a unifying framework for particle entrainment 1 a new framework is presented that describes the hydrodynamic entrainment of sediments the framework is derived from the general equations of particle stability and fluid motion and covers a wide range of scales from that of the individual particle to en masse entrainment at the scale of the stream reach particle entrainment is explicitly shown to be driven by the combination of instantaneous fluid stress and pressure gradients particle surface contact forces particle submerged weight and the weight and motion of particle associated fluid of these factors bed shear stresses across particle differences in pressures and fluxes of momentum and sediment bed characteristics are found to be key factors in particle entrainment the conventional shields entrainment parameter is shown to be applicable to averaged en masse entrainment by steady uniform 2 d flows this nondimensionalized bed shear stress at threshold is further demonstrated to be a function of interparticle forces and bed normal hydrodynamic forces and thereby sediment size and other fluid sediment and bed characteristics observed shields parameter dependencies are discussed the presented framework which includes all relevant particle and fluid characteristics and flow dynamics may aid understanding of entrainment mechanics the design and analysis of further studies of entrainment the design of parameterizations that lead to the solution of entrainment problems and numerical modeling of combined fluid and sediment dynamics

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

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

measurements of turbulence in the benthic boundary layer over a gravel bed and comparison between acoustic measurements and predictions of the bedload transport of marine gravels articles originaux williams j j thorne p d heathershaw a d 1989a sedimentology 36 959 971 1989b sedimentology 36 973 979

incipient sediment motion at high slopes in uniform flow condition 1 this paper reviews the problem of the incipient motion of cohesionless particles in a high sloping water stream a theoretical analysis of the equilibrium of a gravel particle is developed by a deterministic approach with the introduction of the relative degree of exposure of the particle itself to the stream flow the parameter relative degree of exposure relates the positioning of a particle with the upstream one the problem of velocity distribution and the definition of drag coefficient is also revised to better account for low submergence the results of the theory are compared with a series of ad hoc experiments performed by c gregoretti

in situ acoustic measurements of marine gravel threshold and transport measurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles the turbulence in the bottom boundary layer was measured using two electromagnetic current meters and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material visual estimates of bedload were also obtained with an underwater tv camera the acoustic technique unlike a conventional bedload sampler has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected this has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions the results of the study show that of the turbulent bursting events which contribute towards the reynolds stress only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material the measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process

entrainment threshold of cohesionless sediment grains under steady flow of air and water existing formulations for bed sediment entrainment under steady flow are incapable of explaining two well documented observational facts i water flow requires considerably higher dimensionless shear stresses to move the bed grains than air flow and ii under open channel flow steep granular beds are more stable than beds with milder slopes these two facts together with recent direct measurements of forces acting on bed grains giving time mean negative drags schmeeckle et al 2007 question the conventional models of forces used so far here fluid forces acting on bed particles are treated in a new way in order to take into consideration the fundamental interference effects thus obtaining appropriate magnitude estimates that exhibit good agreement with direct force measurements by schmeeckle et al 2007 impulsive pressure fluctuations generated by turbulence are shown to be capable of dislodging the bed grains by saltation under air flow whereas they can only produce a rocking effect under water flow thus explaining the first anomaly on the other hand previous work by the authors allows a direct estimate of space averaged time mean drag and lift forces exerted on bed grains both components have the same order of magnitude but contrary to the common belief the mean lift is downward which provides an explanation for the second anomaly finally spatial disturbances of pressure both positive and negative appear to generate maximum persistent local forces considerably greater than mean forces thus allowing an explanation for the observed negative time mean drag a new formula for predicting incipient motion of sediment under open channel flow is derived which incorporates all dynamically significant effects and gives very good agreement with observation for the entire range of bed slopes

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

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

analysis of bed load motion at high shear stress recent pipeline experiments performed in saskatchewan show the existence of a shear layer in which the solids concentration decreases linearly with height this finding has already been used to calculate the velocity distribution within a constant stress shear layer the analysis has now been extended to obtain the solids discharge per unit breadth of bed the result agrees with the modified meyer peter and muller equation and with the writer s sand data for larger nylon particles the solids discharge per unit breadth fell below that expected on the basis of the sand data and the new analysis shows the reason for this apparent anomaly it also shows that a shear layer several grain diameters thick will not be uncommon in open channel flow contrary to the conventional wisdom the new analysis predicts that the threshold of motion curve on the shields diagram must approach a constant value at sufficiently small abscissae a finding which appears to be in accord with recent experimental evidence

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

channel morphology gradient profiles and bed stresses during flood in a step pool channel abstract the stability of a step pool channel is investigated on the basis of measurements during a median annual flood the study reaches range in gradient from 5 to 10 and the flow was 3 m3 s 1 in an 8 m wide channel the estimated shear stress based on the channel gradient was 102 pa which implies that the channel ought to move half meter class boulders calculations of the shear stress using flow resistance equations yield values of about 30 pa these figures imply that the largest boulders in the channel could be moved in fact the channel is stable the effective stress remains well below the threshold to move the large structure forming boulders the steps are stable structures that result from interlocking of the larger stones they are semi randomly located depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks the high variation in step pool wavelength step height residual pool depth and pool length is indicative of the random nature of step location and structure along a reach there is no convincing evidence that special conditions govern their formation

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

dense nonaqueous phase liquid dnapl source zone characterization influence of hydraulic property correlation on predictions of dnapl infiltration and entrapment 1 the influence of aquifer property correlation on multiphase fluid migration and entrapment was explored through the use of correlated and uncorrelated porosity permeability and capillary pressure saturation pc sat parameter fields in a cross sectional numerical multiphase flow model data collected from core samples in a nonuniform sandy aquifer were used to generate three dimensional aquifer parameter fields porosity was assumed to be uniform or simulated using sequential gaussian simulation sgs permeability k was modeled independently of porosity using sgs as well as simulated geostatistical indicator classes derived from measured grain size distribution curves retention characteristics were assigned employing leverett scaling of a representative pc sat curve to the geostatistical k fields or alternatively on the basis of simulated indicator classes and porosity values ensemble dense nonaqueous phase liquid dnapl infiltration and entrapment behavior for a hypothetical tetrachloroethylene pce spill was simulated in four sets of two dimensional profiles extracted from these realizations comparisons of saturation profiles and spatial moments from point source dnapl infiltration simulations suggest that choices involving the geostatistical algorithm used to model k and the incorporation of variable versus uniform porosity have a smaller influence than choices involving the scaling of capillary retention properties to k from these simulations it is apparent that the degree of spatial correlation in pc sat parameters exerts a controlling influence on predicted dnapl spreading and redistribution in saturated aquifers the resultant distribution of mass within a dnapl source zone will have implications for dnapl recovery and subsequent mass fluxes in remediation operations

nass river on the move radar facies analysis of glaciofluvial sedimentation and its response to sea level change in northwestern british columbia the nass valley of northwestern british columbia is a glacial fiord containing extensive glaciomarine and glaciofluvial sediments two parallel braidplains separated by a bedrock ridge were deposited within the fiord mapping of these deposits led to the hypothesis that the braidplains must have terminated at deltas however a lack of surface exposures meant that ground penetrating radar was needed to investigate these deposits radar facies analysis aided in the identification of braidplain braid delta and glaciomarine depositional environments as well as underlying bedrock several deltas graded to different sea levels were discovered allowing inferences to be made about the relationship of falling sea level to sediment architecture the upper section of the western braidplain is graded to a sea level of 185 m above sea level asl indicating that the proto nass river flowed on the western side of the bedrock ridge when the sea was at that level however the river moved to the east side of the ridge as sea level fell depositing the extensive aiyansh braidplain braid delta which is graded to a 152 m sea level stand several other deltas also formed at this sea level stand avulsion occurred and the river flowed on the west side of the ridge again when sea level fell to 134 m asl the river remained in this position throughout late glacial time and eventually evolved into the modern nass river the coarse grained deposits are indicative of forced regression with both stepped top attached and detached stratal architecture present

pivoting analyses of the selective entrainment of sediments by shape and size with application to gravel threshold measured variations of pivoting angles with grain size shape reliability and angularity and imbrication are employed in analyses of grain threshold to examine how these factors influence selective grain entrainment and sorting with a bed of uniform grain sizes as employed experimentally to establish the standard threshold curves such as that of shields the threshold condition depends on grain shape and fabric the analysis demonstrates quantitatively that there should be a series of nearly parallel threshold curves depending on grain pivoting angles for a given grain size the order of increasing flow strength required for entrainment is spheres smooth ellipsoids depending on their reliability angular grains and imbricated ellipsoids depending on their imbrication angles the relative threshold values for these different grain shapes and fabric are predicted according to their respective pivoting angles but remain to be directly tested by actual threshold measurements the pivoting angle of a grain also depends on the ratio of its size to those it rests upon this dependence permits an evaluation of selective entrainment by size of grains from a bed of mixed sizes the condition generally found in natural sediments the pivoting model predicts systematic departures from the standard threshold curves for uniform grain sizes such departures have been found in recent studies of gravel threshold in rivers and offshore tidal currents the pivoting model is compared with those threshold data with reasonable agreement however more controlled measurements are required for a satisfactory test of the model it is concluded that variations in pivoting angles for grain entrainment are significant to the processes of selective sorting by grain size and shape

representative grain size and equivalent roughness height of a sediment bed abstractthe representative grain size of a sediment bed can be defined as the mean volume diameter which varies with the geometric standard deviation of sediment size the obtained results explain why the representative sediment size can be approximated to be d84 for typical gravel beds the analysis is finally extended to quantify the dependence of the equivalent sand roughness on the geometric standard deviation of grain size distribution

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small