the infiltration of fine matrices into coarse grained alluvial sediments and its implications for stratigraphical interpretation traps designed to simulate the framework gravel of natural streams and installed in a coarse grained channel allow the identification and differentiation of the processes governing the infiltration of fines into bed interstices vertical patterns of matrix distribution reflect the relative importance of variables acting at both the surface of the channel bed and within the bed sediments differences in grain size and therefore pore size of the framework exercise a strong controlling influence an armour layer of coarse particles superimposed on comparatively finer sub surface bed material common to many coarse grained rivers is shown to encourage the clogging of near surface pores with matrix fines below the obstruction interstices may be protected from incursion and the gravel framework is often left open this is the reason for patchy concentrations of matrices often reported in the stratigraphical literature in contrast a uniform size grading of the bed sediment or an upwards fining leads to a more uniform packing of the interstices with matrix flood history is shown to be an important determinant of both the size distribution and the spatial distribution of matrices and is useful in providing a dynamic explanation of the variability of matrices observed from place to place in ancient deposits

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

disturbances in coarse bedload transport in a high mountain stream channel system western tatras poland abstract bedload transport measurements in the formerly glaciated chocholowski catchment located in the western tatras in poland were performed in the period 1975 to 2018 measurement of bedload displacement allowed to determine the role of bedload transport disturbances in both system connectivity and sediment transfer bedload transport occurred as many as triggered up to several times a year the longest distances were observed during rain on snow events between 12 m in headwaters and over 100 m in the fluvial valley bedload became activated along the entire length of the channel system 10 5 km every 2 to 5 years when the stream discharge exceeded 10 m3 s 1 with an average of 1 25 m3 s 1 in such situations bedload dynamics increased downstream and stream power was sufficient to overcome local barriers i e boulder and log steps downstream increases in bedload dynamics may become disturbed and inverted by sudden snowmelt and locally heavy rainfall these types of events caused the dynamics of bedload transport to be 190 to 320 greater in the upper part of the studied catchment and to decline in the downstream direction over a distance of 7 km to yield an attenuation effect the same pattern was observed in local tributaries where the dynamics of bedload transport were 200 higher in the upper part and attenuation was observed along a distance of 1 5 km in the downstream direction all events of this type cause seasonal disturbances in bedload transport however the greatest effect on bedload dynamics was produced by natural deforestation a 16 decrease in forest cover causes a fluvial system disequilibrium manifested in an intensification of hydro geomorphologic processes and formation of new landforms

bed microtopography and entrainment thresholds in gravel bed rivers although increasingly acknowledged as an important factor in bedload transport few data on the effects of bed conditions on entrainment in gravel bed rivers exist this reflects to some degree the substantial difficulties involved in obtaining accurate threshold measurements in such channels using a sensor which allows initial movement of labeled particles to be detected during flood flow the entrainment thresholds of clasts incorporated into the bed microtopography have been compared with those in exposed open plane bed positions the threshold velocity of clasts from cluster bedforms the most prevalent type of bed microtopography is found to be above that of more exposed particles of like size and shape measurements indicate that the initial movement of 70 of particles in gravel bed rivers will be directly influenced by the presence of neighboring grains bed microtopography plays an important role in delaying incipient motion the influence of microtopography may be invoked to account for phenomena such as discontinuous particle movement and variations in the composition of bedload during discharge events

lichenometric application to historical and subrecent dynamics and sediment transport of a corsican stream figarella river france abstract lichenometry has been applied to blocks canyon walls and terrace deposits a growth curve specific to corsica was made based on information obtained from tombstones old monuments from the genoese occupation of corsica and roman megaliths this allowed us to create a curve going as far back as 2000 years the curve is similar to those developed in mediterranean environments cevennes and eastern pyrenees but is very different from those developed in alpine and arctic regions lichens present on boulders of the bedload allow the time at which the boulders were laid down to be determined the lichenometric study of the figarella shows firstly that there is high stream competence blocks approximately 100 cm wide are moved during the 5 year flood recent important flood events were identified in 1973 and 1869 these correspond well with historical sources other older and much more important floods were also identified this analysis has allowed us to extend the specific streampower curves proposed by costa geol soc am bull 94 1983 986 and williams geogr ann 65a 1983 227 to blocks of more than 2 m wide lichens present on the canyon walls and on terrace sides in the plain allowed the last main incision phase to be dated to the little ice age aerial photograph analysis has shown that the present braided system in the lower course of the river is no longer active

morphological characterization of cluster microforms a field study was conducted on two mountain streams in the cascade mountains of washington state on the morphological characterization of cluster microforms morphological characterization of clusters is presented in terms of i cluster shape ii cluster geometric properties and iii the spatial arrangement of clusters in the horizontal plane clusters were differentiated from other microtopography features such as reticulate structures and transverse ribs and identified clusters were categorized by shape as being of pebble line comet heap or ring type the complex spatial arrangement of clusters at the sites was characterized by using a two dimensional correlation function which allowed for measurement of the average cluster spacing properties for the rivers examined pebble shaped clusters were the most frequently observed cluster shape cluster geometric properties were found to be controlled by particles of the largest size fraction in the bed and the projected frontal width of the cluster with cluster length being linearly related to cluster width for cluster width to height ratios 3 5 results of the cluster spacing analysis suggest that cluster spacing increases with cluster size and decreases with local slope application of this principle to the available spacing data shows that cluster spacing scales with the ratio of s d0 such that s d0 constant where s is the local slope and d0 the diameter of the largest particle in the cluster

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

dynamics of bedload transport in turkey brook a coarse grained alluvial channel automatic and continuously recording samplers are deployed in a hertfordshire gravel bed stream to show that bedload transport is related to stream power the pattern is similar to that already established for north american channels but because the record is so detailed it is possible to identify the cause of the considerable scatter that is normal in such relationships a major factor is the occurrence of rhythmic pulses in bedload discharge that are not matched by similar fluctuations in hydraulic variables it is suggested that these pulses reflect downstream differences in the concentration of mobile particles in a slow moving traction carpet and that they may be likened to kinematic waves the record also reveals that the threshold of sediment transport always presumed hithero to be associated with incipient motion is related to the cessation of bedload transport in a river flood indeed the mean value of stream power at the finish of bedload transport is only 20 percent of that prevailing at the moment of incipient sediment motion because of this there is an inevitably poor correlation between actual bedload transport rates and those predicted by bedload equations which rely upon a single traction threshold these new data show that the general inverse relationship between bedload discharge and water depth grain size ratio proposed by bagnold 1977 1980 is not universal transport efficiency for this gravel bed stream is typically 0 05 per cent of available stream power which compares with 1 6 per cent for a river moving both gravel and sand and 5 per cent for another channel where bedload is composed predominantly of sand sized particles it is argued that coarse and fine grained alluvial channels may need to be considered separately by allowing for differences in traction threshold at the beginning and end of bedload events and by averaging bedload discharge flood by flood in order to smooth out the effect of pulses it is possible to achieve a reasonably good prediction of average bedload transport rate in terms of stream power

delineation of gravel bed clusters via factorial kriging abstract gravel bed clusters are the most prevalent microforms that affect local flows and sediment transport a growing consensus is that the practice of cluster delineation should be based primarily on bed topography rather than grain sizes here we present a novel approach for cluster delineation using patch scale high resolution digital elevation models dems we use a geostatistical interpolation method i e factorial kriging to decompose the short and long range grain and microform scale dems the required parameters are determined directly from the scales of the nested variograms the short range dem exhibits a flat bed topography yet individual grains are sharply outlined making the short range dem a useful aid for grain segmentation the long range dem exhibits a smoother topography than the original full dem yet groupings of particles emerge as small scale bedforms making the contour percentile levels of the long range dem a useful tool for cluster identification individual clusters are delineated using the segmented grains and identified clusters via a range of contour percentile levels our results reveal that the density and total area of delineated clusters decrease with increasing contour percentile level while the mean grain size of clusters and average size of anchor clast i e the largest particle in a cluster increase with the contour percentile level these results support the interpretation that larger particles group as clusters and protrude higher above the bed than other smaller grains a striking feature of the delineated clusters is that anchor clasts are invariably greater than the d90 of the grain sizes even though a threshold anchor size was not adopted herein the average areal fractal dimensions hausdorff besicovich dimensions of the projected areas of individual clusters however demonstrate that clusters delineated with different contour percentile levels exhibit similar planform morphologies comparisons with a compilation of existing field data show consistency with the cluster properties documented in a wide variety of settings this study thus points toward a promising alternative dem based approach to characterizing sediment structures in gravel bed rivers

structural properties of mobile armors formed at different flow strengths in gravel bed rivers differences in the structure of mobile armors formed at three different flow strengths have been investigated in a laboratory flume the temporal evolution of the bed surfaces and the properties of the final beds were compared using metrics of surface grain size microtopography and bed organization at both grain and mesoscales measurements of the bed condition were obtained on nine occasions during each experiment to describe the temporal evolution of the beds structured mobile armors formed quickly in each experiment at the grain scale 1 45 mm 9 ds50 17 mm where ds50 is the median surface particle size surface complexity decreased and bed roughness increased in response to surface coarsening and the development of the mobile armor particles comprising the armor also became flow aligned and developed imbrication at a larger scale 100 200 mm the surface developed a mesoscale topography through the development of bed patches with lower and higher elevations metrics of mobile armor structure showed remarkable consistency over prolonged periods of near constant transport demonstrating for the first time that actively transporting surfaces maintain an equilibrium bed structure bed structuring was least developed in the experiments conducted at the lowest flow strength however little difference was observed in the structural metrics of the mobile armors generated at higher flows although the range of transport rates studied was limited the results suggest that the structure of mobile armors is insensitive to the formative transport rate except when rates are low 0 03 where is the dimensionless shear stress

the spatial characterization of turbulence around large roughness elements in a gravel bed river abstract this study characterizes the flow field above and around multiple instream submerged cobbles boulders and pebble clusters in order to obtain a better understanding of the hydrodynamics associated with large roughness elements lres in gravel bed rivers spatially distributed high frequency three dimensional velocity measurements were recorded in situ using acoustic doppler velocimeters at different flow stages the spatial distributions of turbulent kinetic energy ke longitudinal component integral timescales itsu and reynolds shear stresses were characterized and are presented for selected sites the longitudinal vertical reynolds shear stress increased with flow stage more strongly than the longitudinal lateral or lateral vertical reynolds shear stresses and dominate at the highest measured flows canonical redundancy analysis was used to relate lre morphometrics and mean flow conditions to the turbulence parameters estimated in the lre wakes i e ke itsu and reynolds shear stresses lre size and mean unobstructed velocity explained the highest proportion of the variance in the turbulent wake statistics multivariate regression models based on lre width mean unobstructed longitudinal velocity and flow depth are presented offering a tool to predict lre wake turbulence

a two dimensional discrete particle model of gravel bed river systems 1 the formation of bed forms in gravel bed rivers acts as a control on stream ecology and the response of rivers to floods available models do not reproduce the range of observed bed forms and do not consider interactions between the bed and flow hydraulics the model presented here considers a gravel bed river as a complex system in which sediment clasts are represented as discrete elements simple and local rules describe the sediment and flow dynamics using a trimodal sediment distribution irregular forms that scale with particle diameter develop without explicit feedback mechanisms because of the tendency of large particles to roll along the bed surface and collect into chains feedback mechanisms such as imbrication increase the effective entrainment threshold of groups of large particles and increase the stability of these imbricate forms a second type of bed form is associated with saltating grains and emerges where particles are transported at a preferred distance the development and maintenance of larger scale bed forms require feedback between the bed and flow properties by allowing mean velocity to adjust to bed morphology and considering the effect of acceleration on turbulence generation and mean velocity profiles we demonstrate the emergence of forms similar in morphology to gravel sheets dunes and riffle pools the model is best used to complement field based studies and is suitable for testing hypotheses of streambed behavior

sediment transport from continuous monitoring in a perennial mediterranean stream vericat d batalla r j 2010 sediment transport from continuous monitoring in a perennial mediterranean stream catena 82 2 77 86 sponsorship research project sediment export from large semi arid catchments measurements and modelling sesam funded by the deutsche forschungsgemeinschaft dgf

linking spatial patterns of bed surface texture bed mobility and channel hydraulics in a mountain stream to potential spawning substrate for small resident trout abstract in this paper we examined spatial patterns of bed texture bed mobility and channel hydraulics in four reaches of a mountain stream with moderate gradient

a note on cluster bedform behaviour in a gravel bed river abstract the behaviour of cluster bedforms was investigated in the farma r central italy a study reach was surveyed and the three components of the pebble clusters occuring on the surface of a gravel bar were painted using different colours before and after each flood all the clusters were photographed and a comparison was made this simple methodology allowed the evolution of the painted clusters to be followed and also allowed a survey of the dislodgment of the clusters particles and the addition of new ones the experiment shows that the mobility of the wake grains is higher than that of the stoss particles a pebble cluster generally loses the wake grain first followed by the stoss pebbles the obstacel clast is commonly the last to be entrained and acts as a single particle after the wake and stoss particles have been dislodged different explanations of this unexpected behaviour are postulated and the interaction of pebble with bed armouring and bedload size distribution is discussed

mean flow characteristics in a rock ramp type fish pass abstractdetailed mean flow characteristics generated by a staggered arrangement of boulders in a rock ramp type naturelike fish pass were investigated experimentally at three different channel slopes 5 3 and 1 5 the results showed that this type of fish pass can produce adequate water depth and favorable flow velocity suitable for fish passage three different characteristic velocity regions were identified namely downstream of boulders wake region intermediate region and upstream of boulders some general correlations were developed for predicting the flow depth and velocity in a rock ramp fish pass as a function of normalized discharge moreover a flow resistance analysis based on basic concepts for wake interference flow regime in this fish pass has resulted in a general equation for the average velocity this study improves the understanding of the complex flow characteristics in a rock ramp fish pass

the importance of bed sediment sand content for the structure of a static armor layer in a gravel bed river the surface structure of static armor layers generated from water worked gravel bed channels was investigated with primary focus on the influence of sand content and flow rate flume experiments were conducted in which four sediment mixtures with sand contents between 1 and 38 were armored under one of three different flow rates first and second order statistical analyses were applied to digital elevation models of unarmored armored and clustered bed surface areas to identify changes in surface structure results were combined with data from previous research to create an extended data set of armored bed surfaces water worked unarmored bed surfaces established under a dynamic equilibrium flow rate impacted the topographic variability and structure of the armored beds surface complexity decreased with armor formation as surface grains preferentially aligned with the flow direction the bed surface became smoother and where sediment mixture sand content was constant there was greater smoothing of the surface during higher armoring flows as grains rearranged more easily as bulk sand content increased statistical analyses of the expanded data set showed that beds with very little sand content developed static armor layers that remained rough and had greater topographic variability than armor layers from sediments with higher sand contents the bulk sediment sand content exerted a stronger influence over the change in surface roughness and structure upon armoring than that of the flow rate during armor formation when combined with the knowledge of the local flow regime the sand content may aid in predictions related to armored bed surface structure

coarse bed material patch evolution in low order ephemeral channels abstract in river channel beds composed of a wide range of grain sizes the bed material is often arranged in discrete patches discernable by relative texture these bed material patches are the primary source of entrainable coarse sediment within the channel system and their composition and size have been found to influence the composition and rate of sediment transport twelve coarse gravel cobble sediment patches distributed throughout the channel network within a 4 53 ha watershed in southeastern arizona were monitored for 2 years changes in patch area and grain size were measured and painted patch grains were monitored to confirm that patch grains were mobilized during flow individual coarse bed material patches exhibited variable persistence during flows with return frequencies ranging from approximately 1 year to 4 6 years while no patch fully dispersed during the study period two new patches formed most coarse patches remained relatively stable in area and grain size distribution despite the entrainment of patch grains as lost grains were sufficiently replaced with sediment from upstream because of the grain replacement process and the effect of other sediment supply dynamics the changes in patch area and grain size distribution display a complex relationship with the magnitude of predicted grain mobilization within each patch results indicate that relative stability varies from patch to patch influenced by the balance of patch grains transported out of the patch and the deposition of new grains into the patch predictive models of coarse sediment transport and yield that assume the channel bed is a fixed source of sediment supply may not adequately capture the sediment dynamics within patchy channel beds and should be used with caution when applied to these environments due to the possibility of patch instability as documented in this study

the timing and magnitude of coarse sediment transport events within an upland temperate gravel bed river this paper describes the application of a new instrument to continuously measure bedload transport an impact sensor to a 72 km2 test catchment in the yorkshire dales northern england data from a network of impact sensors are linked to repeat surveys of channel morphological response to get a better understanding of the conditions that lead to sediment generation and transfer results suggest certain areas of the catchment act as key sediment sources at the annual time scale with material being quickly delivered to the lower parts of the catchment along the steep bedrock channel sediment transfer within the tributaries occurs in significantly smaller magnitudes than within the main channel but it moves more frequently and at different times of the year with transfer rates being strongly conditioned by larger scale valley geomorphology the lower 5 6 km reach sees a significant reduction in gradient and a widening of the valley this permits significant accumulation within the channel which has persisted for many years this lower reach is very sensitive to changes in sediment supply and there is good agreement between changes in bedload transport data and the surveyed channel response these observations have major implications for how river management projects should be developed in upland environments especially those where large scale geomorphological controls have a major impact upon the sediment transfer process evidence suggests that where river management restricts lateral movement of the channel and transfer of sediment into floodplain storage changes in sediment supply can lead to areas of severe accumulation acceleration of bank erosion and exacerbated flood risk

the influence of lee sediment behind large bed elements on bedload transport rates in supply limited channels a coarse surface layer can help to limit bedload transport rates in channels with cobble and gravel beds in these systems periodic boulder sized clasts often exist with small deposits of fine material in the lee of these large bed elements a combined field and flume study was conducted to investigate the potential impact of lee deposits with distinctly finer sediment sizes behind boulders on bedload transport rates detailed sediment characterizations were performed on surface subsurface and lee sediments in two coarse bedded connecticut channels bedload measurements also were conducted in a series of flows that approached the bankfull level in these two systems to determine transport rates and the size distribution of bedload material a 6 m long 0 5 m wide flume was used to model these systems with fine sediment passing over a fixed bed of sediment particles with uniform sized large bed elements sediment distributions of the lee deposits in the two connecticut channels indicate that lee deposits may be produced from winnowing of sediments from the surface layer lee deposits also exhibit sediment distributions similar to bedload sediment distributions from low to near bankfull flow in one of the two channels bedload sediments in the second channel were finer than lee deposits presumably from selective entrainment of fines flume experiments demonstrate that bedload transport rates are lower for periods of steady flow relative to periods that include either an increase or decrease in discharge the results show that lee sediments establish a metastable deposit behind each obstruction for a given discharge either increases or decreases in discharge disrupt this temporary stability and increase sediment delivery to the main flow the study suggests that the influence of the rate of change in discharge may be as important as the absolute magnitude of discharge on sediment transport rates at moderate and low discharges in sediment limited systems with large bed elements

discussion of bedload and size distribution in paved gravel bed streams by gary parker peter c klingeman and david g mclean april 1982

an electromagnetic device for automatic detection of bedload motion and its field applications a new sensor for the continuous and unmanned detection and recording of bedload motion is described the sensor acts in the same fashion as a conventional metal detector but is elongate and installed permanently in the bed of an alluvial channel artificial clasts are labelled with short lengths of ferrite rod and are seeded upstream from the sensor replacing bed particles the entrainment of seeded clasts takes them over the sensor where they distort the magnetic field and produce a change in inductance that is detected and recorded a field installation demonstrates the value of the sensor by revealing for the first time in coarse grained alluvium the spasmodic nature of particle motion reminiscent of kinematic waves it also illustrates the importance of pebble clusters in delaying particle entrainment

co evolution of coarse grain structuring and bed roughness in response to episodic sediment supply in an experimental aggrading channel this is the accepted version of the following article hassan m a saletti m zhang c ferrer boix c johnson j p l muller t and von flotow c 2020 co evolution of coarse grain structuring and bed roughness in response to episodic sediment supply in an experimental aggrading channel earth surf process landforms 45 948 961 https doi org 10 1002 esp 4788 which has been published in final form at https onlinelibrary wiley com doi abs 10 1002 esp 4788

frequency and magnitude of bedload transport in a mountain river magnitude and frequency of bedload transport was examined in the lainbach river bavaria using magnetically tagged particles during the study 18 small to moderate events and one major event occurred about 1 per cent of the flow period exceeded the entrainment threshold and at least once every year about 50 per cent of the tagged particles were mobile themajor event which occurred during the study period resulted in the deposition of a thick layer of sediment in parts of the channel and passive burial of most of the recovered particles the step pool pattern which characterized the study site prior to the large event was obliterated however the channel recovered quickly and has returned to a new step pool pattern the event changed the boundary conditions by increasing the availability of loose sediment and creating higher river bed slopes in reaches between breached check dams as a result movement of individual particles measured for events of both the same magnitude and duration occurring before and after the large event yielded different values for events which occurred after the large event the range and the mean distance of movement were about ten times higher

the influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers a reply clifford et al 1992 argue that differences in hydraulic conditions at the moment of sediment entrainment in turkey brook flood by flood are attributable to variations in the form drag that arises from cluster microforms while not denying the possibility of variable cluster concentration we show that the time dependent bed consolidation and cementation that arises as a function of interstitial matrix development can also account for variation in the traction threshold clifford et al further postulate that the growth of pebble clusters just prior to bedload entrainment is a cause of the observed differences in traction thresholds at the start and finish of bedload during a flood event this merits further examination however we point out that clusters are not only observed at low flow and therefore already present and able to contribute significant form drag as the next bedload generating flood waxes but that they are known both to survive more or less intact and to entrap mobile particles during bed disturbing events the revelation by clifford et al that flow structure in the river quarme reflects differences in bed rugosity between riffles and pools and that it relates in scale to bed roughness elements perhaps clusters is not surprising there is however some difficulty in distinguishing the precedence of chicken and egg when using this data set to elucidate the cause of cluster spacing nevertheless the inference that flow and form are dynamically linked is in sympathy with our own conclusions that cluster spacing is an equilibrium condition governed by the interplay of flow resistance and sediment transport

sediment transport processes in pool riffle sequences this paper reviews the published data for sediment transport in pool riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport this conclusion is explored in more detail using observations of sediment transport from the regulated river north tyne sediment transport rates at discharges of 30 per cent bankfull are initially highest on riffles as discharge increases sediment transport rates in pools rise sharply and values locally exceed those on riffles tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool riffle sequence coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds an appraisal of the possible mechanisms controlling sediment entrainment transport and deposition in pool riffle sequences is made which reveals evidence for the operation of a combination of hydraulic sedimentological and interactive processes a descriptive model of sediment transport processes in pool riffle sequences is presented

the theoretical foundations and potential for large eddy simulation les in fluvial geomorphic and sedimentological research abstract large eddy simulation les is a method for resolving the time dependent structure of high reynolds number turbulent flows with les it is possible to model and track the behaviour of coherent turbulent structures and study their effect on the flow field hence les is potentially an important research tool in the fluvial sciences where flow mixing sediment entrainment and sediment transport are all affected by the presence of coherent vortices and their interactions with channel boundaries and other flow structures this paper introduces the les methodology discusses a variety of ways for representing small scale processes within les the subgrid scale modelling problem and provides some examples of early work into the use of les in a fluvial context a number of advances in computational power and numerical methods are required before les can be effectively applied at the river reach scale this paper considers some recent developments and their potential for providing validated large eddy simulations of river flow at the channel scale

using multiple bed load measurements toward the identification of bed dilation and contraction in gravel bed rivers 1 this study examines bed load transport processes in a small gravel bed river beard creek quebec using three complementary methods bed elevation changes between successive floods bed activity surveys using tags inserted into the bed and bed load transport rates from bed load traps the analysis of 20 flood events capable of mobilizing bed material led to the identification of divergent results among the methods in particular bed elevation changes were not consistent with the bed activity surveys in many cases bed elevation changes were significant 1 to 2 times the d50 even if the bed surface had not been activated during the flood leading to the identification of processes of bed dilation and contraction that occurred over 10 to 40 of the bed surface these dynamics of the river bed prevent accurate derivation of bed load transport rates from topographic changes especially for low magnitude floods this paper discusses the mechanisms that could explain the dilation and contraction of particles within the bed and their implications in fluvial dynamics bed contraction seems to be the result of the winnowing of the fine sediments under very low gravel transport bed dilation seems to occur on patches of the bed at the threshold of motion where various processes such as fine sediment infiltration lead to the maintenance of a larger sediment framework volume both processes are also influenced by flood history and the initial local bed state and in turn may have a significant impact on sediment transport and morphological changes in gravel bed rivers

reach scale variability of turbulent flow characteristics in a gravel bed river in gravel bed rivers bed roughness is often controlled by the spatial distribution of pebble clusters and large protuberant clasts the turbulent flow structure around these roughness elements has been studied extensively as opposed to the effects of the spatial variability of the bed roughness on the structure of the flow at the reach scale in this paper we examine the effects of roughness elements on the spatial variability of the mean flow velocity profiles and of the properties of turbulent flow structures at the reach scale the experiment was carried out in two reaches of 25 m long in the eaton north river quebec we produced detailed topographic maps of the river reaches to obtain the spatial distribution of the main roughness elements the flow is characterized with velocity time series measured using electromagnetic current meters mounted on a vertical wading rod we collected 65 velocity profiles from which we derived turbulent flow characteristics results showed that in spite of the complex configuration of the bed velocity profiles are predominantly log linear mean and turbulent flow characteristics are only affected at a local scale and over short distances by the presence of roughness elements at the reach scale the distribution of individual roughness elements does not greatly control the spatial variability of turbulent flow properties

determination of bedload transport using naturally magnetic tracers first experiences at squaw creek gallatin county montana pebbles and cobbles constitute an important part of the sediment discharge of many streams data on the movement of this coarse bedload is important to a variety of studies including analysis of total sediment yield resevoir filling rates bridge erosion and trout spawning habitat despite the importance of knowledge regarding coarse bedload transport measurement remains difficult and unreliable a new detector has been constructed which records the movement of naturally magnetic pebbles and cobbles the detector was installed at a log sill in the bed of squaw creek gallatin county montana to evaluate the detector under natural conditions during a flood in may 1981 the detector recorded the passage of magnetic particles larger than 32 mm extrapolation of the data allows estimation of total bedload transport based on the composition of an unvegetated gravel bar upstream of the detector and on measurements of turbidity during the flood bedload accounted for approximately 66 of the solid material leaving the drainage basin since many coarse bed streams probably contain material as magnetic as the andesitic rocks in squaw creek the procedure should be widely applicable

a review of mining subsidence instrumentation and its potential application for seabed monitoring abstract extensive mineable coal reserves exist in the offshore portion of the sydney coalfield nova scotia the near surface portion of these reserves are particularly attractive in economic terms but their extraction is constrained by the amount of subsidence generated at the seafloor field proven undersea subsidence guidelines are therefore required to allow the optimal recovery of these reserves without producing unacceptable risks regarding major water inflows to the mine workings to monitor seafloor subsidence an instrumentation scheme is required that is rugged and capable of operating in the marine environment without maintenance for prolonged periods of time and during bad weather and ice conditions however to identify an optimum scheme it is first necessary to undertake a review and evaluation of existing subsidence monitoring systems that have been used on land and which may with suitable research and development and or modification be capable of use in marine conditions two principal techniques direct and indirect subsidence monitoring systems are reviewed and these include geodetics lasers geotechnical instrumentation and automatic data acquisition systems in addition potential geophysical methods for subsidence monitoring are described along with a review of the development expected with conventional land based and subsurface systems the paper concludes with a discussion of the potential application of these techniques for monitoring seafloor subsidence profiles

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

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

temporal variations in bedload transport rates associated with the migration of bedforms temporal variations in bedload transport rates that occur at a variety of timescales even under steady flow conditions are accepted as an inherent component of the bedload transport process rarely however has the cause of such variations been explained clearly we consider three data sets obtained from laboratory experiments that refer to measurements of bedload transport made with continuously recording bedload traps each data set is characterized by a predominant low frequency oscillation on which additional higher frequency oscillations generally are superimposed the period of these oscillations as isolated through the use of spectral analysts ranged between 0 47 and 168 minutes and was associated unequivocally with the migration of bedforms such as ripples dunes and bars the extent to which such oscillatory behaviour may be recognized in a data set depends on the duration of sampling and the length of the sampling time with respect to the period of a given bedform several theoretical probability distribution functions have been developed to describe the frequency distributions of relative bedload transport rates that are associated with the migration of bedforms einstein 1937b hamamori 1962 carey and hubbell 1986 these distribution functions were derived without reference to a sampling interval we present a modification of hamamori s 1962 probability distribution function generated by monte carlo simulation which permits one to specify the sampling interval in relation to the length of a bedform comparisons between the simulated and observed frequency distributions that were undertaken on the basis of the data described herein are good significant at the 90 per cent confidence level finally the implications that temporal variability which is associated with the migration of bedforms have for the accurate determination of bedload transport rates are considered

comparison of turbulent flows over clusters of varying density abstractgravel bed rivers commonly form an armored surface layer within which cluster bedforms develop clusters can form in isolation or develop cluster groups of increasing density flume experiments are presented in which a clustered armored surface was formed on a gravel bed flows were measured over an area without a cluster and areas with an isolated cluster a coupling of two clusters and a group of three clusters turbulent flow parameters reynolds stresses and turbulent kinetic energy were calculated from acoustic doppler velocimeter measured flows and visualizations of the flow patterns around clusters flow separated over the cluster crest creating an increase in the turbulent flow properties and a high turbulence intensity region when flow reattached downstream turbulent kinetic energy increased around the coupled clusters but around the grouped clusters the flow field was distinguished by a high degree of uniformity the net effect of increasing density of cluster grouping was an incr

simplicity and complexity of bed load response during flash floods in a gravel bed ephemeral river a 10 year field study 1 a decade of data for the nahal eshtemoa an unarmored gravel bed ephemeral river reveals that bed load flux during rain fed flash floods is a simple function of channel average boundary shear stress however the relation is inadequately described by a power function of the type commonly used in predictive bed load equations and a linear function gives a much better fit the success of a linear function is related to the unarmored nature of the bed material typical of ephemeral gravel bed rivers and the ready availability of sediment at all levels of transport stage bed load response to changing shear stress is largely undifferentiated as flash floods wax or wane however while most data conform to a simple relation between bed load and hydraulic stress some for individual floods or flood segments do not reflecting variability in sediment supply and texture three type behaviors are identified that assist in explaining both the general simple pattern and the limited scatter of the bed load shear stress plot type 1 provides the majority of the data giving the general linear relation between bed load flux and boundary shear stress type 2 is rare given the flashy nature of the hydrograph but represents comparatively steady flows well above the entrainment threshold during which bed load sheets or the breakup of grain clusters lead to spasmodic changes in bed load flux type 3 shows a weak relation between shear stress and bed load flux representing flow conditions just above the entrainment threshold

combined satellite gravity field model goco01s derived from goce and grace 1 the satellite only gravity field model goco01s is a combination solution based on 61 days of goce gravity gradient data and 7 years of grace gps and k band range rate data resolved up to degree order 224 of a harmonic series expansion the combination was performed consistently by addition of full normal equations and stochastic modeling of goce and grace observations the model has been validated against external global gravity models and regional gps leveling observations while low to medium degrees are mainly determined by grace significant contributions by the new measurement type of goce gradients can already be observed at degree 100 beyond degree 150 goce becomes the dominant contributor correspondingly with goco01s a global gravity field model with high performance for the complete spectral range up to degree order 224 is now available this new gravity model will be beneficial for many applications in geophysics oceanography and geodesy

threshold of coarse sediment transport in broad and narrow natural streams the threshold of coarse sediment transport has been examined in natural streambeds in an upland pennine u k area threshold values of the total boundary shear stress t0 for a given grain size in a narrow natural stream w d 11 efficiency in the entrainment process is related not only to the overall channel geometry but also varies as a function of discharge in channels characterized by compound roughness empirical curves relating t0 and a mean grain size d5 are presented but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source the reasons for this divergence are discussed the influence of grain shape was found not to be important in the initiation of motion criterion this conclusion may reflect the limited range of natural grain shapes in the study streams but might reasonably apply to other field investigations of similar streams modifications of the shields and yalin diagrams are suggested for practical applications in shallow streams with poorly graded bed material the shields parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth d5 d the increased importance of augmented drag forces in the entrainment process in shallow flows is suggested as the physical explanation for the reduced values of the shields parameter however the relationships presented should not be applied to laboratory experiments concerned with well graded sediments therefore beds with little deviation in level in which the shields parameter may be regarded as constant at high reynolds grain numbers consistent estimated field values of a threshold sediment transport parameter might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the literature

sensitivity of bed shear stress estimated from vertical velocity profiles the problem of sampling resolution bed shear stress in open channel flows is often estimated from the logarithmic vertical velocity profile however most measuring devices used in the field do not allow for flow velocity to be measured very close to the bed the lack of near bed measurements is a critical loss of information which may affect bed shear stress estimates detailed velocity profiles obtained from a field acoustic doppler velocimeter over three different bed roughnesses clearly show that the inclusion of near bed points is critical for the estimation of bed shear stress in a shallow river environment moreover the results indicate that using the full flow depth instead of the bottom 20 per cent of the profile generates an underestimation of the shear stress when flow is uniform 1998 john wiley sons ltd

smartrock transport in a mountain stream bedload hysteresis and changing thresholds of motion bedload movement is fundamentally probabilistic our quantitative understanding of gravel transport is particularly limited when flow conditions just exceed thresholds of motion in part because of

laboratory study on the evolution of gravel bed surfaces in bed armoring processes abstract cluster microforms in bed armoring have important implications for bedload transport in this study two sets of flume experiments were carried out to study the evolution of the cluster microforms in the bed armoring and their influences on the bedload transport dynamically stable armor layers were formed by applying a staged hydrograph on a mobile sediment bed consisted of poorly sorted sand and gravel thus enabling the investigation of the cluster microforms associated with different flow characteristics simulated in these experiments the size and the planimetric positions of the large particles on the bed surface were extracted using photogrammetric technique allowing analysis of surface grain size distribution and particle clustering pattern of the bed the results showed a tendency towards a coarsened and clustered bed surface which is likely responsible for the declining bedload transport rate the cluster microforms evolved with increasing bed shear stress in the following sequence flat bed pebble cluster line cluster heap cluster reticulate structure we found a bed armoring statistics that can partially explain the declining bedload transport rate under constant discharge nonetheless additional parameters are needed to simulate the fluctuation in the bedload transport rate under changing flow conditions

the influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers comment on a paper by marwan a hassan and ian reid data from turkey brook are used to demonstrate that the interaction between gravel bedforms flow resistance and bedload transport is a dynamic one both between and within hydrographs and that creation of a significant component of form drag through construction of microforms pebble clusters may precede the eventual break up of the bed in a transport event this process of drag augmentation which can be seen as a feedback mechanism delaying transport and can be likened to dilation of a soil tested in a direct shear apparatus itself appears to be dependent on the characteristics of turbulence and therefore involves feedback at a finer resolution than envisaged by hassan and reid 1990

role of settling entrainment and dispersive equivalence and of interstice trapping in placer formation grain scale placer mineral concentrating processes are reviewed early explanations of localized enrichment of heavy particles in terms of sorting associated with the settling equivalence of clasts of differing density have been superseded entrainment equivalence is shown to be a more appropriate concentrating factor in both fluvial and littoral sedimentary environments the vertical shuffling of particles of different density that is associated with dispersive equivalence is an ancillary process that presents appropriate particles for entrainment sorting however the role of entrapment of comparatively small grains in the interstices of the stream bed or the beach face is shown to be more important that hitherto envisaged this depositional enrichment is controlled by the relative sizes of the bed sediment pores and the ingressing particles local vertical changes in framework particle size dictate patterns of interstice clogging and hence the quantity of placer mineral rich matrix passing into the bed

the impact of biological bedforms on near bed and subsurface flow a laboratory evaluated numerical study of flow in the vicinity of pits and mounds the complex surface topography of river substrates controls near bed hydraulics and drives the exchange of subsurface and surface flow in rivers the topographic structures that are studied are usually formed by the flow but it is known that many animals also create biogenic bedforms such as pits and mounds here a large eddy simulation les model of flow over a pit and a mound is evaluated with flume experiments the model includes actual bedform topography and the topographic complexity of the surrounding bed surface subsurface grains are organized in a body centered cubic packing arrangement model evaluation showed strong agreement between experimental and modelling results for velocity r2 0 8 and good agreement for reynolds stresses r2 0 7 which is comparable to other similar studies simulation of the pit shows that the length of the downwelling region is smaller than the upwelling region and that the velocity magnitude is higher in the downwelling region simulation of the mound reveals that the flow is forced into the bed upstream of the mound and re emerges near the top of the mound the recirculation zone is limited at the leeside of the mound with increasing reynolds number the depth of the upwelling region at the leeside of the mound increases the analysis of shear stress indicates that sediments on the upstream edge of the pit and on the downstream face of the mound are relatively unstable these results demonstrate the effect of biogenic structures on the near bed flow field hyporheic exchange and sediment stability

the role of sediment specific gravity and availability on cluster evolution the objective of this experimental study was to account for the role of sediment availability and specific gravity on cluster formation and cluster geometric characteristics spacing and size to isolate the effects of sediment availability and specific gravity on cluster evolution mono sized spheres were used to simulate the cluster evolutionary cycle overall twelve experimental runs were carried out in the laboratory flume six of these tests were performed by using glass spheres specific gravity sg 2 58 and the other six by employing an equal combination of glass and teflon spheres sg 2 12 of the same diameter to evaluate the role of specific gravity on cluster evolution the three sediment availability conditions that were investigated here simulated isolated gravel elements pool riffle sequences and densely packed gravel bed an advanced image analysis technique was employed to track the evolution of cluster microforms and provide quantitative information about the size and shape of clusters and the number of clusters per unit area the results of this study showed that 1 sediment availability affects the architecture and size of cluster microforms and 2 clusters consisting of mono sized sediments start disintegrating at twice the incipient conditions by performing complementary tests for the isolated gravel elements case it was found that the evolutionary cycle of individual clusters could be described as follows in order of increasing stress no cluster two particle cluster comet triangle rhomboid break up copyright 2003 john wiley sons ltd

intermittent turbulent events over sandy current ripples a motion picture analysis of flume experiments abstract a series of laboratory flume experiments was conducted in order to elucidate sediment transport processes over sandy current ripples instantaneous cinematography capable of recording up to 30 frames per second was employed to trace the coherent trajectories of the sediment particles and flow analysis of channel side view images suggests that sediment behaviour over a fully developed current ripple can be categorized as distinct stages on both stoss and lee sides near the reattachment point on the stoss side sediment particles exhibit the following intermittent but coherent motions 1 impact of sediment laden fluid on the reattachment point 2 entrainment of particles 3 spiral like turbulent motion and 4 downstream dissipation of particles the sequential sediment behaviour is called micro scale kolk which is a key mechanism responsible for turbulent sediment entrainment over current ripples the salient features of boil were not observed in the crest and lee side region sequential images show the following characteristics 1 detachment of particles and fluid at the crest 2 entrainment and shielding of particles 3 avalanching and rolling of free shear layer and 4 downstream dissipation and recreation of a lee face segregation of heavy minerals was accomplished by successive avalanches along the lee slope and mixing vortices in the separation cell

effect of changes in fine grained matrix on bedload sediment transport in a gravel bed river while clay and silt matrices of gravel bed rivers have received attention from ecologists concerned variously with the deteriorating environments of benthic and hyporheic organisms their impact on sediment entrainment and transport has been explored less a recent increase of such a matrix in the bed of nahal eshtemoa an ephemeral river of the northern negev has more than doubled the boundary shear stress needed to initiate bedload from 7 n m 2 0 027 during the flash floods of 1991 2001 to 15 n m 2 0 059 during those of 2008 2009 the relation between bedload flux and boundary shear stress continues to be well defined but it is displaced the matrix now contains a significant amount of silt and clay size material the reasons for the increased entrainment threshold of bedload are explored large scale laser scanning of the dry bed reveals a reduction in grain scale morphological roughness while artificial in situ tests of matrix integrity indicate considerable cohesion the implications for adopting bed material sampling strategies that account for matrix development are assessed copyright 2012 john wiley sons ltd

bed clusters in humid perennial and mediterranean ephemeral gravel bed streams the effect of clast size and bed material sorting summary a short review of the literature on particle clusters on gravel river beds reveals investigations of both process and form dominated by the intensive study approach using a restricted geographical sample or evidence from flumes an alternative presented here is extensive sampling from three climatic zones several channels in each and at multiple transects at each site it uses insights provided by a more intensively studied base station wittenberg l newson m d 2005 particle clusters in gravel bed rivers an experimental morphological approach to bed material transport and stability concepts earth surface processes and landforms 30 11 1351 1368 transect surveys were completed in each selected reach to establish flow depth bed material size and bed structure a total of more than 5000 sample points reveals the vital presence of bed material of 100 mm d50 for all sub types of clusters to occur thereafter cluster frequency relates directly to the d90 with an improving correlation at d90 256 mm a better integration of data from the diverse hydrological hydraulic regimes can be achieved by correlating cluster frequency with a sorting index for bed material further analysis of hydrological and hydraulic data for all sites is required to develop a dynamic explanation

increase in coarse sediment transport associated with disturbance of gravel river beds by signal crayfish pacifastacus leniusculus there is growing acknowledgement of the interaction between animals and the river bed on which they live and the implications of biological activity for geomorphic processes it has been observed that signal crayfish pacifastacus leniusculus disturb gravel substrates potentially promoting sediment transport and impacting ecological communities however the mechanisms involved and the extent of their impact remain poorly understood especially in relation to other processes that affect grain mobility in gravel bed rivers a series of flume experiments using loose and water worked gravel beds of narrowly graded grain sizes that were exposed to 6 h of crayfish activity under low velocity flows showed a substantial increase in the number of grains entrained by subsequent higher velocity flows when compared with control runs in which crayfish were never introduced crayfish alter the topography of their substrate by constructing pits and mounds which affect grain protrusion when walking and foraging they also alter gravel fabric by reorienting and changing the friction angle of surface grains in water worked surfaces this fabric rearrangement is shown to lead to a statistically significant partial reversal of the structuring that had been achieved by antecedent flow for these previously water worked surfaces the increase in entrainment arising from disturbance by crayfish was statistically significant with grain transport nearly twice as great this suggests that signal crayfish an increasingly widespread invasive species in temperate latitudes beyond their native nw north america have the potential to enhance coarse grained bedload flux by altering the surface structure of gravel river beds and reducing the stability of surface grains this study illustrates further the importance of acknowledging the impact of mobile organisms in conditioning the river bed when assessing sediment entrainment mechanics in the context of predicting bedload flux

the incidence and nature of bedload transport during flood flows in coarse grained alluvial channels a continuous record reveals that the incidence of bedload in a coarse grained river channel changes from flood to flood long periods of inactivity encourage the channel bed to consolidate sufficiently so that bedload is largely confined to the recession limb of the next flood wave but when floods follow each other closely the bed material is comparatively loose and offers less resistance to entrainment in this case substantial amounts of bedload are generated on the rising limb this is confirmed by values of bed shear stress or stream power at the threshold of initial motion which can be up to five times the overall mean in the case of isolated floods or those which are the first of the season this produces a complicated relationship between flow parameters and bedload and explains some of the difficulties in establishing bedload rating curves for coarse grained channels besides this the threshold of initial motion is shown to occur at levels of bed shear stress three times those at the thresholds of final motion this adds further confusion to attempts at developing predictive bedload equations and clearly indicates at least one reason why equations currently in use are unsatisfactory bedload is shown to be characterized by a series of pulses with a mean periodicity of 1 7 hours in the absence of migrating bedforms it is speculated that this well documented pattern reflects the passage of kinematic waves of particles in a slow moving traction carpet the general pattern of bedload including pulsations is shown to occur more or less synchronously at different points across the stream channel