laboratory study of loose saturated and unsaturated decomposed granitic soil weathered soils are used extensively as fill materials in slope construction in tropical and subtropical cities such as hong kong the mechanical behavior of loose decomposed fill materials particularly in the unsaturated state has not often been investigated and is not yet fully understood the objective of this study was to understand the mechanical behavior of loose unsaturated decomposed granitic soil and to study the effects of the stress state the stress path and the soil suction on the stress strain relationship shear strength volume change and dilatancy via three series of stress path triaxial tests on both saturated and unsaturated specimens it was found that loose and saturated decomposed granitic soil behaves like clean sands during undrained shearing strain softening behavior is observed in loose saturated specimens in unsaturated specimens sheared at a constant water content a hardening stress strain relationship and volumetric contractions are observed in the considered range of net mean stresses the suction of the soil contributed little to the apparent cohesion the angle of friction appeared to be independent of the suction in unsaturated specimens subjected to continuous wetting suction reduction at a constant deviator stress the volumetric behavior changed from dilative to contractive with increasing net mean stress and the specimen failed at a degree of saturation far below full saturation it was revealed that the dilatancy of the unsaturated soil depends on the suction the state and the stress path

strength of moist sand controlled by surface tension for tectonic analogue modelling abstract a technique has been developed to control the strength of moistened sand in a quantifiable accurate and reproducible way while other mechanical properties were maintained strength of dry sand was increased through adding a small amount of liquid in order to control the additional cohesion of moist sand the influence of the surface tension of the liquid was investigated direct shear experiments were performed on four granular materials at confining stress levels below 1 kpa it has been found that the surface tension of the added liquid controlled the additional apparent cohesion of sand with high accuracy the mechanical properties of moist sand show dynamic similarity towards natural brittle rock which enables analogue modelling of fault formation fault reactivation and tension fracture formation in the brittle regime with a controlled strength profile furthermore experimental results fit well to shear strength models from this followed the direct proportionality of the unsaturated shear strength parameter b relative to the matrix suction measured at low stress levels moreover shear strength turned out to be also a function of grain size and the grain shape

a microstructurally based effective stress for unsaturated soils current alternative choices of stress state variables in unsaturated soils are described and compared with a special focus on the use of an effective stress experimental data on stiffness and shear strength evolution with suction suggest that the proportion of suction contributing to the effective stress is often much smaller than predicted by the term suction times degree of saturation generally used in effective stress expressions of the bishop type it is suggested that effective stress in unsaturated soils should be related to soil microstructure an effective degree of saturation is defined as describing the volume of water partially filling the soil macroporosity this effective degree of saturation defines the proportion of the prevailing suction that actually contributes to the effective stress two alternative expressions piecewise linear and non linear are proposed for the effective degree of saturation they offer a similar performance available data on stiffness and shear strength varia

channel bed steps along nahal yael negev desert israel abstract the height and spacing of channel bed steps formed in boulder and bedrock in the ephermeral channels of the nahal yael watershed show a strong correlation with the channel slope the distance between steps is inversely proportional to slope at slopes of 0 to 20 but then remains fairly constant at higher slopes step height increases proportionally with channel slope these bed steps are similar to those described on perennial channels elsewhere where step formation has been attributed to the effects of high flows which submerge the clasts forming the steps however the ephemeral flows in the tributary channels of the nahal yael system are not sufficient to submerge the clasts forming the boulder steps using rouse s criteria channel bed steps are creating maximum flow resistance at slopes of 12 to 50 along the channels of nahal yael using davies criteria bed steps are creating maximum flow resistance at slopes of 3 to 10 the bed steps also relate to sediment transport nahal yael has a low sediment yield by world standards and the increase in flow resistance due to step formation may serve to increase sediment movement both the boulder and bedrock steps at nahal yael are hypothesized to reflect the scales of flow turbulence along the channels we were unable to evaluate the role of flow volume or boundary roughness in controlling these scales but channel slope appears to exert some control on the scale of turbulence based on the correlation between slope and bed steps the hypotheses developed from field evidence at nahal yael should be testable in flume simulations

distribution of erosion across bedrock channels lateral erosion in bedrock rivers is an important control on the shape of channel crosssections and the coupling of channels and hillslopes recent observations link lateral erosionto the variability of flow we propose two mechanisms to explain this one is based on changing shear stress distributions within the channel with varying flood level the other on the competition between cover and tool effects in fluvial bedrock erosion we assess these processes for the liwu river taiwan and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case

controls over particle motion and resting times of coarse bed load transport in a glacier fed mountain stream coarse bed load transport is a crucial process in river morphodynamics but is difficult to monitor in mountain streams here we present a new sediment transport dataset obtained from two years of field based monitoring 2014 2015 at the estero morales a high gradient stream in the central chilean andes this stream features step pool bed geometry and a glacier fed hydrologic regime characterized by abrupt daily fluctuations in discharge bed load was monitored directly using bunte samplers and by surveying the mobility of pit passive integrated transponder tags we used the competence method to quantify the effective slope which is the fraction of the topographical slope responsible for bed load transport this accounts for only 10 of the topographical slope confirming that most of the energy is dissipated on macroroughness elements we used the displacement lengths of pit tags to analyze displacement lengths and virtual velocity of a wide range of tracer sizes 38 415 mm bed load transport in the estero morales show to be size selective and the distance between steps influences the displacement lengths of pit tags displacement lengths were also used to derive the statistics of flight distances and resting times our results show that the average length of flight scales inversely to grain size this contradicts einstein s conjecture about the linear relationship between grain size and intervals between resting periods in a steep step pool stream in ordinary flood conditions

the geomorphic significance of step pools in mountain streams abstract this paper develops a conceptual model to define the changing role and significance of step pools as energy dissipators in steep mountain channels although energy dissipation is a significant function of step pools this role changes over time with variations in discharge steps are effective in reducing stream energy at low flows but their effectiveness diminishes with increasing stage accordingly the manner in which energy dissipation occurs also varies with increasing flows spill resistance gives way to a dominance in form and grain resistance the conceptual model for the changing role of step pools is illustrated with data from hydraulic analysis and modeling of step pools in the santa monica mountains of california the model points to the importance of the size of steps in determining their role in energy dissipation and in their interactions with channel hydraulics the model offers a new articulation for the geomorphic significance of step pools in mountain streams and it serves as a useful template for a more complete understanding of step pools over a longer time scale

shear strength characteristics of an unsaturated expansive clay the shear strength characteristics of an expansive clay from china were studied by performing suction controlled direct shear tests on both natural and compacted specimens the tested soil was a silty clay with intermedi ate plasticity and medium expansion potential a modified direct shear apparatus with a newly developed water volume indicator was used for this laboratory study the experimental results clearly show that the dilatancy of the expansive clay increases with an increase in the applied suction for both the natural and compacted specimens matric suction contributes to the shear strength of the expansive clay via two different mechanisms the contribution of capillary force to interparticle normal stress and the effect of suction on soil dilatancy as a result of the second mechanism the con tribution of suction to peak shear strength for the clay is more significant than that to post failure shear strength par ticularly at a high suction range the contribution of suction to post failure shear strength for the natural specimen is basically consistent with that for the compacted specimen the higher peak shear strength and dilatancy for the natural specimen are related to the cementation effect of the iron and manganese oxides the contribution of suction to shear strength for the compacted expansive clay is more significant than that for a compacted kaolin at suctions less than 100 kpa

model analyses of the contribution of in channel processes to sediment concentration hysteresis loops summary sediment concentration sc water discharge q relations in rivers are typically governed by multiple and relatively complex processes due to hysteresis effects sediment discharges can differ for similar or equivalent water discharges which causes scatter in empirical datasets and may decrease the predictive power of sc rating curves such hysteresis effects must therefore be understood and accounted for to make dependable predictions for river system management the overall objectives of this study are to develop modelling approaches suitable for reproducing and predicting hysteresis effects at larger scales and to investigate the possible contribution of in channel processes erosion and deposition to sediment concentration hysteresis loops to investigate relevant field scale conditions we develop a one dimensional dynamic sediment transport model of the downstream tuul river northern mongolia investigating in channel processes along a 141 km stretch during a hydrological year the results show that the present modelling approach can reproduce both anti clockwise and clockwise hysteresis effects importantly in channel processes alone can cause considerable anti clockwise hysteresis effects without being reinforced by catchment processes such as hillslope erosion such specific contributions from in channel processes introduced data scatter into the sediment rating curves decreasing their r 2 values from unity to approximately 0 5 to 0 6 more generally possible changes in the number or magnitude of high flow events caused by climatic or other anthropogenic factors could influence total sediment deposition which was primarily found to occur during relatively short high flow events such potential changes also have important implications for the possible spreading of polluted sediments

influence of bed patchiness slope grain hiding and form drag on gravel mobilization in very steep streams steep streams are a major portion of channel networks and provide a link to transport sediment from hillslopes to lower gradient rivers despite their importance key unknowns remain perhaps foremost of which is evaluating in steep streams empirical laws for fluvial sediment transport developed for low gradient rivers to address this knowledge gap we painted sediment in situ over 3 years to monitor incipient sediment motion and sediment patch development in five small drainage areas of 0 04 2 km 2 and steep slopes of 5 37 tributaries of elder creek california united states we found that channel beds organized into size sorted sediment patches which displayed active fluvial transport of gravel annually consistent year to year patch median grain sizes partial transport of bed material and significantly higher values of critical shields stress for incipient sediment motion compared to that observed for lower gradient rivers the high critical shields stresses up to 0 5 for the median grain size agree within a factor of 3 to theoretical predictions which account for slope dependent hydraulics grain hiding and sediment patches for grains of approximately the same size as the roughness length scale slope dependent hydraulics and bed patchiness are the dominant controls on critical shields stress values while grain hiding is important for grains larger or smaller than the roughness length scale form drag exists in our monitored tributaries but has a smaller influence than the above effects our field observations show fluvial processes contribute to sediment mobilization in steep channels which are often considered to be dominated by debris flows

the partitioning of the total sediment load of a river into suspended load and bedload a review of empirical data the partitioning of the total sediment load of a river into suspended load and bedload is an important problem in fluvial geomorphology sedimentation engineering and sedimentology bedload transport rates are notoriously hard to measure and at many sites only suspended load data are available often the bedload fraction is estimated with rule of thumb methods such as maddock s table which are inadequately field tested here the partitioning of sediment load for the pitzbach is discussed an austrian mountain stream for which high temporal resolution data on both bedload and suspended load are available the available data show large scatter on all scales the fraction of the total load transported in suspension may vary between zero and one at the pitzbach while its average decreases with rising discharge i e bedload transport is more important during floods existing data on short and long term partitioning is reviewed and an empirical equation to estimate bedload transport rates from measured suspended load transport rates is suggested the partitioning averaged over a flood can vary strongly from event to event similar variations may occur in the year to year averages using published simultaneous short term field measurements of bedload and suspended load transport rates maddock s table is reviewed and updated long term average partitioning could be a function of the catchment geology the fraction of the catchment covered by glaciers and the extent of forest but the available data are insufficient to draw final conclusions at a given drainage area scatter is large but the data show a minimal fraction of sediment transported in suspended load which increases with increasing drainage area and with decreasing rock strength for gravel bed rivers whereby in large catchments the bedload fraction is insignificant at 1 for sand bed rivers the bedload fraction may be substantial 30 50 even for large catchments however available data are scarce and of varying quality long term partitioning varies widely among catchments and the available data are currently not sufficient to effectively discriminate control parameters

relationships between chemical weathering indices and physical and mechanical properties of decomposed granite abstract chemical weathering is one of the dominant mechanisms for weathering of granite in subtropical and tropical regions resulting in modifications of its chemical physical and mechanical properties thus the changes in chemical properties may provide insight about the changes in physical and mechanical properties of weathered granite in this paper three quantitative chemical weathering indices are correlated to the dry density of decomposed granite a linear relationship is found between each chemical weathering index and dry density for granitic saprolites mobiles index i mob gives the highest coefficient of correlation among the three chemical weathering indices further investigations were conducted to establish the relationships between i mob and other physical and mechanical properties of granitic saprolites the test results showed that fines content increases with increasing i mob but peak strength increases with decreasing i mob the peak states of granitic saprolites are consistent with rowe s stress dilatancy relationship dilatancy increases with decreasing i mob that contributes to a higher peak strength based on some test results presented in this study it seems that i mob could be a good indicator to quantify the effects of chemical weathering on physical and mechanical properties of granitic saprolites

hydraulic geometry of cohesive channels undergoing base level drop abstract this study extends earlier contributions on dynamic adjustments of fluvial channels to base level changes we have investigated an in situ response of self formed cohesive channels to a base level drop conditions analogous to a gradual change in uplift and or climate empirical hydraulic geometry equations for clayey cohesive natural streams are presented using data from eight channels draining perennial brackish springs and discharge into the dead sea investigation of downstream variations in gradient and stream power relations suggests existence of three distinct reaches in which channel adjustment to base level drop is shared inequitably among hydraulic geometry variables values of the flow velocity exponent m are low 0 11 m 0 24 the mid channel reach having the lowest exponent the depth exponent f has the lowest value f 0 3 for the uppermost channel reaches the rest having higher values f 0 4 the smallest width exponent b 0 35 characterizes the upper reaches these values and their spatial distribution exhibit a regular pattern we show that the lowermost channel reach adjusts by profile steepening and channel narrowing f b the prevailing mechanism in the mid channel reaches is lateral width adjustment cross sections transiently transforming toward equilibrium the uppermost reaches have wide and shallow channel cross sections because of series of bank collapses and resultant sediment aggradation bringing rise to decreased local gradient forcing further channel widening the results of this study not only allow inference about how cohesive channels regulate their geometry but also reveal the means by which hydraulic forces overcome substrate resistance adjusting slope and channel dimensions and as such have implication for reach scale channel morphology and models of stream power

equal mobility bed load transport in a small step pool channel in the ouachita mountains equal mobility transport emt of bed load is more evident than size selective transport during near bankfull flow events in a small step pool channel in the ouachita mountains of central arkansas bed load transport modes were studied by simulating five separate runoff events with peak discharges between 0 25 and 1 34 m3 s 1 0 to 1 6 year recurrence intervals in a natural channel using controlled releases from a storage tank emt occurrence was investigated using four different bed load relationships suggested by previous research with each of these approaches the relationship of a given bed load characteristic dmax distribution percentile displacement distance and skewness to some independent factor c and grain size was assessed to determine which transport mode was evident regression models derived using combinations of these four relationships with different datasets provide seven separate tests five of the seven tests indicate that emt occurred or was predominant several reasons may explain the apparent contradictory results but the confounding effects of changes in the structural arrangements of bed material prior to or during the events seem particularly important

volumetric behavior of unsaturated soils an elastoplastic framework to account for the volumetric behavior of unsaturated soils is proposed herein the proposed equation is based on the effective stress principle and results in a unifying framework for the volumetric behavior for both saturated and unsaturated soils the results of the proposed equation are compared with experimental results published by different researchers these comparisons show that the equation is adequate to account for wetting drying and net stress loading unloading paths in addition the collapse upon wetting phenomenon can be simulated and the critical state for unsaturated soils coincides with the proposed volumetric framework this analysis confirms that the effective stress principle can be applied to the volumetric behavior of unsaturated soils

characteristics of sediment dynamics following large scale sediment supply events in mountain watersheds in japan abstract large scale sediment supply events lses cause not only a sudden increase in sediment discharge immediately after their occurrence but also a long term condition of elevated sediment yield in japan many lses triggered by heavy rainfall or earthquakes have created high risks for sediment related disaster however the difference between sediment dynamics following a lse triggered by rainfall and one triggered by an earthquake is unclear as the sediment dynamics due to rainfall and those due to earthquakes have not been compared using consistent methodology in this study we estimated the volume of sediment supply vl m3 time series of the volume of sediment yield vy m3year 1 and the non dimensional parameter nvy as estimated by dividing vl by the catchment area and annual maximum 3 day rainfall amount using the same methodology for 16 events in eight study basins in japan we used annual maximum 3 day rainfall amounts as a surrogate for discharge using these parameters we estimated the period over which the effect of a lse continues pls sediment retention time residence time tr1 and time for active sediment yield to recover to the background level recovery time tr2a in addition we estimated the recovery time tr2b based on when the maximum of nvy immediately after the lse decreased to a certain extent this calculation allowed us to estimate the recovery time even when background level was unknown both residence and recovery times ranged from

on the stability of step pool mountain streams although step pools are generally considered to be stable bedforms stability is not absolute but depends on size scale and perspective hydraulic analysis of the stability of step pool sequences in the santa monica mountains california indicates that they are active channel features that are generally restructured within 5 to 100 years the degree of mobility depends on step particle size steps are stable within small temporal and spatial scales where they function as independent variables that dissipate stream energy and regulate channel hydraulics but stability decreases at larger scales where step pools become dependent variables that respond to discharge and sediment load hence the role of step pools changes from energy dissipation to one of channel adjustment these results underscore the need to consider larger spatial and temporal scales in order to reveal the complete function and significance of step pools in mountain fluvial systems

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

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

experimental investigation of both the disturbed and undisturbed granitic saprolite soil granitic saprolites are abundant in many regions around the world and they are widely employed in various engineering projects behavior of such kinds of soils is not sufficiently understood and our knowledge about the fundamental behavior of saprolites is to a far less extent than the sedimentary soils owing to the limited knowledge many problems occurred in the engineering applications that used saprolites as a construction material this study presents results of a set of conventional drained and undrained triaxial compression shearing tests on the granitic saprolites in both the disturbed and undisturbed conditions this study showed that the soil fabric was the paramount factor governing on the soil stress strain response rather than the soil bonds under the drained shearing condition the undisturbed soil revealed a strain softening behavior with a dilative response at the low consolidation stress level while a contractive response was present at the high consolidation stress levels however the disturbed soil showed a strain hardening behavior with a contractive response regardless of the consolidation stress levels during the drained shearing the undrained response was complicated as the soil showed both the contractive and dilative responses with phase transformation points regardless of the soil condition i e disturbed or undisturbed and the consolidation stress levels the bond effect of the intact soil persisted during the consolidation stage however its effect did not extend to the large strain controlled shearing stage where the tangential stiffness suddenly decreased at the axial strain of 1 5

a numerical study of steady state evaporative conditions applied to mine tailings the suction profile of a desiccating soil is dependent on the water table depth the soil water retention characteristics and the climatic conditions in this paper an unsaturated flow model whi

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

a field experiment on the development of sedimentary structures in a gravel bed river in gravel bed rivers bed sedimentary structures exert a significant influence on the stability of the bed particle motion and stream dynamics a detailed description of the development of sedimentary structures is essential to define the overall activity of sediment transport this paper aims to improve our understanding of the dynamics of sedimentary structures at the reach scale using a field approach the objectives of the study are 1 to describe the processes of sedimentary structures development from a manipulated bed surface and 2 to show the effect of keystones on the development of the sedimentary structures by monitoring the displacement of individual tagged particles we investigated the dynamics of sedimentary structures in three 15 m long reaches of a small gravel bed river characterized by moderate slope and high relative roughness the bed material is composed of a range of gravel sizes with sparse boulders that remain immobile during most flows we have disrupted the bed surface material of the first reach to create a disorganized bed surface two reaches where the bed material remained undisturbed were used as control sections detailed topographic maps and passive transponders were used to describe the formation processes of sedimentary structures the initial manipulations of the bed led to a more fragmented surface in the experimental section however sedimentary structures developed very quickly with an equilibrium bed surface developing after only two sediment transport events of low magnitude discharge this rapid adjustment of the bed is associated with the development of key sedimentary structures anchored by the large boulders at the reach scale these structures increased the probability of congested areas in the displacement of bed material in both control sections sedimentary structures were maintained during the experiment copyright 2007 john wiley sons ltd

flume study of the effect of relative depth on the incipient motion of coarse uniform sediments an experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0 3 m wide tilting flume the concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport the experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter a revised shields diagram relating critical stress grain reynolds number and depth to grain size ratio has been derived this has implications for the calculation of sediment transport rates design of stable alluvial channels and physical hydraulic modeling

is the critical shields stress for incipient sediment motion dependent on channel bed slope data from laboratory flumes and natural streams show that the critical shields stress for initial sediment motion increases with channel slope which indicates that particles of the same size are more stable on steeper slopes this observation is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force processes that might explain this discrepancy are explored using a simple force balance model including increased drag from channel walls and bed morphology variable friction angles grain emergence flow aeration and changes to the local flow velocity and turbulent fluctuations surprisingly increased drag due to changes in bed morphology does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical shields stress are similar for flume experiments and natural streams and significant variations in bed morphology in flumes is unlikely instead grain emergence and changes in local flow velocity and turbulent fluctuations seem to be responsible for the slope dependency due to the coincident increase in the ratio of bed roughness scale to flow depth i e relative roughness a model for the local velocity within the grain roughness layer is proposed based on a 1 d eddy viscosity with wake mixing in addition the magnitude of near bed turbulent fluctuations is shown to depend on the depth averaged flow velocity and the relative roughness extension of the model to mixed grain sizes indicates that the coarser fraction becomes increasingly difficult to transport on steeper slopes

step pool evolution in the rio cordon northeastern italy the principle that formative events punctuated by periods of evolution recovery or temporary periods of steady state conditions control the development of the step pool morphology has been applied to the evolution of the rio cordon stream bed the rio cordon is a small catchment 5 km2 within the dolomites wherein hydraulic parameters of floods and the coarse bedload are recorded detailed field surveys of the step pool structures carried out before and after the september 1994 and october 1998 floods have served to illustrate the control on step pool changes by these floods floods were grouped into two categories the first includes ordinary events which are characterized by peak discharges with a return time of one to five years 1 8 5 15 m3 s 1 and by an hourly bedload rate not exceeding 20 m3 h 1 the second refers to exceptional events with a return time of 30 50 years a flood of this latter type occurred on 14 september 1994 with a peak discharge of 10 4 m3 s 1 and average hourly bedload rate of 324 m3 h 1 step pool features were characterized primarily by a steepness parameter c h ls s the evolution of the steepness parameter was measured in the field from 1992 to 1998 the results indicate that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events during this period bed armouring dominate the sediment transport response however following an extraordinary flood and unlimited sediment supply conditions the steepness factor can suddenly decrease as a result of sediment trapped in the pools and a lengthening of step spacing the analogy of step spacing with antidune wavelength and the main destruction and transformation mechanism of the steps are also discussed copyright 2001 john wiley sons ltd

field measurements of the energy delivered to the channel bed by moving bed load and links to bedrock erosion 1 impact driven fluvial erosion is directly related to the energy delivered to the channel bed by moving bed load using a novel protocol we have measured this energy in four mountain streams in austria and switzerland similar to bed load transport rates the energy delivered to the bed displays large scatter over several orders of magnitude even for a constant discharge we found that only a small fraction 1 of the total energy available to the stream is delivered to the bed and can be used for erosive work empirical predictive equations can be defined for specific sites but there is large site to site variability prediction of energy delivered to the bed using the saltation abrasion model of bedrock erosion only provides the observed trends when measured bed load transport rates are used as input using an empirical transport law calibrated to the conditions at one of the study streams leads to overprediction of delivered energies by more than 2 orders of magnitude this overprediction decreases with increasing discharge and thus at high discharges better predictive results are obtained we find a correlation between the channels bed slope or characteristic grain sizes of the channel bed and the fraction of energy delivered to the bed of the total energy available to the stream this observation provides a tentative link between fundamental fluvial incision processes to the stream power model family that has widely been used to model fluvial bedrock incision in landscape evolution simulations

behavior of a compacted completely decomposed granite soil from suction controlled direct shear tests a series of single staged consolidated drained direct shear tests are carried out on recompacted completely decomposed granite cdg soil a typical residual soil in hong kong under different matric suctions and net normal stresses matric suction is controlled by applying air pressure in the pressure chamber and water pressure at the bottom of the high air entry ceramic disk the experimental results show that the contribution of suction to shear strength is significant shear strength of cdg soil increases with the increase of matric suction net normal stress has a remarkable influence on the shear strength of unsaturated cdg soil the increase in shear strength due to an increase in matric suction suction envelope is observed as nonlinear i e b value varies with matric suction no soil dilatancy is observed for zero matric suction saturated case but as the suction value is increased higher soil dilatancy is obvious in lower net normal stresses the rate of increase of soil dilatancy is greater

coupled dynamics of interfacial waves and bed forms in fluid muds over erodible seabeds in oscillatory flows recent field investigations of the damping of ocean surface waves over fluid muds have revealed waves on the interface between the thin layer of fluid mud and the overlying much thicker column of clear water accompanied by bedforms on the erodible seabed beneath the fluid mud the frequencies and wavelengths of the observed interfacial waves are qualitatively consistent with the linear dispersion relationship for long interfacial waves but the forcing mechanism is not known to understand the forcing a linear model is proposed based on the layer averaged hydrostatic equations for the fluid mud together with the meyer peter mueller equation for the sediment transport within the underlying seabed both subject to oscillatory forcing by the surface waves if the underlying seabed is non erodible and flat the model indicates parametric instability to interfacial waves but the threshold for instability is not met by the observations if the underlying seabed is erodible the model indicates that perturbations to the seabed elevation in the presence of the oscillatory forcing create interfacial waves which in turn produce stresses within the fluid mud that force a net transport of sediment within the seabed toward the bedform crests thus causing growth of both bedforms and interfacial waves the frequencies wavelengths and growth rates are in qualitative agreement with the observations a competition between mixing created by the interfacial waves and gravitational settling might control the thickness density and viscosity of the fluid muds during periods of strong forcing this article is protected by copyright all rights reserved

frequency and magnitude of bedload transport in a mountain river magnitude and frequency of bedload transport was examined in the lainbach river bavaria using magnetically tagged particles during the study 18 small to moderate events and one major event occurred about 1 per cent of the flow period exceeded the entrainment threshold and at least once every year about 50 per cent of the tagged particles were mobile themajor event which occurred during the study period resulted in the deposition of a thick layer of sediment in parts of the channel and passive burial of most of the recovered particles the step pool pattern which characterized the study site prior to the large event was obliterated however the channel recovered quickly and has returned to a new step pool pattern the event changed the boundary conditions by increasing the availability of loose sediment and creating higher river bed slopes in reaches between breached check dams as a result movement of individual particles measured for events of both the same magnitude and duration occurring before and after the large event yielded different values for events which occurred after the large event the range and the mean distance of movement were about ten times higher

channel morphology gradient profiles and bed stresses during flood in a step pool channel abstract the stability of a step pool channel is investigated on the basis of measurements during a median annual flood the study reaches range in gradient from 5 to 10 and the flow was 3 m3 s 1 in an 8 m wide channel the estimated shear stress based on the channel gradient was 102 pa which implies that the channel ought to move half meter class boulders calculations of the shear stress using flow resistance equations yield values of about 30 pa these figures imply that the largest boulders in the channel could be moved in fact the channel is stable the effective stress remains well below the threshold to move the large structure forming boulders the steps are stable structures that result from interlocking of the larger stones they are semi randomly located depending on the location of keystones that are deposited in the channel by debris flows or fall out of the banks the high variation in step pool wavelength step height residual pool depth and pool length is indicative of the random nature of step location and structure along a reach there is no convincing evidence that special conditions govern their formation

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

a relationship describing the shear strength of unsaturated soils a relationship describing the shear strength profile of a desiccating soil deposit is essential for the purpose of analysis especially when a numerical method is adopted where each zone in a discretised grid is assigned an elevation dependent shear strength value the matric suction profile of a desiccating soil deposit is nonlinear up to the air entry value an increase in matric suction is associated with a linear increase in shear strength beyond air entry as the soil starts to desaturate a nonlinear increase in shear strength occurs the soil water characteristic curve is stress dependent as is the shear strength gain as matric suction increases in this paper a three dimensional nonlinear regression analysis showed that a power additive function is suitable to describe the variation of the shear strength of unsaturated soils with matric suction the proposed function incorporates the effect of normal stress on the contribution of matric suction to the shear strength

continuous measurement of sediment transport in the erlenbach stream using piezoelectric bedload impact sensors we report on bedload transport observations using piezoelectric bedload impact sensors pbis an indirect method of estimating the volume of bedload transport of coarse sediment the pbis device registers vibrations produced by bedload particle diameter 20 mm and records the signal as a sum of the number of impulses per time sediment transport at the erlenbach stream has been continuously monitored with a pbis array starting in 1986 the sensor array spans the width of an entire cross section and is mounted flush with the surface of a check dam immediately upstream of a sediment retention basin we compare pbis data with long term sedimentation records obtained from repeated surveys of material stored in the sediment retention basin with artificial sediment input under controlled conditions in the field and also with laboratory experiments the rate of bedload transport is proportional to the number of impacts on the sensor per unit time the reliability of the calibration relationship increases with the length of the observation period e g for higher numbers of impacts and larger bedload volumes sediment volumes for individual flood events estimated with the pbis method are in agreement with volumes estimated using an independent empirical method based on the effective runoff volume of water the peak water discharge and the critical discharge for the onset of sediment transport copyright 2007 john wiley sons ltd

climate driven bedrock incision in an active mountain belt measurements of fluvial bedrock incision were made with submillimeter precision in the east central range of taiwan where long term exhumation rates and precipitation driven river discharge are independently known they indicate that valley lowering is driven by relatively frequent flows of moderate intensity abrasion by suspended sediment is an important fluvial wear process and channel bed geometry and the presence of widely spaced planes of weakness in the rock mass influence erosion rate and style

critical shear stress of natural sediments the critical shear stress of individual fractions ci in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture for strongly bimodal sediments ci increases with grain size an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes ci to deviate from size independence in the direction of unisize shields values a quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of ci is observed and to predict the variation of ci with grain size in bimodal sediments because properties of sediment grain size distributions other than bimodality appear to have little influence on ci the trends presented here for 14 unimodal and bimodal sediments may be quite general

bed morphology and generation of step pool channels flume experiments have been carried out to study the formation processes and the bed morphology of step pool channels from the experiments different step types and step configurations could be distinguished depending on the stream power these step types can be seen as an image of the generation mechanisms of step pool systems these results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution in contrast to a variety of other studies antidunes did not contribute to the generation of the step structures however the data of the presented study fits well into the region of antidune formation proposed by kennedy for sand bed rivers this observation points out that step pool field data located in the kennedy region do not inevitably prove that antidunes played a role in step development it is rather proposed that in kennedy s region of antidune formation there exist hydraulic conditions where the flow resistance is maximized it is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height independently of whether these are antidunes in sand and gravel bed rivers or step pool units in boulder bed streams the considerations of the kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self stabilization by modifying the step pool structure towards a geometry that provides maximum flow resistance and maximum bed stability copyright 2008 john wiley sons ltd

calculating bed load transport in steep boulder bed channels 1 steep rough channels occupy a large fraction of the total channel length in mountainous regions most sediment mobilized on hillslopes must pass through these streams before reaching lower gradient channels steep channels have wide grain size distributions that are composed of finer more mobile sediment and large rarely mobile grains the large grains can bear a significant portion of the total shear stress and thereby reduce the stress available to move the finer sediment conventional bed load transport equations often overpredict the sediment flux in steep channels by several orders of magnitude we hypothesize that sediment transport equations overpredict the sediment flux because they do not 1 account for the stress borne by rarely mobile grains 2 differentiate between highly and rarely mobile sediment and 3 account for the limited availability of mobile sediment here we modify a conventional bed load transport equation to include these three effects we use measurements of the flow bed properties and sediment flux in a small steep flume to test this equation we supply gravel at a constant rate through fields of regularly spaced immobile spheres and measure the bed coverage by gravel and sphere protrusion the percent of the sphere that protrudes above the gravel deposit for a given sphere spacing the proportion of the bed covered by gravel increases and the sphere protrusion decreases with greater sediment supply thus bed coverage and immobile grain protrusion may serve as proxies for sediment availability in steep rough streams unlike most transport equations that we tested our modified bed load equation predicts sediment fluxes to within an order of magnitude of the measured values our results demonstrate that accurately predicting bed load transport in steep rough streams may require accounting for the effects of local sediment availability coverage by mobile sediment and drag due to rarely mobile particles

continuous catchment scale monitoring of geomorphic processes with a 2 d seismological array 1 distributed activity of geomorphic processes with different spatiotemporal scales is hard to monitor in detail with conventional methods but might be detected with seismometers from july to september 2010 we deployed 14 seismometers to evaluate the ability of a two dimensional array with small interstation distances 11 km to continuously monitor geomorphic processes in a mountain catchment 370 km2 in taiwan spectral analysis of seismic records highlights different sources with high frequency content 1 hz consistent with hillslope and river processes using a common detection algorithm and a location technique based on the timing of seismic amplitude we have located 314 near surface events most of which 69 occurred during daily convective storms event activity was positively correlated with the precipitation intensity but this relation was not uniform in the catchment high resolution satellite images and air photos did not show geomorphic change during the study which did not have any episodes of extreme precipitation a majority of events 61 were collocated with preexisting geomorphic features landslide scars gullies within the uncertainty on location 9 of interstation distance the combination of event location and timing suggests a geomorphic source of recorded signals and most events had the seismic characteristics of rockfall debris avalanches or slides reactivation of prior erosion sites by such processes is difficult to detect with imagery but can possibly be resolved by seismic monitoring when proven this approach will allow a spatially comprehensive survey of geomorphic activity at the catchment scale with temporal detail sufficient to evaluate the exact meteorological conditions under which process events occur

behavior of a loosely compacted unsaturated volcanic soil in this paper the stress strain relationsihp and volumetric behavior of a loosely compacted unsaturated decomposed volcanic soil fill were studied by conducting three series of triaxial stress path tests 1 consolidated undrained on the saturated fill 2 constant water content and 3 a reducing suction under constant deviator stress on the unsaturated fill the last two series of tests were designed to simulate the effects of undrained response and rainfall infiltration in initially unsaturated slopes respectively it was found that the saturated loose volcanic soil behaves like clay under isotropic compression but it resembles sand behavior when it was subjected to undrained shear for isotropically consolidated unsaturated specimens sheared under a constant water content a hardening stress strain and a nonlinear shear strength suction relationship are observed at relatively high suctions both angle of friction and apparent cohesion appear to be independent of suction volumetric contraction during shear is observed in this series of tests on the other hand anisotropically consolidated loose unsaturated specimens subjected to a reducing suction change from contractive to dilative behavior as the net mean stress increases this observed volumetric behavior unlike the shear strength is stress path dependent and cannot be explained by using the existing elastoplastic critical state theoretical framework extended for unsaturated soils

the effects of low magnitude flow conditions on bedload mobility in a steep mountain stream abstract the transport of coarse material strongly controls the stability and evolution of mountain fluvial systems but despite this bedload dynamics are not yet fully understood especially in mountain streams in this sense particular attention was paid on the bedload magnitude volume expressed at event scale and on the long term while few studies were focused on when and how the transport of the coarse material occurs the aim of this work was to investigate the bedload mobility in the rio cordon a mountain stream characterized by cascade and step pool morphologies with a rough streambed here the critical conditions for initiation of motion transport distance and virtual velocity expressed by the coarse streambed material were assessed and their relationships with hydraulic forcing conditions and grain size were analyzed to this end a monitoring program based on bedload tracing was maintained over 7 years allowing to analyze the bedload mobility during persistent high frequency low magnitude flow conditions to investigate the bedload mobility 250 tracers were released between 2011 and 2012 and their propagation along study site was monitored until 2018 overall 14 tracer inventories were realized determining 1697 tracer localizations during the study period the bedload dispersion resulted well described by the peak discharge magnitude qp qp and p c while no significant relationships were observed with duration of competent flow tover and effective runoff volume er transport distance and tracer grain size were negatively correlated whereas virtual velocity increased with increasing particle size in this sense the propagation velocity seems to be affected by the high frequency flows that on the one hand triggered mobilization of the coarser tracers only through limited and impulsive events and on the other hand favored a slowdown on the finer particles due to bedforms disturbance compared to other study sites the rio cordon exhibited accentuated threshold conditions with lower transport distance and virtual velocity confirming that steep mountain streams are generally influenced by a reduced transport efficiency due to protruding bedforms and macro roughness that cause a pronounced energy dissipation interestingly such condition seems to have progressively reduced from under to near bankfull flows the results were compared to the bedload mobility observed in the rio cordon during 1993 1998 enabling to quantitatively assess how the bedload dispersion varied between a setting of stable armouring layer with protruding bedforms 2012 2018 and a partial alteration of these 1993 1998 an evident difference was observed in terms of transport distance while critical conditions did not significantly change lastly the long term bedload tracing investigation highlighted that a certain legacy on the transport efficiency was produced by the persistent high frequency flow conditions over the study period a general decrease of transport distance was observed that could be explained by a progressive stabilization of streambed material previous studies suggested that such condition can be interrupted by high magnitude low frequency floods

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

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

downstream coarsening in headwater channels 1 field data from four mountain drainage basins in western washington document systematic downstream coarsening of median bed surface grain size d50 and a subsequent shift to downstream fining at a drainage area of about 10 km2 analyses of network wide patterns of unit stream power derived from both channel surveys and digital elevation models reveal maximum unit stream power that in all four study areas roughly corresponds with both the grain size maxima and an inflection in the drainage area slope relation thought to represent the transition from debris flow dominated channels to fluvially dominated channels our results support the hypothesis that basin wide trends in d50 are hydraulically controlled by systematic variations in unit stream power in addition to lag deposits forced by mass wasting processes the similar relations found in our four study areas suggest that the tendency for downstream coarsening may be ubiquitous in headwater reaches of mountain drainage basins where debris flow processes set the channel gradient

step pool formation models and associated step spacing step pool bedforms develop under conditions of high flow rates and high sediment influx and transport rates step setting events are capable of mobilizing and rearranging the large step forming grain sizes thus observations of the processes of step formation are limited to flume experiments the results of laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps are presented over 350 step formations were observed and documented creating a large data set for detailing and evaluating step formation processes existing step formation models focus on step development from antidunes particle clusters transverse ribs or depositional berms or from a hydraulic regime that includes cascade flow and hydraulic jumps these models were developed using field measurements of artificial grade control structures or stable step pool systems during low flow conditions or degradational flume experiments where flows elucidated the step pool bedform in the channel profile but did not create equilibrium transport of the large step forming clasts this research evaluates the pre existing formation models and proposes three new models the rough bed exhumation and dune models the rough bed model was dominant throughout the experiments followed by the exhumation model and then the dune model a single model was not responsible for all step formations the rough bed exhumation and dune models all influenced step development and every step sequence was created through a mixture of formation processes this research generated a large data set where each step spacing is paired with the corresponding step formation model making it possible to identify a distinct exclusion zone length for each model thus given knowledge of the step formation model the minimum distance to the next step downstream can be predicted from the final spacing of a step sequence it may be possible to deduce the likely formation mechanisms active during a step resetting event copyright 2007 john wiley sons ltd

flow resistance of rock chutes with protruding boulders steep and short rough channels such as rock chutes are used in river restoration as grade stabilization structures the stability of these channels can be increased using boulders placed over the base material the boulders can be placed with different configurations such as randomly or in rows in each case their presence affects the geometry of the channel and modifies the hydraulic resistance this paper shows the results of tests conducted on rough channels with or without the insertion of protruding boulders the parameters that influence the flow resistance such as slope relative submergence and blocks disposition were systematically investigated to describe the hydraulic behavior of the structure the aim is to define a relationship useful for practical design which relates the flow resistance in terms of both darcy weisbach friction factor and manning coefficient the results were combined with available data for loose rock riprap to determine new experimental relationships

magnitude frequency distributions of boundary shear stress along a rapidly eroding bedrock river 1 the magnitude frequency distribution of boundary shear stress frames erosion rates in bedrock rivers but empirical constraints are rare particularly for extreme floods here we present measurements of mean stress b and its frequency distribution along a fast eroding river in taiwan we construct rating functions of discharge and hydraulic geometry using high resolution satellite images of flood patterns river stage time series topographic profiles and post flood field surveys the method allows us to assess the spatiotemporal variation in b along the channel the boundary shear stress pdf p b has a steep power law tail and includes semiannual floods generating b 100 200 pa and 50 year 3000 m3 s 1 events driving b 300 pa all such floods contribute to modification of the coarse alluvial cover and erosion of the bedrock bed given the steep tail decay in p b the rapid channel incision probably owes more to the exceptional frequency of moderate shear stresses than to the magnitude of the extremes

the influence of landsliding on sediment supply and channel change in a steep mountain catchment abstract rates of sediment supply by landsliding to an alluvial channel in a small catchment in central switzerland were estimated over an 11 month study period fluvial sediment transport in the channel is independently monitored at the upstream and downstream ends of the study reach yielding a unique opportunity to quantitatively compare the volume of sediment supplied to the channel with the volume in fluvial transport landslide derived sediment discharge to the channel was greatest during the winter and spring months while most of the fluvial sediment transport occurred during short intense summer storms approximately 98 m 3 of sediment was delivered directly to the study reach by landsliding 80 m 3 was transported into the reach from upstream and 70 m 3 was transported out of the reach thus there was a net accumulation of 100 m 3 of sediment during the 11 month study decadal scale channel aggradation was independently assessed by comparing channel longitudinal profiles in 1993 and 2004 during this 11 year period a total of 1500 m 3 of sediment has accumulated in the study reach aggradation has occurred largely in two broad zones that correspond with both the locations of major landslide complexes and reaches of high channel slope indicating that hillslope sediment input left an imprint on the morphology of the channel bed that appears to be stable over at least decadal time scales

analysis of self boring pressuremeter sbpm and marchetti dilatometer dmt tests in granite saprolites this paper presents the analyses of the results of the site investigation programme carried out at the kowloon bay site in hong kong the tests consisted of self boring pressuremeter sbpm marchetti dilatometer dmt and laboratory tests carried out in a granite saprolite which can be described as a lightly cemented sand the purpose of this research project is to stimulate the development of methods to interpret data obtained from tests in residual soils in particular the work aims to evaluate the analyses of the sbpm data through a curve fitting technique both the loading and unloading portions of the sbpm curve were analysed and the results compared with those from other tests the advantage of this analysis technique is the possibility of constructing a theoretical curve that reproduces a pressuremeter test from which a set of fundamental parameters can be derived namely the friction angle cohesion intercept lateral stress and shear modulus the dmt proved to be a reliable tool that yielded

threshold of coarse sediment transport in broad and narrow natural streams the threshold of coarse sediment transport has been examined in natural streambeds in an upland pennine u k area threshold values of the total boundary shear stress t0 for a given grain size in a narrow natural stream w d 11 efficiency in the entrainment process is related not only to the overall channel geometry but also varies as a function of discharge in channels characterized by compound roughness empirical curves relating t0 and a mean grain size d5 are presented but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source the reasons for this divergence are discussed the influence of grain shape was found not to be important in the initiation of motion criterion this conclusion may reflect the limited range of natural grain shapes in the study streams but might reasonably apply to other field investigations of similar streams modifications of the shields and yalin diagrams are suggested for practical applications in shallow streams with poorly graded bed material the shields parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth d5 d the increased importance of augmented drag forces in the entrainment process in shallow flows is suggested as the physical explanation for the reduced values of the shields parameter however the relationships presented should not be applied to laboratory experiments concerned with well graded sediments therefore beds with little deviation in level in which the shields parameter may be regarded as constant at high reynolds grain numbers consistent estimated field values of a threshold sediment transport parameter might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the literature

the morphologic structure of step pools in mountain streams abstract step pool sequences in streams in the santa monica mountains of southern california impart a characteristic morphologic structure that varies with slope but despite the strong association with slope that suggests a direct process control analysis of 464 step pools in 13 study reaches reveals the relationship to reflect correlation but not necessarily causality the direct control on step height is particle size whereas discharge is the probable causal process determining step wavelength because both particle size and discharge generally change with channel slope downstream a slope variation with step wavelength and height is the apparent indirect geometric result a conceptual model of process form linkages is proposed based on these results whereby the step pool morphology is produced by positive relationships between wavelength and discharge and between height and particle size the model offers new insights for explaining the step pool morphology and it is potentially useful in channeling future efforts toward the appropriate scales and variables

coarser and rougher effects of fine gravel pulses on experimental step pool channel morphodynamics understanding how steep mountain rivers respond to natural and anthropogenic sediment supply perturbations is important for predicting effects of extreme events e g floods and landslides and for restoring rivers to more natural conditions using flume experiments we show that stabilized step pool like channel beds can respond to pulses of finer gravel by becoming even coarser and rougher than before adding finer gravel initially reduces bed roughness and also increases the mobility of previously stable bed grains small and intermediate diameter clasts are then preferentially winnowed from the bed surface leaving behind higher concentrations of even larger clasts ultimately this results in both a coarser and rougher bed our experiments demonstrate that steep river beds become stable through the coevolution of bed roughness and surface grain size distribution and that these morphological variables can be sensitive to the history of upstream sediment supply

characteristic dimensions of the step pool bed configuration an experimental study 1 the dominant dimension of the step pool bed form is its spacing which is determined by the locations at which large step forming grains are deposited we conducted laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps direct observation of step development indicates that deposition of step forming grains was predominantly associated with bed roughness impeding transport or with earlier flow and transport conditions not contemporaneous with step development neither mechanism is likely to occur at regular locations on the bed measured step spacing was found to follow a poisson distribution supporting the conclusion that steps do not develop in preferred or regular locations this contrasts with previous arguments that regular step spacing develops in response to deposition forced at regular locations beneath water surface waves we observe neither the regular distribution nor the associated depositional mechanism although the observed step distribution is well defined with a strong mode that could be interpreted as indicating regular spacing

bedload transport measurements with impact plate geophones comparison of sensor calibration in different gravel bed streams indirect bedload transport measurements have been made with the swiss plate geophone system in five gravel bed mountain streams these geophone sensors record the motion of bedload particles transported over a steel plate mounted flush with the channel bed to calibrate the geophone system direct bedload transport measurements were undertaken simultaneously at the erlenbach in switzerland a moving basket sampler was used at the fischbach and ruetz streams in austria a helley smith type bedload sampler provided the calibration measurements a bunte type bedload trap was used at the rofenache stream in austria at the nahal eshtemoa in israel reid type slot bedload samplers were used to characterize the response of the geophone signal to bedload particles impacting on the plate geophone summary values were calculated from the raw signal and stored at one second intervals the number of impulses i e the number of peaks above a pre defined threshold value of the geophone output signal correlated well with field measured gravel transport loads and was found to be a robust parameter the relations of impulses to gravel transport loads were generally near linear but the steepness of the calibration relations differed from site to site by comparing the calibration measurements from the different field sites and utilizing insights gained during preliminary flume experiments it has been possible to identify the main factors that are responsible for site specific differences in the calibration coefficient the analysis of these calibration measurements indicates that the geophone signal also contains some information about the grain size distribution of bedload copyright 2013 john wiley sons ltd

the impact of exceptional events on erosion bedload transport and channel stability in a step pool channel sediment transport in the erlenbach a small stream with step pool morphology in the canton of schwyz switzerland has been monitored for more than 20 years during this time three exceptional events events with high sediment yield and long return times that have a large effect on channel morphology have impacted the stream and partly or completely rearranged the existing step pool morphology in the aftermath of the events sediment transport rates at a given discharge and total sediment yield remained elevated for about a year or longer for the last event dated on the 20 june 2007 observations of boulder mobility and step destruction were used to interpret channel stability boulders with median diameters of up to 135 cm and estimated weights of more than 2 5 tons have moved during the 2007 event using hydraulic observations and shear stress calculations boulders up to 65 cm in diameter were predicted to have been fully mobile in peak conditions even if form resistance and increased critical stresses needed for the initiation of motion in steep streams were taken into account for two of the events estimated peak shear stresses at the bed exceeded 1000 pa calculated both from observations of the flow hydraulics and from boulder mobility this suggests that highly energetic flows occur relatively frequently in small steep streams and that large boulders can be transported by fluvial processes in such streams the observations have potential significance for hazard risk mitigation stream engineering and restoration copyright 2009 john wiley sons ltd

a field investigation of the influence of sediment supply on step pool morphology and stability abstract the effect of the sediment supply on step pool morphology and stability was studied through the investigation of 42 step pool reaches eight in the vosges and 34 in the alps france step pool geometry step wavelength l and height h was measured for each reach and the data set split into two distinct groups comprising 25 connected to a sediment source step pool reaches and 17 disconnected step pool reaches the geometry analysis showed differences between the two groups with the shape factors k l hr and the h hr ratios hr is the residual pool depth being lower for disconnected step pools this was interpreted to result from progressively changing geometry over time with disconnected step pool geometry defining the probable asymptotic characteristic associated with this long term process the measured step pool dimensions were in good agreement with equations based on maximum flow resistance abrahams et al 1995 energy of a falling jet comiti et al 2005 and channel spanning deposition chartrand et al 2011 however the average shape factor for the comiti et al equation was k 17 instead of 8 as the authors had proposed the data set was extended to 255 values with data from the literature which confirmed this average value for k field evidence moss cover and stone imbrication indicate that connected step pools are mobile whereas disconnected step pools are stable this was investigated by a hydraulic analysis of threshold motion conditions for the boulder steps comparison with discharges measured at nearby gauging stations indicates that remobilization return periods are 20 50 years for connected step pools and even less when considering scouring effects and greater than 50 years for disconnected step pools this difference in step mobility was ascribed to the fact that with sediment supply the channel can freely adjust its local slope allowing a relatively efficient transfer downstream of large boulders by scouring effects

alluvial architecture in headwater streams with special emphasis on step pool topography alluvial mountain streams exhibit a range of channel forms pool riffle plane bed step pool and cascades previous work suggested that these forms exist within discrete and progressively steeper slope classes measurements conducted at over 100 sites in west central and central idaho confirm that slope steepens progressively as one moves from pool riffle to plane bed to step pool and finally to cascades median slope for pool riffle topography is 0 0060 for plane beds 0 013 for step pools 0 044 and for cascades 0 068 there is substantial overlap in the slopes associated with these channel forms pool riffle topography was found at slopes between 0 0010 and 0 015 plane beds between 0 0010 and 0 035 step pools between 0 015 and 0 134 and cascades between 0 050 and 0 12 step pools are particularly striking features in headwater streams they are characterized by alternating steep and gentle channel segments the steep segments step risers are transverse accumulations of boulder and cobbles while the gentle segments pools contain finer material step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers this result differs from past studies that have reported channel slope to be the dominant control on step wavelength the presumed geometry and froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step pools are formed by antidunes copyright 2000 john wiley sons ltd

shear strength characteristics of two saprolitic soils the saturated and unsaturated shear strength behavior of an undisturbed completely decomposed fine ash tuff and an undisturbed completely decomposed granite from hong kong were studied using direct shear and triaxial tests the completely decomposed fine ash tuff is a fine to medium grained saprolite the completely decomposed granite is a coarse grained saprolite results show that matric suction increases the shear strength of both soils the extent of the increase is the shear strength with matric suction is related to the soil water characteristic curve for the soil and to the amount of dilation during shear the effect of matric suction on the shear strength was more pronounced for the fine to medium grained completely decomposed fine ash tuff than for the coarse grained completely decomposed granite these studies on the saprolitic soils provide insight into the understanding of the shear strength of unsaturated coarse grained soils key words saprolites shear strength matric suction triaxi

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

bedload hysteresis in a glacier fed mountain river sediment transport during flood events often reveals hysteretic patterns because flow discharge can peak before counterclockwise hysteresis or after clockwise hysteresis the peak of bedload hysteresis in sediment transport has been used in the literature to infer the degree of sediment availability counterclockwise and clockwise hysteresis have been in fact interpreted as limited and unlimited sediment supply conditions respectively hysteresis has been mainly explored for the case of suspended sediment transport but it was rarely reported for bedload transport in mountain streams this work focuses on the temporal variability of bedload transport in an alpine catchment saldur basin 18 6 km2 italian alps where bedload transport was monitored by means of an acoustic pipe sensor which detects the acoustic vibrations induced by particles hitting a 0 5m long steel pipe runoff dynamics are dominated by snowmelt in late spring early summer mostly by glacier melt in late summer early autumn and by a combination of the snow and glacier melt in mid summer the results indicate that hysteretic patterns during daily discharge fluctuations are predominantly clockwise during the snowmelt period likely due to the ready availability of unpacked sediments within the channel or through bank erosion in the lower part of the basin on the contrary counterclockwise hysteresis tend to be more frequent during late glacier melting period possibly due to the time lag needed for sediment provided by the glacial and peri glacial area to be transported to the monitoring section however intense rainfall events occurring during the glacier melt period generated predominantly clockwise hysteresis thus indicating the activation of different sediment sources these results indicate that runoff generation processes play a crucial role on sediment supply and temporal availability in mountain streams

prediction of sediment transport in step pool channels 1 in mountainous drainage networks sediment mobilized on hillslopes must first pass through steep streams before reaching lower gradient channels the bed of steep channels is typically composed of large relatively immobile boulders and finer more mobile gravel most sediment transport equations overpredict sediment flux in steep streams by several orders of magnitude because they do not account for the stress borne by immobile grains and the limited availability of the more mobile sediment we previously developed and tested in flume experiments a sediment transport equation that accounts for these two effects here we modify the parker 1990 bed load equation to include the resistance borne by steps and selective transport of the relatively mobile sediment using a range of hiding functions we test a number of resistance equations and hiding functions combined with our modified and the original parker equations against measured flow and sediment transport in three steep channels our modified sediment transport equation generally predicts the transported sediment volumes to within an order of magnitude of the measured values whereas the unmodified equations do not the most accurate sediment flux predictions were obtained from using our modified equation combined with a hiding function that calculates highly selective transport of the relatively mobile sediment furthermore this hiding function has a critical shields stress that is similar to those reported for lower gradient channels the effects of the immobile steps on flow and sediment transport are not adequately captured by simply increasing the critical shields stress to values reported in steep streams

grain size effects on energy delivery to the streambed and links to bedrock erosion fluvial bedrock erosion rates are thought to be proportional to the energy delivered to the channel bed by impacting bed load particles combining methods for field measurements of impact energy and for automatic sensing of grain size distributions in the instrumented channel of the erlenbach we show that the larger a bed load grain is the more efficient it is in transferring energy to the streambed on average the largest grain size class contributes only 9 to the total bed load mass but is responsible for more than 40 of the energy delivery this effect may help to explain the predominance of extreme floods when large grains are mobile in driving long term erosion while the mechanistic saltation abrasion model accounts for hydraulic forcing of energy delivery the observed grain size effect is not captured we suggest that the discrepancy arises from differing transport modes rolling sliding and saltation not currently captured in modeling

morphological effects of local scouring in step pool streams step pool morphology characterizes many high gradient streams in a variety of natural settings but formative processes and evolutionary dynamics are still poorly understood in this paper natural step pool geometry is compared with steep alluvial channels where grade control structures such as check dams and bed sills make the stream profile resemble a natural stepped stream along these channels local scouring due to falling jets forms plunge pools under each structure analogous to natural steps determining the formation of pools in order to test the hypothesis that natural pools are analogous to pools formed below grade control works with respect to their dimensions shape and formative dynamics 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern italian alps pools below grade control works featured a transitional zone between the scour hole and the downstream sloping bed marked by a depositional berm when geometric parameters such as maximum pool depth length and step berm distances are normalized to the jet virtual energy no statistically significant differences were detected between natural and artificial systems these results lend support to an upstream forced cascade model for step pool formation where the energy of falling jets controls the geometry of the pools and is therefore regarded as the most important scaling independent variable copyright 2005 john wiley sons ltd