statistical properties of armored gravel bed surfaces 1 roughness of successively developed armor layers in a large laboratory flume is analyzed considering gravel surfaces as two dimensional random fields of bed elevations statistically derived roughness parameters are evaluated from precise digital elevation models dems of bed surfaces and are discussed with respect to successive development of stable armor layers the key results relate to evolution of the probability distribution of bed elevations and its moments and structure functions of various orders with respect to armoring discharge our results suggest that statistical analysis of bed elevations is a helpful tool for defining roughness parameters quantifying armoring effects and identifying particle orientation flow direction and surface forming processes

high resolution numerical modelling of three dimensional flows over complex river bed topography this paper describes preliminary results from a method for the treatment of complex bed topography for the understanding of three dimensional flow processes it develops an established porosity based method and couples it to high resolution bed topographical data acquired using digital photogrammetry the results provide quantitative data on flow processes that match and perhaps exceed current measurement capabilities and so raise difficult questions for model validation however the detailed flow fields provided by the method allow us to develop current models for flow form interactions in gravel bed rivers of most importance the method will allow us to enhance significantly the application of computational fluid dynamics to complex river bed morphology for large eddy simulation roughness parameterization sediment entrainment and deposition studies and understanding interactions between flow and vegetation in relation to conveyance studies copyright 2002 john wiley sons ltd

topographically controlled fronts in the ocean and their biological influence headlands islands and reefs generate complex three dimensional secondary flows that result in physical and biological fronts mixing and diffusion processes near these reefs and headlands are quite different from these processes in the open sea and classical advection diffusion models that were developed for the open sea are not valid near shore topographically generated fronts affect the distribution of sediments and they aggregate waterborne eggs larvae and plankton this aggregation influences the distribution and density of benthic assemblages and of pelagic secondary and tertiary predators

equivalent roughness of alluvial bed the eguivalent roughness of a movable bed is considered relationships are given which can be used to determine the equivalent bed roughness from the bed material size and the bed form dimensions in the case of a plane bed the equivalent roughness is related to the d 9 0 of the bed material based on flume and field data the equivalent roughness may vary from 1 10 d 9 0 similar values were reported by other research workers in the case of bed forms the equivalent roughness is related to the average height and length of the bed forms both flume and field data were used to determine a functional relationship the proposed relationship yields values which are considerably smaller than other existing relationships

bed microtopography and entrainment thresholds in gravel bed rivers although increasingly acknowledged as an important factor in bedload transport few data on the effects of bed conditions on entrainment in gravel bed rivers exist this reflects to some degree the substantial difficulties involved in obtaining accurate threshold measurements in such channels using a sensor which allows initial movement of labeled particles to be detected during flood flow the entrainment thresholds of clasts incorporated into the bed microtopography have been compared with those in exposed open plane bed positions the threshold velocity of clasts from cluster bedforms the most prevalent type of bed microtopography is found to be above that of more exposed particles of like size and shape measurements indicate that the initial movement of 70 of particles in gravel bed rivers will be directly influenced by the presence of neighboring grains bed microtopography plays an important role in delaying incipient motion the influence of microtopography may be invoked to account for phenomena such as discontinuous particle movement and variations in the composition of bedload during discharge events

distribution of calanoid copepods across the mesoscale frontal zone of tropical subtropical convergence off mexico abstract the distribution of calanoid copepods was investigated in the convergence zone between the california current water and transitional tropical water in june 2010 to understand how this zooplankton community responds to mesoscale environmental structure hydrographic conditions and zooplankton profiles were measured along two transects which crossed a mesoscale front and adjacent structures visible in sea surface temperature and ocean color satellite images the hydrographic structure showed strong temperature 2 c and oxygen 50 mol kg 1 gradients in the frontal zone 90 km associated with a sinking of the thermocline and oxycline from 30 m depth in the transitional tropical water to 90 m depth in the california current water the front was marked by a salinity minimum

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

computational fluid dynamics modelling of boundary roughness in gravel bed rivers an investigation of the effects of random variability in bed elevation results from a series of numerical simulations of two dimensional open channel flow conducted using the computational fluid dynamics cfd code fluent are compared with data quantifying the mean and turbulent characteristics of open channel flow over two contrasting gravel beds boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra grid scale roughness elements e g particle clusters and small bedforms results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed typically at y h 0 1 0 3 this is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed use of the random elevation model to represent supra grid scale roughness also allows a reduction in the height of the near bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness despite these benefits the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution increasing the horizontal mesh resolution results in an increase in the near bed velocity gradient and turbulent kinetic energy effectively roughening the bed varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles but results in substantial changes to the shape of the turbulent kinetic energy profile these findings have significant implications for the application of cfd within natural gravel bed channels particularly with regard to issues of topographic data collection roughness parameterization and the derivation of mesh independent solutions copyright 2001 john wiley sons ltd

subsurface recirculation and larval retention in the lee of a small headland a variation on the upwelling shadow theme 1 the interaction of alongshore coastal currents with large headlands has been shown to increase the retention of planktonic organisms through the formation of headland eddies or upwelling shadows in their lee this study investigates the circulation within bodega bay in the lee of a small headland in an upwelling region and the potential for retention of plankton during the upwelling season of 2004 time series of temperature and velocity were recorded throughout bodega bay conductivity temperature depth ctd surveys were conducted and surface drifters were released during upwelling downwelling and relaxation conditions postlarval settlement was monitored daily over two periods coinciding with ctd surveys under strong upwelling favorable conditions wind driven surface currents were equatorward both offshore and throughout the bay however there was significant current shear along the eastern shore of the bay where cold bottom waters move poleward counter to the direction of the wind driven surface flow during downwelling and relaxation conditions flow was poleward throughout the water column along the eastern shore of the bay postlarvae settled during all wind conditions but greatest settlement was observed at the onset of upwelling favorable conditions while no typical upwelling shadow is evident in the lee of the bodega headland subsurface recirculation driven by the alongshore flow past bodega head may facilitate the retention of plankton in the bay previous studies have generally focused on large headlands however it is likely that other small embayments in the lee of small headlands may also provide retention opportunities for planktonic organisms in upwelling regions

flow resistance estimation in mountain rivers examination of the flow resistance of high gradient gravel and boulder bed rivers using data collected in british mountain rivers with slopes of 0 4 4 shows that there are differences in resistance variation between mountain and lowland rivers and that between site variations do not necessarily reflect at a site variations comparison of data with the familiar resistance equation relating the dracy weisbach friction factor to the logarithm of relative submergence shows that the equation tends to overestimate the resistance in uniform flow the equation also tends to underestimate the rate of change of resistance at a site as discharge varies with high gradients the influences of nonuniform channel profile sediment size distribution channel slope and sediment transport are reviewed but the data do not allow any quantification of these effects instead an empirical approach based on the available data is presented allowing the friction factor to be calculated from the relative submergence with an error of up to n25 to n35 a summary of the field data is included

tracer transport in the greenland ice sheet three dimensional isotopic stratigraphy abstract polar ice cores can provide both a record of climate history and a sharp test of the performance of numerical ice dynamics models the stratigraphic structure of an ice sheet is an expression of its full depositional and dynamic history and thus presents a greater challenge to computer models than merely matching the contemporary ice thickness and areal extent we describe a coupled model of ice and tracer dynamics that is realized by adding a semi lagrangian tracer transport scheme to a conventional thermomechanical ice dynamics model model skill is demonstrated by using ice core data from the grip site near summit greenland to successfully predict the isotopic stratigraphy of ice cores at other deep drilling sites the success of this effort indicates that when compensated for the effects of ice flow and elevation all the deep cores relate a coherent glacial history over the past 120 000 years according to the simulation results the oldest greenland ice lies beneath the grip gisp2 and northgrip sites although comparably old ice may also be found in north greenland and east central greenland

waxing and waning of the perennial north polar h2o ice cap of mars over obliquity cycles abstract the perennial north polar h2o ice cap of mars is investigated with the dynamic thermodynamic ice sheet model sicopolis computational results for flow velocities ice temperatures and surface accumulation ablation rates are presented for the steady state with present conditions as well as for transient scenarios over idealized obliquity cycles with periods of 1 3 myr and 125 kyr the transient simulations lead to a stop and go type dynamics with a cold and almost stagnant modern ice cap and a warmer flowing ice cap for past large obliquity periods the most likely scenario for the present ice cap comprises an ice volume of 1 2 million km3 a maximum thickness of 3 km the absence of pronounced local isostasy h2o ice accumulation rates of the order of tenth mm water equivalent w e per earth year yr flow velocities of the order of mm yr and an ice base far below pressure melting

the importance of bed sediment sand content for the structure of a static armor layer in a gravel bed river the surface structure of static armor layers generated from water worked gravel bed channels was investigated with primary focus on the influence of sand content and flow rate flume experiments were conducted in which four sediment mixtures with sand contents between 1 and 38 were armored under one of three different flow rates first and second order statistical analyses were applied to digital elevation models of unarmored armored and clustered bed surface areas to identify changes in surface structure results were combined with data from previous research to create an extended data set of armored bed surfaces water worked unarmored bed surfaces established under a dynamic equilibrium flow rate impacted the topographic variability and structure of the armored beds surface complexity decreased with armor formation as surface grains preferentially aligned with the flow direction the bed surface became smoother and where sediment mixture sand content was constant there was greater smoothing of the surface during higher armoring flows as grains rearranged more easily as bulk sand content increased statistical analyses of the expanded data set showed that beds with very little sand content developed static armor layers that remained rough and had greater topographic variability than armor layers from sediments with higher sand contents the bulk sediment sand content exerted a stronger influence over the change in surface roughness and structure upon armoring than that of the flow rate during armor formation when combined with the knowledge of the local flow regime the sand content may aid in predictions related to armored bed surface structure

sea surface temperature gradients across blue whale and sea turtle foraging trajectories off the baja california peninsula mexico sea surface temperature sst fronts are integral to pelagic ecology in the north pacific ocean so it is necessary to understand their character and distribution and the way these features influence the behavior of endangered and highly migratory species here telemetry data from sixteen satellite tagged blue whales balaenoptera musculus and sea turtles caretta caretta chelonia mydas and lepidochelys olivacea are employed to characterize biologically relevant sst fronts off baja california sur high residence times are used to identify presumed foraging areas and sst gradients are calculated across advanced very high resolution radiometer avhrr images of these regions the resulting values are compared to classic definitions of sst fronts in the oceanographic literature we find subtle changes in surface temperature between 0 01 and 0 101c km across the foraging trajectories near the lowest end of the oceanographic scale between 0 03 and 0 31c km suggesting that edge detection algorithms using gradient thresholds 40 101c km may overlook pelagic habitats in tropical waters we use this information to sensitize our edge detection algorithm and to identify persistent concentrations of subtle sst fronts in the northeast pacific ocean between 2002 and 2004 the lower gradient threshold increases the number of fronts detected revealing more potential habitats in different places than we find with a higher gradient threshold this is the expected result but it confirms that pelagic habitat can be overlooked and that the temperature gradient parameter is an important one r 2006 elsevier ltd all rights reserved

isotopic balance of the greenland ice sheet modelled concentrations of water isotopes from 30 000 bp to present abstract polar ice cores contain a complex record of climate and ice flow that can be used to decode climate history interpretation of ice core records requires the flow signature to be either simple or removable when the signature is complex computer modelling can become a powerful aid to interpretation by combining an ice dynamics model with tracer deposition and transport models the stratification of individual tracers can be calculated in this way the time evolving three dimensional chemical stratigraphy can be simulated by linking an ice tracer dynamics model with models of atmosphere and ocean tracer dynamics coupled models of the global geochemical cycling of water isotopes can be developed such models hold promise for an integrated interpretation of the marine and ice core climate records as a first step along this path we apply tracer transport modelling to simulate the temporal and spatial variations of water isotopes in the greenland ice sheet over the past 30 000 years and calculate the influence of greenland ice volume and isotopic chemistry on the isotopic content of the ocean we find that the greenland ice sheet maintained its maximum ice volume from 18 000 to 15 000 bp but was isotopically lightest at 11 000 bp the sea level equivalent of ice volume changes occurring over the past 30 000 years did not exceed 2 1 m and the volume averaged influence on ocean chemistry did not exceed 0 025 for 18 o and 0 20 for d and at the last glacial maximum taken as 21 000 bp these values were respectively 1 9 m for ice volume 0 022 for 18 o and 0 17 for d

the transition zone chlorophyll front a dynamic global feature defining migration and forage habitat for marine resources abstract pelagic ecosystem dynamics on all temporal scales may be driven by the dynamics of very specialized oceanic habitats one such habitat is the basin wide chlorophyll front located at the boundary between the low chlorophyll subtropical gyres and the high chlorophyll subarctic gyres global satellite maps of surface chlorophyll clearly show this feature in all oceans in the north pacific the front is over 8000 km long and seasonally migrates north and south about 1000 km in the winter this front is located at about 30 35 n latitude and in the summer at about 40 45 n it is a zone of surface convergence where cool vertically mixed high chlorophyll surface water on the north side sinks beneath warm stratified low chlorophyll water on the south side satellite telemetry data on movements of loggerhead turtles and detailed fisheries data for albacore tuna show that both apex predators travel along this front as they migrate across the north pacific the front is easily monitored with ocean color satellite remote sensing a change in the position of the tzcf between 1997 and 1998 appears to have altered the spatial distribution of loggerhead turtles the position and dynamics of the front varied substantially between the 1998 el nino and the 1999 la nina for example from may to july 1999 the transition zone chlorophyll front tzcf remained between about 35 n and 40 n latitude showing very little meandering whereas in 1998 during the same period the tzcf exhibited considerable meandering and greater monthly latitudinal movement catch rates for albacore were considerably higher in 1998 than in 1999 and we hypothesize that a meandering tzcf creates regions of convergence which enhances the foraging habitat for apex predators along the front

the kuroshio extension bifurcation region a pelagic hotspot for juvenile loggerhead sea turtles satellite telemetry of 43 juvenile loggerhead sea turtles caretta caretta in the western north pacific together with satellite remotely sensed oceanographic data identified the kuroshio extension bifurcation region kebr as a forage hotspot for these turtles in the kebr juvenile loggerheads resided in kuroshio extension current kec meanders and the associated anti cyclonic warm core and cyclonic cold core eddies during the fall winter and spring when the kec water contains high surface chlorophyll turtles often remained at a specific feature for several months however in the summer when the kec waters become vertically stratified and surface chlorophyll levels are low the turtles moved north up to 600 km from the main axis of kec to the transition zone chlorophyll front tzcf in some instances the loggerheads swam against geostrophic currents and seasonally all turtles moved north and south across the strong zonal flow loggerhead turtles traveling westward in the kebr had their directed westward movement reduced 50 by the opposing current while those traveling eastward exhibited an increase in directed zonal movement it appears therefore that these relatively weak swimming juvenile loggerheads are not passive drifters in a major ocean current but are able to move east west north and south through this very energetic and complex habitat these results indicate that oceanic regions specifically the kebr represent an important juvenile forage habitat for this threatened species interannual and decadal changes in productivity of the kebr may be important to the species s population dynamics further conservation efforts should focus on identifying and reducing threats to the survival of loggerhead turtles in the kebr r 2006 elsevier ltd all rights reserved

antarctic subglacial conditions inferred from a hybrid ice sheet ice stream model abstract subglacial conditions of large polar ice sheets remain poorly understood despite recent advances in satellite observation major uncertainties related to basal conditions such as the temperature field are due to an insufficient knowledge of geothermal heat flow here a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots as well as the distribution of subglacial lakes in this way geothermal heat flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available results of the sensitivity experiments show that 55 of the grounded part of the antarctic ice sheet is at pressure melting point calculated basal melt rates are approximately 65 gt year 1 which is 3 of the total surface accumulation although these sensitivity experiments exhibit small variations in basal melt rates the impact on the ice age of basal layers is quite important in areas that are characterized by relatively low melt rates this may lead to standard deviations up to 50 in a model run over a period of 10 6 years subglacial water flow is concentrated underneath the large outlet glaciers of antarctica the larger subglacial lakes are perched at the head of these subglacial water systems an exception however is the lake system of the recovery ice stream catchment that receives a substantial amount of subglacial melt water from the upstream catchment

altitudinal variation of the stable isotopes of snow in regions of high relief a major discontinuity in the variation of 18 o d with altitude in high mountains was first seen in data from mount logan yukon territory canada holdsworth and others 1991 the profile of vs altitude revealed three well defined regions 1 a lower monotonic fractionation sequence below 3 km 2 a middle layer typically 1 2 km thick within which values are nearly constant or stepped with altitude and 3 part of another fractionation sequence in the quasi geostrophic flow region above 5 3 km the middle region was inferred to be a mixed layer combining moisture from regions 1 and 3 this type of structure is now seen to occur on other high altitude mountains including cerro aconcagua argentina where observations reach almost 7 km the new observations confirm the general occurrence of a multi layered atmosphere during precipitation at high altitude glacier sites this structure is linked to synoptic scale polar cyclones where the middle layer is identified as being the signature of the warm front zone these results have implications for the common practice of using a specific spatially derived isotopic thermometer in the time domain for the paleoclimatic interpretation of high altitude ice core records

large scale ice sheet modelling as a means of dating deep ice cores in greenland the three dimensional ice sheet model sicopolis is used to simulate the dynamic thermodynamic behaviour of the entire greenland ice sheet from 250 000 a bp until today external forcing consists of a surface temperature history constructed from 18 o data of the grip core a snowfall history coupled linearly to that of the surface temperature a piecewise linear sea level scenario and a constant geothermal heat flux the simulated greenland ice sheet is investigated in the vicinity of summit the position where the maximum elevation is taken and where the two drill sites grip and gisp 2 are situated 28 km apart from each other in the region the aggreement between modelled and observed topography and ice temperature tuns out to be very good computed age depth profiles for grip and gisp2 are presented which can be used to complete the dating of these cores in the deeper regions where annual layer counting is not possible however artififical diffusion influences the computed ages in a near basel boundary lar of approximately 15 of theice thickness so that the age at the bottom of the cores cannot be predicted yet

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

tracer transport in an isochronal ice sheet model the full history of ice sheet and climate interactions is recorded in the vertical profiles of geochemical tracers in polar ice sheets numerical simulations of these archives promise great advances both in the interpretation of these reconstructions and the validation of the models themselves however fundamental mathematical shortcomings of existing models subject tracers to spurious diffusion thwarting straightforward solutions here i propose a new vertical discretization for ice sheet models that eliminates numerical diffusion entirely vertical motion through the model mesh is avoided by mimicking the real world flow of ice as a thinning of underlying layers a new layer is added to the surface at equidistant time intervals isochronally thus identifying each layer uniquely by its time of deposition and age this new approach is implemented for a two dimensional section through the summit of the greenland ice sheet the ability to directly compare simulations of vertical ice cores with reconstructed data is used to find optimal model parameters from a large ensemble of simulations it is shown that because this tuning method uses information from all times included in the ice core it constrains ice sheet sensitivity more robustly than a realistic reproduction of the modern ice sheet surface

melting and refreezing beneath roi baudouin ice shelf east antarctica inferred from radar gps and ice core data indicate that this reflector pattern is best fit by including basal melting of at least 15 cm a 1 this rate of melting is low compared with rates observed on larger ice shelves in both west and east antarctica ice cores extracted from a rift system close to the ice rise promontory show several meters of marine ice accreted beneath the shelf these observations of low rates of basal melting and limited distribution of accreted marine ice suggest that either antarctic surface water may reach the ice shelf base or that circulation beneath the shelf is likely dominated by the production of high salinity shelf water rather than the incursion of circumpolar deep water implying a weak sub shelf circulation system here many of the ice shelves located along the coast of dronning maud land are like roi baudouin ice shelf characterized by frequent ice rises and promontories therefore it is highly likely that these are also of shallow bathymetry and are subject to similarly weak side shelf basal melting and refreezing

estimation of effective roughness for water worked gravel surfaces abstractthere is no accepted standard equation for predicting flow resistance in gravel rivers this is mainly because of the lack of an effective approach for describing the roughness of the gravel surface this paper aims at describing the roughness of gravel surfaces based on characteristics of the elevation field rather than conventionally used grain size distribution a new parameter the coarseness parameter in addition to the steepness parameter and standard deviation of the elevation field is proposed for characterizing the roughness of gravel surfaces two groups of rough surfaces a and b were generated using unconditional gaussian simulation to study the influence of the coarseness and steepness parameters on effective roughness the effective roughness of groups a and b was calculated by a two equation turbulence model and a quantitative relationship between the effective roughness and proposed parameters was derived the resulting relationship derived from groups a and b was validated using

distribution of sei whales balaenoptera borealis in the subarctic subtropical transition area of the western north pacific in relation to oceanic fronts the subarctic subtropical transition area of the western north pacific is an important summer feeding grounds of sei whales the oceanographic structure and circulation of this area are largely determined by strong oceanic fronts and associated geostrophic currents namely the polar front pf subarctic front saf and kuroshio extension front kef the relationship between the distribution of sei whales and oceanographic fronts was investigated using a generalized additive model gam and the cetacean sighting survey data and oceanographic observations in july from 2000 to 2007 were used in the analysis the number of individual sei whales was used as the response variable while the distances from the pf saf and kef to the whales were used as explanatory variables along with the longitude values sei whales were concentrated north and south of the saf and the areas from 250 to 300 km north and from 100 to 200 km south of the saf were estimated as high density areas of sei whales the entire inter frontal zone between the pf and saf featured an elevated concentration of sei whales and the area south of the pf and along the saf was identified as an important feeding ground of sei whales in july from 2000 to 2007

on characterization of the imbrication of armored gravel surfaces abstract imbrication of static armor surfaces is investigated based on laser scanned dems collected from both flume experiments and the field two approaches are employed to study the imbrication including inclination index and aspect and slope as method the inclination index is found to be a sensitive and valid method for indicating the existence of imbrication application of this method however requires high resolution surfaces a positive relationship is obtained between the inclination index and effective grain size which can be used to deduce the historical armoring shear stress the as method based on the statistics of local gradients describes the imbrication in a more straightforward way however this method is based on the statistics of orientations of grids rather than individual grains an improved as method is proposed in this study based on orientations of grains with the help of automated grain segmentation method the improved as method makes this method more reasonable and increases the applicable range

effects of macro scale bed roughness geometry on flow resistance 1 flow field over macro roughness elements as for example in flow conditions such as of steep mountain streams is investigated by means of a theoretical model and a laboratory study with the main aim of understanding the influence of concentration and planimetric disposition of grain roughness on flow resistance a theoretical model for the average velocity distribution profile over a bed with macro roughness elements is developed herein the model is based on a two layer approach which takes into account the displacement of the main flow due to the drag around the roughness the laboratory study is composed of an extensive set of flume experiments in the case of steep slopes and large relative roughness conditions for a given flow discharge and bed slope various experiments have been carried out with different pattern and spacing of the roughness elements constituted by river pebbles application of the theoretical model to the experimental results enables the determination of how flow resistance is affected by the macro roughness in particular it appears that when the concentration of the roughness is within an optimal range flow resistance is maximum

three dimensional hydrodynamics of meandering compound channels the velocity field in meandering compound channels with overbank flow is highly three dimensional to date its features have been investigated experimentally and little research has been undertaken to investigate the feasibility of reproducing these velocity fields using computer models if computer modeling were to prove successful in this context it could become a useful prediction technique and research tool to enhance our understanding of natural river dynamics in particular an accurate computer prediction of the velocity field could benefit studies of channel morphology and pollution transport in this paper a meandering channel experiment from the u k flood channel facility is simulated using computational fluid dynamics and the predicted velocities compared with the experimental data particular attention is paid to the reproduction of the secondary velocities and the helical motion of the water flowing within the main channel sensitivity tests of mesh design discretization scheme and roughness height are reported together with the turbulence characteristics of the flow

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

spectral behavior of gravel dunes abstract spectral behavior of gravel dunes formed under different flow discharges is analyzed with an attempt to verify the 3 spectral law that has been confirmed extensively for sand dunes a schematic spectrum of gravel dunes is proposed based on the spectral analysis as well as results from the literature the results of spectral analysis show a significant deviation from the 3 spectral law for gravel dunes and the magnitude of deviation correlates with flow discharge possible explanations for the deviation from the 3 spectral law being associated with kinetic and geometrical characteristics have been explored to investigate the kinetic characteristics of gravel dunes a wavelet based method that calculates the celerity of dunes based on a pair of elevational time series is quantitatively tested our results suggest that 1 the kinetic explanation based on the relationship between dune celerity and dune length cannot fully explain the spectral behavior of gravel dunes 2 the geometrical explanation based on the self similarity hypothesis is confirmed by the relationship between dune length and dune height and 3 the development of gravel sheets accounts for the differences in kinetic and geometrical characteristics between gravel dunes and sand dunes

application of structure from motion photogrammetry in laboratory flumes abstract structure from motion sfm photogrammetry has become widely used for topographic data collection in field and laboratory studies however the relative performance of sfm against other methods of topographic measurement in a laboratory flume environment has not been systematically evaluated and there is a general lack of guidelines for sfm application in flume settings as the use of sfm in laboratory flume settings becomes more widespread it is increasingly critical to develop an understanding of how to acquire and process sfm data for a given flume size and sediment characteristics in this study we 1 compare the resolution and accuracy of sfm topographic measurements to terrestrial laser scanning tls measurements in laboratory flumes of varying physical dimensions containing sediments of varying grain sizes 2 explore the effects of different image acquisition protocols and data processing methods on the resolution and accuracy of topographic data derived from sfm techniques and 3 provide general guidance for image acquisition and processing for sfm applications in laboratory flumes to investigate the effects of flume size sediment size and photo overlap on the density and accuracy of sfm data we collected topographic data using both tls and sfm in five flumes with widths ranging from 0 22 to 6 71 m lengths ranging from 9 14 to 30 48 m and median sediment sizes ranging from 0 2 to 31 mm acquisition time image overlap point density elevation data and computed roughness parameters were compared to evaluate the performance of sfm against tls we also collected images of a pan of gravel where we varied the distance and angle between the camera and sediment in order to explore how photo acquisition affects the ability to capture grain scale microtopographic features in sfm derived point clouds a variety of image combinations and sfm software package settings were also investigated to determine optimal processing techniques results from this study suggest that sfm provides topographic data of similar accuracy to tls at higher resolution and lower cost we found that about 100pixels per grain are required to resolve grain scale topography we suggest protocols for image acquisition and sfm software settings to achieve best results when using sfm in laboratory settings in general convergent imagery taken from a higher angle with at least several overlapping images for each desired point in the flume will result in an acceptable point cloud

tracer transport in the greenland ice sheet constraints on ice cores and glacial history the climate history and dynamics of the greenland ice sheet are studied using a coupled model of the depositional provenance and transport of glacier ice allowing simultaneous prediction of the detailed isotopic stratigraphy of ice cores at all the major greenland sites adopting a novel method for reconstructing the age depth relationship we greatly improve the accuracy of semi lagrangian tracer tracking schemes and can readily incorporate an age dependent ice rheology the larger aim of our study is to impose new constraints on the glacial history of the greenland ice sheet leading sources of uncertainty in the climate and dynamic history are encapsulated in a small number of parameters the temperature and elevation isotopic sensitivities the glacial interglacial precipitation contrast and the effective viscosity of ice in the flow law comparing predicted and observed ice layering at ice core sites we establish plausible ranges for the key model parameters identify climate and dynamic histories that are mutually consistent and recover the past depositional elevation of ice cores to ease interpretation of their climatic records with the coupled three dimensional model of ice dynamics and provenance transport we propose a method to place all the ice core records on a common time scale and use discrepancies to adjust the reconstructed climate history analysis of simulated grip ice layering and borehole temperature profiles confirms that the grip record is sensitive to the dynamic as well as to the climatic history but not enough to strongly limit speculation on the state of the greenland ice sheet during the eemian in contrast our study indicates that the dye 3 and camp century ice cores are extremely sensitive to ice dynamics and greatly constrain eemian ice sheet reconstructions we suggest that the maximum eemian sea level contribution of the ice sheet was in the range of 3 5 4 5 m

analysis of small scale gravel bed topography during armoring 1 in evaluating the resistance of sediment particles to entrainment by the action of the flow in a river the grain geometry is usually characterized using representative sizes this approach has been dictated initially by lack of physical insight but more recently by the lack of analytical tools able to describe the 3 d nature of surface grain organization on water worked sediment beds laboratory experiments are presented where mixed grain size beds were mobilized under a range of hydraulic and sediment input conditions detailed bed topography was measured at various stages statistical tools have been adopted which describe the degree of surface organization on water worked sediment bed surfaces the degree of particle organization and the bed stability can be evaluated in relative terms using the properties of the probability density distribution of the bed surface elevations and in absolute terms using a properly defined 2 d structure function the methods described can be applied directly to natural water worked surfaces given the availability of appropriate bed surface elevation data sets

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

a comparison of eulerian and lagrangian methods for dating in numerical ice sheet models accurate dating in ice sheets is required for a correct interpretation of palaeoclimatic records and for incorporation of material characteristics in the flow law which depend on ice age in this paper we make a comparison between a lagrangian and eulerian approach to the ice advection problem in numerical ice sheet models this comparison is first performed for a schematic two dimensional ice sheet of nye vialov typewithaprescribed stationary velocity field severalcasesareexaminedwhichincorp orate basal melting basal sliding and an undulating bed a further comparison is made with an analytical solution for the ice divide both methods are also applied in a thermo mechanical model of the antarctic ice sheet for steady state present day conditions our main conclusion is that for similar discretization parameters the lagrangian method produces less error than an eulerian approach using a second order upwinding finite difference scheme thoughthe difference is small 51 forthe largest partof the model domain however problems withthe lagrangian approach areintroducedby the disper sion of tracers necessitating the use of interpolationproceduresthat are a main source of additional error it is also shown that a cubic spline approximation of lagrangian trajec tories improves accuracy but such a method is computationally hardly applicable in large scale ice sheet models

the application of computational fluid dynamics to natural river channels three dimensional versus two dimensional approaches this paper addresses the recent trend in fluvial geomorphology towards using computational fluid dynamics to explore the adjustment between flow processes sediment transport and river channel morphology it aims to evaluate the extent to which three dimensional 3d models improve predictive ability and prediction utility compared to two dimensional 2d applications this is achieved through comparing the predictions of both 3d and 2d models with high quality field data identical boundary conditions obtained from a confluence within a gravel bed river system with high relative roughness are defined for each model evaluation of the 3d model suggests that there is a fundamental limitation upon model predictive ability due to problems of specifying topographic complexity however comparison with the 2d model shows that the 3d model has a higher predictive ability particularly if the 2d model is not corrected for the effects on flow structure of secondary circulation further the 3d model provides more reliable estimates of bed shear stress and other more useful information such as the three dimensional flow field important for mixing processes this suggests that there is significant merit in the move towards 3d models but that research is required to incorporate methods developed in other fields for dealing with boundary condition uncertainties

evaluation of dem size and grid spacing for fluvial patch scale roughness parameterisation abstract surface roughness is a term used in fluvial research without a unanimous definition clarification of the term and improved parameterisation is needed in future research improvements to the collection of topographic data using photogrammetry have provided accurate digital elevation models dems of field and laboratory gravel bed patches of varied sediment size and surface structure in this study a moving window process is used for analysing spatial variability within dems using this information and in unison the effect of dem size and grid spacing are considered on an extensive range of roughness parameters in order to provide insights for obtaining grain roughness statistics it is shown that dem size influences the calculated roughness statistics and the observation of plateaus in statistics for dem window sizes above 16 d50a in both directions where d50a is the median grain size of the bed surface material suggests this as a minimum dem size for grain scale roughness analysis further the dem grid spacing should be 1 mm or below in order to adequately capture grain roughness as coarser resolutions failed to detect particle imbrication finally variability in roughness parameters was evident due to natural spatial variation in gravel bed microtopography suggesting using a single roughness parameter is not appropriate to holistically describe the roughness of a gravel patch as the importance of topographic information continues to grow in geomorphology the results of this study will assist researchers in the collection and subsequent analysis of dems