reservoir sedimentation i delta and density current deposits in china where rivers draining loess soils have some of the highest sediment concentrations in the world reservoir sedimentation is a serious problem that has been the subject of intensive investigation since 1957 this paper describes patterns of sediment deposition and erosion drawing on field measurements in china at the sanmenxia guanting shanyiujiang and liujiaxia reservoirs as well as the shanshenggorig barrage and xijin hydropower station the longitudinal and lateral geometry of reservoir deltas is described it is shown that a marked change in grain size can occur at the downstream limit of the delta even when this limit is not apparent from geometry density current data for guanting reservoir are presented problems caused by reservoir sedimentation in china are described including aggredation in the backwater region causing increased flood levels elevated ground water levels and navigation impairment deposition from or into a tributary entry of sediments into hydropower turbines a

early devonian eurypterids from the northwest territories of arctic canada a new eurypterid fauna from the lower bear rock formation early devonian emsian of anderson river in the northwest territories of the canadian arctic is described the material comprises an almost complete specimen and five isolated carapaces of erieopterus microphthalmus an incomplete carapace and telson referred to drepanopterus sp and an isolated prosomal appendage of carcinosoma sp associations include actinopterygian sarcopterygian and acanthodian fish as well as lingulids conchostracans ostracodes coprolites and plant material a nearshore marine environment is inferred this assemblage provides the first canadian record of drepanopterus and the youngest canadian occurrences of erieopterid and carcinosomatid eurypterids

modeling decadal bed material sediment flux based on stochastic hydrology estimates of decadal bed material sediment flux and net storage are derived by driving sediment transport calculations with a stochastic hydrology model the resulting estimates represent the whole distribution of sediment flux based on natural variability in channel characteristics gradient width and bed grain size and the magnitude duration and interarrival time of flood events a procedure for calibrating a fractional sediment transport equation of a commonly used form to bed material grain size distributions bmgsds at cross sections is presented the procedure was applied to the sacramento river channel network to compute estimates of annual total and annual peak bed material discharges into and through the main stem over a 30 year period main stem bed material budgets were evaluated to identify reaches in states of net accumulation or scour simulations highlight large imbalances in sand and gravel storage throughout the sacramento river which can be explained by a combination of local hydraulics and bmgsds and for which there is at least some empirical support

yvonaspis nouveau genre d osteostraci vertebrata agnatha du devonien emsien eifelien des gres de gaspe quebec canada all osteostracan species of the gaspe sandstones devonian of canada formerly attributed to the genus cephalaspis are referred here to the new genus yvonaspis the latter is characterized by the very elongated shape of the naso hypophysal opening this monophyletic genus may be the sister group of parameteoraspis among cephalaspidians

numerical modeling of bed deformation in laboratory channels a depth average model using a finite volume method with boundary fitted grids has been developed to calculate bed deformation in alluvial channels the model system consists of an unsteady hydrodynamic module a sediment transport module and a bed deformation module the hydrodynamic module is based on the two dimensional shallow water equations the sediment transport module is comprised of semiempirical models of suspended load and nonequilibrium bedload the bed deformation module is based on the mass balance for sediment the secondary flow transport effects are taken into account by adjusting the dimensionless diffusivity coefficient in the depth average version of the k e turbulence model a quasi three dimensional flow approach is used to simulate the effect of secondary flows due to channel curvature on bed load transport the effects of bed slope on the rate and direction of bed load transport are also taken into account the developed model has been validated by computing the scour hole and the

experimental study of the transport of mixed sand and gravel we measured a wide range of transport rates for five different sand gravel mixtures in a laboratory flume each mixture used the same gravel and sand was added to produce mixtures containing 6 15 21 27 and 34 sand control of other variables allows us to isolate the effect of bed sand content on transport as sand content increases gravel transport rates increase by orders of magnitude even though the proportion of gravel in the bed decreases the increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27 the increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content bed surface grain size was measured at the end of each run surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate this casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate

model analyses of the contribution of in channel processes to sediment concentration hysteresis loops summary sediment concentration sc water discharge q relations in rivers are typically governed by multiple and relatively complex processes due to hysteresis effects sediment discharges can differ for similar or equivalent water discharges which causes scatter in empirical datasets and may decrease the predictive power of sc rating curves such hysteresis effects must therefore be understood and accounted for to make dependable predictions for river system management the overall objectives of this study are to develop modelling approaches suitable for reproducing and predicting hysteresis effects at larger scales and to investigate the possible contribution of in channel processes erosion and deposition to sediment concentration hysteresis loops to investigate relevant field scale conditions we develop a one dimensional dynamic sediment transport model of the downstream tuul river northern mongolia investigating in channel processes along a 141 km stretch during a hydrological year the results show that the present modelling approach can reproduce both anti clockwise and clockwise hysteresis effects importantly in channel processes alone can cause considerable anti clockwise hysteresis effects without being reinforced by catchment processes such as hillslope erosion such specific contributions from in channel processes introduced data scatter into the sediment rating curves decreasing their r 2 values from unity to approximately 0 5 to 0 6 more generally possible changes in the number or magnitude of high flow events caused by climatic or other anthropogenic factors could influence total sediment deposition which was primarily found to occur during relatively short high flow events such potential changes also have important implications for the possible spreading of polluted sediments

probable tetrapod tracks rediscovered in the devonian of n scotland the discovery of probable tetrapod tracks in the upper old red sandstone of easter ross n scotland lends credibility to 19th century claims of reptilian footprints in these rocks these are the only probable amphibian fossils known in the west european devonian and are amongst the oldest anywhere

selecting geomorphic variables for automatic river segmentation trade offs between information gained and effort required abstract identifying river segments with apparently distinct geomorphic characteristics but relatively homogeneous internal features may be critically helpful in designing network analysis for characterization environmental assessment and river management automatic segmentation procedures using geographic tools and statistical methods provide objective and replicable results in particular multivariate procedures may be appropriated for different purposes such as coping with the multiple dimensionality of river systems although there is an increasing number of studies dealing with segmentation using different sets of morphological variables the influence that the selected variables have on segmentation results is rarely assessed in this context we defined five combinations of frequently used geomorphic variables i e channel slope active channel width valley bottom width channel confinement and specific stream power and compared the obtained segmentation results we study the upper esla river network covering a total length of 294 km with the largest two rivers regulated by large dams variables were measured at successive river sections 200 m apart five segmentation results were obtained in which we compared the number and characteristics of the segments and the internal variability and the suitability of predicting river dynamics i e occurrence of bare and vegetated gravel bars the number of segments per kilometer of river and their average length were different among segmentations but varied much more across rivers than across segmentations in general segmentations including channel slope and active channel width performed better in predicting the occurrence of bare gravel bars than segmentations based on stream power or valley confinement when splitting the initial data set into regulated and non regulated segments differences in predicting the occurrence of gravel bars were found with better results in the case of non regulated rivers channel slope and active channel width showed a reduced explanatory power for the regulated reaches finally we conclude that primary geomorphic variables such as channel slope and active channel width were more efficient than secondary variables such as stream power which may encompass more information but needed additional data and the use of empirical models with greater effort and much uncertainty in the case of regulated rivers automatic segmentation including the affected variables e g active channel width may help in detecting differences in geomorphic sensitivity to river adjustments across the resulting segments downstream from the dams our results offer valuable insights into the selection of geomorphic variables for river segmentation analysis in which trade offs between the information gained and the effort required must be considered according to the respective research targets

s to p scattering at the 650 km discontinuity summary a search of seismograms recorded at the warramunga seismic array wra from events occurring below the izu bonin islands shows an arrival on some but not all of the records with an onset at 25 30 s after p which is not predicted by the standard travel time tables the slowness and azimuth of the phase show that it is generated almost in line with p and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source it appears in fact to be an s to p conversion at a depth of 650 700 km which is seen only when the receiver is close to a node of the p radiation pattern and an antinode for s so that its amplitude compared with that of p is at a maximum finally the duration of the phase indicates that it is not simply a refracted wave but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the izu bonin islands

hydraulic controls of grain size distributions of bedload gravels in oak creek oregon usa grain size distributions of gravels transported as bedload in oak creek oregon show systematic variations with changing flow discharges at low discharges the gravel distributions are nearly symmetrical and gaussian as discharges increase the distributions become more skewed and follow the ideal rosin distribution the patterns of variations are established by goodness of fit comparisons between the measured and theoretical distributions and by q mode factor analysis two end members are obtained in the factor analysis having respectively almost perfect gaussian and rosin distributions and the percentages of the two end members within individual samples vary systematically with discharge transformation from gaussian to rosin distribution with increasing discharge may be explained by processes of selective entrainment of grains from a bed of mixed sizes samples of bed material in oak creek follow the rosin distribution at high discharges the transported bedload approaches the grain sizes of that bed material source and mimics its rosin distribution random selection processes must be more important to grain entrainment at lower discharges so that the resulting gaussian distributions of transported bedload reflect similar distributions of bed stresses exerted by the stream flow the results from oak creek demonstrate that the competence of the flow is reflected in the entire distribution of transported gravel sizes a sequence of layers of fluvial gravels modern or ancient might show systematic variations between coarse rosin and finer grained gaussian distributions and these could be used to infer frequencies of various discharges and to establish a relationship to the source sediment with further study analyses of changing bedload grain size distributions and their transport rates will lead to a better understanding of downstream variations in grain sizes of bed sediments and how their distributions reflect the progressive development of textural maturity

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

modeling the evolution of incised streams i model formulation and validation of flow and streambed evolution components a robust computational model for simulating the long term evolution of incised and restored or rehabilitated stream corridors is presented the physically based model simulates the three main processes that shape incised streams hydraulics sediment transport and streambed and bank adjustments a generalized implicit preissmann scheme is used for the spatial and temporal discretization of the flow governing equations to accommodate large time steps and cross sections spaced at irregular intervals the solution method introduces several enhancements that increase its robustness specifically to simulate flashy flows transport of cohesive or cohesionless graded bed material is based on a total load concept and suspended and bed load transport modes are accounted for through nonequilibrium effects the model simulates channel width adjustment by hydraulic erosion and gravitational mass failure of heterogeneous bank material the present paper focuses mainly on the treatment of streamflow hydraulics and evolution of graded streambeds and reports simulations of published experiments on degrading and aggrading channels with graded bed material description and validation of the model s stream bank erosion component and the application of the model to incised stream systems are presented elsewhere

arthropod trackways from the early devonian of south wales a functional analysis of producers and their behaviour abundant arthropod trackways assigned to diplichnites gouldi are described from the lower old red sandstone early devonian lochkovian of pant y maes quarry brecon beacons south wales the trackways are preserved on bedding planes of finely laminated planar and rippled siltstones the sedimentology of the succession indicates that these units represent bar top and marginal deposits in a braided fluvial setting two trackway types are recognized type a and b comparisons with contemporaneous myriapodous producers favour kampecarid and eoarthropleurid myriapods respectively functional analysis of the trackways indicates that the producers were not using the most efficient stable walking techniques but instead utilized in phase swimming stroke like gaits together with their occurrence on rippled surfaces and lateral displacement of some trackways attributed to currents this indicates that they were produced sub aqueously

a one dimensional process based approach to study reservoir sediment dynamics during management operations a comprehensive understanding of redthe dynamics of erosion and sedimentation in reservoirs under different management conditions is required to anticipate sedimentation issues and implement effective sediment management strategies this paper describes a redunique approach combining fluvial geomorphology tools and morphodynamic modeling for analyzing the sediment dynamics of an elongated hydropower reservoir subjected to redmanagement operations the ga nissiat reservoir on the rha ne river redfunctional sub reaches representative of the reservoir morphodynamics were delineated by adapting natural river segmentation methods to elongated reservoirs the segmentation revealed the link between the spatial and temporal reservoir changes and the variability of longitudinal flow conditions during reservoir management operations an innovative modeling strategy incorporating the reservoir segmentation into two sediment transport codes was implemented to simulate the dynamics of erosion and sedimentation at the reach scale during historic events one code used a bed load approach based on the exner equation with a transport capacity formula and the other used a suspended load approach based on the advection dispersion equation this strategy provided a fair quantification of the dynamics of erosion and sedimentation at the reach scale during different management operations this study showed that the reservoir morphodynamics is controlled by bed load transport in upper reaches graded suspended load transport of sand in middle reaches and suspended load transport of fine sediments in lower reaches eventually it allowed a better understanding of the impact of dam management on sediment dynamics

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

modeling of riverbed evolution for bedload sediment mixtures a new computational methodology for fully coupled simulation of unsteady water and sediment movement in mobile bed alluvial rivers has been developed and tested the one dimensional approach treats bedload transport of nonuniform sediment mixtures with sorting and armoring effects considered spatial lag effects in nonequilibrium bedload transport are taken into account through innovative use of the recently developed notion of a loading law the new system of equations for water and sediment movement is solved in a fully coupled implicit manner using the preissmann finite difference scheme implemented to allow for either upstream or downstream induced mobile bed perturbations formal analysis of the coupled system sheds new light on boundary condition requirements for the multiple size class problem the fully coupled implicit solution has proven to be robust and stable and it respects the underlying volumetric conservation laws remarkably well as demonstrated in continuity analyses of computed bed e

ichnology and depositional environment of the middle devonian valentia island tetrapod trackways south west ireland abstract nine tetrapod trackways are described from the middle devonian continental valentia slate formation on the north eastern coast of valentia island in county kerry ireland the trackways occur at three sites dohilla coosadillisk and culoo head the latter two being recorded for the first time morphological and taphonomic analyses of the trackways suggest they were made by populations of similar tetrapods but of varying size body length 0 5 to 1 m width 0 15 to 0 30 m that moved in some cases by terrestrial locomotion and in others possibly by a slow paddling gait while the substrate was submerged by very shallow floodwater sedimentological facies analysis of the trackway bearing sequences has been carried out for the first time and shows that the preserved trackways are associated with the late stages of 1 a principal river channel margin coosadillisk 2 a minor floodplain drainage channel culoo head and 3 a sand rich crevasse splay medial alluvial ridge environment dohilla the valentia island tetrapods inhabited marginal environments of proximal river channels 170 km northwards into the continent from postulated shallow marine habitats as they evolved this capability over a potential 4 5 million year period during the middle devonian

design for hydraulic geometry of alluvial channels a new solution to the stable channel paradox has been derived based on the contribution of secondary flow to the redistribution of boundary shear stress this new solution was then combined with the entropy based bank profile equations the flow continuity condition an appropriate frictional resistance relation and a sediment transport relation to develop a new geometric model for straight stable alluvial channels furthermore the effect of nonuniform boundary particle sizes was introduced to make the new geometric model applicable to a wide range of both laboratory experiments and natural alluvial rivers a fortran program was developed to compute the design procedure the comparisons between the cross sectional shapes depths and widths calculated by the new geometric model and field and laboratory data show that the proposed geometric model is in reasonable agreement with the data both for well graded and for nonuniform bed materials

numerical simulation of contraction scour in an open laboratory channel a finite volume computer code developed at the institute for hydromechanics university of karlsruhe has been used to calculate the flow and sediment transport in a laboratory channel with constriction and movable bed the flow is calculated by solving the fully three dimensional reynolds averaged navier stokes equations with k e turbulence model the bed deformation is obtained from an overall mass balance equation for sediment transport and the bed load transport is simulated with a nonequilibrium model the calculated results for flow and scour development in the laboratory channel are compared with experimental measurements the sensitivity of the simulated results to the nonequilibrian adaptation length parameter in the nonequilibrium bed load transport model is investigated systematically which represents the main contribution of this paper

sediment transport modeling review current and future developments the use of computational models for solving sediment transport and fate problems is relatively recent compared with the use of physical models several considerations govern the choice between physical and computational models namely the nature of the problem that needs to be solved the available resources and the overall cost associated with the problem solution in some specific problems a combination of physical and computational models can be used to obtain a better understanding of the processes under investigation de vries 1973 using computational hydrodynamic sediment transport models in general involves the numerical solution of one or more of the governing differential equations of continuity momentum and energy of fluid along with the differential equation for sediment continuity an advantage of computational models is that they can be adapted to different physical domains more easily than physical models which are typically constructed to represent site specific conditions another advantage of computational models is that they are not subject to distortion effects of physical models when a solution can be obtained for the same flow conditions identical reynolds and froude numbers same length scale in the three directions etc as those present in the field with the rapid developments in numerical methods for fluid mechanics computational modeling has become an attractive tool for studying flow sediment transport and associated pollutant fate processes in such different environments as rivers lakes and coastal areas representative processes in these environments include bed aggradation and degradation bank failure local scour around structures formation of river bends fining coarsening and armoring of streambeds transport of point source and nonpoint

spatial disaggregation and aggregation procedures for characterizing fluvial features at the network scale application to the rhone basin france network scale heterogeneity in fluvial forms and processes has been analyzed for decades through the field based characterization of a reduced number of sites spatially continuous data sets are increasingly available at a national scale e g digital elevation models or aerial orthophotographs and open up new possibilities to investigate spatial structures in stream networks the paper introduces a gis methodological framework to support and automate the large scale characterization of the fluvial system based on spatial disaggregation and aggregation procedures the spatial disaggregation consists in the longitudinal discretisation of geographical objects into relatively high resolution spatial units to characterize fluvial features continuously along stream networks because rivers are heterogeneous and hierarchically organized the spatial aggregation aims to delineate statistically pattern or process based spatial units to answer questions at appropriate scales the paper develops the gis implementation of the disaggregation and aggregation procedures for characterizing stream networks in a geomorphic perspective we illustrate the approach on the rhone network focusing on the characterization of three geographical objects commonly used in classification schemes the streamline the valley bottom and the active channel the latter on a limited area the spatial aggregation is automated using the threshold test of pettitt 1979 to detect change points in univariate data series and delineate homogeneous spatial units throughout the stream network we provide three simple application examples to illustrate the potentialities of the spatial aggregation to investigate spatial heterogeneity in stream networks finally insights limitations and future challenging issues emerging from the application of the methodological framework are discussed

response of the ha ha river to the flood of july 1996 in the saguenay region of quebec large scale avulsion in a glaciated valley modifications to valley form due to extreme flooding in the lower 34 km of the ha ha river were analyzed heavy regional precipitation in july 1996 triggered extreme runoff and the catastrophic drainage of ha ha lake producing discharges of 1100 m3 s 8 times the 100 year flood dominant valley modifications revealed by comparing preflood and postflood topographies derived photogrammetrically were related to two large scale avulsions in particular a deep retrogressive incision which bypassed the 30 m high perron falls exporting 6 106 m3 of glacial stratified drift from a 2 km section of valley and producing massive sedimentation in the reach downstream reconstructed maximum flow power values support the existence of a 300 w m2 threshold for major scouring of the alluvial valley bottom the evidence highlights the potential for massive scour and fill and reorganization of the long profile with potentially catastrophic effects on infrastructure during extreme floods in glaciated valley settings

the gravel sand transition sediment dynamics in a diffuse extension as gravel bedded rivers fine in the downstream direction they characteristically exhibit an abrupt transition from gravel to sand bedded conditions the prevailing theory for why abrupt gravel sand transitions emerge is based on bed load sorting of a bimodal sediment the abruptness is thought to be a consequence of sand overwhelming the gravel sand mixture once it reaches a critical coverage on the bed the role suspension plays in the development of gravel sand transitions has not been fully appreciated the fraser river british columbia is an archetypical abrupt gravel sand transition with a diffuse extension composed of a sand bed with some patches of gravel we examine flow shear stress and suspended sediment flux in the diffuse extension to better understand sediment dynamics where the sand bed emerges sand is carried in suspension upstream of the primary abrupt gravel sand transition but in the diffuse extension sand is moved as both bed load and suspended load we do not observe downstream gradients in shear stress or suspended sand flux through the diffuse extension that would suggest a gradual rain out of sand moving downstream which raises the question how is the sand bed formed sediment advection length scales indicate that with the exception of very fine sand that moves as wash load in the diffuse extension fractions coarser than the median sand size cannot be carried in suspension for more than one channel width this suggests that sand is deposited en masse at the beginning of the diffuse extension forming a sediment slug at low flood flows that is smeared downstream at high flood flows to form the sand reach

a large arthropod trackway from the gaspe sandstone group middle devonian of eastern canada a large arthropod trackway from the cap aux os member of the battery point formation gaspe sandstone group middle devonian from the baie de gaspe eastern canada is described and assigned to the ichnotaxon palmichnium paleohelcura antarcticum gevers et al 1971 a large stylonurid eurypterid or scorpion is considered the most likely producer a shallow water marginal fluvial environment is inferred as the setting the animal making a transition from walking to swimming along the course of the trackway resume la trace d une piste d un grand arthropode dans le membre de cap aux os de la formation de battery point groupe gres de gaspe devonien moyen dans la baie de gaspe dans l est du canada est decrite et assignee a l ichnotaxon palmichnium paleohelcura antarcticum gevers et al 1971 il semble que la trace ait ete produite par un grand eurypteride stylonuride ou un scorpion on presume que c est dans un environnement fluviatile marginal sous une mince tranche d eau que l animal a creuse cette empreinte en marchant vers un lieu de nage traduit par la redaction braddy and milner 1122 a large arthropod trackway was discovered in 1988 milner 1992 along the northern shore of the baie de gaspe grid reference 20u lk 993112 approximately 5 km east of the town of gaspe near the village of cap aux os on the forillon peninsula in the province of quebec eastern can ada fig 1 the locality is within the cap aux os member of the battery point formation gaspe sandstone group approximately 750 m above the base of the formation in the lower part of unit 4 lawrence and rust 1988 fig 7 the battery point formation comprises a coarsening upwards clastic sequence ranging in age from early emsian to early eifelian middle devonian based on abundant spores oc curring throughout the formation mcgregor 1977 the cap aux os member approximately 640 m thick consists mainly of interbedded sandstones and mudstones which were deposited in a meandering fluvial system the prevail ing palaeocurrent direction varied from the north northwest to the west northwest lawrence and rust 1988 sand was deposited in the river channels and finer grained silts and muds on the extensive vegetated floodplain lawrence 1986

the drainage of the lake ha ha reservoir and downstream geomorphic impacts along ha ha river saguenay area quebec canada abstract on july 18 21 1996 a severe rainstorm caused widespread flooding along the north shore of the st lawrence river southern quebec canada particularly along rivers that drain the area just south of the saguenay lake st jean region at the lake ha ha reservoir inadequate available capacity to spill during the storm at the outlet dam resulted in the overtopping and erosion of a nearby earthfill saddle dyke a new outlet formed at the site of the dyke and drained the reservoir over a period of many hours decreasing its area from 8 1 to 4 7 km2 estimates of discharge range from 910 to 1380 m3 s 1 at the site of the eroded dyke to 1080 to 1260 m3 s 1 at a location 27 km downstream about 8 km above the mouth of the river the uncontrolled drainage of the lake ha ha reservoir increased flooding along the lower 35 km of ha ha river where flooding was already in progress because of the rainstorm runoff the flooding caused extensive geomorphic impacts along the river long sections of the river totalling 25 km experienced significant widening locally up to 280 m and channel incision locally up to 20 m while two reaches 6 and 4 5 km long experienced up to several metres of aggradation in general the slope of the valley was the most important variable affecting whether or not the energy of the flow was above or below the erosive threshold of the valley bottom locally a permanent channel diversion now exists where the drainage divide between the main river course and a small ravine was overtopped and extensively eroded communities infrastructure and industry located along the river were extensively damaged by the flood waters the effects of flooding along ha ha river demonstrate that rivers on the canadian shield can undergo severe geomorphic changes caused by very high magnitude flooding

surface sampling in gravel streams the accuracy of studies concerning the characteristics of gravel bed waterways is often dependent upon the techniques used to sample and quantify the material found on the channel boundary due to the vertical stratification present in the riverbed the use of sampling techniques that remove only the particles within a thin surface layer is often necessary attributes of clay and grid sampling are considered in the present work criteria for determining the minimum sample size for a desirable level of accuracy are also presented many of the current surface sampling techniques truncate either the upper or lower size range of particles truncation of part of a size distribution not only limits the available information but can also bias the rest of the distribution the present work rectifies this problem by proposing the use of a hybrid technique that can sample the entire size range of either dry or submerged bed material results of field tests that utilize this new method are described in detail

short period s p conversion in the mantle at a depth near 700 km summary data from 16 deep earthquakes originating in the tonga subduction zone and recorded at the warramunga seismic array wra in central australia show the presence of a distinct later arrival in the coda of p this phase which is not predicted by smooth velocity depth models of the earth has been called x in the paper slownesses and arrival azimuths of x are similar to those of the p wave it is unlikely that x was radiated as the result of multiple focal activity because a corresponding phase was not observed neither in the coda of scp nor on seismograms of seismic stations located at close epicentral distances relative to p the variation in arrival times with focal depth of the earthquakes indicates that x is produced by s p conversion below the source at a depth near 700 km most of the observed x phases have approximately the same signal duration as the p waves thus suggesting a transition zone near 700 km which is only a few kilometres thick synthetic seismograms calculated for a model with a first order seismic discontinuity at a depth of 700 km show reasonable agreement with the observations as demonstrated by the synthetic seismogram modelling the amplitude of x depends on the radiation pattern of the earthquake source variations in the radiation pattern offer a plausible explanation that first x is only intermittently detected at wra and secondly that x has most frequently been observed from the southern part of the tonga subduction zone

early devonian stylonurine eurypterids from arctic canada two new stylonurine eurypterids are described from the peel sound formation early devonian lochkovian of the northern coast of prince of wales island nunavut arctic canada associations including pteraspids and ostracodes indicate a fluvial depositional environment an almost complete stylonurid pagea plotnicki sp nov is recognized by its large size and lack of vaulting on the carapace and it provides evidence that stylonurus and pagea are sister taxa also a smaller incomplete rhenopterid assigned to leiopterella tetliei gen et sp nov is characterized by its broad turbinate carapace and lack of cuticular sculpture this assemblage provides the first canadian record of pagea and the youngest occurrence of a rhenopterid outside the rheno hercynian terrane indicating that these taxa were more geographically widespread than previously supposed

spatial and temporal patterns of sedimentation in an infilling reservoir abstract river dams are ubiquitous globally altering the connection of up and downstream environments in particular they store vast amounts of sediment in their upstream reservoirs many of which are approaching storage capacity there is much interest in sediment management for these systems even though quantifying sedimentation rates remains somewhat challenging this study focuses on one such system conowingo reservoir upstream of the last and largest dam conowingo dam on the susquehanna river before it enters chesapeake bay the trapping capacity of conowingo reservoir has decreased since its creation in 1928 potentially increasing fluvial loading to the bay in addition large flood events can scour much more than the average annual sediment load from the reservoir bottom delivering exceptionally large sediment loads to the bay within days predicting the downstream impacts of both reservoir infill and scour events is essential for management however these predictions are limited by understanding of the complex spatial and temporal variability of sedimentation to evaluate this variability sedimentation rates are determined throughout the reservoir over seasonal to decadal time scales with the naturally occurring radioisotopes 7be and 210pb half lives 53 3 days and 22 3 years respectively as well as anthropogenically produced 137cs half life 30 7 y as an independent geochronometer these rates are placed within the context of sediment character mud content and fluvial sediment supply to the reservoir as well as its geomorphology in general seasonal sedimentation rates scale with fluvial loads and reflect deposition of watershed derived sediment spatial patterns of seasonal sedimentation follow those of river deltas incised by channels while decadal scale sedimentation rates increase downstream episodic event sedimentation in channels is suggested by the character of 210pb profiles which indicate non steady state sedimentation that makes application of sediment age dating models challenging in this system although sediment mud content has clearly decreased as the reservoir has filled observed changes in sedimentation rates may reflect biases of these models rather than physical changes

sedimentation within a regulated river channel change induced by tributary sediment injection to a regulated gravel bed river is described for the river rheidol u k eighteen years after dam closure morphological and sedimentological data are presented for an aggrading reach a primary lobate bar appears to have initiated flow separation and the subsequent construction of channel side berms hydraulic sorting has operated to produce both longitudinal and lateral particle size reduction and selective particle size dispersal appears to be an important process influencing channel change the channel is adjusting to a lower width depth ratio form of smaller capacity and with a steeper slope

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

assessing channel response of a long river influenced by human disturbance article i nfo this paper describes an approach to assess channel changes of a long anthropogenised river the middle loire river over decadal timescales channel changes are evaluated along geomorphically homogeneous river reaches the classic geomorphic parameters active channel width bed slope grain size are complemented with parameters extracted from a 1d hydraulic model width depth ratio effective bed shear stress and specific stream power calculated for the biennial discharge assimilated to bankfull flow conditions the delineation of reaches is undertaken by combining visual inspection with the implementation of simple statistical tests to corroborate discontinuities in flow and sediment transport the 450 km long study area has been divided into 167 homogeneous reaches the general trends observed over the last fiftyyearsarenarrowingoftheactivechannel width and incision of the river bed however changes in bed level and active channel width are not consistently correlated channel changes at the reach scale are mainly controlled by the presence of former sediment extraction sites significant incision is observed at the peak of the in stream sediment mining period this was followed by channel recovery when extractions stopped the 1d numerical model provides a more rigorous manner to derive hydraulic parameters the effective bed shear stress made dimensionless by its critical value and the width depth ratio helps to explain observed channel responses more effectively by relating patterns to geological units along the middle loire

calculation of bed changes in mountain streams simulation of flow and sediment transport in mountain streams is complicated by the presence of high gradients abrupt changes in geometry variations in regime of flow and large roughness elements most of the numerical models to predict aggradation and degradation in alluvial channels have been developed for low gradient rivers this paper is devoted to the development of a numerical model to calculate bed elevation and grain size distribution changes in mountain streams where the maximum bed material size is in the range of boulders an attempt is made to validate the model by using observed field data collected upstream from a small retention dam in a venezuelan stream after calibration of the sediment transport equation reasonable agreement is obtained for the variations in the grain size distribution of the bed surface material an additional application is presented in the cocorotico river a small mountain stream located in the northwest region of venezuela which illustrates the adaptability of the model to handle a case of coarsest bed material removal from the active channel and to simulate the armoring process

modeling the evolution of channel shape balancing computational efficiency with hydraulic fidelity 1 the cross sectional shape of a natural river channel controls the capacity of the system to carry water off a landscape to convey sediment derived from hillslopes and to erode its bed and banks numerical models that describe the response of a landscape to changes in climate or tectonics therefore require formulations that can accommodate evolution of channel cross sectional geometry however fully two dimensional 2 d flow models are too computationally expensive to implement in large scale landscape evolution models while available simple empirical relationships between width and discharge do not adequately capture the dynamics of channel adjustment we have developed a simplified 2 d numerical model of channel evolution in a cohesive detachment limited substrate subject to steady unidirectional flow erosion is assumed to be proportional to boundary shear stress which is calculated using an approximation of the flow field in which log velocity profiles are assumed to apply along vectors that are perpendicular to the local channel bed model predictions of the velocity structure peak boundary shear stress and equilibrium channel shape compare well with predictions of a more sophisticated but more computationally demanding ray isovel model for example the mean velocities computed by the two models are consistent to within 3 and the predicted peak shear stress is consistent to within 7 furthermore the shear stress distributions predicted by our model compare favorably with available laboratory measurements for prescribed channel shapes a modification to our simplified code in which the flow includes a high velocity core allows the model to be extended to estimate shear stress distributions in channels with large width to depth ratios our model is efficient enough to incorporate into large scale landscape evolution codes and can be used to examine how channels adjust both cross sectional shape and slope in response to tectonic and climatic forcing

upper mantle seismic discontinuities and the thermal structure of subduction zones the precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western united states the boundary near 410km depth is locally elevated that near 660 km depressed the depths of these boundaries which mark phase transitions provide an in situ thermometer in subduction zones the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth in addition a reflector near 210 km depth may mark the bottom of the aesthenosphere

mantle phase transitions and layered chaotic convection we report preliminary results from a sequence of very high resolution simulations of the mantle convection process based upon an axisymmetric anelastically compressible spherical model with both olivine spinel and spinel post spinel phase transitions our analyses strongly suggest that at earth like rayleigh numbers near 107 the circulation may be quite strongly layered although a very long timescale transient exists that is characterized by brief excursions into the whole mantle state we suggest the use of a new diagnostic tool that may be employed to determine whether or not the layered state is characteristic of the present day earth

a critical perspective on 1 d modeling of river processes gravel load and aggradation in lower fraser river 1 we investigate how well a width averaged morphodynamic model can simulate gravel transport and aggradation along a highly irregular 38 km reach of lower fraser river and discuss critical issues in this type of modeling bed load equations with plausible parameter values predict a gravel input consistent with direct measurements and a sediment budget simulations using spatially varying channel width and forced by dominant discharge or a 20 year hydrograph match the observed downstream fining well they reproduce major qualitative features of a 47 year sediment budget and match maximum local rates of aggradation and degradation to well within a factor of 2 locations and rates of aggradation and degradation are influenced by channel constrictions runs for single years of unusually high or low peak discharge suggest that the main transfers of gravel occur in different parts of the reach in different years one dimensional morphodynamic modeling of highly nonuniform rivers has serious conceptual limitations but may be a valuable complement to empirical approaches

one dimensional modeling of dam break flow over movable beds a one dimensional model has been established to simulate the fluvial processes under dam break flow over movable beds the hydrodynamic model adopts the generalized shallow water equations which consider the effects of sediment transport and bed change on the flow the sediment model computes the nonequilibrium transport of bed load and suspended load the effects of sediment concentration on sediment settling and entrainment are considered in determining the sediment settling velocity and transport capacity in particular a correction factor is proposed to modify the van rijn formulas of equilibrium bed load transport rate and near bed suspended load concentration for the simulation of sediment transport under high shear flow conditions the governing equations are solved by an explicit finite volume method with the first order upwind scheme for intercell fluxes the model has been tested in two experimental cases with fairly good agreement between simulations and measurements the sensitivities of the model results to parameters such as the sediment nonequilibrium adaptation length manning s roughness coefficient and the proposed correction factor have been verified the proposed model has also been compared to an existing model and the results indicate the new model is more reliable

one dimensional numerical model for nonuniform sediment transport under unsteady flows in channel networks in this study the proposed one dimensional model simulates the nonequilibrium transport of nonuniform total load under unsteady flow conditions in dendritic channel networks with hydraulic structures the equations of sediment transport bed changes and bed material sorting are solved in a coupling procedure with a direct solution technique while still decoupled from the flow model this coupled model for sediment calculation is more stable and less likely to produce negative values for bed material gradation than the traditional fully decoupled model the sediment transport capacity is calculated by one of four formulas which have taken into consideration the hiding and exposure mechanism of nonuniform sediment transport the fluvial erosion at bank toes and the mass failure of banks are simulated to complement the modeling of bed morphological changes in channels the tests in several cases show that the present model is capable of predicting sediment transport bed changes and bed material sorting in various situations with reasonable accuracy and reliability

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

automatic procedures for river reach delineation univariate and multivariate approaches in a fluvial context abstract segmenting the continuum of rivers into homogeneous reaches is an important issue in river research and management automatic procedures provide significance objectivity and repeatability although univariate techniques are frequently used to identify river reaches multivariate approaches offer a more integrative context three nonparametric methods multi response permutation procedures mrpp with an advance in the significance level estimation the pettitt and mann kendall tests are applied for segmenting the river based on three geomorphic variables valley width active channel width and channel slope systematically measured in a gis environment the cited techniques have been applied to the curueno river nw spain to illustrate the methods we analyse reach distribution along the river longitudinal profile the methods successfully characterize the evident transitions along fluvial systems and also others less noticeable the three methods provide more reaches according to valley width and less reaches according to channel slope 18 0 and 3 7 reaches on average respectively in contrast to the mann kendall test mrpp and pettitt tests provide more stable segmentations when significance level varies however the pettitt test provides irregular segmentations for regular patterns the mrpp both univariate and multivariate applications enables a wider scope for the segmentation issue which is useful in diverse aspects of fluvial domain

estimating accumulation rates and physical properties of sediment behind a dam englebright lake yuba river northern california 1 studies of reservoir sedimentation are vital to understanding scientific and management issues related to watershed sediment budgets depositional processes reservoir operations and dam decommissioning here we quantify the mass organic content and grain size distribution of a reservoir deposit in northern california by two methods of extrapolating measurements of sediment physical properties from cores to the entire volume of impounded material englebright dam completed in 1940 is located on the yuba river in the sierra nevada foothills a research program is underway to assess the feasibility of introducing wild anadromous fish species to the river upstream of the dam possible management scenarios include removing or lowering the dam which could cause downstream transport of stored sediment in 2001 the volume of sediments deposited behind englebright dam occupied 25 5 of the original reservoir capacity the physical properties of this deposit were calculated using data from a coring campaign that sampled the entire reservoir sediment thickness 6 32 m at six locations in the downstream 3 4 of the reservoir as a result the sediment in the downstream part of the reservoir is well characterized but in the coarse upstream part of the reservoir only surficial sediments were sampled so calculations there are more uncertain extrapolation from one dimensional vertical sections of sediment sampled in cores to entire three dimensional volumes of the reservoir deposit is accomplished via two methods using assumptions of variable and constant layer thickness overall the two extrapolation methods yield nearly identical estimates of the mass of the reservoir deposit of 26 106 metric tons t of material of which 64 7 68 5 is sand and gravel over the 61 year reservoir history this corresponds to a maximum basin wide sediment yield of 340 t km2 yr assuming no contribution from upstream parts of the watershed impounded by other dams the uncertainties and limitations of the estimates of overall sediment quantities are discussed implications for watershed management and future reservoir sedimentation studies are also presented

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

one dimensional numerical modeling of sediment transport and bed deformation in open channels 1 a one dimensional numerical model for simulating unsteady flow and sediment transport in open channels is presented and tested the flow hydrodynamics is represented by the shallow water equations and the bed morphodynamics is represented by the exner equation and an additional equation describing the nonequilibrium sediment transport sediment size distribution is represented by the median grain diameter and the standard deviation instead of the usual modeling with multiple particle size classes various methods for computing bed elevation changes at a cross section due to erosion or deposition of sediment are proposed and tested including an innovative approach that relates the spatial pattern of erosion and deposition rates to boundary shear stress distribution which is calculated by the merged perpendicular method an explicit finite difference scheme is employed for solving the water and sediment governing equations the pertinence of the model is examined for two hypothetical cases the model is then tested on one set of laboratory experiments on bed degradation under steady flow showing excellent model data fit and indicating that incorporating a nonequilibrium sediment transport equation into the model structure is an important element in reproducing the bed degradation process finally the model is applied to simulate the morphological changes taking place in the ha ha river quebec after the failure of the ha ha dyke on july 1996 relevant results can be obtained in terms of changes in longitudinal bed profile and cross sectional geometry as well as water levels although some discrepancies are obtained between the simulated and surveyed cross sectional geometries mainly because bank failure and channel widening are not modeled

discovery of abundant arthropod trackways in the lower devonian muth quartzite spiti india implications for the depositional environment abstract arthropod trace fossils are described from the lower devonian muth quartzite spiti india a sequence virtually devoid of faunal evidence trace fossils which are abundant and often well preserved mainly consist of arthropod trackways some burrows are present too the ichnogenera of the trackways include numerous palmichnium and rarer diplichnites the first can probably be assigned to eurypterids and the second is probably related to myriapods sedimentological observations point to deposition of the muth quartzite in a foreshore backshore environment this combination of eurypterid and myriapod trackways is a spectacular indication of early terrestrialization

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

control of turbidity currents in reservoirs by solid and permeable obstacles turbidity currents are often the main process for the transport and deposition of the sediments in narrow alpine reservoirs these underwater avalanches with high concentration of suspended sediments follow the thalweg of the lake to the deepest area near the dam where the sediments can affect the operation of the bottom outlet and the intake structures to control the sedimentation within the reservoir the effects of solid and permeable obstacles on the turbidity current were investigated with physical experiments and numerical simulations in the physical experiments the velocity distributions as well as the spatial and temporal evolution of the deposits were measured the investigated measures were also simulated by means of a two dimensional numerical model based on the flow solver cfx 4 4 the investigations showed that turbidity currents can be influenced effectively by properly designed constructive measures based on the results of the physical experiments and numerical simulations some design recommendations for solid and permeable obstacles are proposed

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

a network scale intermediate complexity model for simulating channel evolution over years to decades abstract excessive river erosion and sedimentation threatens critical infrastructure degrades aquatic habitat and impairs water quality tools for predicting the magnitude of erosion sedimentation and channel evolution processes are needed for effective mitigation and management we present a new numerical model that simulates coupled river bed and bank erosion at the watershed scale the model uses modified versions of bagnold s sediment transport equation to simulate bed erosion and aggradation as well as a simplified bank stability and toe erosion model bstem to simulate bank erosion processes the model is mechanistic and intermediate complexity accounting for the dominant channel evolution processes while limiting data requirements we apply the model to a generic test case of channel network response following a disturbance and the results match physical understanding of channel evolution the model was also tested on two field data sets below parker dam on the lower colorado river and the north fork toutle river nftr which responded dramatically to the 1980 eruption of mount st helens it accurately predicts observed channel incision and bed material coarsening on the colorado river as well as observations for the upstream 18 km of the nftr watershed the model does not include algorithms for extensive lateral migration and avulsions and therefore did not perform well in the lower nftr where the channel migrated across a wide valley bottom rem is parsimonious and useful for simulating network scale channel change in single thread systems responding to disturbance

the fascination of a shallow water theory for the formation of megaflood scale dunes and antidunes abstract exceptional megaflood scale bedforms on earth are commonly associated with the catastrophic draining of glacial lakes in the late pleistocene the widest studied events have been the missoula and altai floods with 300 700 m flow depth 1 20 m bedform height and 10 300 m wavelength nowadays the saint venant equations have succeeded at simulating the catastrophic glacial lake drainage process numerically but we still lack a depth averaged morphodynamic theory able to predict the growth of dunes and antidunes the disparity of spatial scales in megafloods prevents the use of non depth averaged rotational flow equations motivating the present shallow water theory for the formation of megaflood scale bedforms we adopt a non equilibrium sediment transport equation rooted in einstein s pioneering work here we prove that the bed instability triggers to form dunes and antidunes simply by lagging the entrainment term for sediment mass conservation or the bottom shear stress with respect to the depth averaged flow velocity we formalise this result using a linear stability theory that captures the existence regions of dune and antidune in addition to the roll wave instability furthermore in the spirit of kennedy annu rev fluid mech vol 1 1969 pp 147 168 we derive a closed form solution of growth rate and wave speed of the bedform the nondimensional groups controlling the linear instabilities are the froude number r the shields parameter sh and the grain roughness relative to flow depth d subsequently we simulate the drainage of the largest missoula flood numerically to explain the formation of giant antidunes in the camas prairie montana us during the late stage of the megaflood also considered are large fields of gravel dunes in the kuray chuja lake basin altai mountains siberia the simulated hydraulic conditions over bedforms in both basins yield values of the nondimensional parameters that lie in the theoretical region of dunes and antidunes according to the proposed theory and in situ measurements in sandy rivers and flume experiments

incipient motion of sand particles on side slopes several criteria have been theoretically proposed for the critical shear stress of noncohesive sediment on side slopes however no laboratory work has ever been done on this problem a theoretical study of the incipient motion of sediment particles on side slopes is presented including the effects of viscous sublayer turbulence and lift force as a special case of level beds the theory describes the experimental values reported by several workers a specially designed rotary wind tunnel permits the evaluation of the critical shear stress on side slopes and the experiment supports the theory these works reveal that the lift force can be neglected in estimating the critical shear stress using the proposed theory if the averaged experimental values are concerned but inclusion of the lift force in the theory is recommended for safety and for describing the lower limit of the possibility of incipient motion