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

a numerical study of the evolving convective boundary layer and orographic circulation around the santa catalina mountains in arizona part i circulation without deep convection abstract the daytime evolution of the thermally forced boundary layer bl circulation over an isolated mountain about 30 km in diameter and 2 km high is examined by means of numerical simulations validated with data collected in the cumulus photogrammetric in situ and doppler observations cupido field campaign two cases are presented one remains cloud free in the simulations and the second produces orographic convection just deep enough to yield a trace of precipitation the weather research and forecasting version 3 simulations at a resolution of 1 km compare well with cupido observations the simulations reveal a solenoidal circulation mostly contained within the convective bl but this circulation and especially its upper level return flow branch are not immediately apparent since they are overwhelmed by bl thermals a warm anomaly forms over the high terrain during the day but it is rather shallow and does not extend over the depth of the convective bl which bulges over the mountain low

mesoscale anticyclonic circulations in the lee of the central rocky mountains composite analyses of terrain forced mesoscale anticyclonic circulations over southern wyoming and northern colorado are constructed these suggest two different types of circulations based on upstream flow direction the air within type 1 circulations originates to the west of the continental divide and contains little moisture type 2 circulations form in more moist northerly flow and are sometimes associated with snowbands both types tend to form during the afternoon and dissipate after sunset although type 2 events may follow frontal passages and occur at night case studies of one event of each type suggest that anticyclonic vorticity generation occurs within the lowest kilometer above ground level when that layer is nearly vertically mixed in both potential temperature and velocity an analogy with vorticity generation in mixed layer models is considered and it is shown that the conditions for generating negative vorticity in those models are satisfied in each observed case the mixed layer mechanism is also favored as it naturally explains the diurnal tendency of the circulations and may therefore explain the observed late day snowfall maximum along the front range of colorado

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

the interaction of katabatic flow and mountain waves part ii case study analysis and conceptual model via numerical analysis of detailed simulations of an early september 1993 case night the authors develop a conceptual model of the interaction of katabatic flow in the nocturnal boundary layer with mountain waves mki a companion paper part i describes the synoptic and mesoscale observations of the case night from the atmospheric studies in complex terrain ascot experiment and idealized numerical simulations that manifest components of the conceptual model of mki presented herein the reader is also referred to part i for detailed scientific background and motivation the interaction of these phenomena is complicated and nonlinear since the amplitude wavelength and vertical structure of the mountain wave system developed by flow over the barrier owes some portion of its morphology to the evolving atmospheric stability in which the drainage flows develop simultaneously katabatic flows are impacted by the topographically induced gravity wave evolution which may include significantly changing wavelength amplitude flow magnitude and wave breaking behavior in addition to effects caused by turbulence including scouring perturbations to the leeside gravity wave structure at altitudes physically distant from the surface based katabatic flow layer can be reflected in the katabatic flow by transmission through the atmospheric column the simulations show that the evolution of atmospheric structure aloft can create local variability in the surface pressure gradient force governing katabatic flow variability is found to occur on two scales on the meso due to evolution of the mountain wave system on the order of one hour and on the microscale due to rapid wave evolution short wavelength and wave breaking induced fluctuations it is proposed that the mki mechanism explains a portion of the variability in observational records of katabatic flow

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

effects of trade wind strength and direction on the leeside circulations and rainfall of the island of hawaii abstract the leeside circulations and weather of the island of hawaii were studied from the fifth generation pennsylvania state university ncar mesoscale model mm5 land surface model simulations for eight strong 7 9 m s 1 and eight weak 5 2 m s 1 trade wind days and for five days with southeasterly trades 7 1 m s 1 during summer 2004 the objective is to investigate the effects of trade wind strength and directions on the leeside circulations and rainfall and the modification of these effects by the land surface thermal forcing for the small wake on the lee side of the kohala mountains 1700 m lower than the trade wind inversion at 2000 m over northern hawaii the hydraulic jump is present with stronger downslope winds and warmer and drier conditions on the lee side and a weaker westerly reversed flow offshore when trades are stronger in contrast the westerly reversed flow along the large wake axis off the central kona leeside coast behind massive mountains with tops 4000 m is 1 1 5 m s

thermally forced convection over a mountainous tropical island abstractobservations from the dominica experiment domex and cloud resolving numerical simulations are used to study a thermally forced convection event over the caribbean island of dominica on 18 april 2011 a clear diurnal cycle of island thermal forcing and cumulus convection was observed with cumuli initiating over the southwestern flank of the ridge and deepening as they drifted eastward apart from errors in cloud fraction and notably precipitation the simulations verified well against the observations provided horizontal grid spacings of 500 m or less were used the simulated flows developed an island scale solenoidal circulation with an organized and intense updraft over the ridge that focused convective initiation sensitivity tests investigated the impacts of topographic forcing subcloud winds and cloud radiative feedbacks on the island scale horizontal inflow and cloud vertical mass flux these experiments confirmed that thermal forcing drove the island convection and that the inflow an

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

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

on thermally forced circulations over heated terrain abstracta combination of analytical and numerical models is used to gain insight into the dynamics of thermally forced circulations over diurnally heated terrain solutions are obtained for two layer flows representing the boundary layer and the overlying free troposphere over an isolated mountainlike heat source a scaling based on the linearized boussinesq system of equations is developed to quantify the strength of thermally forced updrafts and to identify three flow regimes each with distinct dynamics and parameter sensitivities this scaling closely matches corresponding numerical simulations in two of these regimes the first characterized by a weakly stable boundary layer and significant background winds and the second by a strongly stable boundary layer in the third regime characterized by weak winds and weak boundary layer stability this scaling is outperformed by a fundamentally different scaling based on thermodynamic heat engines within this regime the inability of wind ventilation or

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

the interaction of katabatic flow and mountain waves part i observations and idealized simulations abstract the mutual interaction of katabatic flow in the nocturnal boundary layer nbl and topographically forced gravity waves is investigated due to the nonlinear nature of these phenomena analysis focuses on information obtained from the 1993 atmospheric studies in complex terrain field program held at the mountain canyon plains interface near eldorado canyon colorado and idealized simulations perturbations to katabatic flow by mountain waves relative to their more steady form in quiescent conditions are found to be caused by dynamic pressure effects based on a local froude number climatology case study analysis and the simulations the dynamic pressure effect is theorized to occur as gravity wave pressure perturbations are transmitted through the atmospheric column to the surface and through altered horizontal pressure gradient forcing to the surface based katabatic flows it is proposed that these perturbations are a routine feature in the atmospheric record and represent a significant p

evolution of downslope flow under strong opposing trade winds and frequent trade wind rainshowers over the island of hawaii abstract the evolution of downslope flow on the windward side of the island of hawaii during 7 8 august 1990 is investigated this period is characterized by atypical strong 11 m s 1 upstream trade winds and frequent nocturnal rainshowers in the late afternoon the onset of downslope flow first occurs on the upper slope which is frequently dominated by orographic clouds with mean weak surface winds on the windward slopes because of island blocking the downslope flow onset occurs after the slope surface becomes negatively buoyant along the hilo coastal areas that are well exposed to decelerating trade wind flow the offshore flow onset there occurs much later 2 h than the hawaiian rainband project harp mean as the drainage front moves slowly from the windward lowlands toward the coast against the atypical strong incoming trade wind flow the downslope flow onset along the coast occurs after a band of trade wind rainshowers has moved over the coast these rainshowers produce evaporatively coole

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

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

observation and theory of the diurnal continental thermal tide abstract harmonic analysis of summer automated surface observing system asos data over north america shows sun following diurnal temperature and pressure oscillations with amplitudes increasing in the western united states i e 5 8 k and 60 120 hpa respectively due to larger sensible heating in the dryer western terrains the phases of temperature and pressure i e 220 and 110 are constant with longitude after an interfering eastward propagating wave is subtracted tidal amplitudes and phases shift significantly with season a linear boussinesq model with thermal forcing can reproduce these observed oscillations with properly selected parameters the model neglects global effects to focus on a single transect across a single ideal continent a damping parameter ranging from 5 10 5 to 9 10 5 s 1 comparable to the inertia and coriolis parameters is needed to explain the temperature phase lag relative to local solar noon 40 50 c the phase lag between surface pressure minimum and temper

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

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

synoptic versus orographic control on stationary convective banding quasi stationary convective bands can cause large localised rainfall accumulations and are often anchored by topographic features here the predictability of and mechanisms causing one such band are determined using ensembles of the met office unified model at convection permitting resolution 1 5 km grid length the band was stationary over the uk for 3 h and produced rainfall accumulations of up to 34 mm the amount and location of the predicted rainfall was highly variable despite only small differences between the large scale conditions of the ensemble members only three of 21 members of the control ensemble produced a stationary rain band these three had the weakest upstream winds and hence lowest froude number band formation was due to the superposition of two processes lee side convergence resulting from flow around an upstream obstacle and thermally forced convergence resulting from elevated heating over the upstream terrain both mechanisms were enhanced when the froude number was lower by increasing the terrain height thus reducing the froude number the band became more predictable an ensemble approach is required to successfully predict the possible occurrence of such quasi stationary convective events because the rainfall variability is largely modulated by small variations of the large scale flow however high resolution models are required to accurately resolve the small scale interactions of the flow with the topography upon which the band formation depends thus although topography provides some predictability the quasi stationary convective bands anchored by it are likely to remain a forecasting challenge for many years to come

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

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

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

terrain induced turbulence over lantau island 7 june 1994 tropical storm russ case study numerical simulations of terrain induced turbulence associated with airflow over lantau island of hong kong are presented lantau is a relatively small island with three narrow peaks rising to between 700 and 950 m above mean sea level this research was undertaken as part of a project to better understand and predict the nature of turbulence and shear at the new airport site on the island of chek lap kok which is located to the lee of lantau intensive ground and aerial observations were taken from may through june 1994 during the lantau experiment lantex this paper focuses on flow associated with the passage of tropical storm russ on 7 june 1994 during which severe turbulence was observed the nature of the environmental and topographic forcing on 7 june 1994 resulted in the turbulence and shear being dominated by the combination of topographic effects and surface friction high resolution numerical simulations initialized using local sounding data were performed using the clark model the simulation results indicate that gravity wave dynamics played a very minor role in the flow distortion and generation of turbulence as a result of this flow regime relatively high vertical and horizontal resolution was required to simulate the mechanically generated turbulence associated with tropical storm russ results are presented using a vertical resolution of 10 m near the surface and with horizontal resolutions of both 125 and 62 5 m over local nested domains of about 13 24 km on a side the 125 m model resolution simulated highly distorted flow in the lee of lantau with streaks emanating downstream from regions of sharp orographic gradients at this resolution the streaks were nearly steady in time at the higher horizontal resolution of 62 5 m the streaks became unstable resulting in eddies advecting downstream within a distorted streaky mean flow similar to the 125 m resolution simulation the temporally averaged fields changed little with the increase in resolution however there was a three to fourfold increase in the temporal variability of the flow as indicated by the standard deviation of the wind from a 10 min temporal average overall the higher resolution simulations compared quite well with the observations whereas the lower resolution cases did not the high resolution experiments also showed a much broader horizontal and vertical extent for the transient eddies the depth of orographic influence increased from about 200 m to over 600 m with the increase in resolution a physical explanation using simple linear arguments based on the blocking effects of the eddies is presented the nature of the flow separation is analyzed using bernoulli s energy form to display the geometry of the separation bubbles the height of the 80 m2 s22 energy surface shows eddies forming in regions of large orographic gradients and advecting downstream tests using both buoyancy excitation and stochastic backscatter to parameterize the underresolved dynamics at the 125 m resolution are presented as well as one experiment testing the influence of static stability suppressing turbulence development all these tests showed no significant effect implications of these results to the parameterization of mechanically induced turbulence in complex terrain are discussed

the 0 125 degree finite volume general circulation model on the nasa columbia supercomputer preliminary simulations of mesoscale vortices 1 the nasa columbia supercomputer was ranked second on the top500 list in november 2004 such a quantum jump in computing power provides unprecedented opportunities to conduct ultra high resolution simulations with the finite volume general circulation model fvgcm during 2004 the model was run in realtime experimentally at 0 25 degree resolution producing remarkable hurricane forecasts atlas et al 2005 in 2005 the horizontal resolution was further doubled which makes the fvgcm comparable to the first mesoscale resolving general circulation model at the earth simulator center ohfuchi et al 2004 nine 5 day 0 125 degree simulations of three hurricanes in 2004 are presented first for model validation then it is shown how the model can simulate the formation of the catalina eddies and hawaiian lee vortices which are generated by the interaction of the synoptic scale flow with surface forcing and have never been reproduced in a gcm before

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

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

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

the thermal wake of kauai island satellite observations and numerical simulations abstract island thermal effects on the trail cloud band over the central north pacific are investigated for the lee of hawaii using satellite observations and a regional atmospheric model the trail cloud band develops around noon and peaks in cloudiness in the early afternoon the analysis of numerical simulations of the kauai wake suggests that a dynamically induced convergence zone forms in the lee of kauai and oahu maximum elevation at 1 5 and 1 2 km respectively under the trade wind flow the island thermal effect significantly modulates the island wake and creates a diurnal cycle of development and decay in the lee cloud band as solar radiation heats up the island from morning to afternoon warm air moves downstream warm advection from the island in the wake zone increasing the air temperature decreasing the air pressure and enhancing low level wind convergence in favor of the formation of the trail clouds conversely the cold advection during night suppresses cloud formation in the wake t

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

separation of upslope flow over a uniform slope motivated by the importance of understanding mountain weather during periods of thermal convection a laboratory study was conducted to investigate the separation of an upslope anabatic flow on a two dimensional heated mountainous slope in the absence of a background mean flow three flow regimes were identified in the first at slope angles larger than a critical value the separated flow generated a rising plume completely fed by the anterior upslope flow for this case a simple model based on a balance between the opposing vorticities of baroclinicity and shear was proposed to predict the location of the separation point relative to the mountain base the model also predicts the velocity and length scales at separation as well as those of the rising plume after separation in the second regime the volume flow of the separated plume was not fully supplied by the upslope flow requiring entrainment of additional ambient fluid at the base of the plume source the third regime occurred when wherein the plume almost completely engulfed the slope similar to a buoyant plume emanating from a source of finite dimensions thus overshadowing the upslope flow measurements of the separation point conducted during the materhorn field research program were consistent with the results of the laboratory experiments and modelling

flow over small heat islands a numerical sensitivity study abstract a two dimensional nonlinear model with physical parameterizations was applied to simulate the observed diurnal variation on the 5 km wide flat tropical island of nauru in the trade wind zone both the model and atmospheric radiation measurement arm campaign aircraft observations indicate vigorous mixing in the typical sunny daytime conditions leading to a warm plume downstream of the island the model s afternoon wind field displayed rising motion downstream and downwash ahead of the island with gravity wave structure in accordance with linear models of steady flow over a heated island the roughness difference between sea and land added local rising motion above the windward coast and sinking motion above the lee without large scale wind u a weakish sea breeze sb pattern develops during the day in this model over the 5 km wide island peninsula this pure sb circulation intensifies with increasing island width up to 40 km when large scale wind is present the morning leeside sb cell is a

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

the response of bora type flow to sea surface temperature a non linear two dimensional hydrostatic incompressible numerical model with a higher order turbulence closure scheme is used to study the effect of sea surface temperature on the severe downslope wind called bora at the adriatic coast a non linear large amplitude mountain wave is generated and is broken beneath and within its critical layer due to resonant tuning between the initially single layer atmosphere and the terrain the tuning is governed by the froude number a qualitative and sometimes quantitative analogy exists between the wave breaking unsteady stratified flow and the hydraulic jump steady two layer flow it is also known that the strongest adriatic bora appears during the winter season when the sea surface temperature is typically larger than the ground surface temperature firstly a relatively higher lower sea surface temperature means an additional distortion moderation of the mountain wave and consequently a larger smaller area with bora wind maxima for a relatively higher sea surface temperature a propagating hydraulic jump occurs typically bora maxima are about three to four times larger than the related geostrophic wind 8 m s 1 secondly the presence and importance of the inertial oscillation are indicated since the wave breaking is the vital component of the strongest bora cases there is a relatively large elevated area i e the critical layer with substantial flow decelerations and generally low wind speeds the wave breaking area has a rossby number o 1 hence the earth s rotation appears to be an important part of bora evolution the simulations presented consider generalized bora cases which may pertain to other similar orographic flows

geology of the sandaowanzi telluride gold deposit of the northern great xing an range ne china geochronology and tectonic controls abstract the sandaowanzi gold telluride deposit is a new discovery with dominance in gold tellurides in northeastern china the deposit is hosted in quartz veins intruding sub alkaline volcanic rocks there are two types of gold telluride ores in the sandaowanzi ore deposit i e the vein ores and the disseminated ores calaverite krennerite sylvanite petzite empressite hessite and stuetzite are the dominant au ag bearing minerals either in vein ores or in disseminated ores minor amount of other tellurides e g coloradoite altaite and sulfides e g pyrite sphalerite galena chalcopyrite and chalcocite mostly occur in the disseminated ores the gold bearing quartz veins crosscut volcanic host rocks which are dated at 135 3 124 7 ma by la icp ms u pb zircon method the latter unconformably overly a monzogranite dated as 181 6 ma diabase dikes dated at 116 6 ma intrude the gold bearing quartz veins the age data thereby bracket the timing of gold mineralization between 125 3 and 116 6 ma these geochronologic data are consistent with field relations regional comparison reveals that the widespread volcanism in the great xing an range is a product of the subduction of the izanagi plate beneath the eurasian plate gold telluride mineralization was contemporaneous with regional extension and magmatism as the subducting izanagi plate rolled back

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

thermally forced low froude number flow past three dimensional obstacles abstract the present study extends the discussion of the flow of a density stratified fluid past three dimensional obstacles for froude number o 1 to flows past an isolated obstacle with heated cooled surface the study focuses on a response of thermally forced stratified flows representative of mesoscale flows past mountainous islands such as hawaii or taiwan in order to minimize the span of the parameter space uniform ambient wind and stratification axially symmetric bell shaped hills with moderate slopes representative of mesoscale mountains and a simple thermal forcing function that mimics natural effects over mountainous islands are assumed the earth s rotation surface friction viscosity dissipation and moisture are neglected with these simplifications flows can be characterized with two parameters the froude number and a characteristic scale of thermal forcing defined later in this paper the principal question addressed in this study is under what circumstances will a transition occur

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

boundary layer updrafts driven by airflow over heated terrain abstractthis study presents linear and nonlinear scalings for boundary layer ascent forced by airflow over heated terrain and compares them to results from corresponding high resolution numerical simulations close agreement between theory and simulation is found over most of the parameter space considered including variations in background winds boundary layer stability mountain height and diabatic heating rate as expected the linear and nonlinear scalings perform best for linear and nonlinear flows respectively for a convective boundary layer the scalings accurately predict vertical motion for all flows considered including those that extend well into the nonlinear regime thus these scalings may ultimately help to improve the parameterization of subgrid orographic ascent in large scale models the vertical velocity scalings are less accurate for mechanically blocked flows in stable boundary layers for which a simple vertical displacement scaling is superior although the scalings do not tre

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

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

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

numerical simulations of island scale airflow over maui and the maui vortex under summer trade wind conditions abstract the fifth generation pennsylvania state university ncar mesoscale model mm5 coupled with the noah land surface model lsm is employed to simulate island scale airflow and circulations over maui county hawaii under summer trade wind conditions during july august 2005 the model forecasts are validated by surface observations with good agreement in this study it is shown that a previously known closed circulation over the central valley of maui or the maui vortex represents the northern cyclonic vortex of the dual counter rotating vortices in the lee of haleakala which extend up to the base of the trade wind inversion with a westerly reversed flow 2 m s 1 at low levels the northern cyclonic vortex is more pronounced than the southern anticyclonic vortex the asymmetric structure of the dual vortices is related to the shape of haleakala and the flow deflection by the west maui mountains the maui vortex has a relatively narrow east west extent in the lowest levels especially at nigh

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

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 detection and significance of diurnal pressure and potential vorticity anomalies east of the rockies abstract harmonic analysis of pressure temperature and precipitation data from 1000 automated surface observing system asos stations reveals a mix of stationary and east west moving disturbances east of the rockies optimization of the pressure data using a temperature based tide assumption separates a strong sun following continentally enhanced tide from a smaller eastward propagating wave epw the latter signal moves at a similar speed to the previously discovered eastward moving precipitation systems analysis of asos summer precipitation data confirms eastward propagation but east of 90 w it shows nonpropagating diurnal convection at a fixed local time i e 1800 lst analysis of winter days still finds the epw suggesting that it is the cause and not the result of the propagating precipitation a possible mechanism for the epw is developed from the linear bousinesq equations with heating and wind shear solutions show eastward moving diurnal pulses of potential vorticity pv generated by

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

case study of the leeside mesolow and mesocyclone in tamex abstract an observational study was performed on a leeside mesolow case that occurred 8 june 1987 during the taiwan area mesoscale experiment tamex the case developed as prevailing west southwest winds strengthened and interacted with the terrain of taiwan with a froude number fr u nh increasing from below 0 3 to over 0 5 a quasi stationary mesolow formed to the southeast of taiwan with no closed circulation through most of its 13 h life stages until passage of the mei yu front a stationary and localized mesovortex also formed about 90 km southwest of the low center but little adjustment was observed between the mesolow and the vortex results suggest that airflow at lower levels was blocked and moved around the terrain of the southern central mountain range cmr this led to the formation of low level jets lljs both to the northwest and southeast of taiwan the latter branch provided shear vorticity in the background region of the vortex air parcels at higher levels on the other hand ten

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

trade wind rainfall near the windward coast of hawaii a dynamically based mesoscale climatology of rainfall and other data are examined for typical trade wind conditions near hawaii relative distributions of rainfall are deduced from radar reflectivity data for a 4000 km 2 region over the windward island and the upstream ocean the cumulative rainfall data suggest three zones of forcing that are characteristic of 1 the regional ocean 2 upstream island induced divergence and 3 flow reversal near and over the island it is confirmed that most intense rainband amplifications occur over a mesoscale convergence line that separates the easterly trade winds from an island induced westerly flow this flow separation line resembles a classical gravity current when positioned over the windward island and near shore the domain scale cumulative rainfall is statistically associated with the strength of island blocking as defined by a froude number fr when fr is in the ordinary range 0 3 the island maximum is five times greater than the oceanic average when fr is elevated 0 3 the rainfall maximum increases to 10 times the oceanic average overall the windward island maximum is seven times the upstream oceanic average increased rainfall is more strongly correlated with increased wind speed than with a reduction in dry static stability both component variables of fr unlike rainfall amount the position of the island maximum is relatively insensitive to fr except in the hours surrounding sunrise in the upstream divergence zone 20 45 km offshore rainfall is double the oceanic background it is speculated that the cause is blocking related there are very strong diurnal variations in rainfall amount and distribution both over the island and the upstream ocean windward island rainfall is at a maximum in the hours surrounding midnight when breeze and blocking forcings are fully cooperative in the coastal region seemingly unrelated oceanic rainfall exhibits a strong nocturnal maximum the temporal phase of which may be influenced by wind speed a deep minimum in domain scale rainfall occurs near noon

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