effective medium theories for fluid saturated materials with aligned cracks there is general agreement between different theories giving expressions for the overall properties of materials with dry aligned cracks if the number density of cracks is small there is also very fair agreement for fluid filled isolated cracks however there are considerable differences between two separate theories for fluid filled cracks with equant porosity comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples however although the crack number density in the laboratory sample is such that first order theory is unlikely to apply expressions correct to second order in the number density provide a worse fit it also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective medium theories which are valid only when this ratio is large the data show dispersion effects for dry cracks and scattering neither of which will occur at sufficiently long wavelengths data from the water saturated samples indicate that the effect of equant porosity is significant although the two theories differ strongly as to just how significant once again and in spite of the reservations mentioned above a reasonable fit between theory and observation can be shown

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

numerical modeling of flow processes over gravelly surfaces using structured grids and a numerical porosity treatment this article describes the development and validation of a method for representing the complex surface topography of gravel bed rivers in high resolution three dimensional computational fluid dynamic models this is based on a regular structured grid and the application of a porosity modification to the mass conservation equation in which fully blocked cells are assigned a porosity of zero fully unblocked cells are assigned a porosity of one and partly blocked cells are assigned a porosity of between 0 and 1 according to the percentage of the cell volume that is blocked the model retains an equilibrium wall function and an rng type two equation turbulence model the model is combined with a 0 002 m resolution digital elevation model of a flume based water worked gravel bed surface acquired using two media digital photogrammetry and with surface elevations that are precise to 0 001 m the model is validated by comparison with velocity data measured using a three component acoustic doppler velocimeter adv model validation demonstrates a significantly improved level of agreement than in previous studies notably in relation to shear at the bed although the resolution of model predictions was significantly higher than the adv measurements making model assessment in the presence of strong shear especially difficult a series of simulations to assess model sensitivity to bed topographic and roughness representation were undertaken these demonstrated inherent limitations in the prediction of 3 d flow fields in gravel bed rivers without high resolution topographic representation they also showed that model predictions of downstream flux were more sensitive to topographic smoothing that to changes in the roughness parameterization reflecting the importance of both mass conservation i e blockage and momentum conservation effects at the grain and bed form scale model predictions allowed visualization of the structure of form flow interactions at high resolution in particular the most protruding bed particles exerted a critical control on the turbulent kinetic energy maxima typically observed at about 20 of the flow depth above the bed

modelling granular sediment transport over water worked gravels this paper reports a numerical model that calculates the transport paths of individual granular particles over a natural water worked gravel surface surface topography is measured using digital photogrammetry and incorporated into a computational fluid dynamics cfd scheme by the application of a porosity algorithm the predicted hydraulics determine the forces acting on each particle enabling the particle trajectory to be solved by langrangian transport equations the methodology demonstrates the importance of including complex topography in the numerical scheme as topographically induced hydraulic acceleration and deceleration affect both the transport mechanism and subsequent trajectory of the particle hydraulically induced hops where the particle is entrained into a localized shear layer around a bed particle are demonstrated to be important in transporting granular sediment across the bed and in re starting sediment transport if the particle has fallen into a hydraulically sheltered region copyright 2005 john wiley sons ltd

monitoring and modelling particle and reach scale morphological change in gravel bed rivers applications and challenges this paper focuses on developments in topographic data acquisition including airborne remote sensing digital photogrammetry differential gps and laser profiling e g lidar technologies that allow rapid acquisition of high resolution and high precision topographic data sets over a range of spatial scales these developments have offered new opportunities for investigating spatial and temporal patterns of morphological change in gravel bed rivers and have contributed to revitalization in three key areas 1 morphometric estimates of sediment transport and sediment budgeting 2 boundary conditions for numerical models including computational fluid dynamics and cellular modelling 3 three dimensional characterisation of morphology that is independent of flow stage the potential is clear but there remain a number of significant challenges including quality control and the effects of error on specific applications and morphologies this paper presents results from two investigations representing field and laboratory analyses of gravel bed river morphology at different spatial scales and for different applications case study 1 is concerned with monitoring and modelling morphological change in a large braided gravel bed river using ground based gps survey and digital photogrammetry derived from airborne imagery case study 2 is an investigation of the mechanisms for infiltration of fine sediment into gravel bed rivers which applies close range laser altimetry in flume experiments to derive very high resolution digital elevation models dems that are used to quantify and analyse changes in bed texture these case studies highlight the strengths and weaknesses of specific technologies and approaches to analysis of channel and floodplain morphology and change and suggest key areas that remain to be fully resolved in particular the critical need to define a specific threshold level of detection associated with each acquisition method and for different fluvial settings e g bar surface sub aqueous zone is emphasised

geomorphological evidence of the influence of pre volcanic basement structure on emplacement and deformation of volcanic edifices at the cofre de perote pico de orizaba chain and implications for avalanche generation abstract pre volcanic structure of the basement influences volcanism distribution and avalanche generation in volcanic edifices therefore systematic studies of basement structure below volcanic chains are necessary to understand the deformation effects observed in the surface and vice versa based on a compilation of pre existing data interpretation of aerial photographs and satellite images and a collection of structural data we analyzed morphological and structural features of the cofre de perote pico de orizaba cp po volcanic chain and its basement we have identified three sets of regional lineaments that are related to basement trends 1 nw 55 se fractures are parallel to anticline folds observed in cretaceous rocks that originated during laramide shortening these folds present an abrupt morphology observed only in the eastern flank but that is likely to continue below the volcanic chain 2 ne 55 sw fractures are parallel to normal faults at the basement we infer that these basement faults confine the cp po chain within a stepped graben with a total normal displacement of about 400 m these faults have been active through time since they have affected volcanic deposits and induced the emplacement of monogenetic vents notably lineaments of monogenetic vents concentrate where the basement is relatively shallow 3 another set of faults oriented n s has been observed affecting the scarce basement outcrops at the western flank of the chain covered by lacustrine deposits lineaments measured in the volcanic edifice of pico de orizaba correlate with the regional trends in particular the ne 55 sw alignment of monogenetic vents and fractures at pico de orizaba suggest that the same dike trend exists within the volcanic edifice a normal fault with similar orientation was documented at the ne continuation of an alignment crossing the volcanic edifice along the jamapa canyon in the absence of magmatic activity related to collapses the displacement of ne 55 sw faults represents a potential triggering mechanism for generating avalanches at pico de orizaba volcano instability is enhanced by the presence of n s trending fractures crossing the entire volcanic edifice and e w fractures affecting only the present day cone we conclude that mechanical instability of the volcanic chain is influenced by the basement structure heterogeneity but further detailed studies are necessary at individual volcanoes to evaluate their effects on volcano deformation

topographic forcing of flow partition and flow structures at river bifurcations this paper presents the predicted flow dynamics from the application of a reynolds averaged navierstokes model to a series of bifurcation geometries with morphologies measured during previous flume experiments the topography of the bifurcations consists of either plane or bedform dominated beds which may or may not possess discordance between the two bifurcation distributaries numerical predictions are compared with experimental results to assess the ability of the numerical model to reproduce the division of flow into the bifurcation distributaries the hydrodynamic model predicts 1 diverting fluxes in the upstream channel which direct water into the distributaries 2 super elevation of the free surface induced at the bifurcation edge by pressure differences and 3 counter rotating secondary circulation cells which develop upstream of the apex of the bifurcation and move into the downstream channels with water converging at the surface and diverging at the bed when bedforms are not present weak transversal fluxes characterize the upstream channel for almost its entire length associated with clearly distinguishable secondary circulation cells although these may be under estimated by the turbulence model used in the solution in the bedform dominated case the same hydrodynamic conditions were not observed with the bifurcation influence restricted and depth scale secondary circulation cells not forming the results also demonstrate the dominant effect bed discordance has upon flow division between the two distributaries finally results indicate that in bedform dominated rivers consequently we suggest that sand bed river bifurcations are more likely to have an influence that extends much further upstream and have a greater impact upon water distribution this may contribute to observed morphological differences between sand bedded and gravel bedded braided river networks copyright c 2012 john wiley sons ltd

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

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

seismic velocity anisotropy of phyllosilicate rich rocks characteristics inferred from experimental and crack model studies of biotite rich schist summary seismic velocity anisotropy of biotite schist 30 per cent mode biotite was measured under confining pressures up to 150 mpa the rock shows weak orthotropy which was altered from transverse isotropy ti generated by biotite preferred orientation the orthotropy was caused by microfolding in the rock the velocity increase under confining pressure indicates that most crack planes are aligned parallel to the cleavage planes silicate sheet of the oriented biotite minerals the anisotropy of the rock is basically ti due to both the aligned biotite minerals and cracks which have a common symmetry axis we found that other sheet silicate rich rocks have a similar anisotropy with the biotite schist in which the ti type anisotropy is characterized by the slow p and s wave velocities along the symmetry axis this is caused by the preferred orientation of sheet silicate minerals and the extremely slow p and s wave velocities along the axis perpendicular to the silicate sheet compared to the directions along the silicate sheet when rock contains a large percentage of highly oriented sheet silicates the fast and slow shear waves exchange their polarities at some off symmetry axis directions indicating that the qswave quasi s wave velocity exceeds the sh wave velocity the phase velocity distribution of qs wave shows an asymmetry with respect to the angle from the symmetry axis which is characterized by a bulge in this distribution located near the symmetric axis this is inherent to most sheet silicate minerals when crack density of aligned cracks increases the p wave velocity along the symmetry axis decreases considerably the qs wave phase velocity in the off axis directions also decreases in accordance with the decrease of the p velocity along the symmetry axis the asymmetry of the qs wave phase velocity distribution increases as the p wave velocity decreases along the symmetry axis this relationship can be well understood by means of berryman s extended thomsen approach

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

three dimensional numerical model for flow through natural rivers this paper presents a three dimensional numerical model for simulating flow through natural river reaches the model solves the reynolds averaged navier stokes rans equations closed with the standard ik e turbulence model large scale roughness and multiple islands are directly resolved by employing boundary fitted curvilinear coordinates in conjunction with a multiblock approach small scale bed roughness is accounted for using a two point wall functions approach calculations are carried out for flow through a 4 km stretch of the columbia river downstream of the wanapum dam for which detailed field and laboratory measurements were collected for a range of power plant operating conditions measurements at one operating discharge are employed to calibrate the small scale roughness distribution in the numerical model subsequently the calibrated model is validated by comparing the computed results with laboratory and field measurements for other discharge combinations these comparisons demonstrate the ability of the model to capture most experimental trends with remarkable accuracy

velocity distribution and bed roughness in high gradient streams a model for the velocity field over a poorly sorted bed is derived herein and applied to flow in steep streams with coarse gravel beds in such streams large clasts often are comparable in size to stream depth and act as obstacles to the flow the effect of these obstacles on a flow is estimated by partitioning total stress proportional to the depth slope product into a purely fluid component and a form drag component associated with flow around the obstacles an eddy viscosity closure is used with a length scale dependent on both distance from the boundary and wake dimension resulting velocity profiles agree well with profiles measured by marchand et al 1984 in nine mountain streams in colorado model runs for a range of conditions reveal that the velocity distribution depends primarily on d84z and flow depth h d84z is the length of the vertically oriented axis of clasts at the 84th percentile of the grain size distribution calculated values of mean velocity normalized by shear velocity are well represented by a simple log linear relationship in terms of relative roughness d84 h

sensitivity of interfacial hydraulics to the microtopographic roughness of water lain gravels flow within the interfacial layer of gravel bed rivers is poorly understood but this zone is important because the hydraulics here transport sediment generate flow structures and interact with benthic organisms we hypothesized that different gravel bed microtopographies generate measurable differences in hydraulic characteristics within the interfacial layer this was tested using a high density of spatially and vertically distributed velocity time series measured in the interfacial layers above three surfaces of contrasting microtopography these surfaces had natural water worked textures captured in the field using a casting procedure analysis was repeated for three discharges with reynolds numbers between 165000 and 287000 to evaluate whether discharge affected the impact of microtopography on interfacial flows relative submergence varied over a small range 3 5 to 8 1 characteristic of upland gravel bed rivers between surface differences in the median and variance of several time averaged and turbulent flow parameters were tested using non parametric statistics across all discharges microtopographic differences did not affect spatially averaged median values of streamwise velocity but were associated with significant differences in its spatial variance and did affect spatially averaged median turbulent kinetic energy sweep and ejection events dominated the interfacial region above all surfaces at all flows but there was a microtopographic effect with q2 and q4 events less dominant and structures less persistent above the surface with the widest relief distribution especially at the highest reynolds number flow results are broadly consistent with earlier work although this analysis is unique because of the focus on interfacial hydraulics spatially averaged patch scale metrics and a statistical approach to data analysis an important implication is that observable differences in microtopography do not necessarily produce differences in interfacial hydraulics an important observation is that appropriate roughness parameterizations for gravel bed rivers remain elusive partly because the relative contributions to flow resistance of different aspects of bed microtopography are poorly constrained 2014 the authors earth surface processes and landforms published by john wiley sons ltd

effect of strain rate in the distribution of monogenetic and polygenetic volcanism in the transmexican volcanic belt in the transmexican volcanic belt polygenetic and monogenetic volcanism has taken place concurrently with extensional deformation since the late miocene at a regional scale the deformation is manifested by two groups of faults the dominant group consists of normal faults nearly parallel to the arc in the other group are north northwest trending normal faults that cross the arc and in places form the boundaries of crustal blocks the larger stratovolcanoes of the transmexican volcanic belt are aligned in north south volcanic chains along some of these faults whereas monogenetic volcanoes are usually located along arc parallel normal fault systems because the arc parallel faults are 15 oblique to the subduction plate boundary and assuming stretching perpendicular to the trench the extensional deformation field facilitates activation of both arc parallel and arc transverse structures the former having a higher displacement rate than the latter we observe that in the transmexican volcanic belt polygenetic volcanoes develop along faults having small strain rate and monogenetic volcanoes are emplaced along faults having higher strain rate the agreement with the theoretical model in which monogenetic or polygenetic volcanism depends on the magmatic input rate and the regional stress is true only assuming a linear relation between regional differential stress and local strain rate as in a continuous and homogeneous medium we propose that the local strain rate rather than the regional stress field controls the coexistence of both types of volcanism in the transmexican volcanic belt

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

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

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

a pseudo genetic model of coarse braided river deposits a new method is proposed to produce three dimensional facies models of braided river aquifers based on analog data the algorithm consists of two steps the first step involves building the main geological units the production of the principal inner structures of the aquifer is achieved by stacking multiple point statistics simulations of successive topographies thus mimicking the major successive flooding events responsible for the erosion and deposition of sediments the second step of the algorithm consists of generating fine scale heterogeneity within the main geological units these smaller scale structures are generated by mimicking the trough filling process occurring in braided rivers the imitation of the physical processes relies on the local topography and on a local approximation of the flow this produces realistic cross stratified sediments comparable to what can be observed in outcrops the three main input parameters of the algorithm offer control over the proportions the continuity and the dimensions of the deposits calibration of these parameters does not require invasive field measurements and can rely partly on analog data

investigation of controls on secondary circulation in a simple confluence geometry using a three dimensional numerical model recent research into river channel confluences has identified confluence geometry and particularly bed discordance as a control on confluence flow structures and mixing processes and this has been illustrated using both field measurements in natural confluences and laboratory measurements of simplified confluences generalization of the results obtained from these experiments is limited by the number of confluence geometries that can be examined in a reasonable amount of time this limitation may be overcome by numerical models in which confluence geometry is more readily varied and data acquired more rapidly this paper aims to i validate the application of a three dimensional numerical model to a simple confluence geometry ii simulate the effects of different boundary condition values upon flow structures and iii interpret the implications of these simulations for river channel confluence dynamics the model used in this research solves the three dimensional form of the navier stokes equations and is used to simulate the flow in a parallel confluence of unequal depth channels and to investigate the effect of different combinations of velocity and depth ratio between the two tributaries the results generally agree with empirical evidence that secondary circulation is generated in the absence of streamline curvature but only for specific combinations of depth and velocity ratio this research shows how understanding of the interaction of these controls is enhanced if pressure gradients are considered the velocity ratio is the prime determinant of the cross stream pressure gradient that initiates cross stream velocities however for significant secondary circulation to form cross stream velocities must lead to significant transfer of fluid in the cross stream direction this depends on the vertical extent of the cross stream pressure gradient which is controlled by the depth ratio in this study strong secondary circulation occurred for a depth differential of 25 or more as long as the velocity in the shallower tributary was at least as great as that in the deeper channel this provides an important context for interpretation of previous work and for the design of new experiments in both the field and the laboratory 1998 john wiley sons ltd

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

application of a roughness length representation to parameterize energy loss in 3 d numerical simulations of large rivers 1 recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers however measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach length scales that can be monitored similar constraints exist for computational fluid dynamics cfd studies of large rivers requiring maximization of mesh or grid cell dimensions and implying a reduction in the representation of bedform roughness elements that are of the order of a model grid cell or less even if they are represented in available topographic data these subgrid elements must be parameterized and this paper applies and considers the impact of roughness length treatments that include the effect of bed roughness due to unmeasured topography cfd predictions were found to be sensitive to the roughness length specification model optimization was based on acoustic doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths this proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness length treatments however the general spatial flow patterns are effectively predicted by the model changes in roughness length were shown to have a major impact upon flow routing at the channel scale the results also indicate an absence of secondary flow circulation cells in the reached studied and suggest simpler two dimensional models may have great utility in the investigation of flow within large rivers

on large scale flow structures in a gravel bed river abstract previous studies have suggested the presence of large scale flow structures in gravel bed rivers these structures are pictured as intermittent high speed wedges separated by regions of lower velocity however the characteristics of these structures have not been examined in detail through either visualisation techniques or detailed field measurements this paper confirms the presence of large scale flow structures in gravel bed rivers pictures their sequence and patterns and characterises their mean and individual properties the analysis relies on a new technique for displaying velocity fluctuations in a space time matrix that allows one to see the structures as they pass an array of current meters streamwise and vertical velocities were measured simultaneously with an array of three electromagnetic current meters the sampling frequency was 20 hz five velocity profiles of up to 13 1 min series of measurements and one profile of three 20 min measurements were sampled these data suggest the presence of large wedges of faster fluid joined by regions of slower fluid space time correlation analysis confirmed the presence of vertical coherence of the flow the average angle of the front of the wedges is 36 although individual structures are variable in size and shape a new detection technique using all three velocity signals simultaneously showed that their average frequency is nine events per minute and their duration is more than 2 s the high speed wedges display a complex organisation and do not show a preferred sequence of events as was postulated by previous studies because of their duration and size these high speed wedges are likely to play a major role in bedload sediment transport

gravity study of los humeros caldera complex mexico structure and associated geothermal system abstract los humeros caldera lies in the eastern portion of the trans mexican volcanic belt a geothermal system is associated with this silicic caldera the volcano sedimentary products of the caldera itself rest on a basement of calcareous rocks and granitic intrusives a well marked density contrast exists between these geologic units consequently gravity measurements have proved to be a useful tool to estimate depths to the local basement a detailed interpretation of the gravity anomaly of this caldera was conducted digital processing included vertical derivatives downward and upward continuations in this way it was possible to infer the main features of the subsurface structure of this caldera the 3 d structural model so created was also constrained by geology and well data it was tested by 3 d forward modelling two nested depressions are subsurficial expressions of two corresponding collapses the western rim of the caldera complex is constituted by a structural high representing a major weakness zone that controlled the location of several vents in the caldera complex the geothermal system is located along this structure a straightforward correlation is observed between the subsurficial structure nested depressions the minor central and major western structural highs respectively and magnetic and resistivity anomalies mt and d c resistivity studies the enhanced permeability associated with the faulting along this major structural high accounts for the observed correlation with the resistivity data two relatively shallow heat sources are interpreted to exist at depths around 10 km one is related to the actual geothermal production zone the second is located just to the southwest of the caldera complex because several other geological and geophysical parameters point to the possible existence of geothermal activity this zone is considered a target for exploration by means of wells for geothermal fluids

form drag in rivers due to small scale natural topographic features 2 irregular sequences 1 the size shape and spacing of small scale topographic features found on the boundaries of natural streams rivers and floodplains can be quite variable consequently a procedure for determining the form drag on irregular sequences of different sized topographic features is essential for calculating near boundary flows and sediment transport a method for carrying out such calculations is developed in this paper this method builds on the work of kean and smith 2006 which describes the flow field for the simpler case of a regular sequence of identical topographic features both approaches model topographic features as two dimensional elements with gaussian shaped cross sections defined in terms of three parameters field measurements of bank topography are used to show that 1 the magnitude of these shape parameters can vary greatly between adjacent topographic features and 2 the variability of these shape parameters follows a lognormal distribution simulations using an irregular set of topographic roughness elements show that the drag on an individual element is primarily controlled by the size and shape of the feature immediately upstream and that the spatial average of the boundary shear stress over a large set of randomly ordered elements is relatively insensitive to the sequence of the elements in addition a method to transform the topography of irregular surfaces into an equivalently rough surface of regularly spaced identical topographic elements also is given the methods described in this paper can be used to improve predictions of flow resistance in rivers as well as quantify bank roughness

numerical modelling of separated flow in river bends model testing and experimental investigation of geometric controls on the extent of flow separation at the concave bank the occurrence of separated flow in sharply curving river bends and the associated deposition of fine grained sediment is widely recognized but the factors controlling the existence and extent of separated flow have not been clearly identified a three dimensional fixed lid computational fluid dynamics program was used to simulate time averaged flow in idealized channel bends in order to investigate geometric controls on flow separation the code was applied first to a field site it successfully modelled the qualitative features of the flow including a separation zone and was insensitive to choice of turbulence closure or exclusion of lateral variation in water surface topography the simulations of idealized bends demonstrate that the existence and extent of concave bank flow separation can be significantly influenced by changes in bend planform point bar topography and upstream planform 1998 john wiley sons ltd

le pico de orizaba mexique structure et evolution d un grand volcan andesitique complexe volcan pico de orizaba which marks the eastern end of the trans mexican volcanic belt is one of the largest andesitic composite volcanoes in america it is located above a series of crustal distensive faults making the boundary of the coast plains of the gulf of mexico from thealtiplano for this reason the volcano shows an asymmetry from the west its elevation is about 3 000 m whereas on the eastern side it reaches 4 000 to 4 500 m from its base

macroturbulent structure of open channel flow over gravel beds the turbulent structure of open channel flow over a mobile gravel bed was investigated in an 8 m long 0 3 m wide and 0 3 m deep tilting flume a flow visualization technique was used and complemented by measurements of flow velocity fluctuations near the bed the experiments reveal that turbulent flow consists of a sequence of large scale eddies with a vertical size close to the flow depth an average length equal to four to five depths and a width of about two depths the downstream motion of these eddies causes quasiperiodic fluctuations of the local flow velocity components the development of longitudinal troughs and ridges on the mobile bed and preferential transport of bed particles along troughs are related to the effect of the eddies the experimental results indicate that the depth scale eddies are an important turbulence mechanism contributing to sediment transport

large scale volcanic cone collapse the 1888 slope failure of ritter volcano and other examples from papua new guinea a largely submarine avalanche amphitheatre that formed catastrophically in 1888 on ritter volcano has been identified from a bathymetric survey collapse of the volcano in 1888 therefore is considered to have been caused by rapid large scale slope failure rather than by cauldron subsidence as previously supposed escarpments of pre historic slope failures are common on other papua new guinea volcanoes directions of avalanching on some volcanoes in the bismarck volcanic arc appear to be controlled by a regional stress pattern and those for some volcanoes in the fly highlands province on mainland papua new guinea point away from the regional centre of pliocene uplift large amphitheatres such as at doma peaks in the fly high lands province probably originated by multiple collapses

the importance of accurately representing submerged vegetation morphology in the numerical prediction of complex river flow this paper reports a novel method for the incorporation of complex plant morphologies into a computational fluid dynamics cfd model allowing the numerical prediction of flows around individual plants the morphological complexity which comprises the vertical and lateral distribution of individual branches and leaves is captured through terrestrial laser scanning tls and is maintained in the numerical prediction of flow fields this is achieved where the post processed voxelised plant representation is incorporated into a cfd scheme through a mass flux scaling algorithm mfsa flow around prunus laurocerasus has been modelled under foliated and defoliated states following the removal of leaves the complex plant morphologies are shown to produce spatially heterogeneous downstream velocity fields with velocity profiles that deviate significantly from the idealised inflected shape rapid transition between the high velocity free stream zone and the zone of reduced velocity in the plant wake indicate shearing of flow with the point of reattachment extending up to seven plant lengths downstream the presence of leaves significantly modifies the flow field response with development of a second more pronounced wake structure around the dense foliage this approach provides a full flow numerical description of the pressure field enabling the vegetative drag force to be quantified for the example given here drag force is an order of magnitude greater for the foliated state the methodology outlined here demonstrates the importance of accurately representing complex plant morphology in hydraulic models and allows drag forces and coefficients to be calculated for specific plant species this article is protected by copyright all rights reserved

seismic properties of rocks with distributions of small cracks summary hudson s theory of multiple scattering for the dynamic properties of cracked rock is applied to media containing cracks of varying orientation radius and aspect ratio angular variation of velocity and attenuation due to parallel and nearly parallel cracks and cracks with normals at a fixed angle to a given axis are presented anisotropy gradually decreases as the distribution of orientations is widened from exactly parallel to random increasing the aspect ratio of thin saturated cracks generally causes the elastic properties to approach those due to dry gas filled cracks for fluid filled cracks the line singularity where the birefringent shear waves have equal velocity moves closer to the symmetry axis as either the distribution of orientations is widened or the aspect ratio is increased calculated attenuations due to rayleigh scattering and viscous dissipation in crack filling fluids are many orders of magnitude less than reported attenuations in the earth viscous dissipation varies linearly with frequency and aspect ratio of thin fluid filled cracks a power law distribution of crack radii intended to represent that found in the earth s crust has no effect on the angular variations of seismic velocities or viscous attenuation but scattering attenuation is sensitive to the power law exponent lognormal distributions of aspect ratio also corresponding to observations in real rock affect velocity as well as attenuation velocities derived from the theory are compared with those measured in laboratory specimens with known geometries of cracks

characteristics of anisotropy and dispersion in cracked medium abstract in upper crustal conditions where effective pressure is not high enough to close them all rocks contain cracks understanding the effect of cracks on both the elastic anisotropy and the elastic waves frequency dependence dispersion is of major interest to underground storage monitoring reservoir monitoring and fault mechanics the challenge is to extract from geophysical data the maximum information with a minimal number of assumptions and model parameters this is possible if reference data exist at the laboratory scale and if extrapolation from laboratory to field scale is not biased in order to check that investigating few basic idealized cases can provide useful guidelines let us consider an isotropic rock matrix and assume that dispersion is the result of squirt flow in this paper we will not consider crack to pore but rather focus on crack to crack squirt flow three basic cracks configurations are quantitatively investigated for which the elastic symmetry is transverse isotropy ti our aim is to check if from first principles high frequency laboratory data can be used for field data interpretation in these three cases the first case is that of identical cracks aligned in the same direction a sufficiently strong deviatoric stress field in a given plane is expected to induce such a distribution at the field scale 1 2 3 we show that it results in a maximum anisotropy but with virtually no dispersion because there is no dispersion high frequency data obtained in the laboratory mhz ultrasonic range are directly applicable to field data hz to khz range interpretation thomsen parameters provide then a direct proxy for crack density a second case is that of identical cracks with random orientations isotropic distribution such a situation could exist at the field scale if cracks result from the superposition of several independent tectonic episodes associated with different directions of principal stresses in that case 1 2 3 this configuration could also result from thermal cracking in an initially isotropic metamorphic rock obviously there is no overall anisotropy then but we show that dispersion may be large due to dispersion ultrasonic laboratory data are not directly applicable to field data interpretation unless the medium is dry a third intermediate case is that of cracks randomly distributed in zone with a given axis the corresponding symmetry is that of a so called axi symmetric triaxial test which in fact is biaxial it is of interest because it represents a classical laboratory experiment situation it can also exist at the field scale if one principal stress is much larger than both others 1 2

representing the bed roughness of coarse grained streams in computational fluid dynamics computational fluid dynamics cfd applications are increasingly utilized for modelling complex flow patterns in natural streams and rivers although cfd has been successfully implemented to model many complex flow situations in natural stream settings adequately characterizing the effects of gravel and cobble beds on flow hydraulics in cfd is a difficult challenge due to the scale of roughness lengths and the inadequacy of traditional roughness representations to characterize flow profiles in situations with large roughness elements an alternative method of representing gravel and cobble beds is presented appropriate drag forces associated with different grain sizes are computed and included in the momentum equations to account for the influence of a hydraulically rough bed comparisons with field measurements reveal reasonable agreement between measured and modelled profiles of spatially averaged velocity and turbulent kinetic energy and model fidelity to the nonlogarithmic behaviour of the velocity profiles the novel method of representing coarse beds expands the utility of cfd for investigating physical processes in natural channels with large bed roughness copyright 2005 john wiley sons ltd

major holocene block and ash fan at the western slope of ice capped pico de orizaba volcano mexico implications for future hazards abstract a major block and ash fan extends more than 14 km westward from the summit of pico de orizaba volcano in the eastern part of the trans mexican volcanic belt radiocarbon dating of charcoal within the fan deposits yielded holocene ages that range between 4040 80 and 4660 100 y b p stratigraphical sedimentological geochemical and scanning electron microscope studies indicate that this fan originated within a relatively short time span by multiple volcanic explosions at the summit crater this activity produced a series of pyroclastic flows mainly block and ash flows and lahars which were channelized by a glacial cirque and connecting u shaped valleys as they descended toward the base of the volcano a recurrence of a similar eruption today would pose severe hazards to the population of more than 50 000 people who live in a potentially dangerous zone a detailed reconstruction of the sequence of events that led to the formation of the block and ash fan is presented to help mitigate the risk special attention is given to the effects of an ice cap and the role of pre existing glacial morphology on the distribution of products from such an eruption

flow resistance in natural turbulent channel flows the need for a fluvial fluid mechanics in fluvial environments feedbacks among flow bed forms sediment and macrophytes result in a complex fluid dynamics the assumptions underpinning standard tools in hydraulics are commonly violated and alternative approaches must be formulated i argue that we should question the assumption that classical notions in fluid mechanics provide the foundations for the techniques of the future recent work on turbulent dissipation interscale modulation of the dynamics intermittency and the role of complex forcings is discussed an agenda for future work is proposed that involves improving our characterization of complex forcings and developing better understanding of the behavior of the velocity gradient tensor in complex fluvial environments this leads to the formulation of modeling tools relevant to fluvial fluid mechanics rather than a reliance on methods developed elsewhere one avenue by which such methods might be developed is suggested based on the stretched spiral vortex as a baseline topology this would result in a nonequilibrium model for turbulence that has greater potential to capture the dynamics in which we are interested although these ideas are raised in the context of a future fluvial fluid mechanics they are applicable to any situation where turbulent flows are forced in complicated ways

three dimensional flow structure around small scale bedforms in a simulated gravel bed environment pebble clusters are common small scale morphological features in gravel bed rivers occupying as much as 10 per cent of the bed surface important links exist between the presence of pebble clusters and the development of flow structures these links are poorly understood at the three dimensional level particularly neglected has been the effect of clusters on the lateral flow characteristics a laboratory study was conducted using a hydraulic flume within which simulated pebble clusters were superimposed onto a plane bed of gravel material high resolution three dimensional flow data were collected above the bed at two different flow depths using an acoustic doppler velocimeter the results present evidence of the importance of lateral flow in the development of turbulent flow structure narrow regions of high lateral and downstream turbulence intensity exist to both sides of clusters and in a three dimensional separation zone in their lee this may indicate the presence of horseshoe type vortical structures analogous to those identified in less hydraulically rough environments however it is likely that these structures are more complicated given the mutual interference of the surrounding medium the lateral flow was also identified as a key component in the upwelling identified by other authors in the lee of pebble clusters the results of the vertical flow analysis confirm the hypothesis that six regions with distinct vertical flow characteristics exist above clusters flow acceleration up the stoss side of the cluster recirculation behind the cluster in the wake region vortex shedding from the pebble crest and shear layer flow reattachment downstream of the cluster upwelling of flow downstream of the point of reattachment and recovery of flow copyright 2001 john wiley sons ltd

interactions between subgrid scale resolution feature representation and grid scale resolution in flood inundation modelling numerical modelling of flood inundation over large and complex floodplains often requires mesh resolutions coarser than the structural features e g buildings that are known to influence the inundation process recent research has shown that this mismatch is not well represented by conventional roughness treatments but that finer scale features can be represented through porositybased sub grid scale treatments this paper develops this work by testing the interactions between feature representation sub grid scale resolution and mesh resolution it uses as the basis for this testing a 2d diffusion based flood inundation model which is applied to a 2004 flood event in a topologically complex upland floodplain in northern england results showed serious degradation of model predictions without explicit representation of features like walls inclusion of such features through raising mesh cell elevations where intersected by a feature resulted in a major improvement in model predictions in terms of reduced inundation extent to make such treatments physically realistic and notably so that the full potential for floodplain storage is included it was shown that a sub grid scale treatment also needed to be included the effects of this combined treatment was the recovery of more plausible floodplain friction values as well as a sensitivity to friction that allows for more effective representation of floodplain friction effects such as vegetation

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

emergence of coherent flow structures over a gravel surface a numerical experiment 1 gravel bed rivers have complex porous and irregular surfaces characterized by a range of morphological forms these topographical structures determine the flow structures that develop over the river bed primarily by the shedding of vortices in the downstream wake of protruding clasts previous research into these flow structures has come from experimental studies which have used either flow visualization or single point measurements techniques here we present a numerical experiment where large eddy simulation les is used to study the generation evolution and destruction of these flow structures over a naturally water worked gravel surface the numerical simulations reported in this paper show that there are two distinct scales of boundary influence upon the shallow flow and emphasize that the measured flow variability at any one point in a natural river will contain both locally derived and upstream inherited flow structures according to the range of scales of bed topography present

influence of crack distribution of rocks on p wave velocity anisotropy a laboratory and field scale study the purpose of this paper is the comparison of p wave velocity and velocity anisotropy measured at different scales under laboratory and field conditions a shallow seismic refraction survey with shot receiver spacing of up to 10 m was carried out on a flat outcrop of lhertzolite in the southern part of the balmuccia massif oriented rock samples were also obtained from the locality the particular advantage of the laboratory method used is the possibility of measuring velocity in any direction under controlled conditions laboratory tests were made on spherical peridotite samples 50 mm in diameter by ultrasonic velocity measurements in 132 directions meridian and parallel networks under confining stress ranging from atmospheric to 400 mpa the mean p wave velocity of the field and laboratory data differed by between 20 30 in addition p wave velocity anisotropy of 25 was detected in the field data whereas the anisotropy in the laboratory samples in the same orientation as the field surveys was less than 2 this observed scaling factor is related to the different sampling sizes and the difference in frequencies of applied elastic waves with an ultrasonic wavelength of 10 mm laboratory samples represent a continuum the field velocities and velocity anisotropy reflect the presence of cracks which the laboratory rock samples do not contain three sub vertical fracture sets with differing strikes were observed in the field outcrop estimates of fracture stiffness from the velocity anisotropy data are consistent with other published values these results highlight the difficulty of using laboratory velocity estimates to interpret field data

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

morphology of pyroclastic cones and tectonics the relationships between morphology and spatial distribution of 1315 quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings tepic rift mexico ethiopian rift mexican volcanic belt canary archipelago and mount etna field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone the strike of the feeding plane is directly related to 1 the elongation of cone base and crater 2 the location of depressions on the crater rim and 3 the alignment of pyroclastic cones in relation to a given vent spacing in addition the strike and dip of faults affect the direction of cone breaching these relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9 and are especially sensitive to fault escarpment and cone height lava and cone density and fault orientation with respect to the dip of the volcanic substrate topography relations 1 and 2 become more pronounced for regions undergoing extensional tectonics where edifices also have a larger dimension whereas breaching in the direction of the fault dip is more widespread in regions under extension breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics

the theoretical foundations and potential for large eddy simulation les in fluvial geomorphic and sedimentological research abstract large eddy simulation les is a method for resolving the time dependent structure of high reynolds number turbulent flows with les it is possible to model and track the behaviour of coherent turbulent structures and study their effect on the flow field hence les is potentially an important research tool in the fluvial sciences where flow mixing sediment entrainment and sediment transport are all affected by the presence of coherent vortices and their interactions with channel boundaries and other flow structures this paper introduces the les methodology discusses a variety of ways for representing small scale processes within les the subgrid scale modelling problem and provides some examples of early work into the use of les in a fluvial context a number of advances in computational power and numerical methods are required before les can be effectively applied at the river reach scale this paper considers some recent developments and their potential for providing validated large eddy simulations of river flow at the channel scale

application of 3d mobile bed hydrodynamic model the prediction of sediment erosion and deposition magnitudes has challenged many investigators one and two dimensional sediment transport models are commonly used for predicting sediment scour and deposition they are adequate for many engineering applications however for some problems only a three dimensional model should be used a three dimensional model is necessary for predicting sediment erosion and deposition whenever significant secondary currents exist such as in river bends crossings distributaries or diversions in diversions for example the sediment load may not split in the same proportions as the flow a three dimensional model can be used to estimate the sediment split this paper presents a description of the three dimensional numerical model ch3d sed and the governing equations for the model the validation of the model and its application to the deep draft navigation project on the lower mississippi river also are presented

three dimensional measurement of river channel flow processes using acoustic doppler velocimetry this paper describes and assesses i the use of a new instrument for the determination of three dimensional flow velocities in natural rivers the acoustic doppler velocimeter adv and ii a method for positioning and orienting such measurements relative to a single local coordinate system to relate flow velocity vectors with the bed and water surface the adv uses the doppler shift principle to measure the velocity of small particles assuming to be moving at velocities similar to the fluid velocity is resolved into three orthogonal components and measured in a volume 5 cm below the sensor head minimizing interference of the flow field and allowing measurements to be made close to the bed a simple method for positioning and orienting the instrument using digital tacheometry is described and is used to obtain velocity measurements concurrently with measurements of both bed and water surface topography the paper includes a preliminary field assessment of the adv by comparing velocity profiles with those generated from marsh mcbirney electromagnetic current meters and a full field assessment of the position and orientation methodology these results suggest that the recommended methods in combination with an adv are able to provide reliable mean three dimensional velocity field information and accurate bed and surface topography copyright 1998 john wiley sons ltd

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

distinguishing oil and water layers by interpreting acoustic logging data with changing well diameters during surveys water layers may interfere with the detection of oil layers in order to distinguish between oil and water layers research on the properties of well diameters and oil and water layers and their relation to acoustic logging rules is essential using hudson s crack theory we simulated oil and water layers with different well diameters or crack parameters angle and number density we found that when the well radius increases from 0 03 m to 0 05 m the variation ratio of compressional wave amplitude for the oil layer is less than that for the water layer the difference of stoneley wave amplitude between the crack parameters angle and number density is greater in the case of the water layer than in the case of the oil layer the response sensitivity of wave energy is greater for the water layer than that for the oil layer when the well radius increases from 0 05 m to 0 14 m the maximum excitation intensity for oil layer is greater than that for the water layer we conclude that the propagation of an elastic wave is affected by medium composition and well diameter and the influence has certain regularity these results can guide further reservoir logging field exploration work

dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism a set of experimental small scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers initially underplated at the base of the crust the models are driven by a centrifugal body force which simulates the role of gravity in nature the models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension the experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers in particular extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults in the overlying brittle layer associated with the ductile domal uplift stretching of the continental crust induces a reactive migration of viscous layers into the footwall of major normal faults where magma accumulation at the core of the domes takes place this behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults a situation that is often observed in core complex structures although the models presented here are intended to simulate magma emplacement during extension of a two layer brittle ductile system lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts e g the ethiopian rift may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones in the suggested model such volcanoes are related to large basaltic magma reservoirs located at the base of the crust to accommodate the space vacated by lateral flow and thinning of the ductile crust the similarity in structure of the models with natural examples of continental extension modes such as rift systems and core complexes may suggest a close similarity of dynamic processes

flow heterogeneity over 3d cluster microform laboratory and numerical investigation the present study examines the flow around a self occurring cluster bed form and the use of general computation fluid dynamics methods for hydraulic and geophysical flow applications this is accomplished through a comprehensive experimental numerical investigation in the laboratory cluster bed forms are first formed from movable sediment and laser doppler velocimeter measurements of two dimensional fluid velocity are then taken around a formed cluster a three dimensional 3d reynolds averaged navier stokes simulation of the physical cluster and flow conditions is then conducted using near wall shear stress transport sst turbulence modeling with the inclusion of hydraulic roughness ks r 31 150 ks h 0 1 ks 274 i e in the fully rough regime sst near wall modeling is advantageous compared to the more widely used wall functions approach for flows with significant roughness and flow separation because the model equations can be integrated down to the wall therefore sst near wall model

overall properties of heterogeneous material summary small scale heterogeneity in crustal structure can be very complex and difficult to describe in detail and yet at the same time can be very important for the description of for instance tectonic stress and porosity statistical properties of such heterogeneity can be derived from the properties of waves of relatively large wavelength as they propagate through the crust the differences between measured wavelengths and attenuation coefficients and those of compact rock and the variations of these quantities in space and time provide in principle a means of determining quantities like the density and orientations of microfractures and the nature of crack infill material the theoretical basis for inferences of this kind is the concept of the effective or equivalent material based on an averaging process taken over the microstructure a number of methods have been used to calculate the properties of the effective medium several of which are described here

urban fluvial flood modelling using a two dimensional diffusion wave treatment part 1 mesh resolution effects high resolution data obtained from airborne remote sensing is increasing opportunities for representation of small scale structural elements e g walls buildings in complex floodplain systems using two dimensional 2d models of flood inundation at the same time 2d inundation models have been devel oped and shown to provide good predictions of flood inundation extent with respect to both full solution of the depth averaged navier stokes equations and simplified diffusion wave models however these models have yet to be applied extensively to urban areas this paper applies a 2d raster based diffusion wave model to determine patterns of fluvial flood inundation in urban areas using high resolution topog raphic data the aim of this paper is to explore the effects of spatial resolution upon estimated inundation extent and flow routing process this is important as the complexity of urban surfaces is such that chang ing data resolution may have a major effect upon surface representation and hence flow routing similarly few applications to date have explicitly considered the timing of the inundation process the model assumes that the prime source of the flood is fluvial pluvial floods and floods associated with urban drainage systems are not addressed the topographic data are based upon airborne laser altimetry li dar obtained for the city of york u k a series of image processing steps are used to pre prepare the topographic data a one dimensional hydraulic model of the ouse from skelton upstream of the city through to naburn weir downstream of the city is used to provide estimates of flux from the river to the floodplain for a major flood inundation event estimated to have a return period of greater than 100 years in november 2000 validation data were available in the form of inundation patterns obtained using aerial photography at a point on the falling limb of the flood event model response shows that even relatively small changes in model resolution have considerable effects on the predicted inundation extent and the timing of flood inundation timing sensitivity would be expected given the relatively poor representation of inertial processes in a diffusion wave model compared with previous work sensitivity to inundation extent is more surprising and is associated with three connected effects 1 the smoothing effect of mesh coarsening upon input topographical data 2 poorer representation of both cell blockage and surface routing processes as the mesh is coarsened where the flow routing is especially complex and 3 the ef fects of 1 and 2 upon water levels and velocities which in turn determine which parts of the floodplain the flow can actually travel to the combined effects of wetting and roughness parameters can compen sate in part for a coarser mesh resolution however the coarser the resolution the poorer the ability to control the inundation process as these parameters not only affect the speed but also the direction of wet ting thus high resolution data will need to be coupled to more sophisticated representation of the inun dation process in order to obtain effective predictions of flood inundation extent this is explored in a companion paper

a numerical investigation into the importance of bed permeability on determining flow structures over river dunes although permeable sediments dominate the majority of natural environments past work concerning bedform dynamics has considered the bed to be impermeable and has generally neglected flow between the hyporheic zone and boundary layer herein we present results detailing numerically modelled flow which allow the effects of bed permeability on bedform dynamics to be assessed simulation of an isolated impermeable bedform over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside in the form of returning jets that generate horseshoe shaped vortices the returning flow significantly influences the leeside flow modifying the separation zone lifting the shear layer adjoining the separation zone away from the bed simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside with two dunes placed in series the flow over the downstream dune is influenced by the developing boundary layer on the leeside of the upstream dune for the permeable bed case the upwelling flow lifts the separated flow from the bed modifies the shear layer through the coalescence with vortices generated and causes the shear layer to undulate rather than be parallel to the bed these results demonstrate the significant effect that bed permeability has on the flow over bedforms that may be critical in affecting the flux of water and nutrients

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

in situ characterization of grain scale fluvial morphology using terrestrial laser scanning the grain scale morphology of fluvial sediments is an important control on the character and dynamics of river systems however current understanding of its role is limited by the difficulties of robustly quantifying field surface morphology terrestrial laser scanning tls offers a new methodology for the rapid acquisition of high resolution and high precision surface elevation data from in situ sediments to date most environmental and fluvial applications of tls have focused on large scale systems capturing macroscale morphologies application of this new technology at scales necessary to characterize the complexity of grain scale fluvial sediments therefore requires a robust assessment of the quality and sources of errors in close range tls data this paper describes both laboratory and field experiments designed to evaluate close range tls for sedimentological applications and to develop protocols for data acquisition in the former controlled experiments comprising high resolution scans of white grey and black planes and a sphere were used to quantify the magnitude and source of three dimensional 3d point errors resulting from a combination of surface geometry reflectivity effects and inherent instrument precision subsequently a methodology for the collection and processing of grain scale tls data is described through an application to a coarse grained gravel system the river feshie d50 32 to 63 mm this stepwise strategy incorporates averaging repeat scans and filtering scan artefact and non surface points using local 3d search algorithms the sensitivity of the results to the filter parameter values are assessed by careful internal validation of digital terrain models dtms created from the resulting point cloud data the transferability of this methodology is assessed through application to a second river bury green brook dominated by finer gravel d50 18 to 33 mm the factor limiting the resolution of dtms created from this second dataset was found to be the relative sizes of the laser footprint and smallest grains copyright 2009 john wiley sons ltd

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

seismic wave propagation in cracked porous media summary the movement of interstitial fluids within a cracked solid can have a significant effect on the properties of seismic waves of long wavelength propagating through the solid we consider three distinct mechanisms of wave induced fluid flow flow through connections between cracks in an otherwise non porous material fluid movement within partially saturated cracks and diffusion from the cracks into a porous matrix material in each case the cracks may be aligned or randomly oriented leading respectively to anisotropic or isotropic wave speeds and attenuation factors in general seismic velocities exhibit behaviour that is intermediate between that of empty cracks and that of isolated liquid filled cracks if fluid flow is significant in the range of frequencies for which considerable fluid flow occurs there is high attenuation and dispersion of seismic waves fluid flow may be on either a wavelength scale or a local scale depending on the model and whether the cracks are aligned or randomly oriented resulting in completely different effects on seismic wave propagation a numerical analysis shows that all models can have an effect over the exploration seismic frequency range

modeling complex flow structures and drag around a submerged plant of varied posture although vegetation is present in many rivers the bulk of past work concerned with modeling the influence of vegetation on flow has considered vegetation to be morphologically simple and has generally neglected the complexity of natural plants here we report on a combined flume and numerical model experiment which incorporates time averaged plant posture collected through terrestrial laser scanning into a computational fluid dynamics model to predict flow around a submerged riparian plant for three depth limited flow conditions reynolds number 65 000 110 000 plant dynamics were recorded through high definition video imagery and the numerical model was validated against flow velocities collected with an acoustic doppler velocimeter the plant morphology shows an 18 reduction in plant height and a 14 increase in plant length compressing and reducing the volumetric canopy morphology as the reynolds number increases plant shear layer turbulence is dominated by kelvin helmholtz type vortices generated through shear instability the frequency of which is estimated to be between 0 20 and 0 30 hz increasing with reynolds number these results demonstrate the significant effect that the complex morphology of natural plants has on in stream drag and allows a physically determined species dependent drag coefficient to be calculated given the importance of vegetation in river corridor management the approach developed here demonstrates the necessity to account for plant motion when calculating vegetative resistance

numerical simulation of three dimensional flow hydraulics in a braided channel results are presented from a numerical simulation of three dimensional flow hydraulics around a mid channel bar carried out using the fluent uns computational fluid dynamics cfd software package fluent uns solves the three dimensional reynolds averaged form of the navier stokes equations turbulence closure is achieved using a rng k model simulated flow velocities are compared with measured two dimensional velocities downstream and cross stream obtained using an electromagnetic current meter ecm the results of the simulation are qualitatively consistent with the flow structures observed in the field quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two r 0 88 and a mean difference of 0 09 m s 1 deviations between simulated and measured velocities may be identified that are both random and systematic the former may reflect a number of factors including subgrid scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ecm orientation model mesh configuration roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities these results illustrate the potential for using cfd software to simulate flow hydraulics in natural channels with complex configurations they also highlight the need for detailed spatially distributed datasets of three dimensional flow variables to establish the accuracy and applicability of cfd software copyright 1999 john wiley sons ltd

photogrammetric monitoring of small streams under a riparian forest canopy the recent advent of digital photogrammetry has enabled the modeling and monitoring of river beds at relatively high spatial resolution 0 01 to 1 m through the extraction of digital elevation models dems the traditional approach to image capture has been to mount a metric camera to an aircraft although non metric cameras have been mounted to a variety of novel aerial platforms to acquire river based imagery e g helicopters radio controlled motorized vehicles tethered blimps and balloons however most of these techniques are designed to acquire imagery at flying heights above the riparian tree canopy in relatively narrow channels e g 20 m bankfull width streamside trees can obscure the channel and limit continuous photogrammetric data acquisition of both the channel bed and banks while still providing useful information regarding the riparian canopy and even spot elevations of the channel this paper presents a technique for the capture and analysis of close range photogrammetric data acquired from a vertically mounted non metric camera suspended 10 m above the channel bed by a unipod the camera is positioned under the riparian forest canopy so that the channel bed can be imaged without obstruction the system is portable and permits relatively rapid image acquisition over rough terrain and in dense forest the platform was used to generate dems with a nominal ground resolution of 0 03 m dems generated from this platform required post possessing to either adjust or eliminate erroneous cells introduced by the extraction process overhanging branches and by the effects of refraction at the air water interface for submerged portions of the channel bed the vertical precision in the post processed surface generally ranged from 0 01 to 0 1 m depending on the quality of triangulation and the characteristics of the surface being imaged copyright 2010 john wiley sons ltd

development of a vector based 3d grain entrainment model with application to x ray computed tomography scanned riverbed sediment sediment transport equations typically produce transport rates that are biased by orders of magnitude a causal component of this inaccuracy is the inability to represent complex grain scale interactions controlling entrainment grain scale incipient motion has long been modelled using geometric relationships based on simplified particle geometry and two dimensional 2d force or moment balances however this approach neglects many complexities of real grains including grain shape cohesion and the angle of entrainment relative to flow direction to better represent this complexity we develop the first vector based fully three dimensional 3d grain rotation entrainment model that can be used to resolve any entrainment formulation in 3d and which also includes the effect of matrix cohesion to apply this model we use x ray computed tomography to quantify the 3d structure of water worked river grains we compare our 3d model results with those derived from application of a 2d entrainment model we find that the 2d approach produces estimates of dimensionless critical shear stress urn x wiley mma media esp4608 math 0001 that are an order of magnitude lower than our 3d model we demonstrate that it is more appropriate to use the c axis when calculating 2d projections which increases values of urn x wiley mma media esp4608 math 0002 to more closely match our 3d estimates the 3d model reveals that the main controls on critical shear stress in our samples are projection of grains cohesive effects from a fine grained matrix and bearing angle for the plane of rotation the lateral angle of departure from downstream flow that in part defines the grain s direction of pivot about an axis formed by two contact points in 3d the structural precision of our 3d model demonstrates sources of geometric error inherent in 2d models by improving flow properties to better replicate local hydraulics in our 3d model entrainment modelling of scanned riverbed grains has the potential for benchmarking 2d model enhancements

coupled 3 d stream flow and hyporheic flow model under varying stream and ambient groundwater flow conditions in a pool riffle system 1 exchange of water and solutes across the stream sediment interface is an important control for biogeochemical transformations in the hyporheic zone hz in this paper we investigate the interplay between turbulent stream flow and hz flow in pool riffle streams under various ambient groundwater flow conditions streambed pressures derived from a computational fluid dynamics cfd model are assigned at the top of the groundwater model and fluxes at the bottom of the groundwater model domain represent losing and gaining conditions simulations for different reynolds numbers re and pool riffle morphologies are performed results show increasing hyporheic exchange flows m3 d for larger re and a concurrent decrease in residence time rt losing and gaining conditions were found to significantly affect the hyporheic flow field and diminish its spatial extent as well as rates of hyporheic exchange flow the fraction of stream water circulating through the hyporheic zone is in the range of 1 10 5 to 1 10 6 per meter stream length decreasing with increasing discharge complex distributions of pressure across the streambed cause significant lateral hyporheic flow components with a mean lateral travel distance of 20 of the longitudinal flow paths length we found that the relationship between pool riffle height and hyporheic exchange flow is characterized by a threshold in pool riffle amplitude beyond which hyporheic exchange flow becomes independent of riffle height hyporheic residence time distributions rtd are log normally distributed with medians ranging between 0 7 and 19 h

modeling river bed morphology roughness and surface sedimentology using high resolution terrestrial laser scanning 1 recent advances in technology have revolutionized the acquisition of topographic data offering new perspectives on the structure and morphology of the earth s surface these developments have had a profound impact on the practice of river science creating a step change in the dimensionality resolution and precision of fluvial terrain models the emergence of hyperscale survey methods including structure from motion photogrammetry and terrestrial laser scanning tls now presents the opportunity to acquire 3 d point cloud data that capture grain scale detail over reach scale extents translating these data into geomorphologically relevant products is however not straightforward unlike traditional survey methods tls acquires observations rapidly and automatically but unselectively this results in considerable noise associated with backscatter from vegetation and other artifacts moreover the large data volumes are difficult to visualize require very high capacity storage and are not incorporated readily into gis and simulation models in this paper we analyze the geomorphological integrity of multiscale terrain models rendered from a tls survey of the braided river feshie scotland these raster terrain models are generated using a new computationally efficient geospatial toolkit the topographic point cloud analysis toolkit topcat this performs an intelligent decimation of point cloud data into a set of 2 5 d terrain models that retain information on the high frequency subgrid topography as the moments of the locally detrended elevation distribution the results quantify the degree of terrain generalization inherent in conventional fluvial dems and illustrate how subgrid topographic statistics can be used to map the spatial pattern of particle size grain roughness and sedimentary facies at the reach scale

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

flow structures at an idealized bifurcation a numerical experiment river bifurcations are key nodes within braided river systems controlling the flow and sediment partitioning and therefore the dynamics of the river braiding process recent research has shown that certain geometrical configurations induce instabilities that lead to downstream mid channel bar formation and the formation of bifurcations however we currently have a poor understanding of the flow division process within bifurcations and the flow dynamics in the downstream bifurcates both of which are needed to understand bifurcation stability this paper presents results of a numerical sensitivity experiment undertaken using computational fluid dynamics cfd with the purpose of understanding the flow dynamics of a series of idealized bifurcations a geometric sensitivity analysis is undertaken for a range of channel slopes 0 005 to 0 03 bifurcation angles 22 to 42 and a restricted set of inflow conditions based upon simulating flow through meander bends with different curvature on the flow field dynamics through the bifurcation the results demonstrate that the overall slope of the bifurcation affects the velocity of flow through the bifurcation and when slope asymmetry is introduced the flow structures in the bifurcation are modified in terms of bifurcation evolution the most important observation appears to be that once slope asymmetry is greater than 0 2 the flow within the steep bifurcate shows potential instability and the potential for alternate channel bar formation bifurcation angle also defines the flow structures within the bifurcation with an increase in bifurcation angle increasing the flow velocity down both bifurcates however redistributive effects of secondary circulation caused by upstream curvature can very easily counter the effects of local bifurcation characteristics copyright 2011 john wiley sons ltd

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