levee morphology and sedimentology along the lower tuross river south eastern australia levees on the lower tuross river in south eastern australia reflect a complex interplay between depositional and erosional processes stream power conditioned primarily by valley width is the key determinant of levee morphology and sedimentology in this confined valley setting three styles of levee are described the rewlee levee is functionally linked to a flood channel in narrow valley settings 250 m these levees contain a diverse facies assemblage characterized by various scales of erosion surfaces vertical accretion on levees has produced conditions under which stream power values exceed the threshold for catastrophic floodplain stripping the levee at the mortfield site is associated with less confined settings valley width 500 600 m which present lower flood stage and stream power conditions this levee hosts a wide range of facies but erosion surfaces are seldom observed in the more open valley setting at the central site valley width 700 1000 m levees comprise uniform fine grained deposits which grade to pronounced distal floodplains with backswamps as levees reflect a combination of within channel and overbank processes both depositional and erosional these geomorphic features influence the character and sedimentology of adjacent landforms and the associated alluvial architecture of the basin

entrainment and flushing time in the fraser river estuary and plume from a steady salt balance analysis 1 a passenger ferry equipped with oceanographic instrumentation made eight transects across the fraser river plume in the strait of georgia british columbia each day for 4 years estimates of average plume salinity and volume were made from the observations and used in a quasi steady budget to estimate the net upward flux of ambient salt water into the plume this entrainment flux increases from a minimum of 17 000 m3s 1 at low river flow to a maximum of about 27 000 m3s 1 at intermediate flow the entrainment flux then becomes essentially independent of river flow although we argue on physical grounds it must eventually decrease entrainment velocities of a few mm s 1 were estimated from the entrainment flux when assuming that mixing primarily occurs along the salt wedge estuary or in the near field plume these values are consistent with previous estimates this in turn suggests that entrainment is not important over most of the plume s full extent and that bulk plume properties are set in the near field finally the observations are used to estimate a plume freshwater flushing time of 2 2 days with essentially no dependence on river discharge even though discharge varies seasonally by almost an order of magnitude this value lies in between the expected time scales for near field and far field plumes and is long enough for rotation to play an important role there is no evidence however of a coherent bulge as observed in other systems

anatomy of the buried burdekin river channel across the great barrier reef shelf how does a major river operate on a tropical mixed siliciclastic carbonate margin during sea level lowstand abstract a large palaeochannel on the northeastern australian continental shelf has been imaged by a series of shallow seismic reflection profiles the buried channel forms an important pleistocene route of the burdekin river and extends almost continuously for 160 km from the present coast to the outermost reef the channel floor profile steps across the shelf with alternating segments of gentle gradient flats and steeper gradient ramps channel sinuosity as interpreted from seismic records varies among segments between 1 and 1 72 with no consistent relationship between sinuosity and gradient the lower and upper parts of the channel fill have different geometry and reflection character suggesting channel excavation and initial filling occurred during a different regime than final filling in one section of the shelf about the 50 m isobath the channel is difficult to define and appears to have wandered significantly either because it has been modified by shoreface erosion ca 10 5 ka or because the river encountered a change in topography in front of karstified reefs as the channel passes between the numerous outer shelf reefs in water depths of 70 80 m it becomes progressively smaller conspicuously underfilled and absent entirely over the outermost 10 km of the shelf no discrete lowstand river mouth could be recognised on the present shelf edge the elevations of flat segments on the channel floor profile show considerable similarity to published elevations of stillstands or brief rises in sea level attained during the long term drawdown associated with the last glacial cycle 125 20 ka and are interpreted to have formed during this stepwise drop in sea level channels were cut and partially filled during the fall and lowstand and then backfilled during the holocene transgression the ancestral channel of the burdekin river therefore preserves a rare insight into the stratigraphic record of falling sea level during the last glacial

turbulence transition in stratified atmospheric and oceanic shear flows reynolds and prandtl number controls upon the mechanism 1 we provide detailed analyses of the role of secondary instabilities in the transition to turbulence of stratified shear flows at the high reynolds and prandtl numbers appropriate to the atmosphere and ocean our focus is upon a series of recent observations suggestive of a marked change in the mechanism of transition in this laboratory inaccessible regime our results demonstrate the joint dependance of the transition mechanism on these two nondimensional parameters we discuss the primary transition mechanisms of shear aligned convective instability stagnation point instability and shear instabilities of the braid in detail

rheology of bimodal crystals suspensions results from analogue experiments and implications for magma ascent 1 the effect of crystal size of bimodal suspensions on rheology of magmas at strain rates between 0 and 1 s 1 is studied suspensions consist of silicon oil and two populations of natural crystals with size 63 125 and 250 500 m mixed in different proportions the total solid fraction of the mixtures is between 0 25 and 0 5 at 0 4 finer coarser and bimodal suspensions display comparable viscosities at 0 4 the viscosity of the bimodal suspensions is larger than that of the unimodal ones the bimodal suspension made mainly of finer crystals shows a stronger increase of viscosity with the addition of finer crystals to a suspension of coarser ones produces a more pronounced increase of viscosity with respect to suspensions of coarse or fine crystals alone and of finer crystals with added coarser ones the bimodal suspensions of coarser crystals develops yield stress at 0 25 the others at 0 4 it is modeled the ascent velocity in a 20 m wide dike of magmas with bimodal and unimodal populations of crystals of different size for 0 4 the crystal size has not effects on the ascent velocity of magmas for 0 4 the velocity of a magma with growing phenocrystals decreases as increases less than that of a magma with forming microlites and more of a magma with microlites and growing phenocrystals a magma with phenocrystals and forming microlites has the lowest ascent velocity

shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

the influence of crystal size distributions on the rheology of magmas new insights from analog experiments this study examines the influence of particle size distributions on the rheology of particle suspensions by using analogue experiments with spherical glass beads in silicone oil as magma equivalent the analyses of 274 individual particle bearing suspensions of varying modality uni bi tri and tetramodality as well as of polymodal suspensions with specific defined skewness and variance are the first data set of its kind and provide important insights in the relationship between the solid particles of a suspension and its rheological behaviour since the relationship between the rheology of particle bearing suspensions and its maximum packing fraction m is well established by several theoretical models the results of the analogue experiments of this study reveal that the polydispersity exerts the largest influence on m consequently the estimation of the polydispersity of a particle size distribution is essential for estimating the viscosity of that given suspension

viscosity of hydrous etna basalt implications for plinian style basaltic eruptions water dissolved in a silicate melt can strongly influence its physical properties and thus magma behavior during crystallization degassing foaming and fragmentation etna is a basaltic volcano whose activity is dominated by effusive eruptions which have long represented a threat to the densely populated surrounding area recently recognition of the products of a plinian eruption 122 b c has raised further issues for hazard assessment at etna and other basaltic volcanoes constraining the behavior of etna magma under conditions relevant to both effusive and explosive hazards requires viscosity data under conditions near the glass transition here we have investigated the viscosity of hydrous etna lava in order to better understand eruptive processes which characterize this volcano the experimental methods which have been used include piston cylinder synthesis of the hydrated melts micropenetration viscometry for low temperature viscosity measurements and near infrared spectroscopy for the evaluation of sample homogeneity and measurements of water content additionally scanning calorimetric determinations were performed to check whether incipient crystallization had occurred sample compositions were determined using electron microprobe analysis and 57fe mossbauer spectroscopy results from this study are compared with previous reports of trachytic phonolitic and model calc alkaline rhyolite hpg8 compositions the viscosity of the basaltic melt dry and wet has been parameterized in terms of temperature and water content via the non arrhenian equation log10 4 643 5 812 44 427 04 h2o t k 499 31 28 74 ln h2o where is the viscosity in pa s h2o is the water content in wt and t is the temperature in kelvin we observe that the viscosity of alkali basalt at more than 0 5 wt h2o is similar to that of an alkaline trachyte agnano monte spina eruption phlegrean fields and much higher than that of a peralkaline phonolite teide tenerife at similar silica contents and nbo t for water contents above 1 5 wt the viscosity of the basalt is similar to that of rhyolitic melts with similar water contents at temperatures ranging from 1 050 to 1 150 c and with water contents between 0 5 and 2 3 wt eruptive conditions the viscosities calculated using the equation defined in this study are 1 in reasonable agreement with those calculated using shaw s model and 2 much lower than those experimentally determined in a previous study however outside these temperature and water content ranges the agreement with shaw s model 1972 breaks down

rigid migration of unzen lava rather than flow abstract we have reassessed the mode of advance of lava erupted in the 1990s from unzen fugen dake volcano in northwestern kyushu japan the apparent viscosity of unzen lava had been estimated to be 0 9 4 2 1010 pa s from the observed lava surface velocity in 1991 using jeffreys model based on the assumption that the lava was a newtonian or bingham fluid in contrast the viscosity of synthetic groundmass glass derived from unzen lava had been measured in the laboratory at expected lava temperatures 780 c 880 c to be 1011 1013 pa s considerably higher than the viscosity based on field observation if the suspension effect of crystals is taken into consideration for groundmass glass several orders of magnitude higher viscosity than observed should be expected for unzen lava to seek the source of the disparity in viscosity estimates we first performed uniaxial compression viscometry on natural unzen lava samples collected in 1991 and thereby confirmed that the lava viscosity was actually higher than determined by field observations and equal to or higher than the fluid solid threshold viscosity 1012 pa s this suggests that the observational viscosity had been underestimated from reanalysis of the lava migration using video and photographs contemporaneous with the field based viscosity estimation we found that the lava behavior was far from that of a fluid apparently the lava lobe lost its fluidity at the flow front and instead advanced rigidly thereby indicating that the application of jeffreys model had incorrectly led us to underestimate viscosity although we used only the 1991 lava samples and image data we infer that rigid migration was common for all exogenous lava lobes during the 1991 1995 activity our comparative study demonstrates that cooled and or degassed lavas could show higher mobility than expected from experiment or model based viscosity apparently this occurs when the frictional force is smaller between lobe base and underlying slope than the viscous resistance inherent in lava

effects of crystal shape and size modality on magma rheology erupting magma often contains crystals over a wide range of sizes and shapes potentially affecting magma viscosity over many orders of magnitude a robust relation between viscosity and the modality of crystal sizes and shapes remains lacking principally because of the dimensional complexity and size of the governing parameter space we have performed a suite of shear viscosity measurements on liquid particle suspensions of dynamical similarity to crystal bearing magma our experiments encompass five suspension types each consisting of unique mixtures of two different particle sizes and shapes the experiments span two orthogonal subspaces of particle concentration as well as particle size and shape for each suspension type thereby providing insight into the topology of parameter space for each suspension type we determined the dry maximum packing fraction and measured shear rates across a range of applied shear stresses the results were fitted using a herschel bulkley model and augment existing predictive capabilities we demonstrate that our results are consistent with previous work including friction based constitutive laws for granular materials we conclude that predictions for ascent rates of crystal rich magmas must take the shear rate dependence of viscosity into account shear rate dependence depends first and foremost on the volume fraction of crystals relative to the maximum packing fraction which in turn depends on crystal size and shape distribution

temporal and spatial variability of vertical salt flux in a highly stratified estuary 1 the temporal and spatial variability of vertical salt flux in the fraser river estuary british columbia was investigated observationally using several different direct and indirect indicators of buoyancy flux data were collected from the estuary using shipboard instrumentation primarily an acoustic doppler current profiler and a towed conductivity temperature depth unit direct estimates of buoyancy flux were made from along channel control volume analyses and from measurements of overturn scales the spatial and temporal evolution of the salt wedge structure through a tidal cycle was evaluated using the results of these buoyancy flux calculations as well as gradient richardson number froude number and stratification profiles observations indicate that buoyancy flux is driven primarily by interfacial stress and not bottom stress as is common in many estuaries vertical salt flux as opposed to seaward advection of high salinity fluid was found to be the dominant mechanism responsible for removal of salt from the estuarine channel during each daily tidal cycle buoyancy flux was highly variable in time and space however with vertical salt flux during ebb tides on the order of 2 to 3 times greater than that estimated during floods this is due to an increase in the vertical shear of horizontal velocity and a sharp increase in stratification which was observed during early ebb enhanced mixing was observed spatially within a region dominated by a channel constriction in which the channel narrows by approximately 25

mixing by shear instability at high reynolds number 1 shear instability is the dominant mechanism for converting fluid motion to mixing in the stratified ocean and atmosphere the transition to turbulence has been well characterized in laboratory settings and numerical simulations at moderate reynolds number it involves rolling up i e overturning of the density structure within the cores of the instabilities in contrast measurements in an energetic estuarine shear zone reveal that the mixing induced by shear instability at high reynolds number does not primarily occur by overturning in the cores rather it results from secondary shear instabilities within the zones of intensified shear separating the cores this regime is not likely to be observed in the relatively low reynolds number flows of the laboratory or in direct numerical simulations but it is likely a common occurrence in the ocean and atmosphere

flow and sediment suspension events over low angle dunes fraser estuary canada 1 the morphodynamics of large sand bedded rivers and estuaries are ultimately controlled by the way bed material is moved and the development of large subaqueous sand dunes that control hydraulic flow resistance it is widely thought that the primary mechanism for moving sandy bed material in these channels is large scale coherent flow structures that cause suspension events whose properties vary with flow especially in tidally influenced environments here we examine mean flow and sediment suspension events over low angle dunes lee face angle 30 in the unsteady flow of the fraser estuary canada at high tide flow nearly ceased and a salt wedge entered the channel forcing salt water under the downstream moving fresh water the salt wedge persisted in the channel until late in the falling tide causing stratification in the water column and instabilities along the saline fresh water interface at low tide mean velocities peaked and forced the saline water out of the channel flow over the low angle dunes displayed topographically induced patterns previously observed over high angle dunes but permanent flow separation was not observed large scale sediment suspension events dominated sediment flux during low tide and became larger scale yet less frequent as the tide began to rise the suspension events appeared to form over the lower stoss of the dunes and grew up over the bed forms and less commonly emerged downstream of the crest suspension events move 69 of the total sediment in the flow above low angle dunes when they are present

laboratory investigation of the impact of lateral spreading on buoyancy flux in a river plume abstractthe relationship between lateral spreading and mixing in stratified gravity currents is investigated by comparing laterally confined and unconfined currents in a series of laboratory experiments the vertical turbulent buoyancy flux is determined using a control volume approach with velocity and density fields derived from combined particle image velocimetry piv and planar laser induced fluorescence plif lateral spreading is determined in the unconfined experiments based on plan view imaging using the optical thickness method otm the authors find that lateral spreading dramatically modifies the plume structure the spreading plume layer consists of approximately linear density and velocity profiles that extend to the surface whereas the channelized plume profiles are uniform near the surface lateral spreading decreases the average plume density relative to laterally confined currents with similar inflow conditions however the local turbulent buoyancy flux in the spreading experiments i

ultrafine spherical quartz formation during seismic fault slip natural and experimental evidence and its implications abstract in recent works on the determination of pseudotachylyte within the principal slip zone psz of the chelungpu fault taiwan we demonstrated that frictional melting occurred at shallow depths during the 1999 mw 7 6 chi chi earthquake thus the characteristics of melts are of paramount importance to investigate processes controlling dynamic fault mechanics during seismic slips we conducted rock friction experiments on siltstone recovered from the taiwan chelungpu fault drilling project tcdp at a slip rate of 1 3 m s and a normal stress of 1 mpa here we not only target to characterize experimental pseudotachylyte and evaluate the associated frictional behavior but also compare it with natural frictional melts of tcdp our results show that 1 initial shear stress drop was related to the generation of low viscosity melt patches 2 the evolution of shear stress in the postmelting regime was congruent with frictional melt rheology and 3 the slip strengthening presumably resulted from the extremely low content of water of the frictional melt in particular the state of art of in situ synchrotron analyses x ray diffraction and transmission x ray microscope determine the presence of ultrafine spherical quartz usq grains 10 nm to 50 nm in the glassy matrices presumably produced at high temperature our observations confirm that the usq grains formed in rock friction experiments do occur in natural faults we surmise the usq is the result of frictional melting on siltstone and represents the latest slip zones of the chelungpu fault and further infer that the viscous melts may terminate seismic slips at shallow crustal conditions

variations in character and preservation potential of vegetation induced obstacle marks in the variable discharge burdekin river of north queensland australia abstract vegetation induced obstacle marks are described from dalrymple bend in the burdekin river of queensland australia and their preservation potential is discussed the bend is divided into a sand and gravel covered lower bar and a vegetated upper bar obstacle marks comprising an erosional scour and a depositional sediment tail are recognised on the lower bar and the lower margin of the upper bar two types of obstacle marks are dominant in the lower part of the lower bar sediment tail dominated obstacle marks associated with inclined trees of melaleuca argentea silver leaved paperbark and comparatively small obstacle marks around stranded driftwood obstacle marks in the upper part of the lower bar are formed around grass clumps armoured with mud and gravel at the lower margin of the upper bar scours and gravelly sediment tails are recognised around mature tall trees in these cases the scour is 4 m in width and covered with alternating beds of fine sand and mud among the obstacle marks described sediment tail dominated obstacle marks around groves of m argentea have the highest preservation potential obstacle marks in the lower margin of the upper bar are also considered to have high preservation potential obstacle marks around stranded driftwood have the lowest preservation potential obstacle marks around grass clumps survive more than 1 year but their long term preservation potential is nonetheless low implications for the stratigraphic record are also discussed

a relationship between alluvial backswamps and avulsion cycles an example from the willwood formation of the bighorn basin wyoming abstract stratigraphically limited intervals from the lower eocene willwood formation contain laterally extensive carbonaceous shales and ribbon sandstone networks associated with channel avulsion we present data from one such interval that documents the avulsion sequence vertical sections measured along the outcrop of this interval are similar and comprise a basal carbonaceous shale overlain by fine grained deposits on which weakly developed hydromorphic paleosols formed the paleosols enclose and are locally incised by ribbon sandstones some of which cut down to and partly through the carbonaceous shale the ribbons have width thickness ratios between 3 and 13 some ribbons cluster at a particular stratigraphic level which together with paleocurrent trends suggests that they formed channel networks sections are capped by yellow brown paleosols showing moderate pedogenic development we suggest that the carbonaceous shales developed in low lying topogeneous swamps in distal portions of the floodplain far from the trunk channel such a location set limits on the sediment that they received the mudrocks with weakly developed paleosols and associated ribbon sandstones are interpreted as crevasse splay complexes resulting from avulsion of the trunk river the ribbon sandstones represent ancient feeder channels of the avulsion complex the rapid influx of avulsion deposits appears to have been crucial to preserving the organic material and this study reveals an important and as yet uncharacterized link between trunk channel processes and the accumulation of organic rich deposits in distal alluvial swamps similar deposits are found in other stratigraphic units in the rocky mountain region and the development of these and other organic rich deposits should be reassessed in terms of channel avulsion

mixing layer dynamics in separated flow over an estuarine sill with variable stratification 1 we investigate the generation of a mixing layer in the separated flow behind an estuarine sill height h 4 m in the snohomish river washington as part of a larger investigation of coherent structures using remote and in situ sensing during increasing ebb flows the depth d and stratification decrease and a region of sheared flow characterized by elevated production of turbulent kinetic energy develops profiles of velocity and acoustic backscatter exhibit coherent fluctuations of order 0 1 hz and are used to define the boundaries of the mixing layer variations in the mixing layer width and its embedded coherent structures are caused by changes to both the normalized sill height h d and to a bulk richardson number rih defined using the depth of flow over the sill entrainment et and the mixing layer expansion angle increase as stratification and the bulk richardson number decrease this relationship is parameterized as et 0 07rih 0 5 and is valid for approximately 0 1 0 8 due to blocking of flow by the sill

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

the spatial variability of overbank sedimentation on river floodplains abstract river floodplains are important landforms and information on rates and patterns of overbank sediment deposition is essential for understanding their development such information is also needed for validating existing overbank floodplain flow and sediment transport and deposition models the use of bomb derived 137cs for documenting rates of overbank sediment deposition on river floodplains offers the potential for assembling data for a large number of points on a floodplain and therefore for investigating the spatial patterns involved this approach also overcomes some of the operational and sampling problems encountered in traditional monitoring techniques the study reported uses 137cs measurements on sediment cores to investigate the spatial variability of sediment deposition on selected floodplain reaches of five british lowland rivers attention is given both to the overall reach and to more detailed investigations of small areas the observed spatial patterns of sediment deposition are interpreted in terms of the major controls involved which include the floodplain microtopography and morphology the relationship between the grain size composition of deposited sediment and sedimentation rate is also considered

buoyant gravity currents released from tropical instability waves abstracttwo extremely sharp fronts with changes in sea surface temperature 0 4 c over lateral distances of 1 m were observed in the equatorial pacific at 0 140 w and at 0 75 n 110 w in both c

detailed observations of a naturally occurring shear instability simultaneous profiles of microstructure horizontal velocity and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date shear increased rapidly after passing through a lateral constriction which formed a hydraulic control a kilometer long set of 20 m tall billows grew on a middepth density interface where the richardson number fell below 0 25 the velocity interface thickened steadily after the billows formed consistent with rapid momentum mixing across a shear layer with a reynolds number of 3 106 the billows generated large density overturns and dissipation rates greater than 10 5 w kg 1 even within the first large overturn indicating that these structures were fully turbulent early in their development as the billows grew a well mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods 3 times longer than in laboratory studies at low reynolds number variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfaces varied with time where the billows first formed with data from the large overturns within the shear layer we find e n2 3 104 an average root mean square overturn scale lrms of 2 6 m and a buoyancy scale lb of 2 7 m despite having sampled the billows at varying stages of their evolution we find no indication that the ratio lrms lb is ever significantly different than 1 for this shear instability

crystal fabric evolution in lava flows results from numerical simulations abstract the analysis of the preferred orientation of crystals crystal fabric in magmatic rocks has become a widely used technique for the reconstruction of the flow history however little is known about the evolution of the fabric during flow here numerical simulations are used to study the fabric evolution of low concentration laminar magmatic flows e g lava flows the fabric evolution of a particle populations with a specified shape cube tablet prism and transitional shape and b crystal populations from a lava flow is analyzed in different flow geometries simple shear hyperbolic and pure shear flows assuming plane strain results show that fabric analysis of the whole crystal population gives little information about flow kinematics whereas the comparative analyses of crystal sub populations with different shapes allow us to recognize the flow geometry simple shear flow produces oscillating to pseudo stable fabrics the fabric strength is lower with respect to that of hyperbolic and pure shear flows and the preferred orientation of crystals does not coincide with the flow direction except for large strain and specified shapes sub fabrics with opposite sense of shear may also develop depending on the crystal shape and finite strain pure shear and hyperbolic flows show stable to pseudo stable fabrics the preferred crystal orientation may or may not coincide with the flow direction according to whether flow is in pure or hyperbolic shear results from numerical simulations are comparable with those from experimental models and natural examples the fabric strength depends on the number of crystals and caution must be used in extrapolating the results beyond the scale of observation the finite strain in a sample from a lava flow from the aeolian islands is determined by the comparative analysis of the calculated and measured fabric parameters fabric intensity and crystal preferred orientation criteria to discriminate among fabrics produced by different flow types are also provided

propagation speeds of strongly nonlinear near surface internal waves in the strait of georgia 1 a novel aerial observational method for studying internal features in the coastal ocean is developed and tested in a study of large nonlinear internal solitary like waves photogrammetrically rectified oblique photo images from a circling aircraft are used to track a number of internal wave packets for periods of up to one hour in the strait of georgia british columbia canada combining these sequences with coincident water column data allows us to obtain a more complete view of the spatial structure of internal waves highly accurate measurements of wave propagation speeds and directions are possible the applicability of various weakly nonlinear theories in modeling propagation of the observed large amplitude waves is tested the measured wave speeds enable us to differentiate between classic internal wave models the linear kdv korteweg de vries and bo benjamin ono models are applied with and without background shear after background shear effects are included it is found that a continuously stratified bo equation can predict propagation speeds within observational error and that this is not true for other theories the technique may be useful in future studies of oblique internal wave interactions

observation and analysis of shear instability in the fraser river estuary 1 we investigate the occurrence of shear instability in the fraser river estuary instabilities observed with an echo sounder are compared with a linear stability analysis based on observed velocity and density profiles we find that each set of observed instabilities coincides with an unstable mode predicted by the taylor goldstein equation each of these instabilities occurs in a region where the gradient richardson number is less than the critical value of 1 4 both the taylor goldstein predictions and the echo soundings indicate the instabilities are concentrated either above or below the density interface this one sidedness is in contrast to the archetypal kelvin helmholtz instability although the dominant source of mixing in the estuary appears to be caused by shear instability when the tide produces strong near bed velocities small scale overturning due to boundary layer turbulence is apparent throughout the depth

using depth normalized coordinates to examine mass transport residual circulation in estuaries with large tidal amplitude relative to the mean depth abstractresidual subtidal circulation profiles in estuaries with a large tidal amplitude to depth ratio often are quite complex and do not resemble the traditional estuarine gravitational circulation profile this paper describes how a depth normalized coordinate system allows for a more physical interpretation of residual circulation profiles than does a fixed vertical coordinate system in an estuary with a tidal amplitude comparable to the mean depth depth normalized coordinates permit the approximation of lagrangian residuals performance of empirical orthogonal function eof analysis estimation of terms in the along stream momentum equations throughout depth and computation of a tidally averaged momentum balance the residual mass transport velocity has an enhanced two layer exchange flow relative to an eulerian mean because of the stokes wave transport velocity directed upstream at all depths while the observed coordinate profiles resemble gravitational circulation and pressure and fricti

depositional controls in a bedload dominated fluvial system internal architecture of the lee formation kentucky abstract sandstones of the early pennsylvanian lee formation central appalachian foreland basin were deposited in a low sinuosity bedload dominated fluvial system the internal architecture of the sandstones consists of a six fold hierarchy of bounding surfaces which are evaluated with respect to depositional controls minor first second and third order surfaces generally less than a few hundred meters in extent separate deposits of small to intermediate scale bedforms third order surfaces which dip gently in the downflow direction represent reactivation surfaces of downstream accreting macroforms major surfaces fourth fifth and sixth order are hundreds of meters to kilometers in extent fourth order surfaces separate deposits of individual macroforms fifth order bounding surfaces are erosional boundaries of major system scale channels individual sandstone members of the lee formation are bound by sixth order surfaces erosional at the base and gradational at the top deposits overlying major surfaces contain fining upward sequences the trends are more apparent with increased scale of the deposit these sequences represent responses of the fluvial system to various mechanisms that controlled sedimentation both fourth and fifth order surfaces are interpreted as results of autocyclic controls the nature of the lee formation fluvial system was such that a solitary major channel was relatively stable in one position within the alluvial plain for a significant period of time during this time the channel aggraded with the passage of one or more downstream accreting macroforms creating a fourth order erosional surface the channel eventually diverted its flow to another portion of the alluvial plain by avulsion creating a fifth order surface a gross fining upward trend and upward change in fluvial style for individual sandstone members both characteristic of paleovalley fills indicate that sixth order surfaces were controlled by allocyclic processes these characteristics and the stair step positioning of the sandstone members younger away from the alleghenian mountains to the east suggest that tectonic processes were the ultimate control on the system

the rheology of crystal bearing basaltic magmas from stromboli and etna abstract our ability to forecast volcanic hazards at active volcanoes stems from our knowledge of parameters affecting eruption dynamics persistent activity displayed by basaltic volcanic systems such as etna and stromboli is governed mainly by the textural evolution and rheological properties of eruptive products here we investigate the high temperature 1131 c

an expanded model and application of the combined effect of crystal size distribution and crystal shape on the relative viscosity of magmas abstract this study examines the combined effect of crystal size distributions csd and crystal shape on the rheology of vesicle free magmatic suspensions and provides the first practical application of an empirical model to estimate the relative effect of crystal content and csd s on the viscosity of magma directly from textural image analysis of natural rock samples in the form of a user friendly texture rheology spreadsheet calculator we extend and apply established relationships between the maximum packing fraction m of a crystal bearing suspension and both its rheological properties and the polydispersity of a csd by using analogue rotational rheometric experiments with glass fibres and glass flakes in silicone oil acting as magma equivalent this study also provides new insights in the relationship between m and the aspect ratio rp of suspended particles

textural analysis and modelling of a flood deposit river severn u k samples of sediment collected from the severn floodplain between worcester and gloucester following the severe flooding in january and february 1990 were analysed for their grain size distribution the results show that most sand was deposited within 20 m of the channel bank but that fine sand may contribute to flood sediment across the width of the floodplain james 1985 numerical model of overbank sedimentation attempts to predict the transfer of sediment to the floodplain during flooding geometrical and hydraulic data relating to the severn flood are used as input for a computer program of james 1985 model the pattern of sediment concentrations predicted by the model was compared with that obtained from statistical analysis of the flood sediment the patterns were found to be similar so james 1985 model was considered to predict in a relative sense the distribution of flood sediment

the flow of atlantic water through the strait of gibraltar abstract we describe and analyze observations of the water exchange through the strait of gibraltar the primary observations were taken in april 1986 and included data from moored recording instruments at four locations in the strait together with an intensive survey by ship these measurements included extensive ctd profiling acoustic doppler current profiling numerous profiles from expendable instruments and imaging using a high frequency echo sounder the analysis focuses on the internal hydraulics of the strait and in particular the presence of hydraulic controls and their influence on the exchange our observations during april 1986 show that the maximal exchange condition in which a subcritical flow is bounded by supercritical flow at both ends of the strait did occur although with various subtleties not explicitly incorporated in our previous theoretical developments the atlantic water moving east along the surface encounters a control at tarifa narrows in the eastern part of the strait the mediterranean water passes through controls both at camarinal sill and at spartel sill further west at our westernmost mooring at spartel the outflowing mediterranean water is continuously supercritical with little tidal variability the control at camarinal sill is periodically lost due to tidal action and reappears on a reverse flow during a falling spring tide tangier basin bounded by camarinal and spartel sills acts as an internal reservoir for the outflowing mediterranean water the interface rising and falling through each tidal cycle during spring tides the tangier basin interface rises high enough to flood the control at camarinl sill and produces a reverse flow of the lower layer when control is lost at camarinal sill a travelling bore is released this bore may modify the location of the control acting on the surface layer in tarifa narrows the atlantic water enters the alboran sea as a jet which may have a well defined northern boundary where it separates from the european coast this boundary may be identified by the presence of a slick although the internal jump west of camarinal sill is almost certainly an area of intense mixing frictional effects do not appear to be dominant in the subcritical portion of the flow

labyrinthine facies architecture of the tortola fluvial system and controls on deposition late oligocene early miocene loranca basin spain abstract upper oligocene to lower miocene deposits of the tortola fluvial system loranca basin central spain display a labyrinthine stratigraphic architecture comprising isolated sandstone bodies embedded in mudstone and siltstone these sandstone bodies are subdivided into seven types which are interpreted to have formed in rivers with different discharges sediment yields and channel planform patterns on the basis of their characteristics and spatial distributions four facies zones are defined these formed a continuum along the tortola fluvial system profile each being characterized by specific depositional conditions and consequently channel pattern styles gravel rich braided streams dominated zone 1 down gradient these streams graded into wide sand dominated braided streams of zone 2 which contain large bedforms and were typified by significant discharge variations zone 3 comprises belts of mixed load dominated meandering and low sinuosity channels zone 4 represents distal floodplain areas at the toe of the fluvial system depositional conditions in a facies zone varied between an upper and lower bound or geomorphic threshold a crossing of the upper or lower threshold is indicated by relatively sudden but orderly lateral and vertical changes in channel pattern that in turn reflect superposition of facies zones a facies cycle is formed by superposition of two successive downstream facies zones superposition of facies zones is inferred to have been caused by changes in sediment supply and hydraulic regime and consequently the regional gradient of the system it is argued that the tortola fluvial system developed under a weak tectonic regime during the later stages of basin filling and was possibly affected by climatic changes changes in size of the facies zones typified the long term development of the tortola fluvial system and determined the three dimensional architecture of the succession

evolution and mixing of asymmetric holmboe instabilities natural sciences and engineering research council of canada canada research chairs program national science foundation usa oce 0221057

viscosity measurements of subliquidus magmas alkali olivine basalt from the higashi matsuura district southwest japan abstract we carried out viscosity measurements and sampling of a crystal suspension derived from alkali olivine basalt from the matsuura district sw japan at subliquidus temperatures from 1230 c to 1140 c under 1 atm with nno oxygen buffered conditions viscosity increased from 31 to 1235 pa s with a decrease in temperature from 1230 to 1140 c on cooling olivine first appeared at 1210 c followed by plagioclase at 1170 c the crystal content of the sample attained 31 vol at 1140 c plagioclase 22 olivine 9 non newtonian behaviors including thixotropy and shear thinning were pronounced in the presence of tabular plagioclase crystals the cause of such behavior is discussed in relation to shear induced changes in melt crystal textures relative viscosities r s m where s and m are the viscosities of the suspension and the melt respectively were obtained by calculating melt viscosities from the melt composition and temperature at 1 atm using the equation proposed by giordano and dingwell giordano d dingwell d b 2003 non arrhenian multicomponent melt viscosity a model earth and planetary science letters 208 337 349 the obtained relative viscosities are generally consistent with the einstein roscoe relation which represents r for suspensions that contain equant and equigranular crystals even though the crystal suspension analyzed in the present experiments contained tabular plagioclase and granular olivine of various grain sizes this consistency is attributed to the fact that the effect of crystal shape was counterbalanced by the effect of the dispersion of crystal size the applicability of the einstein roscoe equation with respect to crystal shape is discussed on the basis of the present experimental results our experiments and those of sato sato h 2005 viscosity measurement of subliquidus magmas 1707 basalt of fuji volcano journal of mineralogical and petrological sciences 100 133 142 show that the relationship between relative viscosity and crystal fraction is consistent with the einstein roscoe relationship for axial ratios that are smaller than the critical value of 4 6 5 but discrepancies occur for higher ratios

the yield strength of subliquidus basalts experimental results yield strength is an important property of particle fluid suspensions in basaltic lavas that crystallize during flow emplacement the onset of yield strength may result in threshold transitions in flow behavior and flow surface morphology however yield strength crystallinity relations are poorly known particularly in geologic suspensions where difficulties of experimental and field measurements have limited data acquisition in the subliquidus temperature range here we describe two complementary experimental approaches designed to examine the effect of particle shape on the low shear yield strength of subliquidus basalts the first involves melting cubes of holocrystalline basalt samples with different initial textures to determine the temperature crystallinity at which these samples lose their cubic form these experiments provide information on the minimum crystal volume fractions 0 20 0 35 required to maintain the structual integrity of the cube the second set of experiments uses suspensions of corn syrup and neutrally buoyant particles to isolate the effect of particle shape on yield strength development from these experiments we conclude that the shape is important in determining the volume fraction range over which suspensions exhibit a finite yield strength as anisotropic particles may orient during flow the effect of particle shape will be controlled by the orientation distribution of the constituent particles we find that the so called excluded volume can be used to relate results of experiments on anisotropic particles to those of suspensions of spherical particles recent measurements of yield strength onset in basaltic melts at crystal volume fractions near 0 25 are consistent with our observations that crystal frameworks develop at low to moderate crystal volume fractions when crystals are anisotropic e g plagioclase we further suggest that conditions leading to yield strength onset at low crystallinities include rapid cooling increased crystal anisotropy heterogeneous nucleation which promotes extensive crystal clustering and large cluster anisotropy and static conditions random crystal orientations

microstructure observations of turbulent mixing in a partially mixed estuary part i dissipation rate variations of turbulent mixing in the water column and in the benthic boundary layer were observed with a microstructure profiler in the hudson river estuary during two cruises in summer and fall of 1995 variability patterns of stratification shear richardson number ri and turbulent dissipation rates e were similar to those of earlier observations with strong turbulence in the weakly stratified bottom layer weak turbulence in the halocline during neap tides and low ri and high e spanning the water column during spring ebbs depthintegrated turbulent dissipation rates re dz approximately equaled the work done by the tidal pressure gradient force adjusted for the change in tidal kinetic energy alternatively a suitable scaling for re dz is also provided by the product of bottom shear stress and depth average velocity t by a relationship that fails at slack tide however at heights above bottom of z 0 3 m and again excepting slack tides observed e were highly correlated with a law of the wall dissipation rate eb 5 kz ratios e eb were close to 1 here the friction 3 u

gravel antidunes in the tropical burdekin river queensland australia the geological record is punctuated by the deposits of extreme event phenomena the identification and interpretation of which are hindered by a lack of data on contemporary examples it is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle transporting high velocity currents and therefore their characteristics are poorly documented the deposits of such flows are exposed however in the dry bed of the burdekin river queensland australia following tropical cyclone induced floods long wave length up to 19 m gravel antidunes develop during short days high discharge flows in the upper burdekin river maximum recorded discharge near the study reach over 25 600 m s in february 1927 flood water levels fall quickly metres in a day and flow is diverted away from raised areas of the river bed into subchannels exposing many of the high stage bedforms with little reworking by falling stage currents gravel bedforms were observed on the dry river bed after the moderate flows of february 1994 max 7700 m s and january 1996 max 3200 m s the bedforms had wave lengths in the range 8 19 m amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak discharge flow direction the gravel fabric and size sorting change systematically up the stoss and down the lee faces the antidune deposits form erosive based lenses of sandy gravel with low angle downstream dipping lamination and generally steep upstream dipping a b planes the internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses the erosive based lenses of low angle cross bedded gravel with steep upstream dipping a b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes the antidune gravel lenses are associated with thick to 1 m high angle cross bed sets ancient antidune gravel lenses may be diagnostic of episodic high discharge conditions and particularly when they are associated with high angle cross bedded gravelly sand they may be useful for palaeoenvironmental interpretation

plant material deposition in the tropical burdekin river australia implications for ancient fluvial sediments abstract in the deposits of the burdekin river north queensland australia plant material character and directional fabric together with the sedimentary facies character are generally diagnostic of depositional environment plant material is selectively entrained transported and deposited in all flow conditions ranging from dry season minimal flow

coarse grained floodplain deposits in the seasonal tropics towards a better facies model abstract floodplain deposits in semiarid and subhumid settings with erratic rainfall may contain relatively high proportions of sand because of climatically controlled patterns of discharge and vegetation cover ancient formations with abundant floodplain sands may record fluvial systems with extreme discharge variability of this type but are difficult to interpret because of the paucity of published modern analogues the lower burdekin and haughton river alluvial and delta plains of north queensland australia are used here to illustrate this type of fluvial architecture the modern burdekin and haughton channels have gravelly sand beds and transport a mixed load in the wet season the channels have low relief levees that decrease in amplitude downstream across the low gradient floodplains tropical cyclone activity results in high magnitude short duration sediment charged floods that inundate large areas of the coastal lowland bankfull discharge is infrequently exceeded return interval 25 years because the channels are enlarged by major cyclone induced runoff events and between these events flows are too small to modify the channel form significantly suspended sediment loads and the proportions of these that are sand are particularly high when intense rainfall occurs after prolonged drought when vegetation cover is reduced as a consequence of these factors most of the deposition on the levees and floodbasins occurs in very large discharge events with significant suspended sand load these floods carry a lot of sand out of the channel and efficiently in suspension over the floodplains because of deep inundation boreholes drilled in these floodplains encountered sharply bounded and gradationally bounded sand beds that fine upward or show no vertical grain size trend these are interbedded with thin silt clay and minor gravel beds holocene paleochannels make up about 30 of the floodplain area and consist predominantly of coarse sand that is similar to modern channel bed deposits the overbank sand in contrast is predominantly fine medium grained and beds have distinctly different granulometry from each other and from channel floor deposits efficient sediment partitioning in the runoff and extreme variation in flood magnitude explain variations in sand bed granulometry in the alluvial valley and upper delta plain individual sand beds are difficult to correlate between adjacent boreholes even over very short distances tens of meters in contrast on the lower delta plain overbank sands have a more sheet like geometry this is explained by the lower lying landscape negligible levee development greater inundation depths and reduced surface roughness in addition the channel bodies are thinner and wider and may be connected to overbank sand wings

particle size density relationships in pyroclastic deposits inferences for emplacement processes in moving pyroclastic systems particles are sorted as a function of their sizes densities and shapes the analysis of the distribution of these characteristics in particle populations of pyroclastic deposits is a major tool in evaluating properties and regimes of parent transport systems this paper reports a study of size density relationships of component subpopulations from selected quaternary pyroclastic deposits of the latera volcanic complex vulsini italy by means of integrated field and laboratory investigations including grain size and component analyses density measurements and sem crystal morphoscopy three kinds of particle size density relationships characterize the studied deposits as follows 1 a significant correlation of decreasing sizes with increasing component densities exists for direct suspension sedimentation products including plinian fall and pyroclastic surge deposits 2 a wide data dispersion with poor correlation normally occurs for the main zones of deposits from high concentration pyroclastic flows and 3 component subpopulations occur in a narrow size range irrespective of density in granular systems dominated by grain collisions such as traction zones beneath low concentration pyroclastic surges and sheared basal layers of pyroclastic flows the regression coefficients of size density relationships appear as a comparative measure of particle concentration in the parent systems consistent with that inferred from the degree of sorting and abrasion of crystal subpopulations from sequential fragmentation transport analysis of grain size data and quantitative morphoscopy the crystal component becomes less sorted and more abraded as the fit to the size density relationship decreases from plinian fall to pyroclastic surge to pyroclastic flow consistent with increasing particle concentration in the transporting current

effect of glass on the frictional behavior of basalts at seismic slip rates we performed 31 friction experiments on glassy basalts gb and glass free basalts gfb at slip rates up to 6 5 m s 1 and normal stress up to 40 mpa seismic conditions frictional weakening was associated to bulk frictional melting and lubrication the weakening distance dw was about 3 times shorter in gb than in gfb but the steady state friction was systematically higher in gb than in gfb the shorter dw in gb may be explained by the thermal softening occurring at the glass transition temperature tg 500 c which is lower than the bulk melting temperature tm 1250 c of gfb postexperiment microanalyses suggest that the larger crystal fraction measured in gb melts results in the higher steady state friction value compared to the gfb melts the effect of interstitial glass is to facilitate frictional instability and rupture propagation in gb with respect to gfb

tide induced variation of the dynamics of a salt wedge estuary abstract the time variations of the salt wedge intrusion in the fraser estuary british columbia were monitored during a variety of tidal and run off conditions using instruments and sampling methods that provided high resolution of the velocity and water properties in space and time the salt wedge was found to vary considerably in position and vertical structure through the tidal cycle due to the interaction of the tidal flow with the density driven motion of the salt wedge during the flood the salt wedge progressed up the estuary as a gravity current while during the ebb the salinity structure was eroded by shear instability the difference in character of the flow between flood and ebb is attributed to the transition between a subcritical and supercritical internal froude number during flood tide the internal state is subcritical and the estuarine shear flow is maintained with a small amount of vertical mixing while in the supercritical ebb flow vertical shears become so large that the pycnoc

the effect of particle size on the rheology of liquid solid mixtures with application to lava flows results from analogue experiments 1 we investigate the effect of crystal size on the rheology of basaltic magmas by means of a rheometer and suspensions of silicon oil with natural magmatic crystals of variable size from 63 to 0 5 mm and volume fraction from 0 03 to 0 6 at constant finer suspensions display higher viscosities than coarser ones shear thinning flow index n 0 1 0 2 and is more pronounced stronger departure from the newtonian behavior in finer suspensions maximum packing and average crystal size displays a nonlinear positive correlation while yield stress develops at 0 2 0 3 irrespective of the crystal size we incorporate our results into physical models for flow of lava and show that with respect to lava flows containing coarser crystals those with smaller crystals are expected to 1 flow at lower velocity 2 have a lower velocity gradient and 3 be more prone to develop a region of plug flow our experimental results explain the observation that phenocryst bearing and microlite bearing lavas at etna volcano italy show smooth pahoehoe and rough aa surfaces respectively

sedimentology of the upper burdekin river of north queensland australia an example of a tropical variable discharge river the burdekin river is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge in response to seasonal but erratic monsoonal rainfall floods of the burdekin rise abruptly reaching peak discharges of up to 40 000 m s in less than 24 h maintain peak flow for up to a few days and recede exponentially the geomorphology and deposits of these rivers reflect the extreme discharge fluctuations and have not previously been described a stretch of the upper burdekin river comprising four bends and one straight reach was examined near the town of charters towers the river bed is largely exposed for most of any year with a small misfit perennial channel carrying low stage flow major geomorphic elements of bends include point bars with ridge and swale topography three distinct types of chute channels avalanche slipfaces up to 5 m or more high around the downstream edges of bars and on the outer part of one point bar an elevated vegetated ridge straight reaches are flat or gently inclined sand and gravel covered surfaces much of the river bed is covered by well sorted in places gravelly coarse to very coarse grained sand with local accumulations of pebble to boulder gravel lower parts of the river bed are periodically draped by mud which is desiccated on exposure dunes and plane beds are the most commonly occurring bedforms with local development of gravelly antidunes most bank tops and upper vegetated bars are covered by silt and fine grained sand the river bed also hosts a low diversity but locally high abundance flood tolerant flora dominated by the paperbark tree melaleuca argentea which plays an important role in controlling the distribution of sediment the gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major bankfull or near bankfull flows which have a recurrence of about 18 years based on 65 years hydrographic data the initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent non bankfull discharge events

structure and generation of turbulence at interfaces strained by internal solitary waves propagating shoreward over the continental shelf abstract detailed observations of the structure within internal solitary waves propagating shoreward over oregon s continental shelf reveal the evolving nature of interfaces as they become unstable and break creating turbulent flow a persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake this is demonstrated to be due to enhanced density microstructure increased small scale strain ahead of the wave trough compresses select density interfaces thereby locally increasing stratification this is followed by a sequence of overturning high density microstructure and turbulence at the interface which is coincident with the high acoustic backscatter the richardson number estimated from observations is larger than 1 4 indicating that the interface is stable however density profiles reveal these preturbulent interfaces to be o 10 cm thick much thinner than can be resolved with shipboard velocity measurements by as

shear rate dependence of the pāhoehoe to a ā transition analog experiments hawaiian lava flows typically erupt as pahoehoe and transform to a aas they travel away from eruptive vents although the exact conditions that govern this transformation are not well constrained here we describe a set of laboratory experiments that use corn syrup and ellipsoidal rice grains as an analog to lava with crystals to examine the depen dence of the transition on shear rate and particle concentration at a particle volume fraction of 0 3 increasing the shear rate produces a sequence of deformation regimes defined by increasing amounts of shear localization the same regimes may be traversed at constant shear rate by varying the particle concentration at high shear rates the onset of nonlaminar deformation corresponds to the percolation threshold thus providing a link between suspension microstructure and deformation behavior together these results high light the combined importance of shear rate and crystallinity in determining the flow behavior of basaltic lava

preservation of in situ arborescent vegetation and fluvial bar construction in the burdekin river of north queensland australia abstract in sub humid parts of north queensland ne australia certain types of trees are well adapted to living in river bed habitats the bed of the tropical variable discharge upper burdekin river hosts a community dominated by the paperbark melaleuca argentea trees grow preferentially in flow parallel linear groves and engineer their own environment by deflecting currents building sand and gravel bars and stabising banks this is the first study to document in channel bar development resulting from vegetation growth rather than the reverse which has been inferred by previous workers in the burdekin river study site individual melaleuca range from seedlings to mature trees over 100 years old these trees survive regular partial to total submergence and impact damage during wet season runoff events often reaching over 20 000 m3 s 1 at peak discharge partly by adopting structural and growth modifications these modifications include a reclined downstream trailing habit multiple stemmed form modified crown with weeping foliage development of thick spongy bark root regeneration and group strategies notably development of flow parallel linear groves following death in situ remains of trees are preserved within the mainly coarse sand to gravel channel fill either as reclined stems trunks stripped of branches and foliage or as more upright trunks snapped at a height of typically 1 2 m above base both with roots the morphological adapations and styles of preservations of in situ vegetation within the burdekin river are considered distinctive of variable discharge rivers and may be useful in the identification of facies formed in such environments in the rock record particularly when associated with bar development

necessary conditions for a meandering river avulsion there is as yet no rational basis for predicting the conditions leading to avulsion of a meandering river here we present a conceptual model and quantify it under simplifying assumptions as a first step toward the construction of a stability diagram for avulsion initiation it is assumed initially that a rectangular crevasse channel of arbitrary depth is cut into the levee of a meandering river because the water entering the crevasse channel is derived from relatively high in the main flow it contains low concentrations of suspended solids consequently the crevasse flow is under capacity and the entrance is eroded deepening of the entrance increases the crevasse channel discharge and the concentration of suspended solids supplied because more sediment laden waters are tapped from the deeper flow in the main channel the crevasse entrance is predicted to deepen until its sediment carrying capacity is satisfied by the suspended solids entering from the main channel whether an avulsion will occur for a particular combination of initial conditions depends upon whether there exists steady state hydraulic and sediment transport conditions for crevasse channel depths that are equal to or less than the main channel depth this conceptual model is quantified by writing the unsteady gradually varied one dimensional conservation of mass and momentum equations for water and sediment transport through a main and crevasse channel sediment transport is computed as the integral of a cross sectional mean velocity and a rouse concentration profile solutions show that stable crevasse channels exist only for particular combinations of the initial height of the crevasse bed relative to the water depth in the main channel and the ratio of initial crevasse bed slope to main channel slope

strombolian eruptions and dynamics of magma degassing at yasur volcano vanuatu abstract open vent basaltic volcanoes account for a substantial portion of the global atmospheric outgassing flux largely through passive degassing and mild explosive activity we present volcanic gas flux and composition data from yasur volcano vanuatu collected in july 2018 the average volcanic plume chemistry is characterised by a mean molar co2 so2 ratio of 2 14 h2o so2 of 148 and so2 hcl of 1 02 the measured mean so2 flux in the period of 6th to 9th july is 4 9 kg s 1 therefore the mean fluxes of the other species are 7 5 kg s 1 co2 208 kg s 1 h2o and 4 8 kg s 1 hcl the degassing regime at yasur volcano ranges from passive to active styles with the latter including strombolian activity and spattering gases emitted during active degassing are enriched in so2 over hcl and co2 over so2 relative to passive degassing with co2 so2 ratios of 2 85 0 17 so2 hcl of 1 6 0 22 and h2o so2 of 315 78 8 gases emitted during passive degassing have co2 so2 ratios of 1 96 0 12 so2 hcl of 0 50 0 07 and h2o so2 of 174 43 5 we use a model of volatile degassing derived from melt inclusion studies metrich et al 2011 combined with our observations of chemical variations in the outgassing bubbles to propose a mechanism for magma degassing in the conduit at yasur we envisage a shallow conduit filled with crystal rich magma forming a viscous and mobile plug that develops an effective yield strength from the surface to a depth of at least 2000 m in which bubbles are trapped grow ascend towards the surface and burst in a typical strombolian eruption deeper bubbles released during active degassing are enriched in co2 and so2 compared to bubbles released during passive degassing which are sourced from close to the surface and are consequently hcl rich

rheology of magmas with bimodal crystal size and shape distributions insights from analog experiments 1 magmas in volcanic conduits commonly contain microlites in association with preexisting phenocrysts as often indicated by volcanic rock textures in this study we present two different experiments that investigate the flow behavior of these bidisperse systems in the first experiments rotational rheometric methods are used to determine the rheology of monodisperse and polydisperse suspensions consisting of smaller prolate particles microlites and larger equant particles phenocrysts in a bubble free newtonian liquid silicate melt our data show that increasing the relative proportion of prolate microlites to equant phenocrysts in a magma at constant total particle content can increase the relative viscosity by up to three orders of magnitude consequently the rheological effect of particles in magmas cannot be modeled by assuming a monodisperse population of particles we propose a new model that uses interpolated parameters based on the relative proportions of small and large particles and produces a considerably improved fit to the data than earlier models in a second series of experiments we investigate the textures produced by shearing bimodal suspensions in gradually solidifying epoxy resin in a concentric cylinder setup the resulting textures show the prolate particles are aligned with the flow lines and spherical particles are found in well organized strings with sphere depleted shear bands in high shear regions these observations may explain the measured variation in the shear thinning and yield stress behavior with increasing solid fraction and particle aspect ratio the implications for magma flow are discussed and rheological results and textural observations are compared with observations on natural samples

role of straining and advection in the intratidal evolution of stratification vertical mixing and longitudinal dispersion of a shallow macrotidal salt wedge estuary 1 a month of flow observations in the snohomish river estuary reveals the complex intratidal and fortnightly stratification mixing and dispersion dynamics in this macrotidal shallow salt wedge estuary system both salt wedge propagation and concomitant straining of the density field dominate temporal and spatial variations in stratification leading to intratidal variability of shear and mixing that differs in important ways from observations in partially mixed estuaries bottom generated turbulent kinetic energy production is enhanced during spring tides and acts in concert with straining to counteract advection and minimize vertical stratification during the spring flood tides this bottom generated mixing contributes to a buoyancy flux near the top of a well mixed layer during strong flood tides during strong ebb tides interfacial shear production and buoyancy flux occur along the sharp straining enhanced interface just before the system becomes well mixed longitudinal dispersion is less sensitive to the spring neap cycle yet exhibits strong intratidal variability reduced longitudinal dispersion is observed during the large floods relative to the rest of the tidal cycle behavior we attribute to a lack of vertical shear overall ebb tide advection and straining enhance stratification and longitudinal dispersion and allow for interfacial mixing intratidal variability which varies on the spring neap scale is a dominant feature of this estuary suggesting the importance of intratidal processes and tidally varying mixing coefficients in similar strongly stratified strongly forced estuaries

potential causes for the non newtonian rheology of crystal bearing magmas experimental studies indicate that crystal bearing magma exhibits non newtonian behavior at high strain rates and solid fractions we use a zero dimensional 0 d inversion model to reevaluate rheological parameters and shear heating effects from laboratory data on crystal bearing magma the results indicate non newtonian behavior with power law coefficients of up to n 13 5 it has been speculated that finite strain effects shear heating power law melt rheology or plasticity are responsible for this non newtonian behavior we use 2 d direct numerical crystal scale simulations to study the relative importance of these mechanisms these simulations demonstrate that shear heating has little effect on aggregate bulk rheologies finite strain effects result in both strain weakening and hardening but the resulting power law coefficient is modest maximum n 1 3 for simulations with spherical crystals the strain weakening and hardening behavior is related to rearrangement of crystals rather than strain rate related weakening finite strain effects were insignificant in a numerical simulation with naturally shaped crystals strain partitioning into the melt phase may induce microscopic stresses that are adequate to provoke a nonlinear viscous response in the melt large differential stresses and low effective stresses revealed by the simulations are sufficient to cause crystals to fail plastically numerical experiments that account for plastic failure show large power law coefficients n 50 in some simulations we conclude that this effect is the dominant cause of the strong nonlinear viscous response of crystal bearing magmas observed in laboratory experiments

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow