bed material transport estimated from the virtual velocity of sediment this study evaluates the possibility of determining bed material transport using the virtual rate of travel of individual particles dimensions of the active layer of the streambed and porosity and density of streambed material magnetically tagged stones and scour indicators were employed in carnation creek british columbia to quantify transport rates observations cover flows up to 36 m3 s 1 0 081 transport rates ranging from 0 090 to 9 7 kg s 1 0 12 13 2 m3 h 1 display a relatively sensitive trend with maximum stream power as expected error analysis indicates that uncertainty in virtual velocity covers the majority of sample variance an evaluation of the two measurement techniques used to delineate active layer dimensions magnetically tagged stones and scour indicators indicates that they yield comparable depths widths and transport rates over the range of flows observed issues for further study are discussed 1998 john wiley sons ltd

sediment trapping characteristics of a pit trap and the helley smith sampler in a cobble gravel bed river 1 although designed for granule gravels the helley smith hs bed load sampler is often used in streams with much coarser beds to compare magnitudes and grain size distributions of sediment samples collected by a pit trap and a standard hs sampler we conducted 22 sampling events over a wide range of flows during a snowmelt freshet in a stream with a coarse gravel bed each sampling event consisted of three simultaneous measurements one made with a pit trap installed in the bed one with a hs sampler placed on the bed directly beside the pit trap hs i and one with a hs sampler placed on the downstream rim of the pit trap hs p we summed the catches of the pit trap and hs p to estimate actual bed load transport the pit trap and hs i catches were then compared to the summed sample the pit trap yielded a remarkably consistent positively skewed sigmoidal distribution of catch efficiency for all 22 measurements with near 100 efficiency for material larger than 2 8 mm the hs i sampler was more variable in its catch and trapping efficiency exhibiting low trapping efficiency for midrange material 0 71 to 16 mm but high efficiency for finer material the results cast doubt on the accuracy of bed load data sets collected by helley smith samplers in coarse gravel channels

experimental evidence for the simultaneous formation of pseudotachylyte and mylonite in the brittle regime the coexistence of pseudotachylyte a brittle fault rock representing seismic slip and mylonite a plastic or semiplastic fault rock associated with aseismic slip has been debated for its origin in earthquake generating faults deformation experiments on halite gouge at seismic slip rates show that mylonite forms simultaneously with frictional melt pseudotachylyte in the brittle regime equivalent to upper crustal conditions the plastic deformation of halite resulting in mylonite is facilitated by the conduction of heat from a slip concentrated molten layer formed by frictional heating at seismic slip rates thus mylonites which are 1 kinematically consistent and 2 just at the contact with pseudotachylyte might have formed together with the pseudotachylyte in the brittle regime during seismic slip

scour chain employment in gravel bed rivers abstract the dynamics of river beds can be assessed fromscour and fill data obtained from scour chains chains have not been used extensively in gravel bed rivers due to a variety of employment difficulties chain insertion anchoring and relocation can be determined with minimal manpower by utilising a set of tested employment methodologies these are decribed in sufficient detail to allow fluvial geomorphologists ecologists and river engineers to more commonly utilise chains in studies of river stability and river response

grain size and topographical differences between static and mobile armour layers a series of laboratory flume experiments under conditions of sediment starvation zero sediment feeding and recirculation were conducted in order to identify the temporal evolution and surface properties of static and mobile armour layers the experiments were carried out in an 8 m long flume using a bimodal grain size mixture d50 6 2 mm and a range of shear stresses ranging from 4 0 to 8 6 n m 2 the results confirm that a static armour layer is coarser than a mobile one and that the grain size of a mobile armour layer is rather insensitive to changes in the imposed flow strength an analysis of laser scan bed surveys revealed the highly structured and imbricated nature of the static armour layer under these conditions the vertical roughness length scale of the bed diminished and it became topographically less complex at higher forming discharges the topography of mobile armour layers created by rising discharges differed they exhibited a greater roughness length scale and were less organized despite the fact that the grain size of the surface material maintained an approximately constant value during recirculation also the mobile armour tended to create larger cluster structures than static armour layers when formed by higher discharges these differences were mainly due to the transport of the coarser fraction of bed sediments which diminished to zero over the static armour because of being hidden within the bed whereas in the mobile armour the coarser particles protruded into the flow and were actively transported increasing the vertical roughness length scale overall the results show that an examination of the grain size characteristics of armour layers cannot be used to infer sediment mobility and bed roughness detailed elevation models of exposed surfaces of gravel bed rivers are required to provide critical insight on the sediment availability and sedimentation processes copyright 2011 john wiley sons ltd

dynamics of bedload transport in turkey brook a coarse grained alluvial channel automatic and continuously recording samplers are deployed in a hertfordshire gravel bed stream to show that bedload transport is related to stream power the pattern is similar to that already established for north american channels but because the record is so detailed it is possible to identify the cause of the considerable scatter that is normal in such relationships a major factor is the occurrence of rhythmic pulses in bedload discharge that are not matched by similar fluctuations in hydraulic variables it is suggested that these pulses reflect downstream differences in the concentration of mobile particles in a slow moving traction carpet and that they may be likened to kinematic waves the record also reveals that the threshold of sediment transport always presumed hithero to be associated with incipient motion is related to the cessation of bedload transport in a river flood indeed the mean value of stream power at the finish of bedload transport is only 20 percent of that prevailing at the moment of incipient sediment motion because of this there is an inevitably poor correlation between actual bedload transport rates and those predicted by bedload equations which rely upon a single traction threshold these new data show that the general inverse relationship between bedload discharge and water depth grain size ratio proposed by bagnold 1977 1980 is not universal transport efficiency for this gravel bed stream is typically 0 05 per cent of available stream power which compares with 1 6 per cent for a river moving both gravel and sand and 5 per cent for another channel where bedload is composed predominantly of sand sized particles it is argued that coarse and fine grained alluvial channels may need to be considered separately by allowing for differences in traction threshold at the beginning and end of bedload events and by averaging bedload discharge flood by flood in order to smooth out the effect of pulses it is possible to achieve a reasonably good prediction of average bedload transport rate in terms of stream power

marginal bed load transport in a gravel bed stream sagehen creek california marginal bed load transport describes the condition when relatively few bed particles are moving at any time bed particles resting in the shallowest bed pockets will move when the dimensionless shear stress exceeds a value of about 0 020 as increases the number of bed particles moving increases significant motion of bed particles i e when a substantial fraction of the bed particles are moving occurs when exceeds a value of about 0 060 thus marginal bed load transport occurs over the domain 0 020 0 060 marginal bed load transport rates and associated hydraulic characteristics of sagehen creek a small mountain gravel bed stream were measured on 55 days at discharges ranging from slightly less than one half of the bank full discharge to more than 4 times the bank full discharge dimensionless shear stress varied from 0 032 to 0 042 and bed particles as large as the 80th percentile of the bed surface were transported the relation between reference dimensionless shear stress and relative particle protrusion for sagehen creek was determined by varying ri to obtain the best fit of the parker bed load function to the measured transport rates during the period of record water years 1954 1991 the mean annual quantity of bed load transported past the sagehen creek gage was 24 7 tons forty seven percent of all bed load transported during the 38 years of record occurred in just 6 years during 10 of the 38 years of record essentially no bed load was transported the median diameter of bed load was 26 mm compared to 58 mm in the surface bed material

stabilizing self organized structures in gravel bed stream channels field and experimental observations stable reticulate structures which we call stone cells have been observed in cobble gravel channel beds with low bed material transport rates experiments show that such structures develop simultaneously with the armor layer during an extended period when flows do not exceed the shields threshold by more than 2 times so that bed material transport is low they are constructed by particles moving from less stable positions into more stable configurations against each other intermediate developments include clusters and stone lines they reduce sediment transport by orders of magnitude and are evidently a major stability promoting mechanism in gravel channels the timescale for their development suggests that the boundaries of many gravel bed channels are not in equilibrium with recent competent flows but reflect the history of recent dominant flows

bedload entrainment travel lengths step lengths rest periods studied with passive iron magnetic and active radio tracer techniques this paper reports results from bedload transport investigations with active radio and passive iron magnetic tracers in the lainbach a step pool mountain river in bavaria southern germany the spatial distributions of the iron tracers after flood events can be best described by exponential or gamma distributions there is some indication of a tendency of size selective transport of the iron tracers but there is also a considerable amount of scatter in the correlations between weight size and travel length owing to the masking influence of other variables such as the shape of the particles and different positions in the river bed the experiments with artificial magnetic tracers showed that elongated pebbles rods had the longest mean transport distance platy ones discs remained relatively close to the starting points the particles from the pool showed the greatest transport lengths and a 100 per cent chance of being eroded the pebble transmitter system petsy consists of transmitters implanted into individual pebbles a computerized receiver a stationary antenna system with an antenna switchboard and a data logging system the particles do not move continuously but in a series of transport steps and non movement intervals a single value for a given size class is not adequate to describe the critical conditions of entrainment under natural circumstances a probability approach is much more suitable the critical unit discharges total discharge divided by active channel width along the measuring reach are dependent on river bed morphology in the steps bedload needs higher unit discharges to be entrained once entrained the transport of bedload is stochastic in nature and the single particle transport is controlled by the step lengths and the duration of rest periods the distributions of both parameters can be approximated by exponential functions applying the stochastic concept proposed by einstein the mean values of the random variables step length and duration of rest period measured with the petsy technique were used for the simulation of spatial distributions of bedload particles from point sources more field and laboratory data are needed to include varying flow and roughness conditions with tracers representing different particle characteristics

size and distance of travel of unconstrained clasts on a streambed we have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers distance of movement was scaled using the mean distance of movement of the median size group of the surface material size was scaled using the median b axis diameter of the subsurface material the scaled data are fitted with a nonlinear function clasts larger than the median of surface material exhibit a steep decline of mobility with an upper limit for mobility near di d50sub 10 smaller materials exhibit a less sensitive relation we suppose that trapping of small clasts affects their travel distance these results indicate that the movement of free surface stones is size preferred a subordinate analysis of free constrained and buried particles reveals that bed surface structure acts to reduce differential movement

bankfull discharge recurrence interval in gravel bed rivers bankfull discharge was identified in some 30 gravel bed rivers representing in total c 40 gauging stations the catchment sizes vary from 4 km 2 to nearly 2700 km 2 bankfull discharge value increases with basin size in the case of gravel bed rivers developed on an impermeable substratum the following equation emerges q b 0 087 a 1 044 bankfull discharge recurrence interval was determined by fitting maximum annual floods t a into gumbel s distribution and then using the partial duration series t p in this same distribution recurrence interval is below 0 7 years t p for small pebble bed rivers developed on an impermeable substratum it reaches 1 1 to 1 5 years when the catchment size of these rivers exceeds 250 km 2 rivers incised in the soft schists of the famenne show larger channel capacity at bankfull stage a small width depth ratio and thus higher recurrence intervals 1 4 5 3 years with t a and 1 4 4 years with t p baseflow dominated gravel bed streams and sandy or silty rivers experience less frequent bankfull discharges with a recurrence interval higher than 2 or even 3 years t p 1997 by john wiley sons ltd

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

virtual rate and mean distance of travel of individual clasts in gravel bed channels travel distances in gravel bed rivers of tagged particles of various sizes are related to excess stream power estimated for peak discharge mean distance of movement irrespective of grain size is weakly correlated with stream power especially near the threshold of movement there may be several reasons for the weak correlation including variable effects of bed structure varying magnitudes of sediment mobilizing events and sampling problems grain size itself is of marginal significance the virtual rate of travel calculated using total time for which the flow is larger than that needed to initiate clast movement also bears a weak relation to the excess stream power over the period better results are obtained by relating the virtual rate of travel to the first peak of the flow event only this implies that the initial seeding of the tagged particles dominates the observations nonetheless an underlying general relation is indicated by the data which are derived from a wide range of flow regime types

the relationship between shear stress and the shaping of the bed of a pebble loaded river la rulles ardenne summary geomorphological modifications in the bed of a small pebble loaded river were related to different parameters such as flow velocity and shear stress experiments carried out with marked pebbles of known diameter demonstrated that shear stress calculated from the energy grade line slope and the hydraulic radius is a reliable criterion in explaining erosion and transport of the pebble load increase in shear stress with discharge entrainment varies according to site at times of low flow shear stress is weak in the pools allowing transitory accumulation of fine deposits and pebbles these originate mainly from erosion downstream of riffles where shear stress is already considerable at times of discharge near bankfull stage shear stress becomes considerable in the pools so that pebbles deposited at lower discharge rates are then displaced however the coarser elements are halted on the counter slope of the pools and upstream of riffles since shear stress there is then weaker than in the pools riffles thus appears as stopping places with only small elements 1 5 cm diameter able to cross them at times of high discharge the size of these elements thus provides an estimate of the true competence of the river

underlying gravel layers in a large sand bed river and their influence on downstream dam channel adjustment due to the changes in environmental factors during a river s historical development underlying gravel layers are found in many large plain rivers when the buried depth of this gravel layer is within the reach of down cutting by clear water scour after reservoir construction it may be exposed and exert a far reaching influence on channel adjustment in hanjiang river the longest tributary of the yangtze river in china down cutting has been greatly reduced and even stopped due to the total change of bed material composition the exposure of a gravel layer results in a dramatic increase in the bed s hydraulic roughness so the channel slope may increase after a decline during the period prior to the exposure of the gravel layer moreover the exposed gravel layer increases the erosion resistance of bed material making a lower relative erosion resistance of bank to bed material where the bank s erosional resistance is weak this may lead to a tendency towards channel widening

entrainment displacement and transport of tracer gravels the mass and size distribution of grain entrainment per unit bed area may be measured by replacing a volume of the bed with tracer gravels and observing the mass difference before and after a transport event this measure of spatial entrainment is relevant to any process involving size selective exchange of sediment between transport and bed and may be directly used in calculations of sediment transport rate using an elementary relation for fractional transport components presented here this relation provides a basis for evaluating tracer data collected by different methods and may be used to provide physical insight regarding the expected behaviour of tracer grains the variation with grain size of total displacement length l ti depends on the degree of mobilization of the individual fractions on the bed surface l ti is independent of d i for smaller fully mobile sizes and decreases rapidly with d i for larger fractions in a state of partial transport in which a portion of the surface grains remain immobile through the flow event the boundary between fully and partially mobile grain sizes increases with flow strength these inferences are supported by values of l ti calculated from flume experiments and provide a physical explanation for a summary relation between l ti and d i based on field data 1997 john wiley sons ltd

rapidly evolving cirrus clouds modulated by convectively generated gravity waves cirrus clouds can strongly affect earth s radiation balance but questions remain about their growth mechanisms and rates here we show that gravity buoyancy waves generated by a storm in northern australia on 13 november 2015 caused an observable rippling effect on cirrus clouds up to 1 000 km away as seen by the recently launched himawari 8 9 geostationary satellite regional model simulations reproduce the propagation speed of the wave which agrees with theoretical predictions and show that the wave amplitude and timing near the tropopause can account for the cirrus modulation the observed cirrus reach peak optical depths of order 0 3 1 0 and appear roughly in phase with the arrival of the relative humidity maximum providing new evidence that cirrus clouds can respond rapidly 30 min to environmental lifting moreover the edge of a thick anvil cloud attached to the storm itself is observed to expand at the same speed as the wave showing that the lifting mechanism can also apply to optically thicker ice clouds close to convective centers

the transport of gravel in an ephemeral sandbed river the channel dynamics of an ephemeral sandbed river the metsemotlhaba in southeast botswana were studied over a five year period using tagged magnetic particles of pebble and cobble size scour chains and topographic cross sections with particular emphasis on the three dimensional dispersal of gravels patterns of scour and fill and depth of active layer two major flow events of equivalent magnitude occurred moving the tagged particles in december 1987 a mean distance of 837 m at a mean burial depth of 0 40 m and in march 1991 a mean distance of 263 m at a mean burial depth of 0 39 m the volume of mobile sediment based on scour depth and distance of travel was 2 7 times greater in the december 1987 event in which the mean scour depth was almost twice the mean burial depth of the tagged particles the distribution of distance of movement was asymmetrical in this first flood when the tracer started from a surface location but was monotonic thereafter intervening small to medium events yielded limited tracer movement with a mean burial depth equivalent to depth of scour the tracer moved in the low and transitional flow regimes burial depth distribution followed the gamma model field data confirm that longitudinal transport is independent of particle size and shape and strongly skewed with respect to distance whilst depths of scour in excess of 1 m for high magnitude events suggest that scour values predicted from the empirical equation of leopold et al underestimate by an order of magnitude copyright 1999 john wiley sons ltd

the nahal yatir bedload database sediment dynamics in a gravel bed ephemeral stream the nahal yatir bedload transport database arises from the northern negev river sediment monitoring programme and represents the first body of bedload information to be collected during flash floods in desert gravel bed streams bedload flux was established automatically with three slot samplers of the birkbeck type this was complemented by hydraulic measurements that allow sediment transport to be rated against channel average shear stress and specific stream power bed material and bedload grain size distributions are also provided the database is given in both printed and electronic tabular format

when does bedload transport begin in steep boulder bed streams the paper presents the results of field measurements of critical conditions for bedload motion conducted in the rio cordon a steep boulder bed stream in the italian alps under conditions of high reynolds numbers and low relative submergence poorly explored before two methods have been used to determine threshold of motion the displacement of marked clasts and the flow competence approach which uses the largest grain size diameter transported by each flood event the high values of confirm the great relevance of non bedload effective shear stresses in step pool streams given by the additional form drag associated with this morphology relative submergence effects on the dimensionless critical shear stress have been quantified by considering the relative submergence ratio rh d84 and the major effect of relative size on the mobility of each particle in steep widely graded bed mixtures has been evaluated finally the dimensionless critical unit discharge has also been adopted in the regression equations as the critical hydraulic parameter because it may represent an easier parameter to use than the critical shear stress for steep rough mountain rivers copyright 2006 john wiley sons ltd

breakup and reestablishment of the armour layer in a large gravel bed river below dams the lower ebro changes in armour layer during floods under supply limited conditions are little known this paper describes the breakup and the reestablishment of the bed armour layer in the regulated gravel bed ebro river during a flooding period the study was conducted over a 28 km study reach from 2002 to 2004 the surface subsurface and bed load grain size distribution constitute the bases for the analysis of bed armouring dynamics the results indicate that the magnitude of floods controlled the degree of armouring of the river bed the initial mean armouring ratio was 2 3 with maximum values reaching 4 4 floods in the winter of 2002 2003 q8 caused the breakup of the armour layer in several sections this resulted in the erratic bed load pattern observed during the december 2002 flushing flow and in the increase in bed load transport during successive events most grain size classes were entrained and transported causing river bed incision the mean armouring ratio decreased to 1 9 in contrast during low magnitude floods in 2003 2004 q2 the coarsest fractions 64 mm did not take part in the bed load while finer particles were winnowed thus surface deposits coarsened as a result the armour layer was reestablished i e the mean armouring ratio increased to 2 3 and the supply of subsurface sediment decreased the supply and transport of bed material appear to be in balance in the river reach immediately below the dam in contrast the transport of medium and finer size classes in the downstream reaches was higher than their supply from upstream a phenomenon that progressively reduced their availability in the river bed surface hence the armour layer reworking

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

continuous monitoring of bedload flux in a mountain gravel bed river an automatic bedload monitoring station has been established on the perennial tordera river a mediterranean mountain gravel bed stream located in ne spain bedload fluxes were obtained automatically at 1 min intervals using two birkbeck type pit bedload samplers aligned across the width of the channel flow depth and water surface slope were both measured continuously and synchronously with bedload data for five flow events show that bedload flux varies considerably bedload flux is high when it is compared with other perennial streams for a 15 min interval maximum channel average bedload flux was 0 83 kg m 1 s 1 bedload records also show that there is no direct relationship between bedload flux and grain shear stress the low correlation between bedload flux and hydraulics arises due to the different thresholds of initiation of motion for each flow event and the variable character of the bed between events for two individual floods there is a reasonable relationship between bedload flux and grain shear stress in part as a consequence of individual particle entrainment and patch movement over a stable coarse gravel bed during moderately high discharges these results demonstrate that the birkbeck system functions well in a mediterranean mountain gravel bed stream with a longitudinal slope of 2