geomorphological evidence for jokulhlaups from kverkfjoll volcano iceland abstract jokulhlaups glacial outburst floods are known to have drained along the jokulsa a fjollum river in iceland during the holocene however little is known about their number age source and flow characteristics this paper provides detailed geomorphological evidence for jokulhlaups that have routed from the kverkfjoll ice margin and hence into the jokulsa a fjollum erosional evidence of jokulhlaups from kverkfjoll includes gorges cataracts spillways subaerial lava steps and valley wide scoured surfaces depositional evidence includes wash limits boulder bars cataract fill mounds terraces slackwater deposits and outwash fans some of these landforms have been documented previously in association with jokulhlaups however subaerial lava surfaces that have been scoured of the upper clinker gorges within pillow hyaloclastite ridges gorges between pillow hyaloclastite ridges and subaerial lava flows subaerial lava lobe steps cataract fill mounds and boulder run ups are previously undocumented in the literature these landforms may therefore be diagnostic of jokulhlaups within an active volcanic rifting landscape the nature and spatial distribution of these landforms and their stratigraphic association with other landforms suggest that there have been at least two jokulhlaups through kverkfjallarani the biskupsfell eruption occurred between these two jokulhlaups kverkfjallarani jokulhlaups were very strongly influenced by topography geology and interevent processes that together determined the quantity and nature of sediment availability such controls have resulted in jokulhlaups that were probably fluidal turbulent and supercritical over large areas of the anastomosing channel bed kverkfjallarani jokulhlaups would have had highly variable hydraulic properties both spatially and temporally the knowledge of flow characteristics that can be gained from jokulhlaup impacts has implications for recognising jokulhlaups in the rock record and for hazard analysis and mitigation within similar landscapes and upon other glaciated volcanoes

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

predicting incipient motion including the effect of turbulent pressure fluctuations in the bed 1 incipient motion of particles on a river bed can in principle be understood and predicted from a balance of the forces acting on the particles on a natural river bed the exposure of particles to the flow is variable the effect of exposure on the initiation of motion including the case of deep embedding is studied in this paper so far the turbulence induced lifting force has been derived exclusively from examinations of the surface flow the understanding of destabilization processes of the riverbed is improved when turbulence induced vertical pressure gradients in the upper sediment layer are included this is particularly important for particles that lie in the lee of larger ones an analytical solution for the critical shields parameter of spherical particles is found including the effects of pressure fluctuations in the bed macroscale flow structures average bed slope and shallow flow existing laboratory data designed to test the effect of exposure and roughness conditions are in excellent agreement with the new model

analogies between mineral sediment and vegetative particle dynamics in fluvial systems abstract riparian vegetation fragments and propagules form part of the organic sediment load of rivers this review paper draws analogies between the transfer of these vegetative particles and that of mineral sediment within river systems whole plants vegetation fragments and propagules are delivered to rivers by physical processes but the timing of seed release and the morphological characteristics of the plants moderate delivery the transport and deposition of plant material is partly controlled by flow patterns but is also influenced by the buoyancy and morphology of the plant material once deposited the vegetative fragments propagules and the plants that develop from them can have significant local effects on flow hydraulics and sediment erodibility producing complex assemblages of physical features aggregate vegetative particles are identified as being particularly important in accelerating vegetation colonisation of exposed river sediments driving landform development and supporting the rapid development of a diverse cover of plants the geomorphological significance of interactions between vegetative particles and fluvial processes is illustrated at different spatial scales at the catchment scale ribbons of mineral and vegetative particles are transferred downstream with inputs continuously supplied throughout the river s course individual particles experience phases of mobilisation transport and deposition as a result along larger rivers down river changes are typically seen in the size and type of mineral sediment and vegetative particles and also the species composition of the vegetative particles coupled with downstream changes in hydroclimatological conditions within a river s active zone marked downstream changes in interactions between fluvial processes vegetation propagules and landforms can result at the patch scale individual vegetative particles particularly aggregate particles deposited on exposed riverine sediments create local hydraulic complexity that induces the scour and deposition of sediment and further plant propagules resulting in a suite of distinctive landforms which may support many plant species within individual reaches patch scale interactions between mineral sediment and vegetative particles are moderated or sieved by the form of the reach and its flow patterns so that the integration of patch scale effects can support the development of different landforms in different locations

skin friction behind isolated hemispheres and the formation of obstacle marks we have measured the mean magnitude and direction and the rms fluctuation intensity of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long downstream of reattachment the mean skin friction magnitude is about 20 greater than its free stream value along the centre line and comparably reduced to either side of it the horizontal divergence of the skin friction vector field is positive divergent along the centre line and negative convergent to either side of it neither of these conditions favours development of a ridge longer than the separation length along the centre line the development of ridges or tails many obstacle heights long commonly observed in nature requires considerable modification of the simple sediment free wakes we have studied

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

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

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

rapid incision of a small coarse and steep fan delta in response to base level fall the case of nahal qedem the dead sea israel the dead sea has been continuously dropping 0 4 1 0 m yr 1 since the middle of the 20th century and thus provides a unique field laboratory for studying in real time the response of drainage systems to a non tectonic base level fall the aim of this work is to study the short term ongoing erosive response to a rapid base level drop in a small steep fronted erodible fan delta setting the work explores the controls of the steep qedem fan delta guided by its clinoform structure on its incision longitudinal profiles of the fan delta and of its entrenched channel were measured in the field sedimentary facies changes fluviatile shallow lacustrine and beach were followed along exposures the existence of large boulders provided an opportunity to examine the uncertain role of armouring and boulder flux on incision the field study was combined with digital elevation models dems that were extracted from pairs of overlapping aerial photos maps of erosion and deposition were prepared using a change detection algorithm the longitudinal profile of the entrenched channel was found to be steep and linear the outlet temporarily hangs elevated and ungraded above the retreating lake level indicating years without incision flow events which cause lags in response to the rapid lake level drop in spite of the large boulders the small drainage basin and precipitation volume over the basin of the qedem the recorded vertical incision rates in the unconsolidated sediments are as high as 0 8 m yr 1 i e similar to those of the largest wadis draining to the dead sea the steep front of the fan delta is suggested to be a main factor controlling the efficient incision a unique transport mechanism of rolling boulders following undercutting contributed to the entrenchment efficiency copyright 2010 john wiley sons ltd

a froude scaled model of a bedrock alluvial channel reach 1 hydraulics the controls on hydraulics in bedrock alluvial rivers are relatively poorly understood despite the importance of the flow in determining rates and patterns of sediment transport and consequent erosion to measure hydraulics within a bedrock alluvial channel we developed a 1 10 froude scaled laboratory model of an 18 9 m bedrock alluvial river reach using terrestrial laser scanning and 3 d printing in the reported experiments water depth and velocity were recorded at 18 locations within the channel at each of five different discharges additional data from runs with sediment cover in the flume were used to evaluate the hydraulic impact of sediment cover the deposition and erosion of sediment patches in these runs are analyzed in the companion paper in our data 1 spatial variation in both flow velocity and froude number increases with discharge 2 bulk flow resistance and froude number become independent of discharge at higher discharges 3 local flow velocity and reynolds stress are correlated to the range of local bed topography at some but not most discharges 4 at lower discharges local topography induces vertical flow structures and slower velocities but these effects decrease at higher discharges and 5 there is a relationship between the linear combination of bed and sediment roughness and local flow velocity these results demonstrate the control that bedrock topography exerts over both local and reach scale flow conditions but spatially distributed hydraulic data from bedrock alluvial channels with different topographies are needed to generalize these findings

influence of bed patchiness slope grain hiding and form drag on gravel mobilization in very steep streams steep streams are a major portion of channel networks and provide a link to transport sediment from hillslopes to lower gradient rivers despite their importance key unknowns remain perhaps foremost of which is evaluating in steep streams empirical laws for fluvial sediment transport developed for low gradient rivers to address this knowledge gap we painted sediment in situ over 3 years to monitor incipient sediment motion and sediment patch development in five small drainage areas of 0 04 2 km 2 and steep slopes of 5 37 tributaries of elder creek california united states we found that channel beds organized into size sorted sediment patches which displayed active fluvial transport of gravel annually consistent year to year patch median grain sizes partial transport of bed material and significantly higher values of critical shields stress for incipient sediment motion compared to that observed for lower gradient rivers the high critical shields stresses up to 0 5 for the median grain size agree within a factor of 3 to theoretical predictions which account for slope dependent hydraulics grain hiding and sediment patches for grains of approximately the same size as the roughness length scale slope dependent hydraulics and bed patchiness are the dominant controls on critical shields stress values while grain hiding is important for grains larger or smaller than the roughness length scale form drag exists in our monitored tributaries but has a smaller influence than the above effects our field observations show fluvial processes contribute to sediment mobilization in steep channels which are often considered to be dominated by debris flows

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

time transgressive deposits of repeated depositional sequences within interlobate glaciofluvial esker sediments in koylio sw finland abstract evidence of conspicuous repeated seasonal to annual deposition of glaciofluvial and glaciolacustrine sequences within a structurally complex interlobate esker segment in sw finland is presented the time transgressive overlapping depositional sequences consist of deposits from two successive melt seasons including three vertically stacked lithofacies associations 1 massive to stratified coarse gravels summer deposits 2 trough and ripple cross stratified fine grained deposits autumn to winter deposits and 3 sandy stratified beds spring deposits the depositional environment of each lithofacies association involves a transition from subglacial or submarginal tunnel to a subaqueous re entrant environment which then passes to a proglacial glaciolacustrine environment the study also presents evidence of headward extension of subglacial tunnel deposits related to the rapid shifting of a tunnel expansion point during the increasing spring discharge which occupied the old tunnel exit this mode of annual deposition has not been reported previously in esker studies the good preservation of the rhythmic lithofacies associations is suggested as resulting from interlobate depositional conditions associated with rapidly decaying icestreams therefore the depositional model may provide a key to recognizing time transgressive interlobate eskers that form an important geomorphological and sedimentological record of meltwater activity during the last deglaciation of the fennoscandian and laurentide ice sheets the identification of time transgressive interlobate eskers and associated palaeo icestream behaviour is an essential step forward for more accurate models of ice sheet behaviour and palaeoclimatic reconstructions

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

in flow evolution of lahar deposits from video imagery with implications for post event deposit interpretation mount semeru indonesia abstract the hazardous and unpredictable nature of lahars makes them challenging to study yet the in flow processes characterizing these events are important to understand as a result much of the previous research on lahar sedimentation and flow processes has been derived from experimental flows or stratigraphic surveys of post event deposits by comparison little is known on the time dependent sediment and flow dynamics of lahars in natural environments using video footage of seven lahars on the flanks of semeru volcano east java indonesia the present study offers new insights on the in flow evolution of sediment in natural lahars video analysis revealed several distinctive patterns of sediment entrainment and deposition that varied with time related fluctuations in flow these patterns were used to generate a conceptual framework describing possible processes of formation for subsurface architectural features identified in an earlier lateral survey of lahar deposits on semeru volcano gomez and lavigne 2010a the formation of lateral discontinuities was related to the partial erosion of transitional bank deposits followed by fresh deposition along the erosional contact this pattern was observed over the course of several lahar events and within individual flows observations similarly offer potential explanations for the formation of lenticular features depending on flow characteristics these features appeared to form by preferential erosion or deposition around large stationary blocks and by deposition along channel banks during episodes of channel migration or channel constriction finally conditions conducive to the deposition of fine laminated beds were observed during periods of attenuating and surging flow these results emphasize the difficulties associated with identifying process structure relationships solely from post event deposit interpretation and illustrate that an improved understanding of the time dependent sediment dynamics in lahars may be advantageous when interpreting post event structural features

coherent flow structures in a depth limited flow over a gravel surface the influence of surface roughness 1 turbulent flows moving over a gravel bed develop large scale macroturbulent flow structures that are initiated at anchor clasts in the bed and grow and dissipate as they move upward through the flow depth this paper extends previous research in which we investigated the influence of the reynolds number on coherent flow structures generated over a gravel bed by assessing the importance of effective bed roughness here we report on flume experiments in which flows over beds of decreasing surface roughness have been quantified through the application of digital particle imaging velocimetry which allows study of the downstream and vertical components of velocity over the entire flow field these results indicate that as the effective roughness increases 1 the visual distinctiveness of the coherent flow structures becomes more defined throughout the flow depth 2 the upstream angle of slope of the coherent flow structure increases and 3 the reduction in streamwise flow velocity and turbulence intensity toward the upstream side of the structure becomes greater applying standard scaling laws these structures appear shear generated and form through a combination of both wake flapping and the reattachment of localized shear layers associated with flow separation around individual topographic protrusions as the effective protrusion decreases the scale of these coherent flow structures also decreases

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

flume experiments on flow and sediment supply controls on gravel bedform dynamics abstract we present results from a flume experiment in which we investigate how the formation and dynamics of gravel bedforms are affected by changes in discharge and sediment supply we conducted experiments in a straight rectangular sediment feed flume using a unimodal mixture of fine gravel and coarse sand the experiment consisted of three phases 1 equilibrium sediment supply and steady flow 2 equilibrium sediment supply and repeated hydrographs and 3 doubled sediment supply and repeated hydrographs during the experiments low amplitude migrating bedforms resembling alternate bars developed and their dynamics were characterized and tracked via collection of repeated structure from motion topographic data sets channel scale morphology was essentially the same for steady and unsteady flow at the same sediment supply but the bedform celerity was much lower with unsteady flow than it was under constant discharge bedform amplitudes increased on the rising limb of the hydrograph and declined on the falling limb but their wavelength was largely insensitive to flow variation when the sediment supply was increased the dominant channel response was an increase in slope the bedform celerity increased but not to the same rate as under steady flow although our experiment developed alternate bars the width to depth ratio of 8 indicates that present theory would not predict the occurrence of bars for our conditions our observations show that alternate bars may develop under low width to depth conditions and suggest that further theoretical development is needed to identify the mechanism responsible for their formation

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

flow separation a physical process for the concentration of heavy minerals within alluvial channels flow separation is a common feature within alluvial channels that occurs at abrupt changes in the bed geometry at a wide range of scales it is a process that can exert considerable control over the segregation and deposition of heavy mineral grains fluid separation generates a region of high bed shear stress that can entrain heavy minerals and a region of low velocity that is a preferred site for the deposition of denser particles significant concentrations of heavy minerals sometimes up to eight times the background value may be formed by flow separation processes a review of the occurrence of flow separation illustrates that it may be responsible for the accumulation of heavy mineral grains at a range of geomorphological scales

bedload transport in two gravel bedded streams bedload transport measurements in two upland streams are considered as a function of the excess stream power exerted on the bed by the flow during low flows when the framework gravels remain undisturbed fine sediments are winnowed from the bed surface layer once a threshold of 3 4 w m 2 is exceeded and the transport rate is strongly supply limited however for stream powers in excess of 15 w m 2 framework gravels are mobilized and the efficiency of the transport process approaches a local maximum of about 1 per cent for discharges up to 2 3 of bankfull an inverse depth dependence in the efficiency of the transport process was noted but although bedload calibre increased as a function of discharge its influence on efficiency could not be demonstrated however it was suspected that the size sorting relationships of the bed material in a number of rivers in relation to the transport efficiency might profitably be examined further

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

the hydrodynamics of particle clusters and sediment entrapment in coarse alluvial channels particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field this has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows lift and drag forces are determined on clustered hemispherical particles of varying size in the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion on the stoss side of the cluster drag is uniform regardless of the separation of the component particles but lift is shown to increase when particle separation is small so affecting stability this mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape on average 46 of clustered particles are entrained by flood flow compared to 87 of particles in open plane beds the influence of clusters is a major determinant of sedimentary sorting

the application of computational fluid dynamics to natural river channels eddy resolving versus mean flow approaches in the last decade as computing power has increased there has been an explosion in the use of eddy resolving numerical methods in the engineering earth and environmental sciences for complex geomorphic flows where accurate field investigations are difficult to perform and where experiments may be difficult to scale these numerical approaches are beginning to give key insights into the nature of these flows eddy resolving methods such as large and detached eddy simulation les des may be contrasted with the time averaged three dimensional simulations that only really began to be applied seriously in geomorphology fifteen years ago while the potential of les for geomorphology has been examined previously des is a relatively recent method that deserves further consideration in this paper we explain the method and then utilise examples from meander and confluence flows as well as flow near the bed of a gravel bed river to highlight the improvements to both the representation of the mean flow and to the representation of time varying processes that result from the use of les des some suggestions are provided for the future use of such techniques in geomorphology

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

grain size sorting within river bars in relation to downstream fining along a wandering channel the textural variability of river bed gravels at bar scales is poorly understood as are the relations between variability at this scale and at reach and river scales surface and subsurface grain size distributions were therefore examined at reach bar and bedform scales along lower fraser river british columbia canada grain size variations within compound bars are conditioned by longitudinal position elevation and morphological setting surface and subsurface sediments tend to decrease in median size from bar head to bar tail by 33 and 17 respectively sediment size is constrained at some upper limit that is inversely related to bar surface elevation and which is consistent with competence considerations the surface sediments on unit bars are finer and better sorted than the bed materials in bar top channels and along the main bar edges secondary unit bars tend to have a lower sand content than other features a consequence of sediment resorting individual unit bars and gravel sheets exhibit streamwise grain size fining and lee side sand deposition over time significant amounts of cut and fill do not ipso facto cause changes in surface grain sizes yet sediment characteristics can change without any significant morphological adjustment taking place at the reach scale there is a clear downstream fining trend but local variability is consistently high due to within bar variations the surface median grain size range on individual bars is on average 25 of that along the entire 50 km reach but is 68 on one bar while the overall fining trend yields a downstream change in surface median size of 0 76 mm km 1 the average value for head to tail size reduction on individual bars is 6 3 mm km 1 an order of magnitude difference that highlights the effectiveness of bar scale sorting processes in gravel bed rivers possibilities for modelling bar scale variability and the interaction of the different controls that are identified are discussed

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

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

applications of grain pivoting and sliding analyses to selective entrapment of gravel and to flow competence evaluations models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water of special interest is whether such process based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods a detailed derivation is undertaken focusing first on the fluid flow and forces at the particle level the resulting threshold equation for the particle level velocity is then modified to yield the mean entrainment stress for the flow as a whole this approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape the resulting threshold equations contain a number of coefficients e g drag and lift whose values are poorly known but can be constrained by requiring agreement with the shields curve for the threshold of grains in uniform deposits if pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment however if reasonable modifications of those coefficients are made for expected field conditions then the models yield good agreement with the data sliding models where sorting is due entirely to projection distances of the grains above the bed yield somewhat poorer agreement with the field data however the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends the existing measurements lack documentation of the mechanisms of grain movement so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding in spite of such uncertainties the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence

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

the spatial distribution of coarse surface grains and the stability of gravel river beds previous studies of surface particle structures in gravel bed rivers in particular coarse grain clusters conclude that these bedforms increase bed stability and reduce sediment transport rates however few studies have quantitatively analysed the effect of the surface distribution of coarse grains in isolation or arranged into distinct bedforms on particle entrainment focussing on descriptions of bedform formation and mechanical behaviour a novel image analysis technique was employed that allowed recognition of coarse grains and calculation of particle cluster areas under laboratory conditions increasing discharges were applied to a unimodal sand gravel bed the size class containing the d90 size was painted in ultraviolet paint image analysis software was used to identify this size class on digital images of the bed surface taken under ultraviolet light both the total surface area occupied by ultraviolet grains and the area of accumulations of connected grains were related to the threshold shear stress for entrainment as discharge magnitude increased coarse grains actively grouped together forming fewer larger accumulations of coarse particles lower values of critical bed shear stress occurred for beds with higher degrees of clustering beds are more stable when coarse grains are distributed more evenly over the bed surface this implies that using bed surface composition as a descriptor of bed resistance to entrainment may need to be complemented by a descriptor of the spatial distribution of the coarsest grains

ripple formation induced by biogenic mounds abstract bedload and ripple formation at subcritical flows were observed during laboratory studies involving the mound forming head down deposit feeding polychaete cistenides gouldii flume experiments were designed to investigate the conditions resulting in sediment transport within and immediately downstream of biogenic mound assemblages experiments focused on the inputs of mound population density mound height and the presence absence of cistenides the activities of cistenides may generate sediment transport in two ways via particle ejection during feeding or via scour generated by their mounds the first set of experiments were conducted to determine which process was predominantly responsible for ripple formation mounds were required for ripple formation but ripples formed only at the two higher mound densities 150 and 570 per m 2 the second set of experiments used mounds of different densities and heights to investigate density effects one of three final bed states was always attained 1 mounds became and generated migrating ripples 2 mounds changed into ripples which did not migrate beyond the test area called stationary ripples or 3 mounds remained separate the division between migrating ripple and no ripple cases was used to develop a threshold similar to that established by southard and dingler southard j b dingler j r 1971 flume study of ripple propagation behind mounds on flat sand beds sedimentology 16 pp 251 263 for ripple propagation behind isolated mounds our threshold was higher due to the coarser sediments used in this study stationary ripples exhibited a local equilibrium where despite a two fold range of initial mound separation resulting ripples had similar wavelengths this stationary ripple wavelength was smaller than the equilibrium wavelength found for migrating ripples generated from a flat bed the ability of biogenic structures to generate ripples has implications in studies of near bed processes ripples seen in bottom photographs and stratigraphy may not be an absolute indicator of supercritical flows additionally when the topography within patches of mound building worms changes the bed geometry generating interfacial flow and bottom drag is ripples instead of mounds estimates of reworking due to ripple migration greatly outweigh rates of biological reworking observed in the field all of these consequences reinforce the importance of sediment transport induced by biogenic structures

a reappraisal of the significance of obstacle clasts in cluster bedform dispersal the dispersal of cluster bedforms is examined in a short partially braided reach of the river quoich northeast scotland after minor flood events in 1989 and 1990 in all cases the eventual dispersal of clusters occurred without prior removal of the obstacle clast since none of the obstacle clasts was dispersed in 1989 and only two thirds by 1990 obstacle clasts probably form a less significant delay to sediment entrainment than previously assumed and represent a relatively immobile component of bed sediment

modelling gravel bed topography from sediment transport a simple queueing model which generates bed topography consistent with the mechanics of gravel motion is presented the equations on which it is based are derived from the application of simple theoretical ideas and an analysis of published flume data meland and norrman 1966 francis 1973 abbott and francis 1977 the simulation produces an extremely variable bed topography in which at least two scales of bedform may be identified features of 5 10 grain wavelengths similar to pebble clusters are superimposed on other forms whose wavelength is up to 30 40 grains and which resemble step pools systems and antidunes sediment transport shows many of the characteristics found in the field and the model is therefore thought to provide a useful starting point from which to examine the interaction between flow sediment transport and bedforms in gravel bed and cobble streams

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

logjam controls on channel floodplain interactions in wooded catchments and their role in the formation of multi channel patterns the role of wood in the formation and maintenance of complex dynamic floodplain surfaces is important and to date has received relatively limited attention compared to in channel habitat processes this paper explores the role of logjams as important agents of channel floodplain interaction we draw on a specific case study as well as examples from the literature to show that although the processes of interaction differ the resulting dynamic floodplain patchwork is a common feature of rivers with logjams in addition we contend that the presence of logjams is an important factor in the evolution and maintenance of multiple channel patterns in both montane and lowland river environments these observations have important implications for the definition of reference targets for river restoration the specific results of this research show 1 the presence of a range of types of multiple channel network dissecting the floodplains of low order channels that are strongly associated with the presence of logjams 2 the relatively rapid formation of floodplain channels following logjam formation 3 the dynamic nature of logjams within headwater channels on both seasonal and annual timescales that lead to a highly dynamic habitat mosaic on the floodplain surface 4 an increased frequency of overbank flooding and high rates of floodplain sediment accumulation upstream of logjams and along floodplain channel networks that create the complex topography observed in the case study forested floodplains

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

debris flow dominance of alluvial fans masked by runoff reworking and weathering arid alluvial fan aggradation is highly episodic and fans often comprise active and inactive sectors hence the morphology and texture of fan surfaces are partly determined by secondary processes of weathering and erosion in addition to primary processes of aggradation this introduces considerable uncertainty in the identification of formative processes of terrestrial and martian fans from aerial and satellite imagery the objectives of this study are i to develop a model to describe the sedimentological and morphological evolution of inactive fan surfaces in arid settings and ii to assess the relative importance of primary processes of aggradation and secondary processes of weathering and reworking for surface morphology and sedimentology and for the stratigraphic record we studied an alluvial fan characterized by a recently active sector and a long abandoned inactive sector along the coast of the hyperarid atacama desert here rates of primary geomorphic activity are exceptionally low because of extreme aridity while weathering rates are relatively high because of the effects of coastal fogs long term processes of fan aggradation and reworking were determined through sedimentological facies analysis of stratigraphic sections ground surveys for textural and morphological patterns at the fan surface were integrated with remote sensing by an unmanned airborne vehicle uav discharges and sediment transport capacities were calculated to estimate the efficiency of secondary runoff in reshaping the inactive fan sector stratigraphic sections reveal that the fan was dominantly aggraded by debris flows whereas surface morphology is dominated by debris flow signatures in the active sector and by weathering and runoff on the inactive sector on the latter rapid particle breakdown prevents the formation of a coarse desert pavement furthermore relatively frequent local runoff events erode proximal debris flow channels on the inactive sector to form local lag deposits and accumulate fine sediment in low gradient distal channels forming a well developed drainage pattern that would suggest a runoff origin from aerial images nevertheless reworking is very superficial and barely preserved in the stratigraphic record this implies that fans on earth and mars that formed dominantly by sporadic mass flows may be masked by a surface morphology related to other processes

key hydraulic drivers and patterns of fine sediment accumulation in gravel streambeds a conceptual framework illustrated with a case study from the kiewa river australia fine sediment processes in gravel beds may have significant impacts to overall river ecosystem function in addition to gravitational deposition horizontal intragravel transport has been recognized to influence fine sediment accumulation however the specific hydraulic mechanisms and origin of fine sediment movement are not clearly identified the purpose of this study was to investigate key hydraulic drivers and patterns of fine sediment accumulation using a conceptual framework to set the scene we implemented an experimental setup in a gravel lateral bar subject to irregular flow fluctuations in the kiewa river australia we installed nine sets of sediment collector pairs and piezometers into the gravel each pair included one horizontally and one horizontally vertically perforated collector mid range rather than peak flows covering the site in water drove fine sediment deposited in the collectors we estimated horizontal contribution to final deposition as 59 such contribution resulted from shear stresses 3 n m 2 promoting streamwise near bed turbulence at the water sediment interface during flooded conditions despite high subsurface hydraulic gradients intragravel transport in the lower sediment layers via darcy flow did not show any influence to fine sediment deposition our findings contribute to an improved understanding of fine sediment accumulation processes key for overall river ecosystem functioning particularly in regulated rivers

spatial and temporal patterns in fluvial recovery following volcanic eruptions channel response to basin wide sediment loading at mount pinatubo philippines the june 1991 eruption of mount pinatubo philippines was one of the largest volcanic eruptions of the twentieth century emplacing 5 6 km 3 of pyroclastic fl ow material on the fl anks of the volcano the combination of abundant relatively fi ne grained easily erodible material and intense tropical rainfall led to numerous lahars immediately following the eruption even after major lahars ended sediment yields in some basins remained orders of magnitude above pre eruption levels using data collected from 1996 through 2003 we investigated fi ve basins that experienced varying amounts of sediment loading in the 1991 eruption from 1 to 33 of the basin area covered by valley fi lling pyroclasticfl ow deposits from measurements of fl ow and bed characteristics made through time we developed a general conceptual model for channel recovery following basin wide sediment loading initially fi ner grained sediment and pumice are mobilized preferentially through selective transport once the bed is coarse enough for gravel size clasts to interact with one another clast structures develop increasing form roughness and critical shear stress and inhibiting initial clast mobility as sediment inputs continue to decline the channel incises into valley bottom sediments progressively armoring through winnowing at pinatubo incision and armoring occur fi rst as dry season phenomena due to reduced sediment inputs eventually moving to year round low fl ow bed stability observed timing of the onset of in channel biological recovery suggests that reestablishment of channel stability helps catalyze aquatic ecosystem recovery

experiments on surface structure and partial sediment transport on a gravel bed eight flume experiments were conducted to study the development of bed surface texture and structure in the presence of partial bed material transport the experiments have two phases a no feed degradational phase followed by a feeding phase a surface structure of irregular reticulate stone nets and clusters was developed before sediment feeding commenced bed load transport equaled or slightly exceeded the fed supply except at the highest feed rate the bed structure was maintained but bed surface texture fined with increasing sediment load the two phenomena may coexist because the largest grains on the bed moved only very sporadically the actual sediment transport rates were much less than the expected rate calculated from the ratio of bed surface grain size to transported grain size the difference reflects the increase in bed stability introduced by the bed structure between 17 and 47 of the bed shear stress is estimated to be carried by the structure 4 being absorbed by the load while the bed grains carried the balance of the stress bed material transport is exceedingly sensitive to bed surface structure and grain size which raises concerns about the realizable precision of grain size measurements and characterization of the structure

field measurement of boulder flow drag a field method for measuring the flow drag force on individual boulders on a riverbed is identified and tested drag is determined from the momentum deficit in the boulder wake obtained by measuring the flow velocity in a traverse across the wake a field test yielded a mean drag coefficient of 0 36 which is low compared with flume measurements of 1 1 1 5 for cubes this may be the result of higher velocity water near the riverbed being sucked into and reenergizing the wake further tests are needed to define the conditions in which the method can be used and to indicate how well the assumptions behind the method can be accommodated in a natural flow environment

friction angle measurements on a naturally formed gravel streambed implications for critical boundary shear stress we report the first measurements of friction angles for a naturally formed gravel streambed for a given test grain size placed on a bed surface friction angles varied from 10o to over 100o friction angle distributions can be expressed as a function of test grain size median bed grain size and bed sorting parameter friction angles decrease with increasing grain size relative to the median bed grain size and are a systematic function of sorting with lower friction angles associated with poorer sorting the probability distributions of critical shear stress for different grain sizes on a given bed surface as calculated from our friction angle data show a common origin but otherwise diverge with larger grains having narrower and lower ranges of critical shear stresses the potential mobility of a grain as defined by its probability distribution of critical shear stress may be overestimated for larger grains in this analysis because our calculations do not take into account the effects of grain burial and altered near bed flow fields

bioinformatic approaches for objective detection of water masses on continental shelves 1 as part of the 2001 hyper spectral coupled ocean dynamics experiment sea surface temperature and ocean color satellite imagery were collected for the continental shelf of the mid atlantic bight these images were used to develop a water mass analysis and classification scheme that objectively describes the locations of water masses and their boundary locations this technique combines multivariate cluster analysis with a newly developed genetic expression algorithm to objectively determine the number of water types in the region on the basis of ocean color and sea surface temperature measurements then through boundary analysis of the water types identified the boundaries of the major water types were mapped and the differences between them were quantified using predictor space distances results suggest that this approach can track the development and transport of water masses because the analysis combines the information of multiple predictors to describe water masses it is an effective tool in detecting water masses not readily recognizable with temperature or chlorophyll alone

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

boundary skin friction and sediment transport about an animal tube mimic a flume study was made of fluid flow boundary skin friction and sediment transport about an animal tube mimic the effect of a tube on momentum transfer depends most strongly on its height a tube increases the net boundary skin friction locally and tends to promote sediment entrainment near its base flow in this region appears to be governed by the same similarity laws that apply to cylinders with much larger body reynolds number particles travelling as suspended load may be deposited immediately downstream of the tube the net sedimentological effect of any tube i e deposition or scour will depend both on its height and on the boundary shear stress imposed by the external flow the near wake region of a tube is dominated by a strong cross stream exchange of momentum wake perturbations decay downstream seemingly in accordance with similarity laws which also govern mounted two dimensional structures

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

the impact of stress history on bed structure recent research has started to focus on how prolonged periods of sub threshold flows may be capable of imparting structural changes that contribute to increased bed stability to date this effect termed stress history has been found to be significant in acting to increase a bed s critical shear stress at entrainment threshold however it is supported by only limited qualitative and often speculative information on the mechanisms of this stabilisation process in grade specific studies as such this paper uses high resolution laser scanning to quantitatively ascertain the granular mechanics underpinning the relationship between stress history and entrainment threshold for beds of a range of grain size distributions employing a bed slope of 1 200 three grain size distributions with median grain sizes d50 of 4 8mm uniform g d84 d16 0 5 1 13 bimodal g 2 08 and unimodal g 1 63 were exposed to antecedent stress histories of 60 and 960 minutes duration antecedent shear stress magnitude was set at 50 of the critical shear stress for the d 50 when no stress history period was employed two laser displacement scans of the bed surface approximate area 100 x 117mm were taken one prior to the antecedent period and one after this period so that changes to surface topography could be quantified resolution of x 0 10mm y 0 13mm and z 0 24mm rearrangement of bed surface structure is described using statistical analysis and 2d semi variograms to analyse scaling behaviour results reveal vertical settlement changes to bed roughness and particle repositioning however the bed grain size distribution influences the relative importance of each mechanism in determining stress history induced bed stability this is the focus of discussion in this paper

simulation of individual particle movement in a gravel streambed a two dimensional simulation model of travel distances of individual particles in a gravel bed river is presented the model is based on a number of rules which include particle size entrainment trajectory distance of movement and entrapment particle interactions are controlled by resistance fields defined about each obstacle and critical elevation defined in the model resistance fields particle dropping and critical elevation rules control particle interactions the interaction rules cause the particles to develop pebble clusters stone cells and transverse structures transverse ribs the simulated travel distances of individual particles are consistent with reported field results individual particle travel distances were simulated using two different models one without interactions between the individual particles and the stationary bed and one with interactions the case without interactions demonstrates the random nature of sediment transport and narrow ranges of travel distances wider ranges of travel distances similar to those for natural situations were obtained for the cases with interactions the more intense the interaction between the mobile stones and the stationary ones the wider the range of distances of travel for a given particle size modelling the mean travel distance yielded a result similar to that published previously which was based on empirical data well developed bed surface structures were obtained for relatively poorly sorted sediment with intense interactions between particles transverse structures developed when relatively large particles were allowed to move copyright 2002 john wiley sons ltd

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

structural evolution of a crustal section in the western himalaya recent field work on the kohistan complex in the karakorum range of north pakistan has shown its stratigraphy to be repeated by several phases of tight to isoclinal folding this reveals a close association between igneous activity and a late deformation and complicates the previously proposed island arc model

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

stuck at the bar larger than average grain lag deposits and the spectrum of particle mobility larger than average grain deposits in gravel bed rivers potentially exert a distinctive influence upon fluvial morphodynamics and flow resistance they are products of historical contingency sourced from rare events that supply atypically coarse material larger than average grain lag deposits are emblematic attributes of the tongariro river new zealand they are deposited on bar edges and heads derived from lahar valley floor deposits that subsequently became terraces these materials are less likely to be reworked across a range of flows compared to other bar material conceptual models that consider channel configuration and incorporate distributions of particle mobility and flood flows are necessary to assess the role of larger than average grain deposits on river morphodynamics

force chains as the link between particle and bulk friction angles in granular material from sediment transport in rivers to landslides predictions of granular motion rely on a mohr coulomb failure criterion parameterized by a friction angle measured friction angles are generally large for single grains smaller for large numbers of grains and no theory exists for intermediate numbers of grains we propose that a continuum of friction angles exists between single grain and bulk friction angles due to grain to grain force chains physical experiments probabilistic modeling and discrete element modeling demonstrate that friction angles decrease by up to 15 as the number of potentially mobile grains increases from 1 to 20 decreased stability occurs as longer force chains more effectively dislodge downslope keystone grains implying that bulk friction angles are set by the statistics of single grain friction angles both angles are distinct from and generally larger than grain contact point friction with implications for a variety of sediment transport processes involving small clusters of grains