estimating eruption temperature from thermal emission spectra of lava fountain activity in the erta ale ethiopia volcano lava lake implications for observing io s volcanoes 1 we have analysed high spatial resolution and high temporal resolution temperature measurements of the active lava lake at erta ale volcano ethiopia to derive requirements for measuring eruption temperatures at io s volcanoes lava lakes are particularly attractive targets because they are persistent in activity and large often with ongoing lava fountain activity that exposes lava at near eruption temperature using infrared thermography we find that extracting useful temperature estimates from remote sensing data requires a high spatial resolution to isolate lava fountains from adjacent cooler lava and b rapid acquisition of multi color data because existing spacecraft data of io s volcanoes do not meet these criteria it is particularly important to design future instruments so that they will be able to collect such data near simultaneous data at more than two relatively short wavelengths shorter than 1 m are needed to constrain eruption temperatures resolving parts of the lava lake or fountains that are near the eruption temperature is also essential and we provide a rough estimate of the required image scale

statistical properties of armored gravel bed surfaces 1 roughness of successively developed armor layers in a large laboratory flume is analyzed considering gravel surfaces as two dimensional random fields of bed elevations statistically derived roughness parameters are evaluated from precise digital elevation models dems of bed surfaces and are discussed with respect to successive development of stable armor layers the key results relate to evolution of the probability distribution of bed elevations and its moments and structure functions of various orders with respect to armoring discharge our results suggest that statistical analysis of bed elevations is a helpful tool for defining roughness parameters quantifying armoring effects and identifying particle orientation flow direction and surface forming processes

shape sorting of flood transported synthetic clasts in a gravel bed river abstract an experiment was set up in a gravel bed river to test shape sorting of gravel size clasts synthetic clasts representing four shape types were used spheres were entrained in larger numbers than the other shapes and travelled furthest rods and blades were intermediate in mobility discs moved the shortest distance and in the least numbers the angle of repose and therefore the pivotability of clasts in combination with lift and drag forces exerted on the clast will determine if a clast will be entrained or not as well as the shear stress needed for the clast to maintain motion a sphere projects relatively higher than the other shape types of equal volume and will induce a higher degree of flow separation in its wake than other shape types therefore larger components of lift and drag forces exerted by water flow will be present to set this shape type in motion or to maintain motion the prolate clasts have a small angle of repose compared to discoid clasts and will roll much more readily the imbricated orientation assumed by discoid and sometimes blade shaped clasts is a very stable position very high applied shear stress will be needed to dislodge imbricated discs

the role of grain size ratio in the mobility of mixed granular beds abstract the main goal of the study was to understand the effects of grain size distribution on the stability of beds in the sand silt range which is a critical subject for the understanding of geomorphological processes in aquatic environments although theoretical models can explain the mobilization of a mixed bed there is a clear lack in knowledge regarding the stabilizing effect of non cohesive fine material to connect existing findings we analysed bed stability in relation to grain size distribution in laboratory experiments erosion experiments in an annular flume were conducted using beds of different size compositions of spherical glass beads i e a the grain size ratio rd d50 coarse d50 fine the relative size of coarse and fine grains d50 39 367 m and b the amount of fines several glass bead combinations with unimodal and bimodal grain size distributions rd 3 9 5 8 and 9 4 and varying fine fractions 10 40 dry weight were subjected to increasing flow speeds 0 01 0 19 m s 1 using acoustic doppler velocimetry adv and optical backscatter the flow profile in the vicinity of the bed surface the changes in bed morphology and the suspended sediment concentration ssc were measured a new method was developed to evaluate the bed level changes detected by the adv as a proxy for the bed mobility we found different modes of bed mobility depending on the grain size ratio for low grain size ratios an increase in the fine fraction to 40 led to increased bed level changes during the experiment and the mobilization of the mixed bed at the highest flow speed for high ratios an increase in fine fraction to 40 led to a decrease of bed level changes and the beds remained stable i e no bed forms developed even at the highest flow speed therefore increasing the amount of fine particles can lead to different modes of behaviour depending on the grain size ratio for a bimodal sediment bed with spherical grains under unidirectional flow conditions the grain size ratio can be used to estimate the bed mobility i e to evaluate if a bed is likely to mobilize at flow speeds of up to 0 19 m s 1 hence for the stability of a non cohesive mixed bed in the sand silt range the grain size ratio plays a fundamental role while the amount of fine grains is a secondary factor

processes of selective grain transport and the formation of placers on beaches placers are found on many beaches of the oregon coast in the beach that is the focus of this study the principal placer minerals are ilmenite 22 0 epidote 20 0 garnet 18 7 augite 15 5 hypersthene 8 1 hornblende 5 8 and zircon 2 0 the ilmenite fraction actually being partly chromite a series of beach face sand samples was obtained along a profile when sand was being eroded from the upper beach and transported to offshore bars since the processes of grain selective sorting leading to placer formation were probably most active at such a time the landward most sample contained 96 heavy minerals being part of the placer this concentration systematically decreased offshore to a 6 heavy mineral content in the seaward most sample 54 m offshore determinations of individual heavy mineral concentrations revealed that although they all tended to become concentrated within the placer with light quartz and feldspar dominating the offshore transport the sorting processes were m

monitoring active volcanism with the autonomous sciencecraft experiment on eo 1 abstract the ability to monitor and rapidly react to remote detection of volcanic activity has been greatly improved through use of the autonomous sciencecraft experiment ase an advanced software application installed on a spacecraft in earth orbit ase is a nasa new millennium program experiment demonstrating science driven autonomous command and control of a spacecraft flying on the earth observing 1 eo 1 spacecraft ase successfully detected thermal emission from the mt erebus lava lake on 7 may 2004 having analyzed a hyperion hyperspectral data product on board the spacecraft eo 1 was re tasked by ase to obtain a follow up observation 7 h later and sent a notification of detection of volcanic activity to the ground the entire process was carried out autonomously initial acquisition to receipt on the ground of the positive detection took less than 3 h a process that without ase would have taken weeks the ase thermal classifier has detected several styles of effusive volcanic activity active lava lakes pahoehoe flow fields open channel flows and lava domes ase successfully demonstrated that science driven spacecraft operation greatly enhances science return per returned byte through the identification of the most valuable data allowing prioritization of downlink products and the discarding of null data sets this technology has applications on missions elsewhere in the solar system modified thermal classifiers can be used for detecting and monitoring active volcanism on the jovian satellite io the neptunian moon triton and searching for active volcanism on mars and icy satellites the success of ase is an incentive for future instrument and mission designers to consider on board data processing requirements especially data storage capacity number of processors and processor speed and ram in order to take advantage of this flight proven technology

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

inferring maximum geostrophic current velocities in the norwegian greenland sea from settling velocity measurements of sediment surface samples methods application and results abstract critical shear velocities and geostrophic current velocities were calculated from measured grain sizes and settling velocities using a shields type threshold curve of particle movement and assuming bedload transport for particles that were found to be transported by currents because this reconstruction of the current velocities is based on the coarsest transported component that could be recognized in the sediment the resulting current velocities represent a maximum flow speed that was effective during deposition of sediment the shape of the settling velocity distributions identifies the dominant type of deposition that controlled the accumulation of sediment residual well sorted or low energy depositional the resulting pattern of reconstructed maximum geostrophic current velocities in the norwegian greenland sea reflects the known features of present day thermohaline circulation which is influenced by the bathymetry at many places reconstructed current velocities are higher in areas where the cross section of meridional flows is restricted by topographic barriers the distribution of the depositional characters shows that residual and well sorted sediment types are generally associated with higher current velocities in areas of strong geostrophic currents e g fram strait and east greenland continental slope the low energy depositional sediment type is widespread in the basins of the eastern norwegian greenland sea where lower reconstructed maximum current velocities occur the high reconstructed velocities on the spitsbergen bank can be attributed to the disturbance from storm waves in shallower water depths

incipient granular mass flows at the base of sediment laden floods and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands extraordinary floods commonly produce thick deposits as a result of the entrainment and transport often at high concentrations of large volumes of clastic detritus this paper describes the sedimentology of a stratified bouldery sand deposit generated by a 20 km3 breakout flood from intracaldera lake taupo following the 1 8 ka taupo eruption the proximal deposit is characterised by a strongly bimodal grain size distribution with downstream fining boulders enclosed in a poorly sorted and weakly bedded coarse sand to gravel matrix this combination of characteristics is inferred to reflect simultaneous deposition of small and large particles by two distinct mechanisms i accretion of the sandy matrix from an incipient basal granular mass flow as the flood lost capacity and ii progressive emplacement of the boulders as the flood lost the competence to roll them along the channel floor the first mechanism is a product of a nonlinear particle concentration gradient in the sediment laden flow which caused it to segregate into a basal incipient granular mass flow that formed the transient depositional system indicated by the lithofacies the second mechanism is a function of the superjacent turbulent turbid flow that acted as the primary transporting system this pairing of transport and depositional systems is likely to characterise many sediment laden aqueous flows with clay mineral concentrations too low to develop non newtonian rheologies such as jokulhlaups and lahars that entrain unweathered pyroclastic material qualitative and quantitative differences in the way bedload is transported in the presence of this basal granular mass flow as compared to simple water make impossible accurate estimates of flow competence based on shields entrainment functions for beds of both uniform and mixed grain size distribution

size density sorting of sand size spheres during deposition from bedload transport and implications concerning hydraulic equivalence it is well known that sediment sorting according to size shape and density occurs but the exact mechanisms involved are poorly understood to assess the effects of size and density sand size spheres of two densities were transported and deposited under controlled flume conditions observations on the motion of discrete particles show that grains smaller than bed roughness grains move continuously and have the same transport velocities regardless of density for grains near and slightly larger than the roughness movement is intermittent and for a given size heavy particles move more slowly than lights for grains much larger than bed roughness grains movement is continuous over the rough surface and light and heavy grains have nearly the same transport velocities analyses of bulk sediment deposited from plane bed transport show that the size and proportion of heavies decreases and that of lights increases with distance transported for ripple bed transport however the size relations between associated light and heavy grains remains essentially unchanged with transport distance and the proportion of light and heavy grains is extremely variable these results suggest that size density sorting in plane bed transport is a function of the transportabilities identified in the discrete grain studies but that sorting in ripple bed transport is related to deposition on and recycling through the bed forms application of these findings to the concept of hydraulic equivalence implies that some indication of bed configuration may be necessary for the concept to be useful

gradient irregularity in the herbert gorge of northeastern australia the seventy kilometre long herbert gorge of northeastern australia preserves a record of past floods in slackwater deposits and palaeostage indicators step backwater modelling of water surface profiles indicates that discharges ranging from 11000 to 17000 m3s 1 have occurred six times in the gorge during the last 900 years these flood reconstructions provide insight into the role of extreme flows in shaping bedrock channel morphology in particular the hydraulics of extreme flows can be related to boulder transport and to the location of large boulder bars large boulder bars occur throughout the herbert gorge being best developed at loci of stream power minima along the inside of bends at tributary junctions and at obstructions in the channel caused by bedrock highs only the flows exceeding approximately 8000 m3 s 1 are competent to transport the boulders which constitute the bars in the straight channel reaches the boulder accumulations and bedrock highs have a fairly regular spacing which appears to be independent of lithologic or structural controls the bars provide an efficient means of energy dissipation and they are interpreted as a result of the inherent high turbulence of flow in a steep channel the regular spacing of the bars and their correspondence with the hydraulics of large flows suggest that the bars and associated bedrock highs may represent a self regulating mechanism akin to the pool riffle sequence of alluvial channels it may therefore be appropriate to view bedrock channels as deformable on the timescale of extreme discharges

geological and hydrodynamical examination of the bathyal tsunamigenic origin of miocene conglomerates in chita peninsula central japan a conglomerate appears on a rocky coast called tsubutega ura coast located on the southwestern coast near the southern tip of the chita peninsula aichi prefecture central japan the conglomerate belongs to miocene sedimentary rocks termed the morozaki group the conglomerate includes meter scale boulders indicating that it was formed by an extraordinary event in the geological investigation we observed that the conglomerate shows alternate changes of paleocurrent directions between seaward and landward this feature is supposed to be formed by tsunami currents in the hydrodynamical investigation we obtained following results 1 the lowest limit of a current velocity to move a boulder of about 3 m in diameter would be about 2 3 m s 2 the speed of tsunami currents reproduced by tsunami simulation exceeds 3 m s at 300 m in depth when the tsunami is generated by a gigantic earthquake with magnitude 9 0 or more 3 the transport distance of the boulder would be several hundred meters to several kilometers by one tsunami event caused by a gigantic earthquake we conclude that tsunamis best explain the formation of the conglomerate deposited in upper bathyal environments about 200 400 m depth both from geological and hydrodynamical viewpoints

an experimental study of flow bedload transport and bed topography under conditions of erosion and deposition and comparison with theoretical models the nature of flow sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size density sediment under equilibrium and non equilibrium aggradational degradational conditions and compared with theoretical models during each experiment water depth bed and water surface elevation flow velocity bed shear stress bedload transport and bed state were continuously monitored equilibrium uniform flow was established with a discharge of about 0 05 m3 s 1 a flow depth of about 0 01 m a flow velocity of about 0 81 0 88 m s 1 a spatially averaged bed shear stress of about 1 7 2 2 pa and a sediment transport rate of about 0 005 0 013 kg m 1 s 1 i e close to the threshold of sediment transport such equilibrium flow conditions were established prior to and at the end of each aggradation or degradation experiment pebble clusters bedload sheets and low lying bars were ubiquitous in the experiments heavy minerals were relatively immobile and occurred locally in high concentrations on the bed surface as lag deposits aggradation was induced by 1 increasing the downstream flow depth flume tilting and 2 sediment overloading tilt induced aggradation resulted in rapid deposition in the downstream half of the flume of a cross stratified deposit with downstream dipping pebbles pseudo imbricated and caused a slight decrease in the equilibrium mean water surface slope and total bedload transport rate these differences between pre and post aggradation equilibrium flow conditions are due to a decrease in the local grain roughness of the bed sediment overloading produced a downstream fining and thinning wedge of sediment with upstream dipping pebbles imbricated whereas the equilibrium flow and sediment transport conditions remained relatively unchanged degradation was induced by 1 decreasing the downstream flow depth flume tilting and 2 cutting off the sediment feed tilt induced degradation produced rapid downstream erosion and upstream deposition due to flow convergence with little change to the equilibrium flow and sediment transport conditions the cessation of sediment feed produced degradation and armour development a reduction in the mean water surface slope and flow velocity an increase in flow depth and an exponential decrease in bedload transport rate as erosion proceeded a bedload transport model predicted total and fractional transport rates extremely well when the coarse grained or bedform trough areas of the bed are used to define the sediment available to be transported a sediment routing model midas also reproduced the equilibrium and non equilibrium flow conditions total and fractional bedload transport rates and changes in bed topography and texture very well

influence of sand on hydraulics and gravel transport in a braided gravel bed river measurements in a braided reach of the proglacial white river washington are used to compare the bed load capacity and competence of flow over the predominant gravel bed and over a narrow ribbon of mobile sand which extended alongside the talweg analysis of 144 velocity profiles and 222 bed load samples shows that shear stress and roughness height were significantly lower over sand than gravel but maximum bed load diameter and total gravel transport rate significantly higher as were transport rates of given size fractions at similar shear stresses these differences in competence and capacity are explained by relative size effects on the threshold shear stress for entrainment the influence on gravel transport of local variation in bed material size may explain the self sorting of bed load sheets described by other workers however in channels like that described here the continuous longitudinal ribbon of sand allows the flow to develop a lower shear stress over the smoother bed which partly offsets the relative size effects on competence and capacity

structure composition and bulk properties of upland stream gravels upland gravel bedded streams in the u k have received only scant attention from both hydrologists and sedimentologists but are worthy of further investigation the sedimentology of three small streams in teesdale in the pennines has been examined in detail grain size characteristics bedforms structure composition and packing characteristics of these deposits are described and compared where appropriate with published information it is argued that a fuller appreciation of gravel bed composition and morphology should eventually contribute to an improved understanding of sediment transport and deposition mechanisms and hence to improved accuracy in sediment transport and deposition estimates

water worked gravel surfaces high order structure functions at the particle scale 1 high order structure functions are applied at the particle scale for studying water worked gravel surfaces measured at several field sites in new zealand the key findings relate to the shape of the two dimensional structure functions and their scaling behavior the data suggest that at scales comparable to d50 the topography of gravel surfaces is sensitive to the flow direction and exhibits multiscaling behavior we postulate based on our analysis and physical considerations that the high order structure functions may provide information on the orientation and direction of the flow that formed the surface

bedload entrainment travel lengths step lengths rest periods studied with passive iron magnetic and active radio tracer techniques this paper reports results from bedload transport investigations with active radio and passive iron magnetic tracers in the lainbach a step pool mountain river in bavaria southern germany the spatial distributions of the iron tracers after flood events can be best described by exponential or gamma distributions there is some indication of a tendency of size selective transport of the iron tracers but there is also a considerable amount of scatter in the correlations between weight size and travel length owing to the masking influence of other variables such as the shape of the particles and different positions in the river bed the experiments with artificial magnetic tracers showed that elongated pebbles rods had the longest mean transport distance platy ones discs remained relatively close to the starting points the particles from the pool showed the greatest transport lengths and a 100 per cent chance of being eroded the pebble transmitter system petsy consists of transmitters implanted into individual pebbles a computerized receiver a stationary antenna system with an antenna switchboard and a data logging system the particles do not move continuously but in a series of transport steps and non movement intervals a single value for a given size class is not adequate to describe the critical conditions of entrainment under natural circumstances a probability approach is much more suitable the critical unit discharges total discharge divided by active channel width along the measuring reach are dependent on river bed morphology in the steps bedload needs higher unit discharges to be entrained once entrained the transport of bedload is stochastic in nature and the single particle transport is controlled by the step lengths and the duration of rest periods the distributions of both parameters can be approximated by exponential functions applying the stochastic concept proposed by einstein the mean values of the random variables step length and duration of rest period measured with the petsy technique were used for the simulation of spatial distributions of bedload particles from point sources more field and laboratory data are needed to include varying flow and roughness conditions with tracers representing different particle characteristics

geomorphic classification of rivers a new methodology applied in an atlantic region galicia nw iberian peninsula the classification of rivers based on geomorphological criteria played in the past a secondary role for management decision making although in the last years they have experienced a shift from the scientific field to that of the technical currently managers require the most simplified form of classifications in order to use them in plans and planning projects management and restoration on the one hand this means that classifications should be directed toward simplifying the diversity of fluvial environments in a number of manageable types and on the other hand to apply to each geomorphic type a management model in this study we have developed a method of geomorphic classification of rivers according to specific stream power variables and median grain size the new method is dynamic ability to readjust and gain robustness with the incorporation of new data and predictive we obtained six types directly from the method although we added a seventh type bedrock rivers for its special singularity each geomorphic type presents concrete values of specific stream power and median grain size which additionally involves a hydraulic geometry adjusted to terms of balance the change from a geomorphic type to another would reflect a geomorphologic imbalance in the form of greater power and sediment size our results have been compared with six commonly used classifications e g rosgen classification system or river styles framework geomorphic classification of rivers stands as a flexible tool that allows the development of a personalized geomorphic classification for rivers of the same geomorphological province through the temporary revision of various sites that act as control points we can learn should the case arise the intensity and geomorphic change of the site geomorphic classification of rivers acts as an alert system for any geomorphologic disturbance its simple application and interpretation facilitate the implementation in the administrative environment or its attachment to other commonly used classifications

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

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

cooling rate of some active lavas determined using an orbital imaging spectrometer 1 the surface temperature of an active lava flow is an important physical property to measure through its influence on lava crystallinity cooling exerts a fundamental control on lava rheology remotely sensed thermal radiance data acquired by multispectral sensors such as landsat thematic mapper and the terra advanced spaceborne thermal emission and reflection radiometer are of insufficient spectral and radiometric fidelity to allow for realistic determination of lava surface temperatures from earth orbit this paper presents results obtained from the analysis of active lava flows using hyperspectral data acquired by nasa s earth observing 1 hyperion imaging spectrometer the contiguous nature of the measured radiance spectrum in the 0 4 2 5 m region means that although sensor saturation most certainly occurs unsaturated radiance data are always available from even the hottest and most radiant active lava flow surfaces the increased number of wave bands available allows for the assumption of more complex flow surface temperature distributions in the radiance to temperature inversion processes the technique is illustrated by using a hyperspectral image of the active lava lake at erta ale volcano ethiopia a well characterized calibration target a time series of three hyperion images of an active lava flow acquired during a 4 day period at mount etna sicily as well as a lava flow erupted at nyamuragira democratic republic of congo the results provide insights into the temperature radiance mixture modeling problem that will aid in the analysis of data acquired by future hyperspectral remote sensing missions such as nasa s proposed hyspiri mission

the storage of bed material in mountain stream channels as assessed using optically stimulated luminescence dating abstract a detailed understanding of channel forming and maintenance processes in mountain streams requires some measurement and or prediction of bed load transport and sediment mobility traditional field based measurements of such processes are problematic because of the high formative discharges characteristic of such streams the application of optically stimulated luminescence osl dating is proposed here as a new way of determining actual residency times of fine sediments and consequently validating selected predictions for the entrainment of sediment in these streams model predictions of sediment mobility for selected step pool and plane bed channels in a mountain catchment in south eastern australia are initially calculated using equations of hydraulic competence and the one dimensional hec ras model results indicate that floods exceeding bankfull with recurrence intervals up to 13 years are competent to mobilise the maximum overlying surface grain sizes at both sites osl minimum age model results from 7 samples of well bleached quartz in the fine matrix particles indicate general agreement with selected competence equations the apparent long 100 1400 y burial age of most of the mineral quartz however suggests that competent flows are not able to flush all subsurface fine bed material the depth of maximum bed load exchange flushing was limited to twice the depth of the overlying d 90 grain size application of osl in this study provides important insight into the nature of storage and flushing of matrix material in mountain streams

partial entrainment of gravel bars during floods 1 spatial patterns of bed material entrainment by floods were documented at seven gravel bars using arrays of metal washers bed tags placed in the streambed the observed patterns were used to test a general stochastic model that bed material entrainment is a spatially independent random process where the probability of entrainment is uniform over a gravel bar and a function of the peak dimensionless shear stress t of the flood the fraction of tags missing from a gravel bar during a flood or partial entrainment had an approximately normal distribution with respect to t 0 with a mean value 50 of the tags entrained of 0 085 and standard deviation of 0 022 root mean square error of 0 09 variation in partial entrainment for a given t demonstrated the effects of flow conditioning on bed strength with lower values of partial entrainment after intermediate magnitude floods 0 065 t 0 08 than after higher magnitude floods although the probability of bed material entrainment was approximately uniform over a gravel bar during individual floods and independent from flood to flood regions of preferential stability and instability emerged at some bars over the course of a wet season deviations from spatially uniform and independent bed material entrainment were most pronounced for reaches with varied flow and in consecutive floods with small to intermediate magnitudes index terms 1815 hydrology erosion and sedimentation 1821 hydrology floods 1869 hydrology stochastic processes keywords sediment transport bed material entrainment disturbance gravel bars stochastic model

patchy surface stone movement during disturbance in a new zealand stream and its potential significance for the fauna a patchy movement of surface stones during disturbance in streams has been proposed from field observations but few attempts have been made to quantify this phenomenon if spates produce a mosaic of stable and disturbed areas the former could serve as refugia for benthic invertebrates we monitored the stability of surface stones at three geomorphically contrasting sites in a river with an unstable bed during three spates and one large flood stones were marked in situ by drilling holes in them or by scraping them with a chisel for each stone we determined visible surface area embeddedness in the substratum water depth size of surrounding stones and presence or absence of large stones upstream during the first monitoring period which covered one spate and the large flood we marked 400 stones at each 20 m long site in a systematic grid with 40 to 60 cm between stones stones were relocated after each disturbance but each stone set was used to monitor both subsequent events after the large flood few marked stones were left therefore additional sets of 200 stones were marked for a second monitoring period covering the remaining two spates these stones were located at the same sites but the distance between transects was doubled patchy bed movement occurred during all four disturbances especially during the three smaller ones which moved only 33 to 72 of marked stones stability of individual stones was mainly influenced by their size and embeddedness and sometimes by the water depth above the stone larger scale parameters e g the position of the stone across the transect were less important whole site stability differed little among sites during the three smaller events many stable surface stones were available as potential invertebrate refugia in contrast invertebrate refugia may have been restricted to the hyporheic zone and inundated flood plain gravels during the large flood because patchy stone movement was observed in a river with an unstable bed it is likely to be a feature of most rivers therefore small scale experiments may be able to simulate the effects of disturbance on the benthic community more effectively than previously thought

upper regime parallel lamination as the result of turbulent sediment transport and low amplitude bed forms a series of laboratory experiments has been carried out in which parallel laminated deposits were produced from an upper regime plane bed the laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area 0 3 m by 1 5m fluctuations in bed elevation were measured both during deposition and at equilibrium much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence as most theories for lamina formation would require we suggest instead that extremely low amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination all the lamination produced in the laboratory experiments was normally graded using high speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain diameters thick followed by the slow sieving out of a well packed surface layer of finer sand the initial deposition is the result of small scale turbulent fluctuations in boundary shear stress the sieving out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition we term this process glazing the alignment of small scale turbulent scour and fill structures along the paths traced by migrating bedform troughs produces laterally continuous parallel lamination

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

incipient motion conditions for single rock fragments in simulated rill flow laboratory experiments using simulated tilting rill channels with different bed roughnesses have been conducted to determine hydraulic conditions for incipient motion of single rock fragments lying on rill beds these threshold conditions can be described by a modified shields entrainment parameter m and relative depth z adjusted for rill conditions the negative relation found between m and z also fits steep stream channel data collected by other investigators plots of the classical shields entrainment parameter versus both relative depth and relative particle size also showed clear negative trends special care has been taken to avoid false relations that can arise between compound parameters this occurs when the latter are not independent from each other because of their definitions

a comparison between shields threshold criterion and the movement of loosely packed gravel in a tidal channel observations of the threshold of movement of loosely packed gravel in a tidal current are described for gravel with equivalent spherical diameters d in the range 0 2 d 5 0cm the critical friction velocity u c corresponding to the initiation of sediment transport is given by u c 7 0 d0 2 at large values of d within the quoted range the value u c is significantly lower than would be obtained by a shields experiment u c d0 5 by comparing our values of u c with those obtained under well controlled laboratory conditions the discrepancy with shields is shown to be due to the open spacing between and exposure of individual pebbles on the seabed by comparing our results with those from upland gravel streams and flume experiments it is suggested that shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number 0c is valid

a morpho statistical classification of mountain stream reach types in southeastern australia abstract the concepts of sediment transport capacity q c and sediment supply q s have shown promise in broadly differentiating mountain streams the important role of lithology in determining reach characteristics is also noted but as yet not fully included in existing process domain frameworks this study uses topographic and grain size surveys undertaken over 42 channel reaches with slopes between 0 01 and 0 20 m m to describe mountain stream morphology in southeastern australia reaches with two different lithologies are surveyed to specifically address the role of lithology in resultant channel morphology spatial autocorrelation analysis based on moran s i is used to detect the frequency and occurrence of in stream features such as bars and pools these data are combined with the grain size data to describe eight dominant reach morphologies most channel morphologies display broad characteristics similar to previous accounts of mountain streams although a number of intermediate morphologies affecting subtle variations in form and process where evident there was no significant longitudinal arrangement of the observed morphologies particularly with respect to catchment area and slope this is related to the strongly segmented longitudinal profiles characteristic of these streams where increasing catchment area does not result in a commensurate decrease in slope five of the eight channel morphologies identified are lithology dependent with respect to grain size and shape regime diagrams applied to quantify the physical controls on the different reach morphologies identified different q c q s domains for most morphologies however some lithology dependent morphologies could not be differentiated supporting the need to incorporate the effects of dominant grain shape or sphericity in assigning dimensionless critical shear stress values in determining q s overall the differences in mountain stream reach morphologies described in this study reflect the characteristic segmentation of the longitudinal profiles of these streams and contrasts in the supply and breakdown of parent material

the application of computational fluid dynamics to natural river channels eddy resolving versus mean flow approaches in the last decade as computing power has increased there has been an explosion in the use of eddy resolving numerical methods in the engineering earth and environmental sciences for complex geomorphic flows where accurate field investigations are difficult to perform and where experiments may be difficult to scale these numerical approaches are beginning to give key insights into the nature of these flows eddy resolving methods such as large and detached eddy simulation les des may be contrasted with the time averaged three dimensional simulations that only really began to be applied seriously in geomorphology fifteen years ago while the potential of les for geomorphology has been examined previously des is a relatively recent method that deserves further consideration in this paper we explain the method and then utilise examples from meander and confluence flows as well as flow near the bed of a gravel bed river to highlight the improvements to both the representation of the mean flow and to the representation of time varying processes that result from the use of les des some suggestions are provided for the future use of such techniques in geomorphology

geologic influences on fluvial hydrology and bedload transport in small mountainous watersheds northern new mexico u s a discharge characteristics in six adjacent mountainous watersheds in northern new mexico u s a vary substantially between basins underlain by different lithologies relatively resistant gneisses and granites underlie two basins drainage areas 43 and 94 km2 that have high unit discharge 0 010 to 0 14 m3s 1 km 2 high bankfull discharge and sustained high discharge less resistant sandstones and shales underlie four basins drainage areas 96 to 215 km2 that have relatively low unit discharge 0 001 to 0 005 m3s 1 km 2 relatively low bankfull discharge and peak discharges that are not sustained as long as those in the crystalline terrane analysis of snowmelt runoff water budgets suggests that three factors control hydrologic conditions in the basins first area elevation distributions appear to control the timing and amounts of water input these distributions probably reflect the erosional resistance of the different lithologies second lithology appears to control runoff production in areas having minor amounts of storage third glacial deposits in headwater regions control discharge duration and timing via storage and return flow releases the amount of return flow released by glacial deposits however is probably controlled by the permeability of underlying bedrock therefore it appears that the duration timing and magnitude of discharge events in the study area are controlled both directly and indirectly by lithology stream power and shear stress estimates derived from bankfull discharge and bed material size data suggest that higher bedload transport rates and larger bedload particle sizes exist in streams draining crystalline rocks than in streams draining sedimentary terrane it appears that source area lithology by controlling discharge production also influences stream power bedload transport capabilities and therefore total amounts of bedload transport

stabilization of fine gravels by net spinning caddisfly larvae we examined the impact of hydropsychidae caddisfly larvae on the incipient motion of two sizes of narrowly graded fine gravel 4 6 and 6 8 mm this impact was assessed relative to the collective impact of other abiotic and biotic processes that are potentially important conditioning agents of fine gravels trays of gravel were placed in the river soar leicestershire uk where they were colonized to natural densities by caddisfly larvae identical trays that were surrounded by a 250 m mesh were also deployed preventing colonization but allowing field conditioning of sediments including minor reworking of grains and biofilm development after 21 days in the river trays were removed to a laboratory flume where grain entrainment stresses were established in addition to the colonized and conditioned treatments critical shear stresses were measured for identical sediments that were not placed in the river laboratory gravels gravels that were colonized by hydropsychidae required significantly greater shear stresses for entrainment than conditioned trays p 0 002 however there was no significant difference between conditioned and laboratory gravels this implies that the presence of caddisfly can be a more important influence on fine gravel stability than some conditioning processes shields parameter was compared across treatments and across the two gravel size fractions using two way anova no significant differences or interactions were observed indicating that 4 6 mm gravel was stabilized to a similar degree as 6 8 mm gravel by conditioning and colonization processes our results extend earlier studies in two important ways 1 entrainment stresses were established for fine gravels that were colonized at natural densities under natural stream conditions and 2 the caddisfly effect was measured relative to both field conditioned and unconditioned laboratory controls the temporal and spatial distribution of silk spinning caddisfly larvae suggests that they have the potential to influence fine sediment mobility in many rivers worldwide

interactions between lava and snow ice during the 2010 fimmvorðuhals eruption south central iceland 1 the 20 march 12 april basaltic effusive eruption at fimmvor uhals southern iceland was an important opportunity to directly observe interactions between lava and snow ice the eruption site has local perennial snowfields and snow covered ice and at the time of eruption it was covered with an additional 1 3 m of seasonal snow syn eruption observations of interactions between lava and snow ice are grouped into four categories 1 lava advancing directly on top of snow 2 lava advancing on top of tephra covered snow 3 snow ice melting at lava flow margins and 4 lava flowing beneath snow based on syn and post eruption observations in 2010 11 we conclude that the features seen in the lava flow field show only limited and localized evidence for the influence of snow ice presence during the eruption estimated melting rates from radiant and conductive heating at the flow fronts are too slow on the order of 5 m hr to allow for complete melting of snow ice ahead of the advancing lava flows at least during periods of observed rapid lava advance rates 15 55 m hr thus we conclude that during those periods which largely established the aerial extent of the lava flow field lava advanced on top of snow that this likely was the predominant mode of lava emplacement for much of the eruption is supported by many syn eruption field observations examination of the lava flows subsequent to the eruption has so far only found subtle evidence for interactions between lava and snow ice for example locally lava flows have fractured and are collapsing or have developed marginal rubble aprons from melting of snow banks that were partly covered by lava flow margins

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

erratum dynamics of rocky shores cretaceous pliocene pleistocene and recent baja california peninsula mexico rocky shorelines provide an excellent record that can be used to interpret the environmental conditions prevailing in a particular area in time and space former rocky shores are first rate indicators of sea level position and they provide important local to regional information on tectonics and neotectonics in this paper sedimentary units made of gravel size clasts interpreted as having been deposited on a rocky shore provide data for the interpretation of the hydrodynamic conditions under which the units were deposited a model is presented by which to evaluate the minimum wave height associated with rocky shore deposits located on the baja california peninsula both on the pacific and gulf of california margins for upper cretaceous and pliocene shoreface deposits and for a modern beach deposit elements considered under the application of the model are the average clast size and water depth assigned to each deposit based on an equation proposed by earlier workers in conjunction with airy s linear

impact of coarse sediment supply from hillslopes to the channel in runoff dominated dryland fluvial systems sediment supply from hillslopes to channels is an important control on basin functioning and evolution however current theoretical frameworks do not adequately consider processes of runoff driven hillslope sediment supply which affect river channels spatially and temporally mountainous dryland basins exhibit an important manifestation of these processes because their debris mantled hillslopes produce coarse sediment and because rainfall is delivered as infrequent high intensity short duration rainstorms this paper combines field measurements and modeling to explore runoff driven coarse sediment supply from hillslopes to the channel and assesses a range of plausible storms on the longitudinal patterns of sediment load and its caliber over a dryland basin reach our results show that modeled sediment load and its grain size distribution are determined by the nonlinear interaction between rainfall characteristics and hillslope attributes resulting in longitudinal fluctuations in sediment supply the relative magnitude and location of which varies between storms results suggest that long hillslopes are most sensitive to rainfall and they exhibit large variations in supplied sediment load and grain size for different runoff characteristics short and steep hillslopes are less sensitive to rainfall variations as gradient effects dominate over the role of length in modulating runoff accumulation furthermore the signal of the median fraction d50 of modeled sediment supplied by the hillslope is preserved in the coarse fraction of the measured in channel grain sizes d90 finally we propose a simple index which provides new insights into the effectiveness of different rainstorms in terms of the impact of hillslope sediment supply on the channel

measurements in a braided river chute and lobe 2 sorting of bed load during entrainment transport and deposition grain size distributions of bed material bed load and bar head deposition were sampled during the evolution of a chute and lobe in the braided gravel bed sunwapta river alberta although bed shear stress and total bed load transport rate varied substantially within the reach and from day to day ferguson et al this issue the median diameter d50 of bed load was remarkably constant the bed initially fined from chute to bar head but became more uniform sediment deposited on the bar head also had a near constant d50 similar to that of the bed load truncated at the same lower size limit both load and deposition were finer on average than the bed suggesting overall selective transport despite little sign of local sorting in contrast to median diameters maximum sizes dmax of sediment in motion and deposited on the bar head varied substantially and showed a weak but significant dependence on shear stress inferring selective entrainment from this evidence alone would be dubious because dmax was also found to increase systematically with the mass of sediment sampled however two different methods of analysis of fractional transport rates throughout the size range also showed size selective entrainment and transport in the marginal transport conditions of this study size sorting appears to occur but only weakly and mainly in the coarser fractions

volcanic eruption plume top topography and heights as determined from photoclinometric analysis of satellite data photoclinometry a shape from shading technique is applied to satellite data to determine the three dimensional height and morphology of the upper optical surface of a plume from the eruption of redoubt volcano alaska the technique can be applied to visible images of volcanic plumes that have uniform scattering properties and is most effective on images with moderate incident sun angles and plume transport perpendicular to the azimuth of the sun the two most significant sources of error are the finite resolution of the image data and the choice of image data number dn value for a flat plume element provided the plume spans at least 10 image pixels useful results can be obtained for the central portion of the plume the closest approximation to a flat plume element is found to be the visible dn value corresponding to the coldest pixel along a transect errors in the element altitude due to uncertainty in the dn value for a flat pixel amount to 10 20 m per pixel and are cumulative along the plume transect for redoubt this results in an uncertainty of approximately 300 m at the highest points the method indicates that the redoubt plume rose to an altitude of approximately 3 km while traveling a distance of 150 km from the vent and that the surface topography of the plume exhibits influences of the ambient wind through simple wave structures this technique will have a tremendous impact on studies of plume spreading dynamics and the time integrated evolution of plumes

observing iceland s eyjafjallajokull 2010 eruptions with the autonomous nasa volcano sensor web 1 between 24 march and 5 june 2010 the hyperion hyperspectral imager and advanced land imager ali on nasa s earth observing 1 eo 1 spacecraft obtained an unprecedented sequence of 50 observation pairs of the eruptions at fimmvor uhals and eyjafjallajokull iceland this high acquisition rate was possible only through the use of data flow streamlined by using the autonomously operating nasa volcano sensor web vsw the vsw incorporates notifications of volcanic activity from multiple sources to retask eo 1 and process hyperion data to extract eruption parameters from high spatial and spectral resolution visible and short wavelength infrared data physical changes in eruption style and magnitude were charted as the eruptions ran their course rapid data downlink and automatic data processing algorithms generated a variety of products which are compared with estimates from ground based observations and post eruption in situ measurements estimates of effusion rate from heat loss measurements underestimate actual effusion rate while still following broad eruption rate trends but are closer to in situ estimates for effusive eruptions fimmvor uhals than explosive ash rich eruptions eyjafjallajokull during the later stages of the 2010 eruption vsw generated products were rapidly delivered to end users in iceland to aid in the assessment of risk and hazard the success of the vsw led to icelandic meteorological office imo in situ sensors being incorporated into the vsw and in may 2011 an imo seismic alert autonomously triggered eo 1 observations of a new eruption at grimsvotn volcano finally the vsw demonstrates an autonomy driven multi asset spacecraft retasking and data processing system that maximizes science return a desirable capability for future nasa missions

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

pebble and cobble transport on a steep mega tidal mixed sand and gravel beach abstract natural tracer pebbles and cobbles were deployed on a steep slope 0 1 mixed sand and gravel mega tidal beach located at the head of the bay of fundy two experiments were carried out to investigate the response of natural pebble and cobble sized material both to energetic wave conditions during individual storm events and to tidal only forcing conditions between storms net displacement of different tracer sizes was the focus of the first experiment while the effect of different shapes was tested in the second the pebbles and cobbles were painted in fluorescent colors labeled and their positions measured at low tide over periods of 10 12 days the tracers were emplaced at mid tide level on the beach face at low water and later recovered and re deployed on successive low tides net travel distance and direction were measured upon recovery hydrodynamic forcing conditions and local morphology at mid tide level on the beach face were monitored continuously during the experiments in the first experiment recovery rates were 100 during calm and moderate wave conditions hs 0 5 m but decreased significantly in more energetic wave forcing hs up to 1 m recovery rates were lower for smaller tracers during a three day wind wave event net displacements of up to 48 m were measured comparable net displacements were observed for all tracers maximum displacements were 39 m onshore 42 m offshore and 46 m alongshore in calm conditions maximum displacements were less than 1 m indicating that the bottom stresses due to tidal currents alone are too low to cause significant tracer displacement on time scales of weeks despite the high tidal range in the second experiment recovery rates were lower due to more energetic wave forcing significant wave heights reached hs 0 5 m on 80 of the particle tracking days and hs 1 m on 30 of the days flat pebbles and cobbles reached the longest transport distances with a maximum net displacement of 98 m the flat tracers were observed to be flipped by the shore break and to ride the swash bore and were in this manner transported long distances shoreward up to 66 m in both experiments the directions of alongshore displacements were consistent with deviations of the angle of wave incidence from the shorenormal tracer burial and re exposure was observed via rotary imaging sonar with the evolution and migration of m wavelength ripples the study allowed a direct correlation of pebble and cobble tracer displacements to hydrodynamic forcing during individual storm events with consideration of tracer particle size and shape

predictions of steady state and transient landscape morphology using sediment flux dependent river incision models 1 recent experimental and theoretical studies support the notion that bed load in mountain rivers can both enhance incision rates through wear and inhibit incision rates by covering the bed these effects may play an important role in landscape evolution and in particular the response of river channels to tectonic or climatic perturbation we use the channel hillslope integrated landscape development child numerical model with two different bedrock incision models that include the dual role of the sediment flux to explore the transient behavior of fluvial landscapes both models predict that steady state channel slopes increase in landscapes with higher rock uplift rates however the incision models predict different transient responses to an increase in uplift rate and the behavior of each incision model depends on both the magnitude of change in uplift rate and the local drainage area in some cases the transient channel behavior is indistinguishable from that predicted for transport limited alluvial rivers in other cases knickpoints form in some or all of the drainage network as predicted by the detachment limited stream power model in all cases the response in the lower parts of the network is highly dependent on the response in the upper parts of the network as well as the hillslopes as the upper parts of the network send more sediment downstream channel incision rates may rise or fall and slopes in the lower parts of the channel may in fact decrease at times during the transient adjustment to an increase in rock uplift rate in some cases channel incision in the upper parts of the network ceases during the transient while the hillslopes adjust to the new uplift rate drainage density may also change as a function of uplift rate our results suggest that if the sediment flux strongly controls bedrock incision rates then 1 the transient fluvial response will take longer than predicted by the detachment limited stream power model 2 changes in channel slope may be much more complex than predicted by the detachment limited stream power model and 3 changes in the fluvial system will be closely tied to sediment delivery from the hillslopes importantly our results outline quantitative differences in system behavior produced by competing models and provide a framework for identifying locations in natural systems where differences in channel morphology can be used to discern between competing fluvial erosion models

a currents of removal approach for interpreting carbonate sedimentary processes abstract this study develops a currents of removal methodology to examine and quantify the nature of physical transport processes affecting the formation of bioclastic deposits in the cocos keeling islands an indian ocean atoll this approach is based on the hydraulic settling and threshold of entrainment characteristics of bioclastic deposits and measured current energy potential mobility pm analysis of 157 samples quantifies the proportion of deposits able to be transported in each geomorphic zone results show that under mean energy conditions wave induced currents entrain and transport sediment within the atoll and show that transport of sediment is not solely reliant on storm energy conditions gradients of pm values are used to reconstruct sediment transport pathways from the reef flat 85 100 pm through shallow passages 75 100 pm to sand aprons 50 75 pm and shallow 0 20 pm and deep lagoon 0 pm comparison of settling velocity distributions of bed material and sediment retained in sediment trap experiments show that actual mobility levels correspond with pm estimates potential mobility analysis also identifies the immobile portions of deposits which increase lagoonward constituent analysis of immobile fractions and analysis of settling frequency distributions are used to differentiate the importance of physical and biological processes in the formation of deposits throughout cocos and identify the role of each geomorphic zone in the transport system the pattern of sediment is controlled by physical processes between the reef flat the primary sediment production zone with sediment transported through the shallow passage conduits to the sand aprons the formation of shallow and deep lagoon deposits is controlled by autochthonous sediment production and storm deposition potential mobility analysis is a powerful tool enabling physical transport processes within bioclastic sedimentary environments to be quantified the ability to examine an individual deposit s hydraulic behaviour and development also enables depositional processes to be examined at a much finer resolution than previously attempted using conventional textural approaches

depositional environment and origin of the lilaozhuang neoarchean bif hosted iron magnesite deposit on the southern margin of the north china craton the neoarchean lilaozhuang iron magnesite deposit is located in the middle of the huoqiu banded iron formation bif ore belt in anhui province on the southern margin of the north china craton the huoqiu bif is the unique one that simultaneously develops quartz type silicate type and carbonate type magnetite in the region the lilaozhuang deposit is characterized by magnesium rich carbonate magnesite in magnetite ores the bif hosted iron ores include mainly of silicate type and carbonate type with a small amount of quartz type which chiefly exhibit banded and massive structure with minor disseminated structure the magnesite ores occur as crystal like bright white and exhibits massive structure the y ho ratio and rey pattern of both iron and magnesite ores are similar to that of seawater while eu shows positive anomaly which is the sign of seafloor hydrothermal mixture these features suggest that ore forming materials of iron and magnesium in the lilaozhuang deposit are mainly from the mixture of seafloor hydrothermal and seawater both ores do not exhibit negative ce anomaly which indicates that the deposit was formed in an environment showing a lack of oxygen c o isotopic compositions indicate that magnesite ore has been reformed by metamorphism of low amphibolite facies and later hydrothermal alteration based on the comprehensive analysis authors suggest that iron and magnesite ores in the lilaozhuang deposits formed in a confined sea basin on continental margin and was influenced by later complex geological processes

occurrence and formation of water laid placers a placer is a deposit of residual or detrital mineral grains in which a valuable mineral has been concentrated by a mechanical agent the agent is usually running water and the valuable mineral is usually denser than quartz for example gold diamond cassiterite ilmenite or chromite it is difficult to overemphasize the importance of placers as sources of mineral wealth the witwatersrand south africa paleoplacers discovered in 1886 alone have provided over half of all the gold ever mined in the world pretorius 1976 and alluvial placer deposits of cassiterite in southeast asia presently are the world s major source of tin toh 1978 the earliest evidence of placer mining comes from stone carvings in egypt dated at 2500 b c macdonald 1983 and as early as 7 b c strabo pointed out that in turdetania spain more gold was procured from washing sand in the rivers than by digging in the mines given the economic importance of water laid placers and the long history oftheir exploitation it is remarkable that they have received so little attention in sedimentary research mining and exploration practices for the most part have been guided by empirical rules of thumb that are locally useful but lack any firm physical basis of understanding see bateman

threshold of coarse sediment transport in broad and narrow natural streams the threshold of coarse sediment transport has been examined in natural streambeds in an upland pennine u k area threshold values of the total boundary shear stress t0 for a given grain size in a narrow natural stream w d 11 efficiency in the entrainment process is related not only to the overall channel geometry but also varies as a function of discharge in channels characterized by compound roughness empirical curves relating t0 and a mean grain size d5 are presented but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source the reasons for this divergence are discussed the influence of grain shape was found not to be important in the initiation of motion criterion this conclusion may reflect the limited range of natural grain shapes in the study streams but might reasonably apply to other field investigations of similar streams modifications of the shields and yalin diagrams are suggested for practical applications in shallow streams with poorly graded bed material the shields parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth d5 d the increased importance of augmented drag forces in the entrainment process in shallow flows is suggested as the physical explanation for the reduced values of the shields parameter however the relationships presented should not be applied to laboratory experiments concerned with well graded sediments therefore beds with little deviation in level in which the shields parameter may be regarded as constant at high reynolds grain numbers consistent estimated field values of a threshold sediment transport parameter might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the literature

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

friction angle measurements on a naturally formed gravel streambed implications for critical boundary shear stress we report the first measurements of friction angles for a naturally formed gravel streambed for a given test grain size placed on a bed surface friction angles varied from 10o to over 100o friction angle distributions can be expressed as a function of test grain size median bed grain size and bed sorting parameter friction angles decrease with increasing grain size relative to the median bed grain size and are a systematic function of sorting with lower friction angles associated with poorer sorting the probability distributions of critical shear stress for different grain sizes on a given bed surface as calculated from our friction angle data show a common origin but otherwise diverge with larger grains having narrower and lower ranges of critical shear stresses the potential mobility of a grain as defined by its probability distribution of critical shear stress may be overestimated for larger grains in this analysis because our calculations do not take into account the effects of grain burial and altered near bed flow fields

ice volcano interactions during the 2010 eyjafjallajokull eruption as revealed by airborne imaging radar 1 during the eruption of the ice covered eyjafjallajokull volcano a series of images from an airborne synthetic aperture radar sar were obtained by the icelandic coast guard cloud obscured the summit from view during the first three days of the eruption making the weather independent sar a valuable monitoring resource radar images revealed the development of ice cauldrons in a 200 m thick ice cover within the summit caldera as well as the formation of cauldrons to the immediate south of the caldera additionally radar images were used to document the subglacial and supraglacial passage of floodwater to the north and south of the eruption site the eruption breached the ice surface about four hours after its onset at about 01 30 utc on 14 april 2010 the first sar images obtained between 08 55 and 10 42 utc show signs of limited supraglacial drainage from the eruption site floodwater began to drain from the ice cap almost 5 5 h after the beginning of the eruption implying storage of meltwater at the eruption site due to initially constricted subglacial drainage from the caldera heat transfer rates from magma to ice during early stages of cauldron formation were about 1 mw m 2 in the radial direction and about 4 mw m 2 vertically meltwater release was characterized by accumulation and drainage with most of the volcanic material in the ice cauldrons being drained in hyperconcentrated floods after the third day of the eruption meltwater generation at the eruption site diminished due to an insulating lag of tephra

hydrodynamic behaviour of taupo 1800a pumice implications for the sedimentology of remobilized pyroclasts large scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments on land erosion remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run off conversely in the submarine realm both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium despite this and the number of studies devoted to volcaniclastic sedimentation relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves which ought to underpin any assessment of transport or depositional process this is crucial as many volcanic particles exhibit variable density according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and or gaseous phases during transport and deposition investigation of the physical and hydrodynamic properties of taupo 1800a pumice with reference to sedimentary facies developed during the eruption aftermath shows that although buoyant when dry when sufficiently waterlogged cool pumice clasts will sink and behave more like quartzo feldspathic material saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air water exchange in smaller pores low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent sized quartzo feldspathic or crystal lithic particles fine grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles these unusual properties of temporary buoyancy variable saturation low density and size dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies these findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments but also for primary depositional processes during subaqueous explosive volcanism

field application of close range digital photogrammetry crdp for grain scale fluvial morphology studies in situ measurement of grain scale fluvial morphology is important for studies on grain roughness sediment transport and the interactions between animals and the geomorphology topics relevant to many river practitioners close range digital photogrammetry crdp and terrestrial laser scanning tls are the two most common techniques to obtain high resolution digital elevation models dems from fluvial surfaces however field application of topography remote sensing at the grain scale is presently hindered mainly by the tedious workflow challenges that one needs to overcome to obtain high accuracy elevation data a recommended approach for crdp to collect high resolution and high accuracy dems has been developed for gravel bed flume studies the present paper investigates the deployment of the laboratory technique on three exposed gravel bars in a natural river environment in contrast to other approaches having the calibration carried out in the laboratory removes the need for independently surveyed ground control targets and makes for an efficient and effective data collection in the field optimization of the gravel bed imagery helps dem collection without being impacted by variable lighting conditions the benefit of a light weight three dimensional printed gravel bed model for dem quality assessment is shown and confirms the reliability of grain roughness data measured with crdp imagery and dem analysis evidences sedimentological contrasts between gravel bars within the reach the analysis of the surface elevations shows the effect variable grain size and sediment sorting have on the surface roughness by plotting the two dimensional structure functions and surface slopes and aspects we identify different grain arrangements and surface structures the calculation of the inclination index allows determining the surface forming flow direction s we show that progress in topography remote sensing is important to extend our knowledge on fluvial morphology processes at the grain scale and how a technique customized for use by fluvial geomorphologists in the field benefits this progress copyright 2016 john wiley sons ltd

estimating palaeowind strength from beach deposits abstract the geological record of past wind conditions is well expressed in the coarse gravel cobble and boulder beach deposits of quaternary palaeolakes in the great basin of the western usa and elsewhere this paper describes a technique using the particle size distribution of beach deposits to reconstruct palaeowind conditions when the lakes were present the beach particle technique bpt is first developed using coarse beach deposits from the 1986 87 highstand of the great salt lake in utah combined with instrumental wind records from the same time period next the bpt is used to test the hypothesis that wind conditions were more severe than at present during the last highstand of lake lahontan 13 ka which only lasted a decade or two at most the largest 50 beach clasts were measured at nine beach sites located along the north west and south sides of antelope island in the great salt lake all of which formed in 1986 87 at these sites the largest clast sizes range from 10 to 28 cm b axis and fetch lengths range from 25 to 55 km nearshore wave height was calculated by assuming that the critical threshold velocity required to move the largest clasts represents a minimum estimate of the breaking wave velocity which is controlled by wave height shoaling transformations are undertaken to estimate deep water wave heights and ultimately wind velocity wind estimates for the nine sites using the bpt range from 6 5 to 17 4 m s 1 which is in reasonable agreement with the instrumental record from salt lake city airport the same technique was applied to eight late pleistocene beaches surrounding the carson sink sub basin of lake lahontan nevada using the bpt estimated winds for the eight sites range from 9 7 to 27 1 m s 1 the strongest winds were calculated for a cobble boulder beach with a fetch of 25 km instrumental wind records for the 1992 99 period indicate that wind events of 9 12 m s 1 are common and that the strongest significant wind event 9 m s 1 for 3 h reached an average velocity of 15 5 m s 1 based on this preliminary comparison it appears that the late pleistocene western great basin was a windier place than at present at least for a brief time

saltation on gravel beaches tierra del fuego argentina abstract saltation is a common phenomenon on gravel beaches sediment traps across two beaches of the el paramo spit tierra del fuego argentina yielded different results on mean grain size sorting saltating load shape and height above the bed mean clast size increases with height above the bed to an altitude where the saltating size diminishes the sorting of the saltating populations also increases until an altitude where it begins to diminish spheres are preferentially set into saltation although the bed is dominated by platy bladed and elongated gravels much of the behavior of the saltation population is related to local wave dynamics beach steepness and stage of the tide the processes of gravel segregation size and shape on beaches obey to particular phenomena not considered in present day sediment transport equations but of much importance in relation to coastal defense improvements as dynamic revetments

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

evaluation of grain shear stresses required to initiate movement of particles in natural rivers shear stresses were evaluated at different sites on two rivers the first the rulles is characterized by a pebbly bedload and a meandering bed with riffles and pools the second the rouge eau has mainly a sandy rippled bed where meandering is well developed but also flat gravelly sectors without meandering system shear stresses calculated from friction velocities using a redefined y1 roughness height parameter were compared with total shear stresses calculated from the energy grade line and the hydraulic radius divergence between these shear stresses seems to increase in the presence of bedforms and large scale irregularities of the channel the ratio is close to 0 5 in the gravelly sector of the rouge eau and reaches 0 65 in the riffles of the rulles generally located at the inflexion point of the meanders while it is less than 0 3 in the pools of the same river located in the loops and only 0 2 in the sandy rippled sector of the rouge eau grain and bedform shear stresses were evaluated at these same sites by different methods the grain shear stress represents on average 30 per cent of the total shear stress in the riffles of the rulles and the gravelly sector of the rouge eau but less than 15 per cent in the pools in the rulles and the sandy sectors of the rouge eau however it emerges from experiments conducted with marked pebbles and in situ observations of erosion and transport of sandy and gravelly particles that the grain shear stresses are underestimated and cannot explain the movements and modifications actually observed conversely shear stresses calculated from friction velocities at the sites where erosion actually occurred or failed to occur despite very high velocities provide a better explanation of the observed movements

spatial pattern of flow competence and bed load transport in a divided gravel bed river a 14 month continuous record of the spatial variation in the incidence and magnitude of bed load transport was provided by nine pit traps sunk into the bed of a 21 m wide upland gravel bed river the bed load data are related to a simultaneous record of average bed shear stress derived from the depth slope product the study reach is straight and includes a small low relief mid channel bar which represents the divide between two zones of markedly different bed structure and stability he bed of the left channel and bar tail is loose with an open framework while that of the bar head and right channel is tightly interlocking and well imbricated bed surface grain size is similar throughout the reach the two zones of contrasting bed structure have a strong influence on the threshold of sediment movement c and consequently the spatial pattern of bed load transport entrainment thresholds for the tightly structured bed are up to seven times higher than those for the loosely structured bed sediment transport is therefore most frequent in the left channel and over the bar tail where flows with excess shear stresses c peaking at 8 c constitute 19 of the flow record this is in marked contrast to the right channel and bar head where excess shear stresses of maximum 1 4 c total only 1 3 of the flow record however despite the marked difference in the magnitude and duration of competent flows between the two contrasting bed areas little difference in bed load yield is observed between the nine traps when averaged over the 14 month study period this study shows that different zones of bed structure and stability in a channel will control the incidence and spatial pattern of bed load transport but not necessarily the medium and long term bed load yield this has important implications for local and short term bed load sampling programs and the numerical modeling of sediment budgets and zone to zone bed load transfer

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

bed disturbance via foraging fish increases bedload transport during subsequent high flows and is controlled by fish size and species abstract benthic foraging by fish can modify the nature and rates of fine sediment accrual and the structure and topography of coarse grained fluvial substrates with the potential to alter bed material characteristics particle entrainment thresholds and bedload transport fluxes however knowledge of what controls the nature extent and intensity of benthic foraging and the consequent influence of these controls on geomorphic impact remain rudimentary an ex situ experiment utilising barbel barbus barbus and chub leuciscus cephalus extended previous work by considering the role of fish size and species as controls of sediment disturbance by foraging and the implications for bed material characteristics and bedload transport in a laboratory flume changes in bed microtopography and structure were measured when a water worked bed of 5 6 22 6 mm gravels was exposed to four size classes of barbel 4 5 5 6 6 8 8 10 in length and a single size class of chub 8 10 in line with other studies that have investigated animal size as a control of zoogeomorphic agency increasing the size of barbel had a significant effect on measured disturbance and transport metrics specifically the area of disturbed substrate foraging depth and the fish s impact on microtopographic roughness and imbrication all increased as a function of fish size in a comparison of the foraging effects of like sized barbel and chub 8 10 in length barbel foraged a larger area of the test bed and had a greater impact on microtopographic roughness and sediment structure relative to water worked beds that were not foraged bed conditioning by both species was associated with increased bedload transport during the subsequent application of high flows however the bedload flux after foraging by barbel which is a specialist benthivore was 150 higher than that following foraging by chub which feed opportunistically from the bed and the total transported mass of sediment was 98 greater an interesting implication of these results given the abundance and widespread distribution of foraging fish is that numerous fish species belonging to a variety of functional groups may be acting as zoogeomorphic agents in rivers directly affecting bed material conditions and sediment transport fluxes in proportion to their body size and feeding traits

the dispersion of magnetite bedload tracer across a gravel point bar and the development of heavy mineral placers economic concentrations of heavy minerals are often associated with fluvial point bars but prospecting models identifying the heaviest concentrations are poorly developed consequently the dispersal and storage of a heavy mineral bedload tracer magnetite across a rapidly evolving point bar was studied using magnetic susceptibility as a surrogate measure of magnetite concentration the bar head was a preferential area for the development of a placer owing to a lag accumulation of magnetite over an armoured bed surface in contrast when viewed in plan the bar platform and bar tail were regions of tracer dilution owing to downstream dispersion and mixing with shale in the vertical as the bar top rapidly aggraded however in section false bottom placers developed along bedding planes were evident the latter consisted of thin layers of concentrated magnetite resulting from the passage of bedload sheets consisting of a mix of shale and magnetite moving repeatedly from the bar head to accrete over the bar tail differential density sorting of magnetite and shale occurred during transport and deposition such that the heavier magnetite tended to accumulate as a visible concentrated bed layer later to be over run by layers of shale sediment in which magnetite was present diffusely however the placer thickness was greater than that visible because finer fractions of magnetite from each concentrated layer infiltrated the interstices of the top of the shale bed below the placer thickness the infiltration potential of the sediment bed and the actual rate of infiltration of the tracer were determined by fitting a mathematical function to measured variation in magnetic susceptibility with depth in the sediment body finally a simple mathematical model described in the literature as reproducing the plan view of flow and topographic patterns in river bends was found to reproduce patterns of depth velocity shear stress and competence in the point bar environment which were in accordance with the interpretation of the field data it was concluded that such a model when linked to entrainment functions for sediments of mixed density might be suitable for prospecting for economic heavy minerals in the point bar environment

development of a vector based 3d grain entrainment model with application to x ray computed tomography scanned riverbed sediment sediment transport equations typically produce transport rates that are biased by orders of magnitude a causal component of this inaccuracy is the inability to represent complex grain scale interactions controlling entrainment grain scale incipient motion has long been modelled using geometric relationships based on simplified particle geometry and two dimensional 2d force or moment balances however this approach neglects many complexities of real grains including grain shape cohesion and the angle of entrainment relative to flow direction to better represent this complexity we develop the first vector based fully three dimensional 3d grain rotation entrainment model that can be used to resolve any entrainment formulation in 3d and which also includes the effect of matrix cohesion to apply this model we use x ray computed tomography to quantify the 3d structure of water worked river grains we compare our 3d model results with those derived from application of a 2d entrainment model we find that the 2d approach produces estimates of dimensionless critical shear stress urn x wiley mma media esp4608 math 0001 that are an order of magnitude lower than our 3d model we demonstrate that it is more appropriate to use the c axis when calculating 2d projections which increases values of urn x wiley mma media esp4608 math 0002 to more closely match our 3d estimates the 3d model reveals that the main controls on critical shear stress in our samples are projection of grains cohesive effects from a fine grained matrix and bearing angle for the plane of rotation the lateral angle of departure from downstream flow that in part defines the grain s direction of pivot about an axis formed by two contact points in 3d the structural precision of our 3d model demonstrates sources of geometric error inherent in 2d models by improving flow properties to better replicate local hydraulics in our 3d model entrainment modelling of scanned riverbed grains has the potential for benchmarking 2d model enhancements

tracer pebble movement along a concave river profile virtual velocity in relation to grain size and shear stress over 1400 tracer pebbles 16 256 mm in diameter were tracked for 2 years in six reaches of allt dubhaig scotland a small gravel bed river along which shear stress and bed surface grain size decrease toward a local base level pebble movement was size selective both within and between reaches within reaches the decrease in mean travel distance with increasing grain size is strongest in the coarse tail of the size distribution particle shape has a minor secondary effect a nondimensional grain velocity averaged over the duration of competent flow is used to compare different size classes and reaches over 90 of its variance is explained by relative grain size and reach shields stress the pattern of size selectivity is consistent with single event tracer results elsewhere bedload trap data from our distal reach and the concept of partial mobility it provides a mechanism for strong downstream fining by selective transport and deposition along rivers in which stress declines toward base level the nondimensional prediction equation for grain velocity may be of use in other rivers but requires testing

pivoting analyses of the selective entrainment of sediments by shape and size with application to gravel threshold by komar and li 1986 425 436

the role of hydrologic processes and geomorphology on the morphology and evolution of sediment clusters in gravel bed rivers abstract the effect of geomorphic features and hydraulic conditions on the formation evolution and morphology of sediment cluster microforms in an unregulated gravel bed stream were investigated at field sites on the entiat river which drains the eastern slopes of the cascade mountains in central washington state usa sediment clusters were marked described and photographed before and after a series of moderate to high discharges over an 18 month period to quantify the evolution of the cluster morphologies individual sediment particles were tracked to calculate the range of flows and bed shear stresses that maintain and destroy clusters examination of geomorphic settings channel morphology and particle size distributions documented the conditions that favor cluster formation the investigation tested the hypotheses that clusters in this environment delay sediment entrainment and that their morphology and evolution follow a predictable evolution similar to that found in laboratory studies clusters formed on gravel bars adjacent to riffles with slopes 1 poor to moderate sorting and mean and maximum particle size values 1 5 times greater than those associated with noncluster bars clusters were more stable under the bimodal sediment size distribution contributed by a tributary alluvial fan and were destroyed with and without anchor clast mobilization at sites where sediment size was more uniform the six cluster morphologies identified in the field were similar to those in flume studies but did not follow the same evolutionary cycle over multiple flow events this contrast was attributed to the flow events resetting the cluster cycle leading to a high percentage of upstream triangles the dimensionless critical shear stress required to entrain the mean grain size ranged from 0 06 to 0 08 the hydraulic thresholds and geomorphic features that result in stable vs mobilized cluster microforms in this setting can serve as a model for regulated rivers and restoration projects aimed at sustaining instream flows to maintain natural sediment transport conditions

lateral sediment sources and knickzones as controls on spatio temporal variations of sediment transport in an alpine river modern mixed alluvial bedrock channels in mountainous areas provide natural laboratories for understanding the time scales at which coarse grained material has been entrained and transported from their sources to the adjacent sedimentary sink where these deposits are preserved as conglomerates this article assesses the shear stress conditions needed for the entrainment of the coarse bed particles in the glogn river that drains the 400 km2 val lumnezia basin eastern swiss alps in addition quantitative data are presented on sediment transport patterns in this stream the longitudinal stream profile of this river is characterized by three ca 500 m long knickzones where channel gradients range from 0 02 to 0 2 m m 1 and where the valley bottom confined into a 10 m wide gorge downstream of these knickzones the stream is flat with gradients 0 01 m m 1 and widths 30 m measurements of the grain size distribution along the trunk stream yield a mean d84 value of ca 270 mm whereas the mean d50 is ca 100 mm the consequences of the channel morphology and the grain size distribution for the time scales of sediment transport were explored by using a one dimensional step backwater hydraulic model hydrologic engineering centre river analysis system the results reveal that along the entire trunk stream a two to 10 year return period flood event is capable of mobilizing both the d50 and d84 fractions where the shields stress exceeds the critical shields stress for the initiation of particle motion these return periods however varied substantially depending on the channel geometry and the pebble boulder size distribution of the supplied material accordingly the stream exhibits a highly dynamic boulder cover behaviour it is likely that these time scales might also have been at work when coarse grained conglomerates were constructed in the geological past