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

effects of lateral and vertical constrictions on flow in rough steep channels with bedload abstracttwo phase flows occurring at flow constrictions such as bridges or open sediment check dams are complex especially for steep rivers with bedload transport dangerous bedload deposition and

direct measurements of lift and drag on shallowly submerged cobbles in steep streams implications for flow resistance and sediment transport steep mountain streams have higher resistance to flow and lower sediment transport rates than expected by comparison with low gradient rivers and often these differences are attributed to reduced near bed flow velocities and stresses associated with form drag on channel forms and immobile boulders however few studies have directly measured drag and lift forces acting on bed sediment for shallow flows over coarse sediment which ultimately control sediment transport rates and grain scale flow resistance here we report on particle lift and drag force measurements in flume experiments using a planar fixed cobble bed over a wide range of channel slopes 0 004 s 0 3 and water discharges drag coefficients are similar to previous findings for submerged particles c d 0 7 but increase significantly for partially submerged particles in contrast lift coefficients decrease from near unity to zero as the flow shallows and are strongly negative for partially submerged particles indicating a downward force that pulls particles toward the bed fluctuating forces in lift and drag decrease with increasing relative roughness and they scale with the depth averaged velocity squared rather than the bed shear stress we find that even in the absence of complex bed topography shallow flows over coarse sediment are characterized by high flow resistance because of grain drag within a roughness layer that occupies a significant fraction of the total flow depth and by heightened critical shields numbers and reduced sediment fluxes because of reduced lift forces and reduced turbulent fluctuations

a comparison between flume and field bed load transport data and consequences for surface based bed load transport prediction 1 the ability of simple equations to predict bed load transport with limited knowledge of the bed surface material was investigated this was done using a data set consisting of 7 636 bed load transport values from the flume 1 317 data and from 84 river reaches 6 319 field data it was possible to collapse field and flume data by correcting the ratio between the shields number and its critical value with a very simple hiding function proposed as a power law of the d84 d50 ratio in so doing a surface based bed load transport formula was proposed it was successfully tested on an independent data set comprising sand and gravel bed rivers with slope in the range 0 0002 0 08 with 86 of the values predicted within a precision of 1 order of magnitude moreover the formula reproduced the low transport rates well contrary to the usual surface based formulas also tested and is particularly well suited for estimating low transport rates associated with near bankfull flow discharge this new formula is neither time consuming no fractionwise calculation nor data consuming the required parameters are the flow discharge the active width the slope and the surface grain diameters d50 and d84

a mixing layer theory for flow resistance in shallow streams 1 a variety of surface roughness characterizations have emerged from nineteenth and twentieth century studies of channel hydraulics when the water depth h is much larger than the characteristic roughness height ks roughness formulations such as manning s n and the friction factor f can be explicitly related to the momentum roughness height zo in the log law formulation for turbulent boundary layers thereby unifying roughness definitions for a given surface however when h is comparable to or even smaller than ks the log law need not be valid using a newly proposed mixing layer analogy for the inflectional velocity profile within and just above the roughness layer a model for the flow resistance in shallow flows is developed the key model parameter is the characteristic length scale describing the depth of the kelvin helmholtz wave instability it is shown that the new theory originally developed for canopy turbulence recovers much of the earlier roughness results for flume experiments and shallow gravel streams this study is the first to provide such a unifying framework between canopy atmospheric turbulence and shallow gravel stream roughness characterization the broader implication of this study is to support the merger of a wealth of surface roughness characterizations independently developed in nineteenth and twentieth century hydraulics and atmospheric sciences and to establish a connection between roughness formulations across traditionally distinct boundary layer types

hydrodynamics of steep streams with planar coarse grained beds turbulence flow resistance and implications for sediment transport the hydraulics of steep mountain streams differ from lower gradient rivers due to shallow and rough flows energetic subsurface flow and macro scale form drag from immobile boulders and channel and bed forms heightened flow resistance and reduced sediment transport rates in steep streams are commonly attributed to macro scale form drag however little work has explored steep river hydrodynamics in the absence of complex bed geometries here we present theory for the vertical structure of flow velocity in steep streams with planar rough beds that couples surface and subsurface flow we test it against flume experiments using a bed of fixed cobbles over a wide range of bed slopes 0 4 30 experimental flows have a nearly logarithmic velocity profile far above the bed flow velocity decreases less than logarithmically towards the bed and is non zero at the bed surface velocity profiles match theory derived using a hybrid eddy viscosity model in which the mixing length is a function of height above the bed and bed roughness subsurface flow velocities are large 1 m s and follow a modified darcy brinkman forchheimer relation that accounts for channel slope and shear from overlying surface flow near bed turbulent fluctuations decrease for shallow rough flows and scale with the depth averaged flow velocity rather than bed shear velocity flow resistance for rough planar beds closely matches observations in natural steep streams despite the lack of bed or channel forms in the experiments suggesting that macro scale form drag is smaller than commonly assumed in stress partitioning models for sediment transport

effects of check dams on bed load transport and steep slope stream morphodynamics abstract check dams are transversal structures built across morphologically active streams in mountainous regions these structures have been used widely in torrent hazard mitigation for over 150 years thousands of them are regularly maintained by stream managers and torrent control services the stabilization role of these structures is well known i e they durably constrain the stream bed through the creation of vertical and planar fixed points what is not yet clear is to what extent check dams influence bed load transport how do peak solid discharge or flood transported volume change when check dams are added to a reach to address these questions long lasting small scale experiments were conducted in a 4 8 m long flume with either one three or no structures the results show that the addition of structures creates independent compartments in the bed level which have a strong influence on bed surface armouring and stream morphodynamics the consequence is that instantaneous transport intensities are unchanged but peak solid discharge occur more often and for shorter duration this results in the same total transported volume over the long term but reduced volume for a single transport event it reaffirms the observation of pioneering authors of the mid 19th and early 20th century who conceptualized the possible sediment transport regulation function of check dams in addition to stabilizing the stream bed check dams influence bed load transport through a buffering effect releasing frequently and in small doses what in their absence would be transported abruptly en masse during rare extreme events

bed load transport flume experiments on steep slopes experiments were conducted over uniform gravel bed materials to obtain 143 friction factor values under bed load equilibrium flow conditions in an attempt to add to the scarce data available on slopes between 1 and 9 for shields numbers between 0 08 and 0 29 analyses showed that when only flows over flat beds are considered a distinction must be made between flows with and without bed load more particularly fitting flow resistance equations indicated that the roughness parameter increases by a factor of 2 5 from clear water flow to intense bed load transport between these two states the flow resistance can be approximated by a constant for a given slope

bed load effects on hydrodynamics of rough bed open channel flows the extent to which turbulent structure is affected by bed load transport is investigated experimentally using a nonporous fixed planar bed comprising mixed sized granular sediment with a d50 of 1 95 mm three different sizes of sediment d50 0 77 1 99 and 3 96 mm were fed into the flow at two different rates 0 003 and 0 006 kg m s and subsequently transported as bed load particle image velocimetry piv was used to determine the turbulence characteristics over the fixed bed during clear water and sediment feed cases mean longitudinal flow velocities at any given depth were lower than their clear water counterparts for all but one of the mobile sediment cases the exception was with the transport of fine grains at the higher feed rate in this case longitudinal mean flow velocities increased compared to the clear water condition the coarse grains tended to augment bed roughness but fine grains saturated the troughs and interstices in the bed topography effectively causing the influence of bed irregularities to be smoothed the piv technique permitted examination of both temporal and spatial fluctuations in flow variables therefore many results are presented in terms of double averaged quantities in temporal and spatial domains in particular the form induced stress which arises from spatially averaging the reynolds averaged navier stokes equations and is analogous to the reynolds turbulent stress contributed between 15 and 35 of the total measured shear stress in the roughness layer flow around protrusive roughness elements produced a significant proportion of the turbulent kinetic energy shear production suggesting that this process is highly intermittent near rough beds

flow resistance in steep streams an experimental study 1 frequently an assessment of the mean water velocity in a stream is necessary to estimate the discharge associated with a particular flow depth or conversely the mean depth associated with a particular discharge in the absence of a direct measurement of flow velocity a flow resistance approach which establishes the relation between depth and velocity can be applied two approaches have been used in the past traditional approaches based on the use of a resistance coefficient e g darcy weisbach or dimensionless hydraulic geometry approaches to examine if one approach is more appropriate for steep streams data from 31 flume experiments conducted to examine flow resistance in self formed cascade channels were analyzed a dimensionless hydraulic geometry approach developed using at a station data to characterize the q exponent and between site data to characterize the exponent on the channel slope term was more accurate than more traditional approaches the developed relation was similar to the established rational relation v g0 2q0 6s 0 4s0 2 suggesting that the rational relation has merit the approach does not utilize a flow partitioning approach since in steep streams with small relative depths the grains themselves generate form and spill resistance the observation that a single dimensionless hydraulic geometry flow resistance relation can describe measurements across a range of grain sizes and bed slopes 3 21 suggests that steep streams may follow a single scaling relation similar to the regime equations associated with lowland rivers

predicting the equilibrium bed slope in natural streams using a stochastic model for incipient sediment motion in this paper a method to predict the equilibrium bed slope in natural streams based on the incipient motion criterion is proposed the method is based on the criterion suggested by gessler to simulate the grain size distribution of the armour coat using the concept of critical shear stress of a sediment mixture in particular a different expression of the probability for a single particle size to be part of the armour coat is firstly proposed then a simple two steps criterion is suggested to estimate the safety factor required by the proposed approach the method is applied in three different italian regions calabria basilicata and tuscany and required several field campaigns involving the survey of 251 stream reaches upstream of grade control structures the area including five calabrian streams is firstly used to test the suitability of the two steps approach in this area considering the detailed information about the particle size of bed material the effect of using a simplified grain size distribution for each torrent is also checked the method is then applied within the two additional areas basilicata and tuscany in order to check the geographic influence on the prediction of the stable longitudinal profile a comparison between measured and estimated values of the equilibrium bed slope showed a good agreement for the calabrian streams where the number of the investigated stream reaches is greater and satisfactory results for the torrents located in basilicata and tuscany where the field data set is more limited the overall results encourage the extension of the proposed method to additional field data and suggest the use of this approach by hydraulic designers in order to stabilize the longitudinal profile of natural streams copyright 2011 john wiley sons ltd

feedback between bed load transport and flow resistance in gravel and cobble bed rivers 1 to calculate bed load engineers often use flow resistance equations that provide estimates of bed shear stress in these equations on the basis of the estimate of the appropriate hydraulic radius associated with the bed only the bed roughness ks is commonly set as a constant whatever the bed load intensity however several studies have confirmed the existence of feedback mechanisms between flow resistance and bed load suggesting that a flow dependent bed roughness should be used therefore using a data set composed of 2282 flume and field experimental values this study investigated the importance of these feedback effects new flow resistance equations were proposed for three flow domains domain 1 corresponds to no bed load and a constant bed roughness ks d where d is a representative grain diameter whereas domain 3 corresponds to a high bed load transport rate over a flat bed with a constant bed roughness ks 2 6d between these two domains a transitional domain 2 was identified for which the bed roughness evolved from d to 2 6d with increasing flow conditions in this domain the darcy weisbach resistance coefficient f can be approximated using a constant for a given slope the results using this new flow resistance equation proved to be more accurate than those using equations obtained from simple fittings of logarithmic laws to mean values the data set indicates that distinguishing domains 2 and 3 is still relevant for bed load in particular the data indicate a slope dependence in domain 2 but not in domain 3 a bed load model based on the tractive force concept is proposed finally flow resistance and bed load equations were used together to calculate both shear stress and bed load from the flow discharge the slope and the grain diameter for each run of the data set efficiency tests indicate that new equations implicitly taking a feedback mechanism into account can reduce the error by a factor of 2 when compared to other equations currently in use showing that feedback between flow resistance and bed load can improve field bed load modeling

prediction of the el nino of 1982 83 abstract the large growth of sst anomalies during 1982 83 in the equatorial pacific could have been predicted 4 5 months in advance via advanced statistical models the decay of the warmth was not well predicted a result attributed to the need for local forcing and thermodynamic considerations in the modeling process kelvin wave models will likely have this same deficiency the success of the statistical models indicates that the development of the sst anomalies during 1982 83 was not unusual

comparing flow resistance law for fixed and mobile bed rills rills caused by run off concentration on erodible hillslopes have very irregular profiles and cross section shapes rill erosion directly depends on the hydraulics of flow in the rills which may differ greatly from hydraulics of flow in larger and regular channels in this paper a recently theoretically deduced rill flow resistance equation based on a power velocity profile was tested experimentally on plots of varying slopes ranging from 9 to 26 in which mobile and fixed bed rills were incised initially measurements of flow velocity water depth cross section area wetted perimeter and bed slope carried out in 320 reaches of mobile bed rills and in 165 reaches of fixed rills were used for calibrating the theoretical flow resistance equation then the relationship between the velocity profile parameter the channel slope and the flow froude number was separately calibrated for the mobile bed rills and for the fixed ones the measurements carried out in both conditions fixed and mobile bed rills confirmed that the darcy weisbach friction factor can be accurately estimated using the proposed theoretical approach for mobile bed rills the data were supportive of the slope independence hypothesis of velocity due to the feedback mechanism stated by govers the feedback mechanism was able to produce quasicritical flow conditions for fixed bed rills obtained by fixing the rill channel by a glue at the end of the experimental run with a mobile bed rill the slope independence of the flow velocity measurements was also detected therefore an experimental run carried out by a rill bed fixed after modelling flow action is useful to detect the feedback mechanism finally the analysis showed that for the investigated conditions the effect of sediment transport on the flow resistance law can be considered negligible respect to the grain roughness effect

theoretical development on the effects of changing flow hydraulics on incipient bed load motion 1 several decades of flume and field measurements have indicated that in rough turbulent flows the critical shields stress increases with increasing slope and associated decreasing relative depth this result contradicts the usual consideration of a decreased critical shields value on very steep slopes because of increased gravitational effects however recent studies have demonstrated that these experimental results could be reproduced with a force balance model if the classical logarithmic velocity profile was replaced with a velocity profile that was more compatible with available velocity measurements over gravel beds these measurements indicate the existence of a roughness layer that is a zone of almost constant velocity close to the bed whose properties mean velocity and turbulence depended on the flow s relative depth unfortunately velocity profile measurements for low relative depth associated with steep slopes are scarce and it is still difficult to include such flow properties in a force balance model flow resistance data on the basis of depth average velocity measurements are very common and cover a wide range of slopes and relative depths in this paper these data are used to fit a velocity profile including a roughness layer when used in a force balance model for incipient motion it adequately reproduced a data set composed of 270 critical shields values measured in a flume with near uniform sediments the relevance of this research to field problems is discussed using a data set composed of 92 critical shields stresses obtained from field measurements finally a model is proposed for field applications taking into account the slope effect

hyperconcentrated flow and sediment transport at steep slopes in order to simulate a fine material slurry of a debris flow a clay suspension of various concentrations was recirculated in a steep flume the effect of an increasing fluid density and viscosity on the flow behavior and the bed load transport capacity of the flow was examined viscous effects were found to become important below a limiting particle reynolds number of about 10 above this limiting value density effects cause an increase in the bed load transport rates as compared to similar conditions with clear water as transporting fluid the experimental data in this range can be described with conventional newtonian formulas and is analyzed together with other bed load transport data two different calculation schemes are proposed for the steep slope range is n n 10 where the bed load concentration is significant with respect to the total flow depth below the critical particle reynolds number of 10 the bed load transport rates decreased strongly

resistance to flow in steep rough streams a theory of resistance to flow in steep rough streams is developed that takes into account the existence of a highly turbulent wake zone near the bed of a very rough boundary two zones are identified in the flow field the first one close to the top of the bed contains overlapping wakes generated by protruding roughness elements in the second zone located above the first one the velocity distribution is described in terms of a logarithmic profile flume data are used to check the validity of this formulation the compound velocity profile allows for equations to obtain darcy weisbach and chezy factors the proposed approach is compared with the results of four previous theories using data obtained from several mountain rivers although flow in actual mountain rivers is more complicated than the idealized case treated in the study the formulation provides equations that allow accurate prediction of flow resistance with a mean error of 15

slope correction for calculation of bedload sediment transport rates in steep channels abstractclassical theories developed for investigating bedload sediment transport are applicable largely for channel beds of a gentle slope examples are the well known shields diagram for the evaluation of the critical condition for initial sediment motion and the classical bedload transport formulas derived with deterministic or probabilistic approaches how to apply the existing bedload formulas for steep channels remains challenging in this study a simple procedure is proposed for correcting slope effects so that the existing bedload formulas can be extended for steep channels to verify the proposed correction factor two representative bedload formulas are modified and checked against published data of sediment transport rates that were significantly affected by steep slopes

multiple time scales of fluvial processes with bed load sediment and implications for mathematical modeling fluvial bed load transport is often considered to assume a capacity regime exclusively determined by local flow conditions but its applicability in naturally occurring unsteady flows remains to be theoretically justified in addition mathematical river models are often decoupled being based on simplified conservation equations and ignoring the feedback impacts of bed deformation to a certain extent so far whether the decoupling could have considerable impacts on the fluvial processes with bed load transport remains poorly understood this paper presents a theoretical investigation of both issues the multiple time scales of fluvial processes with bed load sediment are evaluated to examine the applicability of bed load transport capacity and decoupled models numerical case studies involving active bed load transport by highly unsteady flows complement the analysis of the time scales it is found that bed load transport can sufficiently rapidly adapt to capacity in line with local flow because sediment

turbulence characteristics of new zealand gravel bed rivers gravel bed river turbulence is investigated on the basis of field measurements in new zealand rivers the data include instant longitudinal velocities obtained by two methods point measurements an

near bed sediment concentration distribution and basic probability of sediment movement sediment concentration distribution and the basic probability of sediment movement near the channel bed are two of the most important and fundamental issues in the study of sediments based on statistical analysis and considering the transport mechanisms the rules of sediment concentration distribution near a channel bed are studied analytical expressions for the near bed sediment concentration distribution and mean sediment concentration are derived and the expression for the mean sediment concentration near the bed is verified by measured data which were obtained from previous experiments with the help of statistical theory the expressions of basic probabilities i e rolling saltating and suspending probabilities for sediment movement near the bed are also derived the expression for starting probability is verified by the measured data the verification shows that the results from the proposed expression agree well with the measured data this research has both theoretical and practical significance for further investigation concerning rules of bed load and suspended sediment transport

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

flow velocity profiles in gravel bed rivers velocity measurements carried out in a rectangular laboratory flume having a gravel bed are presented the measurements are carried out for four different bed shapes characterized from different concentrations of coarser elements and for the two conditions of small and large scale roughness the analysis showed that the dean profile is usable to establish the velocity profile for both hydraulic conditions for small scale hydraulic condition an indirect estimate procedure of the coefficients of dean profile is proposed

bedload transport resistance in rough open channel flows during bedload movement by saltation streamwise momentum is transferred from the ow to the saltating grains when the grains collide with other grains on the bed or in the ow streamwise momentum is reduced and there is a decrease in streamwise ow velocity and an increase in ow resistance herein termed bedload transport resistance fbt based on experiments in two umes with xed and mobile plane beds and previously published data an equation is developed that may be used to predict fbt for both capacity and non capacity ows the variables in this equation are identi ed by dimensional analysis and the coef cients are determined by non linear regression this equation applies to rough turbulent open channel ows where the relative submergence is between 1 and 20 and the entire sediment load moves by saltation an investigation of the relative magnitudes of fbt and grain resistance fc suggests that where dimensionless shear stress is less than 1 and saltation is the dominant mode of bedload transport fbt fc increases with but never exceeds 1 copyright 2004 john wiley sons ltd

average velocity of solitary coarse grain in flows over smooth and rough beds abstracta formula is derived for the prediction of the average velocity of a solitary coarse grain moving over a rigid flat channel bed to verify the theoretical consideration data collected in the present study and reported in the literature 811 sets in total are analyzed for typical bed roughness configurations the results show significant improvements in the prediction of solitary grain velocity for both smooth and rough bed conditions in comparison with other formulas available in the literature from the proposed formula it follows that for rough bed conditions the dimensionless grain velocity depends on the shields number and also the ratio of bed roughness height to the grain diameter the present study is limited to motion of solitary grains but the result obtained implies that bed load transport rate could be also affected by the equivalent bed roughness height even under mobile bed conditions and such effects may become pronounced for nonuniform sediments

resistance equation for large scale roughness a theory of flow resistance for large scale roughness is developed the main points of which are supported by a flume study from which the outline of a process based resistance equation is determined rivers with large scale roughness have steep slopes and depths of the same order of magnitude as the bed material size flow resistance depends on the form drag of the roughness elements and their disposition in the channel theoretical considerations show resistance to be a function of reynolds number froude number roughness geometry and channel geometry these processes are examined using the results of flume experiments based on five different roughness beds and a wide range of flows semi empirical analysis supports the theory and allows the development of a theoretical power law resistance equation which accounts for most of the processes a summary of the data is presented for use in other research studies

bed load fluctuations in a steep channel abstract bedload transport rate fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions even under constant sediment feeding and water discharge the observed bedload transport rate pulses are periodic and a consequence of grain sorting moreover the presence of large relatively immobile boulders such as erratic stones which are often present in mountain streams has an impact on flow conditions the detailed analysis of a 13 hours laboratory experiment is presented in this paper boulders were randomly placed in a flume with a steep slope 6 7 and water and sediment were constantly supplied to the flume along with the sediment transport and bulk mean flow velocity the boulder protrusion boulder surface and number of hydraulic jumps which are indicators of the channel morphology were measured regularly during the experiment periodic bedload transport rate pulses are clearly visible in the data collected during this longduration experiment along with correlated fluctuations in the flow velocity and bed morphology the links among the bulk velocity the time evolution of the morphology variables and the bedload transport rate are analyzed via correlational analysis showing that the fluctuations are strongly related a phase analysis of all observed variables is performed and the average shapes of the time cycles of the fluctuations are shown observations indicate that the detected periodic fluctuations correspond to different bed states furthermore the grain size distribution through the channel which varies in time and space clearly influences these bedload transport rate pulses finally known bedload transport rate formulae are tested showing that only the application of a drag shear stress allows a correct estimation of the time fluctuations

the effect of sediment concentration on bedload roughness bedload roughness is the roughness produced by sediment transported near the bed while it is well established that the magnitude of this roughness is proportional to the thickness of the moving sediment layer the effect of sediment concentration remains largely unknown this paper presents the results of a flume experiment that was designed to investigate the effect of sediment concentration on bedload roughness the experiment consisted of creating flow conditions where bedload transport is supply limited and injecting gravel size particles d 50 7 4 mm at the upstream end of the flume in order to produce bedload transport while keeping flow conditions constant sediment concentration was varied by successively increasing the injection rate of gravel in the flow for each injection rate flow velocity profiles were measured in order to evaluate changes of mean flow velocity u shear velocity u roughness length z 0 and resistance to flow f the results indicate that low sediment concentration affects mainly the near bed portion of the flow where it causes a reduction of mean flow velocity and an increase of shear velocity and roughness length for larger sediment concentration the whole flow velocity profile becomes affected by bedload roughness but the importance of this effect always remain larger near the bed as sediment concentration is augmented mean flow velocity is consistently reduced but shear velocity and roughness length increase until a plateau is reached where these two variables become constant

the effect of bedload transport on mean and turbulent flow properties abstract this paper reports the results of a flume experiment that was designed to investigate the effect of bedload transport on mean and turbulent properties of the flow the experiment consisted of varying the bedload transport rate for a given hydraulic condition and of measuring the flow velocity profiles using an acoustic doppler velocimeter adv for each transport rate in order to allow for comparison bedload transport was produced by injecting gravel size particles d50 7 4 mm with a conveyer belt mounted at the upstream end of the flume the results indicate that the effect of bedload on flow characteristics is complex it is shown that bedload transport causes opposite effects on flow velocity depending on the roughness of the bed and the relative magnitude of flow and sediment transport variables a better understanding of these conflicting results is obtained from the application of an energy budget approach to the analysis of velocity data this approach demonstrates that bedload affects flow velocity by modifying the rate of dissipation of turbulent kinetic energy however the mechanisms responsible for the modification of turbulent dissipation are still unknown

bedload a granular phenomenon bedload the transport of sediment remaining in contact with the stream bed has mainly been studied from the perspective of the correlation between fluid driving forces and the responding sediment flux yet grain grain interactions are important and bedload should also be considered as a granular phenomenon we review progress made recently in the study of granular flows especially on segregation and rheology that better illuminates the nature of bedload granular flows may exhibit gas like or fluid like flow or quasi solid deformation all three conditions might be duplicated in bedload understanding of intense bedload transport occurring continuously in a layer several grains deep typical of sand beds might greatly benefit from results in granular physics as illustrated by grain inspired bedload results however processes restricted to the surface of the bed when particles move intermittently and the bed becomes structured while characteristic in gravel bed channels are not well addressed in granular physics mutual study of these phenomena may benefit both physics and fluvial geomorphology we intend therefore to contribute to an enhanced dialogue between granular physics and bedload science communities copyright 2010 john wiley sons ltd

provenance and tectonic setting of clastic deposits in the devonian xicheng basin qinling orogen central china abstract the qinling orogenic belt is one of world s great collisional orogenic belts which extends from the pamirs west kunlun to the korean peninsula and occupies a key position in understanding of the evolution and assembly of asia this belt separates the north china and south china blocks extensive thick devonian clastic rocks sandwiched between the shangdan and mianlue suture zones are mainly infills of the xicheng basin and provide the principal sedimentary record of tectonism in the qinling orogen devonian infills were subdivided into three subunits the upper devonian dacaotan group the middle upper devonian shujiaba group and the lower upper devonian xihanshui group their provenance and tectonic setting are critical to understanding not only the tectonic evolution of asia but also the global aspects of collisional regions compared with devonian strata in the eastern part of the qinling orogen the xicheng basin was not seriously modified by mesozoic tectonism petrologic studies demonstrate that the dacaotan group consists mainly of feldspathic litharenite with abundant volcanic fragments in contrast the shujiaba and xihanshui groups contain more metamorphic and sedimentary fragments and fewer magmatics than other lithic fragments these relations demonstrate that magmatic plutons were not extensively exposed at the time of deposition of these groups on q f l plots sandstones fall into the field of transitional arcs but on qm p k and lm lv ls diagrams they plot in the fields of continental arc and continental suture rocks the presence of volcanic lithic grains in the sandstones suggests they were derived mainly from a continental arc major element geochemistry of siltstones and mudstones demonstrate that most were derived from a continental arc but some from an oceanic island arc trace element geochemistry favors a continental arc provenance mostly from felsic rocks a high cr content of siltstones and mudstones demonstrates that some mafic or ultramafic rocks were exposed in the source area and provided some detritus rare earth element patterns are very similar to those of sandstones in modern continental arcs devonian paleocurrent indictors demonstrate that this continental arc was located at the northern edge of the basin this deduction demonstrates that the qinling arc in the southern margin of the north china block was the major source of clastic debris in the devonian xicheng basin these relationships suggest that the sediments were most probably deposited in a forearc basin