experimental study of incipient motion in mixed size sediment transport rates of five sediments were measured in a laboratory flume three of these sediments had the same mean size the same size distribution shape and different values of grain size distribution standard deviation the critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate the sorting of the sediment mixture had little effect on the critical shear stress of individual fractions once the median size d50 of the mixture and a fraction s relative size di d50 are accounted for our data combined with previously published data show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction s relative grain size despite a broad variation in the available data of mixture sorting grain size distribution shape mean grain size and grain shape all fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions this result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment recirculating systems as well as for systems where the fractional transport rates are imposed on the system feed systems this equivalence in initial motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination

assessment of bedload equations using data obtained with tracers in two coarse bed mountain streams narcea river basin nw spain abstract this paper evaluates the predictive power of nine bedload equations comparing the results provided by the equations with the bedload rates obtained in a previous field based tracer experiment accomplished in river piguena and river coto two coarse bed streams from nw spain rivers from nw spain draining the northern watershed of the cantabrian mountain range flow into the bay of biscay in a short path 50 60 km in this region they are developed forested catchments featured by fluvial networks with relatively steep slopes single thread sinuous channels and where bed sediment is typically coarse cobble and gravel tagged stones were used to trace bed sediment movement during flood events in river piguena and river coto the two main tributaries of the narcea river basin with the tracer results bedload transport rates between 0 2 and 4 0 kg s were estimated for six flood episodes the tracer based bedload discharges were compared with the bedload rates estimated with the bedload formulae duboys straub schoklitsch meyer peter muller bagbold einstein parker klingeman mclean parker klingeman parker and wilcock crowe our assessment shows that all of the bedload equations tend to overestimate when compared with the tracer based results with the wilcock and crowe 2003 equation the only exception in river piguena we linked these results to the particular geomorphology of coarse bed rivers in humid and forested mountain environments within these rivers armored textures and structural arrangements in the bed are ubiquitous these features together with a low sediment supply coming from upstream forested reaches define a supply limited condition for these channels limiting the potential use of bedload equations the wilcock and crowe 2003 equation introduces complex corrections into the hiding function and this could explain why it performs better

glacially conditioned specific stream powers in low relief river catchments of the southern laurentian great lakes abstract fluvial systems of the southern laurentian great lakes region are carved into a complex glacial landscape shaped by continental ice and meltwater of the late pleistocene these glacially conditioned river catchments are typically small with drainage areas 4 km 2 a 10 m digital elevation model dem is used to map the spatial distribution of stream gradient for 22 major river catchments of peninsular southern ontario which drain to base levels in the lower great lakes huron st clair erie and ontario raw data from the dem show stream gradients that exhibit multiscale variance from real and from artifact sources based on a vertical slice and multiple pass moving window averaging approach slope data are generalised to the river reach scale 1 2 km as a representative spatial scale for fluvial processes operating over holocene timescales models of specific stream power are then compared with glacial landform and surface geology mapping inherited glacial signatures in river slope appear as deviations in a stream length gradient index sl k index where river reaches are frequently oversteepened or understeepened based on a slope area analysis and complementary to theories of channel pattern discrimination constant stream power curves with power law exponent of 0 4 provide a first order approach to stratify river reaches in terms of glacial conditioning and expected planform morphologies however multiple channel planform types are rare and localised in southern ontario indicating that oversteepened reaches with high stream powers may often be moderated by 1 sediment calibre with cobble beds from inherited glacial sediments and or 2 relative bank strength with limited channel widening particularly in gravel and sand bed channels further discrimination of glacially conditioned fluvial process domains will ultimately require consideration of alluvial floodplain characteristics in addition to general observations of river morphology and channel pattern

long term bedload mobility in gravel bed rivers using iron slag as a tracer abstract bedload dispersion is evaluated in gravel bed rivers using slag elements from ironworks established along rivers in the ardenne region between the fourteenth and the nineteenth centuries large quantities of slag were dumped close to these rivers or even directly into the channels for centuries slag elements were dispersed in the bedload and transported by floods of varying importance consequently slag may be considered as a reliable tracer to analyze bedload dispersion over several centuries the size of slag elements was studied along 16 ardenne rivers the longitudinal size trend of the largest slag particles allows the effective competence of these rivers to be determined between 19 and 129 mm for rivers where specific stream power for the bankfull discharge ranges between 20 and 134 w m a direct relationship doesn t exist between these two parameters as the size of slag elements must be considered with regard to the d 50 of the bed selective transport was analyzed directly downstream of the input sites the sorting distance varies from river to river and depends on the velocity of the coarse elements introduced into the river since the inception of the iron industry downstream of two metallurgic sites the slag propagation fronts were located as the periods of activity at these sites are known from historical studies the virtual velocity of bedload movement in these rivers was estimated to be 2 4 km century

river channel slope flow resistance and gravel entrainment thresholds 1 river beds are traditionally assumed to become mobile at a fixed value of nondimensional shear stress but several flume and field studies have found that the critical value is higher in steep shallow flows explanations for this have been proposed in terms of the force balance on individual grains the trend can also be understood in bulk flow terms if total flow resistance has base and additional components the latter due to protruding immobile grains as well as any bedforms and the stress corresponding to additional resistance is not available for grain movement in threshold conditions a quantitative model based on these assumptions predicts that critical shields stress increases with slope critical stream power is near invariant with slope and each has a secondary dependence on bed sorting the proposed slope dependence is similar to what force balance models predict and consistent with flume data and most field data possible explanations are considered for the inability of this and other models to match the very low critical values of width averaged stress and power reported for some low gradient gravel bed rivers

estimating critical stream power for bedload transport calculations in gravel bed rivers the physical basis of bagnold s equation for critical stream power is examined although the general approach is well founded the original equation can be criticised for 1 failing to distinguish between the grain size in transport and the grain size that represents bed roughness 2 requiring a knowledge of critical flow depth in addition to gross channel properties 3 failing to allow for hiding and protrusion effects on the mobility of mixed size stream beds more general equations are presented which overcome these limitations they require a knowledge of channel slope but not depth the analysis suggests that the presence of form resistance in addition to grain roughness does not affect the calculation of critical power critical power is expected to depend more sensitively on grain size between reaches than within reaches

trapping styles and associated hydrocarbon potential in the norwegian north sea exploration has been active in the norwegian north sea for almost 20 years resulting in recoverable reserves that are estimated at 1 5 billion cu m 9 5 billion bbl of oil and 2 5 trillion cu m 88 2 trillion cu ft of gas the 25 fields with reserves of more than 100 million bbl of oil equivalent each together account for 98 of the total recoverable reserves seven fields are giant with more than 1 billion bbl each the distribution of oil and gas fields is intimately linked with the mesozoic rift system of the viking and central grabens that in turn controlled the distribution and maturation of the upper jurassic source rock interval in the norwegian north sea hydrocarbon discoveries to date occur in four forms of traps structural unconformity salt supported compression related and stratigraphic traps end page 585

a crustal scale cross section of the south western alps combining geophysical and geological imagery a geotransect across the south western alps from the pelvoux massif external french alps to the dora maira massif internal italian alps through the monviso ophiolitic complex was investigated in the framework of the geo france 3d alpes programme a new interpretative crustal scale section across the south western alps is proposed combining geological and geophysical 2d 3d data the apulian mantle i e the ivrea body might be divided into two rigid pieces separated by the downward prolongation of the penninic frontal thrust these mantle indenters drive the decoupling of the european crust beneath the high to ultra high pressure metamorphic nappes the deep structure results from the stacking of crustal slices extracted from the european lithosphere the proposed struc tural model provides a basis for discussing the evidence of the crustal scale partitioning of the current strain pattern as well as the location of the seismicity

velocity of debris flow determined by grain composition abstractvelocity is one of the most important parameters of debris flows and it is usually calculated using manning formula in hydraulics mostly ignoring the granular effects due to flow material

modeling bed load rates from fine grain size patches during small floods in a gravel bed river this study investigates the applicability of five bed load transport formulas the meyer peter and muller schoklitsch bagnold smart and jaeggi and rickenmann equations to predict bed load transport rates of frequent low magnitude flood events maximal bankfull discharge for a mountainous poorly sorted gravel bed river characterized by a bimodal sediment size distribution and spatially distributed patches for model parametrization special emphasis was placed on the spatial composition of the grain size distribution gsd to evaluate the impact of preferential removal of sediments from patches with finer sediments on bed load transport three parametrization approaches to the choice of an appropriate sediment size that considered the apparent bimodality of the gsd to varying degrees were tested the modeling study demonstrated that the incorporation of spatial structure of gsd and its bimodal character has an important impact on model performance a unimodal parametrization failed to reproduce meas

comparative geomorphic analysis of surficial deposits at three central appalachian watersheds implications for controls on sediment transport efficiency abstract factors that control the routing and storage of sediments in the appalachian region are poorly understood this study involves a comparative geomorphic analysis of three watersheds underlain by sandstones and shales of the acadian clastic wedge these areas include the fernow experimental forest tucker county west virginia the north fork basin pocahontas county west virginia and the little river basin augusta county virginia gis based analyses of surficial map units allow first order approximation of sediment storage volumes in valley bottoms estimates of volumes are examined in tandem with morphometric analyses and the distribution of bedrock channels to make inferences regarding controls on sediment transport efficiency in the central appalachians the fernow and north fork areas are characterized by v shaped valleys with mixed reaches of alluvial bedrock channels distributed throughout the drainage network in contrast the little river valley is notably wider and gravelly alluvial fill is abundant comparator watershed parameters for the fernow north fork and little river areas include respectively 1 basin area 15 2 km 2 49 3 km 2 41 5 km 2 2 basin relief 0 586 km 0 533 km 0 828 km 3 drainage density 4 2 km 1 3 3 km 1 4 7 km 1 4 ruggedness 2 5 1 7 3 9 5 shreve magnitude 139 287 380 6 total valley bottom area km 2 0 76 km 2 1 86 km 2 3 09 km 2 7 average hillslope gradients 17 2 18 4 22 1 8 total debris fan surface area 0 113 km 2 0 165 km 2 0 486 km 2 and 9 debris fan frequency 2 0 km 2 1 0 km 2 2 8 km 2 the storage volumes in valley bottoms were estimated using map polygon areas and surface heights above channel grade the little river contains significantly higher sediment volumes in floodplain terrace and fan storage compartments total volumes of the valley bottoms are approximately twice that of the fernow and north fork areas combined unit storage volumes for the fernow north fork and little river are 5 2 10 4 m 3 km 2 5 5 10 4 m 3 km 2 and 1 6 10 5 m 3 km 2 respectively a conceptual model postulates that valley width morphometry and style of delivery from hillslopes are the primary factors controlling the efficiency of sediment transport steep debris flow prone hillslopes at the little river deliver high volumes of gravelly sediment at magnitudes greater than transport capacity of the channel patterns of stream power are complex as low order tributaries are under capacity and high order tributaries over capacity with respect to sediment load aggraded alluvial fill insulates valley floor bedrock from vertical erosion and valley widening dominates expansion of the valley width creates a positive response via increased storage capacity and lower unit stream power conversely the fernow and north fork are characterized by diffusive mass movement on hillslopes with incremental bedload transport to higher order tributaries rates of hillslope delivery are balanced by the rate of channel export mixed alluvial bedrock reaches provide the optimal channel configuration for active incision of the valley floor low expansion of valley width promotes high unit stream power and processes of vertical erosion the model implies that the fernow and north fork have been more effective at sediment transport during the late quaternary given similar climatic and tectonic settings variation in bedrock lithofacies is likely the primary factor modulating the efficiency of sediment transport

understanding the influence of slope on the threshold of coarse grain motion revisiting critical stream power abstract this paper investigates the slope dependent variation in critical mean bed shear stress for coarse grain motion and evaluates stream power per unit bed area as an alternative threshold parameter explanations for observed slope dependency and existing approaches for predicting the critical stream power per unit bed area are reviewed an analysis of secondary bed load transport datasets is used to examine the strength of associations between stream power per unit bed area mean bed shear stress and mean velocity with bed load transport rate data from an original flume study are combined with secondary data from similar flume experiments to investigate the effect of slope on both critical stream power per unit bed area and critical mean bed shear stress results suggest that stream power per unit bed area is most closely correlated with bed load transport rate and also that critical stream power per unit bed area is less variable with slope than critical mean bed shear stress alternative solutions to approximating critical stream power are explored these include 1 modifying existing expressions for critical stream power to account for higher critical mean bed shear stresses at higher slopes and 2 applying a constant dimensionless critical stream power criterion

geomorphic effects flood power and channel competence of a catastrophic flood in confined and unconfined reaches of the upper lockyer valley southeast queensland australia flooding is a persistent natural hazard and even modest changes in future climate are believed to lead to large increases in flood magnitude previous studies of extreme floods have reported a range of geomorphic responses from negligible change to catastrophic channel change this paper provides an assessment of the geomorphic effects of a rare high magnitude event that occurred in the lockyer valley southeast queensland in january 2011 the average return interval of the resulting flood was similar to 2000 years in the upper catchment and decreased to similar to 30 years downstream a multitemporal lidar derived dem of difference dod is used to quantify morphological change in two study reaches with contrasting valley settings confined and unconfined differences in geomorphic response between reaches are examined in the context of changes in flood power channel competence and degree of valley confinement using a combination of one dimensional 1 d and two dimensional 2 d hydraulic modelling flood power peaked at 9800 w m 2 along the confined reach and was 2 3 times lower along the unconfined reach results from the dod confirm that the confined reach was net erosional exporting similar to 287 000 m 3 of sediment whilst the unconfined reach was net depositional gaining similar to 209 000 m 3 of sediment 70 of the amount exported from the upstream confined reach the major sources of eroded sediment in the confined reach were within channel benches and macrochannel banks resulting in a significant increase of channel width in the unconfined reach the benches and floodplains were the major loci for deposition whilst the inner channel exhibited minor width increases the presence of high stream power values and resultant high erosion rates within the confined reach is a function of the higher energy gradient of the steeper channel that is associated with knickpoint development dramatic differences in geomorphic responses were observed between the two adjacent reaches of contrasting valley configuration the confined reach experienced large scale erosion and reorganisation of the channel morphology that resulted in significantly different areal representations of the five geomorphic features classified in this study c 2013 elsevier b v all rights reserved

mapping spatial patterns of stream power and channel change along a gravel bed river in northern yellowstone abstract stream power represents the rate of energy expenditure along a river and can be calculated using topographic data acquired via remote sensing or field surveys this study sought to quantitatively relate temporal changes in the form of soda butte creek a gravel bed river in northeastern yellowstone national park to stream power gradients along an 8 km reach aerial photographs from 1994 to 2012 and ground based surveys were used to develop a locational probability map and morphologic sediment budget to assess lateral channel mobility and changes in net sediment flux a drainage area to discharge relationship and dem developed from lidar data were used to obtain the discharge and slope values needed to calculate stream power local and lagged relationships between mean stream power gradient at median peak discharge and volumes of erosion deposition and net sediment flux were quantified via spatial cross correlation analyses similarly autocorrelations of locational probabilities and sediment fluxes were used to examine spatial patterns of sediment sources and sinks energy expended above critical stream power was calculated for each time period to relate the magnitude and duration of peak flows to the total volumetric change in each time increment collectively we refer to these methods as the stream power gradient spg framework the results of this study were compromised by methodological limitations of the spg framework and revealed some complications likely to arise when applying this framework to small wandering gravel bed rivers correlations between stream power gradients and sediment flux were generally weak highlighting the inability of relatively simple statistical approaches to link sub budget cell scale sediment dynamics to larger scale driving forces such as stream power gradients improving the moderate spatial resolution techniques used in this study and acquiring very high resolution data from recently developed methods in fluvial remote sensing could help improve understanding of the spatial organization of stream power sediment transport and channel change in dynamic natural rivers

neogene to ongoing normal faulting in the inner western alps a major evolution of the late alpine tectonics 1 widespread brittle extension occurs in the internal zones of the southwestern alps forming a dense fault network which overprints the compressional structures the extension has been followed by transcurrent motions in terms of paleostress this extension is radial to the belt close to the crustal penninic front and multidirectional in the eastern part of the internal zones the paleostress field of the strike slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults globally we propose that the internal zones of the southwestern alps underwent a single neogene transtensive tectonic regime to strengthen our structural results the current tectonics has been analyzed using seismotectonic tools this approach allowed the recognition of several active faults moreover the ongoing tectonics is coherent with the observed brittle deformation the fault network controls the present day seismicity the combined seismotectonic and structural approaches give coherency and continuity from the neogene to ongoing extensional tectonics in the southwestern alps in the framework of the whole western alps a synthesis of our results with neotectonic related data allowed us to provide an accurate map of the neogene to present kinematics of the belt extension which appears as a major feature of the internal zones has been coeval with the propagation of thrusts in the outer zones during neogene times the geodynamic processes which rule this major tectonic evolution of the belt remain a matter of debate

field based analysis of sediment entrainment in two high gradient streams located in alpine and andine environments abstract this paper presents an analysis of critical thresholds for bedload transport based on field measurements conducted in two small high gradient streams the rio cordon italian alps and the tres arroyos chilean andes the threshold of incipient motion was identified by using marked particles displacement and both flood and flow competence approaches the findings are expressed in terms of shields parameter dimensionless discharge and specific stream power and are used to identify the effects of relative grain size relative depth and bedform resistance overall particle entrainment tends to be size selective rather than exhibiting equal mobility and the high values of dimensionless critical shear stress observed at both study sites confirm the additional roughness effects of step pool morphologies that are very effective in reducing the bed shear stress and causing an apparent increase in critical shear stress

a rational sediment transport scaling relation based on dimensionless stream power the concept of stream channel grade according to which a stream channel reach will adjust its gradient s in order to transport the imposed sediment load having magnitude qb and characteristic grain size db with the available discharge q mackin 1948 geological society of america bulletin59 463 512 lane 1955 american society of civil engineers proceedings81 1 17 is one of the most influential ideas in fluvial geomorphology herein we derive a scaling relation that describes how externally imposed changes in either qb or q can be accommodated by changes in the channel configuration described by the energy gradient mean flow depth characteristic grain size and a parameter describing the effect of bed surface structures on grain entrainment one version of this scaling relation is based on the dimensionless bed material transport parameter w presented by parker and klingeman 1982 water resources research18 1409 1423 an equivalent version is based on a new dimensionless transport parameter e using dimensionless unit stream power this version is nearly identical to the relation based on w except that it is independent of flow resistance both versions of the scaling relation are directly comparable to lane s original relation in order to generate this stream power based scaling relation we derived an empirical transport function relation relating e to dimensionless stream power using data from a wide range of stable bed load dominated channels the form of that transport function is based on the understanding that while grain entrainment is related to the forces acting on the bed described by dimensionless shear stress sediment transport rate is related to the transfer of momentum from the fluid to the bed material described by dimensionless stream power copyright 2010 john wiley sons ltd

critical conditions for particle motion in coarse bed materials of nonuniform size distribution initiation of particle motion in a bed material of nonuniform size distribution may be quantified by qci qcr di dr b where qci is the critical unit discharge at which particle size di enters motion qcr is the critical condition for a reference size dr unaffected by the hiding exposure effects associated with nonuniform size distributions i and r refer to percentiles of the distribution and b varies from 0 equal mobility in entrainment of all particle sizes to 1 5 2 5 full size selective transport currently there is no generally accepted method for predicting the value of b flume and field data are therefore combined to investigate the above relationship thirty seven sets of flume data quantify the relationship between critical unit discharge and particle size for bed materials with uniform size distributions used here to approximate full size selective transport field data quantify the relationship for bed materials of nonuniform size distribution at 24 sites with b ranging from 0 15 to 1 3 intersection of the two relationships clearly demonstrates the hiding exposure effect in some but not all cases dr is close to the median size d50 the exponent has two clusters of values b 1 for sites subject to episodic rain fed floods and data collected by bedload pit trap and tracers and b 1 sites with relatively infrequent bedload transport where particle embedding and consolidation could reduce the mobility of coarser particles and for b 0 7 a looser bed structure with frequent transport events allowing hiding exposure and size selection effects to achieve their balance as yet there is no firm evidence for such a dependency on bed structure but variations in b could potentially be caused by factors outside those determining equal mobility or size selection but appearing to affect b in the same way

characterizing physical habitats in rivers using map derived drivers of fluvial geomorphic processes abstract new understanding of fluvial geomorphological processes has successfully informed flood mitigation strategies and rehabilitation schemes in recent years however well established geomorphological assessments are location specific and demanding in terms of resource and expertise required and their routine application for regional or national river characterization although desirable is unlikely at present this paper proposes a framework based on gis procedures empirical relationships and the self organized map for the analysis and classification of map derived drivers of fluvial morphological processes the geomorphic controls analysed are channel gradient and hydrology specific stream power river order and floodplain extent the case study is a gravel bed river in england using the self organized map we analyse patterns of these controls along the river longitudinal profile and identify clusters of similar configuration the reciprocal relationships that emerge amongst the geomorphic controls reflect the hierarchical nature of fluvial systems and are consistent with the current theoretical understanding of fluvial processes field observations from the river habitat survey are used to prove the influence of geomorphic drivers on reach scale morphological forms six clusters are identified which describe six distinctive channel types these proved to be characterized by distinctive configurations of geomorphic drivers and specific sets of physical habitat features the method successfully characterizes the notable transitions in channel character along the river course the framework is suitable for regional or national scale assessments through automatic gis and statistical procedures with moderate effort

moving boulders in flash floods and estimating flow conditions using boulders in ancient deposits boulders moving in flash floods cause considerable damage and casualties more and bigger boulders move in flash floods than predicted from published theory the interpretation of flow conditions from the size of large particles within flash flood deposits has until now generally assumed that the velocity or discharge is unchanging in time i e flow is steady or changes instantaneously between periods of constant conditions standard practice is to apply theories developed for steady flow conditions to flash floods which are however inherently very unsteady flows this is likely to lead to overestimates of peak flow velocity or discharge flash floods are characterised by extremely rapid variations in flow that generate significant transient forces in addition to the mean flow drag these transient forces generated by rapid velocity changes are generally ignored in published theories but they are briefly so large that they could initiate the motion of boulders this paper develops a theory for the initiation of boulder movement due to the additional impulsive force generated by unsteady flow and discusses the implications keywords

critical specific stream power in gravel bed rivers experiments with marked pebbles were carried out on different sized rivers of the belgian ardenne catchment areas varying from less than 1 km2 to 2700 km2 specific stream power required to cause bedload movement was evaluated and critical values were obtained three types of relationship between critical specific stream power 0 and grain size d were established the values for 0 in the largest river the ourthe were the lowest and were close to the values obtained for mountainous rivers carrying large boulders in medium sized rivers catchment area between 40 and 500 km2 the critical unit stream power was higher it is likely that it is due to the bedform s greater resistance this resistance would use up some of the energy that can cause movement and transport of bedload the amount of resistance of the bedform can be expressed as bedform shear stress determined by the relationship between grain shear stress that determines movement and transport of the bedload and the total shear stress this ratio varies between 0 4 and 0 5 in the medium sized rivers compared to 0 7 in the ourthe in headwater streams less than 20 km2 there is greater loss of energy due to bedform resistance 0 3 critical specific stream power is higher in this third type of river than in the other two

the influence of sediment size relative grain size and channel slope on initiation of sediment motion in boulder bed rivers a lichenometric study sediment transport of four boulder bed rivers is studied using lichenometry the presence of lichens on boulders in the river channel is used to date the last mobilization of the blocks using size frequency diagrams and regional growth curves calibrated with dated reference points it is possible to determine the flood event responsible for the last mobilization of each boulder with lichens present the specific stream power of flood events over the last 60 years is then calculated and thresholds of sediment transport based on the sediment size are calculated the results from the four studied rivers are compared to similar relationships in the literature sediment motion thresholds appear to be very variable within the same type of river mountainous boulder bed rivers the critical specific stream power necessary to mobilize a particle of a given diameter may vary by up to 10 times from one river to the next bed sediment size and river slope may explain this large range of stream powers calculation of the relative size of the transported particles di d50 also shows that both hiding and protrusion effects as well as channels slope are important factors in sediment transport copyright 2010 john wiley sons ltd

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

critical discharge for entrainment of poorly sorted gravel entrainment criteria for gravel sized particles on river beds are usually defined in terms of shear stress but some workers have proposed that critical discharge relationships are more reliable and more convenient the critical shear stress in poorly sorted gravels is known from recent work to depend much more on relative than absolute grain size it is shown that this effect can be included in a semi theoretical equation for the critical unit discharge to move individual grain sizes in a poorly sorted bed comparisons with published data on critical discharges in a variety of rivers show that both the sensitivity of critical discharge to grain size and the absolute levels of critical discharge are predicted quite well however any prediction of critical discharge is sensitive to the assumptions made first about flow resistance and secondly about the critical stress to move average sized particles

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

evaluation of long term bedload virtual velocity in gravel bed rivers ardenne belgium abstract in many gravel bed rivers bed material transfer has been interrupted or perturbed by anthropogenic activities currently restoration projects are being conducted in many countries in order to re establish bedload continuity however until now few studies have provided indications of the velocity of bed material over the long term at least decade to century time scale in the context of river restoration projects e g weir removal addition of spawning gravel these data are nevertheless crucial to predict the downstream propagation of the geomorphological and biological benefits e g supply transport equilibrium morphological and substratum diversity in our study pit tag tracers were used in eight medium sized gravel bed rivers ardenne region belgium to propose a flow competence relationship based on specific stream power on the one hand and to determine the long term virtual velocity of the bed material corresponding to the median diameter d 50 of the surface layer of riffles on the other hand after each flow event that exceeded the threshold for sediment entrainment tagged particles were sought and located even when they were buried in the subsurface layer afterwards all of the data were used to estimate the virtual velocity of the bed material over the long term using three approaches finally the results were compared with long term transport estimations based on iron slag dispersed by the rivers since the end of the middle ages

discussion of understanding the influence of slope on the threshold of coarse grain motion revisiting critical stream power by c parker n j clifford and c r thorne geomorphology volume 126 march 2011 pages 51 65 abstract this paper presents a discussion of parker et al 2011 in particular some theoretical developments are presented that give additional material to apprehend the effects of grain size water depth and slope on the critical shields parameter and critical stream power

selective gravel entrainment and the empirical evaluation of flow competence the concept of flow competence is generally employed to evaluate velocities and bed stresses of river floods from the sizes of the largest sediment particles transported for the most part this evaluation has been empirical combining data from a number of separate flood events in different river systems those data are re examined and compared with empirical equations for the selective entrainment of gravel from deposits of mixed sizes it is found that the competence relationships trend counter to those obtained for selective entrainment indicating that the competence evaluations are affected by varying degrees of selective size entrainment individual data sets which have been employed to establish the flow competence relationships either show no trend on their own or yield a trend which runs counter to the competence equation instead being more compatible with the selective entrainment relationships in most instances the empirical competence equations greatly overestimate the hydraulics of flood flows and it is suggested that the better established selective entrainment equations be used for competence evaluations as well empirical equations are available for this purpose relating the dimensionless shields entrainment function or the bed shear stress to the diameter of the largest grain moved and to the median diameter of the deposit as a whole

unraveling bed slope from relative roughness in initial sediment motion understanding incipient sediment transport is crucial for predicting landscape evolution mitigating flood hazards and restoring riverine habitats observations show that the critical shields stress increases with increasing channel bed slope and proposed explanations for this counterintuitive finding include enhanced form drag from bed forms particle interlocking across the channel width and large bed sediment relative to flow depth relative roughness here we use scaled flume experiments with variable channel widths bed slopes and particle densities to separate these effects which otherwise covary in natural streams the critical shields stress increased with bed slope for both natural gravel s 2 65 g cm 3 and acrylic particles s 1 15 g cm 3 and adjusting channel width had no significant effect however the lighter acrylic particles required a threefold higher critical shields stress for mobilization relative to the natural gravel at a fixed slope which is unexpected because particle density is accounted for directly in the definition of shields stress a comparison with model predictions indicates that changes in local velocity and turbulence associated with increasing relative roughness for lighter materials are responsible for increasing the critical shields stress in our experiments these changes lead to concurrent changes in the hydraulic resistance and a nearly constant critical stream power value at initial motion increased relative roughness can explain much of the observed heightened critical shields stresses and reduced sediment transport rates in steep channels and also may bias paleohydraulic reconstructions in environments with exotic submerged densities such as iron ore pumice or ice clasts on titan

integrating channel form and processes in the gangetic plains rivers implications for geomorphic diversity abstract geomorphic diversity at a variety of spatial and temporal scales has been studied in the western ganga plains wgp india to isolate the dominating factors at each scale that have the potential to cause major geomorphic change the ganga river and its major tributaries draining the wgp have been investigated in terms of longitudinal cross sectional and planform morphology to assess the influence of potential controls such as climate geology topography land use hydrology and sediment transport these data were then compared with those from the rivers draining the eastern ganga plains egp to understand the geomorphic diversity across the ganga plains and the causal factors our investigations suggest that in channel geomorphic diversity over decadal scale in rivers with low width to depth w d ratio is caused by periodic incision aggradation but it is driven by channel avulsion in rivers characterized by high w d ratio similarly planform reach scale parameters such as sinuosity and braid channel ratio are influenced by intrinsic factors such as changes in hydrological conditions and morphodynamics cutoffs small scale avulsion that are in turn impacted by natural and human induced factors finally we have isolated the climatic and hydrologic effects on the longitudinal profile concavity of alluvial trunk channels in tectonically stable and unstable landscapes we demonstrate that the rivers flowing through a tectonically stable landscape are graded in nature where higher discharge tends to create more concave longitudinal profiles compared to those in tectonically unstable landscape at 103 year scale

a fuzzy gis model to determine confluence morphological sensitivity to tributary inputs at the watershed scale abstract confluences are key nodes of river systems that have the potential of disrupting downstream longitudinal trends in the main river through inputs of water sediment wood or ice delivered from tributaries from a geomorphological perspective confluence zones are particularly active and thus susceptible to increased flooding and bed instability which in turn pose serious threats to infrastructure however not all confluences are active despite major advances in our knowledge of confluence dynamics there has been limited progress in the development of low complexity practical tools that use remotely sensed data to predict the spatial distribution of the main river s sensitivity to tributary inputs we have thus developed a novel semi automated gis model that uses a fuzzy approach to integrate key factors unit stream power valley confinement and sediment connectivity potential in order to map the distribution of the confluence morphological sensitivity cms index at the watershed scale the gis model was tested using digital elevation models of high lidar 1 m and coarser 10 m resolution in the coaticook and gaspesie watersheds quebec canada results indicate that the model is useful for detecting geomorphologically active confluences and the resulting sensitivity of the main channel and thus has potential to be used in diverse river management applications e g hazard reduction and freedom space mapping

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

snow cover snowmelt timing and stream power in the wind river range wyoming article i nfo article history earlier onset of springtime weather including earlier snowmelt has been documented in the western united states over at least the last 50 years because the majority n70 of the water supply in the western u s comes from snowmelt analysis of the declining spring snowpack and shrinking glaciers has important implications for the management of streamflow the amount of water in a snowpack influences stream discharge which can also influence erosion and sediment transport by changing stream power or the rate at which a stream can do work such as move sediment and erode the stream bed the focus of this work is the wind river range wrr in west central wyoming ten years of moderate resolution imaging spectro radiometer modis snow cover cloud gap filled cgf map products and 30 years of discharge and meteorological station data are studied streamflow data from streams in wrr drainage basins show lower annual discharge and earlier snowmelt in the decade of the 2000s than in the previous three decades though no trend of either lower streamflow or earlier snowmelt was observed within the decade of the 2000s results show a statistically significant trend at the 95 confidence level or higher of increasing weekly maximum air temperature for three out of the five meteorological stations studied in the decade of the 1970s and also for the 40 year study period as a whole the extent of snow cover percent of basin covered derived from the lowest elevation zone 2500 3000 m of the wrr using modis cgf snow cover maps is strongly correlated with maximum monthly discharge on 30 april where spearman s rank correlation rs 0 89 for the decade of the 2000s we also investigated stream power for bull lake creek above bull lake and found a trend significant at the 90 confidence level toward reduced stream power from 1970 to 2009 observed changes in streamflow and stream power may be related to increasing weekly maximum air temperature measured during the 40 year study period possibly contributing to a reduction in snow cover in addition the strong relationship between percent of basin that was snow covered and maximum monthly streamflow indicates that modis snow cover maps are useful for predicting streamflow and can be used to improve management of water resources in the drought prone western united states published by elsevier b v

sensitivity of a hydraulic model to channel erosion uncertainty during extreme flooding recent research into flood modelling has primarily concentrated on the simulation of inundation flow without considering the influences of channel morphology river channels are often represented by a simplified geometry that is implicitly assumed to remain unchanged during flood simulations however field evidence demonstrates that significant morphological changes can occur during floods to mobilize the boundary sediments despite this the effect of channel morphology on model results has been largely unexplored to address this issue the impact of channel cross section geometry and channel long profile variability on flood dynamics is examined using an ensemble of a 1d 2d hydraulic model lisflood fp of the 1 2000 year recurrence interval floods in cockermouth uk within an uncertainty framework a series of simulated scenarios of channel erosional changes were constructed on the basis of a simple velocity based model of critical entrainment a monte carlo simulation framework was used to quantify the effects of this channel morphology together with variations in the channel and floodplain roughness coefficients grain size characteristics and critical shear stress on measures of flood inundation the results showed that the bed elevation modifications generated by the simplistic equations reflected an approximation of the observed patterns of spatial erosion that enveloped observed erosion depths the effect of uncertainty on channel long profile variability only affected the local flood dynamics and did not significantly affect the friction sensitivity and flood inundation mapping the results imply that hydraulic models generally do not need to account for within event morphodynamic changes of the type and magnitude of event modelled as these have a negligible impact that is smaller than other uncertainties e g boundary conditions instead morphodynamic change needs to happen over a series of events to become large enough to change the hydrodynamics of floods in supply limited gravel bed rivers such as the one used in this research

period and amplitude of bedload pulses in a macro rough channel it is known that bedload fluctuates over time in steep rivers with wide grain size distributions even under conditions of constant sediment feed and water discharge bedload fluctuations which are a consequence of grain sorting are periodic and are related to fluctuations in the flow velocity and channel bed morphology the presence of large relatively immobile boulders such as erratic blocks that are often present inmountain streams has a strong impact on flow conditions and sediment transport however their influence on bedload fluctuations has not been studied sediment transport fluctuations were investigated in this study in a set of 38 laboratory experiments carried out on a steep tilting flume under several conditions of constant sediment andwater discharge for three different slopes s 6 7 9 9 and 13 sediment transport bulkmean flowvelocities and variables describing the channel bed morphologywere measured regularly during the experiments periodic bedload pulses were clearly visible in all of the experiments alongwith flowvelocity and channel bed morphology fluctuations correlation analysis showed that the durations of these cycleswere similar although theywere not necessarily in phase the pulses were characterized by their amplitude and period as a function of various boulder spatial densities and diameters we could showthat for higher streampower the fluctuations decrease in cycle duration and in amplitude the presence of boulders increases the stream power needed to transport a given amount of sediment thus decreasing the bedload fluctuations

counterclockwise rotation of the western alps since the oligocene new insights from paleomagnetic data 1 a paleomagnetic study of penninic units in the southern part of the alpine arc has been carried out more than 200 samples 23 sites were collected in brianconnais ammonitico rosso limestones of the high ubaye valley and the ligurian alps a characteristic component of magnetization of reverse polarity was isolated on most of the sites this component does not pass the fold test and is interpreted as a tertiary overprint related to alpine metamorphism mean directions in geographic coordinates are d 121 i 52 95 11 and d 72 i 48 95 15 for the ubaye and the liguria localities respectively indicating large counterclockwise rotations about vertical axis of 68 and 117 relative to stable europe these rotations in agreement with a previous study conducted in the briancon area together with other paleomagnetic data from the western alps show that the internal alps suffered a large but nonhomogeneous counterclockwise rotation since the oligocene the rotations are in agreement with the combination of earlier separately proposed processes the rotation of the adriatic plate accounts for about 25 of rotation remaining rotation and southward gradient would be related to left lateral shear accommodating the displacement between adria and europe at the southern border of the western alps furthermore the southward extrusion of the western alps south of the simplon fault zone may account for up to 10 of rotation rotations appear therefore as a major process accommodating deformation in the western alps since the oligocene

experiments on surface structure and partial sediment transport on a gravel bed eight flume experiments were conducted to study the development of bed surface texture and structure in the presence of partial bed material transport the experiments have two phases a no feed degradational phase followed by a feeding phase a surface structure of irregular reticulate stone nets and clusters was developed before sediment feeding commenced bed load transport equaled or slightly exceeded the fed supply except at the highest feed rate the bed structure was maintained but bed surface texture fined with increasing sediment load the two phenomena may coexist because the largest grains on the bed moved only very sporadically the actual sediment transport rates were much less than the expected rate calculated from the ratio of bed surface grain size to transported grain size the difference reflects the increase in bed stability introduced by the bed structure between 17 and 47 of the bed shear stress is estimated to be carried by the structure 4 being absorbed by the load while the bed grains carried the balance of the stress bed material transport is exceedingly sensitive to bed surface structure and grain size which raises concerns about the realizable precision of grain size measurements and characterization of the structure

four topic correlation between flood dendrogeomorphological evidence and hydraulic parameters the portaine stream iberian peninsula abstract torrential floods are hazardous hydrological phenomena that produce significant economic damage worldwide flood reconstruction is still problematic in ungauged mountainous areas due to the lack of systematic data so indirect techniques are required this paper presents an integrated palaeoflood study of a pyrenean stream that combines fluvio torrential geomorphology dendrogeomorphology palaeoflood discharge estimation and flow hydraulics the use of a total station and airborne lidar data allows detailed topography for geomorphological mapping and running a one dimensional hydraulic model based on the height of scars on several damaged trees we obtained palaeodischarges of 316 m3 s 1 and 314 m3 s 1 for the 2008 and 2010 floods the hydraulic parameters were related to the geomorphic position of trees showing a positive relation between most energetic geomorphic elements and both flow depth and velocity values the most affected trees are located in intermediate energy geomorphic positions analysing variation in scar height and flow stage differences we suggest that most reliable trees for peak discharge estimation correspond to those in areas related with fluvio torrential processes of intermediate energy this multidisciplinary palaeohydrological study relates flood hydrodynamics with the damage to trees and their geomorphological characteristics focusing on the hydraulic parameters of the peak flow depth velocity and unit stream power which has never been performed before the proposed approach shows strong potential for palaeoflood analysis in ungauged mountain catchments with scarce non systematic data

spatio temporal variability in stream power distribution in the upper kosi river basin central himalaya controls and geomorphic implications abstract river processes in a drainage basin are strongly governed by the spatio temporal variability in stream power recently various approaches have been developed to derive the spatial variability of stream power in a river basin however the temporal variability in stream power due to hydrological changes remains poorly understood an assessment of the temporal variability of stream power is critical for river studies in monsoonal regimes because significant geomorphic work is performed during this period particularly in the himalayan rivers this paper presents a novel methodology to derive the stream power distribution at a basin scale using a physically based hydrological model the soil and water assessment tool swat hydrological changes in the upper kosi also called koshi basin in the central himalaya were analysed and integrated with slope variability and channel width to obtain the basin scale distribution of total stream power and specific stream power most of the tributaries of the kosi river are characterized by a multi peak distribution of stream power stream power distribution in the basin shows no systematic downstream increase or decrease of either total stream power or specific stream power total and specific stream power peaks during the monsoon period are 100 1500 times higher compared to the non monsoon period and exceed the threshold for erosion and sediment transport processes the swat model also helped to assess controls on the stream power distribution in the basin the downstream distribution of stream power between confluence points is influenced by slope variability while major increases at confluence points are due to discharge variability geological characteristics control the spatial distribution of stream power while the annual rainfall distribution influences the seasonal variability of stream power

channel flow competence and sediment transport in upland streams in southeast australia bedload transport data from planebed and step pool reach types are used to determine grain size transport thresholds for selected upland streams in southeast australia morphological differences between the reach types allow the effects of frictional losses from bedforms microtopography and bed packing to be incorporated into the dimensionless critical shear stress value local sediment transport data are also included in a regime model and applied to mountain streams to investigate whether empirical data improve the delineation of reach types on the basis of dimensionless discharge per unit width q and dimensionless bedload transport q b instrumented planebed and step pool sites are not competent to transport surface median grains d50s at bankfull discharge qbf application of a locally parametrized entrainment equation to the full range of reach types in the study area indicates that the majority of cascades cascade pools step pools and planebeds are also not competent at qbf and require a 10 year recurrence interval flood to mobilize their d50s consequently the hydraulic parameters of the regime diagram which assume equilibrium conditions at bankfull are ill suited to these streams and provide a poor basis of channel delineation modifying the diagram to better reflect the dominant transported bedload size equivalent to the d16 of surface sediment made only slight improvements to reach delineation and had greatest effect on the morphologies with smaller surface grain sizes such as forced pool riffles and planebeds likewise the corey shape factor was incorporated into the regime diagram as an objective method for adjusting a base dimensionless critical shear stress c50b to account for lithologically controlled grain shape on bed packing and entrainment however it too provided only minor adjustments to reach type delineation

impacts of permafrost degradation on a stream in taylor valley antarctica abstract the mcmurdo dry valleys mdv of antarctica are an ice free landscape that supports a complex microbially dominated ecosystem despite a severely arid cold environment

downstream fining in large sand bed rivers abstract downstream fining of bed sediments is a well known phenomenon in gravel bed rivers but also occurs in large sand bed rivers the underlying processes however are not necessarily the same due to the difference in sediment mobility grain size distribution width and geographical setting downstream fining studies focusing on large sand bed rivers are scarce nevertheless in this paper the processes that affect downstream fining in gravel bed rivers are reviewed and it is evaluated to what extent they are relevant for large sand bed rivers furthermore several new downstream fining processes are discussed ending in a conceptual model of downstream fining in large sand bed rivers in deriving the conceptual model downstream fining processes are divided into three categories abrasion selective transport and sediment addition extraction abrasion rates in large sand bed rivers are small due to the durable lithologies the small grain size the high degree of rounding the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment only in the upstream part of large sand bed rivers abrasion rates may be somewhat higher selective transport rates in large sand bed rivers everywhere overrun abrasion rates but are small compared to selective transport rates in gravel bed rivers because the unimodal sediments in sand bed rivers exhibit stronger hiding exposure effects during threshold of motion conditions but especially because many sand bed rivers though not all are in state of fully mobilised transport making differences in threshold of motion between coarse and fine grains relatively unimportant selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting which are all processes that act on a relatively large scale within sand bed rivers the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency there are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand bed rivers note that in contrast to abrasion selective transport only produces a stable downstream fining pattern if the river profile is concave generally a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand bed rivers overbank sedimentation for instance may significantly decrease the downstream fining rate in large sand bed rivers because especially fine grains are removed from the bed dredging can also strongly affect the downstream fining trend depending on the volume and the grain size of the sediment involved meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances increasing the downstream fining rate river bifurcations can cause a discontinuity in the downstream fining pattern because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch all these addition and extraction processes not only have a direct effect on the grain size composition of the river bed but also indirectly influence downstream fining by their effect on the longitudinal bed profile other sediment addition and extraction processes such as tributary confluences non alluvial sediment sources and dumping are less common in large sand bed rivers but can locally have a great effect a full determination of the relative importance of all downstream fining processes in large sand bed rivers requires extensive field measurements the construction of detailed fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above

dimensionless critical shear stress in gravel bed rivers this paper first compiles critical shear stress values from 26 studies of gravel bed rivers gbrs worldwide the most frequently proposed value of the shields criterion c is 0 045 but three major groups with c values ranging from 0 100 were identified second dimensionless critical shear stresses the shields criterion were evaluated for 14 gbrs 18 sites with watershed areas ranging from 12 to 3000 km2 different approaches were used to identify the initial movement of the bed material painted and pit tag pebbles sediment traps and bedload samplers the shields criterion c was estimated using the total shear stress and the grain shear stress several shear stresses were also estimated using shear velocities for bedload transport we obtained an average shields criterion c of 0 040 the values were higher in small rivers 0 050 than larger rivers 0 030 because of more significant bedform shear stresses the shields criterion c was lower when the grain shear stress was used and only reached 0 019 different values are also proposed in relation to the type of mobilization the c value for partial transport was 0 025 and exceeded 0 040 for full transport usually reached in association with discharges with a 10 year return period the values based on the results of sediment traps and a bedload sampler were greater than those obtained using tracers but these differences are smaller than those usually reported in the literature

comparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel bed rivers several methods were employed in the ardennian rivers belgium to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events the use of scour chains has shown that the depth of the active layer is systematically less than the b axis of the average particle size d50 of the elements which compose the surface layer of the riffles this indicates that only a partial transport exists during low magnitude floods the bedload discharge has been evaluated by combining data obtained using the scour chains technique and the distance covered by tracers quantities of sediment transported during frequent floods are relatively low 0 02 t km 2 due to the armour layer which protects the subsurface material these low values are also related to the fact that the distance calculated for mobilized bedload only applies to tracers fitted with pit passive integrated transponder tags diameter 20 mm whereas part of the bedload discharge is composed of sand and fine gravel transported over greater distances than the pebbles the break up of the armour layer was observed only once for a decennial discharge during this event the bedload discharge increased considerably 2 t km 2 the use of sediment traps data from dredging and a helley smith sampler confirm the low bedload transport in ardennian rivers in comparison to the bedload transport in other geomorphological contexts this difference is explained by the presence of an armoured layer but also by the imbricated structures of flat bed elements which increase the resistance to the flow finally the use of the old iron industry wastes allowed to quantify the thickness of the bed reworked over the past centuries in the lembree river the river bed contains slag elements up to a depth of about 50 cm indicating that exceptional floods may rework the bed to a considerable depth copyright 2012 john wiley sons ltd

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small