long term slowdown of river tracer pebbles generic models and implications for interpreting short term tracer studies 1 we propose generic approaches to modeling the reduction in mobility that occurs as tracer pebbles seeded on the surface are mixed to less active locations e g beneath the bed or high on bars or advected downstream to reaches with different shear stress or bed grain size advection slowdown which may be negligible is quantified using a regression equation relating virtual velocity to tracer and reach properties mixing slowdown which is always present is simulated using a markov model of exchanges between three stores and assumptions about relative mean velocities of tracers in each store the model can be calibrated from a full search on one date and at least a surface search on another date calibration to six reaches of allt dubhaig scotland in which tracers were mapped after 2 and 8 years suggests mixing took 3 5 years to approach equilibrium in five reaches and was still proceeding in one simulated slowdown through mixing is far greater than that through advection and goes much of the way to explaining the 50 observed reduction in virtual velocity over 8 years compared to 2 years implications for the design and comparison of tracer pebble experiments are considered

surface based transport model for mixed size sediment we present a transport model for mixed sand gravel sediments fractional transport rates are referenced to the size distribution of the bed surface rather than subsurface making the model completely explicit and capable of predicting transient conditions the model is developed using a new data set of 48 coupled observations of flow transport and bed surface grain size using five different sediments the model incorporates a hiding function that resolves discrepancies observed among earlier hiding functions the model uses the full size distribution of the bed surface including sand and incorporates a nonlinear effect of sand content on gravel transport rate not included in previous models the model shares some common elements with two previous surface based transport models but differs in using the full surface size distribution and in that it is directly developed from a relatively comprehensive data set with unambiguous measurement of surface grain size over a range of flow transport rate and sediments

hydraulic geometry river sediment and the definition of bedrock channels a growing literature deals with bedrock channels erosion processes within them and their role in landscape evolution formal definitions currently in use classify channels as bedrock when alluvial cover is discontinuous or thin this is equated with a physical property of the flow using up to date erosion laws and descriptions of channel dynamics we show that existing definitions are difficult to apply and may lead to conflicting classifications we propose the definition a bedrock channel cannot substantially widen lower or shift its bed without eroding bedrock applied to channels in taiwan this leads to a different classification than previous definitions by analysing at a station hydraulic geometry of these channels we show that the classification exposes different controls of sediment effects on dynamics of channels classified as alluvial and bedrock

physically based modeling of bedrock incision by abrasion plucking and macroabrasion 1 many important insights into the dynamic coupling among climate erosion and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision however many questions regarding incision processes and morphology of bedrock streams still remain unanswered a more mechanistically based incision model is needed as a component to study landscape evolution major bedrock incision processes include among other mechanisms abrasion by bed load plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts the purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above to build the model we start by developing a theory of abrasion plucking and macroabrasion mechanisms we then incorporate hydrology the evaluation of boundary shear stress capacity transport an entrainment relation for pluckable particles a routing model linking in stream sediment and hillslopes a formulation for alluvial channel coverage a channel width relation hack s law and exner equation into the model so that we can simulate the evolution of bedrock channels the model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions the results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes this supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes the model is expected to improve the current understanding of the linkage among physically meaningful input parameters the physics of incision process and morphological changes in bedrock streams

a froude scaled model of a bedrock alluvial channel reach 2 sediment cover previous research into sediment cover in bedrock alluvial channels has focussed on total sediment cover rather than the spatial distribution of cover within the channel the latter is important because it determines the bedrock areas that are protected from erosion and the start and end of sediment transport pathways we use a 1 10 froude scaled model of an 18 by 9 m reach of a bedrock alluvial channel to study the production and erosion of sediment patches and hence the spatial relationships between flow bed topography and sediment dynamics the hydraulic data from this bed are presented in the companion paper in these experiments specified volumes of sediment were supplied at the upstream edge of the model reach as single inputs at each of a range of discharges this sediment formed patches and once these stabilized flow was steadily increased to erode the patches in summary 1 patches tend to initiate in the lowest areas of the bed but areas of topographically induced high flow velocity can inhibit patch development 2 at low sediment inputs the extent of sediment patches is determined by the bed topography and can be insensitive to the exact volume of sediment supplied and 3 at higher sediment inputs more extensive patches are produced stabilized by grain grain and grain flow interactions and less influenced by the bed topography bedrock topography can therefore be an important constraint on sediment patch dynamics and topographic metrics are required that incorporate its within reach variability the magnitude and timing of sediment input events controls reach scale sediment cover

interaction among alluvial cover bed roughness and incision rate in purely bedrock and alluvial bedrock channel a major control on bedrock incision is the interaction between alluvial cover and erosive mobile grains the extent of alluvial cover is typically predicted as a function of relative sediment flux sediment supply rate over bed load transport capacity qbs qbc yet little is known about how the bed roughness affects the alluvial cover we performed field experiments with various flow discharges sediment supply rates grain sizes and bed surface topographies we then developed physically based models for estimating the threshold of sediment movement and the extent of alluvial cover so as to include the effect of roughness change the results for the threshold of sediment movement and the extent of alluvial cover obtained from our models show reasonable agreement with the results of the field experiments we explored the sensitivity of the models to variations in sediment supply and bedrock relative roughness bedrock hydraulic roughness height over grain size ksb d the results suggest the following 1 a larger relative roughness yields a greater dimensionless critical shear stress required for initial sediment motion 2 at a given sediment supply rate the extent of alluvial cover is larger when the relative roughness is larger 3 when the sediment supply rate and the relative roughness are small throughput bed load moves over and can abrade a purely bedrock channel with no alluvial cover and 4 the critical value of sediment supply rate below which throughput bed load transport occurs increases with decreasing relative roughness the experimental results and analysis provide a framework for treating the a incisional morphodynamics of purely bedrock rivers by throughput bed load with no alluvial cover b incisional alluvial morphodynamics of mixed bedrock alluvial rivers and c purely alluvial morphodynamics as well as the transition between these states

radarfacies and sedimentological analysis study of sedimentary substrate from an archaeological site shell mound southern brazil integrated results of gpr ground penetrating radar and sedimentological analysis are presented for the jabuticabeira ii archaeological site shell mound santa catarina brazil by means of rada

recrystallization of shell carbonate in soil 14c labeling modeling and relevance for dating and paleo reconstructions abstract mollusk shells are commonly present in a broad array of geological and archaeological contexts the shell carbonate can serve for numerical age determination 14 c and as a paleoenvironmental indicator 18 o 13 c shell carbonate recrystallization in soils however may re equilibrate the carbon c isotopic signature with soil co 2 the equilibration dynamics remain poorly understood because of the absence of suitable experimental approaches here we used the artificial 14 c labeling technique to study the process of shell carbonate recrystallization as a function of time organic free and organic containing shell particles of protothaca staminea were mixed with loess or a carbonate free loamy soil the mixtures were placed in air tight bottles where the bottle air containing 14 co 2 p co 2 2 the 14 c activity of shells was measured over time and related to the recrystallization of shell carbonate recrystallization of shell carbonate already began after one day the recrystallization rates were 10 3 day 1 in organic containing shell embedded in soil and 1 6 10 2 day 1 in organic free shells in loess removal of organic compounds increased shell porosity and so increased the contact surface for exchange with soil solution organic free shells recrystallized much faster in loess 0 56 in 56 days than in other treatments recrystallization was 2 to 7 times higher in loess in the presence and absence of organic compounds respectively than in carbonate free soil loess carbonate itself can recrystallize and accumulate on shells leading to overestimation of shell carbonate recrystallization a model for shell carbonate recrystallization as a function of time was developed the model considers the presence or absence of organic compounds in shell structure and geogenic carbonates in the embedding matrix the model enabled all results to be fitted with r 2 0 98 the modelled time necessary for nearly full recrystallization 95 of shell carbonate was 88 years for organic free shells in loess and up to 770 years for organic containing shells in carbonate free soil after this period the original isotopic signature will vanish completely and will be replaced by a new 13 c and 14 c signature in the shell structure thus shell carbonate recrystallization may proceed relatively rapidly in terms of geologic time this is necessary to consider in the interpretation of dating results and paleoenvironmental reconstructions

modeling the evolution of bedrock channel shape with erosion from saltating bed load 1 the cross sectional shape of a bedrock channel reflects the channel s history of incision although bedrock erosion due to impacts from saltating bed load particles is an important mechanism shaping bedrock channels models of channel shape have thus far neglected the effects of sediment here we present a model in which the cross sectional shape of a bedrock channel evolves in response to abrasion from saltating bed load particles we calculate the distribution of velocity and boundary shear stress with a two dimensional hydrodynamic model and we assume bed load is transported at capacity at the lowest point of the channel localized areas of alluviation emerge depending upon channel geometry and the imposed sediment supply and bedrock erosion occurs in nonalluviated areas where there is bed load transport the model captures the so called tools and cover effects and it simulates the dynamics between sediment supply and channel shape previously observed in experiments

upscaling from grain scale processes to alluviation in bedrock channels using a cellular automaton model 1 the presence of sediment cover in bedrock rivers inhibits saltation driven incision and so accurate predictions of the relationship between bedrock exposure fe and relative sediment flux sediment supply rate over capacity sediment transport rate qs qt are necessary to model incision and hence landscape evolution theoretical predictions of a linear or negative exponential form for this relationship are not consistent with laboratory data that instead demonstrate a range of different relationships here we use a cellular automaton ca model to establish how the relationship between fe and qs qt evolves from the dynamics of and interactions between individual sediment grains moving through a bedrock channel the key model parameter is the probability of grain entrainment which is altered as a function of the number of neighboring grains in order to reproduce the enhanced mobility of isolated grains on bedrock surfaces for each model run an equilibrium sediment cover is attained for a specified sediment input enabling the relationship between fe and qs qt to be established as well as both linear and exponential relationships model runs reproduce other relationships observed in laboratory experiments these other relationships require isolated grains to be more easily entrained than grains in sediment clusters which is consistent with field observations of grain mobility there is therefore a continuum of relationships between fe and qs qt the relationship that is most applicable to a particular reach will depend on the role of channel slope roughness and shear stress in controlling the entrainment of grains from bedrock and alluvial surfaces

geophysical investigations in the service of mount carmel israel prehistoric research abstract geophysical investigations in el wad cave mount carmel israel enabled the mapping of the surface of the bedrock and a reconstruction of the depth of the archeological deposits seismic refraction and ground penetrating radar gpr provided similar results indicating the potential value of geophysical studies when one seeks to reconstruct site features and establish future excavation strategies results of a gpr survey later conducted in the adjacent jamal cave were supported to a high degree by subsequent archeological excavations in the course of which lower and middle paleolithic layers were unearthed a continuous vertical electrical sounding cves geoelectric survey in the collapsed misliya cave indicates a low resistivity layer at the upper central part of the cave suggesting that lithified archeological deposits of ca 4 m are preserved at the site this has been supported by a test excavation recently conducted at the site more intriguing is the case of what on the basis of the outer morphology of the cliff seemed to be another collapsed cave just to the north of el wad the cves survey at the bottom of the cliff and across the hypothesized cave or chamber shows a two layer model with a low resistivity 100 200 ohm m layer overlying a layer of high resistivity 8000 10 000 ohm m a trial trench indicates that here the bedrock is covered by shallow recent surface material with no archeological value all three methods of geophysical investigation proved applicable and resulted in the successful evaluation of sites location depth and spatial extension thus forming essential tools in future prehistoric research

a froude scaled model of a bedrock alluvial channel reach 1 hydraulics the controls on hydraulics in bedrock alluvial rivers are relatively poorly understood despite the importance of the flow in determining rates and patterns of sediment transport and consequent erosion to measure hydraulics within a bedrock alluvial channel we developed a 1 10 froude scaled laboratory model of an 18 9 m bedrock alluvial river reach using terrestrial laser scanning and 3 d printing in the reported experiments water depth and velocity were recorded at 18 locations within the channel at each of five different discharges additional data from runs with sediment cover in the flume were used to evaluate the hydraulic impact of sediment cover the deposition and erosion of sediment patches in these runs are analyzed in the companion paper in our data 1 spatial variation in both flow velocity and froude number increases with discharge 2 bulk flow resistance and froude number become independent of discharge at higher discharges 3 local flow velocity and reynolds stress are correlated to the range of local bed topography at some but not most discharges 4 at lower discharges local topography induces vertical flow structures and slower velocities but these effects decrease at higher discharges and 5 there is a relationship between the linear combination of bed and sediment roughness and local flow velocity these results demonstrate the control that bedrock topography exerts over both local and reach scale flow conditions but spatially distributed hydraulic data from bedrock alluvial channels with different topographies are needed to generalize these findings

a nonlocal theory of sediment buffering and bedrock channel evolution 1 bedrock erosion in mountain river channels ultimately sets the erosion rate of the surrounding hillslopes and the rate of sediment supply to the channels the supply of coarse bed sediment acts as a dampening effect on further erosion by depositing an alluvial cover that temporarily obscures the bedrock for landscapes where the residence time of the alluvial bed cover is comparable to the timescale of bedrock incision coarse sediment supply and transport generate a strong negative feedback on fluvial downcutting and the coupled process of hillslope channel erosion is inherently self buffering here we study a simple model of self buffered bedrock channel erosion that incorporates the spreading of bed sediment cover downstream in a way that allows for a broad tailed power law probability distribution of transport velocities of bed sediment over the long term this leads us to consider a nonlocal transport law fractional advection parameterized by a scaling exponent 0 1 which collapses to local advection for 1 for strong sediment buffering we find that nonlocality 1 has a direct control on the power law scaling of channel slope s with upstream area a giving s a 1 2 at steady state empirical observations of slope area scaling are consistent with 1 and nonlocal transport in general the model predicts linear logarithmic or power law stream profiles depending on the extent of buffering the degree of nonlocality and the scaling of the bedrock erosion law it also predicts somewhat counterintuitively that bed cover should thicken with distance x downstream slower than linearly as x i e the more nonlocal the bed sediment spreading process 0 the slower the bed cover increases downstream we deduce that long range heterogeneous transport of coarse sediment in mixed bedrock alluvial rivers may be a key element of landscape scaling and an important factor in landscape dynamics

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

soft bedrock erosion modeling with a two dimensional depth averaged model many rivers in taiwan have steep slopes are subject to typhoon induced flood flows and contain soft bedrock that is exposed at many locations and easily erodible the occurrence of extensive bedrock erosion has been a major threat to river infrastructure at many locations soft bedrock erosion therefore is an important process to consider for river projects in taiwan in this study bedrock erosion models are reviewed a specific model is proposed by combining two existing models incorporating both the hydraulic and abrasive scour mechanisms the proposed bedrock erosion model is incorporated into a two dimensional mobile bed model and the integrated model is tested by simulating bedrock erosion downstream of the chi chi weir on the choshui river in taiwan a calibration study is performed to determine appropriate values of the model parameters based on two and a half years of measured data the model is then assessed based on a verification study that compares model predictions of bedrock erosion of

reduction of long term bedrock incision efficiency by short term alluvial cover intermittency fluctuations of the sediment volume stored in mountain channels are driven by stochastic variations of discharge and sediment supply and can inhibit bedrock incision if sediment thickness is too large here i study how this short term stochasticity propagates into the long term reduction of bedrock incision efficiency the cover effect at geological time scales i introduce a new numerical model that resolves sediment transport and bedrock incision at daily time scales and is run for thousands of years it incorporates 1 a transport threshold and daily stochastic variations in water discharge and sediment supply 2 a freely evolving channel width and slope and 3 an explicit treatment of alluvial thickness variations and corresponding bed incision reduction for typical mountain river conditions the model predicts that alluvial cover oscillates between complete and negligible incision reduction in this intermittent regime the long term cover effect is mainly set by the fraction of time spent in full cover and the present day extent of alluvial cover is not representative of long term dynamics the long term integrated cover effect law differs strongly from proposed theoretical and experimental models and it is controlled by sediment supply stochasticity rather than the details of cover development at the hydraulic time scale model results also suggest that steady state channel configuration always depends on sediment supply rate while being never limited by transport capacity or strictly detachment limited these results point out that discharge and sediment supply stochasticity should not be considered less important than the intricate details of incision laws to model long term bedrock channel dynamics

alluvial morphodynamics of low slope bedrock reaches transporting nonuniform bed material research on bedrock rivers primarily focused on bedrock incision and to the best of our knowledge morphodynamic models have not yet considered the variability of sediment grain size and the prese

feedbacks between erosion and sediment transport in experimental bedrock channels natural bedrock rivers flow in self formed channels and form diverse erosional morphologies the parameters that collectively define channel morphology e g width slope bed roughness bedrock exposure sediment size distribution all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes to explore the mechanics of river incision we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate sand and cement a single sediment size primarily transported as bedload a single erosion mechanism abrasion and sediment starved transport conditions we find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field sediment transport and bottom roughness abrasion only occurs where sediment impacts the bed and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows however the spatial focusing of erosion results in tortuous flow paths and erosional forms inner channels scoops potholes which dissipate flow energy this energy dissipation is a negative feedback that reduces sediment transport capacity inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load copyright 2007 john wiley sons ltd

eolian depositional episodes controlled by late quaternary relative sea level changes on the imbituba laguna coast southern brazil the late quaternary coastal eolian deposits of the imbituba laguna region santa catarina state southern brazil comprise two eolian depositional sequences separated by an unconformity the older eolian sequence a is correlated to the relative sea level rise of the last interglacial period the eolian sequence b was initiated during the relative sea level rise in the post glacial period and extends up to active dunefields the correspondence between the two eolian depositional episodes represented by these sequences and the relative sea level highstand phases is confirmed by thermoluminescence and optically stimulated luminescence dating this correspondence is explained by a conceptual model that links the availability of coastal sediments for eolian transport with relative sea level changes the link between relative sea level changes and coastal eolian sedimentation can be used to integrate coastal eolian systems to the sequence stratigraphy model

a general power equation for predicting bed load transport rates in gravel bed rivers 1 a variety of formulae has been developed to predict bed load transport in gravel bed rivers ranging from simple regressions to complex multiparameter formulations the ability to test these formulae across numerous field sites has until recently been hampered by a paucity of bed load transport data for gravel bed rivers we use 2104 bed load transport observations in 24 gravel bed rivers in idaho to assess the performance of eight different formulations of four bed load transport equations results show substantial differences in performance but no consistent relationship between formula performance and degree of calibration or complexity however formulae containing a transport threshold typically exhibit poor performance furthermore we find that the transport data are best described by a simple power function of discharge from this we propose a new bed load transport equation and identify channel and watershed characteristics that control the exponent and coefficient of the proposed power function we find that the exponent is principally a factor of supply related channel armoring transport capacity in excess of sediment supply whereas the coefficient is related to drainage area a surrogate for absolute sediment supply we evaluate the accuracy of the proposed power function at 17 independent test sites

a theoretical framework for the morphodynamics of bedrock channels 1 bedrock rivers are characterized by conditions of insufficient sediment supply wherein the amount of sediment supplied to the channel is less than the channel s sediment transport capacity consequently bedrock channels generally exhibit discontinuous sediment cover which current approaches to the morphodynamics of alluvial rivers are unable to handle to fill this gap we present a theoretical framework in which local sediment transport rates are proportional to the areal concentration of sediment available for transport on the bed and the sediment continuity equation is reformulated to account for temporal changes in areal concentration of sediment on nonalluviated surfaces we then perform a linear stability analysis that shows that an initially uniform distribution of the areal concentration of sediment on the bed is unstable to small perturbations this suggests that the sediment cover in bedrock channels tends to concentrate into regions which may eventually be alluviated in general agreement with experimental observations

integrating geoarchaeology and magnetic susceptibility at three shell mounds a pilot study from mornington island gulf of carpentaria australia in coastal areas of the globe open shell matrix sites are commonly used to establish regional chronologies of human occupation and identify patterns of cultural change particularly for the holocene post sea level stabilisation period despite this many basic sedimentary analyses that are routinely applied to rockshelter deposits e g geophysical characterisation particle size etc are rarely applied to these sites magnetic susceptibility occasionally used in rockshelters has never been used to investigate shell matrix sites in australia despite several international studies identifying its efficacy for other types of open sites this paper reports a pilot project applying a range of conventional sedimentary and archaeological analyses as well as magnetic susceptibility at three anthropogenic shell mounds on mornington island gulf of carpentaria australia results are compared to firstly assess site integrity and secondly to ascertain whether magnetic signatures are related to cultural or natural site formation processes the results establish that the mounds were repeatedly visited despite the archaeological evidence including radiocarbon ages suggesting effectively instantaneous deposition this has important implications for studies of other shell mounds where the limitations of radiocarbon dating precision may also mask multiple deposition events

a surface roughness model for predicting alluvial cover and bed load transport rate in bedrock channels partial alluvial cover in bedrock channels influences downcutting rates and mountain river morphodynamics flume experiments have demonstrated that a wide range of stable cover fractions are possible for a given ratio of sediment supply to transport capacity existing cover models impose different unique relationships between cover fraction and reach scale transport variables e g slope discharge sediment supply and grain size individually these models cannot predict the range of cover behaviors observed experimentally suggesting that additional variables may be important i propose a 1 d model in which bedrock surface roughness is a control on partial alluvial cover in the model surface roughness affects both shear stresses and thresholds of grain motion and therefore sediment transport capacity the roughness of the combined bedrock alluvial bed surface varies with the cover fraction model trends are reasonably consistent with previous experiments over ranges of roughness slope and shear stress when bedrock roughness is much greater than grain size cover increases approximately linearly with the ratio of sediment supply to transport capacity when bedrock roughness is much less than the sediment diameter the model tends to predict abrupt shifts between complete bedrock exposure and alluvial cover this runaway alluviation occurs for model solutions that are unstable to perturbations in supply which is the case when transport capacity decreases as alluvial cover increases in addition a model variant in which sediment supply is controlled by the amount of sediment on the bed suggests that a linear cover relation may be sufficient for modeling sediment flux dependent erosion at landscape scales

coarse sediment transport in a bedrock channel with complex bed topography 1 independent lithologic and structural controls in fluvial bedrock systems interact with coarse sediment transport processes to play a key role in bedrock incision processes such as abrasion during a 3 year study on the ocoee river in the blue ridge province of the southern appalachians usa we used painted tracer clasts to measure coarse sediment transport dynamics and address whether different scales of morphologic variability in bedrock channels lead to coarse sediment transport processes that differ from alluvial channels at the reach scale folded metasedimentary units are exposed in the channel bed and appear as linear bedrock ribs that vary in amplitude and orientation to flow under similar flow conditions for which size dependent transport has been observed in alluvial channels dimensionless shields stress within 1 5 2 0 times the dimensionless critical shear stress transport distance was a significant function of grain size where bedrock ribs were longitudinal to flow reach 1 and reach 2 however transport distance was not size dependent where bedrock ribs were oblique to flow reach 3 at different intrareach scales the variables characterizing the local bedrock topography and sediment architecture were the best predictors of transport distance in all three reaches therefore bed load transport processes may be influenced by bed forms in bedrock streams bedrock ribs but at potentially smaller scales than bed forms in alluvial channels the strong influence of bedrock ribs on coarse sediment transport suggested that coarse sediment transport processes are controlled by different factors in bedrock channels only when bedrock ribs cross the channel at a high angle to the flow

numerical simulation of effects of sediment supply on bedrock channel morphology abstractnatural bedrock rivers exhibit diverse erosional morphologies although the formation of alternate bars on bedrock has been noted in previous studies the influence of these alternate bars on bedrock erosion has not been clarified in this study the authors propose a model for bedrock alluvial channels that reproduces both bar formation and erosional morphology in addition the authors report on numerical simulations to evaluate the influence of sediment supply on the state of the bed at and over the bedrock surface the numerical results illustrate the formation of different morphologies for different supply rates when the sediment supply rate is close to transport capacity mixed alluvial bedrock alternate bars form these bars are analogous to purely alluvial alternate bars a meandering thread of alluvial material migrates downstream over a uniformly eroding bedrock surface when the sediment supply rate is well below capacity however multiple incisional troughs grooves form on the bedr

coastal forms and quaternary processes along the arid pilbara coast of northwestern australia abstract the pilbara coast is set in the most arid part of coastal australia this coast and coastal plain composed of riverine plains deltas tidal flats coastal dunes and limestone barriers relic deltas and archipelago ria shores is predominantly a terrigenous and carbonate sediment complex the coastal landforms therein formed during the quaternary through influence of ancestral landforms and fluvial and shoreline accretion coastal erosion and cementation the pre holocene features controlled development of holocene coastal forms and a variety of holocene depositional styles climate also plays a significant part in the coastal processes and in the generation of sedimentary products through high evaporation rates coupled with the limited rainfall cyclonic storms wind wind waves and the limited sediment delivery to the coastal zone this results in a distinctive range of stratigraphic sequences and chemical products as a result this arid coast is characterised by a range of features such as construction of arid zone deltas delta destruction and sediment redistribution during times of sediment depletion cyclone induced erosion and sedimentation mangroves and their associated sedimentary deposits evolution of coastal groundwater hypersalinity formation of salt flats and precipitation and cementation to form beachrocks high tidal crusts and gypsum precipitates the pilbara coastal stratigraphy and geomorphology suggests aridity was intricately involved in the sedimentation geomorphic evolution and pedogenic and diagenetic alteration of this coastal zone throughout the whole of the holocene and pleistocene

cave detection in limestone using ground penetrating radar ground penetrating radar gpr is becoming a more common component of the standard array of geophysical techniques that are used by archaeologists in this paper we report on the use of gpr to survey an area of archaeologically important karst topography at kitley caves in devon u k we describe the use of gpr to detect voids within a limestone outcrop as an aid to locating cave systems which might contain sediments suitable for excavation the performance of the gpr equipment is analysed and the results compared to those obtained with an electrical resistance survey carried out at the same location in particular the depth of penetration of the gpr is estimated and we report discernible echo signals from a much greater depth than is usually reported in archaeological applications of gpr

gpr and inductive electromagnetic surveys applied in three coastal sambaqui shell mounds archaeological sites in santa catarina state south brazil abstract in this article gpr ground penetrating radar and inductive electromagnetic em38 equipment results are presented for three archaeological sites of shell mounds located between laguna and jaguaruna cities mid southern coast of santa catarina state brazil the jabuticabeira ii santa marta iv and encantada iii sites were studied these sites consist of carbonate shell mounds built by societies that inhabited this region during the pre colonial period 7 5 1 3 ky bp geophysical responses obtained in these archaeological sites had allowed the identification of anomalous regions related to archaeological point targets as well as continuous targets correlated to the shell mound s structure and buried geologic features gpr numerical modelling studies were done in order to evaluate the performance of the gpr method in relation to materials found in that environment besides helping interpretations of real results the synthetic gpr model was a good match with real data obtained in the field so the real and synthetic results serve as a reference guide to direct excavation activities in these archaeological and geological structures in the jabuticabeira ii shell mound lithic materials and burials were found amidst archaeological layers characterized by sediment rich in organic material ferruginous concretions and loose shells in the santa marta iv archaeological site a concentration of ceramic materials burials and fireplaces were found in the encantada iii site a strong anomaly characterized by a hyperbolic reflection was related to the presence of a silicified tree root a great example of ambiguity in shallow geophysics interpretation in archaeological applications results functioned as a guide to archaeological excavations allowing cost and time reduction as well as contributing to improve the knowledge of these archaeological sites situated in southern brazil

formation and erosion of sediment cover in an experimental bedrock alluvial channel sediment grains in a bedrock alluvial river will be deposited within or adjacent to a sediment patch or as isolated grains on the bedrock surface previous analysis of grain geometry has demonstrated that these arrangements produce significant differences in grain entrainment shear stress however this analysis neglected potential interactions between the sediment patches local hydraulics and grain entrainment we present a series of flume experiments that measure the influence of sediment patches on grain entrainment the flume had a planar bed with roughness that was much smaller than the diameters of the mobile grains in each experiment sediment was added either as individual grains or as a single sediment pulse flow was then increased until the sediment was entrained analysis of the experiments demonstrates that 1 for individual grains coarse grains are entrained at a higher discharge than fine grains 2 once sediment patches are present the different in entrainment discharge between coarse and fine grains is greatly reduced 3 the sheltering effect of patches also increases the entrainment discharge of isolated grains 4 entire sediment patches break up and are eroded quickly rather than through progressive grain by grain erosion and 5 as discharge increases there is some tendency for patches to become more elongate and flow aligned and more randomly distributed across the bed one implication of this research is that the critical shear stress in bedrock alluvial channels will be a function of the extent of the sediment cover another is that the influence of sediment patches equalises critical shear stresses between different grain sizes and grain locations meaning that these factors may not need to be accounted for further research is needed to quantify interactions between sediment patches grain entrainment and local hydraulics on rougher bedrock surfaces and under different types of sediment supply

sedimentary facies of an arid zone delta gascoyne delta western australia the gascoyne delta is forming on a semi arid wave dominated coast at the northern end of shark bay western australia the gascoyne river flows intermittently the delta has a flattened lobate form with a peripheral strandplain of elongate lagoons and tidal flats separated by beach spits and low beach ridges three major stratigraphic components form the gascoyne delta basal sheet deltaic sequence and plain sequence the basal sheet is a thin transgressive unit deposited during the holocene sea level rise offshore it is shelly quartz sand with a relict shallow water sandflat fauna inshore it contains sediment admixed from contemporary seagrass communities or overlying units the deltaic sequence is a regressive terrigenous wedge overlying the basal sheet it consists of the bar unit the delta bay unit and the strandplain sheet the bar unit is an upward coarsening sand body being deposited on the distributary mouth bars and the shoreface it grades laterally into the delta bay unit a muddy sand body accumulating in the protected embayment to the south of the delta the strandplain sheet overlies both units and is composed of sand and muddy sand with minor mud it is being deposited in beach beach ridge tidal flat and supratidal flat environments the plain sequence consists of three units the channel and levee units and the plain sheet the channel unit is coarse sand and gravel the levee unit is fine sand silt and mud the channel and levee units form a narrow belt of bedded fluvial deposits normal to the strandline the plain sheet is homogeneous red brown mud and sandy mud deposited by sheetwashing the marine deltaic sequence is a laterally extensive upward coarsening sand body typical of other wave dominated deltas e g sao francisco and rhone however the subaerial sequence a narrow axial belt of channel sands and levee silts flanked by extensive red brown muds forms a different cap in contrast both the sao francisco and rhone deltas have sandy deltaic plain sequences other arid zone deltas colorado and new river differ markedly in their process style neither are wave dominated arid climate in the gascoyne does not markedly influence the marine sequence but is reflected in subaerial units by extensive red brown desiccated muds and confinement of marsh deposits to the narrow coastal tidal zone the gascoyne is a potential redbed delta particularly the subaerial sequences reddening of marine sequences is theorized as being dependent on the incorporation of organic matter and possibly on inshore productivity

substrate controls on the longitudinal profile of bedrock channels implications for reach scale roughness 1 in this paper we examine the relationships among bedrock properties and hydraulics in shaping bedrock channel morphology at the reach scale the ocoee river and four other bedrock streams in the blue ridge province of the southeastern united states which have reach scale differences in bedrock erodibility controlled by lithologic and structural variation are the focus of this study we describe a simple conceptual model for concentrated erosion in bedrock channels and test three hypotheses in order to investigate the interactions among rock erodibility characteristics of undulating rib like bed forms reach scale gradient and hydraulic roughness and energy dissipation substrate differences correlate with variation in reach morphology i e gradient bed form orientation and amplitude such that less erodible substrates are associated with steeper reach gradient and with transversely oriented ribs of greater amplitude one dimensional modeling in hec ras indicated that in the reach with the least erodible substrate and greatest bed slope and rib amplitude the reach averaged hydraulic roughness was the greatest increased hydraulic roughness in steeper reaches points to the importance of positive and negative feedbacks in these systems greater substrate erosional resistance limits profile lowering which likely creates steeper bed slopes and greater stream power creating a self enhancing feedback this local increase in stream power is balanced by increased roughness resulting from the erosional processes that produce bedrock ribs which represents a self regulating feedback the overall result reflects quantifiable adjustments between substrate resistance and hydraulic driving forces in bedrock channels

the pilbara coast a riverine coastal plain in a tropical arid setting northwestern australia abstract the unifying feature of the pilbara coast of northwestern australia is that it is a sedimentary repository for a range of rivers that drains a high relief precambrian rocky hinterland and discharges sediments along a coastal plain which fronts a wave dominated environment in a tropical arid climate the combination of fluvial and shoreline accretion processes coastal cementation coastal erosion and ancestral landform architecture such as residual pleistocene limestone ridges and large outcrops of precambrian bedrock has produced a complex coastal system during the quaternary as a result the coast is dominated by active deltas beach dune shores inactive eroding parts of deltas and their barriers limestone barrier coasts bays associated with eroded limestone barriers and archipelago ria coasts quaternary sediments throughout the area while varied in their distribution and history at the smaller scales exhibit a recurring pattern of lithotopes and lithologies in the region there are three main quaternary suites pleistocene red siliciclastic sediments alluvium deltaic sediments and aeolian sand that form an inland zone pleistocene limestones that form local barriers and a holocene system within which are the sedimentary suites of deltas beach dunes tidal flats and tidal embayments

reduction of topography effect in inductive electromagnetic profiles application on coastal sambaqui shell mound archaeological site in santa catarina state brazil we present results of electrical conductivity profiles obtained with inductive electromagnetic geophysical method in santa marta archaeological site santa catarina state southern brazil this site is a sambaqui shell mound in which several human occupation remains are found during pre colonial period such as buried lithic and bone artifacts fire place etc most of these mounds include as well many human burials which in many cases point to funerary ritual as a main agency for mound building a set of profiles of apparent electrical conductivity and magnetic susceptibility was acquired in two sites aiming the identification of geophysical anomalies with potential interest for excavation to enhance conductivity data we applied an effective procedure to remove topographical effects in the apparent conductivity measurements which are rather conditioned by the presence of a variable water table depth or conductive sediment layer a linear dependence among conductivity values and the site elevation provided a simple linear model to remove the influence of topography corrected electric conductivity maps substantially improved the definition of anomalies many of them rather subtle in raw data images corrected maps also show a better adherence with magnetic susceptibility maps both of them identifying archaeological structures of interest a well structured fire place and a concentration of ceramic fragments

evaluating the controls of shear stress sediment supply alluvial cover and channel morphology on experimental bedrock incision rate 1 we explored the dependence of experimental bedrock erosion rate on shear stress bed load sediment flux alluvial bed cover and evolving channel morphology we isolated these variables experimentally by systematically varying gravel sediment flux qs and water discharge qw in a laboratory flume gradually abrading weak concrete bedrock all else held constant we found that 1 erosion rate was insensitive to flume averaged shear stress 2 erosion rate increased linearly with sediment flux 3 erosion rate decreased linearly with the extent of alluvial bed cover and 4 the spatial distribution of bed cover was sensitive to local bed topography but the extent of cover increased with qs qt where qt is flume averaged transport capacity once critical values of bed roughness and sediment flux were exceeded starting from a planar geometry erosion increased bed roughness due to feedbacks between preferential sediment transport through interconnected topographic lows focused erosion along these zones of preferential bed load transport and local shear stresses that depended on the evolving bed morphology finally continued growth of bed roughness was inhibited by imposed variability in discharge and sediment flux due to changes in spatial patterns of alluvial deposition and impact wear erosion was preferentially focused at lower bed elevations when the bed was cover free but was focused at higher bed elevations when static alluvial cover filled topographic lows natural variations in discharge and sediment flux may thus stabilize and limit the growth of roughness in bedrock channels due to the effects of partial bed cover

gpr survey at lapa do santo archaeological site lagoa santa karstic region minas gerais state brazil in this work we present and discuss the results of a novel and timely gpr 2d and gpr 3d survey performed at an archaeological rock shelter site lapa do santo localized in the karstic region of lagoa santa central brazil a total of 113 ground penetrating radar profiles were acquired with 200 mhz and 400 mhz shielded antennas aiming in identifying geological and archaeological anomalies in order to assist archaeologists in an excavation program the gpr results indicated clear geophysical anomalies characterized by hyperbolic reflections and areas with high amplitude sub horizontal electromagnetic waves the anomalies observed by gpr were confirmed by the excavation of test units allowing the identification anthropogenic features such as a fire hearth structure and wooden artifact and natural features such as tree roots and rocky bodies such as speleothems boulders and bedrock the results showed the efficiency of gpr method in identifying potential buried archaeological targets in cave sites within a karstic area and they oriented to archaeological excavations reducing costs and increasing the probability of finding archaeological targets in the initial stages of a project

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

inside a mound applied geophysics in archaeological prospecting at the kings mounds gamla uppsala sweden a combined geophysical prospection with slingram gradiometer and ground penetrating radar is described the prospection is carried out at the eastern mound and the thing mound gamla uppsala middle sweden in an attempt to detect the internal structure of the mounds at the eastern mound we were able to identify the fossil ground surface a centre cairn a possible older grave under the mound and a few other anomalies some of them most possibly depending on historical activities and some more likely on prehistoric human activities the thing mound was found being of geological origin and only prepared to become a grave the project shows that a combined use of several geophysical methods improves and simplifies the interpretation

experimental study of bedrock channel alluviation under varied sediment supply and hydraulic conditions 1 channel incision into bedrock plays a critical role in mountain landscape evolution a bed that is completely alluviated cannot undergo incision as a result incision driven by the collision of bedload grains and bedrock requires transport at below capacity conditions for example where the bed is partially covered by alluvium in a mechanistic model of bedrock incision it is important to have a formula describing the relationship between the bedload transport rate and the degree to which the bed is covered by sediment the purpose of this work is to understand the mechanisms underlying the degree of bed exposure in bedrock rivers by means of an experimental study a number of experiments with various hydraulic and morphological bed conditions were performed in order to characterize this model here we focus on planar bedrock beds with some roughness the results suggest that the sediment supply channel slope hydraulic bed roughness the degree of areal coverage and thickness of antecedent gravel in the channel as well as the presence of boulders in the channel are major factors controlling bedrock exposure for sufficiently low ratios of sediment supply to transport capacity it was found that bedrock roughness can play a role in determining the degree of bedrock exposure for higher sediment ratios on the other hand the composite roughness associated with grain roughness bars and or antidunes dominates so that the underlying bedrock roughness no longer affects the degree of exposure of the bed for lower bed slopes i e less than 0 015 based on our experimental setting bedrock exposure decreases more or less linearly with increasing values of the ratio of sediment supply rate to capacity rate for higher bed slopes i e more than 0 015 there is a range of lower values of the ratio of sediment supply rate to capacity rate where a bedrock surface becomes fully exposed without any alluvial deposit for a given bedrock roughness this range expands solely with increasing slope regardless of shear stress within the upper range of values of the ratio of sediment supply to capacity rate a linear relationship between the degree of bedrock exposure and this supply to capacity ratio still prevails the addition of model boulders can be viewed as very high hydraulic bed roughness to the channel has been found to suppress the overexposure of bedrock and so restore a linear relation between the degree of exposure and the ratio of sediment supply rate to capacity rate formulations for estimating bedrock exposure as a function of sediment supply to capacity ratio under different river channel characteristics are proposed a linear relation between the degree of alluvial cover and the ratio of sediment supply rate to capacity rate is a previously proposed model the present study expands the result for other cases including runaway alluviation some landscape evolution models assume an abrupt shift between fully exposed bedrock and complete alluviation whereas others assume the linear relation the present study shows that both formulations are valid under different settings of river characteristics

modeling sediment transport in ice walled subglacial channels and its implications for esker formation and proglacial sediment yields sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed and eskers hold valuable information about past subglacial hydraulic conditions in their spatial organization geometry and sedimentary structures ultimately eskers are a record of past glacio fluvial sediment transport but there is currently no physical model for this process we develop a 1 d model of morphodynamics in semicircular bedrock floored subglacial channels we adapt a sediment conservation law developed for mixed alluvial bedrock conditions to subglacial channels channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice to which we add the closure or enlargement due to sediment deposition or removal respectively we apply the model to an idealized land terminating glacier and find that temporary sediment accumulation in the vicinity of the terminus or the formation of an incipient esker is inherent to the dynamics of the channelized water flow the alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation we show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supply or transport limited system as has been hypothesized for proglacial sediment yields we also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply but perhaps more importantly ice surface slope and the temporal pattern of water delivery to the bed

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

flow resistance and hydraulic geometry in bedrock rivers with multiple roughness length scales many models of incision by bedrock rivers predict water depth and shear stress from discharge conversely palaeoflood discharge is sometimes reconstructed from flow depth markers in rock gorges in both cases assumptions are made about flow resistance the depth discharge relation in a bedrock river must depend on at least two roughness length scales exposed rock and sediment cover and possibly a third sidewalls a conceptually attractive way to model the depth discharge relation in such situations is to partition the total shear stress and friction factor but it is not obvious how to quantify the friction factor for rough walls in a way that can be used in incision process models we show that a single flow resistance calculation using a spatially averaged roughness length scale closely approximates the partitioning of stress between sediment and rock and between bed and walls in idealised scenarios both approaches give closer fits to the measured depth discharge relations in two small bedrock reaches than can be achieved using a fixed value of manning s n or the chezy friction factor sidewalls that are substantially rougher or smoother than the bed have a significant effect on the partitioning of shear stress between bed and sidewalls more research is needed on how best to estimate roughness length scales from observable or measurable channel characteristics

rates and mechanisms of bedrock incision and strath terrace formation in a forested catchment cascade range washington measurements of channel bed and bank incision into bedrock coupled with mapping and radiocarbon dating of strath terraces in the west fork teanaway river washington provide insight into rates and mechanisms of river incision and strath terrace formation in a forested landscape the west fork drains 102 km 2 of the tectonically quiescent southeastern north cascade range and it is rapidly eroding its bed and creating strath terraces in its lower reach minimum vertical incision measured annually relative to nails embedded in the streambed was greater in the seasonally exposed weathering dominated high flow channel mean 10 9 mm yr 1 than in the perennially wet abrasion dominated low flow channel 3 8 mm yr 1 documenting unsteady lowering of the channel margin ages of radiocarbon dated materials from alluvium on strath terraces 0 1 m to 5 4 m above the water surface suggest three episodes of strath abandonment at maximum ages of ca a d 830 a d 1560 and a d 1890 and average incision rates of 1 3 mm yr 1 1 4 mm yr 1 and 7 4 mm yr 1 for the oldest to youngest surfaces respectively weathering promoted vertical incision in the high flow channel provides a mechanism for top down rapid lateral strath planation in which scour of alluvium on incipient strath terraces incorporates the surface into the high flow channel allowing rapid removal of bedrock weathered during wetting and drying cycles relationships among channel width channel confinement by bedrock terrace risers modeled bankfull shear stress and alluvial bed cover suggest that rapid channel widening could also internally limit vertical incision by slowing incision as shear stresses decline and more alluvium is retained on the bed the timing of the most recent ca a d 1890 strath abandonment corresponds with historic anthropogenic removal of fluvial wood suggesting that the relative abundance of fluvial wood may influence episodes of vertical bedrock incision by affecting the retention of alluvium on streambeds

morphodynamics of a bedrock alluvial meander bend that incises as it migrates outward approximate solution of permanent form in meandering rivers cut into bedrock erosion across a channel cross section can be strongly asymmetric at a meander apex deep undercutting of the outer bank can result in the formation of a hanging cliff which may drive hillslope failure whereas the inner bank adjoins a slip off slope that connects to the hillslope itself here we propose a physically based model for predicting channel planform migration and incision point bar and slip off slope formation bedrock abrasion the spatial distribution of alluvial cover and adaptation of channel width in a mixed bedrock alluvial channel we simplify the analysis by considering a numerical model of steady uniform bend flow satisfying cyclic boundary conditions thus in our analysis sediment supply i e the total volume of alluvium in the system is conserved in our numerical simulations the migration rate of the outer bank is a specified parameter our simulations demonstrate the existence of an approximate state of dynamic equilibrium corresponding to a near solution of permanent form in which a bend of constant curvature width cross sectional shape and alluvial cover distribution migrates diagonally downward at constant speed leaving a bedrock equivalent of a point bar on the inside of the bend channel width is set internally by the processes of migration and incision we find that equilibrium width increases with increasing sediment supply but is insensitive to outer bank migration rate the slope of the bedrock point bar varies inversely with both outer bank migration rate and sediment supply although the migration rate of the outer bank is externally imposed here we discuss a model modification that would allow lateral side wall abrasion to be treated in a manner similar to the process of bedrock incision this article is protected by copyright all rights reserved

stochastic modeling of the cover effect and bedrock erosion 1 several important fluvial bedrock erosion processes are driven by the impact of bed load particles bed load transport rates fluctuate strongly in both nature and experiment and stochastic models of the transport processes have been put forward to describe this behavior in this paper i adapt a model based on a markov chain formulation to derive probability distributions for bed load transport rate over a rock bed only partly covered by sediment i propose a way to calculate the probability distribution of bed cover for given sediment supply using a combinatoric model and combine the two curves to calculate probability distributions for bed cover and erosion rate at constant hydraulics in the proposed model mean bed cover is an exponentially declining function of the number of particles in the control volume the model describes recently published experimental data well but at the moment it is not possible to finally discriminate between the exponential and the previously proposed linear model formulation distributions of erosion rate are fairly broad functions slightly skewed toward high erosion rates

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

inverting topography for landscape evolution model process representation 3 determining parameter ranges for select mature geomorphic transport laws and connecting changes in fluvial erodibility to changes in climate

new insights into the mechanics of fluvial bedrock erosion through flume experiments and theory river incision into bedrock drives the topographic evolution of mountainous terrain and may link climate tectonics and topography over geologic time scales despite its importance the mechanics of bedrock erosion are not well understood because channel form river hydraulics sediment transport and erosion mechanics coevolve over relatively long time scales that prevent direct observations and because erosive events occur intermittently and are difficult and dangerous to measure herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory we also build on this work by providing new theory for rock resistance to abrasion thresholds for plucking by vertical entrainment sliding and toppling and by assessing bedrock analog materials bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength sediment supply has a dominant control over bed roughness and abrasion rates suspended sediment is an efficient agent of erosion and feedbacks with channel form and roughness strongly influence erosion rates erodibility comparisons across rock concrete ice and foam indicate that for a given tensile strength abrasion rates are insensitive to elasticity the few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed and the dominance of block sliding and toppling at knickpoints these observations are consistent with new theory for the threshold shields stress to initiate plucking which also suggests that erosion rates in sliding and toppling dominated rivers are likely transport limited major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river bed parallel waterfall erosion by toppling and plunge pool erosion feedbacks between weathering and physical erosion erosional bedforms and morphodynamic feedbacks between channel form and erosion rates despite scaling challenges flume experiments continue to provide much needed tests of existing bedrock erosion theory force development of new theory and yield insight into the mechanics of landscapes

mobility of river tracer pebbles over different timescales 1 tracer pebbles are widely used to learn about gravel transport along rivers movement over short times and distances is dominated by factors controlling entrainment relative particle size and shear stress movement at longer scales also involves depositional factors burial and reexposure and exchange between channels bars and other depositional environments we mapped mixed size tracers in six reaches of a small scottish river after 2 and 8 years to investigate differences in relative and absolute mobility and infer the importance of burial and exchange patterns of relative mobility according to size and shear stress both within and between reaches did not change significantly some local bunching of tracers was apparent in both surveys with redistribution from pools into riffles and bars the main change was that virtual velocities were 50 lower and estimated gravel fluxes were also lower in the longer term this slowdown is attributed to vertical mixing giving decreased mobility as surface seeded tracers become buried long term storage in bars and other less active parts of the system and in this channel advection of tracers downstream onto a finer bed giving higher relative size

cover effect in bedrock abrasion a new derivation and its implications for the modeling of bedrock channel morphology the sediment load of a bedrock river plays an important role in the fluvial incision process by providing tools for abrasion the tools effect and by covering and thereby protecting the bed the cover effect we derive a new formulation for the cover effect in which the fraction of exposed bed area falls exponentially with increasing sediment flux or decreasing transport capacity and explore its consequences for the model of bedrock abrasion by saltating bed load erosion rates predicted by the model are higher than those predicted by earlier models in a closed system the maximum erosion rate is predicted to occur when sediment supply is equal to transport capacity for a flat bed by optimizing the channel geometry to minimize the potential energy of the stream and using representative values for both discharge and grain size we derive equations for the geometry of a bedrock river and explore how predictions for width slope and bed cover vary as functions of drainage area rock uplift rate and rock strength the equations predict a dependence of channel width on drainage area similar to the relations using a simple shear stress incision law the slope area relationship is predicted to be concave up in a log log regime with a curvature dependent on uplift rate however this curvature does not deviate sufficiently from a straight line to allow discrimination between models using empirical data dependence of channel width and slope on rock uplift rate can be separated into two domains for low uplift rates channel geometry is largely insensitive to uplift rate due to a threshold effect at high uplift rates there is a power law dependence bed cover is predicted to increase progressively downstream and to increase with increasing uplift rate in our model the width to depth ratio is a function of both tectonic and climatic forcing this indicates that the scaling between channel width and bed slope is neither a unique indicator of tectonic forcing at steady state nor a signature of transience or steady state we conclude that sediment effects need to be taken into account when modeling bedrock channel morphology

selective sorting and abrasion of river gravel i theory most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fining is due to some combination of selective sorting by which finer grains are preferentially transported downstream and abrasion by which individual particles are reduced in size here a framework for the simultaneous treatment of both phenomena is developed the analysis is restricted to bed load transport of gravel only abrasion due to binary collisions between moving bed load and stationary bed particles is considered the formulation includes the possibility of differential basin uplift and subsidence due to tectonism and mixtures of rock types with differing coefficients of abrasion the formulation of selective sorting is based on the concept of hiding as applied to a surface based bed load transport relation abrasion is first treated in terms of a lagrangian formulation for individual grains and t

experimental study on coarse grain saltation dynamics in bedrock channels saltation of bed load particles on bedrock surfaces is important for landscape evolution and bedrock incision in steep landscapes however few studies have investigated saltation in bedrock channels where unlike alluvial channels the bed roughness height and the sediment size may be independent to address this data gap we measured the saltation hop height hop length and velocity of gravel saltating over a planar bed using 80 160 readings from high speed photography and direct measurements two separate dimensional analyses are used one leading to a bed shear stress scaling and another leading to a froude number fr scaling our new saltation data coupled with numerous data from previous studies suggest that both shear stress and fr scaling analyses are valid in characterizing bed load saltation dynamics with bed roughness ranging from smooth to alluvial beds however the fr approach has the advantages that 1 there is no need to estimate a critical shields stress c which alone can vary up to 2 orders of magnitude e g 0 001 0 1 due to changes in relative bed roughness and slope and 2 the fr based scaling fits the saltation data set better in a least squares sense results show that the saltation velocity of bed load is independent of grain density and grain size and is linearly proportional to flow velocity saltation height has a nonlinear dependence on grain size saltation length increases primarily with flow velocity and it is inversely proportional to submerged specific density our results suggest that either c or bed roughness coefficient must be properly estimated to yield accurate results in saltation abrasion models

formation of fluvial hanging valleys theory and simulation 1 although only recently recognized hanging tributary valleys in unglaciated tectonically active landscapes are surprisingly common stream power based river incision models do not provide a viable mechanism for the formation of fluvial hanging valleys thus these disequilibrium landforms present an opportunity to advance our understanding of river incision processes in this work we demonstrate that thresholds apparent in sediment flux dependent bedrock incision rules provide mechanisms for the formation of hanging valleys in response to transient pulses of river incision we simplify recently published river incision models in order to derive analytical solutions for the conditions required for hanging valley formation and use these results to guide numerical landscape evolution simulations analytical and numerical results demonstrate that during the response to base level fall sediment flux dependent incision rules may create either temporary or permanent hanging valleys these hanging valleys form as a consequence of 1 rapid main stem incision oversteepening tributary junctions beyond some threshold slope or 2 low tributary sediment flux response during the pulse of main stem incision thus limiting the tributary s capacity to keep pace with main stem incision the distribution of permanent and temporary hanging valleys results from four competing factors the magnitude of base level fall the upstream attenuation of the incision signal the lag time of the sediment flux response and the nonsystematic variation in tributary drainage areas within the stream network the development of hanging valleys in landscapes governed by sediment flux dependent incision rules limits the transmission of base level fall signals through the channel network ultimately increasing basin response time

the influence of sandstone caprock material on bedrock channel steepness within a tectonically passive setting buffalo national river basin arkansas usa bedrock channel profile analysis typically assumes that channels evolve toward a condition of topographic steady state where channel morphology is adjusted to rock erodibility uplift rates and stream power here we use the integral method of channel profile analysis to quantify channel steepness within a large set of tributary channels that incise through layered rocks in the buffalo national river basin in northern arkansas statistical analysis of these channels demonstrates that normalized channel steepness is not a function of local bedrock lithology but is influenced by coarse sediment supply specifically normalized steepness is greatest in reaches of the basin where an interval of pennsylvanian sandstone forms a caprock on the ridges block detachment of the sandstone causes large boulders to be stranded in the upper tributaries where stream power is too low to mobilize or effectively erode the boulders within these channels normalized steepness is correlated with sandstone boulder size and percent boulder coverage rather than local lithology despite strong contrasts in the mechanical strength of the lithologies incised this analysis suggests that removal of caprock material is rate limiting within the landscape and may be responsible for the long term persistence of topography within this tectonically passive setting

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

on why gravel bed streams are paved bedload transport in poorly sorted gravel bed streams downstream of dams is considered bedload and typical bed material subpavement size distributions are observed to be similar it follows that the coarse half of the subpavement moves through a reach at a rate near that of the fine half since coarser grains are intrinsically less mobile than fine grains it follows that some mechanism must act to nearly equalize mobility it is hypothesized that the pavement seen in gravel bed streams at low flow is in fact in place during typical transport events capable of moving all available sizes this pavement can provide the equalizing mechanism by exposing proportionally more coarse grains to the flow field data are used to quantify this concept and to develop a predictive relation for river pavement the model indicates that pavement should be absent in most sand bed streams in agreement with observation

spatial patterns of erosion in a bedrock gorge understanding the physical processes driving bedrock channel formation is essential for interpreting and predicting the evolution of mountain landscapes here we analyze bedrock erosion patterns measured at unprecedented spatial resolution mm over 2 years in a natural bedrock gorge these spatial patterns show that local bedrock erosion rates depend on position in the channel cross section height above the streambed and orientation relative to the main streamflow and sediment path these observations are consistent with the expected spatial distribution of impacting particles the tools effect and shielding by sediment on the bed the cover effect vertical incision by bedrock abrasion averaged 1 5 mm a lateral abrasion averaged 0 4 mm a and downstream directed abrasion of flow obstacles averaged 2 6 mm a however a single plucking event locally exceeded these rates by orders of magnitude 100 mm a and accounted for one third of the eroded volume in the studied gorge section over the 2 year study period hence if plucking is spatially more frequent than we observed in this study period it may contribute substantially to long term erosion rates even in the relatively massive bedrock at our study site our observations demonstrate the importance of bedrock channel morphology and the spatial distribution of moving and static sediment in determining local erosion rates

intertidal boulder based wave hindcasting can underestimate wave size evidence from yorkshire uk large boulder size clasts can represent important archives of high energy erosional wave activity at the coast from tropical coral reefs to high latitude eroding cliffs boulders have been used to hindcast the frequency and magnitude height of waves produced by both storms and tsunami such reconstructions are based on the balance between the hydrodynamic forces acting on individual boulders and the counteracting resistive forces of friction and gravity we test published models on over 900 intertidal boulders on the coast at staithes in north yorkshire u k using repeat high resolution topographic survey data we quantified the predicted versus actual rates of boulder movement in the field over a 7 year period 2007 2014 we assessed the degree to which local geomorphology and biology may affect the resistive forces inhibiting transport we found that 84 of intertidal boulders predicted to be mobile remained stationary over 7 years suggesting that in some contexts boulder movement may be significantly less than predicted regardless of boulder volume shape or location in situ cementation of boulders to the substrate by marine organisms appears to play a key role in retarding or preventing boulder transport the implication is that boulder characteristics may not always provide a reliable estimation of wave height on the coast and reliance solely on hindcasting relationships may result in under prediction of the frequency and magnitude of past storm wave activity we suggest here that more field studies that consider boulder movement are needed set within specific site contexts and that these data are used to improve the applicability and robustness of boulder transport models

bed load transport in bedrock rivers the role of sediment cover in grain entrainment translation and deposition 1 bedrock rivers exert a critical control over landscape evolution yet little is known about the sediment transport processes that affect their incision we present theoretical analyses and field data that demonstrate how grain entrainment translation and deposition are affected by the degree of sediment cover in a bedrock channel theoretical considerations of grain entrainment mechanics and sediment continuity each demonstrate that areas of exposed bedrock and thin sediment depths cause sediment transport to be size independent albeit excluding extreme grain sizes we report gravel and cobble magnetic tracer data from three rivers with contrasting sediment cover the bedrock river calder 20 cover the bedrock south fork eel river 80 and the alluvial allt dubhaig 100 these data sets show that 1 transport distances in the river calder are controlled by sediment patch location whereas in the other rivers transport distances are described by gamma distributions representing local dispersion 2 river calder transport distances are size independent across all recorded shear stresses whereas the other rivers display size selectivity 3 river calder tracers are entrained at a dimensionless shear stress of 0 038 which is relatively low compared to alluvial rivers and 4 virtual grain velocities in the river calder are higher than in a comparable reach of the allt dubhaig these contrasts result from differences in the thicknesses and spatial distribution of sediment in the three rivers and support the theoretical analysis sediment processes in bedrock rivers systematically vary along a continuum between bedrock and alluvial end members citation hodge r a t b hoey and l s sklar 2011 bed load transport in bedrock rivers the role of sediment cover in grain entrainment translation and deposition j geophys res 116 f04028 doi 10 1029 2011jf002032

consistency of scaling relations among bedrock and alluvial channels 1 this paper presents field data on channel geometry and potential control variables from 47 field settings representing a diverse range of environments these data are used to evaluate existing scaling relationships used in models of the evolution of bedrock channel geometry and to test the hypothesis that channel width w increases more slowly and depth d more rapidly in relation to discharge q or drainage area a as substrate resistance increases for this data set w a0 3 w q0 5 d a0 2 and d q0 3 the w a and w q relations are close to those found by previous investigators the d a and d q relations have not previously been reported for bedrock channels examination of trends within the data does not support the hypothesis and instead suggests that the erosional resistance of channel boundaries is not the primary control on scaling relations for channel geometry

the queenston formation shale dominated mixed terrigenous carbonate deposits of upper ordovician semiarid muddy shores in ontario canada the upper ordovician queenston formation constitutes the westernmost part of the taconic clastic wedge of eastern north america in ontario the queenston formation consists of a mixed terrigenous carbonate succession composed primarily of red silty shales which are locally gypsiferous alternating in the middle and lower parts of the formation with thin grey shales calcareous siltstone and sandstone and bioclastic limestone brachiopods some ostracodes and in the lower beds bryozoans constitute the body fossil assemblage diplocraterion skolithos chondrites and occasional large vertical burrows of unknown origin form the ichnofauna plane beds ripple marks and occasional swaley and hummocky cross stratifications are present in the calcareous units mudcracks gypsum nodules occasional halite hopper casts and gilgai like structures typically occur in horizons where shales alternate with siltstone and limestone these features are indicative of prograding muddy and storm and tide affected

the effect of wet dry weathering on the rate of bedrock river channel erosion by saltating gravel abstract previous work has shown that the bedrock erosion rate e because of collisions of saltating bedload can be expressed by e qb 1 pc where qb is the sediment transport rate pc is the extent of alluvial cover and is the abrasion coefficient however the dependence of the abrasion coefficient on the physical characteristics of the bedrock material is poorly known and in particular the effects of wet dry weathering on the saltation abrasion bedrock incision has not been specifically characterized observation suggests that the typical wet dry cycling of exposed bedrock in river beds gives rise to cracks and voids that are likely to alter the incision rate of the material when subjected to impacts of moving sediment in this study flume experiments are performed to develop an understanding of how wet dry cycling affects the rock tensile strength and the bedrock erosion rate to represent the physical effects of weathering boring cores taken from natural bedrock channel are exposed to artificial wet dry cycles the experimental results suggest the following 1 the abrasion coefficient for fresh bedrock is estimated by 1 0 10 4 t 2 d ksb 0 5 where t is the tensile strength d is the diameter of colliding gravel and ksb is the hydraulic roughness height of bedrock 2 the tensile strength of the bedrock decreases exponentially as a result of repeated wet dry cycles t t0 exp ctnwa0 t0 where t0 is the initial tensile strength wa0 is the initial normalized rate of water absorption n is the number of wet dry cycles and ct is a constant 3 the erosion rate of fresh bedrock depends on the inverse of the square of tensile strength but the erosion rate of weathered bedrock depends on the 1 5 power of tensile strength

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

bed load tracer mobility in a mixed bedrock alluvial channel the presence of bare or partially covered rock in an otherwise alluvial river implies a downstream change in transport capacity relative to supply field investigations of this change and what causes it are lacking we used two sets of magnet tagged tracer clasts to investigate bedload transport during the same sequence of floods in fully alluvial bare rock and partial cover reaches of an upland stream high flow shear stresses in different reaches were calculated using stage loggers tracers seeded in the upstream alluvial channel moved more slowly than elsewhere until the frontrunners reached bare rock and sped up tracers seeded on bare rock moved rapidly off it and accumulated just upstream from and later in a partial cover zone with many boulders the backwater effect of the boulder rich zone is significant in reducing tracer mobility tracer movement over full or partial sediment cover was size selective but dispersion over bare rock was not along channel changes in tracer mobility are interpreted in terms of measured differences in shear stress and estimated differences in threshold stress

the micromorphology of unconsolidated sediments this paper aims to describe the use of thin sections micromorphology in the sedimentology of unconsolidated sediments it provides examples of the use of thin sections in a variety of sedimentary environments from fault gouge through caves and volcanics to aeolian fluviatile marine periglacial and glacial it demonstrates that in the latter three fields the use of micromorphology is relatively widespread and that in glacial sedimentology it has revolutionised our way of thinking about subglacial sediments although micromorphology has been mainly descriptive so far methods of quantification observations are demonstrated some of the important aspects of micromorphology are its use for microstratigraphy and the possibility of relating observations to documented processes thereby allowing a more robust sedimentological interpretation of modern and ancient sediments

hillslope derived blocks retard river incision the most common detachment limited river incision models ignore the effects of sediment on fluvial erosion yet steep reaches of mountain rivers often host clusters of large 1 m blocks we argue that this distribution of blocks is a manifestation of an autogenic negative feedback in which fast vertical river incision steepens adjacent hillslopes which deliver blocks to the channel blocks inhibit incision by shielding the bed and enhancing form drag we explore this feedback with a 1 d channel reach model in which block delivery by hillslopes depends on the river incision rate results indicate that incision dependent block delivery can explain the block distribution in boulder creek colorado the proposed negative feedback may significantly slow knickpoint retreat channel adjustment and landscape response compared to rates predicted by current theory the influence of hillslope derived blocks may complicate efforts to extract base level histories from river profiles

field instrumentation for high resolution parallel monitoring of bedrock erosion and bedload transport river incision is fundamental in shaping the earth s surface in mountainous regions with steep river beds fluvial bedrock erosion by bedload transport is an important mechanism forming channels however there are only a few complete field datasets that can be used to improve process understanding and evaluate erosion models especially at the process scale to provide a simultaneous dataset of hydraulics bedload transport and bedrock erosion at high temporal and spatial resolution a new measuring device has been installed in the erlenbach a gauged stream in the swiss pre alps in this stream bedload transport rates can be calculated from surveying deposits and from geophone plate sensors and bedload transport samples can be taken directly by an automated moving basket system to measure bedrock erosion rates simultaneously two natural stone slabs were mounted flush with the channel bed in a steel frame hosting various measurement devices force sensors below the slabs record normal stress and shear stress at a point erosion rates on the slab surfaces are continuously measured at sub millimetre precision at three locations on each slab in addition the slab topography is monitored following erosive flood events in this article i the erosion scale device is described ii data resolution and data quality is assessed by means of tests and event data and iii the first transport event is discussed the erosion scales are confirmed to provide data at high spatio temporal resolution for process analysis the preliminary data show evidence for the tools effect in bedrock erosion the bedrock slabs can be exchanged to obtain measurements for catchments with different lithologies for comparison copyright 2014 john wiley sons ltd

tracer pebble movement along a concave river profile virtual velocity in relation to grain size and shear stress over 1400 tracer pebbles 16 256 mm in diameter were tracked for 2 years in six reaches of allt dubhaig scotland a small gravel bed river along which shear stress and bed surface grain size decrease toward a local base level pebble movement was size selective both within and between reaches within reaches the decrease in mean travel distance with increasing grain size is strongest in the coarse tail of the size distribution particle shape has a minor secondary effect a nondimensional grain velocity averaged over the duration of competent flow is used to compare different size classes and reaches over 90 of its variance is explained by relative grain size and reach shields stress the pattern of size selectivity is consistent with single event tracer results elsewhere bedload trap data from our distal reach and the concept of partial mobility it provides a mechanism for strong downstream fining by selective transport and deposition along rivers in which stress declines toward base level the nondimensional prediction equation for grain velocity may be of use in other rivers but requires testing

flow resistance and hydraulic geometry in contrasting reaches of a bedrock channel assumptions about flow resistance in bedrock channels have to be made for mechanistic modeling of river incision paleoflood estimation flood routing and river engineering field data on bedrock flow resistance are very limited and calculations generally use standard alluvial river assumptions such as a fixed value of manning s n to help inform future work we measured how depth velocity and flow resistance vary with discharge in four short reaches of a small bedrock channel one with an entirely rock bed and the others with 20 70 sediment cover and in the alluvial channel immediately upstream as discharge and submergence increase in each of the partly or fully alluvial reaches there is a rapid increase in velocity and a strong decline in both n and the darcy weisbach friction factor f the bare rock reach follows a similar trend from low to medium discharge but has increasing resistance at higher discharges because of the macro roughness of its rock walls flow resistance at a given discharge differs considerably between reaches and is highest where the partial sediment cover is coarsest and most extensive apart from the effect of rough rock walls the flow resistance trends are qualitatively consistent with logarithmic and variable power equations and with non dimensional hydraulic geometry but quantitative agreement using sediment d84 as the roughness height is imperfect