does flow variance affect bedload flux when the bed is dominated by grain roughness abstract previous studies have shown that spatial variance in fluid and critical shear stress caused by form roughness can increase bedload flux others have revealed that variance in flow velocity increases with relative submergence and that bed mobility is reduced at lower submergences the paper explores the link between these observations and addresses the following questions is grain roughness sufficient to cause variance in fluid shear stress and an increase in bedload flux if this variance changes with submergence does this mean that the increase is dependent on submergence and does this explain the change in mobility with submergence a simple statistical bedload model based on spatial distributions of fluid and critical shear stress has been used to explore these effects over a water worked gravel deposit estimates of spatially distributed fluid shear stress were gained from laboratory flume measurements of near bed flow velocity and a distribution of critical shear stress was simulated using a discrete particle model of the sediment distribution used in the flume the velocity data were used to describe the change in the spatial distribution of near bed velocity with relative submergence which allowed the effects of submergence on flux to also be considered the main conclusions were i spatial variance in fluid shear stress from grain roughness was not sufficient to have an appreciable effect on bedload flux over water worked gravel beds with a spatial distribution of critical shear stress ii spatial variance in critical shear stress caused by grain roughness had a much larger influence and increased bedload flux this was a similar level of increase observed in studies for conditions where form roughness was high iii spatially averaged estimates of fluid and critical shear stress should not be used to estimate bedload flux even if form roughness is low and iv a rise in relative submergence increased bedload flux this was due to changes in the spatial distribution of near bed velocity and not due to a lowering in the local flow velocity as has been suggested by previous studies

topographic disturbance of subaqueous gravel substrates by signal crayfish pacifastacus leniusculus abstract the impact of signal crayfish pacifastacus leniusculus on the topography and fabric of six narrowly graded gravel substrates was investigated using repeat laser scanning of sediment surfaces in still water aquaria digital elevation models dems of the gravel surfaces were obtained before and after exposure to crayfish for five predetermined periods the impact on the substrate was quantified by establishing topographic and volumetric changes using dems of difference dod the presence of an individual medium sized crayfish for 24 h resulted in an average volume change in surface topography of 450 cm 3 over an area of 2400 cm 2 giving a sediment displacement of 1 7 kg m 2 d 1 the majority 78 of this volume change was associated with small scale 1 median grain diameter movements of surface grains this fabric adjustment altered grain orientations and friction angles crayfish also constructed pits and mounds that increased significantly the roughness of the gravel substrates and altered the protrusion of individual grains crayfish were able to move material up to 38 mm in diameter that had a submerged weight six times that of the individuals used in this study by modifying the arrangement of grains on the surface of fluvial substrates signal crayfish may counteract the low flow physical consolidation of gravel beds and reduce the entrainment stresses required to move river bed material the results of this study suggest that signal crayfish an internationally widespread invasive species may have substantial impacts on the physical environment of streams and rivers as well as on local benthic ecological communities

the evaluation of grain shear stress from experiments in a pebble bedded flume initial experiments were conducted in a flat bottomed flume with pebbles median diameter slightly below 20 mm with a view to test the validity of the methods which permit differentiation between shear stress due to grain resistance and that due to bedform resistance it is clearly confirmed that represents the total shear stress the method of laursen 1958 is appropriate for evaluation of grain shear stress in the case of pebbly beds indeed the ratio is close to 1 except when the dimensionless flow depth d d50 is less than 4 a close relationship exists between the grain shear stress and the shear calculated from friction velocities a second series of experiments was then carried out in the same flume after creating differences in bedform alternating riffles and pools the ratio produced by laursen s method represents on average 0 40 the relationship between these grain shear stresses and the shear stresses calculated from shear velocities is relatively good here as well and is similar in presentation to that established in the case of flat bed finally as confirmation of the adequacy of laursen s method under the same initial conditions of bottom slope and discharge the grain shear stress for an undulating bed is very close to the total shear stress of a flat bed

effects of a pebble cluster on the turbulent structure of a depth limited flow in a gravel bed river pebble clusters are important morphological features of gravel bed rivers and have a significant effect on the spatial and temporal response of the flow this paper presents original results from measurements of velocity taken at a high spatial and temporal resolution in a turbulent flow upstream and downstream from a pebble cluster three electromagnetic current meters 0 013 m in diameter mounted on a wading rod were deployed upstream and downstream of an isolated pebble cluster along 31 vertical profiles that contained between 7 and 13 points in a 5 m long section the high sampling resolutions in a natural environment permitted characterization of the complex dynamics of the turbulent flow in the vicinity of the cluster the description of these dynamics is based on the interpretation of contour maps of several turbulence statistics including moments of velocity distributions reynolds shear stress quadrant analysis and autocorrelation function downstream from the cluster flow is decelerated the intensity of turbulence increases and zones of strong vertical motions are present the superimposition of the maps allowed us to decompose the field of turbulent flow around a pebble cluster into distinct regions in a downstream direction from the cluster we observed zones of flow acceleration recirculation vortex shedding flow reattachment fluid upwelling and flow recovering results indicate the existence of highly dynamic boundaries between zones where strong intermittent events are generated they also highlight the difference between shedding and upwelling motions of the flow

delineation of gravel bed clusters via factorial kriging abstract gravel bed clusters are the most prevalent microforms that affect local flows and sediment transport a growing consensus is that the practice of cluster delineation should be based primarily on bed topography rather than grain sizes here we present a novel approach for cluster delineation using patch scale high resolution digital elevation models dems we use a geostatistical interpolation method i e factorial kriging to decompose the short and long range grain and microform scale dems the required parameters are determined directly from the scales of the nested variograms the short range dem exhibits a flat bed topography yet individual grains are sharply outlined making the short range dem a useful aid for grain segmentation the long range dem exhibits a smoother topography than the original full dem yet groupings of particles emerge as small scale bedforms making the contour percentile levels of the long range dem a useful tool for cluster identification individual clusters are delineated using the segmented grains and identified clusters via a range of contour percentile levels our results reveal that the density and total area of delineated clusters decrease with increasing contour percentile level while the mean grain size of clusters and average size of anchor clast i e the largest particle in a cluster increase with the contour percentile level these results support the interpretation that larger particles group as clusters and protrude higher above the bed than other smaller grains a striking feature of the delineated clusters is that anchor clasts are invariably greater than the d90 of the grain sizes even though a threshold anchor size was not adopted herein the average areal fractal dimensions hausdorff besicovich dimensions of the projected areas of individual clusters however demonstrate that clusters delineated with different contour percentile levels exhibit similar planform morphologies comparisons with a compilation of existing field data show consistency with the cluster properties documented in a wide variety of settings this study thus points toward a promising alternative dem based approach to characterizing sediment structures in gravel bed rivers

influence of particle shape on surface roughness dissimilar morphological structures formed by man made and natural gravels abstract static armored gravel surfaces composed of man made and natural gravels have been analyzed using precise digital elevation models the scaling behavior and spatial arrangement of the gravel surfaces were evaluated and discussed with respect to the successive formation of stable armor layers the key results relate to structure functions and imbrication analysis of these surfaces 1 man made gravel surfaces show a similar scaling behavior and spatial correlation pattern to natural gravel surfaces 2 man made gravel surfaces have larger hurst exponents than natural gravel surfaces and 3 man made gravels are poorly imbricated in comparison with natural gravels our results suggest that different morphological structures between man made and natural gravels were attributed to shape features of individual grains

turbulence at a roughness transition in a depth limited flow over a gravel bed turbulence statistics derived from velocity measurements along a longitudinal transect characterized by a roughness transition are presented the bed morphology along the transect is dominated by composite scales of roughness ranging from an armoured bed in the upstream section to the presence of large protuberant clasts superimposed on the bed in the downstream section an array of four electromagnetic current meters operating at a frequency of 20 hz was used to measure the longitudinal and vertical velocities at various heights above the bed surface and at different positions along the transect emphasis is put on the longitudinal and vertical distributions of turbulent flow statistics such as reynolds shear stress uv cross correlation and quadrant analysis of velocity fluctuations the results indicate that the presence of protuberant clasts modifies the turbulence structure increases turbulence intensity and appears to dominate turbulence generation distinct flow zones are observed in particular a clear distinction is made between i a sublayer below the top of the major roughness elements identified by fluid motions predominantly towards the bed surface and characterized by intense eddying with some vortices or rollers possibly being trapped in this near bed flow region and ii an inner layer located immediately above the major protuberant clasts identified by positive vertical velocities and hence upward directed fluid motion the dominant generative mechanism of flow structure appears to be the shedding of vortices from the lee of obstacles interaction of vortices stretched by shear into the main body of flow downstream from major obstacles dominates the turbulence structure in depth limited flows over coarse and irregular bed surfaces

through water terrestrial laser scanning of gravel beds at the patch scale acquiring high resolution topographic data of natural gravel surfaces is technically demanding in locations where the bed is not exposed at low water stages often the most geomorphologically active surfaces are permanently submerged gravel beds are spatially variable and measurement of their detailed structure and particle sizes is essential for understanding the interaction of bed roughness with near bed flow hydraulics sediment entrainment transport and deposition processes as well as providing insights into the ecological responses to these processes this paper presents patch scale laboratory and field experiments to demonstrate that through water terrestrial laser scanning tls has the potential to provide high resolution digital elevation models of submerged gravel beds with enough detail to depict individual grains and small scale forms the resulting point cloud data requires correction for refraction before registration preliminary validation shows that patch scale tls through 200 mm of water introduces a mean error of less than 5 mm under ideal conditions point precision is not adversely affected by the water column the resulting dems can be embedded seamlessly within larger sub aerial reach scale surveys and can be acquired alongside flow measurements to examine the effects of three dimensional surface geometry on turbulent flow fields and their interaction with instream ecology dynamics copyright 2011 john wiley sons ltd

particle clusters in gravel bed rivers an experimental morphological approach to bed material transport and stability concepts structured gravel river beds clearly exert a major influence on bed stability indexing structural stability by field measurements of bed strength neglects the processes operating to entrain and transport bed material in different parts of each structure this study takes a morphological approach to interpreting the critical processes using particle tracing to determine the movement of individual cluster particles over a range of flood event magnitudes and durations the experiment was carried out on the river south tyne uk it uses flow hydrographs measured nearby and also benefits from previous studies of historical development channel morphology and sediment transport at the same site more than 30 clusters were monitored over a seven month period during which clusters occupied 7 16 per cent of the bed threshold flows delimiting three apparently contrasting bed sediment process regimes for cluster particles are tentatively set at 100 m3 s 1 and 183 m3 s 1 durations of flow at these levels are critical for cluster development rather than flow peak values wake particles are transported most easily flow straightening in the wandering channel planform reduces the stability of clusters since mechanical strength is markedly reduced by this change of direction the overall area covered by clusters between significant transport events varies little implying a dynamic equilibrium condition copyright 2005 john wiley sons ltd

effect of non uniformity of flow on velocity and turbulence intensities over a cobble bed non uniform flows encompassing both accelerating and decelerating flows over a cobble bed flume have been experimentally investigated in a flume at a scale of intermediate relative submergence measurements of mean longitudinal flow velocity u and determinations of turbulence intensities u v w and reynolds shear stress ufwf have been made the longitudinal velocity distribution was divided into the inner zone close to the bed and the outer zone far from the bed in the inner zone of the boundary layer near the bed the velocity profile closely followed the log law however in the outer zone the velocity distribution deviated from the log law consistently for both accelerating and decelerating flows and the changes in bed slopes ranging from 2 to 2 had no considerable effect on the outer zone for a constant bed slope s 0 015 the larger the flow rate the smaller the turbulence intensities however no detectable pattern has been observed for u v and w distributions near the bed likewise for a constant flow rate q 0 040 m3 s with variation in bed slope the longitudinal turbulent intensity profile in the longitudinal direction remained concave for both accelerating and decelerating flows whereas vertical turbulent intensity w profile presented no specific form the results reveal that the positions of maximum values of turbulence intensities and the reynolds shear stress depend not only on the flow structure accelerating or decelerating but also on the intermediate relative submergence scale copyright 2009 john wiley sons ltd

comparing different methods of bed shear stress estimates in simple and complex flow fields bed shear stress is a fundamental variable in river studies to link ow conditions to sediment transport it is however dif cult to estimate this variable accurately particularly in complex ow elds this study compares shear stress estimated from the log pro le drag reynolds and turbulent kinetic energy tke approaches in a laboratory ume in a simple boundary layer over plexiglas and over sand and in a complex ow eld around de ectors results show that in a simple boundary layer the log pro le estimate is always the highest over plexiglas the tke estimate was the second largest with a value 30 per cent less than the log estimate however over sand the tke estimate did not show the expected increase in shear stress in a simple boundary layer the reynolds shear stress seems the most appropriate method particularly the extrapolated value at the bed obtained from a turbulent pro le in a complex ow eld around de ectors the tke method provided the best estimate of shear stress as it is not affected by local streamline variations and it takes into account the increased streamwise turbulent uctuations close to the de ectors it is suggested that when single point measurements are used to estimate shear stress the instrument should be positioned close to 0 1 of the ow depth which corresponds to the peak value height in pro les of reynolds and tke shear stress copyright 2004 john wiley sons ltd

estimation of runoff critical shear stress for soil erosion from soil shear strength abstract critical shear stress is an important soil parameter governing detachment by runoff which appears in numerous erosion models soil shear strength has been often presented as the best soil property to predict critical shear stress however only a few studies have documented the relationship between soil shear strength and critical shear stress and the results obtained have sometimes seemed contradictory this is why we have put together all available data about the relationship between critical shear stress and soil shear strength in order to analyse and discuss its significance and its generality results showed that contradictions between different studies mainly arise because total shear stress is a poor predictor of detachment on rough soils when grain shear stress is used rather than total shear stress a significant relationship between saturated soil shear strength and critical shear stress does exist soil shear strength may thus be used to predict critical grain shear stress however it is recalled that in general only total shear stress is predicted by erosion models which makes detachment predictions difficult on rough natural soils

historical biogeography of the cicadas of wallacea new guinea and the west pacific a geotectonic explanation abstract the present day distribution patterns of the cicadas of wallacea new guinea and the west pacific reflect the extremely complex geotectonic history of that area the patterns found in two unrelated monophyletic groups of cicadas insecta homoptera cicadoidea the subtribe cosmopsaltriaria and the tribe chlorocystini with its sister tribe prasiini are analysed and related to data from the geological literature the two groups have a comparable distributional range over sulawesi maluku new guinea the bismarck archipelago and the east melanesian archipelagos to tonga and samoa the occurrence of endemic species species groups and genera in congruently delimited parts of this range made us recognize 14 areas of endemism the areas of endemism coincide with geological entities which have been isolated during a considerable period of time most of the areas originated from oceanic island arcs which developed as a result of subduction along the southern and western edges of the pacific tectonic plate others are rifted microcontinents concentrations of species and especially endemic species of the various cicada genera in different areas of endemism suggest that each of these genera evolved in isolation on one of the afore mentioned geological entities these entities can therefore be regarded as source areas for the genera the island arc fragmented as a result of collisions first with the asian and later with the australian tectonic plate and several of its parts reamalgamated at the northern craton of the australian plate where they now form the greater part of the island of new guinea after reamalgamation of the arc fragments the cicadas of the respective genera that had evolved on these fragments could disperse into adjoining areas but since cicadas are poor dispersers the source areas of the genera can still be recognized in present day distribution patterns the generic relationships in the two groups of cicadas under study indicate area relationships that comply with the latest palaeogeographic reconstructions the main vicariant speciation events in the cicada cladograms correspond with the presumed sequences of fragmentation of the island arc cicada phylogeny and distribution combined reflect the historic area relationships rather than the present ones

characteristics of velocity profiles along riffle pool sequences and estimates of bed shear stress abstract numerous velocity profiles were measured along a riffle pool sequence for three different flow conditions for each flow condition ranging from baseflow to about 2 3 of bankfull depth ten to fifteen velocity profiles were measured along the same transect the objectives of the study were i to differentiate between components of bed roughness and shear stress attributed to surface friction and large scale bed undulations and ii to investigate the effects of fluctuating discharges on the mean vertical flow structure in pools and riffles no previous attempts have been made to investigate scales of bed roughness from velocity profiles along riffle pool sequences and to determine how near bed velocity gradients change with increasing flow discharge at low flows and as expected riffles are characterized by higher near bed velocity gradients than pools similarly average bed shear stresses as determined from measurements of flow depth and energy gradient indicate a greater resistance and bed shear stress in riffle sections invariably however local shear stresses as determined from near bed velocity gradients in pools and in riffles are much smaller than reach averages estimated from measurements of depth and slope the shape of spatially averaged velocity profiles changes as discharges increases the difference in terms of near bed velocities between pools and riffles decreases as discharge increases which suggests that a velocity reversal may occur at or near bankfull further away from the bed surface velocity differences between pools and riffles do remain significant finally the curvature of the velocity profiles is more significant at high flows and surface roughness is maximized when flow conditions are intermediate perhaps a result of increasing density of small scale sedimentary structures

relationships between mesoscale morphological units stream hydraulics and chinook salmon oncorhynchus tshawytscha spawning habitat on the lower yuba river california an expert based approach was used to identify 10 morphological unit types within a reach of the gravel bed regulated yuba river california that is heavily utilized by spawning chinook salmon oncorhynchus tshawytscha analysis of these units was carried out using two dimensional hydrodynamic modeling field based geomorphic assessment and detailed spawning surveying differently classified morphological units tended to exhibit discrete hydraulic signatures in most cases the froude number adequately differentiated morphological units but joint depth velocity distributions proved the most effective hydraulic classification approach spawning activity was statistically differentiated at the mesoscale of the morphological unit salmon preferred lateral bar riffle and riffle entrance units these units had moderately high velocity unit median 0 45 m s 1 and low depth unit median 0 6 m but each exhibited a unique joint depth velocity distribution a large proportion of redds 79 were associated with conditions of convective flow acceleration at riffle and riffle entrance locations in addition to reflecting microhabitat requirements of fish it was proposed that the hydraulic segregation of preferred from avoided or tolerated morphological units was linked to the mutual association of specific hydraulic conditions with suitable caliber sediment that promotes the provision and maintenance of spawning habitat

linking spatial patterns of bed surface texture bed mobility and channel hydraulics in a mountain stream to potential spawning substrate for small resident trout abstract in this paper we examined spatial patterns of bed texture bed mobility and channel hydraulics in four reaches of a mountain stream with moderate gradient

geostatistical analysis of the effects of stage and roughness on reach scale spatial patterns of velocity and turbulence intensity although previous research has documented well organized interactions between the turbulent flow field and an irregular boundary the spatial variability of turbulent flow characteristics at the reach scale remains poorly understood in this paper we present detailed field measurements of three dimensional flow velocities and turbulence intensities in a high gradient cobble bed riffle from three discharges additional data on sediment grain size and bed topography were used to characterize boundary roughness an acoustic doppler velocimeter was used to measure velocities along five cross sections within a 6 in long reach of the north fork cache la poudre river vertical profiles were also measured along the channel thalweg we adopted a spatially explicit stochastic hydraulic approach and focused not on coherent flow structures per se but rather time averaged reach scale variability and spatial pattern scaling velocities and turbulence intensities by the reach averaged friction velocity u accounted for changes in flow depth and enabled comparisons among the three discharges we quantified the effects of stage and roughness by assessing differences among probability distributions of hydraulic quantities and by examining geostatistical metrics of spatial variability we computed semivariograms for streamwise and transverse directions and fit parametric models to summarize the spatial structure of each variable at each discharge cross correlograms were also used to describe the local and lagged effects of boundary roughness on flow characteristics although the probability distributions yielded some insight incorporating spatial information revealed important elements of stage dependent flow structure the development of secondary currents and flow convergence at higher stages were clearly documented in maps and semivariograms in general the spatial structure of the flow field became smoother and more continuous as stage increased and the effects of boundary roughness diminished although roughness elements do influence velocities and turbulence intensities our data suggest that the flow primarily responds to the gross morphology of the channel and that flow depth is the primary control on flow structure the geostatistical framework proved useful and our results indicate that a complete stochastic description must also be explicitly spatial c 2006 elsevier b v all rights reserved

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

coherent flow structures in a depth limited flow over a gravel surface the influence of surface roughness 1 turbulent flows moving over a gravel bed develop large scale macroturbulent flow structures that are initiated at anchor clasts in the bed and grow and dissipate as they move upward through the flow depth this paper extends previous research in which we investigated the influence of the reynolds number on coherent flow structures generated over a gravel bed by assessing the importance of effective bed roughness here we report on flume experiments in which flows over beds of decreasing surface roughness have been quantified through the application of digital particle imaging velocimetry which allows study of the downstream and vertical components of velocity over the entire flow field these results indicate that as the effective roughness increases 1 the visual distinctiveness of the coherent flow structures becomes more defined throughout the flow depth 2 the upstream angle of slope of the coherent flow structure increases and 3 the reduction in streamwise flow velocity and turbulence intensity toward the upstream side of the structure becomes greater applying standard scaling laws these structures appear shear generated and form through a combination of both wake flapping and the reattachment of localized shear layers associated with flow separation around individual topographic protrusions as the effective protrusion decreases the scale of these coherent flow structures also decreases

thresholds of erosion and sediment movement in bedrock channels abstract this study examines the erosive resistance of the discontinuous loose joint blocks of various sizes that comprise the bed material of the etobicoke creek an incised bedrock channel found in an urbanized setting within the city of toronto ontario canada in order to accomplish this both field and laboratory experiments were conducted on the bed material found in this channel the field work involved repeated detailed surveying of the channel bed and estimations of distances of movement of various clasts during many sediment transport events we found that rates of erosion and or deposition can be significant during the snowmelt spring events and that there was a greater loss of bed material over the study reach than was gained in several spots with bed lowering 0 10 m in some locations the field work also showed that the distances moved by the rock clasts scaled directly with shear stresses observed during rain events and that average distances of transport tend to be short the frequency distributions of distance of transport scaled with particle size is log normally distributed flume experiments demonstrated that the clasts offered different levels of resistance to movement depending on their orientation the platy nature of the rock fragments clast orientation and their imbrication significantly affect the threshold of erosion the results have significant implications for the understanding of bedrock rivers from a management perspective especially in the case of those rivers experiencing the effects of urban development in their drainage basins

flow and turbulence structure across the ripple dune transition an experiment under mobile bed conditions current knowledge of flow and turbulent processes acting across the sand bed continuum is still unable to unequivocally explain the mechanism s by which ripples become dunes understanding has been improved by comparative high resolution studies undertaken over fixed bedforms at different stages in the continuum however these studies both ignore the role of mobile sediment and do not examine flow structure during the actual transition from ripples to dunes the aims of the paper are i to describe flow and turbulence characteristics acting above mobile bedforms at several stages across the transition and ii to compare these data with those arising from experiments over fixed ripples and dunes laboratory experiments are presented that examine the turbulence structure across seven distinct stages of the transition from ripples to dunes single point acoustic doppler velocimeter sampling at three flow heights above a developing mobile boundary was undertaken time averaged statistics and the instantaneous quadrant record reveal distinct changes in flow structure either side of the change from ripples to dunes initially shear related high frequency vortex shedding dominates turbulence production this increases until two dimensional 2d dunes have formed thereafter turbulence intensities and reynolds stress decline and three dimensional dunes exhibit values found over 2d ripples this is the result of shear layer dampening which occurs when the topographically accelerated downstream velocity increases at a faster rate than flow depth activity at reattachment increases due to high velocity fluid imparting high mass and momentum transfer at the bed and or wake flapping suspended sediment may also play a role in turbulence dampening and bed erosion ejections dominate over sweeps in terms of event frequency but not magnitude strong relationships between inward interactions and sweeps and ejections and outward interactions suggest that mass and momentum exchanges are dependent upon activity in all four quadrants the results contradict the notion present in most physical models that larger bedforms exhibit most shear layer activity consequently an improved model for the ripple dune transition is proposed

land movements and animal distributions in east wallacea eastern indonesia papua new guinea and melanesia abstract four generalized tracks are identified in eastern indonesia papua new guinea melanesia on the basis of animal distribution patterns two of these tracks are linked to a melanesian rift system that developed to the north and east of australian gondwana during the late cretaceous and early tertiary the other two tracks are related to cretaceous rift systems formed off the west northwest coast of australian gondwana the histories of these rift systems are discussed their present positions are tentatively located in terms of modern geography and animal distributional patterns related to their formation and subsequent histories wallace s line is reinterpreted in terms of a boundary between asian and australian derived terranes and microcontinents distributional patterns that reflect this discontinuity show incongruity with the geological pattern in sulawesi and the banda arcs because of the merging of asian and australian faunas as a direct result of australia s collision with asia indo australian distributions are interpreted in terms of the distribution of ancestral populations prior to the mid mesozoic breakup of the relevant sectors of gondwana

influence of relative submergence on spatial variance and form induced stress of gravel bed flows 1 spatial flow variance has a strong control on sediment transport sediment water interface exchange mechanisms and the distribution and behavior of aquatic organisms in rivers thus being able to quantify spatial flow variance and how it varies with different water levels is important for understanding how fluvial processes change during periods of time varying flow in this paper laboratory flume measurements of near bed flow velocity were used to quantify spatial flow variance and form induced stress and their variation with flow submergence within and above the surface of porous gravel beds with differing grain roughness the analysis revealed spatial flow variance was usually four or five times higher within the roughness layer than above a rise in relative submergence resulted typically in a decrease in spatial variance relative to bed shear velocity in streamwise form induced intensity streamwise turbulence intensity and form induced momentum flux both within and above the roughness layer flow submergence had no consistent influence on spatial variance in the vertical flow direction and in reynolds stress form induced stress was significant within the roughness layer more so at shallow depths the greater significance was driven primarily by higher spatial flow variance in time averaged streamwise velocity at these depths the implication is the relative role of momentum transfer mechanisms within the roughness layer and thus sediment water interface exchange processes in rivers will change during periods of time varying flows

headwater peatland channels in south eastern australia the attainment of equilibrium abstract many small headwater catchments 2 in temperate south eastern australia store sediment in valley fills while accumulation in some of these systems commenced up to 30 000 years ago most did not commence filling with peat or clastic material until at least the mid holocene in such headwater settings many clastic valley fills develop cut and fill channels which contrast to some peatland settings where sinuous equilibrium channels have evolved four peatland systems within this dataset demonstrate stable channel systems which span nearly the full spectrum of observed valley floor slopes we assess new and published longitudinal data from these four channels and demonstrate that each of these channels has achieved equilibrium profiles new and published flow and survey data are synthesised to demonstrate how these peatland systems have attained equilibrium low rates of sediment supply and exceptionally high bank strengths have resulted in low width to depth ratios which accommodate rapid changes in flow velocity and depth with changes in discharge in small peatland channels planform adjustments have been sufficient to counter the energy provided by these hydraulically efficient cross sections and have enabled the achievement of regime energy slopes in larger and higher energy peatland channels large armoured stable bedforms have developed these bedforms integrate with planform adjustments to maintain a condition of minimum variance in energy losses as represented by the slope profiles and therefore a uniform increase in downstream entropy

characterizing spatial variability in velocity and turbulence intensity using 3 d acoustic doppler velocimeter data in a plane bed reach of east st louis creek colorado usa abstract we investigated the influence on flow resistance of flow structure and turbulence at the reach scale in a mountain channel using 3 d velocity measurements and geostatistical analysis to understand the complexity of the flow structure in a reach with limited bed irregularities the increase in flow resistance at low flows in a plane bed reach was not fully explained by grain resistance therefore detailed 3 d velocity measurements were made to investigate spatial variability in velocity and turbulence components and potential controls on flow resistance one plane bed reach was surveyed over two stages in fraser experimental forest colorado using a combination of a total station lidar light detection and ranging and a sontek flowtracker handheld adv acoustic doppler velocimeter lidar was used to capture bank and channel geometry at low flows whereas the water surface and bed data were collected with the total station at all flows we used the standard deviation of bed elevation b within a moving window as an index of roughness height ks and calculated the relative submergence of the bed at different stages as h ks where h is the local flow depth adv measurements were collected on a grid with a 0 3 m to 0 5 m spacing geostatistical analysis of the velocity data indicated that the flow was highly three dimensional and varied based on stage demonstrating that even small irregularities in the bed have a significant influence on the flow characteristics the streamwise component was the largest at both low and high flow but varied more throughout the reach at low flow at high flow the greatest streamwise velocities were located within the thalweg areas of upwelling and downwelling also varied based on stage with this component being strongly influenced by small changes in the morphology at high flow and by protuberant grains at low flows the cross stream velocity and turbulence components were controlled by the flow structure and less by the roughness of the bed the turbulence intensity is significant when considering hydraulics for predicting sediment transport and for habitat assessment the plane bed flow patterns were compared to results from a pool riffle reach in the north fork cache la poudre river a step pool reach in east st louis creek and a step pool reach in italy on the rio cordon the comparison of the 3 d velocities among channel types showed that the plane bed reach has higher streamwise velocities but similar values of cross stream and transverse velocities streamwise turbulence intensities were similar in both the plane bed and step pool reach in east st louis creek the analysis revealed that the connection between the flow characteristics and the channel was not only related to the gross morphology and location of larger clasts but also to the development of shear layers from the convergence of flow therefore it is essential to understand how water moving in one direction can alter the characteristics of another component of flow and how this interaction is connected to the bed morphology

hydrodynamic behaviour of coarse bioclastic sand from shelly cheniers cheniers from mont saint michel bay france are coarse shelly sand ridges migrating on the mudflat up to the salt marshes where they accumulate and merge in a littoral barrier in this macrotidal setting and low wave forcing the cheniers are rarely submerged however they are found to move up to several metres during coincidence of spring tide and wave activity their processes of migration morphology and internal structure composition of the beddings grain size sorting and grain arrangement are thought to be closely related to the hydrodynamic behaviour of the coarse and shelly sediment this paper focuses on the hydrodynamic behaviour of bioclastic sand sampled from the cheniers settling velocities of the shell fragments were measured using a 2 m long sedimentation tube thresholds of motion under unidirectional current velocity and turbulence vertical profiles were characterized in a small recirculating flume using laser doppler anemometry lda the flat shaped bioclastic particles feature low settling velocities and reveal a good resistance to the re suspension effect of the flow when imbricated in a sediment bed the shear stress in the bottom boundary layer has been measured in the viscous and log sub layers nikuradse roughness heights ks for shell debris beds of different sizes have been quantified it is found that ks 2 56d50 this value is close to the ones used for classic rounded sand grains despite their major differences of shape the dual behaviour of the shell fragments low settling velocity good resistance to unidirectional flow should be considered as a key to understanding how this coarse material is transported across the tidal flat and finally accumulated as cheniers further flume experiments including wave activity and tidal fluctuations are necessary to better quantify these complex processes copyright 2010 john wiley sons ltd

river velocities from sequential multispectral remote sensing images 1 we address the problem of extracting surface velocities from a pair of multispectral remote sensing images over rivers using a new nonlinear multiple tracer form of the global optimal solution gos the derived velocity field is a valid solution across the image domain to the nonlinear system of equations obtained by minimizing a cost function inferred from the conservation constraint equations for multiple tracers this is done by deriving an iteration equation for the velocity based on the multiple tracer displaced frame difference equations and a local approximation to the velocity field the number of velocity equations is greater than the number of velocity components and thus overly constrain the solution the iterative technique uses gauss newton and levenberg marquardt methods and our own algorithm of the progressive relaxation of the over constraint we demonstrate the nonlinear multiple tracer gos technique with sequential multispectral landsat and aster images over a portion of the potomac river in md va and derive a dense field of accurate velocity vectors we compare the gos river velocities with those from over 12 years of data at four noaa reference stations and find good agreement we discuss how to find the appropriate spatial and temporal resolutions to allow optimization of the technique for specific rivers

scales of boundary resistance in coarse grained channels turbulent velocity profiles and implications gravel bed surfaces are characterized by morphological features occurring at different roughness scales the total shear stress generated by the flow above such surfaces is balanced by the sum of friction drag grain stress and form drag components created by bed forms to facilitate a better understanding of total resistance and bed load transport processes there is a need to mathematically separate shear stress into its component parts one way to do so is to examine the properties of vertical velocity profiles above such surfaces these profiles are characterized by an inner layer that reflects grain resistance and an outer layer that reflects total resistance a flume based project was conducted to address these concerns through systematically comparing different roughness scales to ascertain how increased roughness affects the properties of vertical velocity profiles great care was taken to create natural roughness features and to obtain flow data at a high spatial and temporal resolution using an acoustic doppler velocimeter average vertical velocity profiles above each roughness scale were clearly segmented the vertical extent of the inner flow region was directly related to the scale of roughness present on the bed and independent of flow depth increasing with increased roughness on a rough but rather uniform plane bed made of heterogeneous coarse sediments with no bed forms the shape of the velocity profile was clearly dominated by the local variations in grain characteristics when pebble clusters were superimposed the average shear stress in the outer flow region increased by 100 from the plane bed conditions the ratio of inner grain shear stress to outer total shear stress for this pebble cluster experiment was 0 18 under shallow flow conditions and 0 3 under deep flow conditions the grain stress component that should be used in bed load transport equations therefore appears to vary in these experiments between 15 and 30 of the total channel stress increasing with decreased resistance roughness height ks d50 values at the grain scale for the plane bed and pebble cluster experiments were 0 73 and 0 63 respectively these are values that should be used in flow resistance equations to predict grain resistance and grain stress for bed load transport modeling

riffle pool maintenance and flow convergence routing observed on a large gravel bed river abstract geomorphologists have studied and debated over the processes responsible for natural riffle pool maintenance for decades most studies have focused on small wadable rivers but they lack much description of overbank flood conditions or a spatially explicit characterization of morphodynamics in this study 1 m horizontal resolution digital elevation models were collected from a riffle pool run sequence before and after an overbank flood with a 7 7 year recurrence interval on the relatively large gravel bed lower yuba river california digital elevation model differencing was used to quantify the magnitude and pattern of flood induced morphodynamic change cross section based analysis and two dimensional hydrodynamic modeling of flows ranging from 0 147 to 7 63 times bankful discharge were completed to evaluate the hydraulic mechanisms responsible for the observed topographic changes one key finding was that riffle pool relief increased by 0 42 m confirming the occurrence of natural hydrogeomorphic maintenance spatially complex patterns of scour and deposition exceeding 0 15 m at the scale of subwidth morphological units were reasonably predicted by the two dimensional mechanistic model that accounts for convective acceleration the one dimensional cross section based method underperformed the two dimensional model significantly consequently multiple scales of channel non uniformity and a dynamic flow regime caused the observed maintenance of the pool riffle morphology through the mechanism of flow convergence routing proposed by macwilliams et al macwilliams m l wheaton j m pasternack g b kitanidis p k street r l 2006 the flow convergence routing hypothesis for riffle pool maintenance in alluvial rivers water resources research 42 w10427 doi 10 1029 2005wr004391

effects of sand addition on turbulent flow over an immobile gravel bed 1 the factors controlling the complex interaction of a coarse streambed with flow and sediment are difficult to measure however planning for reservoir flushing or dam removal requires knowledge of these interactions in both cases impounded sediments are introduced to channel beds that have had fine sediment particles removed without replacement the channel bed pore space interacts with the flow and provides storage for particles in order to address the need for information on such systems an adjustable slope recirculating laboratory flume was used to study the changes in flow and turbulence caused by sand added to an immobile gravel bed detailed measurements were made using an acoustic doppler velocimeter that collected three velocity components at a rate of 200 hz because of the rough nature of the bed individual velocity profiles varied significantly therefore in order to determine general trends the data were spatially averaged over six 10 20 cm planes parallel to the bed with the lowest plane about 2 cm below the maximum gravel elevation the increasing elevation of sand relative to the gravel layer resulted in decreased bed shear stress decreased reynolds stress increased relative turbulence intensity and a near bed shift toward sweep dominated turbulence

the paleogeography of late neogene central crete inferred from the sedimentary record combined with albinaria land snail biogeography abstract phylogenetic and geographic relations of recent land organisms often agree with results from tectonical and paleogeographic reconstructions in the hellenic island arc greece albinaria land snails gastropoda pulmonata clausiliidae are used to provide paleogeographical information in a combined analysis of their present distributional ranges and the sedimentary record late tertiary neogene islands in the region of present day crete existed for 6 9 myear before the arc was uplifted 3 2 myear ago pliocene the albinaria fauna of crete is known to be relictual in central crete an area of 3800 km2 the distributional ranges of 15 parapatrically dispersed albinaria species were determined based on differences in their ecological behavour the species are subdivided into two groups group 1 exclusively living on pre neogene substrate is believed to have largely conserved pre pliocene ranges and group 2 partly living on neogene partly on pre neogene substrates is assumed to have occupied the land which appeared after late neogene uplift events together with the sedimentary record albinaria helps to reconstruct the dynamic paleogeographic developments and block movements in central crete since the serravallian the conclusions imply 1 that some of the involved populations have been separated from each other for up to 11 myear and 2 that providing information on paleoislands recent land snails may contribute to geoscientific questions concerning the rate and timing of uplift and erosion

patterns of seismicity associated with asperities in the central new hebrides island arc in the central part of the new hebrides island arc efate malekula region 16 s 18 6 s a sequence of moderate sized earthquakes their aftershock sequences and other clusters of small earthquakes together form an intricate but coherent time space pattern that probably reveals a major asperity complex along the interplate boundary of the subduction zone this pattern is determined by study of data from a local network the sequences including one event with magnitude mw 7 1 eight events with magnitudes mw between 5 8 and 6 3 and nearly 13 000 smaller events occurred during the six year period 1978 1984 the seismicity is very unevenly distributed in space a sharply defined east west line near 17 2 s separates the very active efate region to the south where nearly 10 500 earthquakes occurred from the less active malekula region to the north where less than 2 500 earthquakes occurred during the six year period in the efate region several spatial patterns are highlighted first the seismic regimes of the updip and the downdip part of the interplate boundary are different the updip part is characterized by a low background activity and main shocks with large aftershock zones while the downdip part is characterized by a very high level of background activity and main shocks with much smaller aftershock zones or no aftershocks this difference in seismic regimes suggests that asperities are located in the updip part of the interplate boundary while the downdip part may slip predominantly by creep a creep episode may have been responsible for the tilt signal observed by periodic relevelings of a l km aperture network of benchmarks on efate island second specific locations along the interplate boubdary are identified either as sites of very intense and repeatedly activated concentrations of hypocenters or as sharp boundaries limiting the spatial development of aftershock zones two areas of concentrated activity particularly stand out one located in the updip part of the interplate boundary and the other in the downdip part most of the clustered activity that occurred during the six years of observation is concentrated in these two areas four boundaries limiting the spatial development of aftershock zones are located in the updip part of the interplate boundary one of these boundaries coincides with one of the areas of intense activity epiceniers of moderately large events that occurred in the region since 1960 are also concentrated along the boundaries the zones of concentrated activity and the boundaries limiting the development of aftershock zones can be interpreted either as locations of penties or as edges of asperities an alternative interpretation is that a giant asperity is located in the ylekula region with the different spatial patterns observed in the efate region being interpreted as the southern edge of that asperity a transition zone between a locked part to the north and normal i subduction to the south study of the temporal patterns of the seismicity shows that the main shocks in tbe downdip part of the interplate boundary are not preceded by obvious long term months to year nor short term days to week precursory activity in the updip part of the interplate boundary short term fipreshocks preceded each main shock but only one of these foreshock sequences is outstanding although no clear cyclic pattern related to the main shocks is evident clustered activity that occurred in the two zones of concentrated activity and two other zones located in the back arc region are correlated ih time with the occurrence of the major sequences located in the updip part of the interplate boundary activation of these four zones always follows the sequences in the updip part of the interplate boundary qith varying time intervals days to months these four zones are also sometimes activated before sequences in the updip part of the interplate boundary in particular one of the zones of concentrated a ctivity shows possibly repeated long term few months precursory activity this phenomenon could b e an indicator of increasing probability of occurrence of main events in the updip part of the interplate b oundary i tuk ukiis lmcui l ilu tl6aj2 qfiztqf

changes in velocity profiles at roughness transitions in coarse grained channels heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness distinct spatial zones in terms of grain size have been reported for example sand ribbons and bedload sheets the transition from areas of finer to coarser grained surface sediment is often abrupt however the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels this paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistance on the structure of the turbulent boundary layer measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface boundary shear stress and roughness length increase considerably downstream from this transition velocity profiles become concave upwards downstream and upstream sections show significant differences in terms of near bed velocities deceleration downstream of the transition velocity gradient and turbulence intensity of the streamwise velocity component comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity or shear stress associated with the pressure drag produced by large and isolated obstacles

turbulent velocity profiles and boundary shear in gravel bed rivers vertical profiles of turbulent streamwise velocities in several new zealand gravel bed rivers are investigated field measurements made at high and low flows with electronic pitot tubes show logarithmic velocity profiles to extend over much of the flow depth for the gravel bed rivers studied the velocity at 0 6 of the total depth was generally a good indicator of depth averaged flow velocity an unambiguous definition of flow depth is adopted to deal with situations where the bed is uneven or moving when hydraulic roughness z sub 0 is defined as a fitted parameter of a logarithmic velocity profile the river data indicate that the profile origin displacement below the tops of roughness elements scales with z sub 0 no direct relation between z sub 0 and bed material size is evident under mobile bed conditions for these conditions a relation between hydraulic roughness and u squared is identified with u also derived as a log profile parameter a flow resistance equation using this relation is verified by comparison with mobile bed laboratory measurements in which u is not fitted from velocity profiles

the trouble with shear stress abstract predicted bedload fluxes often have large errors partly because calculations usually do not include spatial and temporal flow variability sediment motion is driven by a temporal distribution of applied fluid forces which are often simplified into time averaged flow velocities or shear stresses in sediment transport equations spatial variations in these flow parameters are also often neglected given the nonlinearities in the relation between flow and sediment flux the reach averaged sediment flux calculated with an average velocity or shear stress will not necessarily equal the measured spatially averaged bedload flux such spatial and temporal flow variations are ignored with the commonly used reach averaged shear stress ds that is a function of the flow depth and channel slope to demonstrate the uncertainties with using reach averaged shear stresses and even local shear stresses to predict bedload transport rates we have three goals first we outline the derivations and assumptions involved with commonly used estimates of shear stress second we summarize the literature on the uncertainties involved with using shear stress and other space and time averaged flow parameters to predict sediment flux third we use a new data set from experiments to add further support to a growing body of literature that suggests ds may not be linked to the mechanics of bedload transport we show considerable uncertainties exist in estimating a reach averaged shear stress by comparing ds and near bed shear stresses calculated from velocity profiles reynolds stresses and local turbulence for example ds was systematically greater than these near bed stresses even for experiments without large bed roughness we also find that correlations between local time averaged sediment fluxes and local reynolds stresses or turbulence intensities are relatively poor the near bed time averaged local flow velocity has the strongest relation with the time averaged local bedload flux because it is closely related to the forces that actually drive sediment transport however sediment transport rates in individual experiments could still vary by orders of magnitude for a given value of this flow velocity suggesting that additional work is needed to identity a parameter or distributions of parameters that will best predict sediment fluxes

reconstruction of subgrid scale topographic variability and its effect upon the spatial structure of three dimensional river flow a new approach to describing the associated topography at different scales in computational fluid dynamic applications to gravel bed rivers was developed surveyed topographic data were interpolated using geostatistical methods into different spatial discretizations and grain size data were used with fractal methods to reconstruct the microtopography at scales finer than the measurement subgrid scale the combination of both scales of topography was then used to construct the spatial discretization of a three dimensional finite volume computational fluid dynamics cfd scheme where the topography was included using a mass flux scaling approach the method was applied and tested on a 15 m stretch of solfatara creek wyoming united states using spatially distributed elevation and grain size data model runs were undertaken for each topography using a steady state solution this paper evaluates the impact of the model spatial discretization and additional reconstructed variability upon the spatial structure of predicted three dimensional flow the paper shows how microtopography modifies the spatial structure of predicted flow at scales finer than measurement scale in terms of variability whereas the characteristic scale of predicted flow is determined by the cfd scale changes in microtopography modify the predicted mean velocity value by 3 6 for a mesh resolution of 5 cm whereas a change in the computational scale modifies model results by 60 the paper also points out how the spatial variability of predicted velocities is determined by the topographic complexity at different scales of the input topographic model

recent 1999 2003 canadian research on contemporary processes of river erosion and sedimentation and river mechanics contributions by canadian fluvial geomorphologists between 1999 and 2003 are discussed under four major themes sediment yield and sediment dynamics of large rivers cohesive sediment transport turbulent flow structure and sediment transport and bed material transport and channel morphology the paper concludes with a section on recent technical advances during the review period substantial progress has been made in investigating the details of fluvial processes at relatively small scales examples of this emphasis are the studies of flow structure turbulence characteristics and bedload transport which continue to form central themes in fluvial research in canada translating the knowledge of small scale process related research to an understanding of the behaviour of large scale fluvial systems however continues to be a formidable challenge models play a prominent role in elucidating the link between small scale processes and large scale fluvial geomorphology and as a result a number of papers describing models and modelling results have been published during the review period in addition a number of investigators are now approaching the problem by directly investigating changes in the system of interest at larger scales e g a channel reach over tens of years and attempting to infer what processes may have led to the result it is to be expected that these complementary approaches will contribute to an increased understanding of fluvial systems at a variety of spatial and temporal scales

form resistance in gravel channels with mobile beds form resistance which includes pressure drag on bed forms and pressure and viscous drag on bed particles in transport is shown to be a major component of hydraulic roughness in steady quasi uniform two dimensional turbulent flow over mobile gravel beds dimensionless form shear stress exceeds dimensionless particle shear stress for values of about three times critical shields stress in subcritical flow and five times in supercritical flow theoretical analysis provides a physical basis for expressing friction factor as a function of particle friction factor estimated by the keulegan equation and the ratio of form to particle shear stress termed relative form shear stress dimensional reasoning and empirical modeling of flume data demonstrate that relative form shear stress depends on shields stress and relative roughness in subcritical and upper supercritical flow regimes and on relative roughness alone in the lower supercritical regime in particular for given shields stress and increasing rela

neogene sutures in eastern indonesia five suture zones are described from the zone of collision between the eurasian indian australian and pacific philippine sea plates within the eastern indonesia region these are the molucca sorong sulawesi banda and borneo sutures each of these sutures has a relatively short history compared to most pre neogene orogenic belts but each preserves a record of major changes in tectonics including subduction polarity reversals elimination of volcanic arcs changing plate boundaries and important extension within an overall contractional setting rapid tectonic changes have occurred within periods of less than 5 ma many of these events although important would be overlooked in older orogenic belts because the age resolution required to identify them even when the evidence is preserved is simply not possible q 2000 elsevier science ltd all rights reserved

bed forms and bend arc spacings of some coarse bedload channels in upland britain seventy eight riffle to riffle and 80 bend spacings along eleven coarse bedload low sinuosity stream channels in upland britain have been surveyed frequency distributions of these spacings are notably right skewed the most common repeating distances between riffles and bend inflections are between 4 and 6 channel widths although spacings up to 20 widths are also present riffle and pool locations around bends at different stages of planform development indicate that change is largely through increased sinuosity between two consecutive riffles of an original straight reach observed straight segments exhibit alternating riffles and pools evenly spaced at 4 6 widths and most bends have similarly spaced riffles at their inflections in plan with the intermediate pool at their apex however angular deflections between axial lines joining inflections indicate existing sequences of bends did not develop from a single straight reach bends which are significantly longer than 4 6 widths are of low sinuosity and represent variability in naturally irregular planforms rather than arcs in advanced stages of meander growth adjusted bed topography around such long bends takes two forms either a single riffle pool cycle is present with one or both bed forms being longer than average or a 4 6 widths spacing is maintained by more than one riffle pool sequence locally the cross section characteristics of riffles and pools are also influenced by planform location

the origin of southwest sulawesi and other indonesian terranes a biological view abstract a generalised track linking geographically dispersed areas in indonesia with burma is identified from moth and bird distributional data these areas include sulawesi the philippines sabah northern borneo java sumatra mentawai the nicobars and andamman islands a cladistic analysis of sulawesian dacus diptera tehritidae shows that sabah and sulawesi are sister areas and together form a sister area to sulawesi java sumatra thailand cladograms for other taxa are reviewed and these also lend support to the hypothesis that these now diverse indonesian fragments share a closer biotic relationship to each other than they do to other areas the geological history of the region is discussed and evidence that these indonesian fragments are terranes which became sutured to the continental margin of sundaland in the late cretaceous or early tertiary is discussed the congruence between distributional patterns and tectonic history requires further investigation but a working hypothesis suggests that these indonesian terranes are linked to mount victoria land and that all were derived from a gondwanic source area close to india

bar form resistance in gravel bed rivers resistance to flow in unvegetated gravel bed rivers is basically dependent on the skin resistance due to surface bed material and bar form resistance due to accelerations and decelerations in the flow between pools and riffles although flow is generally nonuniform at riffle sections flow is locally uniform it is these sections which principally control the velocity depth characteristics of the reach since hydraulic conditions in the pool result from the backwater effect of the riffle isolating the effect of bar forms on flow resistance is achieved by first establishing the roughness height of the surface bed material on the riffle overall resistance to flow in terms of the total roughness height due to bed forms and grains for equivalent uniform flow is determined from the riffle and reach average flow geometry and the grain roughness height differences between total and grain roughness heights define the bar form effect field data from 62 sites in the united kingdom illustrate that at bankful

reach scale variability of turbulent flow characteristics in a gravel bed river in gravel bed rivers bed roughness is often controlled by the spatial distribution of pebble clusters and large protuberant clasts the turbulent flow structure around these roughness elements has been studied extensively as opposed to the effects of the spatial variability of the bed roughness on the structure of the flow at the reach scale in this paper we examine the effects of roughness elements on the spatial variability of the mean flow velocity profiles and of the properties of turbulent flow structures at the reach scale the experiment was carried out in two reaches of 25 m long in the eaton north river quebec we produced detailed topographic maps of the river reaches to obtain the spatial distribution of the main roughness elements the flow is characterized with velocity time series measured using electromagnetic current meters mounted on a vertical wading rod we collected 65 velocity profiles from which we derived turbulent flow characteristics results showed that in spite of the complex configuration of the bed velocity profiles are predominantly log linear mean and turbulent flow characteristics are only affected at a local scale and over short distances by the presence of roughness elements at the reach scale the distribution of individual roughness elements does not greatly control the spatial variability of turbulent flow properties

oceanic plateaus the fragmentation of continents and mountain building many anomalous rises in today s oceans may be submerged continental fragments detached from previous continents ancient island arcs or basaltic piles formed by hot spots and spreading centers these rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins where such rises are being consumed at present e g the nazca ridge they cause cessation of volcanism disruption of the downgoing slab and possible shifts in plate boundary configuration many past rises including numerous continental fragments have been recognized within mountain belts as allochthonous terranes they constitute a large portion of the orogenic belts in the north pacific from mexico through western north america alaska east siberia japan and in new zealand the orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes many terranes have been accreted with substantial deformation also in the alpine chain well before major continent continent collisions it is suggested therefore that the accretion of fragments may be the common process of the deformation phase of mountain building subduction of normal oceanic crust may be insufficient for deformation whereas full continent continent collision may not be necessary the general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the andes or not most of the accreted fragments with continental affinities in the mesozoic cenozoic orogenic belts of the world can be traced back to the breakup of gondwana beginning with a pacifica domain in the permian through a larger india domain in the early mesozoic and continuing through the separation of the somalia plate in the near future the reasons for this 250 million year breakup process are not known but some kind of thermal process possible of mantle wide scale is implied

terrestrial laser scanning soil surfaces a field methodology to examine soil surface roughness and overland flow hydraulics smith m w cox n j bracken l j 2011 terrestrial laser scanning soil surfaces a field methodology to examine soil surface roughness and overland flow hydraulics hydrological processes 25 6 842 860

water worked gravel beds in laboratory flumes a natural analogue an investigation has been conducted to identify the key parameters that are likely to scale laboratory sediment deposits to the field scale two types of bed formation were examined one where sediment is manually placed and screeded and the second where sediment is fed into a running flume this later technique created deposits through sequential cycles of sediment transport and deposition detailed bed surface topography measurements have been made over a screeded bed and three fed beds in addition bulk subsurface porosity and hydraulic conductivity have been measured by comparing the four beds results revealed that certain physical properties of the screeded bed were clearly different from those of the fed beds the screeded bed had a random organization of grains on both the surface and within the subsurface the fed beds exhibited greater surface and subsurface organization and complexity and had a number of properties that closely resembled those found for water worked gravel beds the surfaces were water worked and armoured and there was preferential particle orientation and direction of imbrication in the subsurface this suggested that fed beds are able to simulate in a simplified manner both the surface and subsurface properties of established gravel bed river deposits the near bed flow properties were also compared it revealed that the use of a screeded bed will typically cause an underestimation in the degree of temporal variability in the flow furthermore time averaged streamwise velocities were found to be randomly organized over the screeded bed but were organized into long streamwise flow structures over the fed beds it clearly showed that caution should be taken when comparing velocity measurements over screeded beds with water worked beds and that the formation of fed beds offers an improved way of investigating intragravel flow and sediment water interface exchange processes in gravel bed rivers at a laboratory scale copyright 2008 john wiley sons ltd

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

effect on flow structure of sand deposition on a gravel bed results from a two dimensional flume experiment 1 particle imaging velocimetry piv was used to quantify the entire two dimensional flow field over a series of fixed test beds in a laboratory flume the test beds used in the experiments provide a two dimensional representation of gravel beds with contrasting roughness the two dimensional form of the beds combined with the nonintrusive nature of piv allowed the velocity field to be quantified right down to the bed surface the three beds used were designed to simulate the progressive effect of sand deposition on a gravel bed with median grain size d50 of 25 mm such a situation is common in bimodal beds but as compared with gravel and sand cases little work has been done to investigate the interaction between the variable roughness of a bimodal bed and the flow structure above it the results demonstrate that as the effective roughness decreases the reverse flow often found in the lee of gravel particles is eliminated furthermore near bed velocities increase while shear stress and turbulent kinetic energy decrease turbulent properties are also diminished higher up in the profile although this is not the case for the mean downstream velocity which remains unaffected except at the bed quadrant analysis reveals that q2 and q4 events become less frequent around high points as effective roughness decreases all these responses to the changing bed conditions are most pronounced in areas where the effective roughness height h approaches 10 mm or less h d50 0 4 such a situation is often found immediately upstream of gravel sand transitions in natural rivers it is hypothesized that the hydraulic response outlined here provides a detailed physically based explanation for the gravel sand transition the combination of reduced bed shear stress and q2 q4 events around gravel particles will result in greater mobility of the sand fraction relative to gravel these experiments thus appear to demonstrate that there is a fundamental threshold between gravel bed and sand bed states that has not previously been quantified

estimation of flow resistance in gravel bedded rivers a physical explanation of the multiplier of roughness length the need to estimate velocity and discharge indirectly in gravel bedded rivers is a commonly encountered problem semilogarithmic friction equations are used to estimate mean velocity using a friction factor obtained from depth and grain size information although such equations have a semi theoretical basis in natural gravel bed channels an empirical constant 6 8 or 3 5 has to be introduced to scale up the characteristic grain size d50 or d84 to represent the effective roughness length in this paper two contrasting approaches are used to suggest that the multiplier of characteristic grain size is attributable to the effect of small scale form resistance reflecting the occurrence of microtopographic bedforms in gravel bedded environments first spatial elevation dependence in short detailed bed profiles from a single gravel bedded river is investigated using semivariogram and zero crossing analyses this leads to objective identification of two discrete scales of bed roughness associated with grain and microtopographic roughness elements second the autocorrelation structure of the three dimensional near bed velocity field is examined to identify regularities associated with eddy shedding and energy losses from larger grains and microtopographic bedforms apart from improving the capacity to determine friction factors for velocity and discharge estimation the findings have implications in general for the initial motion of gravelly bed material

1 min in the life of a river selecting the optimal record length for the measurement of turbulence in fluvial boundary layers in the analysis of instantaneous velocity measurements it is important to consider carefully the properties of the instruments as well as the sampling frequency and the record length although several studies have examined the influence of instrument settings on velocity measurements in rivers little information is available on the effects of the record length on these measurements and on the turbulence parameters that can be extracted from the time series decisions on the record length are often made ad hoc here we quantify the effects of the sampling period on the precision of turbulence statistics e g mean standard deviation skewness reynolds stress quadrant analysis obtained from velocity measurements in fluvial turbulent boundary layers a bootstrapping method is used to determine the standard errors s e of turbulence statistics from 19 time series the time series were sampled in three gravel bed rivers using both electromagnetic current meters ecm and acoustic doppler velocimeters adv the record lengths ranged between 20 and 60 min the sampling frequencies were 20 and 25 hz for the ecms and advs respectively we show that for most turbulence statistics the optimal record length minimum sampling effort to achieve low standard errors ranges between 60 and 90 s we discuss the implication of these record lengths for standard errors estimates and variance losses

the influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers comment on a paper by marwan a hassan and ian reid data from turkey brook are used to demonstrate that the interaction between gravel bedforms flow resistance and bedload transport is a dynamic one both between and within hydrographs and that creation of a significant component of form drag through construction of microforms pebble clusters may precede the eventual break up of the bed in a transport event this process of drag augmentation which can be seen as a feedback mechanism delaying transport and can be likened to dilation of a soil tested in a direct shear apparatus itself appears to be dependent on the characteristics of turbulence and therefore involves feedback at a finer resolution than envisaged by hassan and reid 1990

spatial heterogeneity of near bed hydraulics above a patch of river gravel 1 the spatial heterogeneity of fully turbulent near bed flows above gravel river beds is examined using a realistic replica of a natural gravel patch in a large flume three dimensional velocity time series were obtained at four heights 0 008 0 1 m above the local bed in each of 99 closely spaced verticals for three flows of increasing intensity the spatial heterogeneity of time averaged velocities and root mean square velocity fluctuations increases under stronger flows and closer to the bed however streamwise velocity becomes spatially homogeneous at a distance from the bed of between 2 4 times the median bed elevation heterogeneity in the direction of the velocity vector is independent of mean flow velocity but in all cases it decreases approximately linearly with distance above the surface skewness of the instantaneous velocity distributions suggests that slowly moving fluid emanating from the near bed region impinges upon higher levels with greater frequency and greater spatial coverage as the average flow velocity increases spatial heterogeneity in turbulent kinetic energy increases with flow velocity and maxima occurr at positions that intercept layers of intense vortex shedding in the lee of obstacle crests the spatial organization of the flow properties is nonrandom and consistent across the three flows simple regression models are developed to provide a basis for investigating the heterogeneity of near bed flow at the patch scale 2 m2 in gravel bed rivers

origin of the sula platform eastern indonesia the sula platform is a fragment of continental crust that is generally regarded as a piece of new guinea that was detached from western irian jaya in late cenozoic time the assumption is based on the apparent correlation of the stratigraphy of the sula platform with that of western irian jaya new data from the sula platform and western irian jaya show that this correlation is probably not valid the best correlation of the sula platform stratigraphy with that of the former northern margin of the australian craton is found in central papua new guinea between long 141 and 145 e more than 1200 km farther east than any previously proposed site of origin implying a displacement of more than 2500 km

bedload transport of fine gravel observed by motion picture photography motion pictures taken at duck creek a clear stream 6 5 m wide and 35 cm deep near pinedale wyoming provide detailed quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them bed shear stress was approximately 6 pa 60 dynes cm 2 which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material and transport was almost entirely as bedload the displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones and rarely by brief sliding of large angular ones entrainment was principally by rollover of the larger particles and liftoff of the smaller ones and infrequently by ejection caused by impacts whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones the displacement times averaged about 0 2 0 4 s and generally were much shorter than the intervening repose times the collective motions of the particles were characterized by frequent brief localized random sweep transport events of very high rates of entrainment and transport which in the aggregate transported approximately 70 of the total load moved these events occurred 9 of the time at any particular point of the bed lasted 1 2 s affected areas typically 20 50 cm long by 10 20 cm wide and involved bedload concentrations approximately 10 times greater than background the distances travelled during displacements averaged about 15 times the particle diameter despite the differences in their dominant modes of movement the 8 16 mm particles typically travelled only about 30 slower during displacement than the 2 4 mm ones whose speeds averaged 21 cm s 1 particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely with diffusivities of 4 6 and 0 26 cm 2 s 1 respectively the bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler the 2 4 mm particles were entrained 6 times faster on finer areas of the bed where 8 16 mm particles covered 6 of the surface area than on coarser ones where they covered 12 even though 2 4 and 4 8 mm particles covered practically the same percentage areas in both cases the 4 8 and 8 16 mm particles in contrast were entrained at the same rates in both cases to within the statistical uncertainty the rates of distrainment balanced the rates of entrainment for all three sizes and were approximately proportional to the corresponding concentrations of bedload

the complexity of gravel bed river topography examined with gradual wavelet reconstruction this study analyzes the nonlinear nature of gravel bed elevation data series obtained in a large scale flume facility for four different discharges including spatial transect series and temporal series at a fixed location the goals are to infer the degree of complexity to assess the scales and features in the signals that mostly contribute to this complexity and to discern the difference in the manner in which this complexity is expressed in time and space the asymmetry series magnitude of the first order derivative of the signal over a separation distance raised to the third power of the bed elevation data forms the basis of our nonlinearity analysis an important and novel dimension is the adoption of a recently introduced approach to surrogate data generation gradual wavelet reconstruction this produces partially linearized counterparts of the original series that preserve an increasing degree of the underlying nonlinear structure in the original data while randomizing the rest this allows us to discern the difference in the manner in which the nonlinear asymmetry is expressed in the temporal and spatial data series comparison in the real and phase space of the original and surrogate series is performed and the analysis reveals that the complexity of both spatial and temporal series increases with discharge and that the nature of nonlinearity is quantitatively and qualitatively different for the spatial series asymmetry is expressed at large scales and is shown to result from the organization of intermediate scale features in contrast asymmetry in the temporal series is a smaller scale phenomenon this is a consequence of the scale dependent propagating velocity of topographic features our results have implications for understanding the complexity of geomorphic processes as a function of the space and time scale considered the complexity over the bed form lagrangian time scale is different in nature to that over the flow velocity lagrangian time scale

bed clusters in humid perennial and mediterranean ephemeral gravel bed streams the effect of clast size and bed material sorting summary a short review of the literature on particle clusters on gravel river beds reveals investigations of both process and form dominated by the intensive study approach using a restricted geographical sample or evidence from flumes an alternative presented here is extensive sampling from three climatic zones several channels in each and at multiple transects at each site it uses insights provided by a more intensively studied base station wittenberg l newson m d 2005 particle clusters in gravel bed rivers an experimental morphological approach to bed material transport and stability concepts earth surface processes and landforms 30 11 1351 1368 transect surveys were completed in each selected reach to establish flow depth bed material size and bed structure a total of more than 5000 sample points reveals the vital presence of bed material of 100 mm d50 for all sub types of clusters to occur thereafter cluster frequency relates directly to the d90 with an improving correlation at d90 256 mm a better integration of data from the diverse hydrological hydraulic regimes can be achieved by correlating cluster frequency with a sorting index for bed material further analysis of hydrological and hydraulic data for all sites is required to develop a dynamic explanation

spatially representative velocity measurement over water worked gravel beds 1 recent studies have examined the spatial heterogeneity of velocity fields over natural river boundaries a key challenge is understanding how many velocity measurements are required to provide spatially representative estimates of the flow this paper describes a series of laboratory experiments in which flow velocities have been measured in a detailed spatial pattern over two water worked gravel beds these data have been utilized to deduce the minimum density of measurements required to provide representative estimates of several spatially averaged flow parameters this was coupled with an investigation into the influence of measurement density on the level of error in the estimation of these parameters empirical relationships are developed that can be used to estimate a priori the required minimum measurement density for a known precision and accuracy over macroscopically flat water worked gravel beds within a range of submergences these estimates can be based on measurements of the bed roughness length scale bed shear velocity and flow depth alone for all of the flow parameters the level of error in adopting a lower measurement density than the required minimum and its change with measurement density was especially large at the lower densities given this dependence on measurement density caution should be taken when comparing flow estimates from studies that have used similar flow and bed conditions but different measurement densities

coherent flow structures in a depth limited flow over a gravel surface the role of near bed turbulence and influence of reynolds number 1 in gravel bed rivers the microtopography of the bed exerts a significant effect on the generation of turbulent flow structures although field and laboratory measurements have indicated that flows over gravel beds contain coherent macroturbulent flow structures the origin of these phenomena and their relationship to the ensemble of individual roughness elements forming the bed is not quantitatively well understood here we report upon a flume experiment in which flow over a gravel surface is quantified through the application of digital particle imaging velocimetry which allows study of the downstream and vertical components of velocity over the entire flow field the results indicate that as the reynolds number increases 1 the visual distinctiveness of the coherent flow structures becomes more defined 2 the upstream slope of the structures increases and 3 the turbulence intensity of the structures increases analysis of the mean velocity components the turbulence intensity and the flow structure using quadrant analysis demonstrates that these large scale turbulent structures originate from flow interactions with the bed topography detection of the dominant temporal length scales through wavelet analysis enables calculation of mean separation zone lengths associated with the gravel roughness through standard scaling laws the calculated separation zone lengths demonstrate that wake flapping is a dominant mechanism in the production of large scale coherent flow structures in gravel bed rivers thus we show that coherent flow structures over gravels owe their origin to bed generated turbulence and that large scale outer layer structures are the result of flow topography interactions in the near bed region associated with wake flapping

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small

error propagation for velocity and shear stress prediction using 2d models for environmental management pasternack g b gilbert a t wheaton j m buckland e m 2006 error propagation for velocity and shear stress prediction using 2d models for environmental management journal of hydrology 328 1 2 227 241 sponsorship us fish and wildlife service contracting entity for calfed bay delta ecosystem restoration program cooperative agreement dcn 113322g003