developments in monitoring and modelling small scale river bed topography recent research in fluvial geomorphology has emphasized the spatially distributed feedbacks amongst river channel topography flow hydraulics and sediment transport although understanding of the behaviour of dynamic river channels has been increased markedly through detailed within channel process studies less attention has been given to the accurate monitoring and terrain modelling of river channel form using three dimensional measurements however such information is useful in two distinct senses firstly it is one of the necessary boundary conditions for a physically based deterministic modelling approach in which three dimensional topography and river discharge drive within channel flow hydraulics and ultimately spatial patterns of erosion and deposition and therefore channel change secondly research has shown that an alternative means of estimating the medium term bedload transport rate can be based upon monitoring spatial patterns of erosion and deposition within the river channel this paper presents a detailed assessment of the distributed monitoring and terrain modelling of river bed topography using a technique that combines rigorous analytical photogrammetry with rapid ground survey the availability of increasingly sophisticated terrain modelling packages developed for civil engineering application allows the representation of topographic information as a landform surface intercomparison of landform surfaces allows visualization and quantification of spatial patterns of erosion and deposition a detailed assessment is undertaken of the quality of the morphological information acquired this allow some general comments to be made concerning the use of more traditional methods to monitor and represent small scale river channel morphology

hydrodynamics of a forced riffle pool in a gravel bed river 1 mean velocity and turbulence intensity 1 riffle pools are fundamental units of many gravel bed rivers considerable debate exists as to an appropriate hydrodynamic model for this bed form type sampling designs in previous studies have not always anticipated the degree of spatial variability of flow parameters in riffle pools nor considered the effect of the nonuniform boundary on hydrodynamics the objective of this paper is to detail the distribution of mean velocities and turbulence intensities in a forced riffle pool while limited by the lack of lateral velocity measurements the data set is of high quality and includes measurements during a bankfull flood key observations include the perturbation of velocity profiles from what is observed in uniform flow lateral convergence and divergence of flow and the generation of turbulence away from the channel boundary in the thalweg of the pool head velocities are high near the water surface and low near the bed turbulence intensities are relatively high near the bed and may be significant for sediment entrainment higher mean velocities occur over the side bar and could indicate that sediment is routed around the thalweg the midpool is characterized by the lateral constriction of flow and a shear zone downstream of the forcing element the pool tail has very high velocities and turbulence intensities near the bed a velocity reversal occurs in the pool tail at bankfull flow but does not occur in the pool head a consideration of deceleration and acceleration as a result of vertical expansion and contraction is shown to explain many of the key observations

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

scour downstream of grade control structures a theoretical investigation of local scour downstream of gradecontrol structures based on twodimensional jet diffusion and particle stability is experimentally verified turbulent jet diffusion red

measurement of local bed shear stress in streams using a preston static tube local bed shear stress t w is a fluid dynamic parameter of importance in determining the physical and biological characteristics of stream bed environments unfortunately it is often difficult to measure t w under field conditions herein we describe the use of a preston static tube which is essentially a surface mounted pitot static tube to measure t w in mountain streams our results indicate that it is possible to measure local shear stress quickly consistently and inexpensively in the field this technique also provides high spatial resolution which should allow for detailed in situ studies of local shear stress at scales relevant to lotic organisms such information will be invaluable in studies of benthic organisms and hydraulically relevant phenomena in the near bottom zone of lotic systems

hyporheic flow path response to hydraulic jumps at river steps flume and hydrodynamic models 1 the conceptual model of hyporheic exchange below river steps may oversimplify exchange flow paths if it depicts a uniform pattern of downstream directed upwelling this research used nonmobile porous bed flume experiments and hydrodynamic simulation cfd to characterize hyporheic flow paths below a river step with a hydraulic jump bed slope was 1 step height was 4 cm downstream flow depth was 4 cm substrate was 1 cm median diameter gravel and hydraulic jump length was 25 cm in the flume and cfd experiments with the hydraulic jump flow paths changed to include downwelling beneath the water plunging into the pool and upstream directed upwelling at the base of the step and beneath the length of jump failure to represent the influence of static and dynamic pressures associated with hydraulic jumps leads to erroneous prediction of subsurface flow paths in 75 of the streambed beneath the jump a refined conceptual model for hyporheic flow paths below a step with a hydraulic jump includes reversed hyporheic circulation cells in which downwelling water moves upstream and then upwells and flow reversals in which the larger flow net of downstream directed upwelling encounters a nested flow path of upstream directed upwelling heterogeneity in hyporheic flow paths at hydraulic jumps has the potential to explain field observed mosaics in streambed redox patterns and expand structure function relationships used in river management and restoration

sediment transport through self adjusting bedrock walled waterfall plunge pools many waterfalls have deep plunge pools that are often partially or fully filled with sediment sediment fill may control plunge pool bedrock erosion rates partially determine habitat availability for aquatic organisms and affect sediment routing and debris flow initiation currently there exists no mechanistic model to describe sediment transport through waterfall plunge pools here we develop an analytical model to predict steady state plunge pool depth and sediment transport capacity by combining existing jet theory with sediment transport mechanics our model predicts plunge pool sediment transport capacity increases with increasing river discharge flow velocity and waterfall drop height and decreases with increasing plunge pool depth radius and grain size we tested the model using flume experiments under varying waterfall and plunge pool geometries flow hydraulics and sediment size the model and experiments show that through morphodynamic feedbacks plunge pools aggrade to reach shallower equilibrium pool depths in response to increases in imposed sediment supply our theory for steady state pool depth matches the experiments with an r 2 value of 0 8 with discrepancies likely due to model simplifications of the hydraulics and sediment transport analysis of 75 waterfalls suggests that the water depths in natural plunge pools are strongly influenced by upstream sediment supply and our model provides a mass conserving framework to predict sediment and water storage in waterfall plunge pools for sediment routing habitat assessment and bedrock erosion modeling

processes and forms of an unstable alluvial system with resistant cohesive streambeds as a response to channelization projects undertaken near the turn of the 20th century and in the late 1960s upstream reaches and tributaries of the yalobusha river mississippi usa have been rejuvenated by upstream migrating knickpoints sediment and woody vegetation delivered to the channels by mass failure of streambanks has been transported downstream to form a large sediment debris plug where the downstream end of the channelized reach joins an unmodified sinuous reach classification within a model of channel evolution and analysis of thalweg elevations and channel slopes indicates that downstream reaches have equilibrated but that upstream reaches are actively degrading the beds of degrading reaches are characterized by firm cohesive clays of two formations of palaeocene age the erodibility of these clay beds was determined with a jet test device and related to critical shear stresses and erosion rates repeated surveys indicated that knickpoint migration rates in these clays varied from 0 7 to 12 m a 1 and that these rates and migration processes are highly dependent upon the bed substrate resistant clay beds of the porters creek clay formation have restricted advancement of knickpoints in certain reaches and have caused a shift in channel adjustment processes towards bank failures and channel widening channel bank material accounts for at least 85 per cent of the material derived from the channel boundaries of the yalobusha river system strategies to reduce downstream flooding problems while preventing upstream erosion and land loss are being contemplated by action agencies one such proposal involves removal of the sediment debris plug bank stability analyses that account for pore water and confining pressures have been conducted for a range of hydrologic conditions to aid in predicting future channel response if the sediment debris plug is removed to improve downstream drainage care should be taken to provide sufficient time for drainage of groundwater from the channel banks so as not to induce accelerated bank failures published in 2002 john wiley sons ltd

jet and hydraulic jump near bed stresses below a horseshoe waterfall 1 horseshoe waterfalls are a common but unexplained feature of bedrock rivers as a step toward understanding their geomorphology a six component force torque sensor was used to evaluate clear water scour mechanisms in a 0 91 m broad crested horseshoe waterfall built into a 2 75 m wide flume to measure longitudinal lateral and temporal profiles of the near bed spatially averaged stress vector jet impact caused moderate near bed stress downstream of the impact point the stress vector shifted into a much more forceful upthrust with peak stress beyond the boil crest instantaneous fluctuations in near bed lift exceeded drag by 5 10 times and occurred over a wider range of frequencies suggesting a larger role for particle entrainment predictions of bed stress significantly underestimated actual conditions these results show that horseshoe waterfalls have an extended region of high scour along their centerline corresponding with the three dimensional convergent dynamics associated with both jet impact and jump enhanced lift fluctuations

experiments and modeling of head cut migration in stratified soils 1 a two dimensional model is developed to predict head cut migration rate for the specific case of a relatively erosion resistant soil layer overlying a deep relatively erosive base soil for this geometry head cut migration can occur as a series of discrete mass failures of the surface layer caused by undercutting in a plunge pool formed immediately downstream the length scale of one failure episode is modeled as a cantilevered slab failing in bending the timescale for each failure episode is controlled by the rate and geometry of base layer scour which is modeled independently of cantilevered slab properties an integration of the two model components for the case of steady flow and uniform soil properties predicts linear upstream head cut migration with time occurring in discrete steps bounded by successive mass failure events the model is calibrated and tested in a series of flume experiments representing ideal conditions of different initial geometries and flow conditions head cut migration occurred by the proposed mechanism in a nearly linear rate average error between the observed and predicted retreat rate is 40 most of this error is attributable to prediction of the scour hole shape

monitoring and modelling morphological change in a braided gravel bed river using high resolution gps based survey recent research in fluvial geomorphology has emphasized the direct monitoring of channel topography as a useful tool to better understand the interrelationship between river form and process and in particular to estimate bedload transport rates and reach scale sediment budgets until recently however this morphometric approach has been limited to comparison of repeatedly surveyed cross sections with the associated problems of low frequency sampling and an emphasis on cross stream as opposed to downstream morphological dynamics recent advances in analytical photogrammetry have however permitted the acquisition of data at sufficient resolutions to represent macroform 101 m and microform 10 m channel geometry in fully three dimensions while inherently spatially distributed the photogrammetric approach to land surveying is however restricted to observable areas of the channel and is thus of limited use in the subaqueous zone especially if flows are turbid or highly coloured this paper presents an alternative approach to the study of three dimensional morphological dynamics of a divided reach of the gravelly river feshie scotland in which topographic survey of both exposed and submerged areas of the reach was undertaken using the global positioning system gps survey results from two field programmes in 1998 and 1999 are presented in which a geotronics geotracer 2000 real time kinematic gps receiving unit was used to survey a 200 80 m reach rapid radial survey methods were employed permitting the collection of 2000 survey points per day giving a mean density over the 13 ha reach of 1 1 points per square metre a detailed assessment of survey errors and dem quality is presented and a limit of detection of c 10 cm proposed as an upper threshold for the identification of significant change between dems preliminary interpretation of channel dynamics in 1998 99 reveals relatively little change to the overall macroscale reach structure dem differencing techniques however highlight important local changes including the progradation of submerged bar fronts bar head aggradation and scour along major channel thalwegs and the erosion of fines from sloughs copyright 2000 john wiley sons ltd

submerged and unsubmerged natural hydraulic jumps in a bedrock step pool mountain channel high resolution digital elevation models dems for natural submerged and unsubmerged jump regions were tested against the classical hydraulic jump chj and engineering analogues that have dominated previous geomorphic and engineering research dems were compared for two discharge conditions the data showed bed and water surface features of the natural jump regions differ significantly from chj conditions and engineering analogues with respect to boundary conditions roughness elements were highly irregular in frequency spacing size and orientation and were comprised of form obliquity and bed scour in boulder bed and pure bedrock conditions the data also showed bed and water surface features of natural jump regions have similarities and deviations from chj conditions and engineering analogues with respect to hydraulic characteristics several variations were observed in the hydraulic characteristics between the ballistic and sloping jets revised conceptual models based on idealized corollaries were developed for the natural jump regions based on upstream energy head relative to the downstream tailwater depth and step geometry the dems highlight the importance of recognizing the interrelationship between spatial transcritical flow structures and localized topographic heterogeneities in bedrock channels

combining geologic process models and geostatistics for conditional simulation of 3 d subsurface heterogeneity 1 the goal of simulation of aquifer heterogeneity is to produce a spatial model of the subsurface that represents a system such that it can be used to understand or predict flow and transport processes spatial simulation requires incorporation of data and geologic knowledge as well as representation of uncertainty classical geostatistical techniques allow for the conditioning of data and uncertainty assessment but models often lack geologic realism simulation of physical geologic processes of sedimentary deposition and erosion process based modeling produces detailed geologically realistic models but conditioning to local data is limited at best we present an aquifer modeling methodology that combines geologic process models with object based multiple point and variogram based geostatistics to produce geologically realistic realizations that incorporate geostatistical uncertainty and can be conditioned to data first the geologic features of grain size or facies distributions simulated by a process based model are analyzed and the statistics of feature geometry are extracted second the statistics are used to generate multiple realizations of reduced dimensional features using an object based technique third these realizations are used as multiple alternative training images in multiple point geostatistical simulation a step that can incorporate local data last a variogram based geostatistical technique is used to produce conditioned maps of depositional thickness and erosion successive realizations of individual strata are generated in depositional order each dependent on previously simulated geometry and stacked to produce a fully conditioned three dimensional facies model that mimics the architecture of the process based model we demonstrate the approach for a typical subsea depositional complex

modeling energy dissipation and hydraulic jump regime responses to channel nonuniformity at river steps 1 the river step is an important driver for geomorphic evolution in bedrock rivers but the effect that variations in channel geometry upstream and downstream of a river step have on hydraulic jump regime and energy dissipation has not previously been investigated the associated hydraulic jump is inherent to a river step and its regime is a primary control on step morphodynamics in turn the hydraulic jump regime is controlled by several variables as detailed in a new conceptual framework herein also in this study a parsimonious semianalytical numerical model of step hydraulics is developed to quantify energy dissipation and delineate hydraulic jump regimes accounting for discharge jump submergence and nonuniform channel geometry through a step despite remaining limitations in step theory the model simulates how natural steps respond to a wide range of conditions the model shows that hydraulic jump regime and energy dissipation exhibit greater sensitivity to channel nonuniformity as discharge increases and or step height decreases also channel conditions that create greater jump submergence lead to decreased energy dissipation regardless of the discharge regime the model also reinforces the common observation about gully erosion that downstream channel widening enhances upstream knickpoint migration the new algorithm may be used to aid river engineering involving steps and could be useful for landscape evolution modeling

origin and interpretation of knickpoints in the big south fork river basin kentucky tennessee this study investigates the causes of knickpoints and knickzones in the bedrock controlled streams of the big south fork river basin in kentucky and tennessee knickpoints in the big south fork river area vary in form and apparent origins while some are likely related to base level change and incision in the cumberland river drainage system the locations and drainage relations of the knickpoints are not consistent with transmission of an incision signal throughout the network local controls predominate in forming steeper channel segments with no single factor dominant knickpoints in the study area are characterized by polygenesis and multiple causality though several archetypes can be identified these include rock fall rapids created by mass wasting from adjacent valley slopes structurally controlled headwater cliffs and lithological knickpoints a fourth category local incision knickpoints may be attributable to a variety of factors influencing force resistance relationships these results imply that the simple presence of a knickpoint cannot be attributed to any particular cause or history without consideration of the local controls this further implies that factors such as the spacing of knickpoints may not be an indication of migration rates or that migration has even occurred however the analysis of individual profile convexities can shed light on various controls such as lithology and structure and other processes such as valley side mass wasting and local bed incision important in evolution of fluvially dissected landscapes

fluctuating uplift and lining design in spillway stilling basins the paper deals with the characterization and the control of transient uplift generated by turbulent pressure fluctuations in spillway stilling basins theoretical and experimental analyses are presented defining the maximum instantaneous uplift acting on the linings which lead to a design criterion for suitable floor slabs the thickness of concrete linings not accounting for the possible contribution of anchors required for stability is defined in this paper as a function of 1 the incoming kinetic head and or froude number 2 a dimensionless function of the longitudinal and transversal dimensions of the lining slabs the areal extent of extreme turbulent pressure fluctuations and the anisotropic integral scales of the instantaneous spatial correlation of pressure fluctuations 3 the ratio of specific weights of water and immersed concrete and 4 a dimensionless pressure coefficient qualifying the maximum difference of positive and negative pressure fluctuations about the stationary mean value it is concluded that safe design of protection works is significantly affected by the structure of instantaneous spatial distribution of pressures at the bottom of hydraulic jumps consideration of this factor leads to recommendations for the design of slabs in stilling basins

design of anchored slabs in spillway stilling basins this paper characterizes the temporal behavior of uplift force generated by turbulent pressure fluctuations in spillway stilling basins theoretical and experimental analyses are presented that define the magnitude and temporal evolution of the maximum uplift acting on the lining of such basins analyses for the dynamic behavior of anchored floor slabs are also investigated it is concluded that the applicability of the equivalent thickness criterion based on the balance of the forces acting on the slabs in static condition is unsafe for anchored slabs because this criterion yields an inadequate area for the anchor steel the results lead to a recommendation to double the area of anchor steel as computed by the equivalent thickness criterion for the design of slabs in stilling basins

tdr measurements of hydraulic jump aeration in the south fork of the american river california abstract turbulent pressure and velocity fluctuations associated with hydraulic jump air entrainment likely play an important role in local flow dynamics and morphologic changes of steep bedrock and alluvial rivers previous engineering research indicates that hydraulic jump air entrainment and energy dissipation locally intensify erosion of both bedrock and alluvial elements enhance hydraulic quarrying suppress cavitation and decrease the friction factor of a flow despite studies in artificial settings air content in natural hydraulic jumps remains largely unquantified in this study time domain reflectometry tdr a method commonly used to determine the volumetric water content of porous media for the last two decades was evaluated in the laboratory and then adapted to measure air contents of natural hydraulic jumps laboratory tests were conducted to determine the influence of turbulence temperature and sampling volume on the tdr signal over an air content range of 0 100 based on the results of the laboratory tests an empirical calibration was developed that converts the tdr signal into air content local air contents were then measured in hydraulic jumps occurring along a mixed alluvial and bedrock dominated 8 4 km section of the south fork american river two small jumps had a mean air content of 19 and 23 a coefficient of variation cv of 27 and 28 and a root mean square rms of 19 and 24 respectively a large jump had a mean air content of 42 a cv of 26 and a rms of 44 tdr provides fast accurate repeatable and nondestructive in situ measurements at one tenth the cost of traditional methods and is far more robust for field applications than traditional lab instruments future studies can now expand upon the hydrogeomorphic and ecological significance of jump aeration

recession rates of waterfalls in boso peninsula japan and a predictive equation rates of waterfall recession and major factors that influence the rate were studied using waterfalls in boso peninsula japan the mean rate of waterfall recession was estimated by determining the age and original location the principal factor in determining the rate of waterfall recession is the ratio of the erosive force of stream to the bedrock resistance this is expressed in terms of measurable variables which include the discharge drainage area and precipitation the width and height of the waterfall and the unconfined compressive strength of the bedrock an empirical equation connecting the force resistance ratios and the rates of waterfall recession is derived copyright 2003 john wiley sons ltd

experiments on headcut growth and migration in concentrated flows typical of upland areas experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas in a laboratory channel packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow the rainfall produced a well developed surface seal that minimized surface soil detachment during overland flow soil erosion occurred exclusively at the headcut and after a short period of time a steady state condition was reached where the headcut migrated at a constant rate the scour hole morphology remained unchanged and sediment yield remained constant a fourfold increase in flow discharge resulted in larger scour holes yet aspect ratio was conserved a sediment bed was deposited downstream of the migrating headcut and its slope depended weakly on flow discharge

air concentrations of submerged and unsubmerged hydraulic jumps in a bedrock step pool channel 1 the classical hydraulic jump which is a hydraulic jump formed in a horizontal smooth and prismatic laboratory flume has often been used for estimating hydraulic and morphologic characteristics of mountain river channels however most studies utilizing classical hydraulic jump relationships have neglected the role of aerated flows and jump types prevalent in natural systems by developing a methodology to survey digitize and visualize air concentrations for jumps in bedrock rivers we present new data for submerged jumps and sloping jumps in a bedrock step pool channel as with laboratory research our results indicated peak mean and local air concentrations of natural jumps were skewed by submergence of the jump roller beneath the free surface however unlike laboratory research cross stream variations were significant for both jumps varying up to 20 across the jump width traces through each jump thalweg also indicated the classical hydraulic jump was a poor analogue for estimating surficial air concentrations

evolution of vertical knickpoints waterfalls with resistant caprock insights from numerical modeling 1 vertical knickpoints waterfalls mark a prominent process transition zone whose governing mechanics is not represented by conventional stream power incision models we examine the evolution of vertical knickpoints with resistant caprock utilizing numerical simulations that explicitly represent 1 face failure mechanisms 2 flow acceleration and amplified erosion above a knickpoint lip 3 deposition and removal of coarse debris below the knickpoint and 4 base level lowering or tectonic uplift rates our model demonstrates that knickpoint retreat rate where the subcaprock is weak or vertically jointed and base level fall rates are steady is likely to become tied to downstream conditions and equal to the downstream incision rate divided by channel gradient mechanically this coupling occurs where the subcaprock reaches a threshold height for failure in shear or buckling or where the weathering rate of the subcaprock is higher than the downstream incision wave velocity vi ds the height of the subcaprock face can influence its gravitational stability and the knickpoint lateral erosion rate and lead to a feedback between downstream incision and retreat rate retreat rate can be lower than vi ds during transients which could be long 106 years and set by the weathering rate of the subcaprock or influenced by lag debris evacuation key variables other than discharge can be important in setting retreat rates these include base level lowering rate the rock strength of stratigraphic units downstream of the knickpoint and the size and flux of sediment contributed from above the knickpoint or from the canyon walls two types of oversteepened reaches can form in association with a vertical knickpoint 1 an upstream free fall induced oversteepened reach whose length is longer than the flow acceleration zone and 2 a downstream coarse debris induced oversteepened reach although the model was constructed with caprock type knickpoints in mind some of its elements and insights are also relevant to homogenous substrates

hydraulics of floods upstream of horseshoe canyons and waterfalls horseshoe waterfalls are ubiquitous in natural streams bedrock canyons and engineering structures nevertheless water flow patterns upstream of horseshoe waterfalls are poorly known and likely differ from the better studied case of a one dimensional linear step because of flow focusing into the horseshoe this is a significant knowledge gap because the hydraulics at waterfalls controls sediment transport and bedrock incision which can compromise the integrity of engineered structures and influence the evolution of river canyons on earth and mars here we develop new semiempirical theory for the spatial acceleration of water upstream of and the cumulative discharge into horseshoe canyons and waterfalls to this end we performed 110 numerical experiments by solving the 2 d depth averaged shallow water equations for a wide range of flood depths widths and discharges and canyon lengths widths and bed gradients we show that the upstream normal flow froude number is the dominant control on lateral flow focusing and acceleration into the canyon head and that focusing is limited when the flood width is small compared to a cross stream backwater length scale in addition for sheet floods much wider than the canyon flow focusing into the canyon head leads to reduced discharge and drying in cases across the canyon sidewalls which is especially pronounced for canyons that are much longer than they are wide our results provide new expectations for morphodynamic feedbacks between floods and topography and thus canyon formation

geometric scaling of step pools in mountain streams observations and implications understanding the geometric structuring of alluvial step pools is an enduring problem in mountain stream geomorphology many previous studies propose that the key control parameter governing the step pool form is mean bed slope as explored through several examples however we find that slope control formulations are inconsistently demonstrated for broad data sets on this basis we reexamine the bed form architecture of step pools with a new descriptive parameter termed the aspect ratio active stream width divided by step drop height we test with data collected from three different mountain settings for step pools observed over mean bed slopes ranging from 2 to 22 we find that step wavelength and height are well described by when these dimensions are normalized by the step drop height furthermore we observe that varies inversely with mean bed slope context for our findings is provided by recent field and laboratory work which highlights that the cross stream component to the overall flow structure in step pools and scour pools is significant this is important because it expands upon and relates our work to formative models which are based on jet scour and the associated three dimensional flow structure of step pools our work has practical implications for the design of step pool channels because existing approaches are based in large part on structuring artificial systems according to a narrowly defined geometry we observe however that under certain conditions this approach may yield constructed step pools that are unstable as an alternative approach we propose that the geometric relationships developed here can be applied to identify artificial step pool geometries that will be more fully reflective of natural analogs and thus possibly more stable

sediment and rock strength controls on river incision into bedrock recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply challenging the common assumption that incision rate is simply proportional to stream power our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits maximum erosion rates occur at a critical level of coarse grained sediment supply where the bedrock is only partially exposed fine grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate furthermore we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief

granular nanoparticles lubricate faults during seismic slip nanoparticles are known to form in narrow slip zones in natural and experimental fault zones but their possible role in dynamic weakening of faults during seismic slip remains almost unexplored we conducted friction experiments on periclase mgo nanoparticles 50 nm in average size at rates as high as 1 3 m s 1 a typical speed of seismic slip the nanoparticles were used as the initial gouge to avoid complexities arising from comminution and fluid release the dynamic friction decreased with increasing slip rate and the steady state frictional coefficient reduced to as low as 0 1 at a slip rate of 1 3 m s 1 flash heating is not effective for nanoparticles and we propose that development of slickensides and dominant operation of nanoparticle rolling cause such a weakening nanoparticle lubrication appears to be as effective as melt lubrication and thermal pressurization and the formation of nanoparticles in slip zones may be an important fault lubrication process

convergent hydraulics at horseshoe steps in bedrock rivers abstract horseshoe waterfalls are a common feature of steep bedrock rivers as a first step toward understanding their geomorphology a detailed study of the fluid mechanics at a 0 91 m vertical drop horseshoe waterfall was performed in a 2 75 m wide flume five non dimensional upstream energy levels each with 3 5 non dimensional downstream tailwater depths 21 runs total were assessed for water surface topography via digital elevation modeling flow dynamics via digital videography and overall energy dissipation via an energy and momentum conservation model regardless of tail depth the horseshoe waterfall was found to have three distinct zones beyond the step brink 1 a nappe whose degree of convergence depends on upstream energy and brink configuration 2 a convergence zone whose features vary strongly with upstream energy brink configuration and tail depth and 3 a downstream tailwater region whose dynamics primarily depend on tail depth the centerline nappe profile and brink velocity were reasonably predicted using rouse s jet trajectory equations when h p h 2 peripheral profiles were not predictable using existing equations for any arbitrary broad crested step brink configuration maximum energy dissipation was found to occur when no jump was present and downstream tail depth was exactly critical rather than providing maximal energy dissipation hydraulic jumps below steps provide efficient conversion of kinetic energy to potential energy

transition from hydraulic jump to open channel flow this technical note presents the results of a laboratory study of the transition region from the end of a hydraulic jump to fully developed open channel flow the velocity profile changes from that of a wall jet to the semilogarithmic form of open channel flow in this region the bed shear stress continues to decrease rapidly to eventually become constant the intensity of flow on the centerplane decreases continuously to attain the relevant open channel value the length of this transition region was found to be approximately equal to 10 times the tailwater depth for the range of froude numbers studied which varied from 3 87 to 10 48