computational fluid dynamics as a tool for investigating separated flow in river bends the detailed three dimensional structure of the flow patterns in river bends with separated flow and factors controlling the existence and characteristics of these flow patterns are unclear at present it is shown here firstly how a computational fluid dynamics cfd program may be used to reproduce the qualitative features of the mean flow in a real river bend to allow testing of the model s capabilities it is then shown how the cfd code may be used to construct hypothetical channel bends which allow the experimentation necessary to investigate the controls on the extent of the separated flow

channel planform adjustment and stream power variations on the middle river coe western grampian highlands scotland this paper assesses the spatial variations in types and rates of channel migration over an historical time period on the middle river coe western grampians scotland the study reach represents an exceptional example of an integrated channel system which displays progressive downstream adjustment in both hydraulic geometry and planform characteristics from a mountain torrent environment through a wandering gravel bed river section to a low angle gravel delta within a fluvio lacustrine environment above loch achtriochtan spatial and temporal variations in channel planform adjustment and floodplain reworking are related to variations in bankfull discharge associated stream power and channel competence

prediction of alluvial channel pattern of perennial rivers abstract purely braided meandering and straight channels can be considered as end members of a continuum of alluvial channel patterns several researchers have succeeded in separating channel patterns in fields defined by flow related parameters however the discriminators of the principal channel patterns derived from these diagrams all require some a priori knowledge of the channel geometry in this paper a method is presented which enables prediction of the equilibrium conditions for the occurrence of braided and high sinuosity meandering rivers in unconfined alluvial floodplains the method is based on two almost channel pattern independent boundary conditions median grain size of the river bed material and a potential specific stream power parameter related to bankfull discharge or mean annual flood and valley gradient this can be regarded as a potential maximum of the available flow energy corresponding to the minimum sinuosity condition p 1 based on an analysis of 228 datasets of measurement sites along rivers from many parts of the world an independent discriminating function was found that separates the occurrence of braided rivers and meandering rivers with p 1 5 the function applies to equilibrium conditions of rivers that neither incise nor show rapid aggradation with a bankfull or mean annual flood discharge above 10 m3 s and a median bed material grain size between 0 1 and 100 mm

flow resistance in coarse gravel bed rivers flow resistance relations are deduced from data on coarse gravel bed reaches of new zealand rivers measurements of hydraulic variables were made during reasonably uniform subcritical flows at cross sections regular in confirmation channel bed and bank roughness are not distinguished and surface bedmaterial is characterized by the median size when the channel boundary is rigid the darcy weisbach friction factor is only a statistically significant function of the ratio of hydraulic radius to median size of surface bedmaterial a model of this form explained 60 percent of the friction factor variance inherent in the field data set with active bedload transport conditions and changing bed topography the froude number based on median surface bedmaterial size was statistically significant and 40 percent of friction factor variance was explained by this latter model

factors controlling riffle scale hyporheic exchange flows and their seasonal changes in a gaining stream a three dimensional groundwater flow model 1 subsurface flow within a single riffle of a low gradient gravel bed stream was modeled in three dimensions using modflow a finite difference groundwater flow model model simulations showed that exchange flows can only occur in this low gradient gaining stream because of a zone of alluvial sediment around the stream that has much higher permeability than the surrounding catchment k 10 4 m s 1 compared with k 10 6 to 10 8 m s 1 the key factors controlling exchange flow within the alluvial zone were identified as the hydraulic conductivity of the alluvium the hydraulic gradient between upstream and downstream ends of the riffle and the flux of groundwater entering the alluvium from the sides and beneath in the study riffle each of these factors changes with season causing a reversal of flow paths in the alluvium and a reduction in exchange flows from about 0 2 0 5 m 3 d per meter stream length in summer to about 0 008 0 04 m 3 d 1 per meter stream length during fall to spring the model also revealed that exchange flows are up to twice as strong but more variable at the sides of the stream than near the center and that vertical flow paths beneath the channel are more persistent under the range of conditions modeled than lateral flow paths into the banks

evaluating the reliability of the stream tracer approach to characterize stream subsurface water exchange stream water was locally recharged into shallow groundwater flow paths that returned to the stream hyporheic exchange in st kevin gulch a rocky mountain stream in colorado contaminated by acid mine drainage two approaches were used to characterize hyporheic exchange sub reach scale measurement of hydraulic heads and hydraulic conductivity to compute streambed fluxes hydrometric approach and reachscale modeling of in stream solute tracer injections to determine characteristic length and timescales of exchange with storage zones stream tracer approach subsurface data were the standard of comparison used to evaluate the reliability of the stream tracer approach to characterize hyporheic exchange the reach averaged hyporheic exchange flux 1 5 ml s 1 m 1 determined by hydrometric methods was largest when stream base flow was low 10 l s 1 hyporheic exchange persisted when base flow was 10 fold higher decreasing by approximately 30 reliability of the stream tracer approach to detect hyporheic exchange was assessed using first order uncertainty analysis that considered model parameter sensitivity the stream tracer approach did not reliably characterize hyporheic exchange at high base flow the model was apparently more sensitive to exchange with surface water storage zones than with the hyporheic zone at low base flow the stream tracer approach reliably characterized exchange between the stream and gravel streambed timescale of hours but was relatively insensitive to slower exchange with deeper alluvium timescale of tens of hours that was detected by subsurface measurements the stream tracer approach was therefore not equally sensitive to all timescales of hyporheic exchange we conclude that while the stream tracer approach is an efficient means to characterize surface subsurface exchange future studies will need to more routinely consider decreasing sensitivities of tracer methods at higher base flow and a potential bias toward characterizing only a fast component of hyporheic exchange stream tracer models with multiple rate constants to consider both fast exchange with streambed gravel and slower exchange with deeper alluvium appear to be warranted

a strain energy criterion based on grain dislodgment at borehole wall in poorly cemented sands abstract the breakage of bonding between sand particles at a borehole wall usually precedes the borehole failure and it can be considered as a sign that the onset of the borehole collapse is imminent in granular formations detecting the particle detachment point and introducing an appropriate failure criterion will play a key role in borehole stability analysis to investigate the influence of different factors on the initiation of particle debonding at the borehole wall a series of new laboratory tests was designed and performed on synthetic poorly cemented sand specimens the tests were devised to allow visual observation of the onset of dislodgment of particles from the borehole wall under various stress paths for two different borehole sizes and various cement contents in order to simulate the conducted laboratory tests on twhc a series of numerical modelling has been conducted by discrete element method to estimate the stresses and strains at the borehole wall the total strain energy up to the point of the observed particle debonding was calculated for each stress path and a failure criterion based on the total strain energy was introduced the results showed that the particle detachment point at the borehole wall was reached both before and after the peak strength of the twhc specimens depending on the stress path and cement content also it was concluded that the stress path has a significant effect on the onset of the particle detachment the introduced criterion based on absorbed strain energy will help to design more effective support systems for boreholes

forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction tooth s nanson g c 2004 forms and processes of two highly contrasting rivers in arid central australia and the implications for channel pattern discrimination and prediction geological society of america bulletin 116 7 8 802 816 rae2008

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

determining long time scale hyporheic zone flow paths in antarctic streams in the mcmurdo dry valleys of antarctica glaciers are the source of meltwater during the austral summer and the streams and adjacent hyporheic zones constitute the entire physical watershed there are no hillslope processes in these systems hyporheic zones can extend several metres from each side of the stream and are up to 70 cm deep corresponding to a lateral cross section as large as 12 m 2 and water resides in the subsurface year around in this study we differentiate between the near stream hyporheic zone which can be characterized with stream tracer experiments and the extended hyporheic zone which has a longer time scale of exchange we sampled stream water from green creek and from the adjacent saturated alluvium for stable isotopes of d and 18 o to assess the significance and extent of stream water exchange between the streams and extended hyporheic zones over long time scales days to weeks our results show that water residing in the extended hyporheic zone is much more isotopically enriched up to 11 da nd 2 2 18 o than stream water this result suggests a long residence time within the extended hyporheic zone during which fractionation has occurred owing to summer evaporation and winter sublimation of hyporheic water we found less enriched water in the extended hyporheic zone later in the flow season suggesting that stream water may be exchanged into and out of this zone on the time scale of weeks to months the transient storage model otis was used to characterize the exchange of stream water with the extended hyporheic zone model results yield exchange rates generally an order magnitude lower 10 5 s 1 than those determined using stream tracer techniques on the same stream in light of previous studies in these streams these results suggest that the hyporheic zones in antarctic streams have near stream zones of rapid stream water exchange where fast biogeochemical reactions may influence water chemistry and extended hyporheic zones in which slower biogeochemical reaction rates may affect stream water chemistry at longer time scales copyright 2003 john wiley sons ltd

estimation of discharge from three braided rivers using synthetic aperture radar satellite imagery potential application to ungaged basins analysis of 41 ers 1 synthetic aperture radar images and simultaneous ground measurements of discharge for three large braided rivers indicates that the area of active flow on braided river floodplains is primarily a function of discharge a power law correlation is found between satellite derived effective width we and discharge q where we is the water surface area within a braided reach divided by the reach length synthetic values of we and q generated from a cellular automata model of stream braiding display a similar power law correlation power functions that are fit through plots of we and q represent satellite derived rating curves that can subsequently be used to estimate instantaneous river discharge from space with errors ranging from tens to hundreds of cubic meters per second for ungaged rivers changes in relative discharge can be determined from satellite data alone to determine the shape and timing of annual flows in glacierized basins absolute discharge can probably be estimated within a factor of 2 more accurate estimates will require either 1 one or more ground measurements of discharge acquired simultaneously with a satellite image acquisition or 2 successful parameterization of known morphologic controls such as total sinuosity 5 p valley slope bank material and stability and braid channel hydraulic geometry values of total sinuosity 5 p derived from satellite imagery and field measurements from two rivers of braid channel width depth velocity water surface slope and bed material grain size indicate that while the shape of satellite derived we q rating curves may be influenced by all of these variables the sensitivity of flow area to changing discharge is most dependent upon the degree of braiding efforts to monitor river discharge from space will be most successful for intensely braided rivers with high values of total sinuosity subsampling of existing daily discharge records from the iskut river suggests that satellite return times of about 1 week are sufficient for approximating the shape and timing of the seasonal hydrograph in large glacierized basins although errors are large the presented technique represents the only currently available way to estimate discharge in ungauged braided rivers

morphological controls on the downstream passage of a sediment wave in a gravel bed stream sediment waves in river systems have been widely reported although few studies have examined the interaction between these waves and the morphology of the reaches through which they pass this interaction determines how waves are modified as they propagate downstream this study documents the origin and downstream passage of an avulsion generated sediment wave through a 374 m study reach of the allt dubhaig scotland a nested survey framework was adopted with volumes calculated from cross sections spaced between 10 and 40 m apart documenting the origin and downstream passage of the wave the wave moved through an intensively c 1 m cross section spacing monitored 120 m stretch reach a within the study reach allowing assessment of sediment exchanges between the incoming wave and the local morphology successive surveys show the movement of the wave through and out of the reach and also that areas where wave sediment was deposited did not always correspond with areas of subsequent erosion reach a was divided into three morphologically distinct sub reaches 1a 2a and 3a within which sediment fluxes and the three dimensional distribution of erosion deposition were estimated sediment wave input into 1a and 2a relatively stable sub reaches caused forced bar aggradation and erosion of sediment from elsewhere within the reach which then became part of the wave the downstream transfer of this sediment into unstable 3a caused aggradation and in response widespread erosion which increased the magnitude of the sediment wave as it exited reach a sediment exchange between the recipient reach and the wave depends upon local morphological stability and is a crucial process affecting wave magnitude and attenuation the macroscale sediment wave interacted with rather than overwhelmed the recipient morphology 1998 john wiley sons ltd

channel change bankfull and effective discharges on a vertically accreting meandering gravel bed river along the lower reaches of the waipaoa river new zealand cross section survey data indicate there was a 23 per cent decrease in bankfull width and a 22 per cent reduction in channel cross section area between 1948 and 2000 as the channel responded to increased inputs of fine suspended sediment following deforestation of the headwaters in late c19 and early c20 we determined the bankfull discharge within a 39 km long reach by routing known discharges through the one dimensional mike 11 flow model the model runs suggest that the bankfull discharge varies between 800 and 2300 m3 s 1 and that the average recurrence interval is 4 2 years on the annual maximum series by contrast the effective flow 360 m3 s 1 is equaled or exceeded three times a year the variability in bankfull discharge arises because the banks tend to be lower in places where flood flows are constricted than in reaches where overbank flow is dispersed over a wide area and because scour has counteracted aggradation in some locations there is no downstream variation in shields stress or in relative shear stress within the study reach bankfull shear stress is on average five times greater than the shear stress required to initiate motion at the effective discharge it is more than twice the threshold value the effective discharge probably has more relevance than the bankfull discharge to the overall picture of sediment movement in the lower reaches of the waipaoa river but because width is constrained by the stability and resistance of the bank material to erosion during high flows that also scour the bed the overall channel geometry is likely determined by discharges at or near bankfull copyright 2006 john wiley sons ltd

geostatistical analysis of ground survey elevation data to elucidate spatial and temporal river channel change a digital elevation model dem of a fluvial environment represented landform surface variability well and provided a medium for monitoring morphological change over time elevation was measured above an arbitrary datum using a ground based three dimensional tacheometric survey in two reaches of the river nent uk in july 1998 october 1998 after flood conditions and june 1999 a detailed geostatistical analysis of the elevation data was used to model the spatial variation of elevation and to produce dems in each reach and for each survey period maps of the difference in elevation were produced and volumetric change was calculated for each reach and each survey period the parameters of variogram models were used to describe the morphological character of each reach and to elucidate the linkages between process and the form of channel change operating at different spatial and temporal scales the analysis of channel change on the river nent shows the potential of geostatistics for investigating the magnitude and frequency of geomorphic work in other rivers a flood modified the channel features but low magnitude and high frequency flows rationalized the morphology in spite of relatively small amounts of net flux the channel features changed as a consequence of the reworking of existing material the blocking of chute entrances and redirection of the channel had a considerable effect on the behaviour of the channel such small changes suggested that the distributary system was sensitive to variation in sediment regime plots of the kriging variances against sampling intervals were used to quantify the temporal variation in sampling redundancy ranging between 11 per cent and 93 per cent these curves illustrated the importance of bespoke sampling designs to reduce sampling effort by incorporating anisotropic variation in space and geomorphic information on flow regime variation in the nugget parameter of the variogram models was interpreted as sampling inaccuracy caused by variability in particle size and is believed to be important for future work on surface roughness copyright 2003 john wiley sons ltd

downstream fining in a rapidly aggrading gravel bed river downstream changes in particle size that occur in the waipaoa river a 104 km long gravel bed river in which rapid aggradation in the historic post 1800 period was triggered by the conversion of native forest to pasture are summarized in this paper the textural data presented are unique for a field situation not only because of the spatial resolution and extent of the sampling program but also because they provide information about the pattern of fining at different points in time they are supported by equally comprehensive topographic survey data from which local rates of aggradation can be derived despite variability induced by lateral sediment inputs there is an essentially continuous pattern of fining along the entire length of the river fining occurs in both the fine and coarse size fractions of the bed material the highest rates of fining occur in the larger percentiles of the subsurface bed material and in the surface bed material downstream fining in the waipaoa river appears to be a response to changes in flow hydraulics that are regulated by the concave configuration of the long profile the fining gradient developed rapidly in 45 years it does not appear to be influenced by the rate of aggradation nor the overall rate of sediment supply to the channel system because in the short term aggradation has a negligible impact on the inherited form of the long profile

downstream changes in the channel geometry of a large gravel bed river 1 field data obtained on a nearly contiguous segment of the colorado river in western colorado and eastern utah are used to examine the mechanisms driving downstream changes in channel geometry measurements characterizing the bank full hydraulic geometry bed material grain size and average channel gradient were made at closely spaced intervals in 10 alluvial and quasi alluvial reaches covering 260 km of the river these data indicate that changes in surface and subsurface grain sizes are small in relation to the change in channel slope over the full length of the study area the median grain size of the surface sediment decreases by a factor of a little more than 2 whereas the average channel slope decreases by a factor of about 5 the decreases in slope and median grain size are offset by a large increase in bank full depth relative to width such that the bank full shields stress b is constant downstream for the reach as a whole b averages 0 049 which is roughly 50 higher than the threshold for bed load transport

characteristics of high energy meandering rivers the canterbury plains new zealand observations are described on four natural gravel bedded meandering rivets at two scales that of individual bends and that of relatively long reaches a distinction is made between bends that are overwidened relative to intervening channel traverses and bends that are found in constant width channels imbrication directions on bars in the former show no evidence of bed flow aligned toward the inner bank bar gravel seems to accrete there because of general flow divergence even in constant width channels imbrication patterns in the main part of the bends indicate bed flow toward the outer bank causing intense flow convergence and scour along that zone this scour pattern has implications at the reach level localized scour along the outer bank produces a gravel bar just downstream that then diverts flow against the upvalley bank the result is a premature inflection a reversal of normal meander asymmetry and a frequent tendency for upvalley channel migration avulsion is common eventually arresting such migration and acting to keep over all sinuosity relatively low the similarity between the behavior of these rivers at the reach level and late stage pseudomeandering observed in flumes is noted

trends of grain sizes on gravel bars in the rio chagres panama abstract we examined the trends of grain sizes along the upper 414 km 2 of the mountainous rio chagres drainage basin in panama gravel bars were sampled along 40 km of the rio chagres and five major tributary streams using a transect pebble count of median diameter lithology and clast rounding although previous investigators have found that downstream fining can be obscured by inputs of colluvial sediment and other local controls in mountain drainages we decided to examine the trends of grain sizes along a tropical mountain river where rapid weathering and high capability of transport might be capable of overriding the input effects of colluvium specifically we tested the hypotheses that downstream fining would be present as a result of selective sorting and that weak felsic particles would decrease in size preferentially to strong mafic particles because of abrasion statistical analyses reveal a weak downstream decrease of sediment size on gravel bars along the study reach of the rio chagres with a sternberg diminution coefficient for felsic and mafic grains of 0 013 and 0 017 respectively felsic clasts have thicker weathering rinds and become rounded downstream faster than mafic particles but tumbling mill tests of abrasion show no significant differences in rate of mass loss in relation to lithology and downstream decreases in grain size are similar between lithologies dividing the study reach into six sub reaches bounded by major tributary junctions we further tested the hypothesis that downstream trends in fining might be obscured at the basin scale by sediment input from tributaries but that trends in grain sizes might be more visible at the reach scale between tributaries we did not find any consistent trends in grain size between tributaries stream width appears to assert a local control on grain size coarse particles are associated with narrow channel reaches whereas smaller particles are associated with wide channel reaches

numerical simulation of downstream fining by selective transport in gravel bed rivers model development and illustration a numerical model has been developed for the routing of gravel sized sediment along a river channel which is free to adjust both its long profile and surface texture hydraulic calculations use a step backwater approach and sediment transport is predicted with the method of parker 1990a which uses a low degree of size selectivity exchange of sediment between the surface and subsurface is described using the modified exner equation of parker and sutherland 1990 the model is applied to an idealized channel based on the highly concave allt dubhaig scotland in which fining by particle wear is minor the rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to 102 years under the present flow regime of the river the evolution of the fining pattern during the model run and associated changes in sediment transport and bed aggradation are described it is concluded that strong profile concavity can force rapid downstream fining even though bed load transport is only slightly size selective this run of the model serves as a basis for testing of the sensitivity of downstream fining to alternative choices of parameter values and boundary conditions which are summarized here and will be described in a subsequent paper

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

effect of pressure on stress strain properties of materials the transition from brittleness to ductility in the mechanical behavior of materials with increase in the hydrostatic component of the stress reflects differences in the pressure dependence of the stress needed to initiate particular processes of fracture or flow the simple hypothesis that processes involving ordinary friction in the relative movement of grains or fracture fragments are highly pressure dependent whereas crystallographic slip and twinning are not is examined in the light of recent experimental results on phyllite galena and magnesium oxide there is indication that some slip systems perhaps intrinsically hard ones can be fairly pressure dependent so that the above generalization may not always be valid but more experimental evidence is needed 23 refs

the effect of streambed topography on surface subsurface water exchange in mountain catchments a numerical hydrological simulation suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography at st kevin gulch an effluent stream in the rocky mountains of colorado subsurface hydraulic gradients and movement of ionic tracers indicated that stream water was locally recharged into well defined flow paths through the alluvium stream water filled flow paths in the alluvium referred to as substream flow paths returned to the stream a short distance downstream 1 to 10 m recharge to the substream flow paths occurred where stream water slope increased at the transition from pools 1 to steeper channel units 5 20 return of substream flow paths to the stream occurred where stream water slope decreased at the transition from steeper channel units to pools a net water flux calculation is typically used to characterize water and solute fluxes between surface and subsurface zones of catchments along our study reach at st kevin gulch the net inflow of water from subsurface to stream 1 6 ml s 1 m 1 underestimated the gross inflow 2 7 ml s 1 m 1 by 40 the influence of streambed topography is to enhance hydrological fluxes between stream water and subsurface zones and to prolong water sediment contact times these effects could have important consequences for solute transport retention and transformation in catchments

turbulent characteristics and evolution of sheet flow in an alluvial channel with downward seepage abstract experimental investigation of the flow hydrodynamics and temporal changes in the cross sectional profile of an alluvial channel has been studied in the present work experiments were carried out in a curvilinear cross sectional shaped channel with no seepage and with a downward seepage condition to ascertain seepage effects on channel geometry and turbulent characteristics of flow measures of turbulent characteristics such as time averaged near bed velocities and reynolds stresses were found to increase with the application of downward seepage stream power and the value of shields parameter are increased under the action of downward seepage causing bed particles to move in the form of a sheet layer integral scales of flow suggest that the size of eddies increases with the application of downward seepage which is linked to the evolution of the sheet layer sheet development causes reduction in flow depth and rapid channel widening cross sectional parabolic shape of the threshold channel is transformed into a trapezoidal shape with the presence of the sheet layer with the passage of time 11 h the channel attains another equilibrium geometrical state with the value of shields stress around 0 074 from the no seepage value of 0 0399 an empirical equation for sheet flow rate is derived with the consideration of seepage in the downward direction

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

hyporheic exchange in gravel bed rivers with pool riffle morphology laboratory experiments and three dimensional modeling received 7 june 2005 revised 30 june 2006 accepted 25 august 2006 published 31 january 2007 1 we report the first laboratory simulations of hyporheic exchange in gravel pool riffle channels which are characterized by coarse sediment steep slopes and three dimensional bed forms that strongly influence surface flow these channels are particularly important habitat for salmonids many of which are currently at risk worldwide and which incubate their offspring within the hyporheic zone here we perform a set of laboratory experiments examining the effects of discharge and bed form amplitude on hyporheic exchange with surface subsurface mixing measured directly from the concentration decay of a conservative tracer fluorescein injected into the surface flow near bed pressure measurements were also used to predict hyporheic exchange from a three dimensional pumping transport model comparison of the predicted and observed hyporheic exchange shows good agreement indicating that the major mechanism for exchange is bed form induced advection however the effect of bed forms is modulated by discharge and the degree of topographic submergence we also tested the performance of the hydrostatic pressure as a proxy for the observed near bed pressure in driving hyporheic exchange which would facilitate field measurement and analysis of hyporheic flow in natural rivers we found agreement with measured hyporheic exchange only for low bed form amplitudes and high flows

transient storage as a function of geomorphology discharge and permafrost active layer conditions in arctic tundra streams 1 transient storage of solutes in hyporheic zones or other slow moving stream waters plays an important role in the biogeochemical processes of streams while numerous studies have reported a wide range of parameter values from simulations of transient storage little field work has been done to investigate the correlations between these parameters and shifts in surface and subsurface flow conditions in this investigation we use the stream properties of the arctic namely highly varied discharges channel morphologies and subchannel permafrost conditions to isolate the effects of discharge channel morphology and potential size of the hyporheic zone on transient storage we repeated stream tracer experiments in five morphologically diverse tundra streams in arctic alaska during the thaw season may august of 2004 to assess transient storage and hydrologic characteristics we compared transient storage model parameters to discharge q the darcy weisbach friction factor f and unit stream power across all studied streams permafrost active layer depths i e the potential extent of the hyporheic zone increased throughout the thaw season and discharges and velocities varied dramatically with minimum ranges of eight fold and four fold respectively in all reaches the mean storage residence time tstor decreased exponentially with increasing q but did not clearly relate to permafrost active layer depths furthermore we found that modeled transient storage metrics i e tstor storage zone exchange rate otis and hydraulic retention rh correlated better with channel hydraulic descriptors such as f and than they did with q or channel slope our results indicate that q is the first order control on transient storage dynamics of these streams and that f and are two relatively simple measures of channel hydraulics that may be important metrics for predicting the response of transient storage to perturbations in discharge and morphology in a given stream

downstream patterns of bed material grain size in a large lowland alluvial river subject to low sediment supply 1 a new data set of bimodal subaqueous channel bed sediments was analyzed for longitudinal patterns in grain size it yielded two interesting observations 1 separate fining trends in d50 exist for gravel and fines that overlap for 175 river kilometers and 2 this overlap in fining trends results in a protracted nonabrupt gravel to sand transition these suggest bed patchiness that is interpreted in the context of inherent grain size bimodality localized hydraulic sorting due to spatially heterogeneous and low sediment supply and local deviations in channel gradient the data also show that tributaries have a minor impact on main stem downstream fining due to basin shape and tectonic history in addition subaqueous channel sediments were observed to be finer than nearby sediments collected from exposed bars which may indicate a narrow active zone of transport where bars are disconnected from sediment movement this research has implications for bed material collection and theories of downstream fining

testing links between river patterns and in channel characteristics using mrpp and anova this study tests the assumption that the characteristics of channels within multiple channel rivers are different from those of single channel rivers some river restoration approaches propose radical transformation of river patterns from multiple to single channels based on the link between river patterns and their in channel characteristics determining the links between river patterns and their in channel characteristics is complicated by differences in geology history climate and discharge among rivers furthermore multiple channel rivers are composed of a mosaic of channel types with a range of in channel characteristics this study minimizes these problems by analysing a single river containing neighbouring single and multiple channel patterns with little change in discharge downstream and by analysing all channel types the study addressed two objectives to determine the hydraulic geometry energy and sediment mobility characteristics of neighbouring single and multiple channel river patterns and to test for statistical differences in these characteristics between patterns the renous river shows a wandering pattern for 11 5 km with multiple channels around semipermanent islands and abandoned channels in the flood plain the river displays a single channel river pattern where channels are confined by their valley walls upstream and downstream of wandering the analysis was conducted at three scales first the confined single channel and wandering multiple channel patterns were compared pattern scale second the confined channel pattern was compared to single and multiple channel sections within the wandering pattern section scale third all channel types were compared channel type scale multi response permutation procedure mrpp and analysis of variance anova were used to analyze differences between channels difference tests found no simple discrimination between the single and multiple channel river patterns of the renous river tests between the single confined and multiple wandering channel patterns found few differences in the in channel variables the tests did find differences between multiple channel sections within the wandering pattern and confined single channels however a greater number of differences were found between multiple channel and single channel sections within the wandering pattern highlighting the variability within the wandering pattern two groups emerged when all channel types were tested for differences perennial main channels containing the thalweg and ephemeral side channels therefore side channels define the in channel characteristics of wandering rivers because few differences were found among main channels in either pattern this analysis suggests that all channel types not just main channels should be investigated to obtain a complete picture of a river pattern prior to any restoration efforts engineers must exercise caution when applying the link between river patterns and in channel characteristics to river restoration efforts

numerical modelling of separated flow in river bends model testing and experimental investigation of geometric controls on the extent of flow separation at the concave bank the occurrence of separated flow in sharply curving river bends and the associated deposition of fine grained sediment is widely recognized but the factors controlling the existence and extent of separated flow have not been clearly identified a three dimensional fixed lid computational fluid dynamics program was used to simulate time averaged flow in idealized channel bends in order to investigate geometric controls on flow separation the code was applied first to a field site it successfully modelled the qualitative features of the flow including a separation zone and was insensitive to choice of turbulence closure or exclusion of lateral variation in water surface topography the simulations of idealized bends demonstrate that the existence and extent of concave bank flow separation can be significantly influenced by changes in bend planform point bar topography and upstream planform 1998 john wiley sons ltd

the meandering braided river threshold a reappraisal abstract the concept of a threshold discharge slope q s combination that separates braided from meandering streams is critically re examined partly by review of previous data and partly by analysis of new data from south island new zealand it is argue d that discriminant functions that use mean annual discharge provide a poor physical basis for interpretations based on natural processes while bankfull discharge is also deficient because it is not a fully independent variable the common use of channel rather than valley slope adds a further misleading component because channel slope is partly dependent on channel pattern sinuosity most previous studies have inadequately treated the role of bed particle size in pattern discrimination failing to recognize that active gravel streams must plot higher on a q s chart than sand bed channels irrespective of pattern because of the greater power requirements for bed material movement within any one size class of bed material there is no evidence to indicate a clear discrimination between braiding and meandering only a weak statistical association between pattern and slope discharge values while this association may be explicable in terms of the high shear stress and stream power that accompany high qs values promoting braiding the real prerequisite for braiding appears to be high loads of bed calibre material producing wide shallow channels a factor that is only partly controlled by stream power accordingly the search for a pattern threshold based on discharge and slope seems to be a futile exercise an interpretation of existing pattern discriminant functions from the perspective of the threshold shear stress for particle movement is made it is concluded that in effect these functions merely state that gravel bed streams are more likely to be braided than channels in finer sediment statistically this appears to be valid and reasons for that difference are offered based on the balance between imposed bed material load and channel capacity

impact of heterogeneity bed forms and stream curvature on subchannel hyporheic exchange 1 advection through hyporheic zones hz consisting of heterogeneous channel bend streambed deposits and their equivalent homogenous medium was investigated using finite difference groundwater flow and transport simulations and forward particle tracking the top prescribed head boundary was varied in order to mimic various stream channel head distributions resulting from the presence of bed forms and channel curvature flux calculations show that heterogeneity causes significant additional hz flux compared to an equivalent homogenous medium however the major cause of hz flux is a spatially periodic sinusoidal head distribution along the boundary representing the effect of bed forms the additional influence of heterogeneity on the total channel bed exchange and the overall hz geometry are increased when boundary head sinusoidal fluctuation is more subdued we present dimensionless numbers that summarize these relationships heterogeneity s influence is further magnified by considering the effect of channel curvature on boundary heads the simulations illustrate the dynamic influence of heterogeneity on the hyporheic zone since the various head boundaries employed in our modeling efforts are a proxy for different surface water conditions and bed form states that may occur during a single flood furthermore we show that residence times total tracking times of particles originating from the streambed follow a lognormal distribution in the presence of heterogeneity residence times can decrease or they can increase compared to residence times for homogeneous conditions depending on the relative positions of the heterogeneities and the bed forms hence streambed heterogeneity and stream curvature factors often neglected in previous modeling efforts combine with bed form configuration to dynamically determine hz geometry fluxes and residence time distributions

morphologically based model of bed load transport capacity in a headwater stream 1 field measurements of channel and bed material properties are used to develop a network based model of bed load transport capacity in halfmoon creek a high gradient stream in the rocky mountains of colorado the formerly glaciated watershed contains self formed reaches that are generally disconnected from hillslopes and major sediment perturbations strung together by a sequence of bedrock and cascade channels measurements of channel geometry slope and bed sediment texture surface and substrate were taken in 27 reaches with gradients ranging from 0 6 to 6 to examine interactions among key variables affecting bed load transport the analysis shows that the grain size distribution of the substrate is relatively uniform throughout the basin however the distinction between surface and substrate texture varies systematically with channel gradient and shear stress reaches with high shear stress are characterized by coarse surface layers with a high threshold for transport limiting the mobility of the sediment supplied in reaches with lower gradient and shear stress the difference between surface and substrate texture is less increasing both the frequency of transport and the mobility of the bed load while it might appear that the steeper reaches of halfmoon creek have a high capacity for carrying fine medium gravel the relative mobility of these sizes is affected by the presence of very coarse sediment on the bed surface as a result modeled estimates of bed load transport rate indicate that average annual bed load sediment yields increase downstream nearly linearly with discharge suggesting a simple scaling of bed load transport capacity with discharge and drainage area

quantitative determination of the activity of within reach sediment storage in a small gravel bed river using transit time and response time abstract reach scale sediment storage is rarely quantified in sediment budget studies yet it has a considerable effect on the sediment delivery ratio at the basin scale and on the accuracy of morphological methods of bedload estimation at the reach scale deployment of magnetic tracer particles allows accurate characterisation of sediment fluxes in gravel bed rivers and provides the opportunity to quantify storage activity using reservoir theory activity was quantified at reach and sub reach scales in two reaches of a small gravel bed river and the possibility of quantifying the activity of smaller scale sediment stores is explored reach scale transit time functions were derived from the cumulative output of sediment against age or time the shape of the functions in both reaches was determined by flood incidence and magnitude hydraulics sediment tracer availability grain size and local morphology accurate transit time functions were difficult to determine due to reliance upon tracer output and the associated problems of tracer exhaustion over time and imperfect tracer recovery rates in addition if tracers of different ages are allowed to mix for example where upstream input is possible then the resultant transit time distributions age are not comparable with the timing of the hydraulic processes responsible unless the hydraulic conditions are constant these results suggest that transit time is difficult to determine from tracer studies in gravel bed rivers consequently a refinement of transit time the response time is introduced and is defined as the time after the initial input of tracer sediment when cumulative tracer output exceeds the amount of tracer sediment remaining in storage whilst still based upon tracer output the calculation of response time also utilises storage data and is expressed relative to time since the start of the study rather than age it is therefore directly comparable with the incidence of hydraulic conditions this provides a more informative measure of activity which is readily available from tracer data in gravel bed rivers and allows evaluation of the importance of flood incidence and magnitude grain size morphology and reach characteristics stability like transit time response time estimates are hindered by mixing of sediment of different ages at the sub reach scale however it does account for errors from less than 100 recovery and tracer exhaustion more effectively than transit time quantification of absolute and relative size effects in transport is also possible from response time

the timing and magnitude of coarse sediment transport events within an upland temperate gravel bed river this paper describes the application of a new instrument to continuously measure bedload transport an impact sensor to a 72 km2 test catchment in the yorkshire dales northern england data from a network of impact sensors are linked to repeat surveys of channel morphological response to get a better understanding of the conditions that lead to sediment generation and transfer results suggest certain areas of the catchment act as key sediment sources at the annual time scale with material being quickly delivered to the lower parts of the catchment along the steep bedrock channel sediment transfer within the tributaries occurs in significantly smaller magnitudes than within the main channel but it moves more frequently and at different times of the year with transfer rates being strongly conditioned by larger scale valley geomorphology the lower 5 6 km reach sees a significant reduction in gradient and a widening of the valley this permits significant accumulation within the channel which has persisted for many years this lower reach is very sensitive to changes in sediment supply and there is good agreement between changes in bedload transport data and the surveyed channel response these observations have major implications for how river management projects should be developed in upland environments especially those where large scale geomorphological controls have a major impact upon the sediment transfer process evidence suggests that where river management restricts lateral movement of the channel and transfer of sediment into floodplain storage changes in sediment supply can lead to areas of severe accumulation acceleration of bank erosion and exacerbated flood risk

predicting channel patterns brewer paul lewin j 2001 predicting channel patterns geomorphology 40 pp 329 339 rae2008

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

influence of bank materials bed sediment and riparian vegetation on channel form along a gravel to sand transition reach of the upper tualatin river oregon usa we examined the role of bed sediment size bank materials riparian vegetation and discharge on channel form along a 10 km reach of the upper tualatin river encompassing both gravel and sand bed reaches statistical correlations and analyses of downstream changes of channel morphology reveal that bank materials and bed sediment are the dominant controls on channel cross section form a rapid downstream reduction in bed sediment size in the gravel bed channel is followed by an abrupt transition to a narrower deeper and less sinuous sand bed channel with more cohesive bank materials the simultaneous reductions in channel slope bed sediment size and width to depth ratio indicate the channel crosses a geomorphic threshold to maintain continuity in sediment transport this gravel to sand transition and associated change in channel form are induced by a break in valley slope and an increase in bank resistance from cohesive bank materials bank materials measured as the average percent silt and clay in banks are a primary influence on channel form within both subreaches but demonstrate a greater influence on channel width and vertical stability in the gravel bed channel and on channel depth and lateral stability in the sand bed channel riparian vegetation at current densities and compositions is not a significant control on contemporary channel cross section form and may be responding to the bank and channel stability provided by cohesive bank materials in the laterally dynamic gravel bed channel

channel adjustment and a test of rational regime theory in a proglacial braided stream the upstream reach of the sunwapta river alberta provides a useful quasi experimental field case of channel adjustment in a proglacial stream historically the formation of a proglacial lake deprived the river of its coarse sediment supply for several decades and lead to a dramatic decrease in braiding intensity close to the lake while braiding intensity increased further downstream this response to the reduction of gravel input is consistent with previous experimental results subsequent construction activity and channelization close to the lake have contributed to the continuation of these temporal and spatial trends in channel pattern the current state of adjustment of the river morphology can be explained in the context of these historical changes using rational regime equations the study reach has no tributaries and bed material size decreases twofold along the reach while width and braiding intensity increase yet channel slope decreases by only 10 the absence of any significant change in discharge downstream along the reach allows testing of regime equations under conditions in which discharge is held constant the current downstream trends in slope and fluctuations in width are predicted reliably from rational regime equations but not by the existing empirical hydraulic geometry relations the rational equations incorporate the effect of grain size and slope on channel width and the effect of width and grain size on channel slope the regime equations are successful even though they were devised for single channel gravel streams the small 10 decrease in slope along the reach despite a halving of median grain size is attributed to the counteracting positive effect on slope of the downstream increase in braiding intensity and width the downstream increase in braiding intensity must be largely the result of decreasing grain size this confirms the influence of grain size on channel pattern thresholds and demonstrates using spatial transitions in channel pattern that channel pattern predictions based on stream power alone are inadequate

the mechanical behaviour of synthetic poorly consolidated granular rock under uniaxial compression abstract in order to isolate the effect of grain size and cementation on the mechanical behaviour of poorly consolidated granular rock we prepared synthetic rock samples in which these two parameters were varied independently various proportions of sand portland cement and water were mixed and cast in a mold the mixture was left pressure free during curing thus ensuring that the final material was poorly consolidated we used two natural well sorted sands with grain sizes of 0 22 and 0 8 mm the samples were mechanically tested in a uniaxial press static young s modulus was measured during the tests by performing small stress excursions at discrete intervals along the stress strain curves all the samples exhibited nonlinear elasticity i e young s modulus increased with stress as expected we found that the uniaxial compressive strength increased with increasing cement content furthermore we observed a transition from grain size sensitivity of strength at cement content less than 20 30 to grain size independence above this value the measured values of young s modulus are well explained by models based on rigid inclusions embedded in a soft matrix at high cement content and on cemented grain to grain contacts at low cement content both models predict grain size independence in well sorted cemented sands the observed grain size sensitivity at low cement content is probably due to microstructural differences between fine and coarse grained materials caused by small differences in grain sorting quality

the relationship between shear stress and the shaping of the bed of a pebble loaded river la rulles ardenne summary geomorphological modifications in the bed of a small pebble loaded river were related to different parameters such as flow velocity and shear stress experiments carried out with marked pebbles of known diameter demonstrated that shear stress calculated from the energy grade line slope and the hydraulic radius is a reliable criterion in explaining erosion and transport of the pebble load increase in shear stress with discharge entrainment varies according to site at times of low flow shear stress is weak in the pools allowing transitory accumulation of fine deposits and pebbles these originate mainly from erosion downstream of riffles where shear stress is already considerable at times of discharge near bankfull stage shear stress becomes considerable in the pools so that pebbles deposited at lower discharge rates are then displaced however the coarser elements are halted on the counter slope of the pools and upstream of riffles since shear stress there is then weaker than in the pools riffles thus appears as stopping places with only small elements 1 5 cm diameter able to cross them at times of high discharge the size of these elements thus provides an estimate of the true competence of the river

downstream variation in bed sediment size along the east carpathian rivers evidence of the role of sediment sources taking as an example six main rivers that drain the western flank of the eastern carpathians a conceptual model has been developed according to which fluvial bed sediment bimodality can be explained by the overlapping of two grain size distribution curves of different origins thus for carpathian tributaries of the siret coarse gravel joins an unimodal distribution presenting a right skewness with enhanced downstream fining the source of the coarse material distributions is autohtonous by abrasion and hydraulic sorting mechanisms a second distribution with a sandy mode is in general skewed to the left the source of the second distribution is allohtonous the quantity of sand that reaches the river bed through the erosion of the hillslope basin terrains the intersection of the two distributions occurs in the area of the 0 5 8 mm fractions where in fact the right skewness for gravel and left skewness for sand histogram tails meet this also explains the lack of particles in the 0 5 8 mm interval for rivers where fine sediment sources are low the 0 5 8 mm fractions have a higher proportion than the fractions under 1 mm for the siret river itself bed sediment bimodality is greatly enhanced due to the fact that the second mode is more than 25 of the full sample as opposed to its tributaries the source of the first mode of gravel is allohtonous to the siret river generated by the massive input of coarse sediment through the carpathian tributaries while the second mode of the sands is local in this case we can also observe that the two distributions of particles of different origins overlap in the 0 5 8 mm fraction domain creating the illusion of particle lack in the fluvial bed sediments copyright 2007 john wiley sons ltd

downstream channel adjustment in a low relief glacially conditioned watershed abstract river management practices are often informed by theoretical expectations of downstream channel adjustment which may not be valid in low relief glacially conditioned watersheds such as those in the lower north american great lakes region downstream trends in channel morphology and bed material size within a low relief glacially conditioned watershed are explored here and compared to a theoretical watershed model where slope and grain size are expected to decline exponentially the observed channel morphology is then tested against a theoretical concept of reach scale channel grade the downstream hydraulic geometry relations w bf q bf 0 51 s 0 02 and d bf q bf 0 32 s 0 21 were found to best describe downstream changes in channel morphology and are consistent with some prior studies bed material size varies irregularly down the channel slope controlled downstream fining trends are evident where inputs from glacial materials and tributaries are negligible but are masked by cobble boulder lag deposits where the channel is cut into these glacial deposits the asynchronous variability in slope and grain size produces downstream variations in graded and nongraded understeepened conditions separated at ex 0 graded reaches exist where ex 0 but an upper boundary with nongraded oversteepened reaches is less clear the results emphasize the geomorphic legacy of inherited slopes and sediment sources in dictating the modern downstream patterns of fluvial characteristics and morphologies in glacially conditioned and similarly complex watersheds

measurements in a braided river chute and lobe 2 sorting of bed load during entrainment transport and deposition grain size distributions of bed material bed load and bar head deposition were sampled during the evolution of a chute and lobe in the braided gravel bed sunwapta river alberta although bed shear stress and total bed load transport rate varied substantially within the reach and from day to day ferguson et al this issue the median diameter d50 of bed load was remarkably constant the bed initially fined from chute to bar head but became more uniform sediment deposited on the bar head also had a near constant d50 similar to that of the bed load truncated at the same lower size limit both load and deposition were finer on average than the bed suggesting overall selective transport despite little sign of local sorting in contrast to median diameters maximum sizes dmax of sediment in motion and deposited on the bar head varied substantially and showed a weak but significant dependence on shear stress inferring selective entrainment from this evidence alone would be dubious because dmax was also found to increase systematically with the mass of sediment sampled however two different methods of analysis of fractional transport rates throughout the size range also showed size selective entrainment and transport in the marginal transport conditions of this study size sorting appears to occur but only weakly and mainly in the coarser fractions

fluvial responses to land use changes and climatic variations within the drury creek watershed southern illinois abstract fluvial responses to climatic variation and anglo american settlement were documented for the drury creek watershed southern illinois by examining stratigraphic geomorphic climatic and historical data regional analyses of long term precipitation records document a period of decreasing mean annual precipitation from 1904 to about 1945 and an increasing trend in annual precipitation from 1952 to the present the period between 1945 and 1951 experienced a large number of intense storms that resulted in high annual precipitation totals statistical relationships illustrate that changes in precipitation totals are transferred to the hydrologic system as fluctuations in stream discharge historical records of southern illinois show that a maximum period of settlement and deforestation occurred between the 1860s and 1920s this era ended in the 1940s when large tracts of land were revegetated in an attempt to curtail erosion which had caused extensive upland degradation in response to hillslope erosion at least two meters of fine grained sediments were deposited on valley floors average sedimentation rates determined using decdrochronologic techniques are estimated to be 2 11 cm yr for the period between 1890 and 1988 rates that are 1 to 2 orders of magnitude greater than pre settlement values calculated for other areas of the midwest however botanical data suggest that aggradation was episodic possibly occurring during three periods characterized by greater annual precipitation since the 1940s sedimentation rates have declined reduced rates of sedimentation are related to an episode of channel entrenchment that reduced overbank flooding entrenchment coincided with a period of 1 reduced sediment yields associated with watershed revegetation and the introduction of soil conservation practices and 2 intense storm activity that resulted in long periods of high discharge as a result of channel incision and hillslope erosion newly exposed bedrock in upstream areas currently provides a source of gravel load to the channels the distribution of coarse bedload material along tributary streams combined with downstream decreases in width depth ratios and tractive force estimates suggest that channels in the drury creek watershed are slowly adjusting their configuration to transport coarse bedload material the fluvial response to the increased influx of coarse sediment began more than 45 years ago and continues today

network scale dynamics of grain size sorting implications for downstream fining stream profile concavity and drainage basin morphology we explore the link between channel bed texture and river basin concavity in equilibrium catchments using a numerical landscape evolution model theory from homogeneous sediment transport predicts that river basin concavity directly increases with bed sediment size if the effective grain size on a river bed governs its concavity then natural phenomena such as grain size sorting and channel armouring should be linked to concavity we examine this hypothesis by allowing the bed sediment texture to evolve in a transport limited regime using a two grain size mixture of sand and gravel downstream fining through selective particle erosion is produced in equilibrium as the channel bed texture adjusts downstream so does the local slope our model predicts that it is not the texture of the original sediment mixture that governs basin concavity rather concavity is linked to the texture of the sorted surface layer two different textural regimes are produced in the experiments a transitional regime where the mobility of sand and gravel changes with channel bed texture and a sanddominated region where the mobility of sand and gravel is constant the concavity of these regions varies depending on the median gravel or sand grain size erosion rate and precipitation rate the results highlight the importance of adjustments in both surface texture and slope in natural rivers in response to changes in fluvial and sediment inputs throughout a drainage network this adjustment can only be captured numerically using multiple grain sizes or empirical downstream fining rules copyright 2004 john wiley sons ltd

seasonal variation in surface subsurface water exchange and lateral hyporheic area of two stream aquifer systems abstract we used two dimensional unconfined transient groundwater flow models to investigate the interface between stream and groundwater flow systems or hyporheic zone of two first order streams that drain catchments with distinctly different alluvial sediments and bedrock lithology particle tracking showed that lateral hyporheic area planimetric area of flow paths lateral to the stream that are recharged by and return to the stream with travel times of 10 days or less differed between the two study streams and varied with discharge within each system at the rio calaveras welded tuff lateral hyporheic area ranged from 1 7 to 4 m 2 over the annual cycle in the aspen creek system sandstone lateral hyporheic area 1 1 5 m 2 was restricted to roughly half of that observed at rio calaveras the size of the hyporheic zone lateral to the streams at both sites decreased by approximately 50 during high flows sensitivity analyses indicated that changes in the hydraulic conductivity of alluvial and streambed sediments and variation in recharge rates have greatest impact on the magnitude direction and spatial distribution of stream groundwater exchange

a modelling study of hyporheic exchange pattern and the sequence size and spacing of stream bedforms in mountain stream networks oregon usa studies of hyporheic exchange flows have identified physical features of channels that control exchange flow at the channel unit scale namely slope breaks in the longitudinal profile of streams that generate subsurface head distributions we recently completed a field study that suggested channel unit spacing in stream longitudinal profiles can be used to predict the spacing between zones of upwelling flux of hyporheic water into the stream and downwelling flux of stream water into the hyporheic zone in the beds of mountain streams here we use two dimensional groundwater flow and particle tracking models to simulate vertical and longitudinal hyporheic exchange along the longitudinal axis of stream flow in second third and fourth order mountain stream reaches modelling allowed us to 1 represent visually the effect that the shape of the longitudinal profile has on the flow net beneath streambeds 2 isolate channel unit sequence and spacing as individual factors controlling the depth that stream water penetrates the hyporheic zone and the length of upwelling and downwelling zones 3 evaluate the degree to which the effects of regular patterns in bedform size and sequence are masked by irregularities in real streams we simulated hyporheic exchange in two sets of idealized stream reaches and one set of observed stream reaches idealized profiles were constructed using regression equations relating channel form to basin area the size and length of channel units step size pool length etc increased with increasing stream order simulations of hyporheic exchange flows in these reaches suggested that upwelling lengths increased from 2 7mt o 7 6 m and downwelling lengths increased from 2 9mt o 6 0 m with increase in stream order from second to fourth order step spacing in the idealized reaches increased from 5 3mt o 13 7 m as stream size increased from second to fourth order simulated downwelling lengths increased from 4 3m in second order streams to 9 7 m in fourth order streams with a pool riffle step channel unit sequence and increased from 2 5m to 6 1 m from second to fourth order streams with a pool step riffle channel unit sequence upwelling lengths also increased with stream order in these idealized channels our results suggest that channel unit spacing size and sequence are all important in determining hyporheic exchange patterns of upwelling and downwelling though irregularities in the size and spacing of bedforms caused flow nets to be much more complex in surveyed stream reaches than in idealized stream reaches similar trends emerged relating the average geomorphic wavelength to the average hyporheic wavelength in both surveyed and idealized reaches this article replaces a previously published version hydrological processes 19 17 2915 2929 2005 doi 10 1002 hyp 5790 copyright 2006 john wiley sons ltd

a worldwide correlation for exponential bed particle size variation in subaerial aqueous flows the particle size of the bed sediments in or on many natural streams alluvial fans laboratory flumes irrigation canals and mine waste deltas varies exponentially with distance along the stream a plot of the available worldwide exponential bed particle size diminution coefficient data against stream length is presented which shows that all the data lie within a single narrow band extending over virtually the whole range of stream lengths and bed sediment particle sizes found on earth this correlation applies to both natural and artificial flows with both sand and gravel beds irrespective of either the solids concentration or whether normal or reverse sorting occurs this strongly suggests that there are common mechanisms underlying the exponential diminution of bed particles in subaerial aqueous flows of all kinds thus existing models of sorting and abrasion applicable to some such flows may be applicable to others a comparison of exponential laboratory abrasion and field diminution coefficients suggests that abrasion is unlikely to be significant in gravel and sand bed streams shorter than about 10 km to 100 km and about 500 km respectively copyright c 1999 john wiley sons ltd

stable channels with mobile gravel beds regime type equations for mobile gravel bed rivers are presented based on data obtained from 62 stable gravel bed river reaches in the united kingdom multiple regression techniques are applied to derive equations relating reach average and riffle values of width mean and maximum depth slope velocity sinuosity and riffle spacing to bankfull discharge bed and bank material characteristics valley slope bank vegetation type and an independent estimate of bankfull bed load transport rates although discharge has a dominant control on channel geometry these equations indicate that bed load discharge also has a significant influence particularly with regard to channel slope bank vegetation has a major control on width and velocity while depth velocity and slope are storngly affected by bed material size reasons for these results are considered in terms of the physical processes controlling channel adjustment the application of these regime equations is discussed and particular consideration is given to the design of sinuous channels with a pool riffle bed topograph

sand production simulation under true triaxial stress conditions abstract laboratory sanding experiments were carried out under true triaxial stress conditions the objective was to investigate the effect of state of stresses and fluid flow on the mechanism of sanding and the development of the failure zone around the borehole the experiments were conducted on 100 100 100 mm3 cubic samples of synthetic sandstones the samples were prepared based on an established procedure developed to produce weakly consolidated sandstone samples with identical physico mechanical properties the properties of the synthetic sandstone samples were determined by conducting a series of standard rock mechanics tests on cylindrical plugs using a true triaxial stress cell ttsc cubic samples were subjected to true triaxial stresses and radial fluid flow from the outer boundaries into the borehole the maximum and intermediate principal stresses were applied laterally in both cases while the effect of changing the lateral stresses on the development of the failure zone around borehole was monitored it was observed that the geometry i e width and depth of the failure zone developed around the borehole is a function of the lateral stresses ratio i e lateral stress anisotropy the experiments were also simulated numerically using abaqus in order to validate and interpret the results from the experiments a good agreement was obtained between the results of both methods which confirms the importance of lateral stress anisotropy on the evolution of sanding the observations and results of these experiments and numerical simulations will be presented and discussed

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

estimation of river discharge propagation speed and hydraulic geometry from space lena river siberia 1 moderate resolution imaging spectroradiometer modis derived measurements of lena river effective width we display a high predictive capacity r2 0 81 mean absolute error 60 90 km 2 3 time valley width satellite derived we q rating curves and hydraulic geometry b exponents converge upon stable values b 0 48 indicating transferability of the discharge retrieval method between different locations put another way at length scales exceeding 60 90 km all subreaches display similar behavior everywhere at finer reach length scales e g 0 25 1 km longitudinal extraction of b exponents represents the first continuous mapping of a classical hydraulic geometry parameter from space while at least one gauging station is required for calibration results suggest that multitemporal satellite data can powerfully enhance our understanding of water discharge and flow conveyance in remote river systems

patterns of alluvial rivers the pattern planform of a river can be considered at vastly different scales depending upon both the size of the river and the part of the fluvial system that is under consideration figure 1 for example in the broadest sense river patterns comprise a drainage network dendritic parallel trellis etc figure la the type of pattern is of interest to geomorphologists and geologists who interpret geologic conditions from aerial photographs at another scale a river reach which in figure lb is meandering is of interest to the geomorphologist who is interested in what that pattern reveals about river history and behavior and to the engineer who is charged with maintaining navigation and preventing major instability when a single meander is examined figure 1 c the hydraulics offtow the sediment transport and the potential for bank erosion are of concern in addition the sedimentologist is interested in the distribution of sediment within the bend bed forms within the channel figure ld and sedimentary structures figure ie which also establish a component of roughness for the hydraulic engineer finally the individual grains figure if provide geologic information on the sediment sources the nature of sediment loads and the feasibility of dredging for gravel there is an interaction of hydrology hydraulics geology and geomorphology at all scales which emphasizes the point that the fluvial system as a whole cannot be ignored even though only a component of the system is to be studied in this review only the patterns or planforms of alluvial rivers are discussed although it is apparent that the hydrologic and sediment yield characteristics of the drainage basin figure la as well as its geologic history cannot be ignored in the explanation of the pattern of any river

slope induced changes in channel character along a gravel bed stream the allt dubhaig scotland changes in channel character along a small river in the scottish highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0 02 to o ooo15 because of local baselevel control this decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed the channel pattern alters progressively rather than abruptly in the sequence 1 near braided 2 meandering with active point bar chutes 3 meandering with active outer bank talweg 4 stable equiwidth sinuous the changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical cdncepts than by purely empirical discriminant or trend equations measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload it is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions are typical of many streams in mountainous areas and can be understood in terms of slope induced changes in hydraulic properties

worldwide correlations for subaerial aqueous flows with exponential longitudinal profiles it is shown that very strong worldwide correlations exist between the bed concavity coefficients of a wide range of subaerial aqueous flows with exponential longitudinal profiles and both the corresponding stream segment lengths and exponential bed particle size diminution coefficients the former correlation is complementary to an existing similar correlation for the exponential size diminution coefficients while the latter is consistent with earlier theoretical correlations based on very limited data the data supporting the correlations extend over virtually the whole range of stream lengths solids concentrations and bed sediment particle sizes found on earth this universality strongly suggests that there are underlying mechanisms common to all kinds of mobile bed subaerial aqueous flows however the scatter of the data for the correlations is significant and is mostly attributable to variations in hydraulic conditions and sediment properties rather than measurement errors some of the conditions and properties have been identified but others remain obscure copyright c 1999 john wiley sons ltd

method for estimating spatially variable seepage loss and hydraulic conductivity in intermittent and ephemeral streams 1 a method is presented for estimating seepage loss and streambed hydraulic conductivity along intermittent and ephemeral streams using streamflow front velocities in initially dry channels the method uses the kinematic wave equation for routing streamflow in channels coupled to philip s equation for infiltration the coupled model considers variations in seepage loss both across and along the channel water redistribution in the unsaturated zone is also represented in the model sensitivity of the streamflow front velocity to parameters used for calculating seepage loss and for routing streamflow shows that the streambed hydraulic conductivity has the greatest sensitivity for moderate to large seepage loss rates channel roughness geometry and slope are most important for low seepage loss rates however streambed hydraulic conductivity is still important for values greater than 0 008 m d two example applications are presented to demonstrate the utility of the method

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

a temporal stable isotopic δ18 o δd d excess comparison in glacier meltwater streams taylor valley antarctica in this paper we describe the importance of hyporheic dynamics within andersen creek and von guerard stream taylor valley antarctica from the 2010 11 melt season using natural tracers water collection started at flow onset and continued with weekly hyporheic zone sampling the water 18o and d values were isotopically lighter in the beginning and heavier later in the season d excess measurements were used as an indicator of mixing because an evaporative signature was evident and distinguishable between two primary end members glacier meltwater and hyporheic zone hyporheic zone influence on the channel water was variable with a strong control on streamwater chemistry except at highest discharges this work supports previous research indicating that von guerard stream has a large widespread hyporheic zone that varies in size with time and discharge andersen creek with a smaller hyporheic zone displayed hyporheic zone solute interaction through the influence from subsurface hypersaline flow overall the evolution of taylor valley hyporheic zone hydrology is described seasonally in mid december the hyporheic zone is a dynamic system exchanging with the glacier meltwater in the channel and with diminishing flow in january the hyporheic zone drains back into the channel flow also impacting stream chemistry this work adds new information on the role of hyporheic zone stream interaction in these glacier meltwater streams

simulating unsteady flow anabranching and hyporheic dynamics in a glacial meltwater stream using a coupled surface water routing and groundwater flow model 1 flooding affects ecosystems by transporting water and solutes across aquatic terrestrial interfaces removing nutrient and organic substrate limitations and spurring biogeochemical activity few studies have considered the influence of flooding on surface water groundwater interactions this research examines the temporally variable water storage and exchange in a stream in the mcmurdo dry valleys mdv of antarctica where diel flood pulses occur due to glacial melt several mdv streams display truncated discharge peaks suggesting water storage between the source glacier and the gauging station we tested the hypothesis that stream braids and subsurface water storage contribute to the difference between glacial melt and stream outflow hydrographs by constructing a coupled surface water routing and subsurface water flow model this model routes water into stream braids at high flows and allows this water to infiltrate and return to the stream via subsurface flow paths as flows recede our simulation demonstrates the importance of surface subsurface water interactions in controlling the hydrograph shape maximum simulated discharge was sensitive to storage parameters including aquifer depth and the flooding threshold while minimum discharge was sensitive to hydraulic conductivity subsurface storage volume varied by 38 over a diel cycle and stream subsurface exchange rates varied from 0 to 0 19 m3 h 1 m 1 with exchange from the stream to the subsurface during high flows and vice versa at low flows these results underscore how unsteady flow can increase hyporheic interactions and ecosystem productivity and provide support for maintaining natural stream morphology and flow regimes

statistical models of fluvial systems abstract statistical methods have dominated quantitative analyses of fluvial systems over the last 40 years this paper assesses the utility of statistical models by examining methodological issues associated with empirical analysis of hydraulic and channel geometry relationships it also describes and presents examples of advanced statistical models that promise to enhance our understanding of the structure and dynamics of fluvial systems bivariate and multivariate power function models of fluvial systems provide objective numerical descriptions of the relationship between channel geometry and hydrologic hydraulic sedimentologic and biotic controls traditional regression models are limited in that they only describe simple input output relations between the states of dependent and independent variables advanced statistical models transcend these limitations simultaneous equation models continuously varying parameter models and distributed lag models are statistical tools for explicitly analyzing 1 mutual adjustments among fluvial variables 2 the factors controlling the spatial structure of channel geometry flow relations and 3 the equilibrium or disequilibrium dynamics of fluvial systems respectively these advanced statistical techniques can be used to empirically test conceptual models of fluvial systems at a variety of spatial and temporal scales

pivoting analyses of the selective entrainment of sediments by shape and size with application to gravel threshold measured variations of pivoting angles with grain size shape reliability and angularity and imbrication are employed in analyses of grain threshold to examine how these factors influence selective grain entrainment and sorting with a bed of uniform grain sizes as employed experimentally to establish the standard threshold curves such as that of shields the threshold condition depends on grain shape and fabric the analysis demonstrates quantitatively that there should be a series of nearly parallel threshold curves depending on grain pivoting angles for a given grain size the order of increasing flow strength required for entrainment is spheres smooth ellipsoids depending on their reliability angular grains and imbricated ellipsoids depending on their imbrication angles the relative threshold values for these different grain shapes and fabric are predicted according to their respective pivoting angles but remain to be directly tested by actual threshold measurements the pivoting angle of a grain also depends on the ratio of its size to those it rests upon this dependence permits an evaluation of selective entrainment by size of grains from a bed of mixed sizes the condition generally found in natural sediments the pivoting model predicts systematic departures from the standard threshold curves for uniform grain sizes such departures have been found in recent studies of gravel threshold in rivers and offshore tidal currents the pivoting model is compared with those threshold data with reasonable agreement however more controlled measurements are required for a satisfactory test of the model it is concluded that variations in pivoting angles for grain entrainment are significant to the processes of selective sorting by grain size and shape

the gravel sand transition in a disturbed catchment abstract more than 40 million cubic metres of mining waste were supplied to the ringarooma river between 1875 and 1984 leading to successive phases of aggradation and degradation the natural bed material is gravel but given the volume of introduced load and the fact that much of the input was less than 5 mm in diameter the size composition of the bed changed from gravel to sand during the phase of downstream progressive aggradation a very sharp gravel sand transition developed in which median grain size decreased from over 30 mm to under 3 mm in less than 500 m with upstream supplies of mining debris becoming depleted first degradation followed the same downstream progressive pattern as aggradation causing the transition to migrate downstream by 1984 the river could be regarded as a series of zones each characterized by a particular bed condition a natural cobble gravel bed unaffected by mining inputs 0 32 km pre disturbance bed re exposed by degradation over 35 40 years 32 53 km sandy substrate with a gravel armour produced by differential transport during degradation 53 65 km sand dominated but with developing surface patches of coarser material 65 75 km sandy bed reflecting the size composition of the original mining input 75 118 km although the gravel sand transition itself is sharp the transitional zone is lengthy 53 75 km as degradation continues the gravel sand transition is expected to progress downstream but it has remained in a stable position for 12 years indeed two major floods during the period released large quantities of sand from the sub armour layer and newly formed banks of mine tailings causing fining both above and below the transition surface grain size is an adjustable component in the transitional zone as the river strives to recover from a major anthropogenic disturbance

sensitivity of ice cemented antarctic soils to greenhouse induced thawing are terrestrial archives at risk abstract we modeled the sensitivity of six ice cemented slope deposits from the western mcmurdo dry valleys antarctica to failure by shallow thaw induced planar sliding the deposits examined have purportedly remained physically stable without morphologic evidence for downslope movement for millions of years could they fail in the near future from greenhouse induced warming to address this question we first prescribed various increases in mean summertime soil surface temperature mssst and modeled numerically the resultant changes in soil thaw depths using a one dimensional heat diffusion equation including the effects of latent heat of fusion second we calculated the minimum thaw depths required to facilitate failure by shallow planar sliding for each deposit for all numerical simulations we maintained present soil moisture conditions and used a mohr coulomb based equation of safety factor third we calculated the rate of subsurface meltwater flow assuming darcy s law our results show that although most deposits contain sufficient subsurface ice to induce sliding upon thawing lateral rates of water flow of as much as 40 m day for some colluvial deposits prohibit the build up of requisite pore pressures for failure on the other hand silty deposits that contain gravimetric water 15 occur on slopes 20 and possess low hydraulic conductivities 30 60 cm day common in the dry valleys region could fail if mssst and by inference mean summertime atmospheric temperatures increase by 4 to 9 c this temperature increase is similar to that predicted to occur from greenhouse induced warming in antarctica over the next century

downstream fining in large sand bed rivers abstract downstream fining of bed sediments is a well known phenomenon in gravel bed rivers but also occurs in large sand bed rivers the underlying processes however are not necessarily the same due to the difference in sediment mobility grain size distribution width and geographical setting downstream fining studies focusing on large sand bed rivers are scarce nevertheless in this paper the processes that affect downstream fining in gravel bed rivers are reviewed and it is evaluated to what extent they are relevant for large sand bed rivers furthermore several new downstream fining processes are discussed ending in a conceptual model of downstream fining in large sand bed rivers in deriving the conceptual model downstream fining processes are divided into three categories abrasion selective transport and sediment addition extraction abrasion rates in large sand bed rivers are small due to the durable lithologies the small grain size the high degree of rounding the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment only in the upstream part of large sand bed rivers abrasion rates may be somewhat higher selective transport rates in large sand bed rivers everywhere overrun abrasion rates but are small compared to selective transport rates in gravel bed rivers because the unimodal sediments in sand bed rivers exhibit stronger hiding exposure effects during threshold of motion conditions but especially because many sand bed rivers though not all are in state of fully mobilised transport making differences in threshold of motion between coarse and fine grains relatively unimportant selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting which are all processes that act on a relatively large scale within sand bed rivers the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency there are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand bed rivers note that in contrast to abrasion selective transport only produces a stable downstream fining pattern if the river profile is concave generally a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand bed rivers overbank sedimentation for instance may significantly decrease the downstream fining rate in large sand bed rivers because especially fine grains are removed from the bed dredging can also strongly affect the downstream fining trend depending on the volume and the grain size of the sediment involved meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances increasing the downstream fining rate river bifurcations can cause a discontinuity in the downstream fining pattern because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch all these addition and extraction processes not only have a direct effect on the grain size composition of the river bed but also indirectly influence downstream fining by their effect on the longitudinal bed profile other sediment addition and extraction processes such as tributary confluences non alluvial sediment sources and dumping are less common in large sand bed rivers but can locally have a great effect a full determination of the relative importance of all downstream fining processes in large sand bed rivers requires extensive field measurements the construction of detailed fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above

the gravel sand transition flume study of channel response to reduced slope this paper reports flume experiments with a bimodal gravel sand bed and sediment feed designed to investigate river channel response to declining slope in successive runs an initial near equilibrium channel was subjected to progressively bigger reductions in flume slope this simulated a series of sites along a concave profile approaching a local base level and undergoing a transition from gravel to sand bed it also models the effects of a rise in base level on a fixed site slope reduction led to immediate increase in depth and reduction in velocity and shear stress hence progressive aggradation deposition was size selective and led to the following sequence of bed texture change with increasing severity of slope reduction 1 sand in lee of gravel clusters 2 elongated sandy patches and thin streaks 3 wider sand ribbons covering gravel completely and developing ripples there was feedback from the gradual bed surface changes to hydraulic conditions near bed velocity increased while depth shear stress and roughness height decreased these feedbacks were more pronounced with a bigger reduction in slope these results support the view that the gravel sand transition along river channels with a local base level control is assisted by positive feedback between changes in bed surface texture and near bed flow

hydrologic exchange and chemical weathering in a proglacial watershed near kangerlussuaq west greenland abstract the exchange of proglacial river water with active layer pore water could alter water chemical compositions in glacial outwash plains and oceanic solute fluxes to evaluate effects of this exchange we sampled watson river and adjacent pore water during the 2013 melt season at two sandurs in western greenland one in sandflugtdalen and the other near the confluence with sondre stromfjord we measured temperature specific conductivity and head gradients between the river and bank over a week long period at sandflugtdalen as well as sediment hydraulic conductivity and chemical compositions of waters from both sites specific conductivity of pore water is four to ten times greater than river water as solutes are concentrated from weathering reactions cryoconcentration and evaporation pore water compositions are predominantly altered by carbonate dissolution and sulfide mineral oxidation high concentrations of hco3 and so4 result from solute recycling and dissolution of secondary ca mg carbonate sulfate salts initially formed by near surface evaporation in the summer and at depth by freeze in of the active layer and cryoconcentration in the winter high hydraulic conductivity 10 5 to 10 4 m s and diurnal fluctuations of river stage during our study caused exchange of river and pore water immediately adjacent to the river channel with a net loss of river water to the bank pore water 6 m from the river continuously flowed away from the river approximately 1 8 of the river discharge through the sandflugtdalen was lost to the river bank during our 6 75 day study based on calculations using darcy s law although not sampled some of this water should discharge to the river during low river stage early and late in the melt season elevated pore water solute concentrations in sandurs and water exchange at diurnal and seasonal frequency should impact fluxes of solutes to the ocean although understanding the magnitude of this effect will require long term evaluation throughout the melt season

shear failure analysis of a shallow depth unsupported borehole drilled through poorly cemented granular rock abstract adopting an appropriate failure criterion plays a key role in the borehole stability analysis in this paper the induced stresses on a vertical borehole wall were calculated based on the elastic theory then to predict the stability of a borehole drilled through a poorly cemented sand formation failure envelopes in different failure criterion domains were derived using the results from a series of precise laboratory tests conducted on solid and hollow cylinder specimens the mixture used in specimen preparation was designed to simulate the properties of the samples collected from depths up to 200 m at a drilling site in south australia the hollow cylinder test apparatus was developed by modifying a hoek triaxial cell these modifications allowed observing the process of debonding of sand grains from the borehole wall during the test and consequently acquiring a better understanding on the failure mechanisms of a borehole drilled through poorly cemented sand formations three well known failure criterion domains coulomb drucker prager and mogi were considered versus the laboratory test data to investigate their capability to predict the shear failure of a borehole using the data from hollow cylinder tests the obtained results showed the significance of selecting an appropriate failure domain for predicting the shear failure behavior of poorly cemented sands near the borehole wall the results also showed that the coulomb criterion is not well suited for predicting the borehole failure when there is no pressure acting inside the borehole a failure envelope based on the mogi domain was developed which can be used for the far field stress states the introduced failure envelope allows predicting the stability of a borehole drilled in poorly cemented sands

particle size variations between bed load and bed material in natural gravel bed channels particle sizes of bed load and bed material that represent materials transported and stored over a period of years are used to investigate selective transport in 13 previously sampled natural gravel bed channels the ratio d of median particle size of bed material to the transport and frequency weighted mean of median bed load size decreases to unity with increasing drainage area bank full discharge dimensionless stream power and bed material sorting in channels with high values of d significant volumes of fine bed load are transported during discharges that are less than bank full which is commonly associated with general entrainment of the coarse pavement in many gravel bed channels this indicates transport of fine bed load over a more stable substrate of coarser bed material the apparent breakdown in equal mobility of the bed as a whole may be caused by areal segregation of poorly sorted bed material into superiorly sorted patches of varying mean size likely sources of selectively transported fine bed load include fine patches that have low entrainment thresholds and high virtual particle velocities a simple sediment budget applied to measurements from three channels indicates that velocities of material from fine patches in pools relative to velocities of average bed material are high in low order channels and decrease distally as more bed material that represents the bed as a whole is accessed for bed load by deeper annual scour

hydrologic connectivity and implications for ecosystem processes lessons from naked watersheds abstract hydrologic connectivity has received great attention recently as our conceptual models of watersheds and water quality have evolved in the past several decades however the structural complexity of most temperate watersheds i e connections among shallow soils deep aquifers the atmosphere and streams and the dynamic seasonal changes that occur within them i e plant senescence which impacts evapotranspiration create significant challenges to characterizing or quantifying hydrologic connectivity the mcmurdo dry valleys a polar desert in antarctica provide a unique opportunity to study hydrologic connectivity because there is no vegetative cover and therefore no transpiration and no deep aquifers connected to surface soils or streams glacier melt provides stream flow to well established channels and closed basin ice covered lakes on the valley floor streams are also connected to shallow hyporheic zones along their lengths which are bounded at 75 cm depth by ice cemented permafrost these hydrologic features and connections provide water for and underpin biological communities hence exchange of water among them provides a vector for exchange of energy and dissolved solutes connectivity is dynamic on timescales of a day to a flow season 6 12 weeks as streamflow varies over these timescales the timescales over which these connections occur is also dynamic exchanges between streams and hyporheic zones for example have been estimated to be as short as hours to as long as several weeks these exchanges have significant implications for the biogeochemistry of these systems and the biotic communities in each feature here we evaluate the lessons we can learn about hydrologic connectivity in the mdv watersheds that are simplified in the context of processes occurring and water reservoirs included in the landscape yet are sensitive to climate controls and contain substantial physical heterogeneity we specifically explore several metrics that are simple and or commonly employed in hydrologic analyses and interpret them in the context of connectivity between and among hydrologic features

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

stable channel design in alluvial rivers abstract a theory of stable channel design is deduced from the condition that for any given steady flow total sediment concentration maintains a balanced average in space and time within a stable channel this condition for stability applies during changes in channel position but does not constrain them a stability index is defined in terms of concentration of bedmaterial load a function of steady uniform or quasi uniform water discharge channel bedslope water surface width and a typical diameter of cohesionless granular bedmaterial the index is constant only within a stable channel and its mean magnitude may be determined from observations at cross sections of more or less straight wide reaches two distinct design methods threshold and mobile bed are deduced from the theory design formula are presented for both methods as functions of the channel stability index and are applicable to cross sections or reaches independent of planform of both sand silt and gravel bed rivers a relation describing variation along a channel in the ratio of bedload to suspended sediment discharge for a specified steady flow is also inferred from the stability condition