shallow structure of part of northwestern iberia from short period rayleigh wave observations propagation of 0 2 2 5 s short period rayleigh waves across a part of northwestern iberia is investigated analysis of a seismic profile line with recorded seismograms up to distances of 70 km from the shot point provides group velocities and attenuation coefficients of the fundamental mode first higher mode group velocity has been obtained for only one of the studied cases the studied region has been divided into two subrogions the northwestern and the southeastern side from the shot point inversion procedures applied to the velocity and attenuation data yield the shear velocity and the q 1 models up to a depth of about 1 5 km for the nw and 2 4 for the se side from the shear wave obtained models corresponding to the nw and to the se sides it is possible to infer the existence of some degree of lateral variation of s velocity the nw side presents velocities varying between 1 8 km s 1 at the surface and 3 3 km s 1 at 1 2 km depth the velocities in the se model for the same depths are 2 2 and 3 4 km s 1 respectively using only the fist six traces of the se side it is possible to infer a low velocity region located between 0 4 km and 0 7 km depth it seems that the velocities are higher in the complex geological region of the se side than in the nw one which is in agreement with p velocity models found by others from the tentative anelastic attenuation coefficients q 1 models have been inferred for both sides of the seismic refraction profile although the obtained q 1 models are somehow different in shape the maximum q 1 value is about 0 05 in both cases

experimental study of incipient motion in mixed size sediment transport rates of five sediments were measured in a laboratory flume three of these sediments had the same mean size the same size distribution shape and different values of grain size distribution standard deviation the critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate the sorting of the sediment mixture had little effect on the critical shear stress of individual fractions once the median size d50 of the mixture and a fraction s relative size di d50 are accounted for our data combined with previously published data show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction s relative grain size despite a broad variation in the available data of mixture sorting grain size distribution shape mean grain size and grain shape all fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions this result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment recirculating systems as well as for systems where the fractional transport rates are imposed on the system feed systems this equivalence in initial motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination

the variability of critical shear stress friction angle and grain protrusion in water worked sediments the erodibility of a grain on a rough bed is controlled by among other factors its relative projection above the mean bed its exposure relative to upstream grains and its friction angle here we report direct measurements of friction angles grain projection and exposure and small scale topographic structure on a variety of water worked mixed grain sediment surfaces using a simple analytical model of the force balance on individual grains we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography the friction angle projection and exposure of single grain sizes vary widely from point to point within a given bed surface the variability within a single surface often exceeds the difference between the mean values of disparate surfaces as a result the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution rather than a single value on a given bed the crtitical shear stress distributions of different grain sizes have similar lower bounds but above their lower tails they diverge rapidly with smaller grains having substantially higher median critical shear stresses large numbers of fines trapp ed within pockets on the bed or shielded by upstream grains are effectively lost to the flow our calculations suggest that critical shear stress as conventionally measured is defined by the most erodible grains entrained during transient shear stress excursions associated with the turbulent flow this implies a physical basis for the indeterminacy of initial motion these observations suggest that transport rate shear stress relationships may be controlled in part by the increasing numbers of grains that become available for entrainment as mean shear stress increases they also suggest that bed textures and grain size distributions may be controlled within the constraints of an imposed shear stress and sediment supply regime by the influence of each size fraction on the erodibility of other grain sizes present on the bed

inception sediment transport relationships at high slopes this study proposes relationships between mean streamflow depth and cohesionless particle bed characteristics at the beginning of the sediment transport at high slopes in low submergence conditions the inception of sediment transport is theoretically approached by the incipient motion condition a previous theoretical analysis of the equilibrium of a gravel particle based on the exposure of the particle itself to the streamflow is generalized by introducing the relative position of the exposed part of the particle with respect to the bottom and by considering the case of a partially submerged particle using a low submergence flow velocity profile in the particle balance of forces at the limit of equilibrium leads to a relationship between the mean flow depth and the sediment size another velocity profile of low submergence flow is added to that used in the previous study and two analytical expressions of flow depths corresponding to the beginning of sediment transport are obtained for both fully submerged particles and partially submerged particles the theoretical flow depths given by the analytical expressions are then compared with those from previous experimental tests at the beginning of sediment transport at high slopes with fully submerged particles the comparison shows a satisfactory agreement between theoretical and experimental values of flow depth corresponding to the degree of exposure and the relative position of the exposed part of the surface layer bed particles

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

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

structural properties of mobile armors formed at different flow strengths in gravel bed rivers differences in the structure of mobile armors formed at three different flow strengths have been investigated in a laboratory flume the temporal evolution of the bed surfaces and the properties of the final beds were compared using metrics of surface grain size microtopography and bed organization at both grain and mesoscales measurements of the bed condition were obtained on nine occasions during each experiment to describe the temporal evolution of the beds structured mobile armors formed quickly in each experiment at the grain scale 1 45 mm 9 ds50 17 mm where ds50 is the median surface particle size surface complexity decreased and bed roughness increased in response to surface coarsening and the development of the mobile armor particles comprising the armor also became flow aligned and developed imbrication at a larger scale 100 200 mm the surface developed a mesoscale topography through the development of bed patches with lower and higher elevations metrics of mobile armor structure showed remarkable consistency over prolonged periods of near constant transport demonstrating for the first time that actively transporting surfaces maintain an equilibrium bed structure bed structuring was least developed in the experiments conducted at the lowest flow strength however little difference was observed in the structural metrics of the mobile armors generated at higher flows although the range of transport rates studied was limited the results suggest that the structure of mobile armors is insensitive to the formative transport rate except when rates are low 0 03 where is the dimensionless shear stress

empirical model for probabilistic rock stability on flat beds under waves with or without currents abstract an empirical formula was developed for predicting the stability of isolated quarry rocks on relatively flat beds under waves with or without current in terms of the damage ratio the damage ratio was examined using 100 crushed stones median mass m50 293 g and mass density s 2 62 g cm3 as scale models of quarry rocks and replicas with a lower density s 1 38 g cm3 by placing them on beds of different roughness in a wave flume not only under periodic waves with periods of 2 3 5 s but also in symmetric and asymmetric oscillatory flows periods 8 12 s simulating waves without and with current in a circulating water channel comparison between hydrodynamic forces expressed in terms of three non dimensional mobility indices the maximum velocity maximum semi velocity amplitude ua and maximum acceleration relative to the friction force expressed in terms of the median coefficient of friction suggested that the damage ratio was most closely related to the ua based mobility index 2 nevertheless significant differences remained between data from the wave flume and circulating water channel tests the variation in the damage ratio which included the effects of the oscillatory velocity asymmetry oscillation period superimposed steady current mass density of stones and bottom friction was reasonably well described via the product of 2 and a function of a keulegan carpenter number the results of the field tests on quarry rocks with m50 2 04 t placed on a thin sand layer overlaying hard substrate show that the minimum stable mass is consistent with the prediction

a surface roughness model for predicting alluvial cover and bed load transport rate in bedrock channels partial alluvial cover in bedrock channels influences downcutting rates and mountain river morphodynamics flume experiments have demonstrated that a wide range of stable cover fractions are possible for a given ratio of sediment supply to transport capacity existing cover models impose different unique relationships between cover fraction and reach scale transport variables e g slope discharge sediment supply and grain size individually these models cannot predict the range of cover behaviors observed experimentally suggesting that additional variables may be important i propose a 1 d model in which bedrock surface roughness is a control on partial alluvial cover in the model surface roughness affects both shear stresses and thresholds of grain motion and therefore sediment transport capacity the roughness of the combined bedrock alluvial bed surface varies with the cover fraction model trends are reasonably consistent with previous experiments over ranges of roughness slope and shear stress when bedrock roughness is much greater than grain size cover increases approximately linearly with the ratio of sediment supply to transport capacity when bedrock roughness is much less than the sediment diameter the model tends to predict abrupt shifts between complete bedrock exposure and alluvial cover this runaway alluviation occurs for model solutions that are unstable to perturbations in supply which is the case when transport capacity decreases as alluvial cover increases in addition a model variant in which sediment supply is controlled by the amount of sediment on the bed suggests that a linear cover relation may be sufficient for modeling sediment flux dependent erosion at landscape scales

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

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

spatial variations in surface sediment structure in riffle pool sequences a preliminary test of the differential sediment entrainment hypothesis dseh riffle pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles this preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows however the differential sediment entrainment hypothesis dseh postulates that differential entrainment can instead result from spatial sedimentological contrasts here we use a novel suite of in situ grain scale field measurements from a riffle pool sequence to parameterise a physically based model of grain entrainment field measurements include pivoting angles lift forces and high resolution dems acquired using terrestrial laser scanning from which particle exposure protrusion and surface roughness were derived the entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles this is because pool grains have looser packing hence greater exposure and lower pivoting angles conversely riffle and pool exit grains have denser packing lower exposure and higher pivoting angles a cohesive matrix further stabilises pool exit grains the resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a 2d computational fluid dynamics cfd model of the reach the cfd model predicts that under bankfull conditions pools experience lower shear stresses than riffles and pool exits however the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles resulting in differential entrainment under bankfull flows significantly this differential entrainment does not require a reversal in flow velocities or shear stress suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle pool sequences this finding has implications for the prediction of sediment transport and the morphological evolution of gravel bed rivers

x ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment of fluvial gravels the critical shear stress c for grain entrainment is a poorly constrained control on bedload transport rates in rivers direct calculations of c have been hindered by the inability to measure the geometry of in situ grains i e the shape and location of each grain relative to surrounding grains and the bed surface we present the first complete suite of three dimensional 3 d grain geometry parameters for 1055 water worked grains and use these to parameterize a new 3 d grain entrainment model and hence estimate c the 3 d data were collected using x ray computed tomography scanning of sediment samples extracted from a prototype scale flume experiment we find that 1 parameters including pivot angle and proportional grain exposure do not vary systematically with relative grain size 2 c is primarily controlled by grain protrusion not pivot angle and 3 larger grains experience larger forces as a result of projecting higher into the flow profile producing equal mobility we suggest that grain protrusion is a suitable proxy for assessing gravel bed stability

paleomagnetic record of martian meteorite alh84001 1 the natural remanent magnetization nrm of the martian meteorite alh84001 is predominantly carried by fine magnetite which is found in association with carbonate the magnetite is in epitaxial and topotactic relation with the carbonate and formed from the carbonate in the major impact event at 4 0 ga the nrm will therefore record this field the local preferential crystallographic and shape alignment of the magnetite defines local easy directions of magnetization may account for the observed inhomogeneity of the nrm on a microscopic scale normalizing the intensity of the nrm by the saturation isothermal remanence irms then gives an estimate for the 4 0 ga martian field one order smaller than the present geomagnetic field such a field is unlikely to be strong enough to generate the high intensity martian magnetic anomalies alh 84001 in its pristine state as an orthopyroxenite is not a plausible source rock for the martian anomalies because its magnetite was not formed until the 4 0 ga event

a review of alpine tectonics in portugal foreland detachment in basement and cover rocks abstract the alpine foreland in portugal was deformed by compressional tectonism during the miocene in the nne ssw oriented lusitanian basin most folds and thrusts in the meso cenozoic cover are oriented ene wsw parallel to the alpine front in the betic cordillera and verge towards the north northwest and south southeast the thrusts are connected by lateral ramps most of these are oriented nne ssw to n s and show sinistral movement and some are transpressional the lateral ramps result from reactivation of older extensional faults related to crustal thinning of the continental margin in the e w oriented algarve basin a simpler basin inversion occurred with older e w normal faults reactivated as essentially pure thrusts in both basins alpine structures formed above decollements in the hettangian evaporite clastic complex variscan basement was also deformed by ene wsw reverse faults during miocene time the similarity in orientation and style of the basement structures to those in the cover suggests that they also occurred by detachment but their larger scale indicates that the detachment is deep and involves much of the crust thus we interpret the central cordillera in which basement rocks are thrust over miocene sediments on both sides as a pop up of crustal scale elevated above downward flattening faults that dip towards each other and merge into a single deep detachment alpine structures in the iberian foreland are therefore similar in structural style to those of the appalachian and laramide forelands of north america and the alpine foreland of northwest europe

model experiments on mobile paved gravel bed streams laboratory experiments which model streams with poorly sorted gravel beds at reduced scale reveal that pavement is a mobile bed phenomenon it acts to decrease the inherent difference in mobility between large and small grains by overrepresenting the percentage of large grains exposed to the flow pavement can coexist with the motion of all available grain sizes because motion is sporadic so that at any given time only a small percentage of surface grains are actually in motion particles in the immediate subpavement are only occasionally moved and deeper particles are hardly ever moved the experiments were conducted at constant discharges they provide accurate quantitative models of two field streams in that they obey the same bedload law as field data both sediment feed and recirculating configurations were utilized the only systematic difference between the two is in the behavior of the finer grains

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

particle friction angles in steep mountain channels sediment transport rates in steep mountain channels are typically an order of magnitude lower than predicted by models developed for lowland rivers one hypothesis for this observation is that particles are more stable in mountain channels due to particle particle interlocking or bridging across the channel width this hypothesis has yet to be tested however because we lack direct measurements of particle friction angles in steep mountain channels here we address this data gap by directly measuring the minimum force required to dislodge sediment pebbles to boulders and the sediment weight in mountain channels using a handheld force gauge at eight sites in california with reach averaged bed angles ranging from 0 5 to 23 and channel widths ranging from 2 m to 16 m we show that friction angles in natural streams average 68 and are 16 larger than those typically measured in laboratory experiments which is likely due to particle interlocking and burial results also show that larger grains are disproportionately more stable than predicted by existing models and that grains organized into steps are twice as stable as grains outside of steps however the mean particle friction angle does not vary systematically with bed slope these results do not support systematic increases in friction angle in steeper and narrower channels to explain the observed low sediment transport rates in mountain channels instead the spatial pattern and grain size dependence of particle friction angles may indirectly lower transport rates in steep narrow channels by stabilizing large clasts and channel spanning steps which act as momentum sinks due to form drag

estimating in situ relative density and friction angle of nearshore sand from portable free fall penetrometer tests the friction angle of sand in the nearshore zone of cannon beach yakutat alaska was estimated from the deceleration measured by a portable free fall penetrometer pffp at 72 test locations a c

unraveling bed slope from relative roughness in initial sediment motion understanding incipient sediment transport is crucial for predicting landscape evolution mitigating flood hazards and restoring riverine habitats observations show that the critical shields stress increases with increasing channel bed slope and proposed explanations for this counterintuitive finding include enhanced form drag from bed forms particle interlocking across the channel width and large bed sediment relative to flow depth relative roughness here we use scaled flume experiments with variable channel widths bed slopes and particle densities to separate these effects which otherwise covary in natural streams the critical shields stress increased with bed slope for both natural gravel s 2 65 g cm 3 and acrylic particles s 1 15 g cm 3 and adjusting channel width had no significant effect however the lighter acrylic particles required a threefold higher critical shields stress for mobilization relative to the natural gravel at a fixed slope which is unexpected because particle density is accounted for directly in the definition of shields stress a comparison with model predictions indicates that changes in local velocity and turbulence associated with increasing relative roughness for lighter materials are responsible for increasing the critical shields stress in our experiments these changes lead to concurrent changes in the hydraulic resistance and a nearly constant critical stream power value at initial motion increased relative roughness can explain much of the observed heightened critical shields stresses and reduced sediment transport rates in steep channels and also may bias paleohydraulic reconstructions in environments with exotic submerged densities such as iron ore pumice or ice clasts on titan

sand grain threshold in relation to bed stress history an experimental study besides particle size density and shape the erodibility of a sediment bed depends also upon the exposure to prethreshold velocities in the overlying flow such flow effectively rearranges the grains at and below the bed surface causing them to become more resistant to subsequent erosion the effects of the stress history leading up to the critical condition for sediment movement are investigated for unidirectional flows generated in a recirculating laboratory flume the sediment beds investigated consisted of cohesionless quartz sand grains with mean grain diameters of 0 194 mm fine sand 0 387 mm medium sand and 0 774 mm coarse sand with narrow particle size distributions the critical threshold shear velocity target value for the three beds was established within 2 5 min of increasing the flow from zero velocity the subsequent experiments were performed under prethreshold velocities at 70 for 5 10 20 40 and 80 min exposure duration 80 for 5 10 20 40 and 80 min exposure duration 90 and 95 for 5 10 20 40 80 and 120 min exposure duration of the target value following exposure to these different prethreshold conditions the flow was increased then to reach actual critical conditions within a period of 2 5 min the critical condition for the initiation of sediment movement was established using visual observation supplemented by video recordings according to the yalin criterion the results show that if the exposure duration to prethreshold velocities remains constant then the critical shear velocity increases with increasing prethreshold velocity likewise if the prethreshold velocity remains constant then the critical shear velocity increases with increasing exposure duration in some circumstances the critical shear velocity was found to increase by as much as 27 an empirical formula is proposed to account for the exposure correction to be applied to the critical shear velocities of sand sized sediment beds this is prior to their inclusion into bedload transport formulae for an improved prediction of the magnitude and nature of transport

shallow crustal models in the lisbon area from explosion data using body and surface wave analysis abstract knowledge of the shallow velocity structure is very important to understand the possible site effects which could greatly contribute to the irregular damage distribution observed in lisbon for the 1755 earthquake taking advantage of small charge blasts in the tagus river bed in front of lisbon seismic records were obtained in a scattered array extending to a 8 5 km distance from the shotpoint geological considerations allowed the grouping of traces in wide profiles for three zones west east and south the presence of a distinct group of rayleigh waves made possible the joint modelling of first arrivals and surface waves for the west and east zones the p and s wave velocity models inferred show the existence of two low velocity layers in the west zone which is in agreement with the known geology for the area the east zone structure could not be resolved in detail from refraction data only but surface wave analysis allow the identification of the first low velocity layer for the south zone the study of first arrivals revealed a structure similar to the west zone although velocity values and interface depths are slightly different the interpretation of these models was done according to the age and lithology of the different geological formations the final models obtained represent a significant improvement in detail over previous regional studies for the tagus valley basin mendes victor et al 1980 and over the preliminary interpretation of this data presented in mendes victor 1987

probabilistic description of grain resistance from simultaneous flow field and grain motion measurements experiments were carried out using a mobile gravel bed placed in a tilting flume with a modified particle image velocimetry piv system individual grain movements were surveyed using data from time series of images near bed velocity flow field measurements were made simultaneously above the same area of the sediment surface by applying cross correlation techniques to the collected plan view images statistics of grain motions were collected through a semiautomatic procedure significant changes in the flow field were observed in the proximity of the entrained or deposited particles a strong correlation is shown between the changes in the local streamwise and lateral velocity and the movement of the grains the theory of grass is revisited and developed based on the experimental results the probability distribution of individual grain resistance has been derived from the statistics of the near bed velocity field and of the entrainment risk

propagation of 0 7 2 5 hz rayleigh waves in scotland summary rayleigh waves in the frequency range 0 7 2 5 hz are recorded at stations on the scottish lownet array from local underwater explosions in the kirkcaldy bay region of the firth of forth the fundamental mode is identified by comparing theoretical and observed particle motion plots derived from seismograms at the edinburgh station this being used as a guide for selection at the other stations the selected signals are then analysed to obtain estimates of the group velocity dispersion characteristics along isolated event station paths and this information is interpreted in two ways a the group velocity curves are inverted to obtain shear velocity profiles of each path these penetrating to depths of between 1 2 and 3 km into the crust the models show an increase of shear velocity with depth shear velocity in the upper 400 m of the crust which includes the surface weathering lies between 1 4 and 2 1 km s 1 the regional variation of shear velocity correlates with the surface geology which consists of carboniferous sediments old red sandstone and dalradian rocks there are wide differences in the velocities of the deepest layer ranging from 1 9 to 3 5 km s 1 b the variation of group slowness with the geological expression is utilized in a linear scheme to obtain pure provincial group velocities for several geological provinces the dispersion curves corresponding to the carboniferous sediments and the firth of forth regions decrease smoothly from 1 6 km s 1 at 0 7 hz to 1 2 km s 1 at 1 8 hz the group velocities for the old red sandstone formation are about 0 5 km s 1 faster these results also demonstrate that the water layer and seabed sediments do not have a significant effect on the surface wave propagation

bedload sheets in heterogeneous sediment field observations in streams with beds of coarse sand and fine gravel have revealed that bedload moves primarily as thin migrating accumulations of sediment and coarse grains cluster at their leading edge these accumulations are one or two coarse grains high and are much longer 0 2 0 6 m long in sand 0 5 2 0 m in fine gravel than their height the authors propose the term bedload sheet for these features and the authors argue that they result from an instability inherent to bedload movement of moderately and poorly sorted sediment in essence coarse particles in the bedload slow or stop each other trap finer particles in their interstices and thus cause the coarse particles to become mobile again bedload sheets develop on the stoss side of dunes causing the dune to advance incrementally with the arrival of each sheet successive deposition of coarse sediment from the leading edge followed by fine sediment may generate the grain size sorting that distinguishes cross bedding available flume experiments and field observations indicate that bedload sheets are a common but generally unrecognized feature of heterogeneous sediment transport

experimental study on coarse grain saltation dynamics in bedrock channels saltation of bed load particles on bedrock surfaces is important for landscape evolution and bedrock incision in steep landscapes however few studies have investigated saltation in bedrock channels where unlike alluvial channels the bed roughness height and the sediment size may be independent to address this data gap we measured the saltation hop height hop length and velocity of gravel saltating over a planar bed using 80 160 readings from high speed photography and direct measurements two separate dimensional analyses are used one leading to a bed shear stress scaling and another leading to a froude number fr scaling our new saltation data coupled with numerous data from previous studies suggest that both shear stress and fr scaling analyses are valid in characterizing bed load saltation dynamics with bed roughness ranging from smooth to alluvial beds however the fr approach has the advantages that 1 there is no need to estimate a critical shields stress c which alone can vary up to 2 orders of magnitude e g 0 001 0 1 due to changes in relative bed roughness and slope and 2 the fr based scaling fits the saltation data set better in a least squares sense results show that the saltation velocity of bed load is independent of grain density and grain size and is linearly proportional to flow velocity saltation height has a nonlinear dependence on grain size saltation length increases primarily with flow velocity and it is inversely proportional to submerged specific density our results suggest that either c or bed roughness coefficient must be properly estimated to yield accurate results in saltation abrasion models

laboratory investigations into the threshold of movement of natural sand sized sediments under unidirectional oscillatory and combined flows the threshold of movement of sediment obtained from sandbanks within the bristol channel uk is investigated under unidirectional oscillatory and combined flows the experiments were undertaken in a recirculating unidirectional laboratory flume containing an oscillating plate to simulate wave action with movement along the same axis as the unidirectional flows the sand samples consisted of cohesionless quartz grains with median grain sizes between 0 315 and 0 513 mm the experiments were performed under flow velocities measured at 2 cm above the bed ranging between 0 and 24 cm s 1 and oscillatory currents wave periods of 5 12 and 15 s ranging from 0 to 28 cm s 1 the critical conditions for the initiation of sediment movement were assessed by visual observation using the yalin criterion the results show that under unidirectional flow there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions in the case of oscillatory flows the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves under combined flows wave period is shown to control threshold conditions with the unidirectional and oscillatory flow components combining in a linear fashion for long period 12 s and 15 s waves in contrast in the presence of short period 5 s waves the unidirectional and oscillatory components of the flow appear to decouple for high orbital velocities in both cases the effect of the wave period on threshold diminishes

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

laboratory measurements of pivoting angles for applications to selective entrainment of gravel in a current important to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain a series of experiments has been undertaken to determine how this angle depends on grain shape rollability and angularity on the ratio of the size of the pivoting grain to those beneath and on factors such as imbrication the experiments involved gravel sized spheres ball bearings and marbles natural pebbles selected for their approximately triaxial ellipsoid shapes and angular crushed basalt pebbles the pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined the angle of the board being equal to the pivoting angle at the instant of grain movement the pivoting angles of spheres showed reasonable agreement with a theoretically derived equation showing much better agreement than in previous studies which utilized sand sized spheres a series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size our results are therefore consistent with the earlier studies limited to sand size spheres the cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out similar size dependencies are also found for the ellipsoidal pebbles and angular gravel the experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle being a function of the ratio of the pebble s smallest to intermediate axial diameters this ratio controls the grain s ability to roll and pivot with small ratios of these diameters the pebbles tended to slide out of position whereas with ratios closer to unity circular cross section true pivoting took place and the angles were smaller experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position the pivoting angle being increased approximately by the imbrication angle the angular crushed gravel also required high pivoting angles apparently due to interlocking of the grains resulting from their angularity other factors being equal the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres ellipsoidal grains angular grains and imbricated grains the results obtained here make possible the quantitative evaluation of these shape effects on grain threshold as well as evaluation of the selective entrainment of grains from a bed of mixed sizes

on why gravel bed streams are paved bedload transport in poorly sorted gravel bed streams downstream of dams is considered bedload and typical bed material subpavement size distributions are observed to be similar it follows that the coarse half of the subpavement moves through a reach at a rate near that of the fine half since coarser grains are intrinsically less mobile than fine grains it follows that some mechanism must act to nearly equalize mobility it is hypothesized that the pavement seen in gravel bed streams at low flow is in fact in place during typical transport events capable of moving all available sizes this pavement can provide the equalizing mechanism by exposing proportionally more coarse grains to the flow field data are used to quantify this concept and to develop a predictive relation for river pavement the model indicates that pavement should be absent in most sand bed streams in agreement with observation

laboratory study on the evolution of gravel bed surfaces in bed armoring processes abstract cluster microforms in bed armoring have important implications for bedload transport in this study two sets of flume experiments were carried out to study the evolution of the cluster microforms in the bed armoring and their influences on the bedload transport dynamically stable armor layers were formed by applying a staged hydrograph on a mobile sediment bed consisted of poorly sorted sand and gravel thus enabling the investigation of the cluster microforms associated with different flow characteristics simulated in these experiments the size and the planimetric positions of the large particles on the bed surface were extracted using photogrammetric technique allowing analysis of surface grain size distribution and particle clustering pattern of the bed the results showed a tendency towards a coarsened and clustered bed surface which is likely responsible for the declining bedload transport rate the cluster microforms evolved with increasing bed shear stress in the following sequence flat bed pebble cluster line cluster heap cluster reticulate structure we found a bed armoring statistics that can partially explain the declining bedload transport rate under constant discharge nonetheless additional parameters are needed to simulate the fluctuation in the bedload transport rate under changing flow conditions

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

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

evaluating a 2d image based computerized approach for measuring riverine pebble roundness abstract the geometrical characteristics of pebbles are important features to study transport pathways sedimentary history depositional environments abrasion processes or to target sediment sources both the shape and roundness of pebbles can be described by a still growing number of metrics in 2d and 3d or by visual charts despite new developments existing tools remain proprietary and no pebble roundness toolbox has been available widely within the scientific community the toolbox developed by roussillon et al 2009 automatically computes the size shape and roundness indexes of pebbles from their 2d maximal projection plans using a digital camera this toolbox operates using 2d pictures taken of pebbles placed on a one square meter red board allowing data collection to be quickly and efficiently acquired at a large scale now that the toolbox is freely available for download 1 an evaluation is needed to help end users sample process and interpret the data we first assess the influence of pebble position and of the resolution and enhancement of pictures and errors associated with the sampling size second we run experimental flume tests of roundness trends that confirm the great potential of the toolbox for river geomorphic diagnoses as well as for targeting sediment sources we also validate the correlation between roundness metric trends and particle abrasion

a physically based bedload transport model developed for 3 d reach scale cellular modelling abstract a bedload transport model is outlined which aims to provide a more physically based description of bed surface and bedload grain size distributions gsds than existing empirical bedload models but is simple enough to be incorporated in a reduced complexity modelling framework one motivation for this is the increasing need for 3 d models of decadal reach scale channel and floodplain evolution for use in river management the computational demands of simulation at these scales mean that the appropriate models are necessarily of the reduced complexity type the bedload model is derived through analysis of sediment beds created using a discrete element model these beds are eroded using a probabilistic entrainment algorithm which predicts the bedload gsd there are two parts to the bedload model the first identifies a functional relationship between the applied shear stress and the fraction of the bed undergoing active transport while the second uses this fraction in conjunction with the surface gsd to produce a bedload gsd the model produces bedload coarsening as shear stress increases from a value of one to a value of five times a reference shear stress this model is tested in a series of numerical experiments applied using a simple cellular model of a straight flume which is run in both recirculation and feed modes in both modes the equilibrium gsds of the bedload and surface are consistent with expected flume behaviour suggesting that the model provides a reasonable reproduction of sediment entrainment transport and sorting