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

experiments on saltation of sand in water a high speed video system was used to study saltation of sand in an open channel flow high rates of image acquisition provided the information needed to resolve details of the lagrangian properties of particle motion saltation of sand is described in terms of statistical properties of particle trajectories such as mean values and standard deviations of saltation length height and streamwise particle velocity results are compared with available empirical data saltation height seems to be independent of particle size when made dimensionless with the particle diameter however the dimensionless saltation length appears to increase as the particle size decreases particle collision with the bed is also analyzed observations of this process indicate that most of the interactions between saltating particles and the bed are of the collision rebound type which contradicts previous discussions on the subject the estimated value of the dynamic friction coefficient is about half that proposed by bagnold i

a method for the calculation of the impact parameters of dilute pyroclastic density currents based on deposit particle characteristics 1 a new method for the reconstruction of the physical characteristics of dilute pyroclastic density currents is introduced the velocity density and particle volumetric concentration profiles of the stratified current are calculated together with the profile of dynamic pressure which is a useful parameter for checking resistance of buildings the model links turbulent boundary layer shear flow theory with particle coupling to gas turbulence the working procedure starts with the recognition in the field of the fining upward sequence of layers formed during the time integrated depositional history of an individual current distinct processes of particle transportation and deposition are associated with the different particle modes composing the bedset the system of equations for the solution of the fluid dynamic parameters is implemented in two alternative ways the first one uses data of particles coming from both the basal coarse layer and the overlying laminated layer of the bedset the second uses features of two distinct components of the laminated layer a statistical test is performed for checking model results against experimental data of actual particles model calculations give the average solution as well as solutions corresponding to a range of 68 of probability around the average value the maximum solution can be considered as a safety value for impact parameters the methodology is applied to deposits of recent eruptions of vesuvius and campi flegrei in italy and results are discussed in terms of hazard and expected damage

do critical stresses for incipient motion and erosion really exist the concept of critical stress for the initial motion of noncohesive sediment beds under turbulent flow conditions is reviewed observational definitions of incipient motion are many and not entirely compatible some laboratory flume observations of sediment movement suggest that no true threshold exists current understanding of turbulent fluid motion at a sediment bed suggests that some particle movement must occur at all nonzero time mean velocities a combined model for flume flow and sediment transport having no threshold is seen to explain features of data that have been previously used to support the threshold concept

modes of bed load movement on a smooth rigid surface sediment grains transported as supply limited bed load on a rigid surface move either discretely or collectively as bed forms with significantly different effective grain speeds and active storage volumes the adopted mode has implications for sediment sorting and heavy mineral placer formation dispersal of grain associated pollutants and accumulation and flushing of sediment deposits in unlined canals and sewers the threshold condition between the two modes has been established for a smooth surface from flume experiments with different sediment types flow conditions and sediment supply rates this is expressed in terms of a relationship between the sediment movability number a dimensionless bed load parameter and a grain shear reynolds number

erosion of sand from a gravel bed abstractcleaning of fine sediment out of gravel streambeds has become an important method to restore affected stream habitats introducing the increased flows needed to entrain fine sediments without eroding the coarser fractions of the bed and potentially destroying its usefulness as a habitat requires careful selection of flow strength toward this end a series of flume experiments was conducted on the entrainment of sand from immobile gravel beds to determine the depth to which the sand could be eroded without the gravel being mobilized a series of steady flows was imposed on three gravel beds filled with sand with median sizes of 0 2 0 3 and 0 9 mm the shear stress of the series of flows ranged from just above the critical shear stress for initiation of motion of the sand up to 0 9 times the critical stress of the 36 1 mm gravel the experiments continued until the rate of fine sediment exiting the channel was very small it was found that the cleanout depth of the sand was reasonably predicted b

distinguishing current effects in sediments delivered to the ocean by ice i principles methods and examples abstract there are climatically important ocean flow systems in high latitudes for example the east and west greenland and labrador currents and nordic sea overflows in the north and antarctic circumpolar current in the south for which it would be useful to know history of flow strength most of the sediment records under these flows contain evidence of supply from glacial sources which has led to the supposition that fine sediment records which in other settings provide evidence of vigour of flow from the sortable silt proxy are fatally contaminated by unsorted glacial silt it is suggested here that if the fine fraction 50 because the two are not related end member em decomposition of several records yields variable results in terms of the relationship between em ratios and grainsize parameters although such an approach can generate fine sediment parameters it does not provide a basis for deciding whether or not a record is acceptably current sorted and thus contains a valid flow speed proxy our proposed discrimination between current sorted and unsorted fine fractions is applicable to all fine grained deposits not only high latitude deposits with coarse ird examples from east greenland faroe bank channel gardar drift show mainly well sorted signatures amounts of coarse ird range up to 60 with only those 50 having a consistent impact on sortable silt mean size with the exception of a southern ocean site on the antarctic continental rise where half the record is poorly sorted the silt mean data are sufficiently well sorted to provide credible flow speed histories this bodes well for the extraction of such histories from climatically important high latitude flows such as the east greenland current

progressive incision of the channeled scablands by outburst floods the surfaces of earth and mars contain large bedrock canyons that were carved by catastrophic outburst floods reconstructing the magnitude of these canyon forming floods is essential for understanding the ways in which floods modify planetary surfaces the hydrology of early mars and abrupt changes in climate flood discharges are often estimated by assuming that the floods filled the canyons to their brims with water however an alternative hypothesis is that canyon morphology adjusts during incision such that bed shear stresses exceed the threshold for erosion by a small amount here we show that accounting for erosion thresholds during canyon incision results in near constant discharges that are five to ten fold smaller than full to the brim estimates for moses coulee a canyon in the channeled scablands which was carved during the pleistocene by the catastrophic missoula floods in eastern washington usa the predicted discharges are consistent with flow depth indicators from gravel bars within the canyon in contrast under the assumption that floods filled canyons to their brims a large and monotonic increase in flood discharge is predicted as the canyon was progressively incised which is at odds with the discharges expected for floods originating from glacial lake outbursts these findings suggest that flood carved landscapes in fractured rock might evolve to a threshold state for bedrock erosion thus implying much lower flood discharges than previously thought

what sets the size of current ripples water flowing over sand in fluvial and marine settings often results in the formation of current ripples found in modern and ancient deposits on earth and mars ripple stratification records flow directions and fluid properties that are crucial to interpreting sedimentary records despite decades of observations of current ripples there is no universal scaling relation to predict their size or to distinguish them from dunes here we use dimensional analysis and a new data compilation to develop a scaling relation that collapses data for equilibrium wavelengths of ripples forming under unidirectional flows results show that ripples are larger with more viscous fluids coarser grains smaller bed shear stresses and smaller specific gravity of sediment the scaling relation also segregates ripples from dunes highlighting a narrow regime of transitional bedforms that have morphologic properties and sediment transport conditions that overlap with both ripples and dunes our analysis shows that previous absolute size based definitions of ripples and dunes only hold for certain conditions such as water flows transporting siliciclastic grains on earth the new theory allows estimates of ripple sizes in foreign fluids and on other planets including meter scale ripples in methane flows on titan or in viscous brines on mars

unequal mobility of gravel and sand in weakly bimodal river sediments bed load was trapped during flood events over a 20 month period at the lower end of the allt dubhaig a small river in scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile the channel bed near the trap is predominantly gravel with a secondary sand mode total transport in each event depended mainly on peak shear stress rather than duration over a threshold bed load was mainly sand in smaller events bimodal in intermediate events and mainly gravel in the biggest floods mean and maximum grain diameter both increased with peak shear stress but in different ways analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility the slight selectivity in gravel transport is consistent with previous field studies of near equilibrium unimodal beds and supports assumptions made in the numerical model of hoey and ferguson 1994 which successfully simulates the observed amount of downstream fining over the 2 5 km upstream of the bed load trap

subaqueous barchans and plane beds from deposition of quartz silt abstractthe suspension flow of quartz silt geometric mean grain size of 4 15 m in unfavorable conditions for deposition is the motivation for the development of different bed morphologies particles deposit over a plane nonerodible surface and develop well defined barchan ripples barchanoids and plane beds in flume experiments bedload transport of quartz silt by bedform migration is several orders of magnitude smaller than the suspended transport the final bed morphology is controlled by the suspended sediment concentration and running time of the experiment the average dimensions of the bedforms after 1 day experiments are 4 93 cm wide 10 33 cm long and 0 45 cm high cohesive plane beds appear after 2 day and 3 day experiments with very high sediment concentrations 22 5 kg m3 viscous effects are deemed relevant for the formation of the beds

grains in motion a review abstract this review paper aims to present a practical integrated procedure for the prediction of sediment transport rates in both the continental and marine environment based on the settling velocity of particles such studies have important applications in diverse fields such as civil and coastal engineering sedimentology and environmental geology where they can assist in flood control and the prevention of contaminant dispersal by wind and water as well as the siltation of dams and harbours the formation of placer deposits including gold diamonds and other heavy minerals is also controlled by sediment transport processes so that these studies can form the basis for more efficient exploration programs the first part of the paper discusses some basic principles important in sediment transport followed by an overview of published methods to determine the settling velocity of differently shaped particles including natural grains the application of settling velocity to predict the entrainment threshold of sediments on plane horizontal and inclined beds by unidirectional currents and oscillatory waves is then discussed which finally leads to the determination of sediment transport rates over plane and rippled beds the validity of this approach to natural conditions is tested against published field data where possible other widely used methods are also critically discussed pointing out severe problems in current sampling technology and the calculation of bedload transport in natural environments

experiments on particle turbulence interactions in the near wall region of an open channel flow implications for sediment transport a high speed video system was used to study the interaction between sediment particles and turbulence in the wall region of an open channel flow with both smooth and transitionally rough beds in smooth flows particles immersed within the viscous sublayer were seen to accumulate along low speed wall streaks apparently due to the presence of quasi streamwise vortices in the wall region larger particles did not tend to group along streaks however their velocity was observed to respond to the streaky structure of the flow velocity in the wall region in transitionally rough flows particle sorting was not observed coherent flow structures in the form of shear layers typically observed in the near wall region interacted with sediment particles lying on the channel bottom resulting in the particles being entrained into suspension although there has been some speculation that this process would not be effective in entraining particles totally immersed in the viscous sublayer the results obtained demonstrate the opposite the entrainment mechanism appears to be the same independent of the roughness condition of the bottom wall smooth or transitionally rough in the latter case however hiding effects tend to preclude the entrainment of particles with sizes finer than that of the roughness elements the analysis of particle velocity during entrainment shows that the streamwise component tends to be much smaller than the local mean flow velocity while the vertical component tends to be much larger than the local standard deviation of the vertical flow velocity fluctuations which would indicate that such particles are responding to rather extreme flow ejection events

threshold for particle entrainment into suspension laboratory observations regarding the limit conditions for particle entrainment into suspension are presented a high speed video system was used to investigate conditions for the entrainment of sediment particles and glass beads lying over a smooth boundary as well as over a rough bed the results extend experimental conditions of previous studies towards finer particle sizes a criterion for the limit of entrainment into suspension is proposed which is a function of the ratio between the flow shear velocity and particle settling velocity observations indicate that particles totally immersed within the viscous sublayer can be entrained into suspension by the flow which contradicts the conclusions of previous researchers a theoretical analysis of the entrainment process within the viscous sublayer based on force balance considerations is used to show that this phenomenon is related to turbulent flow events of high instantaneous values of the reynolds stress in agreement with previous observations in the case of experiments with a rough bed a hiding effect was observed which tends to preclude the entrainment of particles finer than the roughness elements this implies that as the ratio between particle and roughness element sizes becomes smaller progressively higher bed shear stresses are required to entrain particles into suspension on the other hand an overexposure effect was also observed which indicates that a particle moving on a smooth bed is more prone to be entrained than the same particle moving on a bed formed by identical particles

risk of sediment erosion and suspension in turbulent flows it is shown how some basic probability tools may help in the characterization of the turbulent processes involved in the mechanics of sediment transport both the initiation of sediment motion and the threshold for sediment suspension are analyzed in a stochastic frame of work isorisk curves are constructed based upon some minimum experimental information the well known shields curve is shown to correspond to an erosion risk of approximately 40 these concepts are believed to be of importance for the design of movable bed channels as well as for future experimental investigations which are indeed necessary for computing the risk for higher values of the particle reynolds number finally an approximate methodology is proposed for computing the risk of erosion

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

effect of lithofacies on the pore system of over mature longmaxi shale in the jiaoshiba area sichuan basin china abstract ten longmaxi shale samples were collected from the jiaoshiba area china where 5 billion cm 3 year of shale gas is produced the samples were evaluated to characterize the pores of various shale lithofacies three shale lithofacies were determined in the jiaoshiba area sichuan basin china based on the shale mineral components namely siliceous lithofacies sl argillaceous lithofacies al and mixed lithofacies ml field emission scanning electron microscopy fe sem which was performed under high and low voltages gri porosity and carbon dioxide and nitrogen adsorptions were used to investigate the bulk shale porosity pore type pore size pore morphology and pore volume among the lithofacies lithofacies affects the shale total organic carbon toc bulk shale porosity pore size shape and volume sl and ml contain high porosity high toc abundant organic matter om hosted pores and high om porosity whereas al contains few om pores and has low toc and low porosity in the same lithofacies samples with various toc values exhibit different pore morphologies for sl the low toc sample has a round and elliptical om pore forms whereas the high toc samples show linear and narrow pore shapes that are probably associated with gas expulsion in ml the higher toc is related to spherical or near spherical om pores whereas the om pore shape collapses in the low toc shale therefore lithofacies affect the shale reservoir and the characteristic and evolution of om pores should be considered as constraints in shale lithofacies in sl sections with a higher toc value are prime targets despite the high expulsion efficiency which does not affect the enrichment and high yield of the shale gas sections of ml and low toc sl should be considered as potential targets because of higher om porosity and low expulsion efficiency whereas sections of al engender risk in exploration and development of shale gas

hyperconcentrated flows and gastroliths sedimentology of diamictites and wackes of the upper cloverly formation lower cretaceous wyoming u s a abstract diamictites and matrix supported wackes of the upper cloverly formation lower cretaceous represent rivers laden with volcaniclastic sediment hyperconcentrated flows the cloverly formation of central wyoming is dominantly fluvial lacustrine and playa deposits the sediments accumulated east of the cordilleran foredeep during the early stages of the sevier orogeny the diamictites and wackes are decimeters thick and stacked vertically forming rock bodies up to 11 m thick pebbles and cobbles within the diamictites and wackes occur as isolated clasts or in poorly defined layers many such clasts exhibit some degree of polish clast lithologies can be correlated to rocks that were exposed in the mountain belt to the west provenance data are consistent with early cretaceous movement on the meade laketown paris willard thrust system the clay fraction of the diamictites and wackes is dominantly illite smectite and smectite characteristic of altered volcanic ash diamictites and matrix supported wackes lack primary sedimentary structures e g cross stratification but do exhibit soft sediment deformation features indicative of dewatering erosional bases and bedding geometries are indicative of channelized flow the diamictites and matrix supported wackes resemble debris flow deposits however maintaining debris flows that behave as bingham plastics on low depositional slopes 200 400 km from the mountain front is improbable rather the diamictites and wackes represent hyperconcentrated flows deposition either en masse by a newtonian or nearly newtonian flow that was turbulent throughout or by progressive sedimentation from a stratified flow with a basal incipient granular mass flow overlain by a turbulent suspension is more consistent with the field data paleohydraulic calculations indicate that grain sizes up to small pebbles could have been transported in suspension with larger clasts transported as bedload consistent with sedimentological evidence much of the finer grained suspended sediment was remobilized volcanic ash likely derived from the idaho batholith to the west northwest the polished extraformational pebbles and cobbles long regarded as dinosaur gastroliths or stomach stones are simply clasts associated with the hyperconcentrated flow deposits the polish exhibited by many of the clasts is attributable to transport in ash laden flows the occurrence of polished extraformational stones in cloverly diamictites and wackes may have implications for presumed gastroliths in other fluvial rocks of the western interior e g the cedar mountain and morrison formations

flume experiments test grain size distribution of onshore tsunami deposits abstract we conducted flume experiments to examine the effects of grain size on the distribution of tsunami deposits using well sorted quartz sand with median diameters d50 of 0 064 0 134 0 215 and 0 250 mm in these experiments we investigated deposits transported onto a flat terrestrial area by a single tsunami like bore without a strong backwash flow the post tsunami distributions of the coarser d50 0 215 and 0 250 mm and finer d50 0 064 and 0 134 mm sands differed markedly the amount of the deposited coarser sand decreased landward across the entire terrestrial area whereas that of the deposited finer sand was either approximately uniform across the terrestrial area or increased gradually landward time series of flow depth and sediment transport observed via composited video frame images showed that suspension of the coarser sand did not reach the height of the maximum flow depth and decreased with distance from the sediment source and that the sediment settled gradually across the terrestrial surface with the amount deposited decreasing landward conversely the finer sand was densely suspended up to the maximum flow depth and was transported over longer distances across the terrestrial area with the amount deposited being relatively uniform or increasing slightly landward advection lengths that we calculated on the basis of our experimental parameters supported the dependence on grain size of the distribution patterns of onshore deposits shown by our flume experiments moreover advection lengths that we calculated for the 2011 tohoku oki tsunami from previously reported tsunami parameters were consistent with a deposit distribution similar to that observed in the field after the tsunami although we ignored some key factors e g geomorphology and sediment availability our flume experiments showed that the distribution patterns of identical tsunami flows differ for sediments of different grain size

statistical assessment of soil water characteristic curve models for geotechnical engineering a number of empirical equations have been proposed for the soil water characteristic curve a nonlinear least squares method was used to determine best fit parameters for several empirical equations that were best fit to 230 water content versus soil suction data sets in addition two proposed correction methods to accommodate high soil suctions up to 1 000 000 kpa were applied to the various soil water characteristic curve equations the data sets of water content versus soil suction were arranged into one of the usda soil classifications based on their relative amounts of sand silt and clay only eight soil classifications had sufficient data for later analysis the quality of fit for each model was compared using the akaike information criterion a series of conclusions were arrived at regarding i the relationship between two and three parameter equations ii the relationship between exponential and sigmoidal type equations and iii the value of correction factors for the high soil suction

aerodynamic entrainment threshold effects of boundary layer flow conditions experimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions new methods are proposed for 1 the unambiguous determination of aerodynamic threshold for any grain population and 2 approximation of the probability density function pdf distributions of threshold shear velocity for aerodynamic entrainment in wind tunnel experiments the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values initial grain dislodgements and subsequent erosion from narrow strips of loose finely fractionated ballotini were recorded photographically as wind speed was increased boundary layer parameters including average threshold shear velocity u t were calculated using the momentum integral method direct observations show that sporadic oscillation of grains preceded dislodgement at slightly higher velocities most grains rolled over their neighbours before entering saltation initial entrainment in spatially semi organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold these observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process for each grain fraction values of u t were found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased a probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the pdf for u t required by the model and for future stochastic treatment of the threshold condition

determination of the wind induced detachment threshold for granular material on mars using wind tunnel simulations experimental results are presented of wind induced grain detachment under mars simulation conditions a simple force balance equation is applied to quantify the wind shear stress required for removal of glass spheres from a sand bed the transport of fine grained martian dust is simulated by the detachment of hollow glass spheres which resemble low mass density dust aggregates observed to form during simulations when using mars analogue material the results agree well with observations of dust removal and wind speed measurements made by the nasa viking landers at the martian surface this work supports the suggestion that dust aggregate fragmentation allows wind induced dust entrainment at substantially lower wind shear than that of solid sand grains and has direct application to the study of global dust transport and martian climatology

on the stabilizing influence of silt on sand beds abstract in marine environments sediments from different sources are stirred and dispersed generating beds that are composed of mixed and layered sediments of differing grain sizes traditional engineering formulations used to predict erosion thresholds are however generally for unimodal sediment distributions and so may be inadequate for commonly occurring coastal sediments we tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two grain fraction silt d50 55 m and sand d50 300 m in a laboratory based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed to mimic recent deposition of particles following large storm events and the longer term result of the incorporation of fines in coarse sediment we designed two suites of experiments 1 the layering experiment in which a sandy bed was covered by a thin layer of silt of varying thickness 0 2 3 mm 0 5 3 7 wt dry weight in a layer 10 cm deep and 2 the mixing experiment where the bed was composed of sand homogeneously mixed with small amounts of silt 0 07 0 7 wt dry weight to initiate erosion and to detect a possible stabilizing effect in both settings we increased the flow speeds in increments up to 0 30 m s results showed that the sediment bed or the underlying sand bed in the case of the layering experiment stabilized with increasing silt composition the increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds combined with in the case of the mixed bed decreasing erosion rates our results show that even extremely low concentrations of silt play a stabilizing role 1 4 silt wt on a layered sediment bed of 10 cm thickness in the case of a mixed sediment bed 0 18 silt wt in a sample of 10 cm depth stabilized the bed both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed these observations are summarized in a conceptual model that suggests that in addition to the effect on surface roughness silt stabilizes the sand bed by pore space plugging and reducing the inflow in the bed and hence increases the bed stability measurements of hydraulic conductivity on similar bed assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22 in the case of a layered bed and by up to 70 in the case of a mixed bed

sensitivity of incipient particle motion to fluid flow penetration depth within a packed bed a discrete element method is applied to a three dimensional analysis related to sediment entrainment on a micro scale sediment entrainment is the process by which a fluid medium accelerates particles from rest and advects them upward until they are either transported as bedload or suspended by the flow modelling of the entrainment process is a critically important aspect for studies of erosion pollutant resuspension and transport and formation of bedforms in environmental flows previous discrete element method studies of sediment entrainment have assumed the flow within the particle bed to be negligible and have only allowed for the motion of the topmost particles at the same time micro scale experimental studies indicate that there is a small slip of the fluid flow at the top of the bed indicating the presence of non vanishing fluid velocity within the topmost bed layers the current study demonstrates that the onset of particle incipient motion which immediately precedes particle entrainment is highly sensitive to this small fluid flow within the topmost bed layers using an exponential decay profile for the inner bed fluid flow the discrete element method calculations are repeated with different fluid penetration depths within the bed for several small particle reynolds numbers for cases with slip velocity corresponding to that observed in previous experiments with natural sediment the predicted particle velocity is found to be a few percent of the fluid velocity at the top of the viscous wall layer which is a reasonable range of velocities for observation of incipient particle motion this method for prescribing the fluid flow within the particle bed allows for the current discrete element method to be extended in future studies to the analysis of sediment entrainment under the influence of events such as turbulent bursting additionally predictions for the slip velocities and fluid flow profile within the bed suggest the need for further experimental studies to provide the data necessary for additional improvement of the discrete element method models

methodology for reconstructing wind direction wind speed and duration of wind events from aeolian cross strata 1 a methodology for reconstructing wind direction speed and event duration from aeolian dune cross strata was developed from analysis of crescentic dunes at white sands new mexico during wind events dune lee faces were surveyed lee face deposits mapped deposition rates measured grain size sampled by stratification type and winds characterized from meteorological and field data the spatial distribution of lee face stratification styles is a function of the incidence angle formed between the wind and the brinkline with secondary controls by wind speed and dune sinuosity and height sets of wind ripple strata form at incidence angles of 25 40 grainfall grainflow foresets over wind ripple bottomsets at 40 70 and grainflow grainfall foresets at 70 90 erosional reactivation surfaces form at incidence angles up to 15 bypass surfaces up to 25 the total sediment load is fractionated within lee face stratification types wind speed can be reconstructed from relationships between grain size transport mode shear velocity and grain settling velocity where the full range of grain transport modes occurs and grain size is limited by shear stress the shear velocity and grain size range in each transport mode can be estimated by assuming the coarse fraction in grainflow strata traveled in creep and the coarse fraction in grainfall traveled in saltation the minimum duration of a wind event can be estimated using measures of shear velocity dune height and dune forward migration method limitations arise with source area control on grain size extremes in wind events and severe truncation of sets of cross strata

spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces application to meridiani planum mars the landscape seen by the mars exploration rover mer opportunity at meridiani planum is dominated by eolian wind blown ripples with concentrated surface lags of hematitic spherules and fragments these ripples exhibit profound spatial grain size sorting with well sorted coarse grained crests and poorly sorted generally finer grained troughs these ripples were the most common bed form encountered by opportunity in its traverse from eagle crater to endurance crater field measurements from white sands national monument new mexico show that such coarse grained ripples form by the different transport modes of coarse and fine grain fractions on the basis of our field study and simple theoretical and experimental considerations we show how surface deposits of coarse grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces activation of meridiani planum coarse grained ripples requires a wind velocity of 70 m s at a reference elevation of 1 m above the bed from images by the mars orbiter camera moc of reversing dust streaks we estimate that modern surface winds reach a velocity of at least 40 m s and hence may occasionally activate these ripples the presence of hematite at meridiani planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments however the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site

particle over passing on depth limited gravel bars an experimental channel is used to examine the transport of mixed sand and gravel bedload over the crestal platform of hump back bars and along the top of planar gravel sheets hydraulic processes result in the simultaneous transport of cobbles and pebbles over a static closely packed bed consisting of like sized and finer particles for prescribed conditions flat upper stage plane sand beds develop over the crestal location with pebbles rolling easily over the sandy bed at the brinkpoint flow separation ensures effective segregation of the gravel from the sand over the slip face the deposition rate of the sand is insufficient to fill fully the interstices within the gravel foresets before rapid deposition of gravel further advances the bed form consequently distinctive vertical assemblages of open work and closed contact framework gravels could be generated as another bar migrates over and preserves the initial structure in respect to the observed mechanisms of sorting over the bars a mathematical expression is developed to explain the critical conditions allowing coarse particle mobility over planar sand or gravel beds under upper stage plane bed conditions on the crestal platform the model then is used to ascertain whether the depositional environment ascribed to certain facies in the bunter pebble beds described in a recent publication is appropriate given the distinctive facies assemblages generated in this experiment and the known hydrodynamic control of the particle segregation process

paleoslope reconstruction in sandy suspended load dominant rivers constraining the depositional gradient of ancient alluvial river systems can aid in reconstructing landscapes and estimating paleodischarge by establishing boundaries on the climatic and tectonic history of continental sequences we present three methods for estimating ancient depositional gradients based on the interpreted mode of sediment transport for the range of particle sizes found on the beds of modern rivers or preserved in channel fills for sandy rivers with suspension as the dominant mode of sediment transport these methods take advantage of observations that can be directly obtained from preserved strata including measurements of paleo flow depth and grain size the first method relates river slope to shields number at bankfull flow the second method is similar to the first but allows for variation in shields number at bankfull flow with grain diameter the final method relies on criteria required for both suspended and bed material load sediment transport in the same system and is the most comprehensive and physically justified method we present each method and test them with modern river datasets to verify accuracy and help constrain uncertainty the first step to adapting them to ancient systems results indicate that all methods estimate slope within a factor of two these methods are potentially very powerful for interpreting sandy fluvial deposits because they can provide reasonably accurate quantitative estimates of paleoslope an elusive yet important environmental variable