surface based transport model for mixed size sediment we present a transport model for mixed sand gravel sediments fractional transport rates are referenced to the size distribution of the bed surface rather than subsurface making the model completely explicit and capable of predicting transient conditions the model is developed using a new data set of 48 coupled observations of flow transport and bed surface grain size using five different sediments the model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions the model uses the full size distribution of the bed surface including sand and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models the model shares some common elements with two previous surface based transport models but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow transport rate and sediments

reenvisioning velocity reversal as a diversity of hydraulic patch behaviours past research investigated the surpassing of mean velocity at riffle cross sections by that at pool cross sections for flows up to bankfull termed velocity reversals to understand one mechanism by which riffle pool relief is maintained this study reenvisioned the classic velocity reversal by documenting stage dependent changes to the locations of peak velocity without cross sections instead the dynamics of peak velocity patches were considered for flows spanning 0 2 to 22 times bankfull discharge through the use of a high resolution dem and two dimensional hydrodynamic modelling a remarkable diversity in peak velocity patch behaviour was found across discharges including gradual expansion and shifting as well as abrupt disappearance and emergence relative to the low flow patch locations these behaviours blended together to varying degrees to produce many reversals in peak velocity across morphological units but it took substantially higher than bankfull discharge for peak velocities to move from riffles and chutes to fast glides and pools the discharges at which reversals occurred among morphological units were significantly higher for the valley confined reach than for the anastomosing reach copyright 2016 john wiley sons ltd

hydrodynamics of a forced riffle pool in a gravel bed river 1 mean velocity and turbulence intensity 1 riffle pools are fundamental units of many gravel bed rivers considerable debate exists as to an appropriate hydrodynamic model for this bed form type sampling designs in previous studies have not always anticipated the degree of spatial variability of flow parameters in riffle pools nor considered the effect of the nonuniform boundary on hydrodynamics the objective of this paper is to detail the distribution of mean velocities and turbulence intensities in a forced riffle pool while limited by the lack of lateral velocity measurements the data set is of high quality and includes measurements during a bankfull flood key observations include the perturbation of velocity profiles from what is observed in uniform flow lateral convergence and divergence of flow and the generation of turbulence away from the channel boundary in the thalweg of the pool head velocities are high near the water surface and low near the bed turbulence intensities are relatively high near the bed and may be significant for sediment entrainment higher mean velocities occur over the side bar and could indicate that sediment is routed around the thalweg the midpool is characterized by the lateral constriction of flow and a shear zone downstream of the forcing element the pool tail has very high velocities and turbulence intensities near the bed a velocity reversal occurs in the pool tail at bankfull flow but does not occur in the pool head a consideration of deceleration and acceleration as a result of vertical expansion and contraction is shown to explain many of the key observations

on the necessity of applying a rotation to instantaneous velocity measurements in river flows in studies on river channel flow turbulence it is often the case that the measured mean vertical velocity is different from zero indicating that the frame of reference of the current meter is not parallel to the flow streamline this situation affects the estimate of reynolds shear stress in the streamwise and vertical planes and consequently the analysis of the flow turbulent structure one way to solve this problem is to correct data by applying a rotation and this is reviewed in the first part of the paper however in fluvial geomorphology the studied flow is often complex and streamlines may exhibit significant changes from one point of measurement to the other in this context applying a rotation complicates the situation more than it simplifies it the second part of this paper examines the question of velocity data correction in complex flows using a field example of the turbulent boundary layer over a very rough gravel bed and a laboratory example taken from flow at a river channel confluence in both cases velocity vectors are spatially variable in the first case errors in the reynolds shear stress estimates are relatively low ranging from 13 to 7 per cent deg while in the second case they are much larger 200 to 164 per cent deg the significance of these errors on the interpretation of turbulence statistics in river channel flows is discussed we propose that corrections should be applied in all clear cases of sensor misalignment and when the frame of reference changes spatially and temporally however no corrections should be used where different flow velocity vector orientations not sensor misalignment are responsible for the mean vertical velocity differing from zero

shear stress and hydrodynamic recovery over bedforms of different lengths in a straight channel abstractpools and riffles are common morphological features in rivers that are frequently used but poorly specified analogs in restoration design here straight two dimensional 2d bedforms are conceptualized as perturbations and flow recovery is measured in a laboratory flume with an array of ultrasonic doppler velocity profilers udvps the objectives are to 1 assess the variation of skin friction turbulent stresses and total stress 2 assess the role of topographical feedback on flow recovery and 3 compare flow recovery in isolated and bedforms in series the results show that the total shear stress and near bed turbulence greatly exceed the skin friction in decelerating flow and the pool and that hydrodynamic recovery tends to occur at length scales similar to geophysical scales despite potential negative feedback from the bed repeating short bedforms can push the flow to a more turbulent and laterally concentrated equilibrium condition implications for sediment entrainment thresholds e

late holocene environmental changes in arid southwestern madagascar a 5000 yr stratigraphic record containing fossil pollen charcoal and bones of the extinct quaternary megafauna from andolonomby a hypersaline pond in arid southwestern madagascar shows evidence for climatic desiccation beginning about 3000 yr b p pollen spectra shift at this time from primarily arboreal taxa characteristic of forests and woodlands of more mesic western madagascar to wooded savanna typical of somewhat drier localities between 3000 and 2000 yr b p the site became increasingly arid charcoal and pollen evidence indicates that increased fire and disturbance occurred at the site beginning ca 1900 yr b p probably signaling the beginning of human settlement in the area the fossil record suggests that various human and natural factors on the island may have interacted in the subsequent millennium to culminate in the extinction of the entire endemic megafauna

modeling forced pool riffle hydraulics in a boulder bed stream southern california the mechanisms which control the formation and maintenance of pool riffles are fundamental aspects of channel form and process most of the previous investigations on pool riffle sequences have focused on alluvial rivers and relatively few exist on the maintenance of these bedforms in boulder bed channels here we use a high resolution two dimensional flow model to investigate the interactions among large roughness elements channel hydraulics and the maintenance of a forced pool riffle sequence in a boulder bed stream model output indicates that at low discharge a peak zone of shear stress and velocity occurs over the riffle at or near bankfull discharge the peak in velocity and shear stress is found at the pool head because of strong flow convergence created by large roughness elements the strength of flow convergence is enhanced during model simulations of bankfull flow resulting in a narrow high velocity core that is translated through the pool head and pool center the jet is strengthened by a backwater effect upstream of the constriction and the development of an eddy zone on the lee side of the boulder the extent of flow convergence and divergence is quantified by identifying the effective width defined here as the width which conveys 90 of the highest modeled velocities at low flow the ratio of effective width between the pool and riffle is roughly 1 1 indicating little flow convergence or divergence at bankfull discharge the ratio of effective width is approximately 1 3 between the pool and downstream riffle illustrating the strong flow convergence at the pool head the effective width tends to equalize again with a ratio of 1 1 between the pool and riffle during a modeled discharge of a five year flood as the large roughness elements above the pool become drowned out results suggest that forced pool riffle sequences in boulder bed streams are maintained by flows at or near bankfull discharge because of stage dependent variability in depth averaged velocity and tractive force

velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions abstract keller keller e a 1971 areal sorting of bed load material the hypothesis of velocity reversal geological society of america bulletin 82 753 756 hypothesized that at high flow near bed velocities in pools exceed velocities in riffles and create pool scour pools however typically have larger cross sectional areas of flow at bankfull discharge this condition raises an inconsistency with keller s velocity reversal hypothesis and the one dimensional continuity of mass equation to address this problem a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit slopes of pools according to the model a narrow zone of high velocity occurs in the center of pools creating scour along the downstream end of pools an uphill climb of particles up the pool exit slope promotes sediment deposition the model is tested with field and flume measurements of velocity water surface elevation and size of bed sediments in recirculating eddy influenced pools local reversals of the water surface gradient were measured in the field and a velocity reversal was created in the flume the reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow the distribution of bed sediments collected in the field also support the proposed model of pool maintenance

simulation of flow over pool riffle topography a consideration of the velocity reversal hypothesis computer flow simulations using the hec 2 step backwater routine are used to demonstrate the effect of systematically varying river channel width riffle spacing and channel roughness on the shear velocity section mean velocity and energy slope in fixed bed pool riffle sequences initial scaling is obtained by utilizing published information on hydraulic parameters within reaches of the river severn subsequently this restriction is relaxed and the effect of varying parameter combinations within realistic limits is explored the purpose of this exercise is to isolate those scenarios which may preclude or promote the occurrence of a competence reversal such that pools scour at high flow whilst deposition occurs on riffles it is concluded that rivers in which pools are hydraulically rougher than riffles are likely to demonstrate a competence reversal for prescribed conditions the critical discharge at which a reversal occurs is a negative function of riffle spacing and riffle width relative to pool width downstream variation in hydraulic roughness also has implications for the phase relationship of shear velocity maxima and minima in relation to the extremes in pool riffle topography

role of near bed turbulence structure in bed load transport and bed form mechanics the interactions between turbulence events and sediment motions during bed load transport were studied by means of laser doppler velocimetry and high speed cinematography sweeps u 0 w 0 w 0 which contribute negatively to the bed shear stress and are relatively rare individually move as much sediment as sweeps of comparable magnitude and duration however and much more than bursts u 0 and inward interactions u 0 w 0 when the magnitude of the outward interactions increases relative to the other events therefore the sediment flux increases even though the bed shear stress decreases thus although bed shear stress can be used to estimate bed load transport by flows with well developed boundary layers in which the flow is steady and uniform and the turbulence statistics all scale with the shear velocity it is not accurate for flows with developing boundary layers such as those over sufficiently nonuniform topography or roughness in which significant spatial variations in the magnitudes and durations of the sweeps bursts outward interactions and inward interactions occur these variations produce significant peaks in bed load transport downstream of separation points thus supporting the hypothesis that flow separation plays a significant role in the development of bed forms

the heartbeat of the oligocene climate system a 13 million year continuous record of oligocene climate from the equatorial pacific reveals a pronounced heartbeat in the global carbon cycle and periodicity of glaciations this heartbeat consists of 405 000 127 000 and 96 000 year eccentricity cycles and 1 2 million year obliquity cycles in periodically recurring glacial and carbon cycle events that climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle solar forcing and glacial events box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass the pronounced 405 000 year eccentricity cycle is amplified by the long residence time of carbon in the oceans

the linkage between velocity patterns and sediment entrainment in a forced pool and riffle unit a field based project was initiated in order to characterize velocities and sediment entrainment in a forced pool and riffle sequence three dimensional velocities and turbulence intensities were measured with an acoustic doppler velocimeter at 222 different points at three similar flows that averaged approximately 4 35 m 3 s 1 within a large pool riffle unit on north saint vrain creek colorado sediment sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow which reached a maximum level of 14 8 m 3 s 1 velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply jet flow flow separation vortex scour and turbulence generation enhance scour in the centre of pools ultimately multiple processes appear to play some role in maintenance of this forced pool and the associated riffle copyright 2008 john wiley and 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

the characteristics of turbulence in a shear zone downstream of a channel constriction in a coarse grained forced pool a field based project was initiated to characterize the influence of varying discharge and bed topography along a pool exit slope on the strength of turbulence generated by vortex shedding velocities were measured with an adv downstream of a boulder constriction within a shear zone of a large pool measurements were repeated for four flows that varied from 20 to 50 of bankfull discharge an additional longitudinal profile was conducted along the pool thalweg plots of velocity demonstrate high near bed velocities and turbulence levels in the pools turbulence levels were highest closer to the constriction in the shear zone turbulent kinetic energy decayed in the downstream direction in response to the bed topography and at lower discharges instantaneous velocities are large enough to temporarily lift pebbles at this depth it appears that instantaneous forces created by vortex shedding may play an important role in scouring pools downstream of constrictions the turbulence may also respond to changes in bed topography in a manner that encourages feedbacks among bed topography characteristics of turbulence and sediment transport

pool spacing in forest channels field surveys of stream channels in forested mountain drainage basins in southeast alaska and washington reveal that pool spacing depends on large woody debris lwd loading and channel type slope and width mean pool spacing in pool riffle plane bed and forced pool riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing lwd loading whereas pool spacing in generally steeper step pool channels is independent of lwd loading although plane bed and pool riffle channels occur at similar low lwd loading they exhibit typical pool spacings of greater than 9 and 2 4 channel widths respectively forced pool riffle channels have high lwd loading typical pool spacing of 2 channel widths and slopes that overlap the ranges of free formed pool riffle and plane bed channel types while a forced pool riffle morphology may mask either of these low lwd loading morphologies channel slope provides an indicator of probable morphologic response to wood loss in forced pool riffle reaches at all study sites less than 40 of the lwd pieces force the formation of a pool we also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear cut forests have lower lwd loading and hence fewer pools than reaches in pristine forests

stage dependent variability in tractive force distribution through a riffle pool sequence high resolution data on spatial and temporal variability in flow hydraulics and sediment transport within riffle pool sequences are required to improve understanding of how fluvial processes maintain these meso scale bedforms this paper addresses this issue by providing velocity and boundary shear stress data over a range of discharges from base flow 0 07 m3 s 1 to just over bankfull 8 52 m3 s 1 from a sequence of four pools and three riffles in the river rede northumberland the data supports the reversal hypothesis of keller geol soc am bull 87 1971 753 as the primary explanation for the maintenance of the riffle pool sequence although they also indicate that spatial variability in tractive force is highly stage dependent and complex section averaged velocity data indicate reversal to be evident at four out of six riffle pool units an equalisation in velocity was found for the other two riffle pool units close to bankfull stage the spatial patterns of tractive force exhibited in the study reach as a result of increased discharge demonstrate that riffle pool units operate independently of one another shear stress reversals were observed in individual riffle pool units at different river stages during a flood hydrograph and in some instances two occurred in the same riffle pool unit during a single flow event pools were characterised by coarser bed sediments and narrower channel widths in comparison to riffles increasing the likelihood of tractive force reversal in the river rede areas of predicted bed sediment entrainment obtained from o c matched observed channel changes in the upper part of the study reach but over estimated change in the middle portion of the reach

unifying criterion for the velocity reversal hypothesis in gravel bed rivers it has been hypothesized that velocity reversals provide a mechanism for maintaining pool riffle morphology in gravel bed rivers an important habitat for salmonids which are at risk in many places worldwide and that are the focus of extensive environmental legislation in europe and north america however the occurrence of velocity reversals has been controversial for over 3 decades we present a simple one dimensional criterion that unifies and explains previous disparate findings regarding the occurrence of velocity reversals results show that reversal depends critically on the ratio of riffle to pool width residual pool depth difference between pool and riffle elevations and on the depth of flow over the riffle suggesting that land management activities which alter channel form or divert water from the channel can have negative impacts on the sustainability of pool riffle habitat in gravel bed rivers

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

9000 years of sea level change along the southern african coastline abstract beachrocks are good indicators of sea level stillstands on microtidal coastlines and data for this sea level curve is mainly based on dating a series of holocene beachrocks and planation episodes on the southeast african coastline sea level reached its present level along the southern african coastline at ca 6500 bp sea level then rose depositing a series of beachrocks at an elevation of 2 75 m this mid holocene sea level highstand persisted for a period of 2500 years with a 3 5 m stillstand dated at 4480 bp being the highest sea level reached during the holocene in post holocene sea level optimum times a regression occurred lowering sea level to its present level at 3880 bp and down to 2 m at approximately 3000 bp subsequently sea level rose to 1 5 m at 1610 bp and attained its present level at about 900 bp the mid holocene sea level highstand appears to have resulted from a combination of isostatic emergence and the steric expansion of seawater relating to warmer ocean temperatures associated with the holocene hypsithermal event on the eastern agulhas bank

jet and hydraulic jump near bed stresses below a horseshoe waterfall 1 horseshoe waterfalls are a common but unexplained feature of bedrock rivers as a step toward understanding their geomorphology a six component force torque sensor was used to evaluate clear water scour mechanisms in a 0 91 m broad crested horseshoe waterfall built into a 2 75 m wide flume to measure longitudinal lateral and temporal profiles of the near bed spatially averaged stress vector jet impact caused moderate near bed stress downstream of the impact point the stress vector shifted into a much more forceful upthrust with peak stress beyond the boil crest instantaneous fluctuations in near bed lift exceeded drag by 5 10 times and occurred over a wider range of frequencies suggesting a larger role for particle entrainment predictions of bed stress significantly underestimated actual conditions these results show that horseshoe waterfalls have an extended region of high scour along their centerline corresponding with the three dimensional convergent dynamics associated with both jet impact and jump enhanced lift fluctuations

a tale of two riffles using multidimensional multifractional time varying sediment transport to assess self maintenance in pool riffle sequences pool riffle sequences play a central role in providing habitat diversity conditions both in terms of flow and substrate in gravel bed streams understanding their capacity to self maintain has been the focus of research for many years starting with the velocity reversal hypothesis this hypothesis relied only on cross sectional averaged flow information but its limited success prompted extensions of the hypothesis and alternative explanations for self maintenance significant advances beyond the velocity reversal hypothesis have been achieved by incorporating more information either on flow or sediment transport characteristics however this has been done in a compartmentalised way with studies either focusing on one or the other aspect this work bridges the gap between these two aspects by using an approximate methodology that combines observed characteristic stage dependent 3 d flow patterns with time varying cross sectional information on bed shear stresses sediment distribution and sediment bed changes during a one year record of continuous discharges from a real stream this methodology allows us to track the behaviour of different sediment size fractions along flow streamlines over time and identify self maintenance conditions due to the combined effect of both flow multi dimensionality and sediment transport we apply this approximate methodology to two contiguous pools and riffles and demonstrate that unexpectedly they may rely on different mechanisms for self maintenance due to differences in geometry and sediment size distribution we also demonstrate that our methodology is potentially overarching and integrative of previous partial approaches based on flow multidimensionality or sediment transport which tend to underestimate the occurrence of self maintenance this article is protected by copyright all rights reserved

flow reversal over a natural pool riffle sequence a computational study a computational study is presented on the hydraulics of a natural pool riffle sequence composed of mixed cobbles pebbles and sand in the river lune northern england a depth averaged two dimensional numerical model is employed calibrated with observed data at the field site from the computational outputs the occurrence of longitudinally double peak zones of bed shear stress and velocity is found in particular at low discharge there exists a primary peak zone of bed shear stress and velocity at the riffle tail in line with the local maximum energy slope in addition to a secondary peak at the pool head as discharge increases the primary peak at the riffle tail at low flow moves toward the upstream side of the riffle along with the maximum energy slope showing progressive equalization to the surrounding hydraulic profiles concurrently the secondary peak due to channel constriction appears to stand at the pool head with its value increasing with discharge and approaching or exceeding the primary peak over the riffle the existence of flow reversal is demonstrated for this specific case which is attributable to channel constriction at the pool head a dynamic equilibrium model is presented to reconstruct the pool riffle morphology a series of numerical modelling exercises demonstrates that the pool riffle morphology is more likely produced by shallow flows concentrated with coarse sediments than deep flows laden with low concentrations of fine sediments it is concluded that channel constriction can but may not necessarily lead to competence reversal depending on channel geometry flow discharge and sediment properties

spatial variations in surface sediment structure in riffle pool sequences a preliminary test of the differential sediment entrainment hypothesis dseh riffle pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles this preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows however the differential sediment entrainment hypothesis dseh postulates that differential entrainment can instead result from spatial sedimentological contrasts here we use a novel suite of in situ grain scale field measurements from a riffle pool sequence to parameterise a physically based model of grain entrainment field measurements include pivoting angles lift forces and high resolution dems acquired using terrestrial laser scanning from which particle exposure protrusion and surface roughness were derived the entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles this is because pool grains have looser packing hence greater exposure and lower pivoting angles conversely riffle and pool exit grains have denser packing lower exposure and higher pivoting angles a cohesive matrix further stabilises pool exit grains the resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a 2d computational fluid dynamics cfd model of the reach the cfd model predicts that under bankfull conditions pools experience lower shear stresses than riffles and pool exits however the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles resulting in differential entrainment under bankfull flows significantly this differential entrainment does not require a reversal in flow velocities or shear stress suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle pool sequences this finding has implications for the prediction of sediment transport and the morphological evolution of gravel bed rivers

flow convergence routing hypothesis for pool riffle maintenance in alluvial rivers 1 the velocity reversal hypothesis is commonly cited as a mechanism for the maintenance of pool riffle morphology although this hypothesis is based on the magnitude of mean flow parameters recent studies have suggested that mean parameters are not sufficient to explain the dominant processes in many pool riffle sequences in this study two and three dimensional models are applied to simulate flow in the pool riffle sequence on dry creek california where the velocity reversal hypothesis was first proposed these simulations provide an opportunity to evaluate the hydrodynamics underlying the observed reversals in near bed and section averaged velocity and are used to investigate the influence of secondary currents the advection of momentum and cross stream flow variability the simulation results support the occurrence of a reversal in mean velocity and mean shear stress with increasing discharge however the results indicate that the effects of flow convergence due to an upstream constriction and the routing of flow through the system are more significant in influencing pool riffle morphology than the occurrence of a mean velocity reversal the hypothesis of flow convergence routing is introduced as a more meaningful explanation of the mechanisms acting to maintain pool riffle morphology

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

oxygen isotope variations in phosphate of biogenic apatites iv mammal teeth and bones groups of rats grown from birth to death in identical conditions but with different 18o of drinking water w were studied for variations of 18o of their body water bw and bone phosphate p there is a high linear correlation r 0 99 between bw and between p and w the regression lines have similar slope coefficients 0 53 and 0 49 values of p of different teeth and bones are the same within the precision of the method 0 5 the isotopic fractionation coefficient between phosphate and body water is 1 0178 and close to the estimated value of 1 0173 derived from the phosphate paleotemperature equation deviations from constant fractionation between bone phosphate and environmental drinking water depend primarily on rates of drinking and metabolism environment conditions have relatively little effect on oxygen isotope fractionation between water sources and bone phosphate

velocity characteristics along a small step pool channel this paper summarizes measurements of velocity along three reaches of a small mountain channel with step pool bedforms a one dimensional electromagnetic current meter was used to record velocity fluctuations at 37 fixed measurement points during five measurement intervals spanning the peak of the annual snowmelt hydrograph measurement cross sections were located upstream from a bed step at the step lip downstream from the step and in a uniform gradient run data analyses focused on characteristics of velocity profiles and on correlations between velocity characteristics and the potential control variables bedform type reach gradient and flow depth to test the hypothesis that velocity characteristics are related to channel bedform types anova and ancova tests were performed for the average velocity and coefficient of variation of point velocity data results indicate that high frequency velocity variations correlate to some degree with both channel characteristics and discharge velocity became more variable as stage increased particularly at low gradient reaches with less variable bed roughness velocity profiles suggest that locations immediately downstream from bed steps are dominated by wake turbulence from mid profile shear layers locations immediately upstream from steps at step lips and in runs are dominated by bed generated turbulence adverse pressure gradients upstream and downstream from steps may be enhancing turbulence generation whereas favourable pressure gradients at steps are suppressing turbulence the bed generated turbulence and skin friction of runs appear to be less effective energy dissipators than the wake generated turbulence and form drag of step pool bedforms copyright 2000 john wiley sons ltd

riffle pool morphometry and stage dependant morphodynamics of a large floodplain river vereinigte mulde sachsen anhalt germany riffle pool sequences are a common feature of gravel bed rivers however mechanisms of their generation and maintenance are still not fully understood in this study a monitoring approach is employed that focuses on analysing cross sectional and longitudinal channel geometry of a large floodplain river vereinigte mulde sachsen anhalt germany with a high temporal and spatial resolution in order to conclude from stage dependant morphometric changes to riffle and pool maintaining processes in accordance with previous authors pool cross sections of the mulde river are narrow and riffle cross sections are wide suggesting that they should rather be addressed as two general types of channel cross sections than solely as bedforms at high flows riffles and pools in the study reaches changed in length and height but not in position pools were scoured and riffles aggraded a development which was reversed during receding flows below the threshold of 0 4qbf 40 bankfull discharge an index for the longitudinal amplitude of riffle pool sequences the bed undulation intensity or bedform amplitude is introduced and proved to be highly significant as a form parameter its first derivative as a process parameter the process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation it is indicated by increasing longitudinal bed amplitudes according to the observed dynamics of bed amplitudes maintenance of riffle pool sequences lags behind discharge peaks maximum bed amplitudes may be reached with a delay of several days after peak discharges increasing bed undulation intensity is interpreted to indicate bed mobility post flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools this type of transport behaviour is referred to as disconnected mobility the comparison of two river reaches one with undisturbed sediment supply the other with sediment deficit suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions it is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload copyright 2011 john wiley sons ltd

a 40 000 yr pollen and diatom record from lake tritrivakely madagascar in the southern tropics abstract links between southern and northern hemisphere climates during the late quaternary are poorly known partly due to the scarcity of continuous climatic records in the southern tropics pollen and diatom evidence from lake tritrivakely 19 47 s provides information on vegetational and hydrological changes in the central highlands of madagascar over the past 40 000 yr most of the record reflects natural environmental variability since humans arrived on the island ca 2000 yr b p during glacial times the migration of mountain plants toward lower altitudes is consistent with a temperature decrease and with reduced atmospheric co2levels in the lake a positive mean annual hydrologic balance from 38 000 to 36 000 and from 17 500 to 9800 cal yr b p coincided with periods of decreasing summer insolation and preceded by several millennia lake rises in the northern tropics a negative hydrologic budget during periods of maximum seasonal contrast in solar radiation is partly attributed to high summer evaporation rate the last glacial maximum was cool and dry the deglacial warming occurred in two steps the first step accompanied by an increase in wetness occurred abruptly at ca 17 000 cal yr b p about two millennia earlier than in the northern hemisphere it is abundantly documented in southern terrestrial data the second step at 15 000 cal yr b p was in phase with the first major temperature change in the northern hemisphere

the amino acid and stable isotope biogeochemistry of elephant bird aepyornis eggshells from southern madagascar abstract a diverse suite of animals became extinct on madagascar during the late holocene as observed on landmasses elsewhere the extinction process broadly coincided with the arrival of people our research on the amino acid racemisation and the carbon and oxygen isotope biogeochemistry of elephant bird aepyornis eggshells from southern madagascar refines models that attempt to explain the extinction process a correlation between the extent of isoleucine epimerisation aile ile and radiocarbon age of eggshells allows aile ile to serve as a proxy for eggshell age the aile ile values indicate the majority 87 of eggshells in this study are holocene with the remainder representing pleistocene aepyornis populations and that further amino acid analyses would help to constrain the timing of aepyornis extinction carbon isotope ratios in the organic and calcite fractions of eggshells indicate that aepyornis primarily browsed c3 vegetation oxygen isotope values are more negative and less variable than in eggshells of ostriches living in semi arid environments suggesting that aepyornis populations relied upon groundwater fed coastal wetlands for their drinking water the isotope results require that the changing abundances of c3 vegetation and groundwater fed watering points be considered in models that aim to understand the extinction of aepyornis in southern madagascar

pool riffle sedimentation and surface texture trends in a gravel bed stream a 3 year field campaign was completed to investigate spatial and temporal variability of sedimentation trends for a single pool riffle pair located in the santa cruz mountains california our measurements represent a range of hydrologic conditions over 11 sediment mobilizing events two different statistical methods were used to explore riffle sedimentation cochran s q and mcnemar s nonparametric tests one method indicate that riffle sediment surface texture was spatially and temporally varied at the transect level for mcnemar s test variation was significant at p 0 05 with several trends evident including strong riffle fining triggered by a 20 year flood event a nonlinear empirical orthogonal function method known as self organizing maps soms the second method shows that riffle sediment surface texture is well described by two characteristic temporal signals and one transitional signal at the sampling node level som mapping to each sampling node clearly shows riffle sediment surface texture change was spatially organized over the 11 sediment mobilizing events observations of pool sediment storage indicate that the pool riffle pair exhibited a coupled sedimentation response i e similar texture trends between pool and riffle following the 20 year flood the coupled response was characterized by a trend toward overall sedimentation conditions that were similar to those measured at the beginning of the study the reported texture trends may be of interest to salmonid habitat studies that examine factors contributing to successful versus unsuccessful fry emergence

understanding pool riffle dynamics through continuous morphological simulations pool riffle dynamics is governed by complex time and spatial interactions between water and sediment flows in the last few decades significant advances have been made in characterizing and modeling the hydrodynamics of pool riffle sequences and this information has been extensively used as the basis of conceptual models to describe or infer pool riffle morphodynamics a lot less attention however has been paid to the coupled dynamics of flow and sediment which is essential to fully understand these complex geomorphic systems this paper uses an unsteady 1 d flow morphology and bed sorting model to analyze pool riffle dynamics the model is first applied to a pool riffle sequence on a 1 1 km reach of the lower bear creek arkansas united states after showing the model s ability to describe the general reach hydrodynamics and morphological evolution over 1 year the detailed sediment and flow information is used to investigate pool riffle dynamics in terms of self maintenance mechanisms two effects that have been only marginally explored in the past i e bed sediment sorting and downstream riffle control are explained and quantified with the help of the model s outputs the results show that self maintenance occurs more frequently than previously thought as a result of grain sorting and that erosion or deposition of contiguous riffles also constitutes a self maintenance mechanism these findings provide the support for a physically based integral description of pool riffle morphodynamics and highlight the importance of flow and sediment variability on pool riffle self maintenance the morphodynamic analysis bridges the gap between observations and current theories based mainly on hydrodynamic information

velocity reversal hypothesis a model approach hydraulic modelling of a riffle pool riffle sequence in a straight reach of dry creek near winters california indicates that while at low flow the mean section average velocity at riffles exceeds that of an adjacent pool during high flow the mean velocity of the pool exceeds that of adjacent riffles this supports the velocity reversal hypothesis which is based on limited field measurements of near bed flow velocity a velocity reversal does not occur at the same discharge in both riffle pool sequences modelled and a more detailed explanation is necessary to account for complex aspects of channel form and process in riffle pool sequences nevertheless the velocity reversal hypothesis helps to explain many of the observations and processes that operate in the riffle pool environment

the relationship between shear stress and the shaping of the bed of a pebble loaded river la rulles ardenne summary geomorphological modifications in the bed of a small pebble loaded river were related to different parameters such as flow velocity and shear stress experiments carried out with marked pebbles of known diameter demonstrated that shear stress calculated from the energy grade line slope and the hydraulic radius is a reliable criterion in explaining erosion and transport of the pebble load increase in shear stress with discharge entrainment varies according to site at times of low flow shear stress is weak in the pools allowing transitory accumulation of fine deposits and pebbles these originate mainly from erosion downstream of riffles where shear stress is already considerable at times of discharge near bankfull stage shear stress becomes considerable in the pools so that pebbles deposited at lower discharge rates are then displaced however the coarser elements are halted on the counter slope of the pools and upstream of riffles since shear stress there is then weaker than in the pools riffles thus appears as stopping places with only small elements 1 5 cm diameter able to cross them at times of high discharge the size of these elements thus provides an estimate of the true competence of the river

estimating local bed shear stress from velocity observations replicate velocity observations using conventional equipment under typical field conditions are used to evaluate the precision of different methods for estimating local boundary shear stress from velocity measurements the bed shear velocity u can be estimated within 3 using the depth averaged velocity in the vertically averaged logarithmic velocity profile to be accurate this method is limited to relatively simple flow geometries which may be expected to have the appropriate velocity structure estimates of u made using a single near bed velocity observation are less precise by a factor of 3 because of the larger uncertainty associated with a single observation accuracy of this method requires appropriate flow conditions only near the bed so it may be applied in a wider range of flow conditions including spatially variable flow estimates of u from the slope of the near bed velocity profile are the least precise and require the most restrictive flow conditions for accuracy but offer the advantage that they may be made without independent knowledge of the bed roughness different methods for estimating roe should be useful in judging the precision of estimates made under similar conditions these estimates of precision together with considerations of convenience and model accuracy provide the basis for select ing the most appropriate or advantageous method for different conditions and purposes

numerical investigation of bearing capacity of spudcan foundations in clay overlying sand under combined loading abstractthe evaluation of the foundation capacity under combined vertical horizontal and moment loading is an essential element of the site specific assessment that ensures mobile jackup rigs are

an appraisal of the velocity reversal hypothesis for stable pool riffle sequences in the river severn england the velocity reversal hypothesis is the linchpin for a number of recent conceptual models pertaining to sediment sorting and the maintenance of pool riffle sequences in gravel bedded streams the literature in support of the hypothesis together with published adverse criticism is reviewed it is concluded that convincing evidence for the ubiquitous occurrence of such a reversal in a range of channel geometries is currently unavailable continuity considerations indicate that riffles need to be considerably wider than pools for a reversal in the mean velocity to occur under conditions of subcritical flow high stage and stable morphology these observations are substantiated by a detailed study of the hydraulic geometry of stable pool riffle sequences in the river severn england neither the sectionally averaged velocity nor the near bed shear velocity is sensibly greater in the pools than over the riffles during bankfull or near bankfull flow instead a tendency towards equalization of the values of average hydraulic variables is noted as discharge increases a detailed investigation of the three dimensional character of the flow is required to demonstrate whether the entrainment forces within pools can locally exceed those over neighbouring riffles unusual behaviour of the energy gradient over riffles during moderate discharges is related to backwater effects as mediated by the spacing of the riffles the hydraulic data are used to comment on the stability of the test reaches in the context of the development of the river severn in the vicinity of shrewsbury

differential bed sedimentology and the maintenance of riffle pool sequences abstract riffle pools are the characteristic reach scale bedforms of gravel and mixed bedded channels of low to moderate slope the purpose of this paper is to clarify several aspects of the sedimentology of riffle pool sequences and to suggest how these both reflect and control riffle pool stability in the presence of spatial differences in the turbulent near bed flow field high frequency velocity measurements obtained using an array of electromagnetic current meters are combined with results from grid and transect sampling of bed sediments from 9 sites on 3 small upland rivers in n exmoor u k riffle sediments are shown to possess greater development of microtopographic structuring than adjacent pools which enhances riffle stability and which may reflect greater turbulent kinetic energy there over most of the flow range data on surface sediment arrangement and turbulent flow characteristics can be combined with different transport threshold equations to explain the maintenance of riffle pool sequences with respect to a particle queuing kinematic wave model without recourse to a competence reversal as conventionally assumed

sediment transport processes in pool riffle sequences this paper reviews the published data for sediment transport in pool riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport this conclusion is explored in more detail using observations of sediment transport from the regulated river north tyne sediment transport rates at discharges of 30 per cent bankfull are initially highest on riffles as discharge increases sediment transport rates in pools rise sharply and values locally exceed those on riffles tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool riffle sequence coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds an appraisal of the possible mechanisms controlling sediment entrainment transport and deposition in pool riffle sequences is made which reveals evidence for the operation of a combination of hydraulic sedimentological and interactive processes a descriptive model of sediment transport processes in pool riffle sequences is presented

stable isotope distribution in maastrichtian vertebrates and paleosols from the haţeg basin south carpathians abstract located in the south carpathians in romania the ha eg basin contains a thick sequence of maastrichtian continental deposits yielding a rich dinosaur and mammalian fauna stable isotope analyses of both calcretes and dinosaur crocodilian and turtle remains from two localities tustea and sibisel were integrated in order to reconstruct environmental conditions during maastrichtian time and to gain further insights into the metabolism and behaviour of the vertebrates the large difference observed between the 18o and 13c of the eggshells and the surrounding mudstones as well as the preservation of the 9 difference between the oxygen isotope composition of the telmatosaurus eggshell and tooth enamel indicate that diagenesis has not significantly altered the primary isotopic signal the presence of smectite in the fraction less than 2 m and the preservation of dinosaur eggshell structure also indicate that diagenesis was not significant in the studied outcrops stable isotope compositions of both calcretes and phosphatic remains suggest warmer conditions during the deposition of the tustea sequence mean annual temperature of around 14 c than during the deposition of the sibisel sequence mean annual temperature of 10 c the intra tooth 18o patterns for zalmoxes and allodaposuchus show different magnitudes of isotopic variation with lower values for tustea and higher for sibisel the calculated 18obody water enrichment for kallokibotion bajazidi is similar to that found in living turtle taxa by contrast in the case of allodaposuchus the isotopic enrichment is higher than for recent taxa this suggests that for allodaposuchus the body water was less buffered by a watery environment which probably indicates more time spent outside water i e more terrestrial habit the 18o values for the teeth of telmatosaurus and zalmoxes are similar to those of allodaposuchus suggesting that at the investigated sites the body temperature of both dinosaurs was similar to that of the crocodile the isotopic composition of calcretes teeth and eggshells indicates a c3 vegetation and diet with 13c values between 27 and 29 pdb and the absence of large scale habitat partitioning between the dinosaurs

holocene carbon cycle dynamics based on co2 trapped in ice at taylor dome antarctica a high resolution ice core record of atmospheric co2 concentration over the holocene epoch shows that the global carbon cycle has not been in steady state during the past 11 000 years analysis of the co2 concentration and carbon stable isotope records using a one dimensional carbon cycle model uggests that changes in terrestrial biomass and sea surface temperature were largely responsible for the observed millennial scale changes of atmospheric co2 concentrations

ams 14c dates for extinct lemurs from caves in the ankarana massif northern madagascar abstract an extensive late quaternary fauna including many extinct giant lemurs has been collected recently in a 110 km system of caves in the ankarana massif of northern madagascar ams 14c dates for the acid insoluble collagen gelatin fraction of bones of the giant lemur megaladapis 26 150 400 and 12 760 70 yr b p confirm its presence in the area during the late pleistocene and provide the first pleistocene 14 c ages from bones of the extinct megafauna of the island the first date from bones of the recently described extinct babakotia radofilai 4400 60 yr b p shows that it was present in northern madagascar in mid holocene times a comparatively recent age of 1020 50 yr b p for the extinct archaeolemur indicates survival of this genus for at least a millennium after the first direct evidence for humans in madagascar this suggests that the island s extinction window may have represented a longer time span than would have been expected under the blitzkrieg model of late quaternary extinctions a mid holocene age 4560 70 yr b p for a bone sample of the small extant lemur hapalemur simus indicates that the disappearance of this now restricted species from the ankarana occurred after this date new data from the ankarana and other sites on the island add to the consensus that major biotic changes occurred on madagascar in the late holocene

a model of two phase bedload transport in an oregon coast range stream a descriptive model for bed material routing in small sand and gravel bedded channels with sequences of pools and armoured riffles was developed aspects of the model are demonstrated at flynn creek a 202 ha third order drainage in the oregon coast range assuming that channel geometry sediment transport competence and the availability of sediments for transport are all non uniform in the downstream direction the model described bedload transport as occurring in two phases phase i involves the transport of fine predominantly sand sized bed materials over stable gravel armoured riffles phase ii occurs at higher flows that can entrain riffle armour and transport riffle sediments in addition to phase i sediments phase i bedload transport was sampled during three moderate flow events a power function related phase i bedload transport to water discharge the relationship was consistent between storms and between two successive years phase ii bedload transport sampled during a 1 8 yr return period streamflow event in february 1979 proved non uniform in a downstream direction and unsteady over time at a given stream discharge transport rates closely correlated with the rapid scour and redeposition of the riffle at the bedload sampling cross section transport peaks corresponded to scour and transport decreased greatly during deposition the transport of large 12 5 mm diam bed material increased up to 12 fold during riffle scour

modelling three dimensional flow structures and patterns of boundary shear stress in a natural pool riffle sequence fluid sediment interactions control river channel forms and processes analysis of spatial hydraulic patterns and the resulting boundary shear stress are required to aid understanding of river system behaviour in this paper the hydraulic processes active in a natural pool riffle sequence are simulated using a three dimensional computational fluid dynamics cfd model methods employed for the prescription of model boundary conditions are outlined model calculations are assessed using comparisons with field observations acquired over a range of flows simulations are then used to illustrate flow structures and patterns of boundary shear stress for a near bankfull and an intermediate flow event results are used to assess existing theories that seek to explain the development and maintenance of pool riffle sequences simulated results suggest that near bed velocities and bed shear stresses decrease on riffles and increase in pools as discharge increases model simulations indicate that secondary flow acts to route near bed flow over the downstream side of riffles and into the pool head away from the centre of pools implications for sediment transport and pool maintenance are discussed

consistent preload calculations for jackup spudcan penetration in clays onshore bearing capacity methods have conventionally been used to predict the penetration of offshore jackup spudcans into the seabed however the brinch hansen depth factor has a step change in value at the point at which the lateral dimension of a footing equals its embedment for soft clay seabeds where penetrations can be greater than the spudcan diameter the step produces a spurious unrealistic prediction of punchthrough a small modification is proposed to the depth factor to avoid this some consistent modifications are proposed for several related calculations

influence of turbulence on bed load sediment transport this paper summarizes the results of an experimental study on the influence of an external turbulence field on the bed load sediment transport in an open channel the external turbulence was generated by 1 a horizontal pipe placed halfway through the depth ih 2 a series of grids with a clearance of about one third of the depth from the bed and extending over a finite length of the flume and 3 a series of grids with a clearance in the range 0 1 1 0 ih from the bed but extending over the entire length of the flume two kinds of experiments were conducted plane bed experiments and ripple covered bed experiments in the former case the flow in the presence of the turbulence generator was adjusted so that the mean bed shear stress was the same as in the case without the turbulence generator in order to single out the effect of the external turbulence on the sediment transport in the ripple covered bed case the mean and turbulence quantities of the streamwise component of the velocity were measured and the shields parameter due to skin friction was determined the shields parameter together with the rms value of the streamwise velocity fluctuations was correlated with the sediment transport rate the sediment transport increases markedly with increasing turbulence level

the determination of late quaternary paleoenvironments at equus cave south africa using stable isotopes and amino acid racemization in ostrich eggshell abstract the stable carbon and nitrogen isotopes from organic compounds and stable oxygen and carbon isotopes from inorganic carbonate in modern and fossil ostrich eggshell oes at equus cave south africa were used to determine relative changes in c 3 and c 4 vegetation rainfall and temperature throughout the last 17 ka amino acid racemization and three new radiocarbon dates were used to assign ages to the individual oes fragments analyzed for stable isotopes and to evaluate the stratigraphic integrity of the samples the amino acid racemization and radiocarbon data indicate that most of the oes fragments are not in correct stratigraphic context and may have been upwardly and downwardly mixed thus each fragment was placed in chronological sequence using radiocarbon calibrated amino acid racemization the stable carbon isotope data from the fossil oes do not change systematically through the last 17 ka the ostriches primarily consumed c 3 plants between 70 and 90 and smaller quantities of c 4 plants between 30 and 10 indicating that a mixture of c 3 and c 4 plants has been in the vicinity of equus cave for the last 17 ka stable nitrogen isotope data from oes indicate that at 17 ka mean annual precipitation map was at a minimum 190 50 mm yr increased steadily to modern values by 6 ka 600 150 mm yr and remained relatively unchanged until present stable oxygen isotope data coupled with nitrogen isotope data indicate that paleotemperatures were at a minimum between 14 and 17 ka and reached their maximum in the latest holocene this study demonstrates the plethora of paleoenvironmental information that can be derived from analysis of three different stable isotopes carbon nitrogen and oxygen in the same sample type oes and presents a new tool with wide applicability for reconstructing paleoenvironments in semi arid and arid regions of africa

body size evolution in palaeognath birds is consistent with neogene cooling linked gigantism abstract palaeognathae is an extant clade of birds with body masses ranging over several orders of magnitude of kilograms including the largest yet described terrestrial avian species most studies have suggested flight loss and increasing body size began early in its evolutionary history prior to the divergence of the major extant lineages however recent phylogenetic work suggests body size increases occurred more recently and independently within many extant lineages here we use linear regression equations to estimate the masses of extinct taxa from both the crown and stem of palaeognathae to test these hypotheses we allocate fragmentary specimens to discrete body mass categories to accommodate additional species for which more precise body mass estimation is not possible the first fossil evidence for an increase in maximum palaeognath body size from approximately that of a rhea 20 kg or smaller to ostrich size 100 kg is miocene in age approximately 20 ma the heaviest taxa those exceeding 120 kg all occur within the last 9 million years molecular divergence dating estimates for the crown ages of palaeognath subclades pre date the oldest preserved material often significantly however plotting the lightest member of each clade at the estimated crown ages does not change the observed pattern these results may be affected by the predictive model used to estimate the mass of extinct taxa but the observed trends in body mass do not change when four models from different studies are used both island dwelling and continental clades show broad temporal congruence in the timing of their estimated gain of large body sizes meaning geographical restriction on islands does not alone explain the observed trend we suggest large and giant palaeognathae body sizes may be more closely linked to global cooling which creates selective pressures on body size for thermoregulation as well as causing landscape changes which alters dietary and locomotory requirements global cooling has received extensive attention as a driver of mammalian body size extremes but has been relatively little discussed with respect to birds

the influence of pool length on local turbulence production and energy slope a flume experiment the in uence of pool length on the strength of turbulence generated by vortex shedding was investigated in a 6 m long recirculating ume the experiment utilized a 38 constriction of ow and an average channel bed slope of 0 007 the base geometry for the intermediate length pool experiment originated from a highly simpli ed 0 10 scale model of a forced pool from north saint vrain creek colorado discharge in the ume was 31 6 l s which corresponds to a discharge in the prototype channel of 10 m3 s three shorter and four longer pool lengths also were created with a xed bed to determine changes in turbulence intensities and energy slope with pool elongation three dimensional velocities were measured with an acoustic doppler velocimeter at 31 40 different 0 6 depth and near bed locations downstream of the rectangular constriction the average velocity and root mean square rms of the absolute magnitude of velocity at both depths are signi cantly related to the distance from the constriction in most pool locations downstream of the constriction in many locations pool elongation results in a non linear change in turbulence intensities and average velocity based on the overall ow pattern the strongest turbulence occurs in the center of the pool along the shear zone between the jet and recirculating eddy the lateral location of this shear zone is sensitive to changes in pool length energy slope also was sensitive to pool length due to a combination of greater length of the pool and greater head loss with shorter pools the results indicate some form of hydraulic optimization is possible with pools adjusting their length to adjust the location and strength of turbulent intensities in the center of pools and lower their rate of energy dissipation copyright 2004 john wiley sons ltd

sediment mobility in a forced riffle pool forced riffle pools occur in gravel bed rivers where a large nonalluvial element leads to local scour and deposition to manage rivers where this type of morphology is found and to specify restoration measures that mimic this process more field data on sediment mobility in forced riffle pools is needed the objectives of this study are to describe the spatial variability of sediment mobility and deposition in a developing forced riffle pool and to use high resolution flow velocity measurements to explain the observed dynamics the field site is a forced riffle pool in moras creek a 6 m wide gravel bed stream with a 1 2 bed slope in quebec canada topography and the movement of sediment particles equipped with passive integrated transponder pit tags are surveyed through a series of competent floods bed shear stress is estimated from near bed measurements of time averaged velocity and turbulent kinetic energy the sediment transport regime of the creek is characterized by partial mobility exponential distributions of path lengths and a negative relation between particle size and path length full mobility occurs in the center of the pool and over the exit slope where flow is accelerated from the constriction of flow during two events above the bankfull discharge partial mobility occurs during the same events over the entrance slope to the pool lateral gradients of deposition and mobility suggest that the majority of sediment in motion is routed over the side bar at the entrance to the pool high levels of turbulence intensity that occur as a result of flow deceleration may explain the removal of finer sediments from the head of the pool

hydrodynamics of a forced riffle pool in a gravel bed river 2 scale and structure of coherent turbulent events 1 in the companion article we present mean velocity and turbulence intensity statistics at a range of discharges in a forced riffle pool unit and show that the distributions of mean velocity and turbulence are characteristic of nonuniform flows the objectives of this paper are to describe the spatial and temporal variability of the structure of turbulence in the forced riffle pool unit and to examine turbulence generation mechanisms and the links between the scales of macrobed forms and coherent turbulent flow structures our results show that while larger scale structures develop outside of the thalweg the scales of coherent turbulent flow structures decrease with velocity and discharge in the forced riffle pool flow separation is shown to be an important factor with full separation occurring in lateral shear zones and intermittent separation near the bed in the pool head these zones generate new turbulence that disrupts a number of coherent turbulent flow structures that are typical of uniform flows coherent turbulent flow structures do not appear to be the source of morphological scaling in the forced riffle pool

sediment routing hypothesis for pool riffle maintenance this paper provides comprehensive evidence that sediment routing around pools is a key mechanism for pool riffle maintenance in sinuous upland gravel bed streams the findings suggest that pools do not require a reversal in energy for them to scour out any accumulated sediments if little or no sediments are fed into them a combination of clast tracing using passive integrated transponder pit tagging and bedload traps positioned along the thalweg on the upstream riffle pool entrance pool exit and downstream riffle are used to provide information on clast pathways and sediment sorting through a single pool riffle unit computational fluid dynamics cfd is also used to explore hydraulic variability and flow pathways clast tracing results provide a strong indication that clasts are not fed through pools rather they are transported across point bar surfaces or around bar edges depending upon previous clast position clast size and event magnitude spatial variations in bedload transport were found throughout the pool riffle unit the pool entrance bedload trap was often found to be empty when the others had filled further supporting the notion that little or no sediment was fed into the pool the pool exit slope trap would occasionally fill with sediment thought to be sourced from the eroding outer bank cfd results demonstrate higher pool shear stresses 140 n m 2 in a localized zone adjacent to an eroding outer bank compared to the upstream and downstream riffles 60 n m 2 at flows of 6 2 m3 s 1 60 of the bankfull discharge and above there was marginal evidence for near bed velocity reversal near bed streamlines produced from velocity vectors indicate that flow paths are diverted over the bar top rather than being fed through the thalweg some streamlines appear to brush the outer edge of the pool for the 4 9 m3 s 1 to 7 8 m3 s 1 between 50 and 80 of the bankfull discharge simulations however complete avoidance was found for discharges greater than this copyright 2013 john wiley sons ltd

phase shifts in shear stress as an explanation for the maintenance of pool riffle sequences the stability of the pool rif e sequence is one of the most fundamental features of alluvial streams for several decades the process of velocity or shear stress reversal has been proposed as an explanation for an increase in the amplitude of pool rif e sequence bars during high ows offsetting gradual scour of rif es and deposition in pools during low ows despite several attempts reversal has rarely been recorded in eld measurements we propose that instead of being reversed maxima and minima in shear stress are phase shifted with respect to the pool rif e sequence bedform pro le so that maximum shear stress occurs upstream of rif e crests at high ow and downstream at low ow such phase shifts produce gradients of shear stress that explain rif e deposition and pool scour at high ow in accord with sediment continuity the proposal is supported by results of a one dimensional hydraulic model applied to the surveyed bathymetry of a pool rif e sequence in a straight reach of a gravel bed river in the sequence studied the upstream phase shift in shear stress at high ow was associated with variations in channel width with width minima occurring upstream of rif e crests approximately coincident with shear stress maxima and width maxima occurring downstream of rif e crests assuming that the width variation is itself the result of ow de ection by rif e crests at low ow and associated bank toe scour downstream low and high ow can be seen to have complementary roles in maintaining alluvial pool rif e sequences copyright 2004 john wiley sons ltd

formation of riffle pool sequences field evidence for an autogenetic process abstract riffle pool sequences are the characteristic reach scale bedforms of mixed and gravel bedded rivers while most research has been directed to the quantitative description of these sequences and to their maintenance comparatively little attention has been given to the processes responsible for their formation in this paper field results from quasi continuous velocity records obtained with an array of electro magnetic current meters are analysed with respect to a model of macro scale structure in the turbulent velocity field of natural channels this model has been suggested as the most plausible basis for riffle pool formation the results demonstrate the occurrence of a variety of coherent flow structures some of which scale on channel dimensions as predicted by the model but which also suggest that some revision of the association between flow structure and riffle pool formation is necessary it is suggested that riffle pool units are initiated with the generation of roller eddies upstream and downstream from a major flow obstacle corresponding to engineering criteria for scour around bridge piers the obstacle persists long enough to fix the flow pattern and hence account for significant modification of channel form but is ultimately removed as part of the process of bed modification via the extension of scour three distinct stages in the process are involved local scour of a single pool creates deposition downstream which then generates the next downstream flow irregularity a riffle pool sequence is thus created autogenetically as the summation of a sequence of irregularities each unit of which is formed and maintained only by flow dynamics operating at the local scale the process is essentially both deterministic and statistical in character it is further suggested that spatial differences in the near bed turbulence field arising from incipient riffle pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism similar reasoning might possibly be extended to the entire range of bedform sizes

undrained bearing capacity factors for conical footings on clay introduction the bearing capacity of circular foundations on undrained clay is of fundamental importance in many geotechnical problems in particular there are a number of designs of offshore foundations where the foundation can be treated approximately as a large circular footing for instance some gravity bases the spudcan foundations of jack up units and the more recently developed caisson foundations in most cases the footing is not placed at the ground surface and it is important to take into account the depth of embedment furthermore the base of a spudcan is generally not flat but approximates a shallow cone for foundations on soft clays the effect of the increase of strength of the soil with depth needs to be taken into account and this is particularly important for large foundations the purpose of this note is to present calculations of bearing capacity factors for shallow circular foundations accounting for embedment cone angle rate of increase of strength with depth and surface roughness of the foundation the results have widespread application particularly in the offshore industry the soil is assumed to be rigid plastic with yield determined by the tresca condition with an undrained strength su the method of characteristics is used for the bearing capacity calculation as described by shield 1955 eason shield 1960 houlsby 1982 and houlsby wroth 1982a for application to undrained axisymmetric problems some previous results have been published for this problem using similar numerical techniques e g houlsby wroth 1982b salencon matar 1982 houlsby wroth 1983 tani craig 1995 martin 2001 but the study presented here involves a much more comprehensive coverage of the parameters where comparisons can be made with the previous solutions the factors differ by up to about 0 5 which gives some indication of the level of accuracy attainable with this numerical technique exceptionally the rough footing results given by tani craig 1995 are higher by up to about 5 but this may be due to a problem with their numerical integration procedures see martin randolph 2001

spontaneous formation and degradation of pool riffle morphology and sediment sorting using a simple fractional transport model many gravel bed streams have a typical bed morphology consisting of pool riffle sequences which provides important habitat diversity both in terms of flow and substrate a complete explanation of pool riffle genesis and self maintenance remains elusive and despite advances in understanding the effects of flow spatial and temporal variability the key sediment processes have been only marginally explored here we use a 1d unsteady multi fraction morphodynamic model to explain the formation and degradation of pool riffle sequences using a 1 year time series of measured flows below bankfull on a stream in which we have removed initial bedforms and sediment sorting our model spontaneously generates pools with finer substrate at narrow sections and riffles with coarser sediment at wider sections closely resembling the natural bed morphology additional experiments show that under our modelling assumptions a variable flow regime is fundamental for development and self maintenance of the longitudinal grain sorting characteristic of pool riffle sequences which could not be obtained or maintained with discharges held constant over relatively long periods