combined visual and quantitative techniques in the study of natural turbulent flows an important difficulty in understanding turbulence in natural flows is that one does not see what is being measured by devices recording velocity fluctuations this study shows how visualization of turbulent structures can be effectively implemented in natural river flows in order to match turbulent flow structures with their velocity signatures flow visualization is combined with high frequency 20 hz velocity measurements from electromagnetic current meters two fluvial environments are examined in a river confluence where the difference in turbidity provides a natural tracer very strong negative velocity fluctuations for the streamwise component are concomitant with an upwelling motion a video recording made from a bridge of the passage of turbulent structures delineated by turbid and clear water allowed us to relate the marked fluctuations in velocity to eddies of a duration of 1 2 s in the mixing layer in a gravel bed river we have examined patterns of eddy shedding in the lee of a pebble cluster these patterns were highlighted by dye milk injected in the separation zone downstream black and white images were obtained from an underwater lens linked to a video camera recorder we observed a complex dynamic characterized by two dominant patterns of eddy shedding one with a clear motion of eddies toward the water surface and another with a downward expansion toward the bed of the structures this pattern is related to the passage of intermittent wedges of low speed fluid separated by wedges of fast moving fluid

tropical zonal momentum balance in the ncep reanalyses the seasonal cycle of the zonal mean zonal momentum balance in the tropics is investigated using ncep reanalysis data it is found that the climatological stationary waves in the tropical upper troposphere which are dominated by the equatorial rossby wave response to tropical heating produce an equatorward eddy flux of westerly momentum in the equatorial belt the resulting westerly acceleration in the tropical upper troposphere is balanced by the advection of easterly momentum associated with the cross equatorial mean meridional circulation the eddy momentum fluxes and the cross equatorial flow both tend to be strongest during the monsoon seasons when the maximum diabatic heating is off the equator and weakest during april may the season of strongest equatorial symmetry of the heating the upper level rossby wave pattern exhibits a surprising degree of equatorial symmetry and follows a similar seasonal progression solutions of the nonlinear shallow water wave equation also show a predominantly equatorially symmetric response to a heat source centered off the equator

shallow water turbulence modeling and horizontal large eddy computation of river flow by introducing the concept of sds subdepth scale turbulence to model three dimensional 3d turbulence with length scales less than the water depth and treating it explicitly with a proper separate modeling an sds 2dh model has been developed to simulate the evolution of horizontal large scale eddies in shallow water applying this model to river flows with transverse shear due to vegetation drag the horizontal large scale hls eddies were found to dominate horizontal momentum mixing the bottom friction and vegetation drag acting as sinks of vorticity play the key roles in the development of the horizontal large scale eddies and in reynolds stress generation the sds 2dh model can directly describe effects of flow geometry such as vegetation layer width on the large scale eddy development and hence predicts turbulence mixing better than the k e model

spatially averaged open channel flow over rough bed in this paper it is suggested that the double averaged in temporal and in spatial domains momentum equations should be used as a natural basis for the hydraulics of rough bed open channel flows especially with small relative submergence the relationships for the vertical distribution of the total stress for the simplest case of 2d steady uniform spatially averaged flow over a rough bed with flat free surface are derived these relationships explicitly include the form induced stresses and form drag as components of the total stress using this approach we define three types of rough bed flows 1 flow with high relative submergence 2 flow with small relative submergence and 3 flow over a partially inundated rough bed the relationships for the double averaged velocity distribution and hydraulic resistance for all three flow types are derived and compared with measurements where possible the double averaged turbulent and form induced intensities and stresses for the case of regular spherical segment type roughness show the dominant role of the double averaged turbulence stresses and form drag in momentum transfer in the near bed region

geographic variability in the export of moist static energy and vertical motion profiles in the tropical pacific 1 column integrated moist static energy mse budgets were calculated using daily data from three reanalyses 1998 2001 and isccp radiative cooling the mse export by the circulation was separated into vertical and horizontal mse advection components in order to examine how observationally derived vertical motion profiles affect gross moist stability calculations in a 2000 km wide region in the central eastern pacific itcz vertical motions import mse implying a negative gross moist stability horizontal mse advection is negative and of similar magnitude to vertical mse advection in rainy regions geographic differences in export of mse by the vertical circulation are primarily due to differences in the shape of the vertical motion profile in the west pacific warm pool rainy regions mean horizontal convergence extends up to 300 mb while in parts of the pacific itcz where meridional sst gradients are strong the vertical motion profile is bottom heavy with convergence below 800 mb and divergence above on daily timescales the mode of vertical motion variability has little dependence on precipitation rate and is consistent with observed regional differences in the vertical structure of trmm pr reflectivities

tropical tropopause transition layer cirrus as represented by calipso lidar observations the spatial and temporal variability of cirrus cloud fraction within the tropical tropopause transition layer ttl is investigated based on three years of data from the cloud aerosol lidar and infrared pathfinder satellite observations calipso mission analyzed in conjunction with fields from the european centre for medium range weather forecasts re analysis era interim and temperature profiles from radiosondes launched at manus island papua new guinea 28s 1478e ttl cirrus is found to be mainly confined to the rising branch of the hadley cell within 158 of the equator with maximum cloud fraction between 14 and 15 km the time varying spatial pattern ofcloud fraction within this beltdoesnot resemble the pattern of cloud fractionin the layer below aswould be expected ifthe ttl cirruswere formed bythe spreading ofthe anvils of convective clouds on the contrary within the stably stratified layer above 13 km cirrus cloud fraction and temperaturebothappear tobemodulatedbytheplanetary scalevertical velocityfield thetime varyingspatial patterns arereminiscentofthevertical propagatingkelvinwaveresponsetoanequatorialheatsource withthe coldest cloudiest air in the ttl centered approximately 308 of longitude to the east of the strongest heating

the influence of tributary flow density differences on the hydrodynamic behavior of a confluent meander bend and implications for flow mixing abstract the goal of this study is to evaluate the influence of tributary flow density differences on hydrodynamics and mixing at a confluent meander bend a detailed field characterization is performed using an acoustic doppler current profiler adcp for quantification of the 3d flow field flow discharge and bathymetry as well as ctd measurements conductivity temperature depth to characterize the patterns of mixing satellite images of the confluence taken at complementary times to the field surveys were analyzed to evaluate the confluence hydrodynamics at different flow conditions the results illustrate the differences in hydrodynamics and mixing length in relation to confluences with equal density tributaries at low density differences and higher discharge ratio qr between the two rivers the flow is similar to equi density confluent meander bends in contrast at high density differences low qr the tributary flow is confined to near the confluence but the density difference causes the flow to move across channel in this case the density difference causes the lateral spread of the tributary flow to be greater than at a greater qr when the density difference is less these results illustrate the potential importance of density differences between tributaries in determining the rate and spatial extent of mixing and sediment dispersal at confluent meander bends

static and dynamic angles of repose in loose granular materials under reduced gravity granular materials avalanche when a static angle of repose is exceeded and freeze at a dynamic angle of repose such avalanches occur subaerially on steep hillslopes and wind dunes and subaqueously at the lee side of deltas until now it has been assumed that the angles of repose are independent of gravitational acceleration the objective of this work is to experimentally determine whether the angles of repose depend on gravity in 33 parabolic flights in a well controlled research aircraft we recorded avalanching granular materials in rotating drums at effective gravitational accelerations of 0 1 0 38 and 1 0 times the terrestrial value the granular materials varied in particle size and rounding and had air or water as interstitial fluid materials with angular grains had time averaged angles of about 40 and with rounded grains about 25 for all effective gravitational accelerations except the finest glass beads in air which was explained by static electricity for all materials the static angle of repose increases about 5 with reduced gravity whereas the dynamic angle decreases with about 10 consequently the avalanche size increases with reduced gravity the experimental results suggest that relatively low slopes of granular material on mars may have formed by dry flows without a lubricating fluid on asteroids even lower slopes are expected the dependence on gravity of angle of repose may require reanalysis of models for many phenomena involving sediment also at much lower slope angles

turbulent mixing in the amazon river the isotopic memory of confluences earth and planetary science letters 290 2010 pp 37 43

effects of bed discordance on flow dynamics at open channel confluences although a discordance in bed elevation between mainstream and tributary channels is a common feature of most river confluences this height differential is largely ignored in many models of junction flow dynamics here we present new laboratory flume data that demonstrate the significant influence of bed discordance on flow at an acute angled confluence a difference in bed height between mainstream and tributary channels is shown to obliterate flow deflection at the bed and create a distortion of the mixing layer between the flows resulting in fluid upwelling at the downstream junction corner this upwelling is responsible for the absence of a flow separation zone near the bed at the downstream junction corner and the lack of a zone of marked flow acceleration in the postconfluence channel these experiments reveal complex flow fields generated by bed discordance and suggest the need for major revisions to current models of river channel confluence flow dynamics and their application to sediment routing at these sites

bed morphology and sedimentology at the confluence of unequal depth channels abstract spatial and temporal variations in the bed geometry and bed material size of a sand bed river confluence with unequal depth channels were monitored during a sequence of floods additionally two other sand bed confluences were surveyed to test the replicability of these observations the confluences studied here have only one avalanche face which corresponds to the front of a tributary mouth bar in the shallower channel and do not exhibit a marked scour zone three distinct morpho sedimentological zones are present 1 an area around the upstream corner of the junction where the sediments are generally finer than the mean 2 a maximum depth zone with coarser than average particles and 3 a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream changes in relative discharge between the two channels had little effect on the grain size of the downstream junction corner bar but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar the downstream junction corner bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels the contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone

an investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel 1 conditions corresponding to the initiation of the erosion and deposition process 1 the present study investigates the flow physics and the role played by the main coherent structures in the scouring processes around a vertical spur dike in a straight channel at conditions corresponding to the start flat bed of the scouring process large eddy simulation les is performed at a relatively low channel reynolds number re 18 000 in the range where most flume studies with clear water scour conditions are conducted similar to these studies the incoming flow is fully turbulent and contains realistic turbulence fluctuations visualization experiments are conducted to better understand the nature of the interactions between the dominant coherent structures playing a role in the erosion process it is found that the structure of the horseshoe vortex hv system at the base of the spur dike changes considerably in time and in vertical sections perpendicular to the trajectory defined by the axis of the main necklace vortex however its intensity is the largest at vertical sections situated around the tip of the spur dike it is in this region that the core of the main necklace vortex oscillates aperiodically between two preferred modes in one of them zero flow mode the necklace vortex is closer to the spur dike and more compact and the near bed jet flow beneath it is weak in the other one back flow mode a strong near bed jet flow convects the primary necklace vortex away from the spur dike and its core is more elongated and less compact this explains the large amplification by about 1 order of magnitude compared to the surrounding turbulent flow of the turbulent kinetic energy and pressure fluctuations inside the hv system in the region situated around the tip of the spur dike and the double peak distribution of the turbulent kinetic energy past the spur dike in the legs of the necklace vortex the intensity of the bimodal oscillations decreases such that they are not observed in spanwise sections situated at more than one channel depth behind the spur dike it is found that the legs of the horseshoe vortices can interact at times with the vortex tubes shed in the detached shear layer dsl and with the tip of the spur dike these events typically result in a significant change in the coherence of the hv system the largest bed shear stress values in the mean flow are present in the strong acceleration region near the tip of the spur dike but high bed shear stress values are also observed beneath the upstream part of the dsl the bed shear stress fluctuations around the local mean values can be very high especially in the region situated beneath the upstream part of the dsl at random times some of the vortices shed in the dsl merge or interact with eddies from the recirculation region this leads to an increase in their strength and to a large increase of the bed shear stress along their path

crustal failure on icy moons from a strong tidal encounter abstract close tidal encounters among large planetesimals and moons should have been more common than grazing or normal impacts using a mass spring model within an n body simulation we simulate the deformation of the surface of an elastic spherical body caused by a close parabolic tidal encounter with a body that has similar mass as that of the primary body such an encounter can induce sufficient stress on the surface to cause brittle failure of an icy crust and simulated fractures can extend a large fraction of the radius of body strong tidal encounters may be responsible for the formation of long graben complexes and chasmata in ancient terrain of icy moons such as dione tethys ariel and charon

field investigation of three dimensional flow structure at stream confluences 2 turbulence stream confluences are among the most highly turbulent locations in fluvial systems this paper examines the three dimensional structure of turbulence at three stream confluences in east central illinois the analysis focuses on the characteristics of turbulence both within the shear layer and in the ambient flow results show that at the upstream end of each confluence the shear layer occupies a limited portion of the flow cross sectional area but turbulence kinetic energy within this layer is 2 3 times greater than the turbulence kinetic energy of the ambient flow which has turbulence characteristics similar to those for flow in straight channels turbulence within the shear layer can be characterized as quasi two dimensional in the sense that large scale turbulence generated by transverse shear is predominantly two dimensional whereas small scale turbulence associated with bed friction is three dimensional spectral analysis suggests that the structure of fluid motion within the shear layer differs for confluences with symmetrical versus asymmetrical planforms the shear layer dissipates rapidly as flow enters the downstream channel even though a well defined mixing interface persists at downstream locations

on playfair s law of accordant junctions geomorphologists since playfair have apparently accepted that fluvial confluences will be morphologically accordant in distinction to the hanging junctions of glaciated troughs a view reinforced by gilbert s law of declivities the issue has been largely ignored despite field instances of discordant fluvial junctions and widespread engineering practice which favours discordant confluences in non erodible materials six categories of fluvial junctions are defined accordant hanging discordant barred impeded disrupted and disturbed the study focuses principally on the first two classes and defines discordance purely morphologically in terms of relative depth of incision of the confluent thalwegs likely controls of accordance discordance are identified as flow magnitudes synchroneity of flow regimes channel boundary materials and debris calibre field investigations in malawi france west malaysia and new zealand in small 100 km2 basins on a variety of rock types are described barred junctions are relatively rare but complex in character a high proportion of confluences are distinctly discordant and there is a general continuum from accordant cases tectonic control is improbable and the principal cause identified is the degree of difference in channel forming discharges estimated by stream magnitude and channel area some evidence for lithologic control is also apparent it appears that many confluences behave as if the tributary junction is non erodible and not in the fashion predicted by the law of declivities it is argued that the observed relation is a necessary result of the general down stream changes in hydraulic geometry limited comparison with glacial trough confluences reveals a difference in degree but not in nature although the relationship between discordance and channel size is affected by cross section geometry a 10 fold difference in the cross sectional areas of channels meeting at a junction produces on average a 2 4 fold difference in mean channel depths for fluvial cases and a 4 fold difference for glacial troughs the role of varying flow regimes seems to require attention nevertheless it is maintained that playfair s law is a misleading description of the morphology of fluvial junctions which behaves fundamentally in the same fashion as glacial troughs

transport in the low latitude tropopause zone diagnosed using particle trajectories abstract recent literature has described a transition zone between the average top of deep convection in the tropics and the stratosphere here transport across this zone is investigated using an offline trajectory model particles were advected by the resolved winds from the european centre for medium range weather forecasts reanalyses for each boreal winter clusters of particles were released in the upper troposphere over the four main regions of tropical deep convection indonesia central pacific south america and africa most particles remain in the troposphere descending on average for every cluster the horizontal components of 5 day trajectories are strongly influenced by the el nino southern oscillation enso but the lagrangian average descent does not have a clear enso signature tropopause crossing locations are first identified by recording events when trajectories from the same release regions cross the world meteorological organization lapse rate tropopause most crossing events occ

numerical simulation of three dimensional time averaged flow structure at river channel confluences current confluence research emphasizes three broad controls on flow structure generation 1 planform curvature 2 topographic steering and 3 anisotropic turbulence associated with flow separation and shear layer dynamics the relative importance of these processes in explaining observed flow structures is controversial a situation that may be related to the fact that different investigators have examined different confluence configurations this paper uses a three dimensional numerical model with a fully elliptic solution a free surface treatment and a turbulence model based on a renormalized group rng to help to provide a physically based explanation of the controls upon flow structure generation for both a laboratory rectangular and a field confluence the confluence of the kaskaskia river and copper slough and to identify the particular conditions under which particular flow structures are observed results suggest that an analogy with back to back meanders is possible for symmetrical configurations but that there will be progressive divergence from this state as confluence asymmetry increases in asymmetric situations a dual cell structure may be limited to the immediate vicinity of the junction because of the effects of streamline curvature and topographic steering these differences can be explained by consideration of the dynamic pressure field which may be specific to each confluence configuration as such this study partially reconciles differing views over what controls time averaged flow structures in river channel confluences although further research into the interaction of these processes with instantaneous velocity fluctuations is required

numerical predictions of surface effects during the 2029 close approach of asteroid 99942 apophis asteroid 99942 apophis close approach in 2029 will be one of the most significant small body encounter events in the near future and offers a good opportunity for in situ exploration to determine the asteroid s surface properties and measure any tidal effects that might alter its regolith configuration resurfacing mechanics has become a new focus for asteroid researchers due to its important implications for interpreting surface observations including space weathering effects this paper provides a prediction for the tidal effects during the 2029 encounter with an emphasis on whether surface refreshing due to regolith movement will occur the potential shape modification of the object due to the tidal encounter is first confirmed to be negligibly small with systematic simulations thus only the external perturbations are taken into account for this work despite this seismic shaking induced by shifting blocks might still play a weak role and we will look into this mechanism in future work a two stage approach is developed to model the responses of asteroid surface particles the regolith based on the soft sphere implementation of the parallel n body gravity tree code pkdgrav a full body model of apophis is sent past the earth on the predicted trajectory to generate the data of all forces acting at a target point on the surface a sandpile constructed in the local frame is then used to approximate the regolith materials all the forces the sandpile feels during the encounter are imposed as external perturbations to mimic the regolith s behavior in the full scenario the local mechanical environment on the asteroid surface is represented in detail leading to an estimation of the change in global surface environment due to the encounter typical patterns of perturbation are presented that depend on the asteroid orientation and sense of rotation at perigee we find that catastrophic avalanches of regolith materials may not occur during the 2029 encounter due to the small level of tidal perturbation although slight landslides might still be triggered in positions where a sandpile s structure is weak simulations are performed at different locations on apophis surface and with different body and spin axis orientations the results show that the small scale avalanches are widely distributed and manifest independently of the asteroid orientation and the sandpile location we also include simulation results of much closer encounters of the apophis with earth than what is predicted to occur in 2029 showing that much more drastic resurfacing takes place in these cases

structure of the annual mean equatorial planetary waves in the era 40 reanalyses abstract the three dimensional structure of the annual mean equatorial planetary waves in the 40 yr ecmwf re analysis era 40 is documented the features in the free atmosphere are predominantly equatorially symmetric driven by east west heating gradients the geopotential height and wind perturbations are strongest at or just below the 150 hpa level below the level of maximum amplitude the circulations in the waves are thermally direct with latent heat release in deep convective clouds and radiative cooling in the intervening cloud free regions within the overlying capping layer the wave related circulations are thermally indirect with rising of the coldest air and sinking of air that is less cold at the cold point just above the 100 hpa 17 km level the era 40 annual mean vertical velocity in the equatorial belt ranges up to 3 mm s 1 over the equatorial western pacific during the boreal winter implying diabatic heating rates of up to 3 c day 1 an order of magnitude larger than typical clear

near at eros imaging and spectral results eros is a very elongated 34 kilometers by 11 kilometers by 11 kilometers asteroid most of the surface of which is saturated with craters smaller than 1 kilometer in diameter the largest crater is 5 5 kilometers across but there is a 10 kilometer saddle like depression with attributes of a large degraded crater surface lineations both grooves and ridges are prominent on eros some probably exploit planes of weakness produced by collisions on eros and or its parent body ejecta blocks 30 to 100 meters across are abundant but not uniformly distributed over the surface albedo variations are restricted to the inner walls of certain craters and may be related to downslope movement of regolith on scales of 200 meters to 1 kilometer eros is more bland in terms of color variations than gaspra or ida spectra 800 to 2500 nanometers are consistent with an ordinary chondrite composition for which the measured mean density of 2 67 0 1 grams per cubic centimeter implies internal porosities ranging from about 10 to 30 percent

mixing layer distortion at the confluence of channels of different depth channel confluences have received considerable recent attention in the fields of geomorphology1 3 sedimentology4 9 and hydraulic engineering10 13 where channels join rapid changes in fluid velocity turbulence intensity channel hydraulic geometry and bed geometry may occur14 17 previous models of junction dynamics have tended to assume that the confluent channels are of equal depth10 18 a condition that may be found only rarely in natural junctions more often the depths of confluent channels are different19 this discordance is commonly the result of the forma tion at the mouth of each confluent channel of bars that possess steep avalanche faces which dip into a central junction scour1 2 5 20 here we suggest that when channels of different depth join the mixing layer that forms at their confluence is distorted by interacting with a separation zone which forms in the lee of the mouth of the shallower channel this mixing layer distortion imparts a complex three dimensional flow to the junction promoting vertical fluid upwelling and significantly enhancing rapid mixing within the flow of the shallow tributary while retarding the rate of mixing across the flow of the deeper channel suspended sediments or contaminants within the deeper channel may be transferred rapidly across the flow of the shallower tributary upwelling downstream of the junction

flow and sediment dynamics in channel confluences confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large low gradient main channel commonly occur in nature but they have not yet been investigated measurements of the three dimensional velocity field turbulence sediment transport bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the upper rhone river switzerland the difference between the low flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone due to this bed discordance the tributary flow penetrates into the main channel mainly in the upper part of the water column whereas the main channel flow is hardly hindered by the tributary in the lower part of the water column giving rise to a two layer flow structure in the confluence zone in confluences with high supply of coarse sediment from the tributary the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity the sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near bed flow originating from the main channel the sediment transport capacity is further increased by the three dimensionality of the flow which is characterized by maximum velocities occurring near the bed and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary a conceptual model is proposed for the hydro morpho sedimentary processes and compared to existing conceptual models for confluences with different characteristics

numerical evaluation of the effects of planform geometry and inflow conditions on flow turbulence structure and bed shear velocity at a stream confluence with a concordant bed this study numerically investigates the effects of variations in inflow conditions and planform geometry on large scale coherent flow structures and bed friction velocities at a stream confluence with natural bathymetry and concordant bed morphology several numerical experiments are conducted in which either the kelvin helmholtz mode or the wake mode dominates within the mixing interface mi between the two confluent streams as the junction angle and alignments of the tributaries are altered in the kelvin helmholtz mode the mi contains mostly corotating vortices driven by the mean transverse shear across the mi while in the wake mode the mi contains counterrotating vortices forming by the interaction of the separated shear layers on the two sides of a zone of stagnant fluid near the junction corner a large angle between the two incoming streams is not necessary for the development of strongly coherent streamwise oriented vortical sov cells in the immediate vicinity of the mi results show that such sov cells can develop and produce high bed friction velocities even for cases with a low angle between the two tributaries and for cases where the downstream channel is approximately aligned with the axes of the two tributaries low curvature cases sov cells tend not to develop only when the incoming streams are parallel and aligned with the downstream channel junction angle of zero and the incoming flows produce a strong kelvin helmholtz mode under such conditions quasi 2 d mi vortices play the primary role in mixing and the production of high bed shear velocities simulations with and without natural bed morphology local bank line irregularities indicate that planform geometry and inflow conditions primarily govern the development of coherent flow structures but that bathymetric and bank line effects can locally modify details of these structures

the global monsoon as seen through the divergent atmospheric circulation a comprehensive description is given of the global monsoon as seen through the large scale overturning in the atmosphere that changes with the seasons and it provides a basis for delimiting the monsoon regions of the world the analysis focuses on the mean annual cycle of the divergent winds and associated vertical motions as given by the monthly mean fields for 1979 93 reanalyses from the national centers for environmental prediction national center for atmospheric research ncep ncar and european centre for medium range weather forecasts ecmwf which are able to reproduce the dominant modes a complex empirical orthogonal function analysis of the divergent circulation brings out two dominant modes with essentially the same vertical structures in all months of the year the first mode which depicts the global monsoon has a simple vertical structure with a maximum in vertical motion at about 400 mb divergence in the upper troposphere that is strongest at 150 mb and decays to zero amplitude above 70 mb and convergence in the lower troposphere with a maximum at 925 mb ecmwf or 850 mb ncep however this mode has a rich three dimensional spatial structure that evolves with the seasons it accounts for 60 of the annual cycle variance of the divergent mass circulation and dominates the hadley circulation as well as three overturning transverse cells these include the pacific walker circulation an americas atlantic walker circulation both of which comprise rising motion in the west and sinking in the east and a transverse cell over asia the middle east north africa and the indian ocean that has rising motion in the east and sinking toward the west these exist year round but migrate and evolve considerably with the seasons and have about a third to half of the mass flux of the peak hadley cell the annual cycle of the two hadley cells reveals peak strength in early february and early august in both reanalyses a second monsoon mode which accounts for 20 of the variance features relatively shallow but vigorous overturning with the maximum vertical velocities near 800 mb outflow from 750 to 350 mb and inflow peaking at 925 mb it is especially strong over africa where the shallow mostly meridional overturning migrates back and forth across the equator with the seasons it influences the middle east has a signature over australia and is also an important component of the overturning in the tropical eastern pacific and atlantic and thus of the convergence zones in these regions the relationship of the global monsoon to the regional monsoons is described over six zonal sectors africa australia asia north america south america and the pacific and atlantic oceans only the two ocean areas do not undergo a seasonal reversal required for monsoons although they have direct overturning cells and they nevertheless participate in the global monsoon through the changes in large scale overturning the regional meridional cross sections highlight the importance of the shallow overturning cell in lower troposphere monsoon activity the steadiness of the overturning circulation is determined by comparing the signal of the seasonal mean vertical motions at 500 mb with the standard deviation of the transient daily variations locations where this signal exceeds 60 of the daily noise correspond closely with the regional centers of the monsoon

impact excitation of a seismic pulse and vibrational normal modes on asteroid bennu and associated slumping of regolith we consider an impact on an asteroid that is energetic enough to cause resurfacing by seismic reverberation and just below the catastrophic disruption threshold assuming that seismic waves are not rapidly attenuated in asteroids with diameter less than 1 km we identify a regime where rare energetic impactors can excite seismic waves with frequencies near those of the asteroid s slowest normal modes in this regime the distribution of seismic reverberation is not evenly distributed across the body surface with mass spring model elastic simulations we model impact excitation of seismic waves with a force pulse exerted on the surface and using three different asteroid shape models the simulations exhibit antipodal focusing and normal mode excitation if the impulse excited vibrational energy is long lasting vibrations are highest at impact point its antipode and at high surface elevations such as an equatorial ridge a near equatorial impact launches a seismic impulse on a non spherical body that can be focused on two additional points on an the equatorial ridge we explore simple flow models for the morphology of vibration induced surface slumping we find that the initial seismic pulse is unlikely to cause large shape changes long lasting seismic reverberation on bennu caused by a near equatorial impact could have raised the height of its equatorial ridge by a few meters and raised two peaks on it one near impact site and the other near its antipode

dynamics of a river channel confluence with discordant beds flow turbulence bed load sediment transport and bed morphology 1 river channel confluences play a major role in the dynamics of all fluvial systems and yet our understanding of bed load routing at these sites is very sparse the dynamics of confluences are a function of the momentum ratio between the combining flows and the three dimensional geometry of the junction recent experiments have shown that discordance in bed height between the confluent rivers increases turbulence intensity and enhances upwelling of flow within the confluence however the significance of these flow characteristics on sediment transport is still unknown to examine the relations between flow and sediment transport we have measured near bed flow turbulence bed load transport rates and changes in bed morphology for eight different flow conditions at a sand bed discordant confluence detailed analysis of the near bed flow patterns reveals that within the shear layer low mean flow velocities are combined with the highest values of reynolds shear stresses and that turbulence generation is associated with intense upward movements of flow high sediment transport rates are found at the edges of the shear layer region where horizontal vertical cross stresses uw are high these patterns match changes in bed morphology where erosion occurs along the shear layer the relation between the shear layer and sediment transport confirms the role of bed discordance on the dynamics of the confluence migration of the shear layer into the confluence as a result of a change in momentum ratio modifies local near bed flow characteristics sediment transport rates and the spatial distribution of deposition and erosion zones

fine scale characterization of the turbulent shear layer of an instream pebble cluster this study characterizes the shear layer and associated vortex shedding around an isolated submerged pebble cluster in a gravel bed river the approach combines flow visualization and high frequency three dimensional velocity acoustic doppler velocimeter measurements two vortex shedding modes in the wake of the cluster were identified a small scale high frequency initial instability mode and a lower frequency mode that scales with cluster height the lower frequency mode arose from the intermittent interaction and amalgamation of the small scale instability vortices reynolds shear stresses velocity spectra and coherence functions indicated a dominance of longitudinal vertical shedding vortices in the wake of the cluster simultaneous flow visualization was required to determine the nature and behavior of the shedding modes quadrant analysis revealed that q2 and q4 events contributed 80 of the local longitudinal vertical component reynolds shear stress and demonstrated a dominance of ejection events in the wake of the cluster through flow visualization the behavior of the shear layer was seen to vertically expand and contract with the passage of q2 and q4 events respectively

structure of flow over alluvial bedforms an experiment on linking field and laboratory methods this paper presents results of a field study designed to examine the structure of flow over mobile and fixed bedforms in a natural stream and to compare the results with findings of previous laboratory studies within the framework of double time space averaging approach measurements of turbulence were obtained in a small river in illinois usa over a fine spatial grid of sampling points above a mobile sandy bedform and its artificially moulded replica flow structure over the artificial bedform is similar to that observed in laboratory studies but is markedly different from the flow structure over natural bedforms these differences are most pronounced in the roughness sublayer whereas flow in the logarithmic layer over natural and artificial sand waves is fairly similar and exhibits spatial uniformity the double time space averaged distributions of turbulence statistics conform to the multilayer model of flow structure over bedforms mean velocity distributions indicate neither classical flow recirculation nor substantial reduction of velocities in the lee of bedform crests however vertical patterns of turbulence statistics over depth suggest that stacked wakes similar to those observed in laboratory studies exist above the bedforms thus despite the absence of flow separation wake development seems to be induced by the systematic influence of upstream bedforms on the vertical structure of turbulence copyright 2006 john wiley sons ltd

resolving two dimensional flow structure in rivers using large scale particle image velocimetry an example from a stream confluence large scale particle image velocimetry lspiv has emerged as a valuable tool for measuring surface velocity in a variety of fluvial systems lspiv has typically been used in the field to obtain velocity or discharge measurements in relatively simple one dimensional flow detailed two dimensional or three dimensional characterization of flow structure has been relegated to laboratory settings because of the difficulty in controlling piv limiting factors such as poor particle seeding the need for camera rectification and challenging field conditions in this study we implement a low cost lspiv setup using a high resolution action camera mounted above a stream confluence and water seeded with recycled landscape mulch time averaged 2d velocities derived from lspiv are compared with those measured with an acoustic doppler velocimeter adv in the camera s field of view we also assess the capabilities of this setup to resolve both turbulent and time averaged flow structures at a stream confluence our results reveal that even in challenging field conditions a basic lspiv setup can yield accurate data on velocity and resolve in detail the temporal evolution of flow structures on the surface of rivers the resulting dataset contains velocity information at high spatial and temporal resolution a significant advance in understanding flow processes at stream confluences our lspiv analysis provides support for previous numerical modeling studies that have distinguished between kelvin helmholtz and wake modes of turbulent behavior within the mixing interface at confluences this study shows that lspiv should be considered as both an alternative for traditional methods and a tool that can provide unprecedented levels of resolution of surface velocity patterns on rivers that can be used to evaluate numerical predictions of flow structure in complex fluvial environments this article is protected by copyright all rights reserved

coupled adcps can yield complete reynolds stress tensor profiles in geophysical surface flows 1 we introduce a new technique to measure profiles of each term in the reynolds stress tensor using coupled acoustic doppler current profilers adcps the technique is based on the variance method which is extended to the case with eight acoustic beams methods to analyze turbulence from a single adcp rely on questionable assumptions on turbulence anisotropy ratios and on the requirement of perfect vertical alignment which are no longer necessary with the coupled system we apply the technique successfully to data collected in the mahakam river east kalimantan indonesia citation vermeulen b a j f hoitink and m g sassi 2011 coupled adcps can yield complete reynolds stress tensor profiles in geophysical surface flows geophys res lett 38 l06406 doi 10 1029 2011gl046684

shallow meridional circulation in the tropical eastern pacific abstract based on four independent datasets this study presents observational evidence of a shallow meridional circulation cell in the eastern tropical pacific in this shallow meridional circulation cell the northerly cross equatorial return flow from the itcz into the southern hemisphere is found immediately above the atmospheric boundary layer in contrast to the classic concept of the hadley type deep meridional circulation whose northerly return flow resides in the upper troposphere the strength and depth of this shallow meridional circulation undergo a distinct annual cycle possible causes and climatic implications of this shallow meridional circulation are discussed

the vertical structure of tropical convection and its impact on the budgets of water vapor and ozone abstract convective clouds in the tropics that penetrate the boundary layer inversion preferentially detrain into a shallow outflow layer 2 5 km or a deep outflow layer 10 17 km the properties of these layers are diagnosed from a one dimensional model of the tropics constrained by observed mean temperature and water vapor profiles the mass flux divergence of the shallow cumuli 2 5 km is balanced by a mass flux convergence of evaporatively forced descent downdrafts while the mass flux divergence of deep cumulonimbus clouds 10 17 km is balanced by a mass flux convergence of clear sky radiative descent the pseudoadiabatic temperature stratification of the midtroposphere 5 10 km suppresses cloud outflow in this interval the detrainment profile in the deep outflow layer is shifted downward by about 1 5 km from the profile one would anticipate based on undilute pseudoadiabatic ascent of air from the boundary layer the main source of water vapor to most of the tropical troposphere is evaporativ

spectral properties and composition of potentially hazardous asteroid 99942 apophis abstract the known close approach of asteroid 99942 apophis in april 2029 provides the opportunity for the case study of a potentially hazardous asteroid in advance of its encounter the visible to near infrared 0 55 to 2 45 m reflectance spectrum of apophis is compared and modeled with respect to the spectral and mineralogical characteristics of likely meteorite analogs apophis is found to be an sq class asteroid that most closely resembles ll ordinary chondrite meteorites in terms of spectral characteristics and interpreted olivine and pyroxene abundances although we cannot rule out some degree of partial melting a meteorite analog allows some estimates and conjectures of apophis possible range of physical properties such as the grain density and micro porosity of its constituent material composition and size similarities of apophis with 25143 itokawa suggest a total porosity of 40 as a current best guess for apophis applying these parameters to apophis yields a mass estimate of 2 10 10 kg with a corresponding energy estimate of 375 mt for its potential hazard substantial unknowns most notably the total porosity allow uncertainties in these mass and energy estimates to be as large as factors of two or three

a new hybrid framework for simulating hypervelocity asteroid impacts and gravitational reaccumulation abstract we present a hybrid approach for simulating hypervelocity impacts onto asteroids the overall system response is separated into two stages based on their different characteristic timescales first the short timescale fragmentation phase is simulated using a modified version of the tonge ramesh material model implemented in a material point method framework then a consistent hand off to an n body gravity code is formulated to execute the long timescale gravitational reaccumulation calculation we demonstrate this hybrid approach by considering the 5 km s head on impact of a 1 21 km diameter basalt impactor on a 25 km diameter target asteroid the impact event resulted in the fragmentation but not complete disruption of the entire target a granular core is observed at the end of the fragmentation simulations which acts as a gravity well over which reaccumulation occurs in the n body simulations our results suggest that disruption thresholds for rocky asteroids are higher when energy dissipating mechanisms such as granular flow and pore collapse are included

fragments of late eocene earth impacting asteroids linked to disturbance of asteroid belt the onset of earth s present icehouse climate in the late eocene coincides with astronomical events of enigmatic causation at similar to 36 ma ago the 90 100 km large popigai and chesapeake bay impact structures formed within similar to 10 20 ka enrichments of he 3 in coeval sediments also indicate high fluxes of interplanetary dust to earth for 2 ma additionally several medium sized impact structures are known from the late eocene here we report from sediments in italy the presence of abundant ordinary chondritic chromite grains 63 250 pm associated with the ejecta from the popigai impactor the grains occur in the 40 cm interval immediately above the ejecta layer element analyses show that grains in the lower half of this interval have an apparent h chondritic composition whereas grains in the upper half are of l chondritic origin the grains most likely originate from the regoliths of the popigai and the chesapeake bay impactors respectively these asteroids may have approached earth at comparatively low speeds and regolith was shed off from their surfaces after they passed the roche limit the regolith grains then settled on earth some 100 to 1000 yrs after the respective impacts further neon and oxygen isotopic analyses of the grains can be used to test this hypothesis if the popigai and chesapeake bay impactors represent two different types of asteroids one can rule out previous explanations of the late eocene extraterrestrial signatures invoking an asteroid shower from a single parent body breakup instead a multi type asteroid shower may have been triggered by changes of planetary orbital elements this could have happened due to chaos related transitions in motions of the inner planets or through the interplay of chaos between the outer and inner planets asteroids in a region of the asteroid belt where many ordinary chondritic bodies reside were rapidly perturbed into orbital resonances this led to an increase in small to medium sized collisional breakup events over a 2 5 ma period this would explain the simultaneous delivery of excess dust and asteroids to the inner solar system independent evidence for our scenario are the common cosmic ray exposure ages in the range of ca 33 40 ma for recently fallen h and l chondrites the temporal coincidence of gravity disturbances in the asteroid belt with the termination of ice free conditions on earth after 250 ma is compelling we speculate that this coincidence and a general correlation during the past 2 ga between k ar breakup ages of parent bodies of the ordinary chondrites and ice ages on earth suggest that there may exist an astronomical process that disturbs both regions of the inner asteroid belt and earth s orbit with a potential impact on earth s climate c 2015 the authors published by elsevier b v this is an open access article under the cc by nc nd license

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

hydrodynamics of mountain river confluences and its relationship to sediment transport mountain confluences are characterized by narrow and steep tributaries that provide important sediment load to the confluence whereas themain channel supplies the dominant flow discharge this results in a marked bed discordance between the tributary and main channel which induces key differences in the hydromorphodynamics of the confluence when compared to concordant bed and some common types of discordant bed confluences the processes of initiation and maintenance of the morphology of confluences are still unknown and research linking morphodynamics and hydrodynamics of river confluences is required to understand these phenomena in this paper eddy resolving simulations based on laboratory experiments made in a live bed model of a mountain river confluence are used to provide a detailed description of flow hydrodynamics and implications for morphodynamics the test case study corresponds to a confluence with an angle of 70 numerical simulations are performed for two extreme bathymetric conditions those at the start of the experiment and when equilibriummorphological conditions are reached results of the simulations and experimental observations are used tomake inferences on erosion mechanisms during the initial and final states of the erosion deposition process the relationship between the coherent structures present in the near bed region in the instantaneous and mean flow fields and sediment entrainment transport is described the present paper demonstrates the critical role played by large scale turbulence generated in the shear layers forming on the side of the tributary flow as it enters the main channel by the main vortex forming over the discordant bed region surrounding the downstream end of the tributary as well as by several near bed vortices induced by the deflection of the tributary flow by the incoming flow in the main channel the predicted patterns of bed shear stress are linked to pathways of sediment movement

a warming tropical central pacific dries the lower stratosphere the amount of water vapor in the tropical lower stratosphere tls which has an important influence on the radiative energy budget of the climate system is modulated by the temperature variability of the tropical tropopause layer ttl the ttl temperature variability is caused by a complex combination of the stratospheric quasi biennial oscillation qbo tropospheric convective processes in the tropics and the brewer dobson circulation bdc driven by mid latitude and subtropical atmospheric waves in 2000 the tls water vapor amount exhibited a stepwise transition to a dry phase apparently caused by a change in the bdc in this study we present observational and modeling evidence that the epochal change of water vapor between the periods of 1992 2000 and 2001 2005 was also partly caused by a concurrent sea surface temperature sst warming in the tropical central pacific this sst warming cools the ttl above by enhancing the equatorial wave induced upward motion near the tropopause which consequently reduces the amount of water vapor entering the stratosphere the qbo affects the tls water vapor primarily on inter annual timescales whereas a classical el nino southern oscillation enso event has small effect on tropical mean tls water vapor because its responses are longitudinally out of phase this study suggests that the tropical central pacific sst is another driver of tls water vapor variability on inter decadal timescales and the tropical sst changes could contribute to about 30 of the step wise drop of the lower stratospheric water vapor from 1992 2000 to 2001 2005

cirrus outflow dynamics abstract cirrus outflow from deep convection are analyzed as dynamically and thermodynamically active systems the initial outflow is considered as an analog to wake collapse in which a neutrally buoyant flow intrusion is flattened and stretched by its stratified environment and the initially isotropic turbulence within it is converted to other forms the early spread of the outflow is predicted on the basis of analytic and numerical calculations by dugan et al strong radiative heat flux curvature then leads to maintenance or regeneration of buoyant turbulence in the collapsed outflow plume mixed layer analysis allow predictions of entrainment rates in the case of strong net radiative heating the mixed layer model predicts an encroachment condition in which the upper boundary attains nearly the same temperature as the environment and grows into it rapidly this tendency is countered by bodily ascent of the warm mesoscale plumes the rate of which is predicted with the aid of a theory of drag on a f

tropical stationary waves in a nonlinear shallow water model with realistic basic states abstract the nonlinear shallow water equations are used to study the tropical stationary wave response to steady thermal forcing near the equator in earthlike zonally symmetric basic states a thin 200 m fluid layer is superimposed over a large 1500 m zonally symmetric topography distribution that decreases smoothly from the tropics to the poles thus providing the large meridional height gradients required to maintain realistic zonal mean zonal winds without introducing unrealistically large tropical wave speeds a mean meridional circulation is maintained by relaxing the fluid toward its initial global mean depth both hemispherically symmetric equinoctial and hemispherically asymmetric solstitial basic states are considered stationary waves are generated by adding a fixed mass source sink distribution near the equator the presence of westerly zonal mean winds in the subtropics amplifies the steady eddy response to the tropical mass forcing and shifts the rossby gyres poleward and eastward r

kelvin wave variability near the equatorial tropopause observed in gps radio occultation measurements 1 temperature fields in the equatorial upper troposphere and lower stratosphere derived from gps radio occultation measurements for 2001 2002 show evidence for planetary scale kelvin waves these waves have a characteristic eastward phase tilt with height and typical vertical wavelengths of 4 8 km the kelvin waves exhibit coherent vertical structure over 12 25 km with maximum amplitudes near the tropical tropopause 17 km the waves are often quasi stationary near the tropopause but exhibit regular eastward propagation in the lower stratosphere with periods near 20 days the quasi stationary waves modulate the climatological cold tropopause over indonesia the transient lower stratospheric waves show enhanced amplitudes coincident with the descending westerly shear phase of the quasi biennial oscillation qbo correlations with outgoing long wave radiation olr data show that global temperature patterns over 12 17 km with characteristic kelvin wave structure vary coherently with transient deep convection over indonesia

impact of confluence on hydrological and morphological characters of the trunk stream a study on the manu river of north east india tributaries play a significant role in changing flow structure of the main river through their additional discharge a study was conducted at the point of convergence of the river deo with the river manu using simple geospatial technique and field work to justify the hydrological and morphological changes in downstream of the confluence point the spatio temporal change of both the river channels for 10 km length in upstream and downstream of the confluence point was considered moreover field measurements and post field work were carried out to examine the spatial variation of hydrodynamic characteristics like flow velocity depth water discharge wetted perimeter and hydraulic radius of the selected stretches sieving method was applied for grain size analysis of river bed sediment samples the result revealed that both the aggradation and degradation processes were equally active in the upstream segment of the manu river but in downstream segment aggradation exceeded the degradation activity though all the hydrodynamic variables were boosted up in downstream except flow velocity the present research highlighted that the steeper gradient of the r deo had enhanced its competency to transport medium sized grains to the r manu where fine grains were commonly found moreover increased wetted perimeter in downstream specified more friction between channel bed and its flow consequent upon reduced flow velocity with extra sediment load accumulation

the implications of tropical rossby waves for tropical tropopause cirrus formation and for the equatorial upwelling of the brewer dobson circulation this study puts forward a mechanism for the observed upwelling in the tropical upper troposphere and lower stratosphere in this hypothesis the tropical upwelling is driven by momentum transport by rossby waves that are generated by tropical convection to test this hypothesis model runs are conducted using an axisymmetric global primitive equation model in these runs the effect of rossby waves is included by driving the model with observed fields of large scale eddy momentum flux convergence the resulting overturning circulation includes both meridional flow from the intertropical convergence zone itcz to the equator and rising motion in the tropical tropopause transition layer ttl this circulation therefore helps to explain the transport of moisture from the lower portion of the ttl in the itcz to the equatorial cold point tropopause where tropopause cirrus layers frequently occur

spatial and temporal structure of shear layer turbulence at a stream confluence 1 the shear layer that develops at the interface between converging flows is a prominent but poorly understood hydrodynamic feature at stream confluences to examine the spatial and temporal characteristics of turbulence and coherent turbulent structures within a shear layer three dimensional velocity measurements were obtained at a small stream confluence in illinois using two acoustic doppler velocimeters spaced at various separation distances results indicate that coherent structures can be readily identified through cross correlation analysis of the velocity time series for different measurement locations maximum correlations decrease relatively slowly over short separation distances but decrease rapidly for separation distances greater than about 2 times the flow depth convective velocities of coherent structures derived from the correlation analysis differ from local mean velocities and from spatially averaged mean velocities indicating that coherent structures evolve as they are transported along the shear layer a finding consistent with dye tracing experiments despite this evolution correlation and spectral analyses indicate that the frozen turbulence hypothesis holds for frequencies corresponding to length scales of at least some shear generated coherent vortices a domain of quasi two dimensional turbulence energy at low frequencies appears to be associated with large scale intrusions of fluid from one stream into the other along the mixing interface

why are the climatological zonal winds easterly in the equatorialupper troposphere abstract using 16 years of ncep ncar reanalysis data on the 200 mb surface it is shown that in the deep tropics the horizontal transient eddy momentum flux accelerates the zonal mean zonal wind this acceleration is mainly due to transient eddies of intraannual and interannual timescales and to those associated with the madden julian oscillation mjo the interannual timescale eddy fluxes are dominated by eastward propagating disturbances with zonal wavenumber 2 and a period of 2 4 yr suggesting that these eddy fluxes may be tied to the el nino southern oscillation in the deep tropics the single most important factor that decelerates the zonal mean zonal wind is the horizontal momentum flux divergence due to the transient meridional circulation associated with seasonal cycle of the hadley cells the deceleration by the transient meridional circulation is much greater than the acceleration due to the transient eddies this result indicates that a nonzero obliquity of the earth is crucial for maintai

tropical wave driving of the annual cycle in tropical tropopause temperatures part ii model results abstract the atmospheric response to a localized distribution of tropical heating is examined in terms of the stationary waves excited and how these impact the mean flow near the tropical tropopause this is done by examining nonlinear simulations of the gill model with a primitive equation model that extends from the surface up into the stratosphere the model produces strong cooling of zonal mean temperatures near the tropical tropopause when the heating is on the equator but weaker cooling with the heating at 15 n the model shows that equatorial rossby waves that penetrate the lower stratosphere and changes in ep flux divergence that correspond to the observed changes between december and august it is suggested that ascent in the upper tropical troposphere is driven by vorticity advection or equivalently potential vorticity fluxes due to these equatorial rossby waves particularly when the heating is close to the equator the model results provide support to the hypothesis that the annual cycle in tr

response of bed morphology and bed material texture to hydrological conditions at an asymmetrical stream confluence changes in bed morphology and sedimentology at stream confluences in relation to specific hydrological events have not been documented extensively moreover few if any studies have investigated the effect of net hydrological conditions on channel change at a confluence over a period of many years repeat cross section surveys and sampling of bed material at a small asymmetrical stream confluence reveal the short term effects of hydrological variability on bed morphology and spatial patterns of bed material texture survey data over a 15 year period illustrate the long term response of channel morphology at the confluence to net hydrological conditions results confirm that this confluence is a dynamic fluvial environment involving shifts in the zone of scour and repeated deposition and erosion of a bar complex at the downstream junction corner as the discharge ratio changes in response to hydrological variability in the tributary streams persistent faster runoff response times of the lateral tributary compared to the main stem have resulted in long term net erosion of the channel bank opposite this tributary the documented morphological and sedimentological changes are consistent with and can be explained by results of experimental studies and by past investigations at this confluence that have documented changes in patterns of mean flow structure and turbulence in response to variation in the discharge ratio of contributing streams

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

vegetated mixing layer around a finite size patch of submerged plants 1 theory and field experiments 1 dynamics of a vegetated mixing layer around a patch of submerged plants was examined theoretically and experimentally in a lowland river theoretical analysis explored analogy with hydrodynamic mixing layers and introduced equations that describe expansion of the layers and alterations in mean flow field experiments provided empirical data for validation of the theory and examination of the effects of population density on the mixing layer dynamics detailed measurements assessed the structure of turbulent flow plant morphology biomechanical properties bending and streamlining due to interaction with the flow the theory was compared with the field experiments and an agreement was concluded it was found that near the upstream edge of the patch the vegetated mixing layer expands similar to the canonical mixing layers although its statistics scale on the velocity differential characteristic for the stabilized part downstream mean momentum flux reduces in magnitude and reverses in direction while turbulence became dominant this was observed to stabilize the velocity differential and increase the mean velocity of the layer that result in its limited growth described by a logarithmic function the population density of vegetation was found to control the flow in dense vegetation the analogy with mixing layers resembled best while in sparse vegetation the flow behaved similar to the boundary layers this theory can be used for quantitative assessment of seasonal effects in natural vegetated rivers because the population density of vegetation varies during the vegetation season the companion paper focuses on the structural properties of turbulence in the vegetated shear layers

annual interannual and intraseasonal variability of tropical tropopause transition layer cirrus cloud fields based on the first three years of data from the cloud aerosol lidar and infrared pathfinder satellite observations calipso mission are used to investigate the relationship between cirrus within the tropical tropopause transition layer ttl and the madden julian oscillation mjo the annual cycle and el nino southern oscillation enso the ttl cirrus signature observed in association with the mjo resembles convectively induced mixed kelvin rossbywavesolutionsabovethepacificwarmpoolregion thissignatureiscenteredtotheeastofthe peak convection and propagates eastward more rapidly than the convection it exhibits a pronounced east ward tilt with height suggestive of downward phase propagation and upward energy dispersion a cirrus maximumisobservedoverequatorialafricaandsouthamericawhentheenhancedmjo relatedconvection enters the western pacific tropical mean ttl cirrus is modulatedby the mjo with more than twice as much ttl cirrus fractional coverage equatorward of 108 latitude when the enhanced convection enters the pacific than a few weeks earlier when the convection is over the indian ocean the annual cycle in cirrus clouds around the base of the ttl is equatorially asymmetric with more cirrus observed in the summer hemisphere higher in the ttl the annual cycle in cirrus clouds is more equatorially symmetric with a maximum in the boreal winter throughout most of the tropics the enso signature in ttl cirrus is marked by a zonal shift of the peak cloudiness toward the central pacific during el nino and toward the maritime continent during la nina

turbulent flow structure at a discordant river confluence asymmetric jet dynamics with implications for channel morphology only a handful of field studies have examined turbulent flow structure at discordant confluences the dynamics of flow at such confluences have mainly been examined in the laboratory this paper reports results of a field based investigation of turbulent flow structure at a discordant river confluence these results support the hypothesis that flow at a discordant alluvial confluence with a velocity ratio greater than two exhibits jet like characteristics scaling analysis shows that the dynamics of the jet core are quite similar to those of free jets but that the complex structure of flow at the confluence imposes strong effects that can locally suppress or enhance the spreading rate of the jet this jet like behaviour of the flow has important implications for morphodynamic processes at these types of confluences the highly energetic core of the jet at this discordant confluence is displaced away from the riverbed thereby inhibiting scour however helical motion develops adjacent to the jet particularly at high flows which may promote scour numerical experiments demonstrate that that the presence or absence of a depositional wedge at the mouth of tributary can strongly influence detachment of the jet from the bed and the angle of the jet within the confluence

the residual mean circulation in the tropical tropopause layer driven by tropical waves abstractlatent heating estimates derived from rainfall observations are used to construct model experiments that isolate equatorial waves forced by tropical convection from midlatitude synoptic scale waves these experiments are used to demonstrate that quasi stationary equatorial rossby waves forced by latent heating drive most of the observed residual mean upwelling across the tropopause transition layer within 15 of the equator the seasonal variation of the equatorial waves and the mean meridional upwelling that they cause is examined for two full years from 2006 to 2007 changes in equatorial rossby wave propagation through seasonally varying mean winds are the primary mechanism for producing an annual variation in the residual mean upwelling in the tropical tropopause layer averaged within 15 of the equator and between 90 and 190 hpa the annual cycle varies between a maximum upwelling of 0 4 mm s 1 during boreal winter and spring and a minimum of 0 2 mm s 1 during boreal summer this variabilit

estimating bed shear stress from remotely measured surface turbulent dissipation fields in open channel flows synoptic information on bed shear stress is necessary in predicting the transport of sediments and environmental contaminants in rivers and open channels existing methods of estimating bed shear stress typically involve measuring vertical profiles of streamwise velocity or reynolds stress and taking advantage of the logarithmic or the constant stress region respectively to determine friction velocity and subsequently bed shear stress while effective these methods yield local measurements of bed shear stress only direct measurements of bed shear stress can also be obtained through measurements with a drag plate however this method yields average shear stress information over the area of the plate and is impractical for large scale implementation in the field here we present a method capable of providing continuous synoptic measurements of bed shear stress over a large field of view a series of large scale particle image velocimetry lspiv and acoustic doppler velocimetry adv measurements were made in a variety of flows generated in a wide open channel facility turbulent dissipation is calculated on the free surface from the 2 d lspiv results and is correlated with near surface adv measurements of turbulent dissipation in the water column the adv results are consistent with the nezu 1977 established relationship for the vertical variation of turbulent dissipation in the water column knowledge of the correlation between free surface and near surface dissipation values coupled with nezu s 1977 relationship allow a robust and accurate estimate of friction velocity to be made and subsequently shear stress at the bed can be estimated this article is protected by copyright all rights reserved

rates and patterns of thermal mixing at a small stream confluence under variable incoming flow conditions confluences are important locations for river mixing within drainage networks yet few studies have examined in detail the dynamics of mixing within confluences this study examines the influence of momentum flux ratio the scale of the flow cross sectional area and the density differences between incoming flows on thermal mixing at a small stream confluence results reveal that rates and patterns of thermal mixing depend on event specific combinations of the three factors the mixing interface at this confluence is generally distorted towards the mouth of the lateral tributary by strong helical motion associated with curvature of flow from the lateral tributary as it aligns with the downstream channel as the momentum flux from the lateral tributary increases mixing is enhanced because helical motion from the curving tributary flow expands over the width of the downstream channel the cross sectional area of the flow is negatively correlated with mixing rates suggesting that the amount of mixing over a fixed distance downstream of the confluence is inversely related to the scale of the flow density differences are not strongly related to rates of mixing results confirm that mixing rates within the region of confluent flow interaction can be highly variable among flow events with different incoming flow conditions but that in general length scales of mixing are short and rates of mixing are high at this small confluence compared with those typically documented at large river confluences copyright 2015 john wiley sons ltd

on the tropical atmospheric signature of el nino abstractthe linear atmospheric signature of enso obtained by regressing fields of geopotential height z wind vertical velocity and rainfall upon the nino 3 4 sea surface temperature sst index is partitioned into zonally symmetric and eddy components the zonally symmetric component is thermally forced by the narrowing and intensification of the zonally averaged equatorial rain belt during el nino and mechanically forced by the weakening of the upper tropospheric equatorial stationary waves and their associated flux of wave activity the eddy component of the enso signature is decomposed into barotropic bt and baroclinic bc contributions the latter into first and second modal structures bc1 and bc2 separable functions of space x y and pressure p using eigenvector analysis bc1 exhibits a nearly equatorially symmetric planetary wave structure comprising three dumbbell shaped features suggestive of equatorial rossby waves with out of phase wind and geopotential height perturbations in the

time averaged flow structure in the central region of a stream confluence previous process oriented field studies of stream confluences have focused mainly on fluvial dynamics at or immediately downstream of the location where the confluent flows enter the downstream channel this study examines in detail the spatial evolution of the time averaged downstream velocity cross stream velocity and temperature fields between the junction apex where the flows initially meet and the entrance to the downstream channel a well defined vertically oriented mixing interface exists within this portion of the confluence suggesting that lateral mixing of the incoming flows is limited the downstream velocity field near the junction apex is characterized by two high velocity cores separated by an intervening region of low velocity or recirculating fluid in the downstream direction the high velocity cores move inwards towards the mixing interface and high velocity fluid progressively extends downwards into a zone of scour resulting in an increase in flow velocity in the centre of the confluence the cross stream velocity field is dominated by flow convergence but also includes a component associated with a consistent pattern of secondary circulation this pattern is characterized by two surface convergent helical cells one on each side of the mixing interface the helical cells appear to be the mechanism by which high momentum fluid near the surface is advected downwards into the zone of scour for transport ineffective flows the dimensions and intensities of the cells are controlled by the momentum ratio of the confluent streams and by the extant bed morphology within the confluence although the flow structure of formative events was not measured directly in this study documented patterns of erosion and deposition within the central region of the confluence suggest that these events are dynamically similar to the measured flows except for the fact that formative flows are not constrained by but can reshape the bed morphology the results of this investigation are consistent with and augment previous findings on time averaged flow structure in the downstream portion of the confluence 1998 john wiley sons ltd

braided rivers perspectives and problems progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems although many advances have been made within recent years in interpreting the mechanics of flow transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed in particular several areas demand attention the mechanisms of braid bar initiation confluence diffluence dynamics the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales this paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration 1993 the geological society

dynamics of shallow lateral shear layers experimental study in a river with a sandy bed shallow lateral shear layers forming between flows with different velocities though essential for mixing processes in natural streams have been examined only in laboratory settings using smooth fixed bed channels this paper reports the results of an experimental study of a shear layer in a straight reach of a natural river where the layer in contrast to the two dimensional structure observed in the laboratory is highly three dimensional the study included pronounced transverse pressure gradients which influenced shear layer structure compared to flume experiments it also introduces an analysis that complements conventional theory on mixing layers the lateral velocity gradient between the flows downstream from a splitter plate placed in the river the principal controlling factor was adjusted for three experimental runs to determine the influence of different gradients on shear layer dynamics in each run detailed three dimensional measurements of mean and turbulent characteristics were obtained at five cross sections downstream from the splitter plate although experimental results agreed with conventional mixing layer theories with respect to turbulence the dynamics of the shear layers were dominated by the mean lateral fluxes of momentum after re examining the governing equations we developed a parabolic equation describing the shear layer evolution and several scaling relations for essential terms of the energy budget mean and turbulent lateral fluxes of momentum turbulent kinetic energy and dissipation rates the study also provides insight into the spectral dynamics of turbulence in the shear layer and clarifies previous observations reported for confluences in natural streams

turbulence structure in a river reach with sand bed measurements and analysis of the three dimensional turbulence structure in a straight lowland river reach are presented accurate measurements of velocity profiles were taken with an acoustic doppier velocimeter adv and a multichannel micropropeller based system it is shown that analytical expressions derived from investigations of laboratory open channel flows agree well with the measured data only in the central part of the river where flow can be considered weakly three dimensional at the same time clear differences of empirical parameters between river and laboratory flows are detected it was established that river turbulence is isotropic for spatial scales smaller than the river depth the data also allowed the detection and analysis of the turbulence anisotropy impact on the formation of secondary currents special attention is paid to the investigation of coherent structures their spatial scales were evaluated applying the conditional average procedure based on uw quadrant analysis on average the scales of ejection and sweep events are as large as 1 5 times the flow depth

case study effect of submerged aquatic plants on turbulence structure in a lowland river interactions of aquatic plants with turbulent flows in fluvial systems have attracted considerable interest while there have been recent advances in theories that describe vegetation flow interactions in idealized laboratory flows their practical application is still problematic due to limited knowledge of effects caused by heterogeneously patchy distributed plants in naturally formed vegetative mosaics in rivers this paper reports on a study in a lowland river aimed at quantification and parameterization of vegetation effects on redistribution of mean and turbulent characteristics of the flow and their consequences for hydraulic resistance the measurements were carried out in summer on a river reach with a patchy mosaic dominated by submerged flexible aquatic plants and repeated at the same water level in early spring before the plants start growing this design of the study allowed for quantitative evaluation of the effects caused by flow plants interactions on bulk flow parameters at comparable submergences of riverbed roughness elements sediment grains and sand bars the study indicates that symmetrical quasi two dimensional open channel flow structure in unvegetated riverbed was transformed into highly fragmented complex flow pattern spatially arranged by patches and free paths in the mosaic despite complexities and three dimensionality of the flow normalized mean velocity profiles in the patches were satisfactory described by hyperbolic tangent function while flow in the free paths similarly to unvegetated channel was in a reasonable agreement with the conventional logarithmic law

the rubble pile asteroid itokawa as observed by hayabusa during the interval from september through early december 2005 the hayabusa spacecraft was in close proximity to near earth asteroid 25143 itokawa and a variety of data were taken on its shape mass and surface topography as well as its mineralogic and elemental abundances the asteroid9s orthogonal axes are 535 294 and 209 meters the mass is 3 51 1010 kilograms and the estimated bulk density is 1 9 0 13 grams per cubic centimeter the correspondence between the smooth areas on the surface muses sea and sagamihara and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting itokawa is considered to be a rubble pile body because of its low bulk density high porosity boulder rich appearance and shape the existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re agglomeration into a rubble pile object

the planform mobility of river channel confluences insights from analysis of remotely sensed imagery river channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment and which are locations for the preservation of thick fluvial deposits overlying a basal scour despite their importance there has been little study of the stratigraphic characteristics of river junctions or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential as a result although it is known that confluences can migrate through time models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location this is problematic for a number of reasons not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes such as confluence scour and that driven by allocyclic controls such as sea level change this paper investigates the spatial mobility of river confluences by using the 40 year record of landsat imagery to elucidate the styles rates of change and areal extent over which large river confluence scours may migrate on the basis of these observations a new classification of the types of confluence scour is proposed and applied to the amazon and ganges brahmaputra meghna gbm basins this analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally thus in the gbm basin a high sediment supply large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80 of large confluences are mobile over this 40 year window conversely this figure is 40 for the amazon basin these results highlight that i the potential areal extent of confluence scours is much greater than previously assumed with the location of some confluences on the jamuna brahmaputra river migrating over a distance of 20 times the tributary channel width ii extensive migration in the confluence location is more common than currently assumed and iii confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield

equatorial planetary waves and their signature in atmospheric variability abstractequatorial planetary waves are a fundamental component of the tropical climate system previous studies have examined their structure in the climatological mean circulation their role in the climatological mean momentum balance of the tropics and their contribution to the climatological mean upwelling across the tropical tropopause in this study the authors focus on the contribution of the equatorial planetary waves to variability in the tropical circulation about its climatological mean state the equatorial planetary waves that dominate the climatological mean exhibit considerable variability on intraseasonal and interannual time scales variability in the amplitude of the equatorial planetary waves is associated with a distinct pattern of equatorially symmetric climate variability that also emerges from empirical orthogonal function analysis of various tropical dynamical fields variability in the equatorial planetary waves is characterized by variations in 1 convection in the deep tropics

space time variability of equatorial kelvin waves and intraseasonal oscillations around the tropical tropopause 1 using the ecmwf 40 year reanalysis data large scale variations of temperature and zonal wind around the tropical tropopause are investigated from the space time spectral analysis two dominant spectral regions in the eastward propagating domain are found one for kelvin waves and the other for intraseasonal oscillations isos to investigate activities of kelvin waves and isos we reconstructed the grid data for the two spectral windows and calculated the square amplitude at each grid point kelvin wave activities in zonal wind are vigorous during two seasons around february and july in the upper troposphere up to 100 hpa however the longitudinal maxima are found in the eastern hemisphere at 100 hpa and in the western hemisphere at 150 hpa and below activities calculated from the outgoing longwave radiation data for the kelvin wave spectral window are different from those in zonal wind about their maximum longitudes and seasons as the climatological background zonal wind in the upper troposphere is easterly in the eastern hemisphere and westerly in the western hemisphere it seems that the difference of kelvin wave activities between 100 hpa and 150 hpa is closely related to the background wind field affecting kelvin wave propagation and dissipation iso activities in zonal wind at 100 hpa are vigorous in the western pacific during northern winter and we may find relation to those in olr this indicates that the eastward moving disturbance with the iso time scale is coupled with the organized convective system such as the madden julian oscillation

transport of water through the tropical tropopause total water was measured in the high troposphere and low stratosphere over panama during ten aircraft flights the results show that convective storms provide the means of transporting water into the stratosphere from a consideration of the anvil heights over different areas of the tropical zone it follows that a negative gradient of water vapor mixing ratio with altitude must exist over most of the lower stratosphere

convective influence on the heat balance of the tropical tropopause layer a cloud resolving model study the tropical tropopause layer ttl and in particular the cold point tropopause has been previously suggested as a feature decoupled from convection using a cloud resolving model the authors demonstrate that convection in fact has a cooling effect in the ttl that significantly affects its thermal structure in particular the cold point is found to be strongly tied to the convective cooling maximum the authors interpret these as natural features of an entrainment layer such as the ttl the recognition that the cold point tropopause is strongly tied to rather than decoupled from convection suggests that dehydration processes at the cold point cannot be assumed as gradual and the effect of convection may not be ignored

shallow mixing layer downstream from a sudden expansion abstractthis study aims at investigating the mixing layer located at the interface between the free stream and recirculation zone downstream from a sudden lateral open channel expansion specific a

experimental investigation of shallow recirculating flows experiments were conducted to study the bed friction influence in the recirculating zone of a shallow open channel flow velocity measurements were made using a hot film anemometer the recirculating flow rate and the length of the recirculating zone were determined from the measurements and they are related to a horizontal length scale and a bed friction length scale for deep flows the results are independent of the bed friction length scale for shallow flows the results are independent of the horizontal length scale turbulence in the recirculating flows is generated on one hand by the bed friction and on the other hand by the transverse shear the bed generated component is proportional to the bed friction velocity the transverse component is confined to a transverse mixing region in the limit of shallow water depth when the flow is under strong bed friction influence both the transverse mixing region and the recirculating zone have the same length of about one friction length scale

flow and particle paths at a natural river confluence with coarse bed material abstract an investigation of flow and particle trajectories was conducted at a gravel bed confluence where the junction angle is gradually reduced from 60 to 20 as the tributary joins the main channel a shallow and elongated scour zone is present at the mouth of the tributary three flow stages ranging from 1 7 to 1 2 bankfull were recorded flow velocity vectors were measured from five bridges installed across the channels sediment transport paths were determined by following the progression of seeded marked particles through the confluence from may to october 1986 the results show that flow structure is responsive to stages at lower stages less than 1 3 bankfull flow velocity vectors are controlled by local bed topography and specifically by the steeper slopes around the scour zone at higher stages velocity vectors become aligned with the plan geometry of the confluence for all stages surface flow velocities are higher in the scour zone bed shear stress computed from the law of the wall for the deepest flow is higher at the bottom of the scour zone compared to its sides head and exit the trajectories of particles seeded on the tributary pass through the scour zone particles coming from the main channel displayed a dual pattern first moving laterally towards the scour zone early in the season then following paths that are consistent with the plan geometry of the confluence this shift in pattern reflects an increase in the peak discharges during the survey period these results highlight the role of stage in controlling the flow dynamics and particle paths at the confluence

on the signatures of equatorial and extratropical wave forcing in tropical tropopause layer temperatures abstracttemperatures in the tropical tropopause layer ttl play an important role in stratosphere troposphere exchange and in the formation and maintenance of thin cirrus clouds many previous studies have examined the contributions of extratropical and equatorial waves to the ttl using coarse vertical resolution satellite and reanalysis data in this study the authors provide new insight into the role of extratropical and equatorial waves in the ttl using high vertical resolution gps radio occultation data the results examine the influence of four different wave forcings on the ttl extratropical waves that propagate vertically into the stratosphere extratropical waves that propagate meridionally into the subtropical stratosphere extratropical waves that propagate meridionally into the subtropical troposphere and the equatorial planetary waves the vertically and meridionally propagating extratropical stratospheric waves are associated with deep zonally symmetric temperature anomalies that extend a

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

effect of clay fraction on turbulence characteristics of flow near an eroded bank abstract stream bank erosion in the alluvial channel is a matter of great concern particularly for the irretrievable loss of fertile land this in effect has direct impact on the agriculture environment economy and social issues at large a series of laboratory experiments were carry out to study the turbulence structure over eroded bank topography modulation of mean turbulent flow characteristics and the benefaction of turbulent bursting structures to the total reynolds shear stress due to the changed geometry of the eroded bank wall are described for three different clay fraction variations a side looking micro acoustic doppler velocimeter adv was used to record the detailed high frequency velocity data near the bank wall to understand the coupled dynamics of flow and erosion on sediment water interface the information on clay and fine sand content of natural river bank was obtained from field observation results from the present study indicate that higher turbulence intensities and reynolds shear stress have prominent role in the removal of bank materials from the bank wall and continues the dislodgement of material from the bank face until mass failure quadrant analysis to the random velocity fluctuation in the vicinity of eroded stream bank shows that downstream bank material movement is strongly correlated to the presence of sweep events probability density function pdf kinetic energy fluxes and coefficients of skewness and kurtosis of velocity fluctuations show significant changes with the change in clay fraction of the bank material composition though the velocity spectral densities are in general observed to be similar for all the three different bank undercut depths for different clay fraction of bank however spectral energy shows signature of higher energy peak for lower clay fraction of the bank material

flow and turbulence structure around a spur dike in a channel with a large scour hole 1 the flow and turbulence structure around a vertical wall spur dike that extends over the whole depth of a straight channel are investigated using detached eddy simulation des the channel reynolds number in des and the corresponding experiment is red 2 4 105 which is typical of applications in small rivers and outside the range where well resolved large eddy simulation les can be conducted the bathymetry at equilibrium scour conditions is obtained from a loose bed experiment this paper discusses the main mechanisms which drive the growth of the scour hole upstream and downstream from the spur dike during the later stages of the scour process and how these mechanisms change between the initial flat bed conditions and later stages of the scour process scale effects are investigated by comparing simulation results at red 2 4 105 with those from simulations performed at a much lower reynolds number red 18 000 results show that while the structure of the horseshoe vortex hv system changes with respect to the case in which the bed is flat the main necklace vortex of the hv system is still subject to large scale aperiodic oscillations similar to the ones observed in flows past in stream bluff body obstacles mounted on a flat surface present results show that the amplification of the horizontal vorticity within the lower part of the separated shear layer ssl and the associated formation of streaks of high bed shear stress below the region where the ssl eddies are convected in the near bed region is a general feature of high reynolds number flow past a vertical wall spur dike placed in a loose bed channel at all stages of the scour process