effects of lateral and vertical constrictions on flow in rough steep channels with bedload abstracttwo phase flows occurring at flow constrictions such as bridges or open sediment check dams are complex especially for steep rivers with bedload transport dangerous bedload deposition and

direct measurements of lift and drag on shallowly submerged cobbles in steep streams implications for flow resistance and sediment transport steep mountain streams have higher resistance to flow and lower sediment transport rates than expected by comparison with low gradient rivers and often these differences are attributed to reduced near bed flow velocities and stresses associated with form drag on channel forms and immobile boulders however few studies have directly measured drag and lift forces acting on bed sediment for shallow flows over coarse sediment which ultimately control sediment transport rates and grain scale flow resistance here we report on particle lift and drag force measurements in flume experiments using a planar fixed cobble bed over a wide range of channel slopes 0 004 s 0 3 and water discharges drag coefficients are similar to previous findings for submerged particles c d 0 7 but increase significantly for partially submerged particles in contrast lift coefficients decrease from near unity to zero as the flow shallows and are strongly negative for partially submerged particles indicating a downward force that pulls particles toward the bed fluctuating forces in lift and drag decrease with increasing relative roughness and they scale with the depth averaged velocity squared rather than the bed shear stress we find that even in the absence of complex bed topography shallow flows over coarse sediment are characterized by high flow resistance because of grain drag within a roughness layer that occupies a significant fraction of the total flow depth and by heightened critical shields numbers and reduced sediment fluxes because of reduced lift forces and reduced turbulent fluctuations

a benchmark dataset of in situ antarctic surface melt rates and energy balance abstract surface melt on the coastal antarctic ice sheet ais determines the viability of its ice shelves and the stability of the grounded ice sheet but very few in situ melt rate estimates exist to date here we present a benchmark dataset of in situ surface melt rates and energy balance from nine sites in the eastern antarctic peninsula ap and coastal dronning maud land dml east antarctica seven of which are located on ais ice shelves meteorological time series from eight automatic and one staffed weather station neumayer ranging in length from 15 months to almost 24 years serve as input for an energy balance model to obtain consistent surface melt rates and energy balance results we find that surface melt rates exhibit large temporal spatial and process variability intermittent summer melt in coastal dml is primarily driven by absorption of shortwave radiation while non summer melt events in the eastern ap occur during fohn events that force a large downward directed turbulent flux of sensible heat we use the in situ surface melt rate dataset to evaluate melt rates from the regional atmospheric climate model racmo2 and validate a melt product from the quikscat satellite

spatially averaged open channel flow over rough bed in this paper it is suggested that the double averaged in temporal and in spatial domains momentum equations should be used as a natural basis for the hydraulics of rough bed open channel flows especially with small relative submergence the relationships for the vertical distribution of the total stress for the simplest case of 2d steady uniform spatially averaged flow over a rough bed with flat free surface are derived these relationships explicitly include the form induced stresses and form drag as components of the total stress using this approach we define three types of rough bed flows 1 flow with high relative submergence 2 flow with small relative submergence and 3 flow over a partially inundated rough bed the relationships for the double averaged velocity distribution and hydraulic resistance for all three flow types are derived and compared with measurements where possible the double averaged turbulent and form induced intensities and stresses for the case of regular spherical segment type roughness show the dominant role of the double averaged turbulence stresses and form drag in momentum transfer in the near bed region

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

relation between flow surface layer armoring and sediment transport in gravel bed rivers this study investigates trends in bed surface and substrate grain sizes in relation to reach scale hydraulics using data from more than 100 gravel bed stream reaches in colorado and utah collocated measurements of surface and substrate sediment bankfull channel geometry and channel slope are used to examine relations between reach average shear stress and bed sediment grain size slopes at the study sites range from 0 0003 to 0 07 bankfull depths range from 0 2 to 5 m and bankfull widths range from 2 to 200 m the data show that there is much less variation in the median grain size of the substrate d50s than there is in the median grain size of the surface d50 the ratio of d50 to d50s thus decreases from about four in headwater reaches with high shear stress to less than two in downstream reaches with low shear stress similar trends are observed in an independent data set obtained from measurements in gravel bed streams in idaho a conceptual quantitative model is developed on the basis of these observations to track differences in bed load transport through an idealized stream system the results of the transport model suggest that downstream trends in total bed load flux may vary appreciably depending on the assumed relation between surface and substrate grain sizes copyright 2007 john wiley sons ltd

simple method for calculating reach averaged bed load transport abstracta simple and robust method is proposed for calculating reach averaged bed load transport in sand and gravel bed rivers with no calibration the data required are the bed surface d50 and d84 measured with the nontruncated pebble count technique the slope the width and the flow depth or discharge the method is illustrated by comparison with bed load samples measured in 15 river reaches

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

hydrodynamics of steep streams with planar coarse grained beds turbulence flow resistance and implications for sediment transport the hydraulics of steep mountain streams differ from lower gradient rivers due to shallow and rough flows energetic subsurface flow and macro scale form drag from immobile boulders and channel and bed forms heightened flow resistance and reduced sediment transport rates in steep streams are commonly attributed to macro scale form drag however little work has explored steep river hydrodynamics in the absence of complex bed geometries here we present theory for the vertical structure of flow velocity in steep streams with planar rough beds that couples surface and subsurface flow we test it against flume experiments using a bed of fixed cobbles over a wide range of bed slopes 0 4 30 experimental flows have a nearly logarithmic velocity profile far above the bed flow velocity decreases less than logarithmically towards the bed and is non zero at the bed surface velocity profiles match theory derived using a hybrid eddy viscosity model in which the mixing length is a function of height above the bed and bed roughness subsurface flow velocities are large 1 m s and follow a modified darcy brinkman forchheimer relation that accounts for channel slope and shear from overlying surface flow near bed turbulent fluctuations decrease for shallow rough flows and scale with the depth averaged flow velocity rather than bed shear velocity flow resistance for rough planar beds closely matches observations in natural steep streams despite the lack of bed or channel forms in the experiments suggesting that macro scale form drag is smaller than commonly assumed in stress partitioning models for sediment transport

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

transporting capacity of overland flow on plane and on irregular beds in this paper the transporting capacity of thin flows in the laminar and transitional flow regime is studied experiments were carried out on irregular as well as on plane beds using two totally different set ups the results of these two types of experiment were convergent in both cases sediment concentration was clearly related to grain shear velocity and unit stream power expressed as the product of mean velocity and slope yang 1973 the data agreed with those of kramer and meyer 1969 for a sandy bed the unit stream power relationship was able to predict reasonably well the sediment concentrations measured on a mulched surface for laminar and transitional flows both the unit stream power and the shear velocity are related in the same way to slope and unit discharge the unit stream power is a parameter which in particular can be very easily measured and might therefore become useful in obtaining a quick estimate of the transporting capacity of a thin flow however before a sediment transport equation for thin flows can be developed more information is needed about the influence of the flow regime and grain size and density

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

strain accumulation along the denali fault at the nenana river and delta river crossings alaska surveys of trilateration networks across the denali fault at the nenana river in 1982 1984 and 1988 and at the delta river in 1975 1979 1982 and 1984 indicate a minor 0 10 0 04 strain yr northeastward uniaxial extension the component of right lateral shear strain accumulation across the fault is not significant at the two standard deviation level at the delta river network the strain accumulation rate decreases rapidly with distance from the fault but evidence for a similar decrease with distance from the fault is lacking at the nenana river network the strain accumulation rates inferred from trilateration are consistent with the very long baseline interferometry vlbi measurement reported by ma et al 1990 and support their contention that significant right lateral shear is not accumulating along the denali fault at the present time savage et al 1981 had earlier concluded erroneously that preliminary geodetic measurements at the delta river network demonstrated right lateral shear strain accumulation the absence of significant right lateral deformation across the denali fault in the 1975 1988 interval is in marked contrast with the abundant geomorphic evidence for holocene right lateral secular slip at the rate of 10 20 mm yr on the denali fault in this sector

velocity distribution and bed roughness in high gradient streams a model for the velocity field over a poorly sorted bed is derived herein and applied to flow in steep streams with coarse gravel beds in such streams large clasts often are comparable in size to stream depth and act as obstacles to the flow the effect of these obstacles on a flow is estimated by partitioning total stress proportional to the depth slope product into a purely fluid component and a form drag component associated with flow around the obstacles an eddy viscosity closure is used with a length scale dependent on both distance from the boundary and wake dimension resulting velocity profiles agree well with profiles measured by marchand et al 1984 in nine mountain streams in colorado model runs for a range of conditions reveal that the velocity distribution depends primarily on d84z and flow depth h d84z is the length of the vertically oriented axis of clasts at the 84th percentile of the grain size distribution calculated values of mean velocity normalized by shear velocity are well represented by a simple log linear relationship in terms of relative roughness d84 h

structural properties of mobile armors formed at different flow strengths in gravel bed rivers differences in the structure of mobile armors formed at three different flow strengths have been investigated in a laboratory flume the temporal evolution of the bed surfaces and the properties of the final beds were compared using metrics of surface grain size microtopography and bed organization at both grain and mesoscales measurements of the bed condition were obtained on nine occasions during each experiment to describe the temporal evolution of the beds structured mobile armors formed quickly in each experiment at the grain scale 1 45 mm 9 ds50 17 mm where ds50 is the median surface particle size surface complexity decreased and bed roughness increased in response to surface coarsening and the development of the mobile armor particles comprising the armor also became flow aligned and developed imbrication at a larger scale 100 200 mm the surface developed a mesoscale topography through the development of bed patches with lower and higher elevations metrics of mobile armor structure showed remarkable consistency over prolonged periods of near constant transport demonstrating for the first time that actively transporting surfaces maintain an equilibrium bed structure bed structuring was least developed in the experiments conducted at the lowest flow strength however little difference was observed in the structural metrics of the mobile armors generated at higher flows although the range of transport rates studied was limited the results suggest that the structure of mobile armors is insensitive to the formative transport rate except when rates are low 0 03 where is the dimensionless shear stress

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

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

an evaluation of stream characteristics in glacial versus fluvial process domains in the colorado front range abstract many of the conceptual models developed for river networks emphasize progressive downstream trends in morphology and processes such models can fall short in describing the longitudinal variability associated with low order streams a more thorough understanding of the influence of local variability of process and form in low order stream channels is required to remotely and accurately predict channel geometry characteristics for management purposes and in this context designating process domains is useful we define process domains with respect to glacial versus fluvial valleys and lateral confinement of valley segments we evaluated local variability of process domains in the colorado front range by systematically following streams categorizing them into stream morphologic type and process domain and evaluating a number of channel geometry characteristics we evaluated 111 stream reaches for significant differences in channel geometry among stream types and process domains location and clustering of stream types on a slope drainage area s a plot and downstream hydraulic geometry relationships although individual channel geometry variables differed significantly between individual stream types in glacial and fluvial process domains no single channel geometry variable consistently differentiated all stream types between process domains hypothetical s a boundaries between bedrock and alluvial bed channels proposed in previous studies did not reliably divide bedrock and alluvial reaches for our study sites although downstream hydraulic geometry relationships are well defined using all reaches in the study area reaches in glacial valleys display much more variability in channel geometry characteristics than reaches in fluvial valleys less pronounced downstream hydraulic geometry relationships and greater scatter of reaches on an s a plot local spatial variability associated with process domains at the reach scale 10 1 10 3 m overrides progressive downstream relationships in low order mountain streams of the colorado front range

in situ determination of particle friction angles of fluvial gravels particle friction angles represent the physical resistance to initial movement of a sediment particle and are therefore useful for relating initiation of motion to particular flows we determined over 8000 friction angle values at five natural rivers by applying a new method that uses a digital load cell to directly measure the force fd required to pivot or slide a particle out of its natural resting place within each site median values were very similar to previously reported relations yet different enough between sites that a location general predictive empirical relation would produce errors in of 10 degrees for di ks 1 furthermore within a di ks class at a given site the range in was as large as 80 much greater than the range of median values between classes for natural sediment mixtures using estimates of c from extensive bed load measurements made by andrews and erman 1986 at sagehen creek and the in situ measurements made in this study together with a theoretical model developed by wiberg and smith 1987 we show that measurements made with this new method can be used to accurately predict c for natural water worked sediments additionally these results confirm that a value 50 is more appropriate for predicting c of a given size class

applicability of bed load transport models for mixed size sediments in steep streams considering macro roughness in steep mountain streams macro roughness elements typically increase both flow energy dissipation and the threshold of motion compared to lower gradient channels reducing the part of the flow energy available for bed load transport bed load transport models typically take account of these effects either by reducing the acting bed shear stress or by increasing the critical parameters for particle entrainment here we evaluate bed load transport models for mixed size sediments and models based on a median grain size using a large field data set of fractional bed load transport rates we derive reference shear stresses and bed load transport relations based on both the total boundary shear stress and a reduced or effective shear stress that accounts for flow resistance due to macro roughness when reference shear stresses are derived from the total boundary shear stress they are closely related to channel slope but when they are derived from the effective shear stress they are almost invariant with channel slope the performance of bed load transport models is generally comparable when using the total shear stress and a channel slope related reference shear stress or when using the effective shear stress and a constant reference shear stress however dimensionless bed load transport relations are significantly steeper for the total stress approach whereas they are similar to the commonly used fractional wilcock and crowe wc transport model for the effective stress approach this similarity in the relations allows the wc model developed for lower gradient streams to be used in combination with an effective shear stress approach in steep mountain streams

controls on downstream variation in surficial sediment size of an outwash braidplain developed under high arctic conditions kaffioyra svalbard abstract the changes in morphology and surficial sediments of an outwash braidplain developed under high arctic conditions are closely related to geomorphic processes tectonic conditions and climate variations and are affected by the existence of permafrost these factors control the changes in river channel slope their morphology and active braidplain width which influence the downstream variation in grain size of outwash surficial sediments interdependence between these parameters has not previously been considered in relation to the spatial distribution of surficial sediments of outwash braidplain developed in the forefield of a retreating subpolar glacier in this study controls affecting the surficial lithology of the waldemar river outwash nw spitsbergen svalbard were recognised and indicated multiple correlations between the downstream changes in sediment texture the geomorphometry of distributary channels and outwash surface slope due to this the techniques of multivariate regression modelling artificial neural network and multivariate adaptive regression splines were used results show that observed relations between the sedimentary and morphological features indicate the existence of thresholds for downstream variation in bed material size which are not constant moreover the processes of sediment transport in an outwash braidplain are strictly related to shields stress which is proportional to changes in braidplain slope median grain diameter and sediment sorting in the proximal and middle zones of the outwash but is inversely proportional to median grain diameter in the distal zone contrary to shields bed material threshold entrainment model the results of this study will help understand outwash fan and plain evolution in paraglacial landscapes under conditions of changeable rate and source of sediment supply and transport capacity

flow resistance in steep streams an experimental study 1 frequently an assessment of the mean water velocity in a stream is necessary to estimate the discharge associated with a particular flow depth or conversely the mean depth associated with a particular discharge in the absence of a direct measurement of flow velocity a flow resistance approach which establishes the relation between depth and velocity can be applied two approaches have been used in the past traditional approaches based on the use of a resistance coefficient e g darcy weisbach or dimensionless hydraulic geometry approaches to examine if one approach is more appropriate for steep streams data from 31 flume experiments conducted to examine flow resistance in self formed cascade channels were analyzed a dimensionless hydraulic geometry approach developed using at a station data to characterize the q exponent and between site data to characterize the exponent on the channel slope term was more accurate than more traditional approaches the developed relation was similar to the established rational relation v g0 2q0 6s 0 4s0 2 suggesting that the rational relation has merit the approach does not utilize a flow partitioning approach since in steep streams with small relative depths the grains themselves generate form and spill resistance the observation that a single dimensionless hydraulic geometry flow resistance relation can describe measurements across a range of grain sizes and bed slopes 3 21 suggests that steep streams may follow a single scaling relation similar to the regime equations associated with lowland rivers

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

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

incipient sediment motion at high slopes in uniform flow condition 1 this paper reviews the problem of the incipient motion of cohesionless particles in a high sloping water stream a theoretical analysis of the equilibrium of a gravel particle is developed by a deterministic approach with the introduction of the relative degree of exposure of the particle itself to the stream flow the parameter relative degree of exposure relates the positioning of a particle with the upstream one the problem of velocity distribution and the definition of drag coefficient is also revised to better account for low submergence the results of the theory are compared with a series of ad hoc experiments performed by c gregoretti

unequal mobility of gravel and sand in weakly bimodal river sediments bed load was trapped during flood events over a 20 month period at the lower end of the allt dubhaig a small river in scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile the channel bed near the trap is predominantly gravel with a secondary sand mode total transport in each event depended mainly on peak shear stress rather than duration over a threshold bed load was mainly sand in smaller events bimodal in intermediate events and mainly gravel in the biggest floods mean and maximum grain diameter both increased with peak shear stress but in different ways analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility the slight selectivity in gravel transport is consistent with previous field studies of near equilibrium unimodal beds and supports assumptions made in the numerical model of hoey and ferguson 1994 which successfully simulates the observed amount of downstream fining over the 2 5 km upstream of the bed load trap

evaluation of flow resistance in gravel bed rivers through a large field data set 1 a data set of 2890 field measurements was used to test the ability of several conventional flow resistance equations to predict mean flow velocity in gravel bed rivers when used with no calibration the tests were performed using both flow depth and discharge as input since discharge may be a more reliable measure of flow conditions in shallow flows generally better predictions are obtained when using flow discharge as input the results indicate that the manning strickler and the keulegan equations show considerable disagreement with observed flow velocities for flow depths smaller than 10 times the characteristic grain diameter most equations show some systematic deviation for small relative flow depth the use of new definitions for dimensionless variables in terms of nondimensional hydraulic geometry equations allows the development of a new flow resistance equation the best overall performance is obtained by the ferguson approach which combines two power law flow resistance equations that are different for deep and shallow flows to use this approach with flow discharge as input a logarithmic matching equation in terms of the new dimensionless variables is proposed for the domains of intermediate and large scale roughness the field data indicate a considerable increase in flow resistance as compared with the domain of small scale roughness the ferguson approach is used to discuss the importance of flow resistance partitioning for bed load transport calculations at flow conditions with intermediate and large scale roughness in natural gravel cobble and boulder bed streams

theoretical development on the effects of changing flow hydraulics on incipient bed load motion 1 several decades of flume and field measurements have indicated that in rough turbulent flows the critical shields stress increases with increasing slope and associated decreasing relative depth this result contradicts the usual consideration of a decreased critical shields value on very steep slopes because of increased gravitational effects however recent studies have demonstrated that these experimental results could be reproduced with a force balance model if the classical logarithmic velocity profile was replaced with a velocity profile that was more compatible with available velocity measurements over gravel beds these measurements indicate the existence of a roughness layer that is a zone of almost constant velocity close to the bed whose properties mean velocity and turbulence depended on the flow s relative depth unfortunately velocity profile measurements for low relative depth associated with steep slopes are scarce and it is still difficult to include such flow properties in a force balance model flow resistance data on the basis of depth average velocity measurements are very common and cover a wide range of slopes and relative depths in this paper these data are used to fit a velocity profile including a roughness layer when used in a force balance model for incipient motion it adequately reproduced a data set composed of 270 critical shields values measured in a flume with near uniform sediments the relevance of this research to field problems is discussed using a data set composed of 92 critical shields stresses obtained from field measurements finally a model is proposed for field applications taking into account the slope effect

debris flow initiation by runoff in a recently burned basin is grain by grain sediment bulking or en masse failure to blame postwildfire debris flows are frequently triggered by runoff following high intensity rainfall but the physical mechanisms by which water dominated flows transition to debris flows are poorly understood relative to debris flow initiation from shallow landslides in this study we combined a numerical model with high resolution hydrologic and geomorphic data sets to test two different hypotheses for debris flow initiation during a rainfall event that produced numerous debris flows within a recently burned drainage basin based on simulations large volumes of sediment eroded from the hillslopes were redeposited within the channel network throughout the storm leading to the initiation of numerous debris flows as a result of the mass failure of sediment dams that built up within the channel more generally results provide a quantitative framework for assessing the potential of runoff generated debris flows based on sediment supply and hydrologic conditions

critical flow constrains flow hydraulics in mobile bed streams a new hypothesis a new hypothesis predicts that in mobile bed river channels interactions between the channel hydraulics and bed configuration prevent the froude number fr from exceeding 1 for more than short distances or periods of time flow conditions in many steep competent streams appear to be close to critical froude numbers of steep slope 0 01 sand bed streams with considerable freedom to adjust boundaries oscillate between 0 7 and 1 3 over 20 to 30 s cycles with an average of 1 0 at the channel thalweg criticalflow in these streams is maintained by the interaction between the mobile bed and free water surface at highfr which results in a cyclical pattern of creation and destruction of bed forms field observations support that a similar mechanism offlow bed form interaction constrains fr 1 in active bed braided gravel rivers step pool streams laboratory rills lahar runout channels and even some bedrock channels empirical and analytical results show that as slope increases competentflows tend to asymptotically approach criticalflow an assumption of criticalflow would dramatically simplify paleohydraulicflow reconstructions and modeling offlow hydraulics in high gradient streams

runoff generated debris flows observations and modeling of surge initiation magnitude and frequency 1 runoff during intense rainstorms plays a major role in generating debris flows in many alpine areas and burned steeplands yet compared to debris flow initiation from shallow landslides the mechanics by which runoff generates a debris flow are less understood to better understand debris flow initiation by surface water runoff we monitored flow stage and rainfall associated with debris flows in the headwaters of two small catchments a bedrock dominated alpine basin in central colorado 0 06 km2 and a recently burned area in southern california 0 01 km2 we also obtained video footage of debris flow initiation and flow dynamics from three cameras at the colorado site stage observations at both sites display distinct patterns in debris flow surge characteristics relative to rainfall intensity i we observe small quasiperiodic surges at low i large quasiperiodic surges at intermediate i and a single large surge followed by small amplitude fluctuations about a more steady high flow at high i video observations of surge formation lead us to the hypothesis that these flow patterns are controlled by upstream variations in channel slope in which low gradient sections act as sediment capacitors temporarily storing incoming bed load transported by water flow and periodically releasing the accumulated sediment as a debris flow surge to explore this hypothesis we develop a simple one dimensional morphodynamic model of a sediment capacitor that consists of a system of coupled equations for water flow bed load transport slope stability and mass flow this model reproduces the essential patterns in surge magnitude and frequency with rainfall intensity observed at the two field sites and provides a new framework for predicting the runoff threshold for debris flow initiation in a burned or alpine setting

hyperconcentrated flow and sediment transport at steep slopes in order to simulate a fine material slurry of a debris flow a clay suspension of various concentrations was recirculated in a steep flume the effect of an increasing fluid density and viscosity on the flow behavior and the bed load transport capacity of the flow was examined viscous effects were found to become important below a limiting particle reynolds number of about 10 above this limiting value density effects cause an increase in the bed load transport rates as compared to similar conditions with clear water as transporting fluid the experimental data in this range can be described with conventional newtonian formulas and is analyzed together with other bed load transport data two different calculation schemes are proposed for the steep slope range is n n 10 where the bed load concentration is significant with respect to the total flow depth below the critical particle reynolds number of 10 the bed load transport rates decreased strongly

width adjustment in experimental gravel bed channels in response to overbank flows 1 we conducted a series of flume experiments to investigate the response of self formed gravel bed channels to floods of varying magnitude and duration floods were generated by increasing the discharge into a channel created in sand and gravel sized sediment with a median grain size of 2 mm flooding increased the shields stress along the channel perimeter causing bank erosion and rapid channel widening the sediment introduced to the channel by bank erosion was not necessarily deposited on the channel bed but was rather transported downstream a process likely facilitated by transient fining of the bed surface at the end of each experiment bank sediments were no longer in motion partial bed load transport characterized the flat bed portion of the channel and the shields stress approached a constant value of 0 056 about 1 2 times the critical shields stress for incipient motion furthermore the discharge was entirely accommodated by flow within the channel the creation of a stable channel entirely eliminated overbank flows we speculate that similar processes may occur in nature but only where bank sediments are non cohesive and where channel narrowing processes cannot counteract bank erosion during overbank flows we also demonstrate that a simple model of lateral bed load transport can reproduce observed channel widening rates suggesting that simple methods may be appropriate for predicting width increases in channels with non cohesive unvegetated banks even during overbank flows last we present a model for predicting the equilibrium width and depth of a stable gravel bed channel with a known channel forming shields stress

distribution of the pacific north america motion in the queen charlotte islands s alaska plate boundary zone 1 we present gps data that constrain the distribution of the relative pacific north america motion across the queen charlotte islands alaska panhandle margin nw north america velocities from a network of 22 campaign and permanent sites indicate that the pacific north america transpressive motion is mostly accommodated along the locked queen charlotte fairweather fault a significant portion 6 7 mm yr of the relative plate motion is taken up by distributed dextral shear across a 200 km wide region of the margin two models have been proposed to describe how the pacific north america convergence is accommodated off the queen charlotte islands internal shortening vs underthrusting of the pacific plate although the gps data cannot discriminate between the models they provide strong constraints on the convergence distribution the significant non transient motion of gps sites along the central british columbia southern alaska margin has implications for seismic hazard and tectonic evolution models of the canadian cordillera

bedload transport in the instrumented catchment of the rio cordon part ii analysis of the bedload rate abstract sediment transport is monitored by a specially designed instrumentation installed on a steep mountain streams the rio cordon the variability of bedload rate and yield reflects the variability in flow discharge bedload and flow data for ordinary low peak flow floods and exceptional sediment transport events are used to test the range of application of the bedload equations proposed for high gradient streams such equations overestimate bedload rate for ordinary events while their reliability increases when used to predict high bedload transport rates like those recorded during the exceptional flood that affected rio cordon on september 14 1994 the best agreement between computed and measured values of cumulated bedload volumes is found by using the bagnold bagnold r a 1956 the flow of cohesionless grains in fluids philos trans r soc london 249a 235 297 smart and jaeggi smart j m jaeggi m n r 1983 sediment transport on steep slopes mitteil 64 versuchsanstalt fur wasserbau hydrologie und glaziologie eth zurich switzerland 191 pp and rickenmann rickenmann d 1991 hyperconcentrated flow and sediment transport at steep slopes j hydraul eng zurig 117 11 1419 1439 formulas the schoklitsch equation schoklitsch a 1962 handbuch des wasserbaues 3rd edn springer wien also provides good estimations for instantaneous bedload rates at high sediment transport intensity and unlimited sediment supply conditions the formal type of the last equation is still valid and can be calibrated for low intensity bedload rates

foehn winds link climate driven warming to ice shelf evolution in antarctica rapid warming of the antarctic peninsula over the past several decades has led to extensive surface melting on its eastern side and the disintegration of the prince gustav larsen a and larsen b ice shelves the warming trend has been attributed to strengthening of circumpolar westerlies resulting from a positive trend in the southern annular mode sam which is thought to promote more frequent warm dry downsloping foehn winds along the lee or eastern side of the peninsula we examined variability in foehn frequency and its relationship to temperature and patterns of synoptic scale circulation using a multidecadal meteorological record from the argentine station matienzo located between the larsen a and b embayments this record was further augmented with a network of six weather stations installed under the u s nsf larsen ice shelf system antarctica project significant warming was observed in all seasons at matienzo with the largest seasonal increase occurring in austral winter 3 71 c between 1962 1972 and 1999 2010 frequency and duration of foehn events were found to strongly influence regional temperature variability over hourly to seasonal time scales surface temperature and foehn winds were also sensitive to climate variability with both variables exhibiting strong positive correlations with the sam index concomitant positive trends in foehn frequency temperature and sam are present during austral summer with sustained foehn events consistently associated with surface melting across the ice sheet and ice shelves these observations support the notion that increased foehn frequency played a critical role in precipitating the collapse of the larsen b ice shelf

predicting grain size in gravel bedded rivers using digital elevation models application to three maine watersheds riverbed grain size controls suitability of spawning habitat for threatened fish species motivated by this relationship we developed a model that uses digital elevation models dems to predict bed grain size we tested the accuracy of our model and two existing models with channel measurements from high resolution airborne light detection and ranging lidar dems all three models assume that bed grain size is a function of reach average high flow channel hydraulics measured by shear stress or stream power our test data are field measurements of median grain size d 50 at 276 stations along four rivers in maine pleistocene continental glaciation strongly influences the longitudinal profiles which have alternating steep and gradual segments we exploit the resulting variations in sediment supply to understand the controls on model success or failure in predicting bed grain size results show that all three models have 70 success in predicting d 50 within a factor of two overall and better where the rivers are coarse gravel bedded 80 success where d 50 16 mm this similarity is unsurprising given that the models primarily rely on channel gradient s and drainage area as inputs measurements of s from lidar dems yield only a modest improvement in model success over those from topographic maps we find that our model works best in sediment starved steep reaches model failures fall into two broad categories 1 relatively fine grained d 50 d 50 we argue that models based on airborne infrared lidar dems may reach a maximum around 80 85 accuracy due to these sub reach scale factors which cannot be easily measured from dems the overall success of the models in predicting grain size indicates that the morphology of these channels has adjusted to the imposed s and sediment load during the 15 k y since deglaciation and through the period of anthropogenic channel change over the past three centuries

firn air depletion as a precursor of antarctic ice shelf collapse since the 1970s the sudden rapid collapse of 20 of ice shelves on the antarctic peninsula has led to large scale thinning and acceleration of its tributary glaciers the leading hypothesis for the collapse of most of these ice shelves is the process of hydrofracturing whereby a water filled crevasse is opened by the hydrostatic pressure acting at the crevasse tip this process has been linked to observed atmospheric warming through the increased supply of meltwater importantly the low density firn layer near the ice shelf surface providing a porous medium in which meltwater can percolate and refreeze has to be filled in with refrozen meltwater first before hydrofracturing can occur at all here we build upon this notion of firn air depletion as a precursor of ice shelf collapse by using a firn model to show that pore space was depleted in the firn layer on former ice shelves which enabled their collapse due to hydrofracturing two climate scenario runs with the same model indicate that during the 21st century most antarctic peninsula ice shelves and some minor ice shelves elsewhere are more likely to become susceptible to collapse following firn air depletion if warming continues into the 22nd century similar depletion will become widespread on ice shelves around east antarctica our model further suggests that a projected increase in snowfall will protect the ross and filchner ronne ice shelves from hydrofracturing in the coming two centuries

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

foehn jets over the larsen c ice shelf antarctica previously unknown foehn jets have been identified to the east of the antarctic peninsula ap above the larsen c ice shelf these jets have major implications for the east coast of the ap a region of rapid climatic warming and where two large sections of ice shelf have collapsed in recent years during three foehn events across the ap leeside warming and drying is seen in new aircraft observations and simulated well by the met office unified model metum at 1 5 km grid spacing in case a weak southwesterly flow and an elevated upwind inversion characterise a highly nonlinear flow regime with upwind flow blocking in case c strong northwesterly winds characterise a relatively linear case with little upwind flow blocking case b resides somewhere between the two in flow regime linearity the foehn jets apparent in aircraft observations where available and metum simulations of all three cases are mesoscale features up to 60 km in width originating from the mouths of leeside inlets through back trajectory analysis they are identified as a type of gap flow in cases a and b the jets are distinct being strongly accelerated relative to the background flow and confined to low levels above the larsen c ice shelf they resemble the shallow foehn of the alps case c resembles a case of deep foehn with the jets less distinct the foehn jets are considerably cooler and moister relative to adjacent regions of calmer foehn air this is due to a dampened foehn effect in the jet regions in case a the jets have lower upwind source regions and in the more linear case c there is less diabatic warming and precipitation along jet trajectories due to the reduced orographic uplift across the mountain passes

the impact of a changing southern hemisphere annular mode on antarctic peninsula summer temperatures since the mid 1960s rapid regional summer warming has occurred on the east coast of the northern antarctic peninsula with near surface temperatures increasing by more than 2 degrees c this warming has contributed significantly to the collapse of the northern sections of the larsen ice shelf coincident with this warming the summer southern hemisphere annular mode sam has exhibited a marked trend suggested by modeling studies to be predominantly a response to anthropogenic forcing resulting in increased westerlies across the northern peninsula observations and reanalysis data are utilized to demonstrate that the changing sam has played a key role in driving this local summer warming it is proposed that the stronger summer westerly winds reduce the blocking effect of the antarctic peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern antarctic peninsula when this occurs a combination of a climatological temperature gradient across the barrier and the formation of a fohn wind on the lee side typically results in a summer near surface temperature sensitivity to the sam that is 3 times greater on the eastern side of the peninsula than on the west sam variability is also shown to play a less important role in determining summer temperatures at stations west of the barrier in the northern peninsula similar to 62 degrees s both at the surface and throughout the troposphere this is in contrast to a station farther south similar to 65 degrees s where the sam exerts little influence

self adjustment of stream bed roughness and flow velocity in a steep mountain channel understanding how channel bed morphology affects flow conditions and vice versa is important for a wide range of fluvial processes and practical applications we investigated interactions between bed roughness and flow velocity in a steep glacier fed mountain stream riedbach ct valais switzerland with almost flume like boundary conditions bed gradient increases along the 1 km study reach by roughly 1 order of magnitude s 3 41 with a corresponding increase in streambed roughness while flow discharge and width remain approximately constant due to the glacial runoff regime streambed roughness was characterized by semivariograms and standard deviations of point clouds derived from terrestrial laser scanning reach averaged flow velocity was derived from dye tracer breakthrough curves measured by 10 fluorometers installed along the channel commonly used flow resistance approaches darcy weisbach equation and dimensionless hydraulic geometry were used to relate the measured bulk velocity to bed characteristics as a roughness measure d84 yielded comparable results to more laborious measures derived from point clouds flow resistance behavior across this large range of steep slopes agreed with patterns established in previous studies for both lower gradient and steep reaches regardless of which roughness measures were used we linked empirical critical shear stress approaches to the variable power equation for flow resistance to investigate the change of bed roughness with channel slope the predicted increase in d84 with increasing channel slope was in good agreement with field observations

austral summer foehn winds over the mcmurdo dry valleys of antarctica from polar wrf foehn winds are a prominent feature of the mcmurdo dry valleys mdvs climate and are responsible for periods of strong winds and warming the foehn mechanism determined from a case study presented in earlier work is shown here to be robust for a set of the mdvs summer foehn events over the 1994 2009 period using output from the polar weather research and forecasting model polar wrf gap flow south of the mdvs is evidenced by the positive relationship between the pressure gradient and near surface wind speed along the gap subsequently mountain waves are generated and result in adiabatic warming and the downward transport of warm air into the mdvs and differences in mountain wave characteristics depend on the ambient wind direction and the degree of flow nonlinearity pressure driven channelling then brings warm foehn air downvalley although a large range of synoptic scale circulation patterns can drive foehn events the warmest foehn events are typically associated with blocking highs over the australian sector of the southern ocean leading to warm air advection over continental antarctica the episodic nature of foehn events and the tenuous connections between such events and interannual modes of climate variability suggests that intraseasonal variability may be more important for determining their frequency and magnitude the extraordinarily warm austral summer of 2001 2002 across antarctica shows that advection of warm maritime air into the continental interior and strong flow aloft result in warm foehn conditions and significant melt for the mdvs

particle friction angles in steep mountain channels sediment transport rates in steep mountain channels are typically an order of magnitude lower than predicted by models developed for lowland rivers one hypothesis for this observation is that particles are more stable in mountain channels due to particle particle interlocking or bridging across the channel width this hypothesis has yet to be tested however because we lack direct measurements of particle friction angles in steep mountain channels here we address this data gap by directly measuring the minimum force required to dislodge sediment pebbles to boulders and the sediment weight in mountain channels using a handheld force gauge at eight sites in california with reach averaged bed angles ranging from 0 5 to 23 and channel widths ranging from 2 m to 16 m we show that friction angles in natural streams average 68 and are 16 larger than those typically measured in laboratory experiments which is likely due to particle interlocking and burial results also show that larger grains are disproportionately more stable than predicted by existing models and that grains organized into steps are twice as stable as grains outside of steps however the mean particle friction angle does not vary systematically with bed slope these results do not support systematic increases in friction angle in steeper and narrower channels to explain the observed low sediment transport rates in mountain channels instead the spatial pattern and grain size dependence of particle friction angles may indirectly lower transport rates in steep narrow channels by stabilizing large clasts and channel spanning steps which act as momentum sinks due to form drag

alluvial floodplain classification by multivariate clustering and discriminant analysis for low relief glacially conditioned river catchments river classifications provide useful frameworks to understand complex fluvial landscapes and to manage freshwater ecosystems alluvial floodplains for rivers in low relief glacially conditioned catchments of southern ontario canada are classified and tested using a sequence of multivariate statistical analyses an original dataset of 109 floodplain sites is investigated using k means clustering principal component analysis and discriminant analysis statistical approaches four primary floodplain types are proposed representing basic morphological stratigraphical and sedimentological characteristics classifications are successfully discriminated by two principal dimensions 1 stream power resistance and 2 floodplain sedimentology the latter is most efficiently represented by the availability of alluvial sand and specifically a new variable defined as floodplain sand equivalent fse floodplain types are generally consistent with previous river classifications however the glacial legacy requires refined classifications which account for inherited cobble bed materials and patterns of sand supply representing the residual variability of stream power resistance correlations a third explanatory dimension of sediment transport is suggested and may explain some within class variability in channel morphology balancing the opposing concepts of fluvial process domains and landform continuums the potential for transitional floodplain types is also explored the proposed first order alluvial floodplain classifications provide a basis from which to further investigate geomorphological diversity within the context of complex glacial legacy effects in low relief settings future research to reveal the spatial arrangement and linkages of distinct morphological groups within a regional landscape mosaic is expected to provide insights into patterns of post glacial fluvial adjustment copyright 2014 john wiley sons ltd

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

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

critical shear stress for incipient motion of sand gravel streambeds results of an experimental study of the incipient motion of streambeds composed of sand gravel sediment mixtures are reported and compared with the earlier findings for uniform sediments the experiments were conducted in an 8 m long by 0 30 m wide glass walled tilting flume and an 18 m long by 0 80 1 10 m wide trapezoidal concrete channel a reference transport method is used to define the beginning of bed material movement the experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size intergranular effects mixture standard deviation effect of the shape of grain size distribution absolute value of median size absolute size effect and bed slope effect of relative depth on overall flow resistance the shear stress at incipient motion of median sized grains in mixtures is found to be the same as for uniform sediment of this size the present findings are consistent with available flume and field data a technique for calculating the critical shear stress of different grain sizes in coarse uniform sediments and unimodal weakly bimodal sediment mixtures is proposed

coarse sediment transport in mountain streams in colorado and wyoming usa since the early 1990s us forest service researchers have made thousands of bedload measurements in steep coarse grained channels in colorado and wyoming usa in this paper we use data from 19 of those sites to characterize patterns and rates of coarse sediment transport for a range of channel types and sizes including step pool plane bed pool riffle and near braided channels this effort builds upon previous work where we applied a piecewise regression model to 1 relate flow to rates of bedload transport and 2 define phases of transport in coarse grained channels earlier the model was tested using bedload data from eight sites on the fraser experimental forest near fraser colorado the analysis showed good application to those data and to data from four supplementary channels to which the procedure was applied the earlier results were however derived from data collected at sites that for the most part have quite similar geology and runoff regimes in this paper we evaluate further the application of piecewise regression to data from channels with a wider range of geomorphic conditions the results corroborate with those from the earlier work in that there is a relatively narrow range of discharges at which a substantial change in the nature of bedload transport occurs the transition from primarily low rates of sand transport phase i to higher rates of sand and coarse gravel transport phase ii occurs on average at about 80 per cent of the bankfull 1 5 year return interval discharge a comparison of grain sizes moved during the two phases showed that coarse gravel is rarely trapped in the samplers during phase i transport moreover the movement and capture of the d16 to d25 grain size of the bed surface seems to correspond with the onset of phase ii transport particularly in systems with largely static channel surfaces however while there were many similarities in observed patterns of bedload transport at the 19 studied sites each had its own bedload signal in that the rate and size of materials transported largely reflected the nature of flow and sediment particular to that system published in 2005 by john wiley sons ltd

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

14c and 10be dated late holocene fluctuations of patagonian glaciers in torres del paine chile 51 s and connections to antarctic climate change abstract southern hemisphere holocene glacier chronologies are important for unraveling past climate change mid to high latitude teleconnections and regional to global climate forcing at present a significant number of glacier chronologies for patagonia are based on 14c dating which may afford only maximum or minimum limiting dates here we combine geomorphology and stratigraphy with radiocarbon 14c and beryllium 10 10be surface exposure age dating at three outlet glaciers zapata tyndall and pingo these glaciers drain the southernmost tip of the southern patagonian ice field at torres del paine national park chile 51 s after an expansion that we date at 3200 yr b p the torres del paine glaciers expanded to their last major late holocene maxima at 600 and 340 yr b p with the final dated readvance after 190 yr b p we use these data together with other patagonian glacier records to define the early mid and final glacial stages of the last millennium these cold events were separated by warm conditions that allowed the nothofagus forest to colonize deglaciated land the presence of near concurrent glacier fluctuations in patagonia and antarctic peninsula indicates widespread cooling punctuated the late holocene including the last millennium across much of the extratropical southern hemisphere we link this cooling to cold oceanographic atmospheric conditions forced by a northern shift or intensification of the southern westerly winds such scenario increased northward advection of cold antarctic circumpolar water along western patagonia and favored decreased upwelling of warm circumpolar deep water together with expanded sea ice around the antarctic peninsula

effects of macro scale bed roughness geometry on flow resistance 1 flow field over macro roughness elements as for example in flow conditions such as of steep mountain streams is investigated by means of a theoretical model and a laboratory study with the main aim of understanding the influence of concentration and planimetric disposition of grain roughness on flow resistance a theoretical model for the average velocity distribution profile over a bed with macro roughness elements is developed herein the model is based on a two layer approach which takes into account the displacement of the main flow due to the drag around the roughness the laboratory study is composed of an extensive set of flume experiments in the case of steep slopes and large relative roughness conditions for a given flow discharge and bed slope various experiments have been carried out with different pattern and spacing of the roughness elements constituted by river pebbles application of the theoretical model to the experimental results enables the determination of how flow resistance is affected by the macro roughness in particular it appears that when the concentration of the roughness is within an optimal range flow resistance is maximum

predicting the type location and magnitude of geomorphic responses to dam removal role of hydrologic and geomorphic constraints abstract using a dam removal on the ashuelot river in southern new hampshire we test how a sudden spatially non uniform increase in river slope alters sediment transport dynamics and riparian sediment connectivity site conditions were characterized by detailed pre and post removal field surveys and high resolution aerial lidar data and locations of erosion and deposition were predicted through one dimensional hydrodynamic modeling the homestead dam was a 200 year old 4 m high 50 m wide crib dam that created a 9 5 km long relatively narrow reservoir following removal an exhumed resistant bed feature of glaciofluvial boulders located 400 m upstream and 2 5 m lower than the crest of the dam imposed a new boundary condition in the drained reservoir acting as a grade control that maintained a backwater effect upstream during the 15 months following removal non uniform erosion in the former reservoir totaled 60 000 m3 equivalent to 9 3 cm when averaged across the reservoir net deposition of 10 700 m3 was measured downstream of the dam indicating most sediment from the reservoir was carried more than 8 km downstream beyond the study area the most pronounced bed erosion occurred where modeled sediment transport increased in the downstream direction and deposition occurred both within and downstream of the former reservoir where modeled sediment transport decreased in the downstream direction we thus demonstrate that spatial gradients in sediment transport can be used to predict locations of erosion and deposition on the stream bed we further observed that bed incision was not a necessary condition for bank erosion in the former reservoir in this characteristically narrow and shallow reservoir lacking abundant dam induced sedimentation the variable resistance of the bed and banks acted as geomorphic constraints overall the response deviated from the common conceptual model of knickpoint erosion and channel widening due to dam removal with thousands of dams likely to be considered for removal or repair in the coming decades this study helps to advance predictions of the geomorphic response to dam removal and contributes to a broader understanding of the variability in both style and timing of fluvial responses to disturbances

the effects of kinetic sorting on sediment mobility on steep slopes in poorly mobile static armour sorting is usually considered the result of hiding exposure effects we called this effect static sorting in opposition to very efficient grain to grain mechanisms produced by a mobile mixture called kinetic sorting we hypothesized that kinetic sorting can be an important contributor to the morphodynamics of mountain streams and attempted to demonstrate this with new flume experiments two long runs were produced with natural poorly sorted sediments and with transport stages of the coarse fraction defined by the ratio between the shear stress and the critical shear stress for transport smaller and higher than 1 respectively both runs produced an efficient transfer downstream of the injected material but with a major difference the first run no kinetic sorting produced permanent armour figuring clusters akin to what has already been observed in similar experiments the second run with kinetic sorting also produced bed armouring but this armour was periodically totally destroyed leading to substantial bed erosion this phenomenon was explained by kinetic sorting the effects of which are to produce an efficient downward migration of fine materials and bed surface armouring the consequence is that fine materials are hidden to the flow during aggradation allowing the slope to attain values much steeper than would have been expected at equilibrium for the mixture however whereas the surface armouring tends to stabilize the bed construction of a layer of fine sediments at the subsurface also contributes to making it very unstable these two contradictory effects explain the complex bed behaviours and the existence of very large bedload and slope fluctuations copyright 2014 john wiley sons ltd

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

foehn warming distributions in nonlinear and linear flow regimes a focus on the antarctic peninsula the structure of lee side warming during foehn events is investigated as a function of cross barrier flow regime linearity two contrasting cases of westerly flow over the antarctic peninsula ap are considered one highly nonlinear the other relatively linear westerly flow impinging on the ap provides one of the best natural laboratories in the world for the study of foehn owing to its maritime setting and the larsen c ice shelf lcis providing an expansive homogeneous and smooth surface on its east side numerical simulations with the met office unified model at 1 5 km grid size and aircraft observations are utilized in case a relatively weak southwesterly cross peninsula flow and an elevated upwind inversion dictate a highly nonlinear foehn event with mountain wave breaking observed the consequent strongly accelerated downslope flow leads to high amplitude warming and ice shelf melt in the immediate lee of the ap however this foehn warming diminishes rapidly downwind due to upward ascent of the foehn flow via a hydraulic jump in case c strong northwesterly winds dictate a relatively linear flow regime there is no hydraulic jump and strong foehn winds are able to flow at low levels across the entire ice shelf mechanically mixing the near surface flow preventing the development of a strong surface inversion and delivering large fluxes of sensible heat to the ice shelf consequently in case c ice melt rates are considerably greater over the lcis as a whole than in case a our results imply that although nonlinear foehn events cause intense warming in the immediate lee of mountains linear foehn events will commonly cause more extensive lee side warming and over an ice surface higher melt rates this has major implications for the ap where recent east coast warming has led to the collapse of two ice shelves immediately north of the lcis

friction angle measurements on a naturally formed gravel streambed implications for critical boundary shear stress we report the first measurements of friction angles for a naturally formed gravel streambed for a given test grain size placed on a bed surface friction angles varied from 10o to over 100o friction angle distributions can be expressed as a function of test grain size median bed grain size and bed sorting parameter friction angles decrease with increasing grain size relative to the median bed grain size and are a systematic function of sorting with lower friction angles associated with poorer sorting the probability distributions of critical shear stress for different grain sizes on a given bed surface as calculated from our friction angle data show a common origin but otherwise diverge with larger grains having narrower and lower ranges of critical shear stresses the potential mobility of a grain as defined by its probability distribution of critical shear stress may be overestimated for larger grains in this analysis because our calculations do not take into account the effects of grain burial and altered near bed flow fields

the impact of coarse grain protrusion on near bed hydrodynamics in steep rivers sediment is often transported over immobile cobbles and boulders previous studies of such conditions have observed that the entrainment rate of the mobile sediment strongly depends on the level of protrusion of the immobile grains here experiments are conducted in a laboratory flume in order to quantify how different levels of protrusion of large aggregates above a fixed fine sediment bed modeled as a patch of hemispheres modify the local hydrodynamics near the fine sediment bed five protrusion levels defined by p k r 0 20 40 60 80 were investigated where k is the protrusion height and r the radius for small protrusion p 20 enhanced shear stress and turbulence intensity on the fine sediment bed is observed as the mixing layer generated at the hemisphere top impacts the fine sediment bed moreover sweep events generated near the top of the hemispheres reach the fine sediment bed for large protrusions p 60 the mixing layer generated near the top of the hemispheres does not reach the fine grains with the consequence that the shear stress drops the remaining turbulence near the fine sediment bed although enhanced by the wakes generated by the hemispherical caps is quasi isotropic the transition between these two distinct near bed flow regimes is found to be around p 40 corresponding to the protrusion levels observed by grams and wilcock 2014 https doi org 10 1002 2013jf002925 above which the erosion of fine sediment ceases

constraining the relative importance of raindrop and flow driven sediment transport mechanisms in postwildfire environments and implications for recovery time scales national geographic society proposal002929 2009 0046 1 national science foundation 02 39749 09 34131 u s geological survey usgs landslide hazards program

the spatial distribution and temporal variability of fohn winds over the larsen c ice shelf antarctica the eastern side of the antarctic peninsula ap mountain range and the adjacent ice shelves are frequently affected by fohn winds originating from upwind of the mountains six automatic weather stations aws and archived model output from 5km resolution antarctic mesoscale prediction system amps forecasts have been combined to identify the occurrence of fohn conditions and their spatial distribution over the larsen c ice shelf lcis from 2009 to 2012 algorithms for semi automatic detection of fohn conditions have been developed for both aws and amps data the frequency of fohn varies by location being most frequent at the foot of the ap and in the north of the ice shelf they are most common in spring when they can prevail for 50 of the time the results of this study have important implications for further research investigating the impact of fohn on surface melting and the surface energy budget of the ice shelf this is of particular interest due to the collapse of larsen a and b ice shelves in 1995 and 2002 respectively and the potential instability issues following a large calving event on larsen c in 2017

variation in the reference shields stress for bed load transport in gravel bed streams and rivers 1 the present study examines variations in the reference shear stress for bed load transport r using coupled measurements of flow and bed load transport in 45 gravel bed streams and rivers the study streams encompass a wide range in bank full discharge 1 2600 m3 s average channel gradient 0 0003 0 05 and median surface grain size 0 027 0 21 m a bed load transport relation was formed for each site by plotting individual values of the dimensionless transport rate w versus the reach average dimensionless shear stress the reference dimensionless shear stress r was then estimated by selecting the value of corresponding to a reference transport rate of w 0 002 the results indicate that the discharge corresponding to r averages 67 of the bank full discharge with the variation independent of reach scale morphologic and sediment properties however values of r increase systematically with average channel gradient ranging from 0 025 0 035 at sites with slopes of 0 001 0 006 to values greater than 0 10 at sites with slopes greater than 0 02 a corresponding relation for the bank full dimensionless shear stress bf formulated with data from 159 sites in north america and england mirrors the relation between r and channel gradient suggesting that the bank full channel geometry of gravel and cobble bedded streams is adjusted to a relatively constant excess shear stress bf r across a wide range of slopes

variable source areas of bedload in a gravel bed stream abstract bedload texture is dependent on the spatially variable availability of mobile surface bed material distributed in patches or in other bedforms we measured bedload transport continuously with an automatic bedload station on the gravel bedded tordera river we also studied bed patchiness from pre flood vertical aerial photographs in a 40 m section upstream of the bedload station the results suggest that patches are the main source of sediment during the beginning of the flow event the median bedload size in low magnitude events depends on the percentage of total area occupied by patches of a given size patch areal extent and texture depend on the magnitude and general character of the previous flow event

coulomb stress transfer and tectonic loading preceding the 2002 denali fault earthquake pre 2002 tectonic loading and coulomb stress transfer are modeled along the rupture zone of the m 7 9 denali fault earthquake dfe and on adjacent segments of the right lateral denali totschunda fault system in central alaska using a three dimensional boundary element program the segments modeled closely follow for about 95 the arc of a circle of radius 375 km centered on an inferred asperity near the northeastern end of the intersection of the patton bay fault with the alaskan megathrust under prince william sound the loading model includes slip of 6 mm yr below 12 km along the fault system consistent with rotation of the wrangell block about the asperity at a rate of about 1 m y as well as slip of the pacific plate at 5 cm yr at depth along the fairweather queen charlotte transform fault system and on the alaska megathrust the model is consistent with most available pre 2002 global positioning system gps displacement rate data coulomb stresses induced on the denali totschunda fault system locked above 12 km by slip at depth and by transfer from the m 9 2 prince william sound earthquake of 1964 dominated the changing coulomb stress distribution along the fault the combination of loading 70 85 and coseismic stress transfer from the great 1964 earthquake 15 30 were the principal post 1900 stress factors building toward strike slip failure of the northern denali and totschunda segments in the m 7 9 earthquake of november 2002 postseismic stresses transferred from the 1964 earthquake may also have been a significant factor the m 7 2 7 4 delta river earthquake of 1912 carver et al 2004 may have delayed or advanced the timing of the dfe depending on the details and location of its rupture the initial subevent of the 2002 dfe earthquake was on the 40 km susitna glacier thrust fault at the western end of the denali fault rupture the coulomb stress transferred from the 1964 earthquake moved the susitna glacier thrust fault uniformly away from thrust failure by about 100 kpa the initiation of the denali fault earthquake was advanced by transfer of 30 50 kpa of positive coulomb stress to the susitna glacier fault anderson and ji 2003 by the nearby m 6 7 nenana mountain foreshock of 23 october 2002 the regional tectonic loading model used here suggests that the semidi alaska peninsula segment of the megathrust that ruptured in 1938 m 8 2 may be reloaded and approaching failure

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

three dimensional elastic dislocation modeling of the postseismic response to the 1964 alaska earthquake 1 gps velocity observations from the kenai peninsula and kodiak island alaska display a pattern of spatial variability suggesting the presence of multiple active processes on the underlying pacific north american plate interface velocities of sites on the eastern kenai peninsula and kodiak island are consistent with elastic strain accumulation at the plate interface whereas velocities of sites on the western kenai peninsula and cook inlet are oriented in the opposite direction and suggest an ongoing postseismic response to the 1964 alaska earthquake we can reproduce the observed velocities using a three dimensional elastic dislocation model to estimate the variation in coupling between the pacific and north american plates two different inversion schemes are examined to check the robustness of the results we find that the gps data can be satisfied by the presence of a locked area near southwest prince william sound a locked area near southwest kodiak island and an area of postseismic reverse slip beneath and north of the western kenai peninsula the shallow plate interface trenchward of the western kenai peninsula does not appear to be locked the locked areas correspond to the prince william sound and kodiak island asperities that ruptured in 1964 and the area of postseismic slip lies downdip of the 1964 rupture area the correspondence between the present shallow coupling distribution and the 1964 slip distribution suggests that the locked regions repeat from one earthquake cycle to another

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

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis