origin of chondrule rims and interchondrule matrices in unequilibrated ordinary chondrites optical microscopy electron probe microanalysis scanning electron microscopy and analytical transmission electron microscopy were used to study the interchondrule matrix and opaque chondrule rims in bishunpur krymka semarkona chainpur all ll3 tieschitz and sharps both h3 the size distribution of grains in matrix in bishunpur obeys a power law rims around chondrules tend to be finer grained than matrix and may be layered all rims and matrices studied contain clasts of forsteritic olivine and low ca pyroxene in bishunpur the clasts may be cemented by amorphous glue rich in normative albite and partly altered to smectite rims in krymka sharps chainpur and tieschitz have a densely packed olivine groundmass with a grain size of about 0 1 m in krymka groundmass olivine is fo20 30 and forms interlocking dendrites various fe oxides are present in rims and matrices grain size distribution and chemical data indicate that clastic olivines and pyroxenes are derived from chondrules from analyses matrices rims in the ll group apparently are enriched in a feldspathic component tieschitz rims are slightly enriched but here the feldspathic component is present as white matrix sharps rims are not enriched in a feldspathic component opaque interchondrule matrices and chondrule rims probably formed from the products of fragmentation of chondrules partly induced by contraction of protopyroxene on inversion to clinopyroxene fragmented silica rich chondrule mesostases reacted with fe oxides and na to form the groundmass of fayalitic olivine and feldspathic glue a low temperature nebular or pre solar component is limited to 3 vol of each meteorite so equilibrium condensate was not the carrier of volatiles such as tl and bi

measuring flow velocity and sediment transport with an acoustic doppler current profiler an acoustic doppler current profiler adcp measures three dimensional velocity profiles within the water column using the doppler shift principle whilst the bottom tracking function and acoustic backscatter can be used to measure bed load velocity and estimate suspended sediment concentration the adcp offers many advantages over traditional single point current meters and sediment samplers including deployment from a moving launch a single instrument for both velocity and sediment transport measurements profiles of three dimensional velocity and suspended sediment and the ability to map an entire flow field limitations of adcps include a large sampling diameter close to the bed coarse measurement of vertical velocity fragmented bottom track records a poor understanding of the relation between bottom tracking and the mechanisms of sediment transport and sensitivity of the acoustic backscatter to particle size

length of flow separation over dunes dimensional analysis was used to define the length of the flow saparation for the case of two dimensional uniform flow over dunes which was expressed in terms of bed form geometry and the sand size of the bed material experiments were conducted in a rectangular flume using artificial dunes and a flow visualization method to measure the flow separation length when dunes are flat length of the flow separation is affected by the sand grain roughness when dunes are steep the length of the flow separation is due mainly to the bed form geometry and is relatively constant the reverse rectangular steps can be considered as dunes of infinite length and the length of the flow separation for such steps will always be greater than that obtained for dunes of finite length

morphology and flow fields of three dimensional dunes rio parana argentina results from simultaneous multibeam echo sounding and acoustic doppler current profiling 1 most past studies of river dune dynamics have concentrated on two dimensional 2 d bed forms with constant heights and straight crest lines transverse to the flow and their associated turbulent flow structure this morphological simplification imposes inherent limitations on the interpretation and understanding of dune form and flow dynamics in natural channels where dune form is predominantly three dimensional for example studies over 2 d forms neglect the significant influence that lateral flows and secondary circulation may have on the flow structure and thus dune morphology this paper details a field study of a swath of 3 d dunes in the rio parana argentina a large 0 35 km wide 1 2 km long area of dunes was surveyed using a multibeam echo sounder mbes that provided high resolution 3 d detail of the river bed simultaneous with the mbes survey 3 d flow information was obtained with an acoustic doppler current profiler adcp revealing a complicated pattern of dune morphology and associated flow structure within the swath dune three dimensionality appears intimately connected to the morphology of the upstream dune with changes in crest line curvature and crest line bifurcations junctions significantly influencing the downstream dune form dunes with lobe or saddle shaped crest lines were found to have larger more structured regions of vertical velocity with smaller separation zones than more 2 d straight crested dunes these results represent the first integrated study of 3 d dune form and mean flow structure from the field and show several similarities to recent laboratory models of flow over 3 d dunes

flow separation in the lee side of transverse dunes a numerical investigation we investigate flow separation in the air flow over transverse sand dunes numerical simulations of the air flow over differently shaped dunes are performed the length of the recirculation region after the brink of the dune is found to depend strongly on the shape of the dune a phenomenological expression for the separation length is presented suitably non dimensionalized it depends linearly on the angle of the dune at the slip face brink we propose that the shape of the separating streamline is well approximated by an ellipse

on the relationship between flow and suspended sediment transport over the crest of a sand dune rio parana argentina the links between large scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades with past studies illustrating the origin of such turbulence and its influence on flow resistance sediment transport and bedform morphology in this study of turbulence and sediment suspension over large sand dunes in the rio parana argentina time series of three dimensional velocity and at a point suspended sediment concentration and particle size were measured with an acoustic doppler current profiler and laser in situ scattering transmissometer respectively these time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment the analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections regions of high suspended sediment concentration are found to correlate well with such events the frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations such a finding suggests that a combination and interaction of vortex shedding and wake flapping changing length of the lee side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures including information on the topology of dune related turbulent flow structures at the flow stage investigated the turbulent flow events and their associated high suspended sediment concentrations are seen to grow with height above the bed until a threshold height ca 0 45 flow depth is reached above which they begin to decay and dissipate

mechanics of flow over ripples and dunes a simple predictive model for velocity and boundary shear stress fields over two dimensional bedforms of finite amplitude is presented herein and the results of that model are compared with flume data the production of a wake due to separation of the flow near the bedform crest and the subsequent interaction of this wake region with a developing nonuniform boundary layer downstream of the flow reattachment point are treated using the method of mclean and smith 1986 their model is modified in order to take account of the flow response to short wavelength obstructions more accurately and to include the effects of wakes from upstream bedforms as these wakes coalesce in the interior the theory is also extended to include the region between the bedform crest and the reattachment point thereby providing a spatially complete field of velocity and shear stress results of the calculations are found to be in good agreement with laboratory measurements taken over immobile bedforms using a laser doppler velocimeter our finite amplitude flow model is shown to yield accurate predictions of the fluid dynamical effects ultimately responsible for the morphology of well developed natural bedforms

hydraulic and morphological characteristics of middle and upper reaches of the parana river argentina and brazil the parana river basin is a 2 600 000 km2 south american fluvial system most of which is in brazil and argentina the channel is about 4000 km long the mean annual discharge is 17 000 m3 s 1 and the annual sediment load is more than 150 000 000 tons the basin of the paraguay river its main tributary covers about 38 of the parana basin this paper compares the upper and middle channels of the parana hydraulic morphological and sedimentological features of the parana river upstream and downstream of the confluence with the paraguay were analyzed in order to evaluate the changes in these variables the effect of the paraguay on water and suspended sediment discharges was examined the paraguay river contribution explains most of the variation in channel conditions upstream and downstream of the confluence the channel pattern however remains braided the high bed load of the parana river in proportion to the total sediment discharge 21 in the upper course 25 in the middle course determines the similarity in drainage

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

a field study of turbulence and sediment dynamics over subaqueous dunes with flow separation this study examines flow turbulence and sand suspension over large dunes in canoe pass a distributary channel of the fraser river delta canada dune morphology is characterized by a symmetrical shape and steep leeside slopes over 30 velocity was measured with an electromagnetic current meter and suspended sand concentration with four optical backscatter obs probes the general patterns of time averaged velocity and sand suspension are consistent with previous studies including an increase in mean velocity and decrease in turbulence intensity and sand concentration with height above the bed reversed flow with high turbulence intensity and high sand concentrations in the leeside flow separation zone and an increase in near bed velocity and sand concentration along the stoss side of the dune frequency spectra of near bed velocity and obs records from leeside separation zones are composed of two distinct frequencies providing field confirmation of scale relations based on flume experiments the low frequency spectral signal probably results from wake flapping and the high frequency signal from vortex shedding the wake flapping frequency predominates outside the separation zone and is linked to turbulent structures that suspend sand predictions from a depth scale strouhal law show good agreement with measured wake flapping frequencies cross correlations of obs records reveal that turbulent sand suspension structures advect downstream at 23 25 above the horizontal these advection angles are similar to coherent flow structures measured in flumes and to sand suspension structures visualized over large dunes in the field

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

burst like sediment suspension events in a sand bed river in a much quoted paper jackson 1976 hypothesized that turbulent bursting motions such as those documented in laboratory boundary layers play a major role in alluvial sediment suspension to date the hypothesis remained largely untested due to difficulties in monitoring turbulent suspension in rivers this study provides field data documenting burst like turbulent motions over a sandy bed channel and quantifying the role of these motions in sand suspension the data were collected in a 10 m deep channel of the fraser river near mission british columbia canada turbulent fluctuations of both flow components downstream and normal to the bed along with the output of an optical suspended sediment sensor were monitored 1 m above the river bed typical flow velocities averaged 0 9 ms 1 at the sensors where mean suspended sediment concentrations were 500 mgl 1 decimetre height small dunes on the backs of larger half metre amplitude dunes covered the channel bed in the area brief but intense burst like ejection and inrush events were identified in the flow records where they are responsible for a high degree of intermittency in shear stress over the dunes 80 per cent of the turbulent momentum exchange across the 1 m level can be ascribed to such brief 3 8 s duration events active under 12 per cent of the time in addition the record of fluctuating sediment concentrations reveals these burst like motions to be highly effective in vertically mixing suspended sediment and thus ultimately in maintaining suspended sediment transport above the dune bed the bulk 60 and 90 per cent in two deployments of the vertical sediment mixing was accomplished by intense events active some 10 per cent of the time no discrete recurrence timescale for these burst like mixing events is evident however rather a continuous variation of return periods was observed as a function of the magnitude of vertical mixing event considered to that extent conceptual models of sediment transport in terms of burst events with a predictable recurrence such as proposed by jackson 1976 may be misleading

mean flow turbulence structure and bed form superimposition across the ripple dune transition 1 this paper describes a laboratory study of the dynamics of flow associated with three different stages of bed form amalgamation across the ripple dune transition measurements of flow velocity were obtained over simplified fixed bed forms designed to simulate conditions at the ripple dune transition using a 2d laser doppler anemometer this yielded information detailing the mean velocity field turbulence statistics local turbulence production local deviations from the mean pressure caused by dynamic effects turbulent kinetic energy and the contributions to the total reynolds stresses from different coherent turbulent events the results broadly confirm previous hypotheses that as bed form amalgamation proceeds across the ripple dune transition a superimposed bedstate can induce a series of critical changes to the flow structure with higher reynolds stresses being produced near and downstream of flow reattachment in particular quadrant 4 events turbulent flow structures with a downstream velocity greater than average and directed towards the bed dominate the vertical turbulent diffusion of longitudinal momentum in near bed regions close to the crest of the next downstream ripple thus providing the potential for increased erosion and sediment transport these experiments using simple fixed beds also provide support for recent measurements that document increased suspended sediment concentrations across the ripple dune transition schindler and robert 2004 2005 and that at the transition the largest bed forms are not necessarily those associated with the most intense shear layer activity schindler and robert 2005 bed form superimposition and amalgamation may thus significantly alter the mean and turbulent flow field as compared to bed forms without superimposition

spectral analysis of turbulent flow and suspended sediment transport over fixed dunes laboratory measurements of turbulent fluctuations in velocity and suspended sediment concentration were obtained synchronously over fixed two dimensional dunes in a sediment starved flow contour maps of turbulent flow parameters demonstrate that the flow separation cell and a perturbed shear layer are the main sources of turbulence production and that the distribution of suspended sediment is controlled by spatially dependent macro turbulent flow structures spectral analysis reveals that peak spectral energies generally occur at 1 2 hz for the stream wise velocity component and 2 4 hz for the cross stream and vertical velocity components spectra show larger and better defined energy peaks near the shear layer peak spectral energies for suspended sediment concentration occur near 1 hz throughout the flow squared coherency values for cospectral analysis of velocity and sediment concentration are insignificant integral timescales for velocity range from 0 20 s for the streamwise component to 0 06 s for the cross stream and vertical components integral length scales for velocity range from 0 065 to 0 135 m for the streamwise component which is comparable to flow depth and from 0 020 to 0 030 m for the crossstream and vertical components which is comparable to dune height for suspended sediment concentration integral timescales and length scales are similar to the streamwise velocity component

measuring velocity and shear stress over dunes with acoustic doppler profiler this study evaluates the use of an acoustic doppler profiler adp for measurement of velocity profiles over dunes in an estuary the adp can be deployed from a moving launch and provides three dimensional velocity profiles that can be used to describe flow structure and to provide rough estimates of shear stress quadratic stress models are recommended for shear stress because they estimate skin friction and form stress whereas the law of the wall only reliably estimates total stress over dunes the most serious limitation of the adp is a beam geometry that results in a large sampling diameter close to the bed that does not adequately measure velocity in dune troughs

turbulent flow over three dimensional dunes 2 fluid and bed stresses 1 dunes formed in response to fluid flow exert a total boundary stress on the fluid that is made up of form drag and skin friction the latter of which is generally considered important for predicting sediment transport and dune evolution previous research has used various methods to estimate the total stress and its subcomponents with recent work suggesting the application of a spatial average to the equations of motion a complete three dimensional 3 d form of this analysis is derived and applied to recent measurements of turbulent open channel flow over 3 d dunes spatial averages of reynolds shear stresses over 3 d dunes cannot be used to predict total boundary shear stress however estimations of total boundary shear stress from the spatially averaged equations of motion agree with not only the sum of form drag and skin friction but also the direct measurements of average free surface slope form induced stresses from secondary circulations augment the relatively low reynolds shear stresses over the 3 d dunes these low turbulence levels have ramifications for sediment transport predictions in that use of the total boundary shear stress in prediction of sediment transport over 3 d dunes must account for these low turbulence levels so as not to overpredict transport rates

export production of coccolithophores in an upwelling region results from san pedro basin southern california borderlands a seven month long time series sediment trap project was carried out in san pedro basin southern california borderlands in order to evaluate the response of calcareous nannoplankton to seasonal hydrographic changes this region is periodically influenced by upwelling particularly during the spring and early summer the highest fluxes of both whole coccospheres and individual coccoliths occurred during winter january february a period when the fluxes of diatoms and planktic foraminifera were low the highest coccolithophore fluxes were recorded in the mid february with 860 106 coccoliths m 2 day 1 8 106 whole coccospheres m 2 day 1 and 80 mg of coccolith carbonate m 2 day 1 coccolith carbonate fluxes in january and february account for most of the total carbonate fluxes measured during this period the season of maximum coccolithophore production in this region winter is correlated with weak stratification of the upper water column low total primary production low nutrient contents and low temperatures emiliania huxleyi and florisphaera profunda are the two most abundant species in this region while e huxleyi displays no distinct seasonal changes in flux f profunda shows a clear preference for cold low nutrient water conditions and low light levels helicosphaera spp flux is positively correlated to the total coccosphere fluxes and is indicative of high coccolithophore productivity

the morphodynamics of fluvial sand dunes in the river rhine near mainz germany ii hydrodynamics and sediment transport the dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply limited reach of the river rhine during daylong surveys suspended sediment concentration bedload transport rate water depth flow velocity turbulence intensity near bed shear stress and water temperature were recorded over individual isolated dunes this paper considers the hydrodynamic environment and sediment transport over the dunes a companion paper details the sedimentology and morphology of the dunes flow over the flat gravel lag upstream of large dunes is more uniform than that over dunes and gravels are rarely entrained by in bank discharges unsteady and non logarithmic velocity profiles are common within the boundary layer above the stoss side of large dunes and the near bed flow demonstrates evidence of large scale coherent low frequency flow structures these may reflect stacked sequences of separated boundary layers generated by secondary dunes however the low amplitude morphology of large dunes does not affect the statistical properties of turbulence production over the stoss sides bed roughness and near bed shear stress commonly increase steadily over the stoss of dunes but may decrease near the crest especially where a crestal platform exists that is devoid of secondary bedforms bed roughness scales with the physical size of bed roughness elements however variability in roughness lengths is large owing to the composite nature of bed roughness bedload transport over stoss slopes is spatially variable but often shows an increase with increasing bed shear stress over the stoss well formed wakes only develop downstream of lee slopes which are close to the angle of repose otherwise separation is weak and suspension and settling of fine sediments is of little consequence to dune evolution wake flow is characterized by turbulence production one order of magnitude greater than over the stoss side which may be related to vortex shedding from the dune however wake current speeds are extremely low and variable in direction reverse flow is not sustained and no retrogressive bedforms occur in the lee of large dunes near bed shear stress and bed roughness are usually low within the wake reflecting low current speeds weak wake flow reattachment occurs at a variable distance downstream up to several times the duneform height a two dimensional numerical model for flow over dune topography calibrated using average parameter values provided a reasonable description of the flow upstream and over the backs of large dunes as far as the crestal region wake flow could not be modelled however the temporal and spatial complexity of natural three dimensional flow over the dunes resulted in variance in parameter estimates this variance precluded modelling of flow and bedload transport consequently it was not possible to model dune evolution in a deterministic sense

field measurements of flow fields and sediment transport above mobile bed forms 1 field measurements of hydrodynamic conditions bed forms and suspended sediments obtained using a suite of approximately colocated sensors above a mobile bed composed of coarse sand d50 1 28 mm in a strong tidal current with superimposed waves are used to investigate the mean and turbulent flow fields bed forms and the resuspension and transport of sediments in suspension and as bed load while there is no evidence for flow separation properties of the local turbulent flow field are found to vary spatially in response to the bed morphology time averaged suspended sediment concentration profiles c profiles are found to be spatially variable with higher concentrations observed in the trough regions between bed form crests well known expressions used to predict c profiles are found to agree well with measured time averaged c profiles the instantaneous suspended sediment concentration field sscf is found to comprise a series of semiperiodic resuspension events interspersed by relatively quiescent periods using a modified quadrant analysis scheme these events are shown to be correlated closely with times when current and wave induced flows combine and exceed the threshold for suspension the effectiveness of a given combined wave current flow event in resuspending bed sediments is found to depend upon its location relative to bed morphology it is considered that local bed response may be influenced by spatial differences in grain size and compaction brought about by the processes of bed load transport over the bed forms the total mass of sediment in suspension is found to be less than the potential transport capacity of the flow suggesting strongly that the supply of fine sediments is source limited while measured bed form geometry agrees well with predicted dimensions for ripples grain size alone suggests only dunes should be present these were not detected bed load transport qb calculated from measured bed form migration speeds and the measured suspended sediment flux qs was found to agree favorably with empirical predictions

macroturbulence generated by dunes fraser river canada abstract a selectable frequency acoustic profiler and electromagnetic current meter were deployed in the lower fraser river canada to examine large turbulent structures features were generated on the lower stoss sides of dunes and appeared at the surface tens of metres downstream as strong upwellings boils they were identified with acoustic clouds in the acoustic profile event recurrence periods measured from acoustic time series and from ejection events identified in the current meter record had similar two component frequency distributions but the acoustic values lacked the highest frequencies a comparison of measured periods with predictions from the strouhal law indicate that both dune amplitude and the internal boundary layer developed over the dunes are significant length scales associated with the generation of the macroturbulence our data give no support to the notion that boils develop from microturbulent bursts