stabilization of a gravel channel by large streamside obstructions and bedrock bends jacoby creek northwestern california jacoby creek bed width 12 m bankfull discharge 32 6 m 3 s contains stationary gravel bars that have forms and positions controlled by numerous large streamside obstructions bedrock outcrops large woody debris and rooted bank projections and bedrock bends bank projection width and bar volume measured in 104 channel segments 1 bed width long are significantly cross correlated at lags of 1 3 and 4 indicating the tendency for large obstructions and bends to form bars 3 to 4 bed widths downstream and 1 bed width upstream all of the 18 bars downstream of large obstructions or bends in the study reach were along the obstruction side of the channel or outside bank of the bend most of the pools 85 were next to large obstructions or in bends conversely 92 of large obstructions or bends had pools comparison of the volume of four bars with volumetric bar changes and volume of bedload transported during four high flow events suggests that rates of sediment transport were sufficient to cause major changes in bars during bankfull events the only important channel changes observed in 4 yr however have been associated with the movement of large woody debris and with changes in the angle at which the flow approaches an obstruction a general model is proposed that large obstructions and nonalluvial bends stabilize the form and location of gravel bars bars are stabilized by two related mechanisms 1 large obstructions and bends cause intense quasi steady secondary circulation in scour holes that terminate upstream bars at fixed locations obstruction width channel deflection scour hole width and bed width were measured at 26 obstructions these data show that obstructions wider than approximately one third of the bed form pools spanning the entire channel and thus terminating bars smaller obstructions form scour holes contained within a single bar 2 bars are deposited upstream of large obstructions and sharp bends because of backwater reductions in stream power bars are deposited downstream because flow energy is expended around obstructions and bends and because the flow expands downstream of constrictions that result from large obstructions the formation of bars and pools inherent in many gravel channels can thus be enhanced and fixed in position by flow structures set up around large obstructions and bends formed of resistant materials

reenvisioning velocity reversal as a diversity of hydraulic patch behaviours past research investigated the surpassing of mean velocity at riffle cross sections by that at pool cross sections for flows up to bankfull termed velocity reversals to understand one mechanism by which riffle pool relief is maintained this study reenvisioned the classic velocity reversal by documenting stage dependent changes to the locations of peak velocity without cross sections instead the dynamics of peak velocity patches were considered for flows spanning 0 2 to 22 times bankfull discharge through the use of a high resolution dem and two dimensional hydrodynamic modelling a remarkable diversity in peak velocity patch behaviour was found across discharges including gradual expansion and shifting as well as abrupt disappearance and emergence relative to the low flow patch locations these behaviours blended together to varying degrees to produce many reversals in peak velocity across morphological units but it took substantially higher than bankfull discharge for peak velocities to move from riffles and chutes to fast glides and pools the discharges at which reversals occurred among morphological units were significantly higher for the valley confined reach than for the anastomosing reach copyright 2016 john wiley sons ltd

numerical simulation of overbank processes in topographically complex floodplain environments this article presents results from an investigation of the hydraulic characteristics of overbank flows on topographically complex natural river floodplains a two dimensional hydraulic model that solves the depth averaged shallow water form of the navier stokes equations is used to simulate an overbank flow event within a multiple channel reach of the river culm devon uk parameterization of channel and floodplain roughness by the model is evaluated using monitored records of main channel water level and point measurements of floodplain flow depth and unit discharge modelled inundation extents and sequences are assessed using maps of actual inundation patterns obtained using a global positioning system observational evidence and ground photographs simulation results suggest a two phase model of flooding at the site which seems likely to be representative of natural floodplains in general comparison of these results with previous research demonstrates the complexity of overbank flows on natural river floodplains and highlights the limitations of laboratory flumes as an analogue for these environments despite this complexity frequency distributions of simulated depth velocity and unit discharge data closely follow a simple gamma distribution model and are described by a shape parameter that exhibits clear systematic trends with changing discharge and floodplain roughness such statistical approaches have the potential to provide the basis for computationally efficient flood routing and overbank sedimentation models copyright 2002 john wiley sons ltd

experimental study of incipient motion in mixed size sediment transport rates of five sediments were measured in a laboratory flume three of these sediments had the same mean size the same size distribution shape and different values of grain size distribution standard deviation the critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate the sorting of the sediment mixture had little effect on the critical shear stress of individual fractions once the median size d50 of the mixture and a fraction s relative size di d50 are accounted for our data combined with previously published data show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction s relative grain size despite a broad variation in the available data of mixture sorting grain size distribution shape mean grain size and grain shape all fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions this result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment recirculating systems as well as for systems where the fractional transport rates are imposed on the system feed systems this equivalence in initial motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination

sediment from hydraulic mining detained by englebright and small dams in the yuba basin recent initiatives to find ways to reintroduce anadromous fish to the central valley of california have identified the yuba river as one of the best potential watersheds for expanding spawning habitat of spring run chinook salmon and steelhead trout salmon spawning in the yuba river would require substantial modifications or removal of englebright dam a large dam 86 million m 3 capacity built by the u s army corps of engineers in 1941 an extensive on going feasibility study by local state and federal organizations therefore is examining aspects of various dam treatment scenarios that range from no action to complete dam removal this paper examines the extraordinary history of the watershed and resulting conditions pertinent to the feasibility of altering englebright dam it seeks to accomplish four goals first historical geomorphic changes in the watershed are outlined that influence the physical context of the feasibility study the yuba watershed is centered in the hydraulic gold mining region made

lacustrine basin dynamics and organosulphur compound origin in a carbonate rich lacustrine system late oligocene mequinenza formation se ebro basin ne spain abstract the late oligocene mequinenza formation se ebro basin ne spain records the development of extensive shallow low gradient lacustrine zones which evolved under a subtropical warm climatic regime affected by humidity aridity cycles carbonate dominated inner lacustrine sequences which include thin coal seams are laterally related to muddy carbonate and siliciclastic marginal lacustrine assemblages where evaporites occur water solute concentration of the lacustrine zones changed during the alternative high and low stand episodes with lacustrine waters ranging mainly from fresh to oligosaline nonsulphurised nsc and sulphurised osc organic compounds occur in the inner lacustrine facies and record significant contributions from aquatic and terrestrial higher plants bacteria cyanobacteria photosynthetic anaerobic bacteria and algae these biomarkers show that the lacustrine system was eutrophic and the water column was poorly oxygenated and sometimes stratified as evidenced by occasional finely laminated facies and gammacerane organic matter s c ratio lower in carbonate than in coal and coal related facies suggests significant variations in the lacustrine autotrophic production and or the allochthonous organic matter contribution higher plant biomarkers were recorded mainly as normal nonsulphurised lipids by contrast bacterial and algal lipids hopanoids steroids and isoprenoids under intense sulphate reduction conditions incorporated sulphur in their hydrocarbon skeleton and gave rise to osc with distributions of thienyl and thiolanylhopanoids epithiosteranes and polyprenoids linear thiols are new biomarkers which probably were generated by the early incorporation of sulphur into funtionalised precursors e g alkanols intense methanogenesis was enhanced in some lacustrine zones owing to high organic matter input and as a consequence biomarkers of methanotrophic and methylotrophic bacteria related to methane emission are also found

sedimentological and stable isotope evolution of lakes in the vestfold hills antarctica the sedimentological chemical and isotopic characteristics of sediment cores from three slightly saline to hypersaline lakes highway ace and organic lakes and two marine inlets ellis fjord and taynaya bay in the vestfold hills antarctica have been examined sections of the cores deposited in marine environments are characterized by uniform regularly laminated fine grained organic rich sediments with uniform organic 13c values 18 0 to 19 4 vs pdb and sulfur contents in contrast sediments deposited in lacustrine environments are extremely heterogeneous varying from finely laminated mat like sequences to poorly sorted clastic rich sediments authigenic monohydrocalcite and aragonite occur in some lake sediments the 13c values of organic matter in the lacustrine sediments exhibit an extremely wide range 10 5 to 25 3 that can be related to variations in physico chemical conditions in the lake waters strongly negative organic 13c values coupled with high sulfur contents are indicative of an anoxic zone in the overlying lake waters whereas less negative organic 13c values coupled with low sulfur contents are indicative of well mixed oxic conditions particularly high organic 13c values result during high levels of microbial activity in the lakes due to high rates of photosynthetic co2 fixation the large shifts in organic 13c are not necessarily accompanied by any change in macroscopic sedimentological characteristics illustrating the utility of isotopic investigations in these environments the 13c composition of authigenic carbonate in hypersaline organic lake sediments provides a record of changes in palaeoproductivity while the 18o of the carbonate provides information on rates of meltwater input and evaporation in the lake 14c dating suggests that highway lake was isolated from the sea by isostatic uplift at least 4600 yr before present b p whereas organic lake was isolated at approximately 2700 yr b p apparent emergence rates calculated from the 14c ages range from 1 0 to 2 1 mm yr 1 the reservoir effect in the lacustrine and marine environments is variable but probably does not exceed 1000 yr in any of the lakes examined

flood driven topographic changes in a gravel cobble river over segment reach and morphological unit scales regulated rivers generally incise below dams that cut off sediment supply but how that happens and what the consequences are at different spatial scales is poorly understood modern topographic mapping at meter scale resolution now enables investigation of the details of spatial processes in this study spatial segregation was applied to a meter scale raster map of topographic change from 1999 to 2008 on the gravel cobble regulated lower yuba river in california to answer specific scientific questions about how a decadal hydrograph that included a flood peak of 22 times bankfull discharge affected the river at segment reach and morphological unit scales the results show that the river preferentially eroded sediment from floodplains compared to the channel and this not only promoted valley wide sediment evacuation but also facilitated the renewal and differentiation of morphological units especially in the channel at the reach scale area of fill and mean net rate of elevational change were directly correlated with better connectivity between the channel and floodplain while the mean rate of scour in scour areas was influenced by the ratio of slope to bankfull froude number a ratio indicative of lateral migration versus vertical downcutting hierarchical segregation of topographic change rasters proved useful for understanding multiscalar geomorphic dynamics this article is protected by copyright all rights reserved

skin friction behind isolated hemispheres and the formation of obstacle marks we have measured the mean magnitude and direction and the rms fluctuation intensity of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long downstream of reattachment the mean skin friction magnitude is about 20 greater than its free stream value along the centre line and comparably reduced to either side of it the horizontal divergence of the skin friction vector field is positive divergent along the centre line and negative convergent to either side of it neither of these conditions favours development of a ridge longer than the separation length along the centre line the development of ridges or tails many obstacle heights long commonly observed in nature requires considerable modification of the simple sediment free wakes we have studied

effects of a pebble cluster on the turbulent structure of a depth limited flow in a gravel bed river pebble clusters are important morphological features of gravel bed rivers and have a significant effect on the spatial and temporal response of the flow this paper presents original results from measurements of velocity taken at a high spatial and temporal resolution in a turbulent flow upstream and downstream from a pebble cluster three electromagnetic current meters 0 013 m in diameter mounted on a wading rod were deployed upstream and downstream of an isolated pebble cluster along 31 vertical profiles that contained between 7 and 13 points in a 5 m long section the high sampling resolutions in a natural environment permitted characterization of the complex dynamics of the turbulent flow in the vicinity of the cluster the description of these dynamics is based on the interpretation of contour maps of several turbulence statistics including moments of velocity distributions reynolds shear stress quadrant analysis and autocorrelation function downstream from the cluster flow is decelerated the intensity of turbulence increases and zones of strong vertical motions are present the superimposition of the maps allowed us to decompose the field of turbulent flow around a pebble cluster into distinct regions in a downstream direction from the cluster we observed zones of flow acceleration recirculation vortex shedding flow reattachment fluid upwelling and flow recovering results indicate the existence of highly dynamic boundaries between zones where strong intermittent events are generated they also highlight the difference between shedding and upwelling motions of the flow

velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions abstract keller keller e a 1971 areal sorting of bed load material the hypothesis of velocity reversal geological society of america bulletin 82 753 756 hypothesized that at high flow near bed velocities in pools exceed velocities in riffles and create pool scour pools however typically have larger cross sectional areas of flow at bankfull discharge this condition raises an inconsistency with keller s velocity reversal hypothesis and the one dimensional continuity of mass equation to address this problem a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit slopes of pools according to the model a narrow zone of high velocity occurs in the center of pools creating scour along the downstream end of pools an uphill climb of particles up the pool exit slope promotes sediment deposition the model is tested with field and flume measurements of velocity water surface elevation and size of bed sediments in recirculating eddy influenced pools local reversals of the water surface gradient were measured in the field and a velocity reversal was created in the flume the reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow the distribution of bed sediments collected in the field also support the proposed model of pool maintenance

valley width variation influences riffle pool location and persistence on a rapidly incising gravel bed river abstract decades of research have demonstrated that gravel bed rivers exhibit multiple spatial scales of landform nonuniformity landform features such as valley wall oscillations alluvial fans terraces river meanders point bars boulder clusters and logjams contribute to river corridor heterogeneity further studies have shown that the locations of riffles and pools in gravel bed rivers are closely associated with the spatial pattern of river corridor morphological variability these studies imply but do not directly test that persistence of riffles and pools through time is also conditioned by landform variability mechanistically it may be hypothesized that overbank flows interact hydraulically with oscillating valley walls by way of a mechanism known as flow convergence routing in such circumstances can variation in valley width influence channel morphology forcing riffle pool location and persistence to address this question research was performed on a valley confined wandering gravel bed reach of the regulated lower yuba river california usa that is incising from reduced sediment supply and flow regulation after a historic period of mining related valley sedimentation this study quantified stream planform change 1984 2006 and elevation change 1999 2006 using eight aerial photo sets and two digital elevation models and it qualitatively assessed channel change in older aerial photos going back to 1937 analysis revealed that the river has undergone rapid incision 0 11 m year and planform change in response to frequent floods yet eight riffles persisted in the same locations which coincided with the locally wide areas of the valley longitudinal profiles revealed that deep pools were associated with long valley constrictions geomorphic analysis and design need to explicitly account for multiple scales of landform nonuniformity in order to rehabilitate natural fluvial processes

sediment budget for salmonid spawning habitat rehabilitation in a regulated river bed elevation feature adjustments and spawning use were monitored at three chinook salmon oncorhynchus tschawytscha spawning habitat rehabilitation sites to measure project longevity in a regulated river sites enhanced with 649 1323 m 3 of gravel lost from 3 20 of remaining gravel volume annually during controlled flows of 8 70 m 3 s and 2 6 4 6 of placed material during a short duration 19 days release of 57 m 3 s the oldest site lost 50 of enhancement volume over 4 years of the mechanisms monitored gravel deflation was the greatest contributor to volumetric reductions followed by hydraulic scour spawning local scour around placed features and oversteepened slopes contributed to volumetric changes as sites matured volumetric reductions decreased sites captured as much large woody debris as was lost while complexity is an extremely important aspect of ecological function artificial production of highly diverse and complex habitat features may lead to limited longevity without natural rejuvenation d 2005 elsevier b v all rights reserved

numerical modeling of poorly sorted depositional turbidity currents 1 the characteristics of poorly sorted turbidity currents are investigated by numerical simulation a multiphase approach is utilized to express the mass and momentum conservation equations for the bulk fluid the exner equation for multiple size classes of sediment is solved for the evolution of the bottom boundary the model is applied to simulate experiments on poorly sorted gravity currents and reproduced the vertical structures of velocity sediment concentration of individual size class and the total deposition depth satisfactorily the numerical simulation further reveals that the normalized vertical profiles of concentration of all size classes coincide at the position where the maximum longitudinal velocity occurs below this position in the logarithmic velocity region all concentration profiles are upwardly convex and coarser particle size classes are of larger concentration above it in the exponentially decaying velocity region all concentration curves are upwardly concave and finer particle size classes are of higher concentration the model also predicts that the position of maximum longitudinal velocity corresponds to a region of low turbulent kinetic energy the model has also been applied to study three dimensional poorly sorted depositional turbidity currents debouching from a straight channel into a horizontal unconfined area model results reveal the levee and channel building process in which the channel bed slope aggrades at a faster rate than that in the overbank area the simulation also shows that the contour of mean size of deposited sediment is closely related to the contour of deposit thickness and elongated somewhat in the flow direction

formation and maintenance of a forced pool riffle couplet following loading of large wood abstract pool riffle maintenance has been documented in numerous studies but it has been almost impossible to characterize detailed natural pool riffle formation mechanisms because of the lack of baseline data prior to pool establishment in 2013 a study was conducted on the blackledge river in connecticut to document the formation of a new pool riffle couplet on a section of river that had previously been studied from 1999 to 2001 in 2001 the study reach contained a scour hole with a residual depth of 0 08 0 09 m downstream of a 1930s paired deflector with no identifiable riffle immediately downstream at this time a large severely undercut hemlock tree was noted along the left bank sometime between fall 2001 and 2004 the tree fell perpendicular to flow across the channel and formed a large wood lw jam and new pool riffle couplet several meters downstream of the old scour hole pool spacing along the reach decreased from 4 47 bankfull widths bfw in 1999 to 3 83 bfw after the new pool riffle couplet formed the new pool has a residual depth the water depth of the streambed depression below the elevation of the immediate downstream hydraulic control of 1 36 0 075 to 1 59 0 075 m which resulted from a combination of 1 32 0 09 m or less of incision below the old scour hole 95 6 or less of the depth increase and up to 0 18 0 09 m of downstream deposition and associated backwater formation 13 2 or less of the depth increase to assess dynamic stability of the pool riffle couplet over several flood cycles surficial fine sediment and organic material along the reach were quantified the 23 m long pool stores 25 7 of the surficial fine grained sediments and 15 4 of organic material along a 214 m long reach that includes one additional artificially created pool an adjacent 50 m long secondary channel impacted by the lw jam stores 65 3 of the surficial fine grained sediments and 54 8 of organic material along the full reach

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

significant decadal channel change 58 67 years post dam accounting for uncertainty in topographic change detection between contour maps and point cloud models abstract construction of digital elevation models dems and the subtraction of dems between different points in time as a method to determine temporal patterns of scour and fill is a highly valuable procedure emerging in geomorphology these dems of differences dods must be assessed for error in order to distinguish actual topographic change from uncertainty and surface error current methods include 1 uniformly excluding all values that fall below a minimum threshold 2 using a spatially variable approach such as the construction of minimum level of detection lod grids or 3 the creation of a fuzzy inference system although spatially variable methods for determining error have been more accurate in excluding noise without discarding large amounts of meaningful data a challenge remains in performing dods against preexisting contour based maps for which no original point data are available the goals of this study were to 1 develop a method that overcomes the unknown point density of contour and other historical data sets and allows for some assessment of dod uncertainty on the basis of information on topographic variability 2 perform comprehensive uncertainty analysis testing to understand the opportunities and constraints associated with this new method and 3 report and interpret the overall pattern and volume of decadal topographic change for a regulated river 67 years post dam in light of alternate conjectured mechanisms of post dam longitudinal profile adjustment the key feature of the new approach is the introduction of a high density artificial point grid that samples the topographic variability evident in the available historical data set the testbed used to develop and assess this new dod method was the 37 5 km lower yuba river california historical data consisted of 0 6 m contours from a 1999 survey while a more detailed point cloud was available for the most recent survey in 2006 2008 to evaluate uncertainty in the method this study applied seven different uncertainty metrics of varying strictness t 1 t 1 96 uniform 0 3 m exclusion t 1 plus uniform 0 3 m exclusion t 1 with lod minimum at 0 3 t 1 96 plus uniform 0 3 m exclusion and t 1 96 with lod minimum at 0 3 and two different dod adjustment methods exclusion and subtraction a stringent approach involving joint use of the contour half interval and the spatially distributed lod grid at a 95 confidence limit excluded 44 3 of the study area from spatial assessment of channel change and volumetric change computation this preferred approach yielded an estimated 2 518 million m 3 of total scour and 2 455 million m 3 of total fill in 7 9 years after considering different mapping epochs the net annual average export was 17 000 m 3 32 500 tons to the feather river and a river valley sediment yield of 2205 tons km 2 year this amount is 36 of the post dam annual yield of gravel and cobble to the upstream reservoir that blocks sediment conveyance into the study domain suggesting that the lowland system is still highly dynamic 67 years after the dam was built the scientific significance of this is that the response of rivers to dams can be far more long lasting and complex depending on the suite of cumulative societal impacts to rivers the ability to account for spatially explicit dod uncertainty i e data retention when uncertainty is low and data removal when uncertainty is high when comparing historic contour based and modern point cloud based dems will allow for more detailed and reliable dods and sediment budgets in these cases

relationships between mesoscale morphological units stream hydraulics and chinook salmon oncorhynchus tshawytscha spawning habitat on the lower yuba river california an expert based approach was used to identify 10 morphological unit types within a reach of the gravel bed regulated yuba river california that is heavily utilized by spawning chinook salmon oncorhynchus tshawytscha analysis of these units was carried out using two dimensional hydrodynamic modeling field based geomorphic assessment and detailed spawning surveying differently classified morphological units tended to exhibit discrete hydraulic signatures in most cases the froude number adequately differentiated morphological units but joint depth velocity distributions proved the most effective hydraulic classification approach spawning activity was statistically differentiated at the mesoscale of the morphological unit salmon preferred lateral bar riffle and riffle entrance units these units had moderately high velocity unit median 0 45 m s 1 and low depth unit median 0 6 m but each exhibited a unique joint depth velocity distribution a large proportion of redds 79 were associated with conditions of convective flow acceleration at riffle and riffle entrance locations in addition to reflecting microhabitat requirements of fish it was proposed that the hydraulic segregation of preferred from avoided or tolerated morphological units was linked to the mutual association of specific hydraulic conditions with suitable caliber sediment that promotes the provision and maintenance of spawning habitat

flow reversal over a natural pool riffle sequence a computational study a computational study is presented on the hydraulics of a natural pool riffle sequence composed of mixed cobbles pebbles and sand in the river lune northern england a depth averaged two dimensional numerical model is employed calibrated with observed data at the field site from the computational outputs the occurrence of longitudinally double peak zones of bed shear stress and velocity is found in particular at low discharge there exists a primary peak zone of bed shear stress and velocity at the riffle tail in line with the local maximum energy slope in addition to a secondary peak at the pool head as discharge increases the primary peak at the riffle tail at low flow moves toward the upstream side of the riffle along with the maximum energy slope showing progressive equalization to the surrounding hydraulic profiles concurrently the secondary peak due to channel constriction appears to stand at the pool head with its value increasing with discharge and approaching or exceeding the primary peak over the riffle the existence of flow reversal is demonstrated for this specific case which is attributable to channel constriction at the pool head a dynamic equilibrium model is presented to reconstruct the pool riffle morphology a series of numerical modelling exercises demonstrates that the pool riffle morphology is more likely produced by shallow flows concentrated with coarse sediments than deep flows laden with low concentrations of fine sediments it is concluded that channel constriction can but may not necessarily lead to competence reversal depending on channel geometry flow discharge and sediment properties

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

engineered channel controls limiting spawning habitat rehabilitation success on regulated gravel bed rivers in efforts to rehabilitate regulated rivers for ecological benefits the flow regime has been one of the primary focal points of management strategies however channel engineering can impact channel geometry such that hydraulic and geomorphic responses to flow reregulation do not yield the sought for benefits to illustrate and assess the impacts of structural channel controls and flow reregulation on channel processes and fish habitat quality in multiple life stages a highly detailed digital elevation model was collected and analyzed for a river reach right below a dam using a suite of hydrologic hydraulic geomorphic and ecological methods results showed that despite flow reregulation to produce a scaled down natural hydrograph anthropogenic boundary controls have severely altered geomorphic processes associated with geomorphic self sustainability and instream habitat availability in the case study given the similarity of this stream to many others we concluded that the potential utility of natural flow regime reinstatement in regulated gravel bed rivers is conditional on concomitant channel rehabilitation

geospatial organization of fluvial landforms in a gravel cobble river beyond the riffle pool couplet abstract morphological units mu are landforms with distinct local form process associations at 1 10 channel widths scale that may be the fundamental building blocks describing the geomorphic structure of a river past research has disproportionately focused on the two mus of pool and riffle conjecturing that they are the central linked couplet in the process form association the goal of this study was to delineate and map spatially explicit fluvial landforms in two dimensional planview within a gravel cobble bed river using two dimensional hydrodynamic delineation and then to statistically examine mu geospatial patterns for indicators of deterministic geomorphic control this procedure is not discharge dependent like mesohabitat methods but gets at the geometry of underlying landforms statistical testing confirmed that eight delineated in channel mu types comprise a complex and diverse channel morphology in which pools and riffles are not directly coupled specifically gravel cobble river channels 1 exhibit nonrandom spatial organization of their longitudinally and laterally variable landform morphology 2 consist of a variety of mu types not just pools and riffles and 3 show distinct mu collocations and avoidances with riffles linked to chutes and runs while pools are linked to slackwaters and glides planview mu delineation with two dimensional hydrodynamic modeling provides a bottom up approach to understanding and linking channel morphology with ecosystem services and geomorphic processes and is being used to guide river management and rehabilitation strategies

flow convergence routing hypothesis for pool riffle maintenance in alluvial rivers 1 the velocity reversal hypothesis is commonly cited as a mechanism for the maintenance of pool riffle morphology although this hypothesis is based on the magnitude of mean flow parameters recent studies have suggested that mean parameters are not sufficient to explain the dominant processes in many pool riffle sequences in this study two and three dimensional models are applied to simulate flow in the pool riffle sequence on dry creek california where the velocity reversal hypothesis was first proposed these simulations provide an opportunity to evaluate the hydrodynamics underlying the observed reversals in near bed and section averaged velocity and are used to investigate the influence of secondary currents the advection of momentum and cross stream flow variability the simulation results support the occurrence of a reversal in mean velocity and mean shear stress with increasing discharge however the results indicate that the effects of flow convergence due to an upstream constriction and the routing of flow through the system are more significant in influencing pool riffle morphology than the occurrence of a mean velocity reversal the hypothesis of flow convergence routing is introduced as a more meaningful explanation of the mechanisms acting to maintain pool riffle morphology

high resolution two dimensional spatial modelling of flow processes in a multi thread channel this paper describes application and testing of a two dimensional numerical flow model in a multi thread reach of a proglacial stream the model solves the depth averaged form of the navier stokes equations for open channel flow incorporating a two equation turbulence closure an analytical correction for the effects of secondary circulation and a rigid lid approximation the model requires as input the channel bed topography the water surface bed roughness and inflow discharge information and predicts the spatial distribution of depth averaged velocity and eddy viscosity the results have been subjected to intensive testing using simple assessment of numerical performance e g conservation of mass and momentum numerical convergence distributed sensitivity analysis and comparison of model predictions with field measurements of velocity the results are encouraging particularly given some of the difficulties in obtaining accurate distributed cross stream and downstream velocities distributed sensitivity analysis allowed more detailed consideration of the necessary development of the model this suggested that significant errors in the velocity predictions were largely a result of uncertainty in the specification of both the magnitude and the spatial variation of bed roughness secondly the two equation turbulence closure was observed to have little effect upon model predictions except in the vicinity of the side walls in applications of models of this type to irregular coarse bedded channels improvements in the specification of the topographic boundary condition specification and bed roughness are likely to be more important than a sophisticated turbulence closure scheme thirdly although the secondary circulation correction was observed to reproduce some of the expected streamwise transfer of momentum the effects were seen to be relatively small given the intensity of secondary circulation defined in field contexts the inability of the model to correct effectively for the momentum transfer associated with secondary circulation processes related to topographic discordance and shear generated turbulence suggests that further work is required in this respect 1998 john wiley sons ltd

sources of environmental sulfur in the groundwater system southern new zealand abstract sulfide minerals commonly occur in sediments and basement rocks in southern new zealand as authigenic precipitates from groundwater below the oxygenated surface zone there are two principal potential sources for sulfur in the groundwater system weathering of sulfide minerals in the metamorphic basement and rainwater derived marine aerosols we present data for these two key sulfur sources metamorphic sulfide and associated hydrothermal au bearing veins within the otago schist average 34s 1 8 2 4 and an inland saline lake s derived entirely from rainwater 34s 21 4 0 8 we use these two end member 34s values to estimate the contributions of these sources of sulfur in authigenic groundwater sulfide minerals and in waters derived from oxidation of these sulfide minerals across a range of environments we show that authigenic groundwater pyrite along joints in the otago schist is derived primarily from metamorphic basement sulfur in contrast authigenic groundwater pyrite cementing miocene recent aquifers shows a substantial marine aerosol component and represents a distinct hydrogeological system we suggest that marine aerosols represent a significant flux to the terrestrial sulfur cycle that has been present through the groundwater system in otago over the past 20 million years

use of slope creation for rehabilitating incised regulated gravel bed rivers 1 gravel bedded channels often become incised and degraded below dams gravel can be added to the channel to rehabilitate hydrogeomorphic conditions including those promoting salmon spawning when implemented without increasing bed slope gravel addition at downstream riffles back floods upstream riffles a 2 year gravel augmentation project was done to test the efficacy of a new method for slope creation riffle to riffle slope was raised from 0 002 to 0 008 by adding gravel to the most upstream riffle when gravel was added to the next downstream riffle a year later riffle to riffle slope decreased to the sought after 0 004 after the study the area of high quality chinook salmon spawning habitat increased 471 the number of redds observed went from 62 to 161 during the study despite a 50 decline of in river spawners this eliminates variations in migrant population size and hatchery take as alternative explanations slope creation can be a useful aid for rehabilitating regulated rivers

evidence of density currents with the potential to promote meromixis in ice covered saline lakes abstract ice covers all but the most saline of the vestfold hills lakes for about 8 12 months of each year and precludes wind induced turbulent mixing over the winter when strong winds are most frequent nevertheless the progressive growth of ice volume through the austral winter and spring months generates haline convection capable of mixing the water column to a depth at least as great as that achieved by wind induced turbulence in the summer ice free period further cold and very saline brines may form at the shallow periphery of a lake and flow downslope penetrating to the lake centre below the convectively mixed layer detailed temperature profiles of hypersaline organic lake provide the first evidence for these density currents in saline lakes of the vestfold hills these data indicate a dynamic responsiveness to periods of relatively cold weather and that the resulting density currents may be sufficiently small in volume to have little effect on the anoxia of bottom layers in these meromictic lakes alternating periods of negative and positive water balance may also be significant in the formation and destruction of meromixis in these saline lakes which lack outflow streams if a lake has been through a period of negative water balance becoming relatively more saline and then begins to be diluted during a subsequent period of positive water balance the winter haline convection will penetrate to progressively shallower depths and deeper layers may stagnate marking the onset of meromixis an increase in the water level of organic lake over ten years indicates the speed with which the vestfold hills lakes may experience significant change in salinity despite the generally small catchments of these lakes

modelling floodplain flows using a two dimensional finite element model a prototype two dimensional finite element flow model for depth averaged free surface flows was developed for floodplain environments limited refinement of the model s physical representation was undertaken and the enhanced scheme applied to an 11 km river channel floodplain reach in the u k preliminary model results indicate that this modelling approach can be used to identify dynamic variations in the flow field parameters over length scales of the order of 10 100 m potentially such data have the ability to permit detailed analysis of short term floodplain sedimentary dynamics

two pool to pool spacing periods on large sand bed rivers mega pools on the madeira and mississippi abstract the madeira river thalweg includes two pool to pool spacing periods first standard pool crossing sequences form 6 to 8 m deep pools these pool crossing sequences recur approximately every six channel widths 9 7 km or 6 8 w on average however the madeira study reach between manicore and porto velho in northern brazil is relatively straight making these pools more frequent than meanders second much deeper pools 13 to 30 m deep also form at relatively regular but much longer spacing 50 60 km about 40 channel widths apart the deeper mega pools occur where the river reaches the edge of the holocene alluvium encountering resistant units e g granite mud clasts and conglomerates in the confining pleistocene units these two pool scales also occur on the lower mississippi river downstream of tarbert landing this study identified pool formation processes responsible for these two pool depth and spacing scales on two large sand bed rivers the analyses computed pool statistics and correlated the deepest pools with planform controls two distinct pool formation hypotheses explain the two pool types the standard pools are rhythmic features developed by macro turbulent feedbacks while the mega pools are forced features imposed by the regional geology the former are fundamentally periodic the latter are not however periodogram analysis indicates that mega pools on the madeira form a second relatively regular pool period the regional geology including the confining pleistocene bluffs and potentially orthogonal neotectonic lineaments imposed periodic regularity to these mega pools on the madeira river that is not observed on the mississippi river a comparably sized sand bed river with mega pools

reduced sulfur gases in saline lakes of the vestfold hills antarctica a survey of reduced sulfur gases in the lakes of the vestfold hills antarctica was undertaken to elucidate the environmental factors affecting the distribution of these compounds the oxygenated water of all lakes was found to contain low levels of dimethylsulfide dms 0 30 nm and no other sulfur compounds the meromictic lakes show considerable variation in the speciation and concentration of the reduced sulfur compounds present in the anoxic bottom water meromictic lakes of low salinity 80 possessed anoxylimnia nearly devoid of dimethylsulfide dms but with high concentrations of h2s in these lakes it appeared that rates of degradation of dms were as fast as production lakes of intermediate salinity 80 185 had high concentrations of both sulfur species while lakes of salinity greater than 185 had no h2s but high concentrations of dms the absence of hydrogen sulfide was attributed to the absence of sulfate reducing bacteria the observed dms concentrations were the result of the balance between the production and degradation of dms by bacteria at high salt concentrations either the degradation processes became relatively less efficient or more dms was produced in response to increased salinity

riffle pool maintenance and flow convergence routing observed on a large gravel bed river abstract geomorphologists have studied and debated over the processes responsible for natural riffle pool maintenance for decades most studies have focused on small wadable rivers but they lack much description of overbank flood conditions or a spatially explicit characterization of morphodynamics in this study 1 m horizontal resolution digital elevation models were collected from a riffle pool run sequence before and after an overbank flood with a 7 7 year recurrence interval on the relatively large gravel bed lower yuba river california digital elevation model differencing was used to quantify the magnitude and pattern of flood induced morphodynamic change cross section based analysis and two dimensional hydrodynamic modeling of flows ranging from 0 147 to 7 63 times bankful discharge were completed to evaluate the hydraulic mechanisms responsible for the observed topographic changes one key finding was that riffle pool relief increased by 0 42 m confirming the occurrence of natural hydrogeomorphic maintenance spatially complex patterns of scour and deposition exceeding 0 15 m at the scale of subwidth morphological units were reasonably predicted by the two dimensional mechanistic model that accounts for convective acceleration the one dimensional cross section based method underperformed the two dimensional model significantly consequently multiple scales of channel non uniformity and a dynamic flow regime caused the observed maintenance of the pool riffle morphology through the mechanism of flow convergence routing proposed by macwilliams et al macwilliams m l wheaton j m pasternack g b kitanidis p k street r l 2006 the flow convergence routing hypothesis for riffle pool maintenance in alluvial rivers water resources research 42 w10427 doi 10 1029 2005wr004391

scales of boundary resistance in coarse grained channels turbulent velocity profiles and implications gravel bed surfaces are characterized by morphological features occurring at different roughness scales the total shear stress generated by the flow above such surfaces is balanced by the sum of friction drag grain stress and form drag components created by bed forms to facilitate a better understanding of total resistance and bed load transport processes there is a need to mathematically separate shear stress into its component parts one way to do so is to examine the properties of vertical velocity profiles above such surfaces these profiles are characterized by an inner layer that reflects grain resistance and an outer layer that reflects total resistance a flume based project was conducted to address these concerns through systematically comparing different roughness scales to ascertain how increased roughness affects the properties of vertical velocity profiles great care was taken to create natural roughness features and to obtain flow data at a high spatial and temporal resolution using an acoustic doppler velocimeter average vertical velocity profiles above each roughness scale were clearly segmented the vertical extent of the inner flow region was directly related to the scale of roughness present on the bed and independent of flow depth increasing with increased roughness on a rough but rather uniform plane bed made of heterogeneous coarse sediments with no bed forms the shape of the velocity profile was clearly dominated by the local variations in grain characteristics when pebble clusters were superimposed the average shear stress in the outer flow region increased by 100 from the plane bed conditions the ratio of inner grain shear stress to outer total shear stress for this pebble cluster experiment was 0 18 under shallow flow conditions and 0 3 under deep flow conditions the grain stress component that should be used in bed load transport equations therefore appears to vary in these experiments between 15 and 30 of the total channel stress increasing with decreased resistance roughness height ks d50 values at the grain scale for the plane bed and pebble cluster experiments were 0 73 and 0 63 respectively these are values that should be used in flow resistance equations to predict grain resistance and grain stress for bed load transport modeling

three dimensional flow structure around small scale bedforms in a simulated gravel bed environment pebble clusters are common small scale morphological features in gravel bed rivers occupying as much as 10 per cent of the bed surface important links exist between the presence of pebble clusters and the development of flow structures these links are poorly understood at the three dimensional level particularly neglected has been the effect of clusters on the lateral flow characteristics a laboratory study was conducted using a hydraulic flume within which simulated pebble clusters were superimposed onto a plane bed of gravel material high resolution three dimensional flow data were collected above the bed at two different flow depths using an acoustic doppler velocimeter the results present evidence of the importance of lateral flow in the development of turbulent flow structure narrow regions of high lateral and downstream turbulence intensity exist to both sides of clusters and in a three dimensional separation zone in their lee this may indicate the presence of horseshoe type vortical structures analogous to those identified in less hydraulically rough environments however it is likely that these structures are more complicated given the mutual interference of the surrounding medium the lateral flow was also identified as a key component in the upwelling identified by other authors in the lee of pebble clusters the results of the vertical flow analysis confirm the hypothesis that six regions with distinct vertical flow characteristics exist above clusters flow acceleration up the stoss side of the cluster recirculation behind the cluster in the wake region vortex shedding from the pebble crest and shear layer flow reattachment downstream of the cluster upwelling of flow downstream of the point of reattachment and recovery of flow copyright 2001 john wiley sons ltd

investigating two dimensional finite element predictions of floodplain inundation using fractal generated topography two dimensional finite element hydraulic models have been developed to simulate river flood flows at high spatial and temporal resolutions over river reach lengths of 1 60 km such models have been shown to be capable of simulating bulk flow however model redesign to predict spatially distributed hydraulic variables has been constrained by lack of suitable topographic and hydraulic data here we begin this development process using a hypothetical river channel floodplain domain where the topographic surface is parameterized using scaling information derived from a fractal analysis of a real floodplain dtm this is used to test the relative effect of the boundary friction calibration numerical model grid resolution topography sampling error and floodplain relative height on model predictions of outflow discharge inundation extent and local hydraulic variables simulations indicate that model calibration is the dominant factor affecting the above three quantities moreover model sensitivity to spatially uniform change is shown to be simple for bulk flow and inundation extent but spatially complex for local hydraulics the study has a number of implications for model calibration and set up procedures as well as indicating the need to develop a new suite of analysis techniques for this class of model 1998 john wiley sons ltd

effects of lidar derived spatially distributed vegetation roughness on two dimensional hydraulics in a gravel cobble river at flows of 0 2 to 20 times bankfull abstract the spatially distributed effects of riparian vegetation on fluvial hydrodynamics during low flows to large floods are poorly documented drawing on a lidar derived meter scale resolution raster of vegetation canopy height as well as an existing algorithm to spatially distribute stage dependent channel roughness this study developed a meter scale two dimensional hydrodynamic model of 28 3 km of a gravel cobble bed river corridor for flows ranging from 0 2 to 20 times bankfull discharge with and without spatially distributed vegetation roughness results were analyzed to gain insight into stage dependent and scale dependent effects of vegetation on velocities depths and flow patterns at the floodplain filling flow of 597 49 m 3 s adding spatially distributed vegetation roughness parameters caused 8 0 and 7 4 increases in wetted area and mean depth respectively while mean velocity decreased 17 5 vegetation has a strong channelization effect on the flow increasing the difference between mid channel and bank velocities it also diverted flow away from densely vegetated areas on the floodplain vegetation stands caused high velocity preferential flow paths that were otherwise unaccounted for in the unvegetated model runs for the river as a whole as discharge increases overall roughness increases as well contrary to popular conception

pool riffle sedimentation and surface texture trends in a gravel bed stream a 3 year field campaign was completed to investigate spatial and temporal variability of sedimentation trends for a single pool riffle pair located in the santa cruz mountains california our measurements represent a range of hydrologic conditions over 11 sediment mobilizing events two different statistical methods were used to explore riffle sedimentation cochran s q and mcnemar s nonparametric tests one method indicate that riffle sediment surface texture was spatially and temporally varied at the transect level for mcnemar s test variation was significant at p 0 05 with several trends evident including strong riffle fining triggered by a 20 year flood event a nonlinear empirical orthogonal function method known as self organizing maps soms the second method shows that riffle sediment surface texture is well described by two characteristic temporal signals and one transitional signal at the sampling node level som mapping to each sampling node clearly shows riffle sediment surface texture change was spatially organized over the 11 sediment mobilizing events observations of pool sediment storage indicate that the pool riffle pair exhibited a coupled sedimentation response i e similar texture trends between pool and riffle following the 20 year flood the coupled response was characterized by a trend toward overall sedimentation conditions that were similar to those measured at the beginning of the study the reported texture trends may be of interest to salmonid habitat studies that examine factors contributing to successful versus unsuccessful fry emergence

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

coupling channel evolution monitoring and rfid tracking in a large wandering gravel bed river insights into sediment routing on geomorphic continuity through a riffle pool sequence bedload transport and bedform mobility in large gravel bed rivers are not easily monitored especially during floods large reaches present difficulties in bed access during flows for flow measurements because of these logistical issues the current knowledge about bedload transport processes and bedform mobility lacks field based information while this missing information would precisely match river management needs the lack of information linking channel evolution and particle displacements is even more striking in wandering reaches the durance river is a large wandering gravel bed river catchment area 14 280 km2 mean width 240 m located in the southern french alps and highly impacted by flow diversion and gravel mining in order to improve current understanding of the link between sediment transport processes and river bed morphodynamics we set up a sediment particle survey in the channel using radio frequency identification rfid tracking and topographic surveys gps rtk and scour chains for a 4 year recurrence interval flood by combining topographic changes before and after a flood intraflood erosion deposition patterns from scour chains differential routing of tracer particles and spatial distribution of bed shear stress through a complex reach this paper aims to define the critical shear stress for significant sediment mobility in this setting gravel tracking highlights displacement patterns in agreement with bar downstream migration and transport of particles across the riffle within this single flood event because no velocity measurements were possible during flood a telemac three dimensional model helped interpret particle displacements by estimating spatial distribution of shear stresses and flow directions at peak flow although rfid tracking in a large wandering gravel bed river does have some technical limitations burial recovery process time consuming it provides useful information on sediment routing through a riffle pool sequence

velocity reversal hypothesis a model approach hydraulic modelling of a riffle pool riffle sequence in a straight reach of dry creek near winters california indicates that while at low flow the mean section average velocity at riffles exceeds that of an adjacent pool during high flow the mean velocity of the pool exceeds that of adjacent riffles this supports the velocity reversal hypothesis which is based on limited field measurements of near bed flow velocity a velocity reversal does not occur at the same discharge in both riffle pool sequences modelled and a more detailed explanation is necessary to account for complex aspects of channel form and process in riffle pool sequences nevertheless the velocity reversal hypothesis helps to explain many of the observations and processes that operate in the riffle pool environment

objective identification of pools and riffles this paper presents a new technique for objectively identifying pools and riffles the technique is termed the bed form differencing technique and as its data base has bed elevations surveyed at a fixed interval along the channel these elevations are differenced and the difference or elevation change values are used to identify pools and riffles where the cumulative elevation change since the last bed form exceeds a certain tolerance value a new bed form is considered to exist the type of bed form being determined by the direction of the elevation change analysis of a small stream in western new york suggests that the most realistic pool riffle sequence is obtained where the tolerance value is 0 75 times the standard deviation of the difference values the bed form differencing technique can be used to identify bed forms other than pools and riffles such as ripples and dunes by simply modifying the surveying interval to reflect the scale of the bed forms

revealing the natural complexity of topographic change processes through repeat surveys and decision tree classification title revealing the natural complexity of topographic change processes through repeat surveys and decision tree classification short title decision tree classification of topographic change processes authors joshua r wyrick a gregory b pasternack b a lafayette college 740 high street easton pa 18042 b university of california davis one shields drive davis ca 95616 corresponding author tel 1 610 330 5735 e mail wyrickj lafayette edu fax 1 keywords topographic change dem differencing river morphology regulated rivers geomorphic change abstract topographic change processes tcps are the mechanisms by which a landscape is interpreted to be experiencing landform deformation and are defined by the specific actions occurring within a contiguous localized region that cause sediment to be either deposited or eroded past topographic change studies have mostly been focused at the site scale the goal of this study was to identify and delineate spatially explicit tcp

flow and turbulence redistribution in a straight artificial pool 1 multiple hypotheses have been advanced to explain the occurrence of pools in gravel bed rivers these hypotheses were developed without a hydrodynamic model of how open channel flow is affected by pools and it is not clear why and when the flow phenomena they describe might occur laboratory experiments are warranted to improve our understanding of how a gradual convective deceleration and acceleration of the flow without flow separation redistributes flow and turbulence in an open channel experiments are conducted in a 1 5 m wide flume with a 0 25 m deep 7 29 m long straight pool entry and exit slopes of 5 vertical side walls and gravel sediment d50 9 9 mm three dimensional velocity components are recorded at 50 hz using nortek vectrinos velocity and reynolds stress profiles in the channel centerline agree with previous results in nonuniform flow and include increased reynolds stress during deceleration and high velocity near the bed during acceleration lateral flow convergence occurs where depth is increasing which demonstrates that convergence is induced during flow deceleration and does not require a lateral flow constriction turbulence during deceleration is characterized by sweeps angled toward the sidewall of the channel an effect that could lead to the formation of a nonuniform pool depth through lateral gradients in the deposition of mobile sediment a conceptual model of pool hydrodynamics is proposed that includes increased turbulence near bed acceleration and lateral flow convergence as linked aspects of convective deceleration and acceleration due to depth changes in the pool

flow and form in rehabilitation of large river ecosystems an example from the lower missouri river abstract on large intensively engineered rivers like the lower missouri the template of the physical habitat is determined by the nearly independent interaction of channel form and flow regime we evaluated the interaction between flow and form by modeling four combinations of modern and historical channel form and modern and historical flow regimes the analysis used shallow slow water shallow water habitat swh defined as depths between 0 and 1 5 m and current velocities between 0 and 0 75 m s as an indicator of habitat that has been lost on many intensively engineered rivers and one that is thought to be especially important in rearing of young fishes two dimensional hydrodynamic models for modern and historical channels of the lower missouri river at hermann missouri indicate substantial differences between the two channels in total availability and spatial characteristics of swh in the modern channel swh is maximized at extremely low flows and in overbank flows whereas the historical channel had substantially more swh at all discharges and swh increased with increasing discharge the historical channel form produced 3 7 times the swh area of the modern channel regardless of flow regime the effect of flow regime is evident in increased within year swh variability with the natural flow regime including significant seasonal peaks of swh associated with spring flooding comparison with other reaches along the lower missouri river indicates that a channel form is the dominant control of the availability of habitat even in reaches where the hydrograph is more intensively altered and b rehabilitation projects that move toward the historical condition can be successful in increasing topographic diversity and thereby decreasing sensitivity of the availability of habitat to flow regime the relative efficacy of managing flow and form in creating swh is useful information toward achieving socially acceptable rehabilitation of the ecosystem in large river systems

effect of cross section interpolated bathymetry on 2d hydrodynamic model results in a large river two dimensional 2d hydrodynamic models have been increasingly used to quantify aquatic habitat and stream processes such as sediment transport streambed morphological evolution and inundation extents because river topography has a strong influence on predicted hydraulic conditions 2d models require accurate and detailed bathymetric data of the stream channel and surrounding floodplains besides collection of mass points to construct high resolution three dimensional surfaces bathymetries may be interpolated from cross sections however limited information is available on the effects of cross section spacing and the derived interpolated bathymetry on 2d model results in large river systems here we investigated the effects of cross section spacing on flow properties simulated with 2d modeling at low medium and high discharges in two morphologically different reaches a simple almost featureless with low sinuosity and a complex presenting pools riffles runs contractions and expansions reach of the snake river idaho usa the tenth largest river in the united states in terms of drainage area we compared the results from 2d models developed with complete channel bathymetry acquired with multibeam sonar data and photogrammetry with 2d model results that were developed using interpolated topography from uniformly distributed transects results indicate that cross sections spaced equal to or greater than 2 times the average channel width w smooths the bathymetry and suppresses flow structures conversely models generated with cross sections spaced at 0 5 and 1 w have stream flow properties sediment mobility and spatial habitat distribution similar to those of the complete bathymetry furthermore differences in flow properties between interpolated and complete topography models generally increase with discharge and with channel complexity copyright 2013 john wiley sons ltd

an appraisal of the velocity reversal hypothesis for stable pool riffle sequences in the river severn england the velocity reversal hypothesis is the linchpin for a number of recent conceptual models pertaining to sediment sorting and the maintenance of pool riffle sequences in gravel bedded streams the literature in support of the hypothesis together with published adverse criticism is reviewed it is concluded that convincing evidence for the ubiquitous occurrence of such a reversal in a range of channel geometries is currently unavailable continuity considerations indicate that riffles need to be considerably wider than pools for a reversal in the mean velocity to occur under conditions of subcritical flow high stage and stable morphology these observations are substantiated by a detailed study of the hydraulic geometry of stable pool riffle sequences in the river severn england neither the sectionally averaged velocity nor the near bed shear velocity is sensibly greater in the pools than over the riffles during bankfull or near bankfull flow instead a tendency towards equalization of the values of average hydraulic variables is noted as discharge increases a detailed investigation of the three dimensional character of the flow is required to demonstrate whether the entrainment forces within pools can locally exceed those over neighbouring riffles unusual behaviour of the energy gradient over riffles during moderate discharges is related to backwater effects as mediated by the spacing of the riffles the hydraulic data are used to comment on the stability of the test reaches in the context of the development of the river severn in the vicinity of shrewsbury

experiments on the morphological controls of velocity inversions in bedrock canyons a better understanding of bedrock incision mechanisms and processes is essential to the study of long term landscape evolution yet little is known about flow dynamics in bedrock rivers limiting our ability to make realistic predictions of local bedrock incision rates a recent investigation of flow through bedrock canyons of the fraser river revealed that plunging flows defined by the downward directed movement of near surface flow toward the channel bed occur in channels that have low width to depth ratios plunging flows occur into deep scour pools which are often coincident with lateral constrictions and channel spanning submerged ridges sills a phenomenological investigation was undertaken to reproduce the flow fields observed in the fraser canyons and to explore morphological controls on the occurrence and relative strength of plunging flow in bedrock canyons our observations show that the plunging flow structure can be produced along a scour pool entrance slope by accelerating the flow at the canyon entrance either over submerged sills or through lateral constrictions plunging flow appears to be a function of convective deceleration into a scour pool which can be enhanced by sill height the amount of the channel width that is constricted pool entrance slope discharge and a reduction in channel width to depth ratio plunging flow greatly enhances the potential for incision to occur along the channel bed and is an extreme departure from the assumptions of steady uniform flow in bedrock incision models highlighting the need for improved formulations that account for fluid flow

revealing the natural complexity of fluvial morphology through 2d hydrodynamic delineation of river landforms abstract fluvial landforms at the morphological unit scale 1 10 channel widths are typically delineated and mapped either by breaking up the one dimensional longitudinal profile with no accounting of lateral variations or by manually classifying surface water patterns and two dimensional areal extents in situ or with aerial imagery mapping errors arise from user subjectivity varying surface water patterns when the same area is observed at different discharges and viewpoints and difficulty in creating a complete map with no gaps or overlaps in delineated polygons this study presents a new theory for delineating and mapping channel landforms at the morphological unit scale that eliminates in field subjective decision making adds full transparency for map users and enables future systemic alterations without having to remap in the field delineation is accomplished through a few basic steps first near census topographic and bathymetric data are used in a two dimensional hydrodynamic model to create meter scale depth and velocity rasters for a representative base flow second expert judgment and local knowledge determine the number and nomenclature of landform types as well as the range of base flow depth and velocity over each type this step does require subjectivity but it is transparent and adjustable at any time third the hydraulic landform classification is applied to hydraulic rasters to quickly completely and objectively map the planform pattern of laterally explicit landforms application of this theory will reveal the true natural complexity yet systematic organization of channel morphology

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

modelling three dimensional flow structures and patterns of boundary shear stress in a natural pool riffle sequence fluid sediment interactions control river channel forms and processes analysis of spatial hydraulic patterns and the resulting boundary shear stress are required to aid understanding of river system behaviour in this paper the hydraulic processes active in a natural pool riffle sequence are simulated using a three dimensional computational fluid dynamics cfd model methods employed for the prescription of model boundary conditions are outlined model calculations are assessed using comparisons with field observations acquired over a range of flows simulations are then used to illustrate flow structures and patterns of boundary shear stress for a near bankfull and an intermediate flow event results are used to assess existing theories that seek to explain the development and maintenance of pool riffle sequences simulated results suggest that near bed velocities and bed shear stresses decrease on riffles and increase in pools as discharge increases model simulations indicate that secondary flow acts to route near bed flow over the downstream side of riffles and into the pool head away from the centre of pools implications for sediment transport and pool maintenance are discussed

experiments on hydraulic jumps in turbidity currents near a canyon fan transition the point at which a submarine canyon debouches on its associated abyssal fan is generally characterized by a drop in channel slope turbidity currents of the kind responsible for the genesis of the canyon and fan should display an internal hydraulic jump near the slope transition no direct field observations of any such jump appear however to have been made experiments on the nature of the jump and the resulting sedimentary deposits indicate that the thickness of the deposits just downstream of the jump tends to increase as the ratio of bed shear velocity immediately behind the jump to particle fall velocity decreases

investigating the behaviour of two dimensional finite element models of compound channel flow this paper describes results from a recent study concerning the numerical modelling of compound channel flow using two generalized two dimensional finite element codes specifically adapted to floodplain studies rma 2 and telemac 2d by application to an 11 km reach of the river culum devon uk simulations are developed to investigate the impact of numerical technique mesh resolution and topographic parameterization on model results the research is shown to raise a number of issues concerning the construction calibration and validation of two dimensional finite element models for this flow problem 1997 by john wiley sons ltd

hydraulic jumps in sediment driven bottom currents laboratory experiments were conducted to observe the behavior of turbidity currents in the vicinity of a slope transition both sediment laden and saline hydraulic jumps were produced the vertical structure of the currents was found to depend on flow regime the saline and turbid hydraulic jumps showed similar characteristics the amount of water entrained by the flows through a jump was small the change in flow regime caused a marked reduction of the bed shear stress downstream of the jump in nature a turbidity current experiencing a hydraulic jump will drop most of its bedload immediately downstream from the jump while the suspended load will respond more gradually to the change in flow regime and will deposit sediment over a distance far exceeding 1 000 times the jump height

numerical model of turbidity currents with a deforming bottom boundary a numerical model of turbidity currents with a deforming bottom boundary has been developed the model predicts the vertical structure of the flow velocity and concentration as well as change in the bed level due to erosion and deposition of suspended sediment the reynolds averaged navier stokes equations for dilute suspension have been solved using a finite volume method the bottom boundary and the grid system are allowed to adjust in response to sediment deposition and entrainment during the computation the model has been applied to simulate the evolution of a conservative saline density current and turbidity currents along an 11 6 m long flume that includes a slope followed by a horizontal bed the model successfully simulates the evolution of the currents model results have been compared with the experimental data good similarity profiles of velocity and excess density or suspended sediment concen tration are obtained at both the upstream supercritical and the downstream subcritical flow regions a turbulent schmidt number larger than one has been found to be appropriate for providing a good match with the experimental data changes in bed level predicted by the model have also been found to be in agreement with the experiment data

virtual manipulation of topography to test potential pool riffle maintenance mechanisms in this study numerical experimentation with two dimensional hydraulic modeling of pool riffle river topography drawing on the testbed data from the classic keller 1971 study was used to investigate the effect of synthetically manipulating topography on the occurrence and magnitude of velocity and shields stress reversals in a pool riffle sequence reversals in velocity and shear stress have been used to explore mechanisms of pool riffle maintenance while shields stress a combined measure of transport capacity and substrate erodibility is emerging in importance the original site topography was modeled alongside six altered ones to evaluate the sensitivity of hydraulic reversals to subtle morphology five incrementally wider pools and a filled pool the caamano 2009 criterion a simplified geometric threshold for predicting velocity reversals was applied to each terrain to evaluate its utility the original pool riffle topography was just over the threshold for a velocity reversal and well over the threshold for a strong shields stress reversal overall pool widening caused a predominantly local response with change to pool hydraulics and no change in section averaged velocity in the riffle beyond the initial widening of 10 filling in the pool significantly increased the magnitude of reversals whereas expanding it eliminated the occurrence of a reversal in mean velocity though the shields stress reversal persisted because of strong differentiation in bed material texture using shields stress as a reversal parameter enabled the quantification of pool modification effects on pool riffle resiliency the caamano 2009 criterion accurately predicted reversal occurrence for the altered terrains with exaggerated effects but failed to predict the weak reversal for the original topography two dimensional modeling coupled with previously accepted hydrologic geomorphic and engineering analyses is vital in project design and evaluation prior to construction

spontaneous formation and degradation of pool riffle morphology and sediment sorting using a simple fractional transport model many gravel bed streams have a typical bed morphology consisting of pool riffle sequences which provides important habitat diversity both in terms of flow and substrate a complete explanation of pool riffle genesis and self maintenance remains elusive and despite advances in understanding the effects of flow spatial and temporal variability the key sediment processes have been only marginally explored here we use a 1d unsteady multi fraction morphodynamic model to explain the formation and degradation of pool riffle sequences using a 1 year time series of measured flows below bankfull on a stream in which we have removed initial bedforms and sediment sorting our model spontaneously generates pools with finer substrate at narrow sections and riffles with coarser sediment at wider sections closely resembling the natural bed morphology additional experiments show that under our modelling assumptions a variable flow regime is fundamental for development and self maintenance of the longitudinal grain sorting characteristic of pool riffle sequences which could not be obtained or maintained with discharges held constant over relatively long periods

conceptual model of sediment processes in the upper yuba river watershed sierra nevada ca this study examines the development of a conceptual model of sediment processes in the upper yuba river watershed and we hypothesize how components of the conceptual model may be spatially distributed using a geographical information system gis the conceptual model illustrates key processes controlling sediment dynamics in the upper yuba river watershed and was tested and revised using field measurements aerial photography and low elevation videography field reconnaissance included mass wasting and channel storage inventories assessment of annual channel change in upland tributaries and evaluation of the relative importance of sediment sources and transport processes hillslope erosion rates throughout the study area are relatively low when compared to more rapidly eroding landscapes such as the pacific northwest and notable hillslope sediment sources include highly erodible andesitic mudflows serpentinized ultramafics and unvegetated hydraulic mine pits mass wasting dominates surface erosion on the hillslopes however erosion of stored channel sediment is the primary contributor to annual sediment yield we used gis to spatially distribute the components of the conceptual model and created hillslope erosion potential and channel storage models the gis models exemplify the conceptual model in that landscapes with low potential evapotranspiration sparse vegetation steep slopes erodible geology and soils and high road densities display the greatest hillslope erosion potential and channel storage increases with increasing stream order in channel storage in upland tributaries impacted by hydraulic mining is an exception reworking of stored hydraulic mining sediment in low order tributaries continues to elevate upper yuba river sediment yields finally we propose that spatially distributing the components of a conceptual model in a gis framework provides a guide for developing more detailed sediment budgets or numerical models making it an inexpensive way to develop a roadmap for understanding sediment dynamics at a watershed scale published by elsevier b v

synthetic river valleys creating prescribed topography for form process inquiry and river rehabilitation design abstract the synthesis of artificial landforms is complementary to geomorphic analysis because it affords a reflection on both the characteristics and intrinsic formative processes of real world conditions moreover the applied terminus of geomorphic theory is commonly manifested in the engineering and rehabilitation of riverine landforms where the goal is to create specific processes associated with specific morphology to date the synthesis of river topography has been explored outside of geomorphology through artistic renderings computer science applications and river rehabilitation design while within geomorphology it has been explored using morphodynamic modeling such as one dimensional simulation of river reach profiles two dimensional simulation of river networks and three dimensional simulation of subreach scale river morphology to date no approach allows geomorphologists engineers or river rehabilitation practitioners to create landforms of prescribed conditions in this paper a method for creating topography of synthetic river valleys is introduced that utilizes a theoretical framework that draws from fluvial geomorphology computer science and geometric modeling such a method would be valuable to geomorphologists in understanding form process linkages as well as to engineers and river rehabilitation practitioners in developing design surfaces that can be rapidly iterated the method introduced herein relies on the discretization of river valley topography into geometric elements associated with overlapping and orthogonal two dimensional planes such as the planform profile and cross section that are represented by mathematical functions termed geometric element equations topographic surfaces can be parameterized independently or dependently using a geomorphic covariance structure between the spatial series of geometric element equations to illustrate the approach and overall model flexibility examples are provided that are associated with mountain lowland and hybrid synthetic river valleys to conclude recommended advances such as multithread channels are discussed along with potential applications

observations of flow and sediment entrainment on a large gravel bed river constant discharge reservoir releases on the trinity river california provide an unusual opportunity to unambiguously relate flow and gravel entrainment on a large gravel bed river bed shear stress 0 was estimated using local observations of depth averaged velocity gravel entrainment was measured using large tracer gravel installations lateral variability of 0 is large even for straight channels with simple trough like geometry no simple relation exists between local and cross section mean values of 0 fine grains less than 8 mm 20 30 of the bed material are transported at lower discharges than coarse grains scour to the base of the bed surface layer occurs at a dimensionless shear stress g 0 035 for g formed using local 0 and the median grain size of the gravel portion of the bed the dimensionless reference transport rate w 0 002 often used as a surrogate for the threshold of grain motion occurs at nearly the same g at smaller g entrainment and transport rates decrease rapidly becoming vanishingly small at g 0 031 even at very small gravel transport rates all sizes are transported although the coarsest sizes are in a state of partial transport in which only a portion of the exposed grains are entrained both entrainment and cumulative transport observations suggest that maximum scour depth for plane bed transport is slightly less than twice the surface layer thickness

the critical densimetric froude number of subaqueous gravity currents can be non unity or non existent abstract for decades the critical densimetric froude number of subaqueous gravity flows has been regarded to be unity or close to unity the same as the critical froude number for open channel flows based on the analysis of specific energy a concept borrowed from hydraulic engineering it is shown that two correction terms the discharge correction and the concentration correction appear in the formulation of the critical densimetric froude number for subaqueous gravity flows in a channel as a result the critical densimetric froude number of a subaqueous gravity flow in a channel can be non unity or non existent depending on the degree of entrainment of ambient fluid and the density variation in the currents furthermore it is shown that the lower the critical densimetric froude number is in a supercritical subaqueous gravity flow the bigger the internal jump is if the flow experiences a change in flow regime

hydrologic and topographic variability modulate channel change in mountain rivers summary the relationships between flow hydrology topography and channel change in mountain rivers is important to understanding landscape evolution the structure and persistence of aquatic habitat and also the physiochemical cycling of upstream derived organic and inorganic materials there is a paucity of detailed studies that analyze the joint roles of hydrology and topography in controlling multiple mechanisms of channel change in mountain rivers in this study gravel and cobble channel change in a bedrock river canyon were analyzed in light of a controlled yet natural experiment where 4491 metric tonnes of rounded gravel and cobble was augmented below a sediment barrier dam in a 1200 m long mountain river reach that had no prior sources of rounded gravel or cobble and still experiences floods above the bankfull discharge the overall study goal was to investigate how flow hydrology can modulate multiple channel change processes depending on the topographic features engaged by the flow channel change was assessed via differencing of high resolution repeat topographic and bathymetric surveys along with cm scale aerial photography post injection statistical tests used to implicate topographic feature specific mechanisms of channel change that vary with discharge included analyzing geomorphic covariance structures of flow dependent width bed elevation and channel change as well as autocorrelation of flow width spatial series stage dependent topographic steering was inferred from associations of erosion and deposition with changes in 2d model derived flow directions at multiple discharges a variety of mechanisms of channel change were qualitatively and quantitatively confirmed including particle hiding topographic steering eddying and flow convergence no single mechanism explained the observed patterns of channel change but rather it is thought that process blending occurs as modulated by the interactions of flow hydrology with complex topography results from this study suggest that both existing channel boundary variability and input hydrologic variability work together to create hydrodynamic spatial patterns that control the fate and transport of sediments in mountain rivers and ultimately their spatial structure

the application of computational fluid dynamics to natural river channels three dimensional versus two dimensional approaches this paper addresses the recent trend in fluvial geomorphology towards using computational fluid dynamics to explore the adjustment between flow processes sediment transport and river channel morphology it aims to evaluate the extent to which three dimensional 3d models improve predictive ability and prediction utility compared to two dimensional 2d applications this is achieved through comparing the predictions of both 3d and 2d models with high quality field data identical boundary conditions obtained from a confluence within a gravel bed river system with high relative roughness are defined for each model evaluation of the 3d model suggests that there is a fundamental limitation upon model predictive ability due to problems of specifying topographic complexity however comparison with the 2d model shows that the 3d model has a higher predictive ability particularly if the 2d model is not corrected for the effects on flow structure of secondary circulation further the 3d model provides more reliable estimates of bed shear stress and other more useful information such as the three dimensional flow field important for mixing processes this suggests that there is significant merit in the move towards 3d models but that research is required to incorporate methods developed in other fields for dealing with boundary condition uncertainties

error propagation for velocity and shear stress prediction using 2d models for environmental management pasternack g b gilbert a t wheaton j m buckland e m 2006 error propagation for velocity and shear stress prediction using 2d models for environmental management journal of hydrology 328 1 2 227 241 sponsorship us fish and wildlife service contracting entity for calfed bay delta ecosystem restoration program cooperative agreement dcn 113322g003