thermal state of the taranaki basin new zealand the taranaki basin is an active margin basin that has been significantly affected by miocene subduction tectonics along the pacific australian plate boundary we have analyzed its present day thermal state using 354 bottom hole temperatures bhts from 115 wells distributed throughout the basin the measured temperatures were corrected using an exact solution to bullard s equation rather than the horner approximation thereby allowing for recovery dependence on well diameter and correction for some bhts at early time after circulation had ceased thermal conductivity measurements were completed on 256 samples from eight wells and matrix conductivities were determined for six end member lithologies by inversion formation conductivities are based on the conductivity and relative proportion of each end member component corrected bhts in situ thermal conductivity and estimates of sediment heat production were combined to compute the present day steady state heat flow the average heat flow is 60 mw m 2 but important geographic variations are present heat flow on the western platform is remarkably consistent at 53 60 mw m 2 attesting to its relative stability since the late cretaceous heat flow in the southern part of the basin is 65 70 mw m 2 due to as much as 3 km of late miocene erosion on the southern onshore and to the south of the peninsula heat flow is 50 3 mw m 2 possibly due to the heat sink effects of crustal thickening heat flow is highest at 74 mw m 2 on the northern peninsula adjacent to the taranaki volcanic zone suggesting a causal relationship between quaternary volcanism and high heat flow

interface resolved direct numerical simulations of sediment transport in a turbulent oscillatory boundary layer the flow within an oscillatory boundary layer which approximates the flow generated by propagating sea waves of small amplitude close to the bottom is simulated numerically by integrating the navier stokes and continuity equations the bottom is made up of spherical particles free to move which mimic sediment grains the approach allows one to fully resolve the flow around the particles and to evaluate the forces and torques that the fluid exerts on their surface then the dynamics of sediments is explicitly computed by means of the newton euler equations for the smallest value of the flow reynolds number presently simulated the flow regime turns out to fall in the intermittently turbulent regime such that turbulence appears when the free stream velocity is close to its largest value but the flow recovers a laminar like behaviour during the remaining phases of the cycle for the largest value of the reynolds number turbulence is significant during almost the whole flow cycle the evaluation of the sediment transport rate allows one to estimate the reliability of the empirical predictors commonly used to estimate the amount of sediments transported by sea waves for large values of the shields parameter the sediment flow rate during the accelerating phases does not differ from that observed during the decelerating phases however for relatively small values of the shields parameter the amount of moving particles depends not only on the bottom shear stress but also on flow acceleration moreover the numerical results provide information on the role that turbulent eddies have on sediment dynamics

experiments on saltation of sand in water a high speed video system was used to study saltation of sand in an open channel flow high rates of image acquisition provided the information needed to resolve details of the lagrangian properties of particle motion saltation of sand is described in terms of statistical properties of particle trajectories such as mean values and standard deviations of saltation length height and streamwise particle velocity results are compared with available empirical data saltation height seems to be independent of particle size when made dimensionless with the particle diameter however the dimensionless saltation length appears to increase as the particle size decreases particle collision with the bed is also analyzed observations of this process indicate that most of the interactions between saltating particles and the bed are of the collision rebound type which contradicts previous discussions on the subject the estimated value of the dynamic friction coefficient is about half that proposed by bagnold i

the elements and richness of particle diffusion during sediment transport at small timescales the ideas of advection and diffusion of sediment particles are probabilistic constructs that emerge when the master equation a precise probabilistic description of particle conservation is approximated as a fokker planck equation the diffusive term approximates nonlocal transport it looks upstream and downstream for variations in particle activity and velocities whose effects modify the advective term high resolution measurements of bed load particle motions indicate that the mean squared displacement of tracer particles when treated as a virtual plume primarily reflects a nonlinear increase in the variance in hop distances with increasing travel time manifest as apparent anomalous diffusion in contrast an ensemble calculation of the mean squared displacement involving paired coordinate positions independent of starting time indicates a transition from correlated random walks to normal fickian diffusion this normal behavior also is reflected in the particle velocity autocorrelation function spatial variations in particle entrainment produce a flux from sites of high entrainment toward sites of low entrainment in the case of rain splash transport this leads to topographic roughening where differential rain splash beneath the canopy of a desert shrub contributes to the growth of a soil mound beneath the shrub with uniform entrainment rain splash transport often described as a diffusive process actually represents an advective particle flux that is proportional to the land surface slope particle diffusion during both bed load and rain splash transport involves motions that mostly are patchy intermittent and rarefied the probabilistic framework of the master equation reveals that continuous formulations of the flux and its divergence the exner equation represent statistically expected behavior analogous to reynolds averaged conditions key topics meriting clarification include the mechanical basis of particle diffusion effects of rarefied conditions involving patchy intermittent motions and effects of rest times on diffusion of tracer particles and particle borne substances this article is protected by copyright all rights reserved

fluid and sediment dynamics of upper stage plane beds to understand more fully the fluid and sediment dynamics of upper stage plane beds laboratory experiments were conducted using mobile and fixed beds where turbulent motions of fluid and sediment were measured using laser anemometry bed elevation fluctuations on mobile upper stage plane beds reveal millimeter high bed waves vertical profiles of flow velocity mixing length and eddy viscosity diffusivity are represented well by the law of the wall for the mobile bed von karman s 0 33 and equivalent sand roughness to mean bed grain size varies from 9 to 17 because of the presence of bed load and low relief bed waves for fixed beds with no sediment transport 0 41 and equivalent sand roughness is equal to the mean bedgrain size the decrease in for mobile beds is related to the relative motion of grains and fluid mobile bed turbulence intensities are greater than those for sediment free fixed beds because of enhanced wake formation from the lee side of near bed grains and low relief bed waves sediment diffusivities es calculated in a similar way to fluid diffusivities e indicate that es e sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to e in near bed regions y d 0 3 suggesting that larger more energetic turbulent eddies are responsible for sediment suspension higher in the flow

a decade s investigation of the stability of erodible stream beds this contribution is a remake of the one published under the same title by reynolds nord hydrol vol 7 1976 pp 161 183 as in that paper attention is given to the latest developments in the field of river morphodynamics and in particular on the formation of bed patterns stimulated by the work by kidanemariam and uhlmann j fluid mech vol 750 2014 r2 the opportunity is taken to examine the most recent developments in terms of sediment transport models

numerical simulation of sand waves in a turbulent open channel flow we develop a coupled hydro morphodynamic numerical model for carrying out large eddy simulation of stratified turbulent flow over a mobile sand bed the method is based on the curvilinear immersed boundary approach of khosronejad et al adv water resour vol 34 2011 pp 829 843 we apply this method to simulate sand wave initiation growth and evolution in a mobile bed laboratory open channel which was studied experimentally by venditti church j geophys res vol 110 2005 f01009 we show that all the major characteristics of the computed sand waves from the early cross hatch and chevron patterns to fully grown three dimensional bedforms are in good agreement with the experimental data both qualitatively and quantitatively our simulations capture the measured temporal evolution of sand wave amplitude wavelength and celerity with good accuracy and also yield three dimensional topologies that are strikingly similar to what was observed in the laboratory we show that near bed sweeps are responsible for initiating the instability of the initially flat sand bed stratification effects which arise due to increased concentration of suspended sediment in the flow also become important at later stages of the bed evolution and need to be taken into account for accurate simulations as bedforms grow in amplitude and wavelength they give rise to energetic coherent structures in the form of horseshoe vortices which transport low momentum near bed fluid and suspended sediment away from the bed giving rise to characteristic boil events at the water surface flow separation off the bedform crestlines is shown to trap sediment in the lee side of the crestlines which coupled with sediment erosion from the accelerating flow over the stoss side provides the mechanism for continuous bedform migration and crestline rearrangement the statistical and spectral properties of the computed sand waves are calculated and shown to be similar to what has been observed in nature and previous numerical simulations furthermore and in agreement with recent experimental findings singh et al water resour res vol 46 2010 pp 1 10 the spectra of the resolved velocity fluctuations above the bed exhibit a distinct spectral gap whose width increases with distance from the bed the spectral gap delineates the spectrum of turbulence from the low frequency range associated with very slowly evolving albeit energetic coherent structures induced by the migrating sand waves overall the numerical simulations reproduce the laboratory observations with good accuracy and elucidate the physical phenomena governing the interaction between the turbulent flow and the developing mobile bed

investigations of continuous bed load saltating process particle saltating motions during bed load transport are dominated by the forces acting upon particles and the random process of the particles impacting on and rebounding from the channel bed a real time flow visualization technique is developed in this study to measure particle saltating trajectories corresponding velocities and impacting and rebounding angles based on the experimental data regression equations for the dimensionless saltating length height and velocity were obtained a numerical model that is able to simulate the continuous saltating process of a single particle was developed the model was calibrated and verified with the experimental data and the results were satisfactory the model was also used to generate a series of synthetic data based on these data and the flume data collected by previous investigators a bed load equation was derived

empirical relationships between elevation and the stable isotope composition of precipitation and surface waters considerations for studies of paleoelevation change a compilation of 68 studies from throughout many of the world s mountain belts reveals an empirically consistent and linear relationship between change in elevation and change in the isotopic composition of precipitation along altitudinal transects the isotopic composition of precipitation decreases linearly with increasing elevation in most regions of the world except in the himalayas and at elevations 5000 m there are no significant differences in isotopic lapse rates from most regions of the world 0 28 permil 100 m except at the extreme latitudes where isotopic lapse rates are higher given information on past changes in the isotopic composition of precipitation preserved in pedogenic or authigenic minerals this global isotopic lapse rate can be used to place numerical constraints on the topo graphic development of some ancient mountain belts or plateaus there are many complicating factors that can confound interpretation of paleoel evation change based on stable isotopes and many of these are unique to specific mountain belts or time periods relevant to all stable isotope based paleoelevation change studies is the temperature dependent isotope fractionation between a pedo genic or authigenic mineral and the water from which it forms in cases where isotopic proxy minerals are sampled from localities where temperature will change simulta neously with elevation change the apparent change in the isotopic composition of precipitation may be dampened by several permil this suggests that samples taken from the rainshadow side of an emerging orographic barrier may be more likely to preserve isotopic changes resulting from mountain uplift than samples taken from atop a rising mountain range or plateau

initiation of bed forms on a flat sand bed the results of a series of 47 bed development experiments are combined with appropriate experimental results of other authors the generation of initial bed waves termed sand wavelets on a plane sand bed is found to be instigated by the occurrence of a random pileup of sediment this pileup subsequently growing as further sediment is trapped by the pileup when the height of the pileup approaches the bed roughness height further pileups are generated at preferred spacings downstream the wavelength l of the sand wavelets so generated is relatively insensitive to the applied bed shear stress and primarily a function of the size d g of the sediment this wavelength can be described by the empirical formula l dg r c0 2 10 2 5 from a theoretical viewpoint this wavelength is approximately described by a reassessment of the flow system instability analyses of richards both ripples and dunes are found to develop from the sand wavelets

direct numerical simulation of pattern formation in subaqueous sediment we present results of direct numerical simulation of incompressible fluid flow over a thick bed of mobile spherically shaped particles the algorithm is based upon the immersed boundary technique for fluid solid coupling and uses a soft sphere model for the solid solid contact two parameter points in the laminar flow regime are chosen leading to the emergence of sediment patterns classified as small dunes while one case under turbulent flow conditions leads to vortex dunes with significant flow separation on the lee side the wavelength amplitude and propagation speed of the patterns extracted from the spanwise averaged fluid bed interface are found to be consistent with available experimental data the particle transport rates are well represented by available empirical models for flow over a plane sediment bed in both the laminar and the turbulent regimes

revisiting the linear theory of sand dune formation a linear stability theory for dune and antidune formation is presented that implements a rotational two dimensional flow model as in previous linear theories the phase lag between sediment transport and bed elevation remains the main mechanism driving instability however it is shown that this phase lag varies significantly in a neighbourhood of the bed moreover since the layer in which sediments are moving has a finite though small thickness it is assumed that the perturbations of the fluid stress driving bedload transport should be evaluated at the top of the layer itself it is shown that such an apparently minor modification of the classical approach alters remarkably the balance between stabilizing and destabilizing effects that drives the instability process this also allows a clarification of the debated role of bedslope on the formation of dunes and antidunes following the above ideas antidunes are shown to form in the absence of suspended sediment load and without any effect associated with sediment inertia the present analysis ultimately allows a successful unification of the theories of dune and antidune formation

potential flow instability theory and alluvial stream bed forms the present work constitutes a reassessment of the role of potential flow analyses in describing alluvial bed instability to facilitate the analyses a new potential flow description of unsteady alluvial flow is presented with arbitrary phase lags between local flow conditions and sediment transport permitted implicitly in the flow model based on the present model the explicit phase lag between local sediment transport rate and local flow conditions adopted for previous potential flow models is shown to be an artificial measure that results in model predictions that are not consistent with observed flow system behaviour previous potential flow models thus do not provide correct descriptions of alluvial flows and the understanding of bed wave mechanics inferred based upon these models needs to be reassessed in contrast to previous potential flow models the present one without the use of an explicit phase lag predicts instability of flow systems of rippled or dune covered equilibrium beds instability is shown to occur at finite growth rates for a range of wavelengths via a resonance mechanism occurring for surface waves and bed waves travelling at the same celerity in addition bed wave speeds are predicted to decrease with increasing wavelength and bed waves are predicted to grow and move at faster rates for flows of larger froude numbers all predictions of the present potential flow model are consistent with observations of physical flow systems based on the predicted unstable wavelengths for a given alluvial flow it is concluded that bed waves are not generated from plane bed conditions by any potential flow instability mechanism the predictions of instability are nevertheless consistent with instances of accelerated wave growth occurring for flow systems of larger finite developing waves potential flow description of alluvial flows should however no longer form the basis of instability analyses describing bed form sand wavelet generation from flat bed conditions

spectral analysis of sand bed instability in order to clarify the role of sand bed instability mechanism and the significance of linear instability analysis which has been one of the most interesting subjects of alluvial hydraulics sand bed instability analysis of a fourier transformed version was devised by coupling with the stochastic model for bed load motion and the theoretical prediction for sand wave development was compared with some experiments in the laboratory sand wave development from an initially flattened bed was experimentally investigated by using the time variation of wave number spectrum of sand bed these investigations clarified that an instability mechanism brought about a wave selection process from initially formed bed disturbance due to random motion of bed load particles and particularly that waves to satisfy the airy relation became dominant through the mechanism of bed instability the latter fact is a characteristic of sand wave development in open channel and it has been also certified by experiments without

bed and flow dynamics leading to sediment wave initiation 1 new piv based experiments show that the nascent seed waves from which both ripples and dunes develop are generated on planar mobile sediment beds in a two stage process the first stage comprises the motion of random sediment patches that reflect the passage of sediment transport events caused by attached eddies these eddy transport events propagate at speeds that are proportional to their size and less than overhead eddy convection velocities but potentially larger than local average fluid and sediment velocities in the second stage interactions of the moving patches result in a bed disturbance that exceeds a critical height and interrupts the bed load layer quasi regular seed waves are then generated successively downstream of this stabilised growing disturbance via a scour deposition wave that arises from the requirement of sediment mass conservation and the sediment transport and bed stress distributions downstream of a bed perturbation seed waves are thereby of preferred lengths that scale with the grain size i e length o 130 grain diameters agreeing with compiled measurements this two stage generation mechanism is valid for fully turbulent hydraulically smooth and rough bed flows of small to large sediment transport rates it is furthermore valid for laminar flows although the critical disturbances leading to seed wave generation arise through bed discontinuities and not eddy based sediment transport events the identified generation mechanism which accounts for turbulence effects explains the observed similar scaling of alluvial closed conduit and lightweight sediment seed waves the present measurements highlight further aspects of the flow dynamics preceding seed wave generation including decreases in von karman s constant due to bed mobility near bed eddy convection speeds in excess of local double averaged in time and space streamwise velocities and the validity of the four range spectral scaling model for open channel flows proposed earlier by nikora

paleo climate of the boise area idaho from the last glacial maximum to the present based on groundwater δ2h and δ18o compositions abstract a 30 ka paleo climate record of the boise area idaho usa has been delineated using groundwater stable isotopic compositions groundwater ages are modern cold batholith 5 15 ka thermal batholith 10 20 ka frontal fault and 20 30 ka snake river plain thermal the stable isotopic composition of groundwaters have been used as a surrogate for the stable isotopic composition of precipitation using 2h and 18o compositions local groundwater lines lgwl s were defined for each system each lgwl has been evaluated with defined slopes of 6 94 and 8 respectively and resulting deuterium excess values d were found for each groundwater system for each slope time dependent changes in moisture source humidity and temperature and boise area recharge temperatures calculated from stable isotopic data and the deuterium excess factors agree with previous paleo climate studies results indicate that from the last glacial maximum to the present time the humidity over the ocean moisture source increased by 9 sea surface temperature at the moisture source increased 6 7 c and local boise temperature increased by 4 5 c a greater increase of temperature at the moisture source as compared to the boise area may impart be due to a shift in the moisture source area

closed conduit bed form initiation and development nonintrusive measurement of closed conduit erodible bed development was undertaken for 12 experiments of ranges of flow strengths and sediment solids sizes analogous to open channel flows wavelets on the sediment bed of a closed conduit are instigated by discontinuities in the bed with wavelet lengths for laminar and turbulent open channel and closed conduit flows given by 175d0 75 where and sediment size d are in millimeters for closed conduit flows ripples and dunes grow from these wavelets at rates increasing with increasing flow strength and utilizing the mechanisms of bed form coalescence and throughpassing to limiting lengths heights steepnesses and bed friction factors that are approximately maintained or possibly decrease thereafter limitation of free surface deformation results in increased rates of bed wave development for closed conduit flows in comparison to open channel flows measured results indicate that equilibrium closed conduit ripple and dune magnitudes can be pre

direct numerical simulations of ripples in an oscillatory flow sea ripples are small scale bedforms which originate from the interaction of an oscillatory flow with an erodible sand bed the phenomenon of sea ripple formation is investigated by means of direct numerical simulation in which the sediment bed is represented by a large number of fully resolved spherical grains i e the flow around each individual particle is accounted for two sets of parameter values differing in the amplitude and frequency of fluid oscillations among other quantities are adopted which are motivated by laboratory experiments on the formation of laminar rolling grain ripples the knowledge of the origin of ripples is presently enriched by insights and by providing fluid and sediment related quantities that are difficult to obtain in the laboratory e g particle forces statistics of particle motion bed shear stress in particular detailed analysis of flow and sediment bed evolution has confirmed that ripple wavelength is determined by the action of steady recirculating cells which tend to accumulate sediment grains into ripple crests the ripple amplitude is observed to grow exponentially consistent with established linear stability analysis theories particles at the bed surface exhibit two kinds of motion depending on their position with respect to the recirculating cells particles at ripple crests are significantly faster and show larger excursions than those lying in ripple troughs in analogy with the segregation phenomenon of polydisperse sediments the non uniform distribution of the velocity field promotes the formation of ripples the wider the gap between the excursion of fast and slow particles the larger the resulting growth rate of the ripples finally it is revealed that in the absence of turbulence the sediment flow rate is driven by both the bed shear stress and the wave induced pressure gradient the dominance of each depending on the phase of the oscillation period in phases of maximum bed shear stress the sediment flow rate correlates more with the shields number while the pressure gradient tends to drive sediment bed motion during phases of minimum bed shear stress

flow turbulence over fixed and weakly mobile gravel beds characteristics of turbulence structure in quasi 2d flows with static and weakly mobile gravel beds are presented three sets of measurements with acoustic doppler velocimeters in an irrigation canal were used two with subcritical bed shear stress static beds and one with the bed shear stress o close to critical oc weakly mobile bed the analyses included vertical distributions of local mean velocities turbulence intensities turbulent shear stresses velocity auto and cross spectra the quadrant method and high order velocity moments a number of properties of turbulence intensities high order moments streamwise bursting parameters and velocity spectra appeared to be similar for all three flows but some properties were different the most important one was an observed reduction in the von karman constant for the flow with weakly mobile bed comparison of these results with other studies and analogies with drag reducing flows suggest that at o oc 1 the drag on the bed for a given granula

flume experiments on lower flow regime bed forms in coarse sand thirty five runs were made in a recirculating channel 11 5 m long and 0 92 m wide with four sizes of very well sorted coarse sand to study geometry migration and hydraulics of bed configurations in equilibrium with steady uniform open channel flows at flow depths of about 15 cm equilibrium energy slope and sediment transport rate were measured as well bed phases kinds of bed configurations studied were ripples low flow velocity flat beds with sand transport and dunes identical with megaripples current velocities in none of the runs were high enough for development of an upper flow regime flat bed a plot of mean flow velocity versus sand size for all the runs shows that at low flow velocities the ripple field narrows with increasing sand size and pinches out at about 0 7 mm the field for flatbed transport narrows with decreasing sand size and pinches out at about 0 5 mm just above the ripple field at high flow velocities dunes are the stable phase for all sand sizes studied dune configurations are divided into two subphases two dimensional dunes 2d dunes with fairly straight continuous even crests and no strong localized scour in troughs are formed at relatively low flow velocities three dimensional dunes 3d dunes with strongly sinuous discontinuous uneven crests and strong localized scour pits in troughs are formed at relatively high flow velocities 3d dunes tend to be higher than 2d dunes have larger height spacing ratios and show less variability in height spacing and migration rate dunes can be viewed as kinematic shock waves differences between 2d dunes and 3d dunes lie in the differing importance of shock wave coupling and of sand transport in bed form troughs in both geometry and hydraulic ralationships the 2d dunes and 3d dunes in our flume runs correspond to the relatively regular megaripples at low flow velocities and relatively irregular megaripples at high flow velocities described by other authors from intertidal environments

bed form initiation from a flat sand bed 1 bed form initiation in unidirectional flow is examined on a flat bed composed of a homogeneous 0 5 mm sand velocity profiles taken prior to bed form development indicate that the examined flows are typical of fully turbulent uniform open channel flows under these conditions two separate modes of bed form initiation are observed defect and instantaneous initiation defect initiation occurs at lower flow stages where sediment transport is sporadic and patchy and is characterized by defect propagation associated with flow separation instantaneous initiation occurs at larger flow strengths where sediment transport is general and widespread this form of bed form initiation begins with the imprinting of a cross hatch pattern on the flat sediment bed which leads to chevron shaped forms that migrate independently of the initial pattern the chevrons eventually align to form incipient crest lines this mode of bed form initiation does not appear to be linked to turbulent structures but integral scales derived from velocity measurements prior to bed form development are similar to the initial bed form length scales

comparison of precipitation collectors used in isotope hydrology abstract many hydrologic studies require data on the oxygen and hydrogen stable isotope composition 18o 2h of precipitation and various collector designs have been suggested for gathering corresponding samples yet it is crucial that these collectors also known as totalizers prevent evaporation and associated isotope fractionation surprisingly we were unable to find a comprehensive collector intercomparison in the literature and much less one that addressed hot and arid conditions in this study we tested six different collector designs over a period of 32 days they were filled to 20 of their total volume with water of known isotope composition and placed in a modified laboratory drying oven with a low relative humidity 5 and a diurnal temperature change of 26 to 45 c evaporative mass losses were determined gravimetrically daily while samples for isotope analyses were collected every four days the classic oil collector featuring a layer of paraffin oil to prevent evaporation showed the smallest mass losses and no detectable isotope shift the tube dip in water collector with pressure equilibration tube also performed well although we noted somewhat larger mass losses and isotope shifts the latter accounted for 0 28 18o and 0 9 2h after 32 days which is significant but in view of the extreme conditions these changes should be still acceptable for most studies the remaining collectors ball in funnel floating balls and float based all failed under the prevailing conditions the tube dip in water collector with pressure equilibration tube seems to represent a good oil and thus contamination free compromise nonetheless also in this system small precipitation amounts are problematic this may necessitate modifications of the standard design in terms of funnel diameter and bottle geometry we strongly advise researchers to conduct own tests with their intended collector before field deployment

a model for flow over two dimensional bed forms when non cohesive sediment is set in motion by a unidirectional flow waves of sand often result moreover these waves typically are asymmetrical with steep lee faces that produce flow separation behind each wave a wake region forms which grows in height and decays in strength with distance downstream producing a near bottom acceleration the no slip condition at the bed however requires an internal boundary layer to form beneath the wake region which retards the flow there the interaction of these flow processes produces a local maximum in the boundary shear stress downstream of a bump even over an otherwise flat bed because the erosion rate is proportional to the stress divergence for bed load and weak suspended load transport erosion will occur upstream of this point and desposition will occur downstream of it thus influencing the bottom shape herein the equations of motion are solved for the accelerating internal boundary layer beneath a wake the resulting velocity and boundary shear str

ripple and dune formation in rivers a linear stability analysis for dune and ripple formation is presented that implements a rotational two dimensional flow model valid in the smooth as well as in the transitional and rough flow regimes sediment is assumed to be transported as bedload disregarding the role of suspension therefore the main mechanism driving instability for both ripples and dunes is the phase lag between bed shear stress and bed elevation ripples are shown to be confined to relatively low values of the shields parameter and of the particle reynolds number for higher values of the shields parameter and of the particle reynolds number and thus of the froude number and of the roughness reynolds number ripples are replaced by dunes the present analysis ultimately allows for a successful unification of the theories of dune and ripple formation and for a clarification of the debated role of ripples on the formation of dunes a good agreement between predicted and observed wavelengths for both ripples and dunes is found

sediment dynamics part 2 dune formation in pipe flow we present a phase diagram of the different dune patterns observed when a bed composed of spherical particles is subjected to a pipe flow while the threshold for incipient motion is determined by the shields number that for dune formation seems to be controlled by the reynolds number a simple linear stability analysis based on a particle flux derived by ouriemi aussillous guazzelli j fluid mech 2009 accounts reasonably well for the experimental observations

late glacial holocene atmospheric circulation and precipitation in the northeast united states inferred from modern calibrated stable oxygen and carbon isotopes as global climate changes because of anthropogenic influences it has become critical to better understand past climate and its various forcing mechanisms as a baseline for future comparison to this end we present a continental isotopic record from an 11 2 m long wetland piston core sampled at 10 50 yr resolution the core was taken in the heavily populated economically vibrant northeastern united states adjacent to fayetteville green lake and spans 14 600 3200 cal yr b p we use a historically based correlation between d 18 ocalcite obtained from individual varves in a box core from fayetteville green lake and winter atmospheric circulation over the northeast united states to examine the way in which changes in winter circulation have influenced d 18 o in precipitation from 14 600 to 3200 cal yr b p our correlation analysis suggests that in periods during which the circumpolar westerlies are expanded storms track more frequently from the gulf of mexico region delivering precipitation with relatively high d 18 o values to the study site by contrast contracted westerlies result in more frequent low d 18 oprecipitation cross continental storms by using this relationship we model winter vortex latitudes over the northeast united states for the prehistoric oxygen isotope record focusing on millennial scale

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

shape and dimensions of stationary dunes in rivers the shape and dimensions length height of sand dunes in rivers are calculated by use of the measured bed shear stress distribution downstream a rearward facing step the transport of sediment is split up into bed load and suspended load which makes it possible to explain the transition to plane bed the model can predict flow resistance curves in alluvial streams

on interfacial instability as a cause of transverse subcritical bed forms 1 recent experimental results suggest there are at least two distinct bed form initiation processes bed forms may be generated from local bed defects that are propagated down and across stream by flow separation processes when sediment transport is patchy and sporadic alternatively bed form development may occur over the whole bed at once when sediment transport is general and widespread herein we critically test a simple model for this latter bed form initiation mode that was originally presented by h k liu in 1957 but not tested because of the complex nature of the measurements required the theory is based on the idea that a moving sand bed might be likened to a dense fluid and that a hydrodynamic instability develops at the interface between the sediment transport layer and the near bed fluid leading to the organization of the transport system into laterally extended discrete bed forms our predicted initial bed form lengths are within 10 of the lengths observed in a series of flume experiments suggesting that transverse waveforms on sand beds sheared by unidirectional currents can be initiated as an interfacial hydrodynamic instability of kelvin helmholtz type when the current is sufficiently vigorous to produce general sediment movement that is to create a sediment transport layer that acts as a pseudofluid with density composed of solid and fluid components

fluvial dunes initiation characterization flow structure research into fluvial dunes spans disciplines studies at grain to reach scales and methodological approaches that include theoretical experimental numerical and field investigations despite significant research efforts to date it remains difficult to provide definitive answers to some fundamental questions regarding dunes this paper reviews three notable challenges that remain regarding fluvial dunes namely scale consistent linking of bed morphologies with turbulent flow fields the intriguing question of what causes trains of highly ordered sediment waves to form in beds of river sediments and how to define the important characteristics of a dune covered bed including lengths shapes and their statistical nature in each case the particular challenge is discussed and then recent research and ways forward are presented copyright 2010 john wiley sons ltd

prediction of thermal conductivity of rocks by soft computing the transfer of energy between two adjacent parts of rock mainly depends on its thermal conductivity knowledge of the thermal conductivity of rocks is necessary for the calculation of heat flow or for the longtime modeling of geothermal resources in recent years considerable effort has been made to develop artificial intelligence techniques to determine these properties present study supports the application of artificial neural network ann in the study of thermal conductivity along with other intrinsic properties of rock due to its increasing importance in many areas of rock engineering agronomy and geoenvironmental engineering field in this paper an attempt has been made to predict the thermal conductivity tc of rocks by incorporating uniaxial compressive strength density porosity and p wave velocity using artificial neural network ann technique a three layer feed forward back propagation neural network with 4 7 1 architecture was trained and tested using 107 experimental data sets of various rocks twenty new data sets were used for the validation and comparison of the tc by ann multivariate regression analysis mvra has also been done with same data sets of ann ann and mvra results were compared based on coefficient of determination cod and mean absolute error mae between experimental and predicted values of tc it was found that cod between measured and predicted values of tc by ann and mvra were 0 984 and 0 914 respectively whereas mae was 0 0894 and 0 2085 for ann and mvra respectively

the importance of splat events to the spatiotemporal structure of near bed fluid velocity and bed load motion over bed forms laboratory experiments downstream of a backward facing step flow separation reattachment on the lee side of alluvial bedforms is known to produce a complex turbulence field but the spatiotemporal details of the associated patterns of bedload sediment transported remain largely unknown here we report turbulence resolving simultaneous measurements of bedload motion and near bed fluid velocity downstream of a backward facing step in a laboratory flume two synchronized high speed video cameras simultaneously observed bedload motion and the motion of neutrally buoyant particles in a laser light sheet 6 mm above the bed at 250 frames s downstream of a 3 8 cm backward facing step particle imaging velocimetry piv and acoustic doppler velocimetry adv were used to characterize fluid turbulent patterns while manual particle tracking techniques were used to characterize bedload transport octant analysis conducted using adv data coupled with markovian sequence probability analysis highlights differences in the flow near reattachment versus farther downstream near reattachment three distinct flow patterns are apparent farther downstream we see the development of a dominant flow sequence localized intermittent high magnitude transport events are more apparent near flow reattachment these events are composed of streamwise and cross stream fluxes of comparable magnitudes transport pattern and fluid velocity data are consistent with the existence of permeable splat events wherein a volume of fluid moves toward and impinges on the bed sweep causing a radial movement of fluid in all directions around the point of impingement outward interaction this is congruent with flow patterns identified with octant analysis proximal to flow reattachment

carbon isotope stratigraphy and carbonate production during the early middle jurassic examples from the umbria marche sabina apennines central italy abstract the carbon isotopic signature of carbonates depends on secular variations of organic carbon and carbonate carbon production burial rates a decrease in carbonate productivity makes the organic carbonate carbon ratio unstable up to the point that even minor variations in the organic carbon reservoirs can provoke carbon isotopic shifts the 13c positive shifts of the middle carixian early pliensbachian and the early bajocian recorded in the umbria marche sabina domain represent a good example of this mechanism both sedimentology and lithostratigraphy of pelagic platform basin carbonate systems in this area show that important changes in the source of carbonates correspond to the observed isotopic shifts the middle carixian event is in fact well correlatable to the drastic reduction of benthic carbonate production on rift related intrabasinal highs which then became pelagic carbonate platforms the early bajocian event is concomitant with the beginning of a long hiatus on the pelagic carbonate platforms and with a drop of the biodiversity of calcareous organisms followed by the onset of biosiliceous sedimentation in basins

thermohydrological conditions and silica redistribution near high level nuclear wastes emplaced in saturated geological formations evaluation of the thermohydrological conditions near high level nuclear waste packages is needed for the design of the waste canister and for overall repository design and performance assessment most available studies in this area have assumed that the hydrologic properties of the host rock are not changed in response to the thermal mechanical or chemical effects caused by waste emplacement however the ramifications of this simplifying assumption have not been substantiated we have studied dissolution and precipitation of silica in liquid saturated hydrothermal flow systems including changes in formation porosity and permeability using numerical simulation we compare predictions of thermohydrological conditions with and without inclusion of silica redistribution effects two cases were studied namely a canister scale problem and a repository wide thermal convection problem and different pore models were employed for the permeable medium fractures with uniform or nonuniform cross sections we find that silica redistribution in water saturated conditions does not have a sizeable effect on host rock and canister temperatures pore pressures or flow velocities

ripples on stream bed some 30 years ago john f kennedy laid the foundations for the description of the geometry of sand waves on stream beds by the potential flow model since then the analysis has been extensively refined and extended but no rigorous description of the small features called ripples exists the present study attempts a stock taking of the data and the various behavioral concepts with the aim of encouraging researchers to improve our knowledge of the physics of features called ripples various trends are identified from the data it is deduced that ripples act as roughness elements of the boundary the ripple troughs are filled with the energy consuming lee vortices the main flow is over the ripple crests and lee vortices and does not interact with the bed geometry the roughness layer of ripples and vortices is within the constant shear layer and is affected by the flow depth only as far as it affects the velocity distribution and alters the shear stress on the bed

modes of stepwise eastward migration of the northern tyrrhenian sea back arc extension evidences from the northern latium offshore italy the structural architecture of a portion of the eastern side of the tyrrhenian sea back arc basin has been defined reinterpreting a public data set made up by seismic reflection profiles a detailed reconstruction of the extensional front migration trough time across the area has been achieved migration has been defined by identifying the generation of unconformities in the graben like basins developed in the area as due to the activation and deactivation of normal faults such phenomena are part of a tectonic process that also involved blocks rotation crustal thinning and stretching syn rift extension affected the area since the early to middle miocene our estimate principally interesting the north westernmost sectors during late miocene and throughout the plio quaternary the extensional front moved stepwise toward the southeastern and eastern sectors widespread postrift conditions established over the whole area since the late pliocene to early pleistocene since the quaternary the locus of active extension further migrated toward the present onshore area where a pronounced volcanic phase developed in this evolutionary framework the rates of observed migration could be correlated with the acceleration of calabrian slab rollback since pliocene furthermore direct evidences of magmatism were not recognized in the offshore although the observed crustal thinning the absence of volcanic bodies in the study area could be related to normal faults which were unable to sufficiently extend the crust in association with lower crust and mantle processes unable to produce melts

mineralogy porosity and fluid control on thermal conductivity of sedimentary rocks summary estimates of thermal conductivity of the main non clay and clay sedimentary minerals are presented especially for the case of argillaceous ones these estimates are averaged estimates based on the interpretation of laboratory conductivity porosity and mineralogy measurements performed on small volumes of the order of 100cm3 of water saturated moist and air saturated samples the sampling is representative of the main sedimentary rocks sandstones carbonates evaporites and shales and it is composed of samples considered as structurally isotropic material first we experimentally verify the first order control of mineralogy porosity and fluid content on bulk conductivity and we demonstrate that such influences may be predicted accurately using a geometric mean model as long as isotropic samples are used then we interpret the data using an inverse method in order to estimate the average mineral conductivities of the main non clay and clay minerals which give the best fit to the individual laboratory measurements through the geometric mean model the analysis is based on measurements on 82 non clay samples and 29 clay samples taken on oil well cores or cuttings on outcrops and on artificially recompacted samples estimates of average mineral conductivities returned by the inversion process for the main non clay minerals are similar to generally admitted values 7 7 w m k for quartz 3 3 w m i k for calcite 5 3 w m l k for dolomite and 6 3 w m k for anhydrite values obtained for the clay minerals are systematically lower than those obtained for non clay ones they are of the order of 1 9 w m k for illite and smectite 2 6 w m k for kaolinite and 3 3 w m l k for chlorite these estimates combined with porosity and mineralogy data are used with the geometric mean model in order to verify its first order validity for predicting thermal conductivity of small volumes of porous isotropic sedimentary rocks the bulk conductivity is predicted with an accuracy of the order of f10 per cent for moist or water saturated samples and of the order of f20 per cent for air saturated samples

bed form development based upon the results of a series of 47 bed development experiments the evolution of bed features subsequent to their initial generation from flat bed conditions is examined for each such experiment bed profiles were measured at frequent time intervals as sand wave configurations developed under the action of open channel water flow the two sediments used for these experiments were of geometric mean size 0 20 mm and 0 82 mm respectively in line with the bed form unification model of bed development postulated by saudkivi and witte in 1990 the principle of bed form propagation speed decreasing with increasing bed form height and the mechanisms of bed form coalescence and bed form throughpassing are all found to be central to bed evolution processes a quantitative relation between bed form speed and bed form height is presented bed development predictions based upon bed form unification models can be seen to clearly parallel the present experimental results stability analyses based upon potential

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

the correlation between 18o 16o ratios of meteoric water and surface temperature its use in investigating terrestrial climate change over geologic time abstract correlations between mean annual temperature mat and the weighted average oxygen isotope composition of yearly precipitation 18opt are well known but the utility of modern relations to make reliable estimates of temperature change over geological time is uncertain this question has been addressed by using seasonal subsets of the global data base of temperature and isotopic measurements to represent two different climate modes a comparison of middle to high latitude 18opt temperature relations for each climate mode reveals 1 a significant offset between them and 2 a difference in the strength of their correlations the offset in relations is due to differences in temperature and water vapor budget in the tropics and can lead to serious underestimates of temperature change differences in the strength of correlations arise from the influence of climate mode specific non temperature factors on 18opt the overall result is that no single relation can be used in all cases to make unambiguous temperature estimates using a temporal record of 18opt values one way to overcome these problems is to reconstruct 18opt temperature relations for the time periods being investigated if an appropriate proxy for 18opt is available it may also be possible to estimate temperature without relying on 18opt temperature relations a promising alternative to these options is to use records of 18opt to test predictions of global climate models an approach that may allow a reliable and more complete reconstruction to be made of climate change over geologic time

reconstruction of rocks petrophysical properties as input data for reservoir modeling the worldwide increasing energy demand triggered studies focused on defining the underground energy potential even in areas previously discharged or neglected nowadays geological gas storage co2 and or ch4 and geothermal energy are considered strategic for low carbon energy development a widespread and safe application of these technologies needs an accurate characterization of the underground in terms of geology hydrogeology geochemistry and geomechanics however during pre feasibility study stage the limited number of available direct measurements of reservoirs and the high costs of reopening closed deep wells must be taken into account the aim of this work is try to overcome these limits proposing a new methodology to reconstruct vertical profiles from surface to reservoir base of i thermal capacity ii thermal conductivity iii porosity and iv permeability through integration of well log information petrographic observations on inland outcropping samples and flow and heat transport modelling as case study to test our procedure we selected a deep structure located in the medium tyrrhenian sea italy obtained results are consistent with measured data confirming the validity of the proposed model notwithstanding intrinsic limitations due to manual calibration of the model with measured data this methodology represents a useful tool for reservoir and geochemical modellers that need to define petrophysical input data for underground modelling before the well reopening this article is protected by copyright all rights reserved

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

the stability of ripples and dunes abstract the stability of ripples and dunes in flows of finite depth are investigated for the linear small amplitude case and also for the more realistic finite amplitude situation for the former perturbation approach is taken leading to the general result that without the inclusion of some lag between the sediment transport rate and the boundary shear stress all wavelengths are unstable this results from an upstream shift in the stress relative to the bedform leading to deposition at the crest by assuming a realistic lag based on the mechanics of bed load transport a fastest growing wave at ripple scales results similarly if suspended sediment were included longer lags result and a dune scale instability could result some investigators have invoked a gravitational effect arguing that sediment transport is enhanced downslope and retarded upslope this creates an effective downstream shift in the transport rate and some argue that both ripple and dune instabilities result however a realistic evaluation of this effect shows that it is much too small to be effective naturally occurring bedforms are of finite amplitude typically they are asymmetrical often causing flow separation this highly non linear process creates a vastly different flow environment from that assumed in the initial stability case a wake like flow structure develops downstream of the separation zone and an internal boundary layer develops beneath the complex interaction of these two different flow regimes as well as the effect induced by the topography of the bedform itself produces a shear stress field that dictates the stability of the feature itself

the north tyrrhenian northern apennines post collisional system constraints for a geodynamic model the north tyrrhenian northern apennines post collisional system ntas has been analysed on the basis of marine reflection profiles and bibliographic sources with the aim of providing an up to date geotectonic and evolutive framework consistent with structural data the continental rifting began to affect the most internal western side of the system in the middle upper miocene while the most external eastern zones were touched by rifting only in pleistocene times the birth and development of extensive asymmetric basins took place with a progressive speeding up of rifting towards the external sectors a diachronous regionally extended and eastward rejuvenating unconformity recognizable within the neogene quaternary basins marks the passage from the syn rift to the post rift tectonic regime while the first is everywhere dominated by extension the second one produces vertical movements reflection terminations related to this unconformity are interpreted in terms of vertical tectonic component three zones that experienced different post rift vertical movements are thus recognized in the ntas subsidence in the north tyrrhenian deep water region tectonic stability or alternation of moderate subsidence and uplift in the continental shelf and western tuscany strong uplift in eastern tuscany the updated structural data demonstrate that the ntas upper crust is crosscut by both tyrrhenian dipping and apennine dipping low angle detachment planes above which the upper crustal blocks rotated and experienced extensional transport along two opposite directions to the west and to the east respectively the coexistence of opposite dipping crustal surfaces bears two fundamental implications for the ntas geodynamics first it stresses the importance of the anti apennine transfer faults since they separate crustal blocks of opposite extension polarity secondly it testifies that the extension tectonics not only re utilized former crustal thrust planes but also gave origin to new significant e and ne dipping detachment planes plan and crossview geometries of faults together with considerations on the role of the metamorphic core complexes point to an interpretation of the ntas structure that is more consistent with the anastomosing shear model rather than the simple shear or the delamination ones

on the temperature correlation of δ18o in modern precipitation reevaluation of modern precipitation temperature and isotope data permits reconciliation of previous disparate values for the correlation between 18o of modern precipitation and surface temperature past analysis has used the mean surface temperature over the time interval of sample collection e g mean weekly monthly or annual temperature to calculate temperature coefficients and different approaches at mid latitudes yield different temperature coefficients 18o t spatial correlations among geographically distinct sites yield 0 55 k seasonal variations at single sites yield 0 2 0 4 k and 12 month running averages yield 0 5 1 k however there are systematic differences in temperature during precipitation events vs time averaged surface temperature means correction for this bias using hourly weather and monthly isotope data from u s sites yields a single value of 0 55 k for all three approaches revised temperature coefficients based on surface observations are also commensurate with coefficients obtained using cloud base temperatures and with theoretical distillation models 0 5 0 7 k these coefficients provide a consistent basis for validation of general circulation models that incorporate stable isotopes of precipitation and for comparison to independent estimators of the isotopic response to climate change

on the entrainment of sediment and initiation of bed defects insights from recent developments within turbulent boundary layer research sediment entrainment and the initiation of bed defects has commonly been ascribed to the impact of high velocity sweeps upon mobile sand beds results of visualization experiments suggest that these sweep impacts are grouped and may define patches of entrainment upon a mobile bed that may be wider than an individual sweep impact additionally the presence of longitudinal ribs of sediment generated by sweep impacts may stabilize the position of sweeps and low speed streaks these observations are interpreted in the light of recent boundary layer research which suggests the formation of multiple hairpin shaped vortices within turbulent flows these are postulated to generate multiple sweeps that manifest themselves as entrainment patches on a mobile bed these features of the turbulent boundary layer and its modification by sweep generated sediment ridges can be used to propose a model for bed defect formation and the subsequent development of current ripples