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

erratum experimental evidence for statistical scaling and intermittency in sediment transport rates journal of geophysical research 2009 114 f01025 doi 10 1029 2007jf000963

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

experimental study of the transport of mixed sand and gravel we measured a wide range of transport rates for five different sand gravel mixtures in a laboratory flume each mixture used the same gravel and sand was added to produce mixtures containing 6 15 21 27 and 34 sand control of other variables allows us to isolate the effect of bed sand content on transport as sand content increases gravel transport rates increase by orders of magnitude even though the proportion of gravel in the bed decreases the increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27 the increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content bed surface grain size was measured at the end of each run surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate this casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate

channel width flow discharge relationships for rills and gullies abstract research conducted during the first half of the last century has shown that a strong power relationship exists between channel width and total flow discharge in streams recent studies have shown that this power relationship can be theoretically derived for bankfull discharge in channels the relationship has been extended empirically to rills and gullies revealing that the discharge exponent for rills and gullies is significantly smaller than that for rivers however water flow in rills and gullies is only rarely bankfull indicating that the theoretical explanation for the power relationship found for rivers does not apply to rills and gullies in order to investigate the width discharge relationships for rills and gullies a new method is proposed based on field measurements of widths of concentrated flow erosion channels both upstream and downstream of channel junctions although the method only allows the determination of the exponent of the power relationship it is easy and inexpensive to apply a total of 322 rill and gully channel junctions with various soils and land use types were investigated in belgium italy and spain the obtained data confirmed the existence of the power relationship for rills and gullies with the exponent varying from 0 43 for small rills about 3 cm in width to 0 5 for gullies about 50 to 100 cm in width the data did not allow deciding whether the exponent varies consistently with channel width or in a step wise fashion the exponent values obtained in this study are larger than those reported in previous studies but this may result from differences in the definition of the discharge that eroded the channel to its current width

interaction between bed roughness and flow hydraulics in eroding rills govers 1992 showed that the flow velocity in rills eroding loose nonlayered materials could be predicted from knowledge of discharge only without significant slope or soil effect the objective of this paper is to investigate to what extent the observed slope independence of flow velocity in eroding rills can be explained by the interaction between rill bed roughness and flow hydraulics in a laboratory study two situations were compared 1 rills which can freely erode a uniform soil layer and 2 rills with a fixed bed geometry during the experiments rill discharge and flow velocity were recorded after each experiment a detailed topographic survey of the rill bed was carried out using a laser scanner from these data the main hydraulic variables mean values of flow depth wetted perimeter and hydraulic radius were estimated the experiments confirmed the slope independence of rill flow velocities on mobile beds when the bed is fixed the flow velocity in rills is clearly slope dependent the slope independence of flow velocity on mobile beds is due to a feedback between rill bed morphology and flow conditions the roughness amplitude which was assessed from the standard deviation of corrected height values and the frequency of macroroughness elements which was assessed by counting the number of peaks per unit length in a previously simplified longitudinal profiles both increase and tend to counteract the effect of the increase in slope gradient on rill flow velocity the final rill flow velocity appears to be characterized by a constant average froude number

a long flume study of the dynamic factors affecting the resistance of a loamy soil to concentrated flow erosion experiments were carried out in a 20 m long flume to assess the variation of the runoff erosion resistance of a loamy soil as a function of initial moisture content and compaction the results of seven experimental runs show that the runoff erosion resistance of a loamy material is extremely sensitive to variations in initial moisture content and to a somewhat lesser extent in bulk density the very low resistance to runoff erosion of initially dry material is explained by structural changes which take place at the time of wetting slaking and microfissuration cause a considerable decrease of the soil s shear strength and therefore of its resistance to runoff erosion a first analysis shows that during a given rainfall event initially dry soils may well show significantly more erosion than initially wet soils despite the increase of infiltration with decreasing initial moisture content

multiscale statistical characterization of migrating bed forms in gravel and sand bed rivers received 14 october 2010 revised 24 october 2011 accepted 25 october 2011 published 20 december 2011 1 migrating bed forms strongly in uence hydraulics transport and habitat in river environments their dynamics are exceedingly complex making it difcult to predict their geometry and their interaction with sediment transport acoustic instrumentation now permits high resolution observations of bed elevation as well as ow velocity we present a space time characterization of bed elevation series in laboratory experiments of sand and gravel transport in a large 84 m long 2 75 m wide ume we use a simpleltering and thresholding methodology to estimate bed form heights and report that the shape of their probability density function pdf remains invariant to discharge for both gravel and sand and has a positive tail slightly thicker than gaussian using a wavelet decomposition we quantify the presence of a rich multiscale statistical structure and estimate the scale dependent celerity of migrating bed forms showing the faster movement of smaller bed forms relative to the larger ones the nonlinear dynamics of gravel and sand bed forms is also examined and the predictability time i e the interval over which one can typically forecast the system is estimated our results demonstrate that ow rate as well as bed sediment composition exert a signicant in uence on the multiscale dynamics and degree of nonlinearity and complexity of bed form evolution

comparison of rill flow velocity regimes between developing and stationary rills abstract flow velocity is fundamental in calculating related parameters of physical soil erosion models it has been well known that rill flow velocity varies with developing rill form which complicates and impedes the development of soil erosion models in filed rainfall erosion frequently occurs with pre existing rills not damaged by tillage practices however the rill flow velocity in this case has received little attention in this study we investigated rill flow velocity regimes for both developing and stationary rills using 5 m length plots in the laboratory the experiments included two consecutive rainfalls which had similar runoff rate the first rainfall had an intensity of 90 mm h 1 which generated well developed rills the second had an intensity of 60 mm h 1 and did not damage the rills formed by the first rainfall the results show that for a developing rill rill flow velocity is higher in the lower slope than that in the upper slope and rill flow velocity in the lower slope fluctuates much more over time as a result of the variation of rill morphology following rill development with rill development the rill flow velocity of the lower slope decreased responding to the stabilizing rill channel and exhibited a similar trend to that in upper slope for a stationary rill the rill flow velocities at both sections varied little over time demonstrating that rill flow velocity is primarily influenced by rill morphology hence the results suggested that the strength of fluctuation in rill flow velocity could be used to characterize the instantaneous intensity of soil erosion and active level of rill development the channel of the stationary rill simply acted as transport pathways of sediment supplied by inter rill erosion the physical models of soil erosion should take into account the micro topography on land surface related to the antecedently formed rills

water worked gravel surfaces high order structure functions at the particle scale 1 high order structure functions are applied at the particle scale for studying water worked gravel surfaces measured at several field sites in new zealand the key findings relate to the shape of the two dimensional structure functions and their scaling behavior the data suggest that at scales comparable to d50 the topography of gravel surfaces is sensitive to the flow direction and exhibits multiscaling behavior we postulate based on our analysis and physical considerations that the high order structure functions may provide information on the orientation and direction of the flow that formed the surface

large eddy simulation of sediment transport currents over ripples a large eddy simulation has been performed to study the detailed sediment dynamics that occur in channel flow over ripples the code solves the navier stokes equations and an advection diffusion equation for the sediment and has been verified to produce quantitatively accurate hydrodynamic results for the flow domains used in this study the main features of interest in this flow are seen because of the high resolution of the grid especially near the ripple boundary this yields a detailed bottom shear stress distribution properly identified coherent structures and resolved sharp gradients in the sediment concentration globally the sediment is carried up into the flow from locations where the shear stress is high on the upslopes of ripple crests and is advected downstream by the current and upward by the vertical velocity component regions of positive vertical velocity are associated with gortler vortices which represent a significant sediment transport mechanism in this flow

response of bed surface patchiness to reductions in sediment supply 1 river beds are often arranged into patches of similar grain size and sorting patches can be distinguished into free patches which are zones of sorted material that move freely such as bed load sheets forced patches which are areas of sorting forced by topographic controls and fixed patches of bed material rendered immobile through localized coarsening that remain fairly persistent through time two sets of flume experiments one using bimodal sand rich sediment and the other using unimodal sand free sediment are used to explore how fixed and free patches respond to stepwise reductions in sediment supply at high sediment supply migrating bed load sheets formed even in unimodal sand free sediment yet grain interactions visibly played a central role in their formation in both sets of experiments reductions in supply led to the development of fixed coarse patches which expanded at the expense of finer more mobile patches narrowing the zone of active bed load transport and leading to the eventual disappearance of migrating bed load sheets reductions in sediment supply decreased the migration rate of bed load sheets and increased the spacing between successive sheets one dimensional morphodynamic models of river channel beds generally are not designed to capture the observed variability but should be capable of capturing the time averaged character of the channel when applied to our experiments a 1 d morphodynamic model rte bookagdegnormgravmixpw xls predicted the bed load flux well but overpredicted slope changes and was unable to predict the substantial variability in bed load flux and load grain size because of the migration of mobile patches our results suggest that 1 the distribution of free and fixed patches is primarily a function of sediment supply 2 the dynamics of bed load sheets are primarily scaled by sediment supply 3 channels with reduced sediment supply may inherently be unable to transport sediment uniformly across their width and 4 cross stream variability in shear stress and grain size can produce potentially large errors in width averaged sediment flux calculations

morphology and flow fields of three dimensional dunes rio parana argentina results from simultaneous multibeam echo sounding and acoustic doppler current profiling 1 most past studies of river dune dynamics have concentrated on two dimensional 2 d bed forms with constant heights and straight crest lines transverse to the flow and their associated turbulent flow structure this morphological simplification imposes inherent limitations on the interpretation and understanding of dune form and flow dynamics in natural channels where dune form is predominantly three dimensional for example studies over 2 d forms neglect the significant influence that lateral flows and secondary circulation may have on the flow structure and thus dune morphology this paper details a field study of a swath of 3 d dunes in the rio parana argentina a large 0 35 km wide 1 2 km long area of dunes was surveyed using a multibeam echo sounder mbes that provided high resolution 3 d detail of the river bed simultaneous with the mbes survey 3 d flow information was obtained with an acoustic doppler current profiler adcp revealing a complicated pattern of dune morphology and associated flow structure within the swath dune three dimensionality appears intimately connected to the morphology of the upstream dune with changes in crest line curvature and crest line bifurcations junctions significantly influencing the downstream dune form dunes with lobe or saddle shaped crest lines were found to have larger more structured regions of vertical velocity with smaller separation zones than more 2 d straight crested dunes these results represent the first integrated study of 3 d dune form and mean flow structure from the field and show several similarities to recent laboratory models of flow over 3 d dunes

coupled dynamics of the co evolution of gravel bed topography flow turbulence and sediment transport in an experimental channel a series of flume experiments were conducted in a large experimental channel at the st anthony falls laboratory to understand the coupled dynamics of flow and bed forms above the sediment water interface simultaneous high resolution measurements of velocity fluctuations bed elevations and sediment flux at the downstream end of the channel were made for a range of discharges the probability density functions pdfs of bed elevation increments and instantaneous reynolds stress reveal a power law tail behavior and a wavelet cross correlation analysis depicts a strong dependence of these series across a range of scales indicating a feedback between bed form dynamics and near bed turbulence these results complement our previous findings in which the signature of bed form evolution on the near bed velocity fluctuations was confirmed via the presence of a spectral gap and two distinct power law scaling regimes in the spectral density of velocity fluctuations we report herein a strong asymmetry in the probability distribution of bed elevation increments and instantaneous reynolds stresses the latter being further analyzed and interpreted via a quadrant analysis of velocity fluctuations in the longitudinal and vertical directions we also report the presence of intermittency multifractality in bed elevation increments and interpret it in view of the asymmetric nature of the pdfs as the result of scale coupling in other words the geometric asymmetry at the bed form scale gets transferred down to a probabilistic asymmetry at all smaller scales indicating a local anisotropy in the energy transfer finally we propose a predictive relationship between bed form averaged sediment transport rates and bed form averaged instantaneous reynolds stress and validate it using our experimental data

unequal mobility of gravel and sand in weakly bimodal river sediments bed load was trapped during flood events over a 20 month period at the lower end of the allt dubhaig a small river in scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile the channel bed near the trap is predominantly gravel with a secondary sand mode total transport in each event depended mainly on peak shear stress rather than duration over a threshold bed load was mainly sand in smaller events bimodal in intermediate events and mainly gravel in the biggest floods mean and maximum grain diameter both increased with peak shear stress but in different ways analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility the slight selectivity in gravel transport is consistent with previous field studies of near equilibrium unimodal beds and supports assumptions made in the numerical model of hoey and ferguson 1994 which successfully simulates the observed amount of downstream fining over the 2 5 km upstream of the bed load trap

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

equal mobility of gravel in streams the remains of the day 1 the concept of equal mobility of gravel transport is 2 decades old the weak form of the hypothesis states that in order to transport the coarse half of the mean annual gravel load through a river reach at a rate equal to the fine half the coarser material must be overrepresented on the bed surface giving rise to a mobile bed armor the degree of mobile bed armoring necessary to accomplish this diminishes with increasing bed shear stress the strong form of the hypothesis states that the grain size distribution of the gravel portion of the bed load should be similar to that of the substrate and finer than that of the surface layer here the hypothesis is subjected to a rigorous test in the laboratory under controlled conditions which are nevertheless chosen to be as representative as possible of actual field conditions the grain size distributions used in the experiment included both model gravel and model sand at the end of the experiment the entire surface layer and a significant depth of substrate were removed from an entire reach of the channel and sieved the results of the experiment confirmed the weak form of equal mobility the surface layer was significantly coarsened compared with the bed load even when the sand was excluded from the size distributions the strong form of equal mobility was also supported by the experiment in that the size distribution of the gravel substrate was nearly identical to the size distribution of the gravel portion of the load the gravel substrate was however slightly but measurably coarser than the gravel load a tendency that has been observed in the field in cases for which the strong form is not satisfied

morphometric and hydraulic geometry assessment of a gully in sw spain abstract gully erosion represents one of the most significant types of land degradation in the mediterranean areas giving place to important on and off site effects in this paper a second order gully located in sw spain is analyzed along the gully 28 cross sections were established and measured with a leica tcrm1102 laser total station approximately every 6 months from 2001 to 2007 the sections were located at variable distance placing them in areas where active erosion was evident in total 13 field measurements were carried out and the geometric characteristics of 28 cross sections were obtained morphometric analyses were carried out in both the main gully and a tributary reach by applying an empirical relationship between channel length and eroded volume morphometric variables of the gully sections were combined into two dimensionless groups and a morphological similarity between different linear erosion landforms rills ephemeral and permanent gullies was obtained then the coefficient of variation of the calculated volumes was used to compare the instability between the main gully and the tributary reach finally the hydraulic geometry of the gully was assessed by calibrating three empirical power equations which relate bankfull discharge with mean flow velocity cross sectional depth and width the hydraulic characterization of the main gully and the tributary reach was investigated for each field survey and a different behavior was detected the hydraulic analysis also demonstrated that higher values of discharge provide better predictions of flow velocity the size of the main and tributary gullies affects the discharge width relationship and that gully depth is the variable which can be predicted with the highest accuracy

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

two approaches to modeling the initiation and development of rills in a man made catchment 1 surface erosion rills are dominant structures in young developing ecosystems and as such they fundamentally affect water flows for example by channeling surface runoff or by locally changing the infiltration capacity of the soil to successfully model the systems hydrology it is indispensable to take the development of erosion rills into account the goal of this study was to model the emergence of an erosion rill network with total length of 6 km formed during the last few years in the 6 ha large artificial catchment chicken creek near cottbus germany the length and connectivity of the evolving network were quantified from aerial images this analysis showed that the major rill network established already during the first year of ecosystem development we adapted two model approaches to simulate the emergence of erosion rills first we used a self organized critical network approach with soil erosion and sediment deposition governed by a local critical shear stress the second approach based on manning s equation was developed to compare the results of the self organized critical network approach with results from a simple but more physically based approach erosion was triggered in the case of high local shear stress and deposition was calculated from settling velocity of the suspended particles both models were able to reproduce the observed rill network geometric characteristics such as network length and rill depth were simulated in the right order of magnitude the models also managed to describe the position and temporal evolution of the main erosion rills in the catchment

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

vertical sorting and the morphodynamics of bed form dominated rivers an equilibrium sorting model a modeling framework is developed for taking into account the effects of sediment sorting in the morphodynamic modeling of bed form dominated rivers for the case of equilibrium or stationary conditions dominated by bed load transport to this end the blom and parker 2004 framework for sediment continuity is reduced to an equilibrium sorting model the predicted equilibrium sorting profile is mainly determined by the probability density function pdf of bed form trough elevations and by a lee sorting function the pdf of trough elevations needs to be known from either model predictions or measurements a simple formulation for the lee sorting function is suggested yet data on the avalanche mechanism down lee faces of dunes is required so as to improve the function and make it generic the equilibrium sorting model is calibrated and verified using data from flume experiments the agreement between the predicted and measured equilibrium sorting profiles is reasonable although the model does not reproduce an observed coarse top layer in a hydraulic morphodynamic model this equilibrium sorting model may be applied instantaneously if the timescale of large scale morphological changes is much larger than the ones of changes in vertical sorting and dune dimensions

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

soil erosion by surface water flow on a stony semiarid hillslope soil erosion on hillslopes occurs by processes of soil splash from raindrop impacts and sediment entrainment by surface water flows this study investigates the process of soil erosion by surface water flow on a stony soil in a semiarid environment a field experimental method was developed whereby erosion by concentrated flow could be measured in predefined flow areas without disturbing the soil surface the method allowed for measurements in this study of flow erosion at a much wider range of slopes 2 6 to 30 1 per cent and unit discharge rates 0 0007 to 0 007 m2 s 1 than have been previously feasible flow velocities were correlated to discharge and hydraulic radius but not to slope the lack of correlation between velocity and slope might have been due to the greater rock cover on the steeper slopes which caused the surface to be hydraulically rougher and thus counteract the expected effect of slope on flow velocity the detachment data illustrated limitations in applying a linear hydraulic shear stress model over the entire range of the data collected flow detachment rates were better correlated to a power function of either shear stress r2 0 51 or stream power r2 0 59 published in 1999 by john wiley sons ltd

using infrared satellite data to drive a thermo rheological stochastic lava flow emplacement model a method for near real time volcanic hazard assessment 1 we combine two flow simulation models to provide a computationally efficient means of predicting the final dimensions of active lava flows by using a stochastic approach to predict all the downhill paths that a lava flow could possibly follow and a thermo rheological model to terminate these paths once the predicted cooling limited length for the flow has been attained this approach allows for rapid assessment of the area likely to be inundated by lava the 1991 1993 eruption of mount etna is used as a case study lava effusion rate exerts a strong control on the maximum length a lava flow can attain and we demonstrate how the model can be driven by time varying effusion rates obtained from infrared satellite data as such data are obtained in near real time this approach allows flow simulations to be updated in a timely manner in response to changing eruption conditions

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

hydraulics and erosion in eroding rills rills often act as sediment sources and the dominant sediment and water transport mechanism for hillslopes six experiments were conducted on two soils and a uniform sand using three experimental methodologies the results of this study challenge the assumption often used in hydrologic and erosion models that relationships derived for sheet flow or larger channel flow are applicable to actively eroding rills velocity did not vary with slope and reynolds number was not a consistent predictor of hydraulic friction this result was due to interactions of slope gradient flow rate erosion and the formation of rill roughness bed structures and head cuts a relationship for rill flow velocities was proposed stream power was found to be a consistent and appropriate predictor for unit sediment load for the entire data set while other hydraulic variables were not the data for stream power and sediment load fit the form of a logistic curve r2 0 93 which is promising relative to recently proposed erosion models which are based on probabilistic particle threshold theory

a comparative study of the turbulent flow field with and without a pebble cluster in a gravel bed river 1 this paper investigates the processes and spatial patterns of the turbulent flow around a naturally formed in situ pebble cluster and the interactions with large scale coherent flow structures instantaneous high frequency velocities were measured using the simultaneous deployment of four acoustic doppler velocimeters with the pebble cluster and after the cluster was removed results indicate a twofold increase in turbulent kinetic energy over the sampling area when the pebble cluster is present individual vertical component velocity spectra in the wake of the pebble cluster contain a marked peak which decreases in frequency from 1 8 hz near the cluster to 1 0 hz farther downstream this peak is associated with eddies shedding from the shear layer in the lee of the cluster and it agrees well with the values estimated using a strouhal number of 0 18 space time correlations indicate the presence of coherent depth scale macroturbulent flow structures these structures in the intermediate flow region are relatively unaffected by eddies shedding from the pebble cluster the results suggest the effects of the pebble cluster on the surrounding flow dynamics are quite localized and limited to the near bed region

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

relationship between discharge velocity and flow area for rills eroding loose non layered materials a relationship between discharge flow velocity and flow area in rills is established using data from four field and laboratory studies the proposed relationship is shown to predict successfully flow velocities measured in six other studies although slopes range from 0 035 to 0 45 and soil materials range from stony sands over silt loams to vertisols mean flow velocity can be well predicted from discharge alone thus there is no important influence of slope and or soil material characteristics on flow velocities in rills the proposed relationship may be used to improve performance of deterministic flow routing models when applied to rilled catchments furthermore it allows the calculation of unit stream power which has been shown to be related to the transporting capacity of overland flow in terms of slope and discharge

rill hydraulics on a semiarid hillslope southern arizona seventy field experiments were conducted in seven rills located on a semiarid rangeland hillslope underlain by gravelly soils at walnut gulch arizona the rills which are characterized by wide shallow cross sections and gravel covered beds have mean at a station hydraulic geometry exponents of b 0 33 f 0 34 and m 0 33 although the differences between these values and typical values of b 0 30 f 0 40 and m 0 30 for cropland rills are not statistically significant they are thought to be real as cropland rills often have more rectangular cross sections and steeper sides than the rangeland rills under study for rills formed in silty loamy soils govers developed an empirical relation between mean flow velocity and discharge emphasizing the generality of this relation he suggested that it may be used as a simple means of routing runoff through rills he also noted that this relation appeared to be unaffected by either slope or soil materials the present data represent rills underlain by coarser and somewhat more varied gravel rich soils these data do not conform to govers relation and a multiple regression analysis reveals that slope and soil materials either directly or indirectly through bed roughness exert almost as much influence on flow velocity as does discharge three alternative methods are developed for predicting flow velocity in the rills under study all three methods give good results with the largest root mean square deviation being 3 115 cm s 1

the influence of migrating bed forms on the velocity intermittency structure of turbulent flow over a gravel bed 1 modeling turbulent flows at high reynolds number requires solving simplified variants of the navier stokes equations the methods used to close the resulting reynolds averaged or eddy simulation equations usually follow classical theory and at small enough scales postulate universal scaling for turbulence that is independent of the velocity itself this may not be the best way to conceptualize geophysical turbulence turbulent intermittency may be defined in terms of the local roughness of the velocity signal as measured by pointwise holder exponents this study investigates the joint velocity intermittency structure of flow over a gravel bed surface with migrating bed forms we report clear velocity intermittency dependence and quantify its nature above the moving bed form profile our results imply differences in energy transfer close to bed forms at shorter wavelengths than those forced directly hence progress in modeling flows of geophysical relevance may require a reconsideration of the principles on which turbulence closures are based

spectral analysis of turbulent flow and suspended sediment transport over fixed dunes laboratory measurements of turbulent fluctuations in velocity and suspended sediment concentration were obtained synchronously over fixed two dimensional dunes in a sediment starved flow contour maps of turbulent flow parameters demonstrate that the flow separation cell and a perturbed shear layer are the main sources of turbulence production and that the distribution of suspended sediment is controlled by spatially dependent macro turbulent flow structures spectral analysis reveals that peak spectral energies generally occur at 1 2 hz for the stream wise velocity component and 2 4 hz for the cross stream and vertical velocity components spectra show larger and better defined energy peaks near the shear layer peak spectral energies for suspended sediment concentration occur near 1 hz throughout the flow squared coherency values for cospectral analysis of velocity and sediment concentration are insignificant integral timescales for velocity range from 0 20 s for the streamwise component to 0 06 s for the cross stream and vertical components integral length scales for velocity range from 0 065 to 0 135 m for the streamwise component which is comparable to flow depth and from 0 020 to 0 030 m for the crossstream and vertical components which is comparable to dune height for suspended sediment concentration integral timescales and length scales are similar to the streamwise velocity component

size shape and dynamics of large scale turbulent flow structures in a gravel bed river in this paper we present a detailed investigation of the size scale and dynamics of macro turbulent flow structures in gravel bed rivers we used an array of seven electromagnetic current meters with high resolution in both space and time to measure the streamwise velocity fluctuations in a gravel bed river the array was deployed successively in various configurations in order to quantify the vertical lateral and longitudinal extent of the flow structures and to estimate their advecting velocities to depict the spatial and temporal properties of the flow structures we used space time velocity matrices space time correlation analysis and coherent structure detection schemes the results show that the large scale turbulent flow structures in a gravel bed river occupy the entire depth of the flow and that they are elongated and narrow the length of the structures is 3 to 5 times the flow depth while the width is between 0 5 and 1 times flow depth in spite of the high roughness of the bed these values are similar to those reported in the literature for laboratory experiments on large scale turbulent flow structures the dynamics of the large scale turbulent flow structures investigated using flow visualization highlight the interactions between the outer flow region and the near bed region our evidence suggests that large scale flow incursions trigger ejections in the near bed region that can develop into megabursts that can reach the water surface

landscape reorganization under changing climatic forcing results from an experimental landscape understanding how landscapes respond to climate dynamics in terms of macroscale average topographic features and microscale landform reorganization is of interest both for deciphering past climates from today s landscapes and for predicting future landscapes in view of recent climatic trends although several studies have addressed macro scale response only a few have focused on quantifying smaller scale basin reorganization to that goal a series of controlled laboratory experiments were conducted where a self organized complete drainage network emerged under constant precipitation and uplift dynamics once steady state was achieved the landscape was subjected to a fivefold increase in precipitation transient state throughout the evolution high resolution spatiotemporal topographic data in the form of digital elevation models were collected the steady state landscape was shown to possess three distinct geomorphic regimes unchannelized hillslopes debris dominated channels and fluvially dominated channels during transient state landscape reorganization was observed to be driven by hillslopes via accelerated erosion ridge lowering channel widening and reduction of basin relief as opposed to channel base level reduction quantitative metrics on which these conclusions were based included slope area curve correlation analysis of spatial and temporal elevation increments and wavelet spectral analysis of the evolving landscapes our results highlight that landscape reorganization in response to increased precipitation seems to follow an arrow of scale major elevation change initiates at the hillslope scale driving erosional regime change at intermediate scales and further cascading to geomorphic changes at the channel scale as time evolves

a generalized richards equation for surface subsurface flow modelling summary an approach for modelling surface subsurface flow and transport in a fully integrated way is presented the diffusive wave approximation used to model runoff is formulated as a nonlinear darcy law and coupled with the richards equation for variably saturated flow the water dynamics in the three physical domains land surface vadose zone and saturated zone is described through a single richards type equation with domain dependent parameters this approach naturally provides pressure and fluxes continuity along land surface an advective diffusive transport equation is incorporated in the model in order to follow and identify pre event and event water this multi domain richards equation is solved with a mixed hybrid finite element formulation the time discretization is implicit and the nonlinear equations are solved within a sequential iterative picard scheme this model can describe both infiltration excess or saturation excess runoff in order to assess this modelling approach several classical validation verification and application test cases are presented for overland flow solely the model is compared to an analytical solution and to commonly used hydrological models the integrated model is then validated with a sandbox laboratory experiment finally the dynamic response of a saturated contributing area at the toe of a 3d virtual hillslope is simulated to illustrate the applicability of the model to 3d situations

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

turbulent flow and drag over fixed two and three dimensional dunes 1 detailed measurements of turbulent flow were obtained over a fixed flat bed two dimensional 2 d dunes and four types of three dimensional 3 d dune morphologies including 1 full width saddles 2 full width lobes 3 sinuous crests and 4 irregular shaped crests the time averaged turbulence structure over the fixed flat bed was dynamically similar to flow over a flat bed with active sediment transport the flow field over 2 d dunes conforms with previous observations of flow over mobile and fixed bed forms bed form three dimensionality significantly altered the flow field observed over the 2 d dunes lobe shaped dune crest lines curved crests with topographic highs that bowed downstream enhanced the level of turbulence producing a better defined wake structure and more vigorous mixing in the separation cell than observed over 2 d dunes saddle shaped dune crest lines curved crests with topographic highs that bowed upstream diminished the level of turbulence suppressing a well defined wake structure and mixing in the flow separation cell flow over sinuous crested bed forms with lobes and saddles was dynamically similar to flow over full width bed forms development of in line paired saddles and lobes in the downstream direction appears to be controlled by near bed velocity gradients which were larger over saddles and smaller over lobes these gradients control local boundary shear stress patterns that should promote scour and high transport rates over saddles and deposition on lobes two dimensional and 3 d bed forms with the same height and length offered different levels of flow resistance the flow observations support the idea that saddles decrease flow resistance and lobes increase flow resistance relative to a 2 d dune bed form crests composed of irregularly spaced lobes and saddles suppressed turbulent flow structure development and reduced drag by 20 below levels for 2 d or sinuous crested dunes the results suggest that bed form crest shape needs to be accounted for in flow resistance calculations and that a single drag coefficient is inadequate where the bed can evolve through 2 d 3 d and irregular 3 d bed form states

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