the catchment area had no or minor effects in the study reach the most recent changes i e widening and slight aggradation are explained as a response to past disturbances that have produced a remarkable change of channel geometry and an increase of unit stream power in the reach magnitude and frequency of formative discharges are a key driving factor of the intensity of recent changes while this was not the case during the previous phases of adjustment

comparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel bed rivers several methods were employed in the ardennian rivers belgium to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events the use of scour chains has shown that the depth of the active layer is systematically less than the b axis of the average particle size d50 of the elements which compose the surface layer of the riffles this indicates that only a partial transport exists during low magnitude floods the bedload discharge has been evaluated by combining data obtained using the scour chains technique and the distance covered by tracers quantities of sediment transported during frequent floods are relatively low 0 02 t km 2 due to the armour layer which protects the subsurface material these low values are also related to the fact that the distance calculated for mobilized bedload only applies to tracers fitted with pit passive integrated transponder tags diameter 20 mm whereas part of the bedload discharge is composed of sand and fine gravel transported over greater distances than the pebbles the break up of the armour layer was observed only once for a decennial discharge during this event the bedload discharge increased considerably 2 t km 2 the use of sediment traps data from dredging and a helley smith sampler confirm the low bedload transport in ardennian rivers in comparison to the bedload transport in other geomorphological contexts this difference is explained by the presence of an armoured layer but also by the imbricated structures of flat bed elements which increase the resistance to the flow finally the use of the old iron industry wastes allowed to quantify the thickness of the bed reworked over the past centuries in the lembree river the river bed contains slag elements up to a depth of about 50 cm indicating that exceptional floods may rework the bed to a considerable depth copyright 2012 john wiley sons ltd