an experimental investigation of mechanisms involved in bed load sheet production and migration 1 field measurements indicate that in gravel bed rivers bed load may not be a one to one response to shear stress but may instead fluctuate a great deal over time for a given flow condition both in flume and field experiments these fluctuations were often associated with migration of low relief bed forms called bed load sheets whereas several studies have described bed load sheets as a consequence of grain sorting little is known about the mechanisms responsible for their production and migration these were investigated in flume experiments a set of 20 experiments was conducted under constant feeding rate conditions with mixtures of different uniform sediments and for slopes varying from 0 8 to 9 except for runs performed in high flow conditions we observed periodic bed load sheet production and migration associated with fluctuations of bed slope bed state bed fining and paving and bed load observations allowed us to conclude that bed load sheets resulted from very efficient vertical and longitudinal grain sorting that produced periodic local bed aggradation and erosion clearly observed in the upstream section of the flume fractional transport rates were measured in one run combined with the results of experiments previously conducted by authors with uniform sediments this experiment showed that the highest peak solid discharges were essentially caused by the much greater mobility of the coarser gravels when transported within bed load sheets a scenario is proposed for the mechanisms involved in bed load sheet production and migration

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

physically based probabilistic analysis of sediment deposition in open channel flow abstractthis paper proposes a physically based probabilistic model for predicting the longitudinal distance of sediment deposition in an open channel flow numerical simulations were performed usin

experimental study of the transport of mixed sand and gravel we measured a wide range of transport rates for five different sand gravel mixtures in a laboratory flume each mixture used the same gravel and sand was added to produce mixtures containing 6 15 21 27 and 34 sand control of other variables allows us to isolate the effect of bed sand content on transport as sand content increases gravel transport rates increase by orders of magnitude even though the proportion of gravel in the bed decreases the increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27 the increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content bed surface grain size was measured at the end of each run surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate this casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate

infiltration of fine sediment into a coarse mobile bed a phenomenological study experiments were undertaken to study the nature of granular interaction in running water by examining the influence of fine grain inputs to a coarser sediment bed with a mobile surface video recordings of grain sorting by both kinetic sieving and spontaneous percolation are used to diagnose the critical processes controlling the overall bed response kinetic sieving takes place in the mobile bed surface with the finer sediment moving to the bottom of the bedload transport layer at the interface with the underlying quasi static coarse bed we show that the behavior at this interface dictates how a channel responds to a fine sediment input if by spontaneous percolation the fine sediment is able to infiltrate into the underlying quasi static bed the total transport increases and the channel degrades however if the fine sediment input rate exceeds the transport capacity or is geometrically unable to infiltrate into the underlying bed it forms a quasi static layer underneath the transport layer that inhibits entrainment from the underlying bed resulting in aggradation and an increase in bed slope this article is protected by copyright all rights reserved

morphology and controls on the position of a gravel sand transition fraser river british columbia alluvial river channels often exhibit a relatively abrupt transition from gravel to sand bedded conditions the phenomenon is well documented but few prior studies have analyzed the spatial variability through reaches where transitions occur the downstream fining pattern observed in the fraser river is cited as a classic example of an abrupt gravel sand transition in a large alluvial channel however important questions regarding the exact location of the transition its sedimentology and morphology and what controls its location remain unanswered here we present observations of the downstream change in bed material grain size river bed topography and channel hydraulics through the reach within which the transition occurs these observations indicate that the gravel sand transition is characterized by a terminating gravel wedge but there are patches of gravel downstream of the wedge forming a diffuse extension we show that there is a dramatic decrease in shear stress at the downstream end of the wedge and a consequent cessation of general gravel mobility we argue that the patches of gravel observed beyond the wedge are the result of enhanced mobility of fine gravel over a sand bed we also find that sand in suspension declines rapidly at the downstream end of the wedge suggesting that sand is delivered to the bed completing the sedimentary conditions for a gravel sand transition we propose that the break in river slope associated with the transition is a consequential feature of the transition

channel change bankfull and effective discharges on a vertically accreting meandering gravel bed river along the lower reaches of the waipaoa river new zealand cross section survey data indicate there was a 23 per cent decrease in bankfull width and a 22 per cent reduction in channel cross section area between 1948 and 2000 as the channel responded to increased inputs of fine suspended sediment following deforestation of the headwaters in late c19 and early c20 we determined the bankfull discharge within a 39 km long reach by routing known discharges through the one dimensional mike 11 flow model the model runs suggest that the bankfull discharge varies between 800 and 2300 m3 s 1 and that the average recurrence interval is 4 2 years on the annual maximum series by contrast the effective flow 360 m3 s 1 is equaled or exceeded three times a year the variability in bankfull discharge arises because the banks tend to be lower in places where flood flows are constricted than in reaches where overbank flow is dispersed over a wide area and because scour has counteracted aggradation in some locations there is no downstream variation in shields stress or in relative shear stress within the study reach bankfull shear stress is on average five times greater than the shear stress required to initiate motion at the effective discharge it is more than twice the threshold value the effective discharge probably has more relevance than the bankfull discharge to the overall picture of sediment movement in the lower reaches of the waipaoa river but because width is constrained by the stability and resistance of the bank material to erosion during high flows that also scour the bed the overall channel geometry is likely determined by discharges at or near bankfull copyright 2006 john wiley sons ltd

comparison of sediment load and riverbed scour during floods for gravel bed and sand bed reaches of intermittent rivers case study abstracthydraulic scour is one of the major causes of bridge and embankment failure during typhoon induced or monsoon induced floods in steep intermittent rivers in taiwan in this study field data for sediment load and riverbed scour measured in gravel bed and sand bed reaches between 2003 and 2013 are analyzed to determine their differences under nonequilibrium sediment transport conditions the riverbed scouring potential was extremely high during the fast rising stage of a large flood in the bimodal gravel bed reach by contrast the unimodal sand bed reach with a higher suspended load typically demonstrated a lower scouring rate although it was still higher than that of a perennial river existing bridge scour formulas suitable for intermittent rivers were examined by separating short term general scour and localized scour a maximum general scour depth formula is developed in this study for use as a potential tool for emergency evacuations and bridge closure purposes

the effect of hydrographs on bed load transport and bed sediment spatial arrangement 1 field measurements of sediment transport in gravel bed rivers often reveal hysteretic effects due to differences in sediment availability between the rising and falling limbs of a flood hydrograph however only a small number of flume studies have analyzed the dynamics of sediment transport during hydrographs three types of stepped hydrographs with contrasting durations and magnitudes are simulated here under sediment recirculation conditions bed load transport rate and grain size have been measured continuously the dynamic behavior of the surface armor layer has been explored by analyzing digital photographs and laser scanner surveys of the bed surface taken during hydrographs the results indicate that sediment transport during the falling limb was lower than during the rising limb in all of the three types of hydrographs this reduction is more evident for the low magnitude hydrographs the grain size of the bed remained virtually constant throughout the hydrographs but the grain size of transported sediments exhibited a counterclockwise hysteresis also a significant increase in the reference shear stress for sediment entrainment was measured during the falling limb of hydrographs additionally a detailed analysis of partial transport dynamics of the bed surface sediments reveals a reduction in sediment mobility during the falling limb of the hydrographs the difference in sediment entrainment and transport before and after the peak of the hydrographs appears to be caused by a change in the organization of the surface sediments an analysis of detailed laser scan bed surveys reveals a phase of bed restructuring lower vertical roughness clast rearrangement during the falling limb of hydrographs consequently changes in the degree of organization and complexity of the bed surface are likely the cause of the reduced mobility of sediments and thus of the reduced sediment transport rate during the falling limb of the hydrographs

experiment on temporal variation of bed load transport in response to changes in sediment supply in streams a flume experiment was conducted to study channel adjustment to episodic sediment supply in mountain streams the bulk sediment used for the bed and feed included grain sizes 0 5 64 mm with geometric mean dg bulk of 5 7 mm water discharge was held constant for 40 h and 300 kg of sediment was supplied through a range of scenarios bed slope sediment storage sediment transport and bed surface texture responded to sediment supply during the first of seven runs bed slope decreased from 0 022 m m flume slope to 0 018 m m due to sediment starvation bed slope increased beginning in the second run as the bed aggraded due to preferential storage of grains 8 mm transport rate and bed surface particle size were significantly affected by magnitude frequency of sediment feed under constant feed transport rate increased gradually and dg surface ranged between 12 15 mm instead sediment pulses caused a pronounced increase in sediment transport rate and surface fining trends that were inverted as sediment evacuated at the run scale sediment transport and storage behaved as with constant feed if pulse relaxation time exceeded time between pulses the increase in transport rate and surface fining were proportional to pulse size after the 300 kg pulse transport rate reached 100 g m 1 s 1 and dg surface was 12 mm textural differences on the initial bed surface influenced the patterns of sediment transport channel adjustment was controlled by magnitude frequency of sediment feed and not by total feed this article is protected by copyright all rights reserved

grain size distributions of bed load inferences from flume experiments using heterogeneous sediment beds abstract the grain size fractions in the bedload transported over the five heterogeneous sediment beds of different values of bed roughness were computed from the flume experiments the existence of an entrapment factor associated with the sorting observed from the bed to active layer was modeled based on the modified critical shear stress to estimate the grain size fractions in the transport layer under given hydraulic conditions the efficiency of these models was tested with the observed data subsequently the patterns of observed grain size distributions in the transport layer were tested to identify the distributions developed in the active layer due to sorting using three probability density functions pdf such as log normal log hyperbolic and log skew laplace tests indicated that a log skew laplace distribution fitted best for 49 a log hyperbolic for 31 and a log normal for 20 out of forty five bedload samples collected under unidirectional flow with changes in flow discharge and bed roughness the results of this study would be useful to specify the grain size distributions in the bedload formed under different hydrodynamic conditions in various sedimentary environments

the gravel sand transition sediment dynamics in a diffuse extension as gravel bedded rivers fine in the downstream direction they characteristically exhibit an abrupt transition from gravel to sand bedded conditions the prevailing theory for why abrupt gravel sand transitions emerge is based on bed load sorting of a bimodal sediment the abruptness is thought to be a consequence of sand overwhelming the gravel sand mixture once it reaches a critical coverage on the bed the role suspension plays in the development of gravel sand transitions has not been fully appreciated the fraser river british columbia is an archetypical abrupt gravel sand transition with a diffuse extension composed of a sand bed with some patches of gravel we examine flow shear stress and suspended sediment flux in the diffuse extension to better understand sediment dynamics where the sand bed emerges sand is carried in suspension upstream of the primary abrupt gravel sand transition but in the diffuse extension sand is moved as both bed load and suspended load we do not observe downstream gradients in shear stress or suspended sand flux through the diffuse extension that would suggest a gradual rain out of sand moving downstream which raises the question how is the sand bed formed sediment advection length scales indicate that with the exception of very fine sand that moves as wash load in the diffuse extension fractions coarser than the median sand size cannot be carried in suspension for more than one channel width this suggests that sand is deposited en masse at the beginning of the diffuse extension forming a sediment slug at low flood flows that is smeared downstream at high flood flows to form the sand reach

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

the decrease in shear stress and increase in transport rates subsequent to an increase in sand supply to a gravel bed channel abstract it is generally accepted that a gravel bed river will aggrade if the supply of sediment to the river is increased in a series of flume experiments using constant discharge and gravel feed rate sand feed rates were increased to 6 1 times that of gravel the slope of the bed decreased with increasing sand supply indicating that the increased sediment load could be transported at the same rate due to a decrease in shear stresses these results extend previous experiments to a wider range of boundary conditions a recent surface transport model is used to predict the changes in bed composition and transport using the same sediment supply composition and feed rates as in the laboratory experiments this model reasonably predicts a decrease in the reference shear stresses of the sand and gravel fractions as the sand supply is increased an increase in sand supply can increase the mobility of gravel fractions in the stream bed which can lead to bed degradation and preferential evacuation of these sediments from the river

quantifying the relative importance of flow regulation and grain size regulation of suspended sediment transport α and tracking changes in grain size of bed sediment β to predict changes in sediment transport it is essential to know whether transport is regulated mainly by changes in flow or by changes in grain size of sediment on the bed in flows where changes in suspended sediment transport are regulated purely by changes in flow grain size of bed sediment is constant increases in flow strength cause increases in both concentration and grain size of sediment in suspension because stronger flows are able to suspend more sediment and coarser grains under this constraint of constant grain size of bed sediment concentration and median diameter of suspended sediment are positively correlated in contrast where transport is regulated purely by changes in grain size of sediment on the bed concentration and median diameter of suspended sediment are negatively correlated because increasing the median diameter of the bed sediment causes the concentration to decrease while causing the median grain size in suspension to increase where both flow strength and grain size on the bed are free to vary the relation between concentration and grain size in suspension can be used to quantify the importance of grain size regulation relative to flow regulation of sediment transport a measure defined as to predict sediment transport in systems that are regulated dominantly by changes in grain size on the bed it is more useful to measure sediment input events or changes in grain size on the bed than to measure changes in flow more commonly grain size of bed sediment may be secondary to flow in regulating transport but may nevertheless be important the relative coarseness of bed sediment can be measured directly or like can be calculated from measurements of concentration and grain size of suspended sediment

land use changes and environmental dynamics in the upper rhone valley since neolithic times inferred from sediments in lac moras lac moras is a small lake located on a low elevation plateau in the upper rhone valley 304 m a s l the upper 5 m of accumulated sediment in the lake span 7500 years and offer a detailed record of environmental perturbations and land use history at a local scale a multiproxy analysis pollen charcoal and geochemical parameters led to the establishment of four periods of landscape dynamics the first evidence of human impacts was recorded during the neolithic and bronze age 6000 2700 cal bp these impacts were temporary and most likely affected small areas the second period in the iron age classical antiquity 2200 1900 cal bp appears to be a key period in which the intensification of anthropogenic pressure primarily grazing with localised areas of cultivation caused high intensity erosion events and deeply affected soil stability during the middle ages wheat rye and hemp cultures as well as tree farming walnut chestnut were intensively developed from 50 cal bp the 19th century on

can coarse bedload pass through weirs abstract restoring active bedload transfer in human impacted rivers has received increasing attention in recent years notably in response to the european water framework directive wfd which requires that the continuity of rivers not be disturbed by anthropogenic features such as dams or weirs the bocq river 233 km2 a moderate gradient stream in wallonia belgium has a hydraulic resource that was formerly largely exploited with 74 weirs up to 2 3 m high along 43 km we examined the effects of seven old abandoned weirs on bedload transport for three different types of weirs defined by the presence and position of the sluice gate system first the volume estimates of bedload stored in reservoirs indicated that despite their old age the reservoirs were not completely filled between 25 and 50 filled compared to the reservoir volume capacity and did not evolve very much since 1990 second the grain size analysis of bed material upstream downstream and within the reservoirs and the direct measurements of sediment transport slag particles and pit tagged pebbles demonstrated that bedload continues to be transported out of the reservoir even though the selective trapping of coarser elements was observed within the reservoir particles in the range of the median can pass over the crest of weirs but the coarser elements tend to remain in the reservoirs this trapping effect is mitigated when the weir has open or collapsed flushing gates that facilitate bedload transfer this indicates that weirs act as leaky barriers that allow bedload to pass through although the individual geomorphic setting plays a primary role in determining the local sediment continuity these findings suggest that river connectivity is less impacted than initially thought and is likely to increase over time as old weirs gradually fall into disrepair this needs to be acknowledged when planning barrier removal projects

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

sand transport over an immobile gravel substrate abstractexperiments were conducted in a laboratory flume channel to evaluate the effects of increasing amounts of sand on its transport over and through an immobile coarse gravel bed detailed measurements of sand transport rate bed texture and bed topography were collected for four different discharges at approximately the same flow depth of 0 2 m for 11 different elevations of sand in the gravel bed sand transport was measured using both physical samples and a density cell for a given flow rate increases in the elevation of sand relative to gravel resulted in decreases of bed shear stress from 32 44 and increases in sand transport by three orders of magnitude for the highest two discharges the sand merged into a small number of long and low bed forms that translated through and over the gravel bed a collapse of the transport data was accomplished by relating the sand transport rate to the bed shear stress scaled by the cumulative probability distribution function of the gravel surface evaluated

a field investigation of the influence of sediment supply on step pool morphology and stability abstract the effect of the sediment supply on step pool morphology and stability was studied through the investigation of 42 step pool reaches eight in the vosges and 34 in the alps france step pool geometry step wavelength l and height h was measured for each reach and the data set split into two distinct groups comprising 25 connected to a sediment source step pool reaches and 17 disconnected step pool reaches the geometry analysis showed differences between the two groups with the shape factors k l hr and the h hr ratios hr is the residual pool depth being lower for disconnected step pools this was interpreted to result from progressively changing geometry over time with disconnected step pool geometry defining the probable asymptotic characteristic associated with this long term process the measured step pool dimensions were in good agreement with equations based on maximum flow resistance abrahams et al 1995 energy of a falling jet comiti et al 2005 and channel spanning deposition chartrand et al 2011 however the average shape factor for the comiti et al equation was k 17 instead of 8 as the authors had proposed the data set was extended to 255 values with data from the literature which confirmed this average value for k field evidence moss cover and stone imbrication indicate that connected step pools are mobile whereas disconnected step pools are stable this was investigated by a hydraulic analysis of threshold motion conditions for the boulder steps comparison with discharges measured at nearby gauging stations indicates that remobilization return periods are 20 50 years for connected step pools and even less when considering scouring effects and greater than 50 years for disconnected step pools this difference in step mobility was ascribed to the fact that with sediment supply the channel can freely adjust its local slope allowing a relatively efficient transfer downstream of large boulders by scouring effects

sediment detachment and transport processes associated with internal erosion of soil pipes subsurface flow can be an important process in gully erosion through its impact on decreasing soil cohesion and erosion resistance as soil water content or pressure increases and more directly by the effects of seepage forces on particle detachment and piping the development of perched water tables in duplex soils fosters lateral flow that can result in seepage at the surface of hillslopes and or formation of soil pipes by internal erosion of lateral preferential flow paths continued internal erosion of soil pipes can lead to gullies dam and levee failures however the processes involved in particle and aggregate detachment from soil pipe walls and transport processes within soil pipes have not been well studied or documented this paper reviews the limited research on sediment detachment and transport in macropores and soil pipes and applies the knowledge learned from the much more extensive studies conducted on streams and industrial pipes to hydrogeologic conditions of soil pipes knowledge gaps are identified and recommendations are made for future research on sediment detachment and transport in soil pipes

transport of moderately sorted gravel at low bed shear stresses the role of fine sediment infiltration a reliable estimation of sediment transport in gravel bed streams is important for various practical engineering and biological studies e g channel stability design bed degradation aggradation restoration of spawning habitat in the present work we report original laboratory experiments investigating the transport of gravel particles at low bed shear stresses the laboratory tests were conducted under unsteady flow conditions inducing low bed shear stresses with detailed monitoring of the bed topography using a laser scanner effects of bed surface arrangements were documented by testing loose and packed bed configurations effects of fine sediments were examined by testing beds with sand artificial fine sand or cohesive silt infiltrated in the gravel matrix analysis of the experimental data revealed that the transport of gravel particles depends upon the bed arrangement the bed material properties e g size and shape consolidation index permeability and the concentration of fine sediments within the surface layer of moving grains this concentration is directly related to the distribution of fine particles within the gravel matrix i e bottom up infiltration or bridging and their transport mode i e bedload or suspended load compared to loose beds the mobility of gravel is reduced for packed beds and for beds clogged from the bottom up with cohesive fine sediments in both cases the bed shear stress for gravel entrainment increases by about 12 on the other hand the mobility of gravel increases significantly bed shear stress for particle motion decreasing up to 40 for beds clogged at the surface by non cohesive sand particles

influence of sediment supply on mountain streams bedload transport abstract sediment transport and storage in mountain stream channels are closely dependent on hillslope activities and bank erosion this paper analyses bedload measured in 13 mountain streams 989 values with slope higher than 5 in an attempt to investigate how much the sediment supply condition impacts bedload transport rates first the sediment supply condition of each stream was evaluated qualitatively using information available in the original studies in the second step the data set was analysed by comparing bedload transport rates considering a shields stress ratio and by comparing transport rate efficiency with consideration of a dimensionless stream power the two analyses indicate higher sediment transport rates for the streams identified as connected to an active source the consequences for bedload transport prediction are discussed with a nonthreshold bedload equation even though predicting the exact sediment supply condition and the associated transport rate is not yet possible this study suggests that defining an envelope delimiting minimum and maximum transport for a given river reach should be possible

effect of bed sand content on the turbulent flows associated with clusters on an armored gravel bed surface abstractas a gravel bed river armors a bed surface often develops clusters as part of its structure the influence of sand on armoring and the impact of clusters on the surrounding flow processes were investigated through flume experiments armored beds were created from four different sediments which progressively increased from 1 38 sand in the bed sediment extending from gravel framework to sand matrix beds turbulent kinetic energy tke and xz direction reynolds stress calculated from velocity profiles showed the clusters increased turbulence and induced formation of recirculation cells similar recirculation cells formed downstream of every cluster but the strength and orientation of these cells varied with bed condition the increase in sediment sand content and change in bed condition from gravel framework to sand matrix correlated to an increase in the energy and momentum in the recirculation cell but a reduction in cluster influence across the flow profile results show that the sand conten

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

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

spectral behavior of gravel dunes abstract spectral behavior of gravel dunes formed under different flow discharges is analyzed with an attempt to verify the 3 spectral law that has been confirmed extensively for sand dunes a schematic spectrum of gravel dunes is proposed based on the spectral analysis as well as results from the literature the results of spectral analysis show a significant deviation from the 3 spectral law for gravel dunes and the magnitude of deviation correlates with flow discharge possible explanations for the deviation from the 3 spectral law being associated with kinetic and geometrical characteristics have been explored to investigate the kinetic characteristics of gravel dunes a wavelet based method that calculates the celerity of dunes based on a pair of elevational time series is quantitatively tested our results suggest that 1 the kinetic explanation based on the relationship between dune celerity and dune length cannot fully explain the spectral behavior of gravel dunes 2 the geometrical explanation based on the self similarity hypothesis is confirmed by the relationship between dune length and dune height and 3 the development of gravel sheets accounts for the differences in kinetic and geometrical characteristics between gravel dunes and sand dunes

the effects of kinetic sorting on sediment mobility on steep slopes in poorly mobile static armour sorting is usually considered the result of hiding exposure effects we called this effect static sorting in opposition to very efficient grain to grain mechanisms produced by a mobile mixture called kinetic sorting we hypothesized that kinetic sorting can be an important contributor to the morphodynamics of mountain streams and attempted to demonstrate this with new flume experiments two long runs were produced with natural poorly sorted sediments and with transport stages of the coarse fraction defined by the ratio between the shear stress and the critical shear stress for transport smaller and higher than 1 respectively both runs produced an efficient transfer downstream of the injected material but with a major difference the first run no kinetic sorting produced permanent armour figuring clusters akin to what has already been observed in similar experiments the second run with kinetic sorting also produced bed armouring but this armour was periodically totally destroyed leading to substantial bed erosion this phenomenon was explained by kinetic sorting the effects of which are to produce an efficient downward migration of fine materials and bed surface armouring the consequence is that fine materials are hidden to the flow during aggradation allowing the slope to attain values much steeper than would have been expected at equilibrium for the mixture however whereas the surface armouring tends to stabilize the bed construction of a layer of fine sediments at the subsurface also contributes to making it very unstable these two contradictory effects explain the complex bed behaviours and the existence of very large bedload and slope fluctuations copyright 2014 john wiley sons ltd

colorado river sediment transport 1 natural sediment supply limitation and the influence of glen canyon dam analyses of flow sediment transport bed topographic and sedimentologic data suggest that before the closure of glen canyon dam in 1963 the colorado river in marble and grand canyons was annually supply limited with respect to fine sediment i e sand and finer material furthermore these analyses suggest that the predam river in glen canyon was not supply limited to the same degree and that the degree of annual supply limitation increased near the head of marble canyon the predam colorado river in grand canyon displays evidence of four effects of supply limitation 1 seasonal hysteresis in sediment concentration 2 seasonal hysteresis in sediment grain size coupled to the seasonal hysteresis in sediment concentration 3 production of inversely graded flood deposits and 4 development or modification of a lag between the time of a flood peak and the time of either maximum or minimum depending on reach geometry bed elevation analyses of sediment budgets provide additional support for the interpretation that the predam river was annually supply limited with respect to fine sediment but it was not supply limited with respect to fine sediment during all seasons in the average predam year sand would accumulate and be stored in marble canyon and upper grand canyon for 9 months of the year from july through march when flows were dominantly below 200 300 m3 s this stored sand was then eroded during april through june when flows were typically higher after closure of glen canyon dam because of the large magnitudes of the uncertainties in the sediment budget no season of substantial sand accumulation is evident because most flows in the postdam river exceed 200 300 m3 s substantial sand accumulation in the postdam river is unlikely

bedforms and sediment transport processes in the ephemeral streams of kobo basin northern ethiopia abstract bedforms and channel deposits architecture were studied in the ephemeral streams of the structural basin of kobo in northern ethiopia the froude numbers calculated at bankfull flow for a few of the study reaches are in agreement with the extensive occurrence of horizontal planar lamination small bedforms not previously reported in the literature are found and described as produced mainly by scouring rather than accretion processes equations to predict particle entrainment proved that large individual boulders standing scattered on the stream bed can be transported at bankfull flow for long distances direct inspection of the old alluvium and the modern channel deposits allow to identify a typical arrangement within a single bed consisting of divisions with specific characteristics this model of division association is explained in terms of vertical distribution of shear stress and compared with hyperconcentrated flow and traction carpet models in contrast with recent theories of flat asymmetrical bed waves progradation at froude numbers close to unity

mobilization of coarse surface layers in gravel bedded rivers by finer gravel bed load 1 additions of sand to gravel beds greatly increase the mobility and flux of gravel however it is not known how additions of finer gravel to coarser gravel beds will affect the mobility of bed material here we examine the effect of fine gravel pulses on gravel bed material transport and near bed flow dynamics in a series of flume experiments bed material refers exclusively to sediment in the channel prior to the pulse introduction the observations indicate that fine sediment pulses tend to migrate downstream in low amplitude waves as the waves pass over the gravel bed the interstitial pockets in the bed material surface fill and coarse gravel particles are entrained this increases bed material transport rates and causes a distinct shift from a selective mobility transport regime where particles coarser than the bed material median 8 mm make up 30 of the load to an equal mobility transport regime where bed materials coarser than 8 mm and finer than 8 mm are transported in equal proportions the only possible source for this coarser bed load material is the sediment bed suggesting that portions of the coarse surface layer are being mobilized observations of near bed velocity and turbulence suggest that fine gravel pulses cause fluid acceleration in the near bed region associated with a reduction in the level of turbulence produced at the sediment boundary this accelerated fluid at the bed increases drag exerted on coarse surface layer particles promoting their mobilization our findings suggest that in general finer bed sediment not just sand can mobilize coarser sediment and that expressions for the influence of sand on bed mobility need to be generalized on the basis of grain ratios

influence of the sediment supply texture on morphological adjustments in gravel bed rivers the role played by the texture of the sediment supply on channel bed adjustments in gravel bed rivers is poorly understood to address this issue an experimental campaign has been designed flume experiments lasting 96 h in a 9 m long 0 60 m wide have been performed with different sand gravel mixtures as feed textures the response of the surface texture has been found to be highly dependent on the grain size distribution of the feed when the feed texture included gravel the finest fractions of the sediment supply infiltrate beneath the surface conversely sand remains on the surface when the feed texture lacks gravel this different textural response becomes obscured when water discharge increases further the sediment transport rate approaches the feed rate differently depending on the content of gravel in the feed texture when a small proportion of gravel is part of the feed texture bed load transport rate asymptotically approaches the feed rate however when a significant fraction of gravel is part of the feed grain size distribution bed load transport rate approaches the feed rate by following an oscillatory path these findings have been verified in terms of a one dimensional numerical model this modeling reveals that the higher the differences in mobility among the grain sizes contained in the feed texture the more evident is the nonasymptotic transient trend toward equilibrium

sediment transport and channel adjustments associated with dam removal field observations 1 this study documents changes in channel geometry bed level profile and bed grain size distribution and their relations with the sediment transport at the reach scale following the removal of a low head dam after the removal net sediment deposition occurred downstream of the dam and net erosion occurred in the reservoir but approximately less than 1 of the sediment stored in the reservoir was transported downstream no bank erosion was evident either upstream or downstream of the dam bed deposition and scouring in the reservoir accounted for a decrease in the bed slope of 30 the stations downstream of the dam had surface bed material sizes at least 40 finer than preremoval conditions however the sediment transport rates downstream of the dam were not significantly different from predam to postdam removal or from an upstream control overall the removal of the dam had only minor effects on the channel adjustment downstream of the dam a simple analysis linking transport to channel geometry explains this effect

can magic sand cause massive degradation of a gravel bed river at the decadal scale shi ting river china abstract massive bed degradation 20 m in 7 years has been observed in the shi ting river sichuan province china since the 2008 wenchuan ms 8 0 earthquake the reason for the massive bed degradation has not been well understood a hypothesis has been proposed that relates bed degradation to the augmentation of sand supply after the earthquake the effect of sand on gravel mobility magic sand effect has long been observed in laboratory experiments in this paper we study whether the augmentation of sand supply and its magic sand effect can lead to the observed massive degradation at decadal scales a one dimensional river morphodynamic model is implemented to study the problem in general at field scale and in context of the gravel bed shi ting river sediment transport is calculated with the wilcock and crowe 2003 relation in which the magic sand effect is explicitly embedded in terms of a function relating reference shields number to the surface sand fraction we find that the augmentation of gravel supply leads to bed aggradation whereas the augmentation of sand supply can indeed lead to bed degradation and surface fining the magnitude and timescale of bed degradation are not sensitive to the sand supply rate but are sensitive to the flood intermittency factor i e the fraction of time the river is in flood however an unrealistic flood intermittency factor 0 3 would be required in order to match the observed timescale of bed degradation thus indicating that the magic sand effect might not be the governing reason for massive bed degradation in the shi ting river and by implication in gravel bed rivers in general our simulation results also indicate that despite the fact that magic sand effects are not explicitly included in most sediment transport relations they are at least partly built in via the hiding function that is contained in most sediment transport relations for gravel sand mixtures while here we use the shi ting river as an example our results have applicability to gravel bed rivers subjected to augmentation in sand supply in general

temporal dynamics of suspended sediment transport in a glacierized andean basin abstract suspended sediment transport can affect water quality and aquatic ecosystems and its quantification is of the highest importance for river and watershed management suspended sediment concentration ssc and discharge were measured at two locations in the estero morales a chilean andean stream draining a small basin 27 km2 hosting glacierized areas of about 1 8 km2 approximately half of the suspended sediment yield 470 t year 1 km 2 was transported during the snowmelt period and half during glacier melting the hysteresis patterns between discharge and ssc were calculated for each daily hydrograph and were analysed to shed light on the location and activity of different sediment sources at the basin scale during snowmelt an unlimited supply of fine sediments is provided in the lower and middle part of the basin and hysteresis patterns tend to be clockwise as the peaks in ssc precede the peak of discharge in daily hydrographs instead during glacier melting the source of fine sediments is the proglacial area producing counterclockwise hysteresis it is suggested that the analysis of hysteretic patterns over time provides a simple concept for interpreting variability of location and activity of sediment sources at the basin scale

size distributions in suspension over sand pebble mixture an experimental approach experiments were conducted in a laboratory flume to estimate the grain size distributions in suspension at different heights over five heterogeneous sediment beds of different values of bed roughness the analysis had been performed to study the influences of velocity bed roughness and ripple height on the size sorting which developed the specific grain size distribution in suspension the observed size distributions in suspension were tested using three statistical distributions such as log normal log hyperbolic and log skew laplace tests indicated that log hyperbolic distribution does not fit in suspension even the original beds grain size distributions were log hyperbolic it is observed from the experimental findings that the flow velocity influences the overall grain size distribution leading from log normal to log skew laplace distribution with the increase of velocity but the bed roughness does not have any role in changing the size distribution from log normality the results of this study would be valuable to hydraulic engineers and sedimentologists to predict the nature of bed material from the size distributions of suspended materials

effect of sand supply on transport rates in a gravel bed channel in a series of flume experiments using constant discharge flow depth and gravel feed rate sand feed rates were varied from 0 16 to 6 1 times that of gravel the bed slope decreased with increasing sand supply indicating that the gravel could be transported at the same rate along with increasing amounts of sand at smaller shear stresses prediction of river response to an increase in sediment supply requires prediction of mutual changes in bed composition and transport and therefore a transport model defined in terms of the grain size of the bed surface a recent model provides satisfactory prediction of the experimental observations and indicates the general response of gravel beds to increased sand supply an increase in sand supply may increase the sand content of the river bed and the mobility of gravel fractions which can lead to bed degradation and preferential evacuation of these sediments from the river

kinematic wave model for transient bed profiles in alluvial channels under nonequilibrium conditions 1 transient bed profiles in alluvial channels are generally modeled using diffusion or dynamic waves and assuming equilibrium between detachment and deposition rates equilibrium sediment transport can be considerably affected by an excess or deficiency of sediment supply due to mostly flows during flash floods or floods resulting from dam break or dike failure in such situations the sediment transport process occurs under nonequilibrium conditions and extensive changes in alluvial river morphology can take place over a relatively short period of time therefore the study and prediction of these changes are important for sustainable development and use of river water this study hence developed a mathematical model based on the kinematic wave theory to model transient bed profiles in alluvial channels under nonequilibrium conditions the kinematic wave theory employs a functional relation between sediment transport rate and concentration the shear stress approach for flow transport capacity and a relation between flow velocity and depth the model satisfactorily simulated transient bed forms observed in laboratory experiments

does the summer arctic frontal zone influence arctic ocean cyclone activity abstractextratropical cyclone activity over the central arctic ocean reaches its peak in summer previous research has argued for the existence of two external source regions for cyclones contributing to this summer maximum the eurasian continent interior and a narrow band of strong horizontal temperature gradients along the arctic coastline known as the arctic frontal zone afz this study incorporates data from an atmospheric reanalysis and an advanced cyclone detection and tracking algorithm to critically evaluate the relationship between the summer afz and cyclone activity in the central arctic ocean analysis of both individual cyclone tracks and seasonal fields of cyclone characteristics shows that the arctic coast and therefore the afz is not a region of cyclogenesis rather the afz acts as an intensification area for systems forming over eurasia as these systems migrate toward the arctic ocean they experience greater deepening in situations when the afz is strong at midtropospheric levels