mechanisms of riverbank failure along the arno river central italy riverbanks along the arno river have been investigated with the aims of de ning the main mechanisms of failure and retreat their spatial distribution and their causes geomorphological aspects were investigated by a reconnaissance of riverbank processes for a number 26 of representative sites laboratory and in situ tests were then performed on a selected number of riverbanks 15 based on the material characteristics six main typologies of riverbanks have been de ned with homogeneous ne grained and composite banks representing the most frequent types slab type failures are the most frequent mechanism observed on ne grained banks while cantilever failures prevail on composite banks the role of river stage and related pore water pressure distributions in triggering the main observed mechanisms of failure has been investigated using two different types of stability analysis the rst was conducted for 15 riverbanks using the limit equilibrium method and considering simpli ed hypotheses for pore water pressure distribution annulment of negative pore pressures in the portion of the bank between low water stage and peak stage stability conditions and predicted mechanisms of failure are shown to be in reasonably good agreement with eld observations three riverbanks representative of the main alluvial reaches of the river were then selected for a more detailed bank stability analysis consisting of a de nition of characteristic hydrographs of the reach with different return periods b modelling of saturated and unsaturated ow using nite element seepage analysis and c stability analysis with the limit equilibrium method by adopting pore water pressure values derived from the seepage analysis the results are compared to those obtained from the previous simpli ed analysis and are used to investigate the different responses in terms of stability to different hydrological and riverbank conditions copyright 2003 john wiley sons ltd

magnitude and variation in the contribution of bank erosion to the suspended sediment load of the river severn uk the wide range of studies describing the role of bank erosion in fluvial sediment supply have mostly lumped amounts of bank erosion into coarse temporal units such as years this paper investigates sediment yields from individual bank erosion events within the upper river severn uk basin area 380 km2 manual erosion pins and photo electronic erosion pins were used to estimate bank erosion and turbidity meters were used to determine suspended sediment transport at the annual time scale the silt clay fraction of bank derived sediment accounted for an equivalent of 17 per cent of the suspended load increasing to an average of 38 per cent at the monthly timescale and then to an average of 64 per cent at the event timescale this research highlighted that for an upland catchment bank erosion was an important supply of suspended sediment and that for some flood events bank erosion can supply more sediment than is transported 1997 john wiley sons ltd

timing of millennial scale climate change at siple dome west antarctica during the last glacial period using atmospheric methane and the isotopic composition of o2 as correlation tools we place the dd record of ice from the siple dome west antarctica ice core on a precise common chronology with the gisp2 greenland ice core for the period from 9 to 57 ka the onset of major millennial warming events in siple dome preceded major abrupt warmings in greenland and the pattern of millennial change at siple dome was broadly similar though not identical to that previously observed for the byrd ice core also in west antarctica the addition of siple dome to the database of well dated antarctic paleoclimate records supports the case for a coherent regional pattern of millennial scale climate change in antarctica during much of the last ice age and glacial interglacial transition

transport and dispersal of organic debris peat blocks in upland fluvial systems this paper assesses the mechanisms and pathways by which peat blocks are eroded and transported in upland fluvial systems observations and experiments from the north pennines uk have been carried out on two contrasting river systems mapping of peat block distributions and appraisal of reach based sediment budgets clearly demonstrates that macro size peat is an important stream load component in small streams block sizes can approximate the channel width and much of the peat is transported overbank local peat jams and associated mineral deposition may provide an important component of channel storage in larger systems peat blocks rapidly move down channel and undergo frequent exchanges between bed and bank storage results of peat block tracing using painted blocks indicate that once submerged blocks of all sizes are easily transported and blocks break down rapidly by abrasion vegetation and bars play an important role in trapping mobile peat in smaller streams large block transport is limited by channel jams smaller blocks are transported overbank but exhibit little evidence of downstream fining in larger rivers peat blocks are more actively sorted and show downstream reduction in size from source a simple model relating peat block diameter dp to average flow depth d suggests three limiting transport conditions flotation dp d 2 and deposition dp d 2 experiments demonstrate that peat block transport occurs largely by rolling and floating and the transport mechanism is probably controlled by relative flow depth d dp ratio transport velocity varies with transport mechanism rolling is the slowest mode and transport lengths increase as flow depth increases abrasion rates vary with the transport mechanism rolling produces greater abrasion rates and more rounded blocks abrasion rates vary from 0 to 10 g m 1 for blocks ranging in mass from 10 to 6000 g copyright 2001 john wiley sons ltd

an empirical model of event scale cohesive bank profile evolution the highly stochastic nature of riverbank erosion has driven the need for spatially explicit empirical models detailed bank profile surveys along a meander bend of the brandywine creek in pennsylvania usa before and after 28 high flow events over a 2 5 year period are used to develop an empirical model of cohesive bank profile erosion two hundred and thirty six bank erosion observations are classified as hydraulic erosion or subaerial erosion threshold conditions required to initiate bank erosion cannot be defined based on field measurements using the near bank velocity and the number of freeze thaw cycles as predictors regression equations are derived for hydraulic erosion that specify the length thickness and location on the bank face of eroded blocks an empirical discriminant function defines the critical geometry of overhang failures and the volumes removed by overhang failures are computed using another regression equation all the regression equations are significant but have low correlation coefficients suggesting that cohesive bank erosion has a strong stochastic component individual events typically remove small masses of soil average volume 0 084 m3 m a few centimeters thick median 0 057 m and a few decimeters in length median 0 50 m from the lower third of the bank hydraulic erosion is responsible for 87 of all erosion when applied to three survey sites not used in its development the profile model predicts the total volume of erosion with errors of 23 5 and 1 twenty four percent of computed erosion volumes for single events are within 50 of observed volumes at these three sites extending the approach to decadal timescales and to entire bends will require three dimensional observations of bank failure and spatially and temporally explicit methods to account for the influence of individual large trees on bank failures and near bank hydraulic processes copyright 2009 john wiley sons ltd

aspects of alluvial river bank recession some examples from the niger delta the recession of alluvial channel banks occurs as a sequence of mass movements or by gradual dissolution of bank materials criteria for recognizing and interpreting bank recessions due to mass movement and or erosion are presented using back analysis technique slope development processes are reconstructed as a sequence of bank failure episodes and erosional events various mechanisms of mass bank failure and recession as well as causative agents have been identified and analyzed a deterministic sensitivity technique based on partial differentiation can be used for evaluating the contribution of factors to bank recession and as a basis for selecting bank erosion remedial measures

modeling the evolution of incised streams ii streambank erosion incision and ensuing widening of alluvial stream channels is widespread in the midsouth and midwestern united states and represents an important form of channel adjustment streambanks have been found to contribute as much as 80 of the total suspended load the location timing and magnitude of streambank erosion are difficult to predict results from field studies to characterize the resistance of fine grained materials to hydraulic and geotechnical erosion the impact of pore water pressures on failure dimensions and shearing resistance and the role of riparian vegetation on matric suction streambank permeability and shearing resistance are used to enhance the channel evolution model concepts conservational channel evolution and pollutant transport system this paper discusses the conceptualization of the above mentioned physical processes and demonstrates the ability of the derived model to simulate streambank failure processes the model is tested against observed streambank erosion of a bendway on goodwin creek miss between march 1996 and march 2001 where it accurately predicts the rate of retreat of the outside bank of the bendway the observed change in average channel width within the central section of the bendway is 2 96 m over the simulation period whereas a retreat of 3 18 m 7 4 larger is simulated the observed top bank retreat within the central section of the bendway is 3 54 m over the simulation period whereas a retreat of 3 01 m 15 smaller is simulated doi 10 1061 asce0733 94292008134 7905 ce database subject headings bank erosion limit equilibrium mathematical models sensitivity analysis numerical analysis

feasibility of reconstructing paleoatmospheric records of selected alkanes methyl halides and sulfur gases from greenland ice cores seven short lived atmospheric trace gases were measured in 25 ice core samples from summit greenland samples were selected from contemporaneous sections of fluid and dry drilled ice cores to examine what effects using n butyl acetate as the drill fluid would have on the measurements the gases include three light alkanes c2h6 c3h6 and n c4h10 two methyl halides ch3cl and ch3br and two sulfur compounds ocs and cs2 with gas ages from 125 to 325 years before present alkane levels are comparable to measurements in modern arctic air although c2h6 exhibits greater variability than expected compared with c3h6 and n c4h10 these results are not consistent with the idea that the alkanes are primarily of anthropogenic origin suggesting that the ice cores may not truly record a paleoatmospheric signal with respect to these gases the ch3br results are consistent with previous observations of excess ch3br in greenland firn air in situ production processes appear to overwhelm the paleoatmospheric signal of this gas ch3cl exhibits the same effect to a lesser extent ocs levels are similar to those in antarctic ice cores and appear to reflect paleoatmospheric levels cs2 results are similar to the limited database of modern atmospheric measurements only c3h8 and n c4h10 exhibit clear evidence of contamination because of the presence of the drill fluid the results indicate that it is possible to analyze many trace gases in fluid and dry drilled ice samples however it appears that in situ production may significantly alter the levels of some trace gases in greenland ice cores

stability of composite river banks the stability of a river bank depends on the balance of forces motive and resistive associated with the most critical mechanism of failure many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size geometry and structure of the bank the engineering properties of the bank material the hydraulics of flow in the adjacent channel and climatic conditions rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt clay bank erosion occurs by fluvial entrainment of material from the lower cohesionless bank at a much higher rate than material from the upper cohesive bank this leads to undermining that produces cantilevers of cohesive material upper bank retreat takes place predominantly by the failure of these cantilevers three mechanisms of failure have been identified shear beam and tensile failure the stability of a cantilever may be analysed using static equilibrium and beam theory and dimensionless charts for cantilever stability constructed application of the charts requires only a few simple measurements of cantilever geometry and soil properties in this analysis the effects of cracks and fissures in the soil must be taken into account these cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface following mechanical failure blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue hence the rate of bank retreat is fluvially controlled even though the mechanism of failure of the upper bank is not directly fluvial in nature this cycle of bank erosion undermining cantilever failure and fluvial scour of the toe operates over several flood events and has important implications for river engineering channel changes and the movement of sediment through fluvial systems

a new distinct mechanism of river bank erosion in a forested catchment abstract it has been thought that the two main processes responsible for river bank retreat are the removal of particles or layers of material by flow erosion or sloughing and collapse by slumping a third process involving the development of a subsurface cavity after washout of gravels which is succeeded by slow subsidence of a segment of the former channel bank has been identified at a site in the new forest southern england the process was monitored over a 9 month period and was observed to be assisted by the hydrological effects of a dam of coarse woody debris across the channel as these dams occur frequently the process is likely to have occurred elsewhere along channels in forested areas

subaerial river bank erosion processes and their interaction with other bank erosion mechanisms on the river arrow warwickshire uk river bank erosion occurs primarily through a combination of three mechanisms mass failure fluvial entrainment and subaerial weakening and weathering subaerial processes are often viewed as preparatory processes weakening the bank face prior to fluvial erosion within a river basin downstream process domains occur with subaerial processes dominating the upper reaches fluvial erosion the middle and mass failure the lower reaches of a river the aim of this paper is to demonstrate that a subaerial processes may be underestimated as an erosive agent and b process dominance has a temporal as well as spatial aspect bank erosion on the river arrow warwickshire uk was monitored for 16 months december 1996 to march 1998 using erosion pins variations in the rate and aerial extent of erosion are considered with reference to meteorological data throughout the first 15 months all erosion recorded was subaerial resulting in up to 181 mm a 1 of bank retreat compared with 13 to 27 mm a 1 reported by previous researchers while the role of subaerial processes as preparatory is not contended it is suggested that such processes can also be erosive the three bank erosion mechanisms operate at different levels of magnitude and frequency and the river arrow data demonstrate this thus the concept of process dominance has a temporal as well as spatial aspect particularly over the short time periods often used for studying processes in the field perception of the relative efficacy of each erosive mechanism will therefore be influenced by the temporal scale at which the bank is considered with the advent of global climate change both these magnitude frequency characteristics and the consequent interaction of bank erosion mechanisms may alter it is therefore likely that recognition of this temporal aspect of process dominance will become increasingly important to studies of bank erosion processes copyright 2001 john wiley sons ltd

on the origin and timing of rapid changes in atmospheric methane during the last glacial period we present high resolution records of atmospheric methane from the gisp2 greenland ice sheet project 2 ice core for four rapid climate transitions that occurred during the past 50 ka the end of the younger dryas at 11 8 ka the beginning of the bolling allerod period at 14 8 ka the beginning of interstadial 8 at 38 2 ka and the beginning of interstadial 12 at 45 5 ka during these events atmospheric methane concentrations increased by 200 300 ppb over time periods of 100 300 years significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record we suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change we find no evidence for rapid massive methane emissions that might be associated with large scale decomposition of methane hydrates in sediments with additional results from the taylor dome ice core antarctica we also reconstruct changes in the interpolar methane gradient an indicator of the geographical distribution of methane sources associated with some of the rapid changes in atmospheric methane the results indicate that the rise in methane at the beginning of the bolling allerod period and the later rise at the end of the younger dryas were driven by increases in both tropical and boreal methane sources during the younger dryas a 1 3 ka cold period during the last deglaciation the relative contribution from boreal sources was reduced relative to the early and middle holocene periods

bank erosion processes in regulated navigable rivers vessel induced waves affect the morphology and ecology of banks and shorelines around the world in rivers used as waterways ship passages contribute to the erosion of unprotected banks but their short and long term impacts remain unclear this work investigates the effects of navigation on bank erosion along a reach of the regulated meuse river with recently renaturalized banks we apply uav sfm photogrammetry rtk gps acoustic doppler velocimetry aerial and terrestrial photography soil tests and multibeam echosounding to analyze the progression of bank retreat after riprap removal after having analyzed the effects of ship generated waves and currents floods and vegetation dynamics a process based model is proposed to estimate the long term bank retreat the results show that a terrace evolves in length and depth across the bank according to local lithology which we clustered in three types floods contribute to upper bank erosion inducing mass failures while near bank flow appears increasingly ineffective to remove the failed material due to terrace elongation vegetation growth at the upper bank toe reduces bank failure and delays erosion but its permanence is limited by terrace stability and efficiency to dissipate waves the results also indicate that long term bank retreat is controlled by deep primary waves acting like bores over the terrace understanding the underlying drivers of bank evolution can support process based management to optimize the benefits of structural and functional diversity in navigable rivers

laboratory investigation of beach scarp and dune recession due to notching and subsequent failure analytical models to calculate notch development and subsequent mass failure of dunes are presented the notch evolution model is based on a transport equation for sediment from the dune and the sediment volume conservation equation whereas the models of mass failure are derived using elementary engineering statics and soil mechanics an empirical transport coefficient in the model describing the notch growth rate is found to be related to the hydrodynamic forcing at the dune normalized by geotechnical parameters describing the resistive strength of the dune two modes of mass failure are modeled whereby the overhang generated by the removal of material from the dune foot notching slides downward or topples over following the development of a tensile crack some distance shoreward of the maximum notch depth the accuracy of the notch evolution and mass failure models are assessed by comparing calculated recession distances against measurements from a small scale laboratory experiment

prediction of tension crack location and riverbank erosion hazards along destabilized channels riverbank erosion associated sedimentation and land loss hazards are a land management problem of global significance and many attempts to predict the onset of riverbank instability have been made recently osman and thorne 1988 have presented a culmann type analysis of the stability of steep cohesive riverbanks this has the potential to be a considerable improvement over previous bank stability theories which do not account for bank geometry changes due to toe scour and lateral erosion however in this paper it is shown that the existing osman thorne model does not properly incorporate the influence of tension cracking on bank stability since the location of the tension crack on the floodplain is indirectly determined via calculation or arbitrary specification of the tension crack depth furthermore accurate determination of tension crack location is essential to the calculation of the geometry of riverbank failure blocks and hence prediction of land loss and bank sediment yield associated with riverbank instability and channel widening in this paper a rational physically based method to predict the location of tension cracks on the floodplain behind the eroding bank face is presented and tested a case study is used to illustrate the computational procedure required to apply the model improved estimates of failure block geometry using the new method may potentially be applied to improve predictions of bank retreat and floodplain land loss along river channels destabilized as a result of environmental change

basal melt at northgrip modeled from borehole ice core and radio echo sounder observations from temperature measurements down through the 3001 m deep borehole at the north greenland icecore project northgrip drill site it is now clear that the ice at the base 3080 m below the surface is at the pressure melting point this is supported by the measurements on the ice core where the annual layer thicknesses show there is bottom melting at the site and upstream from the borehole surface velocity measurements internal radio echo layers borehole and ice core data are used to constrain a time dependent flow model simulating flow along the north northwest trending ice ridge flow line leading to the northgrip site also time dependent melt rates along the flowline are calculated with a heat flow model the results show the geothermal heat flow varies from 50 to 200 mw m 2 along the 100km section of the modeled flowline the melt rate at the northgrip site is 0 75 cm a 1 but the deep ice in the northgrip core originated 50 km upstream and has experienced melt rates as high as 1 1 cm a 1

effect of the development of notches and tension cracks on instability of limestone coastal cliffs in the ryukyus japan abstract notches cut by waves are currently developing at the base of vertical coastal limestone cliffs in okinawa japan the cliff height varies from 3 0 22 1 m and the maximum notch depth is 8 8 m many rectangular or cubic blocks which appear to have originated from cliff failures are found on platforms in front of the cliffs on the flat top surface of the cliff tension cracks often run parallel to the cliff face the vertical face of the cliffs displays small undulations but no sliding striation suggesting that cliff failures have been caused by toppling rather than by shearing or sliding we use slope stability analysis to determine the critical condition for toppling failure physical and mechanical properties of the cliff material were first obtained from laboratory tests the results indicate that the strength of limestone shows a scale effect such that the strength decreases with increasing size of the test specimens based on this result we estimated the strength of a rock mass corresponding to the size of the coastal cliff cliff stability was then analyzed using a cantilever beam model comparison of the stability analysis and the dimension of fallen blocks indicates that toppling failure is strongly associated with the development of notches and tension cracks

surficial failures of alluvial stream banks erosion of alluvial stream banks is of scientific and economic significance the principal focus of this investigation was stream bank erosion caused by piping of sand seams which leads to collapse of overlaying strata analyses were performed to determine which parameters of the riverbank system are most important in controlling the amount and rate at which piping occurs among the failure mechanisms studied were wedge failure of large masses and surficial shearing slabbing failure of bank faces analyses showed that the most important factors governing piping were permeability and capillary suction in sand seams slope of sand seams and water in tension cracks behind the bank face flood hydrograph parameters height of rise and duration of event were less important though significant whereas unit weight cohesion angle of internal friction and water in tension cracks influenced wedge failures only unit weight and cohesion directly influenced slabbing failures however those parameters influencing piping indirectly also influenced slabbing failures results of the analyses agreed closely with field observations

the cenozoic lower rhine basin rifting sedimentation and cyclic stratigraphy in the cenozoic the lower rhine basin formed as a rift at the southeastern terminus of the dutch german central graben while the rhenish massif was uplifted the study focusses on the marginal marine and fluvial fill of the lower rhine basin a basin model is developed support for this study was given by extensive industry outcrop and well data by new stratigraphical and sedimentological observations the ingression and subsequent regression of the cenozoic north sea is analysed using the concept of base level cyclicity as the geohistory of the basin was complex a subsidence curve is constructed furthermore an attempt is made to trace the simultaneous uplift of the rhenish massif

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

the anthropogenic nature of present day low energy rivers in western france and implications for current restoration projects abstract as in other european countries western france has seen an increase in river restoration projects in this paper we examine the restoration goals methods and objectives with respect to the long term trajectory and understanding of the contemporary dynamics of the small low energy rivers typical of the lowlands of western europe the exhaustive geomorphological paleoenvironmental and historical research conducted in the seulles river basin normandy provides very accurate documentation of the nature and place of the different legacies in the fluvial systems we have inherited the sedimentation rate in the seulles valley bottom has multiplied by a factor of 20 since the end of the bronze age and has generated dramatic changes in fluvial forms hydraulic control of the rivers and valley bottoms drainage throughout the last millennium has channelized rivers within these deposits the single meandering channel which characterizes this river today is the legacy of the delayed and complex effects of long term exploitation of the river basin and the fluvial system bring to light that the naturalness of the restored rivers might be questioned our research emphasizes the gap between the poor knowledge of the functioning of these rivers and the concrete objectives of the restoration works undertaken including dam and weir removal account of the long term history of fluvial systems is required not only to produce a pedagogic history of the river degradation but more fundamentally i to situate the current functioning of the fluvial system in a trajectory to try to identify thresholds and anticipate the potential turning points in a context of climate and land use change ii to understand the role of morphosedimentary legacies on the current dynamics iii to open the discussion on reference functioning or expected states and iv to open discussion on the sustainability of ecological restoration to conclude we point out the necessity to take into account the hybrid nature of low energy rivers in rural environments and to develop specific evaluation protocols which would include both biophysical processes and usual human activities which could be a way to share the evaluation and overcome conflicts between socioeconomic needs and environmental issues

radiation and atmospheric circulation controls on carbonyl sulfide concentrations in the marine boundary layer a potential closure of the global carbonyl sulfide cos or ocs budget has recently been attained through a combination of remote sensing modeling and extended surface measurements however significant uncertainties in the spatial and temporal dynamics of the marine flux still persist in order to isolate the terrestrial photosynthetic component of the global atmospheric ocs budget tighter constraints on the marine flux are needed we present 6 months of nearly continuous in situ ocs concentrations from the north atlantic during the fall and winter of 2014 2015 using a combination of research vessel and fixed tower measurements the data are characterized by synoptic scale 100 pmol mol 1 variations in marine boundary layer air during transitions from subtropical to midlatitude source regions the synoptic ocs variability is shown here to be a linear function of the radiation history along an air parcel s trajectory with no apparent sensitivity to the chlorophyll concentration of the surface waters that the air mass interacted with this latter observation contradicts expectations and suggests a simple radiation limitation for the combined direct and indirect marine ocs emissions because the concentration of ocs in the marine boundary layer is so strongly influenced by an air parcel s history marine and atmospheric concentrations would rarely be near equilibrium and thus even if marine production rates are held constant at a given location the ocean atmosphere flux would be sensitive to changes in atmospheric circulation alone we hypothesize that changes in atmospheric circulation including latitudinal shifts in the storm tracks could affect the marine flux through this effect

modification of meander migration by bank failures meander migration and planform evolution depend on the resistance to erosion of the floodplain materials to date research to quantify meandering river adjustment has largely focused on resistance to erosion properties that vary horizontally this paper evaluates the combined effect of horizontal and vertical floodplain material heterogeneity on meander migration by simulating fluvial erosion and cantilever and planar bank mass failure processes responsible for bank retreat the impact of stream bank failures on meander migration is conceptualized in our rvr meander model through a bank armoring factor associated with the dynamics of slump blocks produced by cantilever and planar failures simulation periods smaller than the time to cutoff are considered such that all planform complexity is caused by bank erosion processes and floodplain heterogeneity and not by cutoff dynamics cantilever failure continuously affects meander migration because it is primarily controlled by the fluvial erosion at the bank toe hence it impacts migration rates and meander shapes through the horizontal and vertical distribution of erodibility of floodplain materials planar failures are more episodic however in floodplain areas characterized by less cohesive materials they can affect meander evolution in a sustained way and produce preferential migration patterns model results show that besides the hydrodynamics bed morphology and horizontal floodplain heterogeneity floodplain stratigraphy can significantly affect meander evolution both in terms of migration rates and planform shapes specifically downstream meander migration can either increase or decrease with respect to the case of a homogeneous floodplain lateral migration generally decreases as result of bank protection due to slump blocks and the effect on bend skewness depends on the location and volumes of failed bank material caused by cantilever and planar failures along the bends with possible achievement of downstream bend skewness under certain conditions

experiments and modeling of head cut migration in stratified soils 1 a two dimensional model is developed to predict head cut migration rate for the specific case of a relatively erosion resistant soil layer overlying a deep relatively erosive base soil for this geometry head cut migration can occur as a series of discrete mass failures of the surface layer caused by undercutting in a plunge pool formed immediately downstream the length scale of one failure episode is modeled as a cantilevered slab failing in bending the timescale for each failure episode is controlled by the rate and geometry of base layer scour which is modeled independently of cantilevered slab properties an integration of the two model components for the case of steady flow and uniform soil properties predicts linear upstream head cut migration with time occurring in discrete steps bounded by successive mass failure events the model is calibrated and tested in a series of flume experiments representing ideal conditions of different initial geometries and flow conditions head cut migration occurred by the proposed mechanism in a nearly linear rate average error between the observed and predicted retreat rate is 40 most of this error is attributable to prediction of the scour hole shape

flow in alluvial river curves uniform flow in curved wide erodible bed channels is formulated on the basis of the conservation of flux of moment of momentum to obtain relations for the vertical distributions of radial plane velocity and radial shear stress the expression for the radial stress exerted on the bed is utilized in a force equilibrium analysis of the moving bed layer to obtain relations for the average transverse slope of the bed and for the radial bed profile the reduction of primary bed shear stress due to the net radial transport of streamwise momentum toward the outer concave bank is then calculated by introducing the derived expressions for the velocity components into the momentum equation for the primary flow direction it is found that the stress reductions in deep narrow channels can exceed 50 bed profiles and velocity distributions measured in natural and laboratory streams are found to be in good conformity with those calculated from this moment based theory

river width adjustment i processes and mechanisms in 1993 a task committee tc of the asce was established to study the hydraulics bank mechanics and modeling of width adjustment in alluvial channels the work of the tc in reviewing width adjustment processes and mechanisms is reported in this paper a companion paper presents the findings of the tc with regard to width adjustment modeling this paper first establishes the geomorphic context within which width adjustments occur and it demonstrates that width adjustment may take place over a wide range of scales in time and space in the past engineering analyses of channel width have tended to concentrate on prediction of the equilibrium width for stable channels most commonly the regime extremal hypothesis and rational mechanistic approaches are used and these are reviewed herein more recently attention has switched to channels that are adjusting their morphology either due to natural instability or in response to changes in watershed land use river regulation or channel engineering chara

time and the persistence of alluvium river engineering fluvial geomorphology and mining sediment in california abstract river managers need to understand fluvial systems as they change through time many river systems are presently in a state of flux as a result of substantial anthropogenic changes to water and sediment regimes and channel hydraulics yet historical approaches to understanding river systems rarely receive adequate attention because historical methodologies are not conducive to the application of quantitative analysis while there is limited precision in most historical reconstructions the information derived from these studies constrains other interpretations and is essential to a full understanding of the behavior of fluvial systems geomorphology provides a perspective on river systems in which time at various scales is interwoven into practical and theoretical aspects of scientific inquiry thus geomorphology is important to our understanding of not only physical systems but also fundamental concepts of time this study examines channel morphological changes in the bear and american basins brought about by two episodes of sedimentation from hydraulic gold mining the primary event was the production of more than 1 billion m3 of sediment throughout the northern sierra nevada from 1853 to 1884 which caused aggradation in many channels across the sierra foothills and sacramento valley assumptions by both engineers and geomorphologists that morphologic responses to this event were ephemeral that sediment loads have returned to previous levels and that deposits have stabilized are not borne out by field and historical data in the sacramento valley a secondary sedimentation event not previously studied was the production of at least 24 million m3 of sediment during a period of licensed mining from 1893 to 1953 this episode of sedimentation has been largely overlooked as a geomorphic hydrologic or water quality event yet channel morphologic responses in phase with mining during this period are demonstrated systematic changes in stage discharge relationships reflect channel morphological changes that are relevant to flood risk assessments stability of engineering structures on floodplains and geomorphic interpretations

bank toe processes in incised channels the role of apparent cohesion in the entrainment of failed bank materials numerous processes may instigate bank retreat and the consequent collection of failed cohesive materials at the bank toe cohesion between the failed material and the substrate can provide additional strength to resist direct fluvial entrainment failed cohesive material can act as a form of natural bank toe protection by consuming and diverting flow energy that may otherwise be used to further scour the basal zone of incising channels investigations in goodwin creek mississippi have revealed the existence of apparent cohesion between failed cohesive blocks and their underlying surface the method used to assess this cohesion involved a pulley system mounted on a tripod and supporting a load cell mean and maximum apparent cohesion values of 1 08 kpa and 2 65 kpa respectively were measured in this way identifying a source that bonds blocks to the underlying surface cohesion values and types vary spatially and temporally tensiometric tests beneath blocks suggest that cohesion resulting from matric suction alone may be as much as 3 5 kpa in summer and 1 8 kpa in winter apparent cohesion is believed to have been sufficient to help prevent removal of the largest blocks by a peak flow of 66 4m3ss on 23 september 1997 maximum excess shear stress required to entrain a d75 block can be augmented by as much as 97 by the presence of apparent cohesion at the block substrate interface when compared with a condition with zero apparent cohesion at the block underside given these findings it is no longer sufficient to estimate block entrainment in the basal area from block size or bed roughness alone as in a shields type approach copyright 2001 john wiley sons ltd

effect of gypsum content on soil water retention summary many gypsiferous soils occur in arid lands where the water retention capacity of the soil is vital to plant life and crop production this study investigated the effect of gypsum content on the gravimetric soil water retention curve wrc we analyzed calcium carbonate equivalent cce equivalent gypsum content eg soil organic carbon content soc and electrical conductivity of 43 samples collected from various horizons in soils in the ebro valley ne spain the wrc of the fine earth was determined using the pressure plate method pressure heads 0 33 100 200 500 and 1500 kpa and the gravimetric water retention curves were fitted to the unimodal van genuchten function soil gypsum content had a significant effect on water retention soils that had high gypsum content made wrc with higher water retention at near saturation conditions and steeper wrc slopes the eg threshold at which gypsum content had an effect on wrc was about 40 and eg was positively and negatively correlated with the and n parameters of the wrc respectively

palaeochannels in mine workings in the high hazles coal westphalian b nottinghamshire coalfield england palaeochannels have numerous deleterious effects on coal mining including the formation of washouts and their detection and delineation in the subsurface is of paramount importance a palaeochannel system in the roof of the high hazles coal westphalian b at gedling colliery nottinghamshire is described the roof strata of this seam are interpreted as the deposits of a lacustrine delta complex formed on an upper delta plain with the palaeochannels constituting minor deltaic distributaries the lacustrine delta front deposits consist of a coarsening upwards sequence of siltstones and sandstones which form a sheet like body of coalescing distributary mouth bars that were laid down by friction dominated effluents the distributary channels were filled by a variety of processes giving rise to complex channel deposits bedload deposition of sand on bars collapse and sloughing of channel banks and deposition of silts and muds from suspension were important mechanisms the nature of the deposits reflects the mode of channel abandonment rather than equilibrium channel processes overbank flood deposits and prodelta lacustrine mudstones are also described from the high hazles roof based on this study criteria are suggested that may enable the successful detection and delineation of palaeochannels in mine workings elsewhere

atmospheric co2 over the last 1000 years a high resolution record from the west antarctic ice sheet wais divide ice core 1 we report a decadally resolved record of atmospheric co2 concentration for the last 1000 years obtained from the west antarctic ice sheet wais divide shallow ice core the most prominent feature of the pre industrial period is a rapid 7 ppm decrease of co2 in a span of 20 50 years at 1600 a d this observation confirms the timing of an abrupt atmospheric co2 decrease of 10 ppm observed for that time period in the law dome ice core co2 records but the true magnitude of the decrease remains unclear atmospheric co2 variations over the time period 1000 1800 a d are statistically correlated with northern hemispheric climate and tropical indo pacific sea surface temperature however the exact relationship between co2 and climate remains elusive due to regional climate variations and or uneven geographical data density of paleoclimate records we observe small differences of 0 2 0 6 ppm among the high precision co2 records from the law dome epica dronning maud land and wais divide antarctic ice cores however those records share common trends of co2 change on centennial to multicentennial time scales and clearly show that atmospheric co2 has been increasing above preindustrial levels since 1850 a d

an experimental investigation on mass failures occurring in a riverbank composed of sandy gravel abstract this paper reports the results and interpretation of laboratory experiments carried out on a model of a relatively coarse sandy gravel riverbank with the aim of investigating the basic processes and possible factors of instability after a series of initial tests three main experiments were carried out in a glass walled tank where a bank model was built with bank angles varying from 75 to 90 bank height of 70 cm and same sediment mixture 60 gravel 40 sand but with the addition of 1 of cement in the third experiment only during the experiments the bank was subject to a given hydrograph associated with a static oscillation of the water level and corresponding variations in pore water pressures were measured results show the occurrence of a large variety of processes including erosion and failures because of loss of matric suction cantilever slab and slide failures and granular flows during the ascending phase of the hydrograph bank instability was related to a superimposition of initial geometric factors and progressive reduction of apparent cohesion during the rising water stage apparent cohesion was sufficient to maintain a stable bank in loose material but only for low bank height and or slopes whereas a limited percentage of cement is able to explain a markedly different response in terms of stability and mechanisms of failure

riverbank stability analysis ii applications bank retreat occurs by a combination of lateral erosion by the flow and mass failure under gravity a new analysis of bank erosion and failure is developed using a critical shear stress concept to account for lateral erosion and a slope stability criterion for mass failure in this paper we apply the analysis to two problems of bank retreat often encountered by practicing engineers dealing with alluvial channels the first application is to the prediction of degradation downstream of a dam for the case in which bed lowering causes bank instability we show that rapid bank retreat can occur once the threshold height for mass failure of the banks is reached this supplies sediment to the flow tends to limit the depth of degradation and drives complex response downstream the second application is to the modeling of flow in channel bends and the prediction of the equilibrium cross section we show that scour depth at the outer bank may be limited by the critical bank height if scouring causes the outer

pore space characteristics as indicators of soil behaviour in gypsiferous soils abstract in order to study the physical properties of gypsiferous soils gypsic xerochrepts and typic xerofluvents from an irrigated semi arid area in catalonia ne spain and their relationships to their pore characteristics twenty horizons with gypsum contents ranging between 0 and 90 were sampled for micromorphological and physical analyses the micromorphological study consisted of thin section description and of image analysis of uncovered thin sections of samples impregnated with a uv fluorescent dye eight fields of each horizon were studied two images were obtained from each field 2 1 3 1 cm 2 one illuminated with incident uv light and the other with transmitted white light image processing with an ibas system allowed measurement of the geometric characteristics of three types of objects actual pores eq diam 100 m transparent objects infilled or not with gypsum and pores located in these transparent objects the physical characteristics of the same horizons considered in the study were the water characteristic curves and the saturated hydraulic conductivity although differences in total porosity between 6 and 16 are not related to other properties an increase of gypsum content leads in the studied horizons to the replacement of large pores by smaller packing pores between gypsum crystals in infillings which are interconnected and do not reduce saturated hydraulic conductivity available water is affected by the degree of mixing of the gypsum infillings with the groundmass besides the crystal size distribution the results stress the importance of pore clogging by gypsum in these materials as a major constraint for root development as well as the need of describing the type of gypsum accumulation when assessing the suitability of these soils for plant growth

carbonyl sulfide hydrolysis in antarctic ice cores and an atmospheric history for the last 8000 years 2014 american geophysical union all rights reserved carbonyl sulfide cos was measured in antarctic ice core samples from the byrd siple dome taylor dome and west antarctic ice sheet divide sites covering the last 8000 years of the holocene cos levels decrease downcore in most of these ice cores the magnitude of the downcore trends varies among the different ice cores and is related to the thermal histories of the ice sheet at each site we hypothesize that this is due to the temperature dependent hydrolysis of cos that occurs in situ we use a one dimensional ice flow and heat flux model to infer temperature histories for the ice core samples from different sites and empirically determine the kinetic parameters for cos hydrolysis we estimate e folding lifetimes for cos hydrolysis ranging from 102 years to 106 years over a temperature range of 0 c to 50 c the reaction kinetics are used to estimate and correct for the in situ cos loss allowing us to reconstruct paleoatmospheric cos trends during the mid to late holocene the results suggest a slow long term increase in atmospheric cos that may have started as early as 5000 years ago given that the largest term in the cos budget is uptake by terrestrial plants this could indicate a decline in terrestrial productivity during the late holocene

genesis of channel bank overhangs in the niger delta and analysis of mechanisms of failure abstract the instability of channel banks carries a significant economic implication because of its effect on navigation and channel commercial development the impact of bank instability in the niger delta is accentuated by the dense network of rivers and creeks the occurrence of overhangs in channel banks is not restricted to stratified banks in which clay overlies sand field observation and analysis indicate that factors such as stratigraphy local climate bio technical and fluvio tidal processes act singly or in combination to generate overhangs in channel banks the failure of channel banks with overhangs involves one of four mechanism namely rotation beam shear and tension failure a mathematical basis for the discrimination of these failure mechanisms is described the significance of identifying the critical location of tension cracks for the failure of the overhang is discussed although the study is based on experience in the niger delta the principles applied and the conclusions reached undoubtedly apply to areas of similar geological and environmental conditions

the 18o isotope exchange rate between firn air co2 and the firn matrix at three antarctic sites 1 having measured 18o of firn air co2 for three antarctic sites dronning maud land dome concordia and berkner island we established that at depth the 18o co2 values are systematically shifted away from atmospheric values toward isotope equilibrium with h2o from precipitation firn matrix the 18o co2 shifts increase with depth and thus with co2 age the equation of a first order reaction is used to describe and calculate co2 oxygen isotope exchange rates the calculated exchange rates clearly show a correlation with firn temperatures assuming that the same mechanism operates at different sites and at different temperatures a pseudoactivation energy of 7 8 0 8 kcal mol is estimated the occurrence of exchange means that firn is not chemically inert in that sense that some form of co2 hydration takes place possible mechanisms of co2 oxygen isotope exchange are discussed and liquid like layers on firn grains are the most likely interface where this exchange takes place other gases e g cos may likewise be sensitive to hydration or related reactions and this may reduce their integrity in firn a formal translation of the reaction rates of co2 oxygen isotope exchange in firn to a cos hydration rate appears to indicate cos preservation on the scale of tens of thousands of years we suggest that the value of co2 oxygen exchange rate in firn expressed as half reaction time t may be used as a new firn characteristic giving a measure of hydration activity in firn an indirect measure of firn metamorphism and be correlated with other firn processes near a pore close off depth t values may possibly correlate with the process of he and ne diffusion expulsion during the close off of pores in firn as well as the release of 14co produced in situ in the firn matrix

electrical stratigraphy of the wais divide ice core identification of centimeter scale irregular layering multi track electrical conductivity measurements ecm imaged a continuous record of the two dimensional electrical stratigraphy for the deepest 40 of the wais divide ice core 1956 m to 3405 m 11 5 to 68 ka the electrical stratigraphy showed clear banding driven primarily by annual variations centimeter scale pinched layers and other irregularities were concentrated between 2700 m and 2900 m 27 ka to 33 ka below 2900 m decreasing amplitude of conductance variations likely due to diffusion prevented confident interpretation of both annual and irregular layering the effective diffusivity at 30 c is 2 2 10 8 m2 yr 1 approximately 5 times greater than for self diffusion of water molecules implying diffusion at grain boundaries the irregular layering indicates that the cm scale layering was disturbed in sections even though other records such as atmospheric methane indicate meter and larger layering is not compromised preservation of irregular layering at deposition is unlikely to be the cause of the identified irregular layering instead the irregular layering likely arises from variations in the deformation of ice

synchronous climate changes in antarctica and the north atlantic central greenland ice cores provide evidence of abrupt changes in climate over the past 100 000 years many of these changes have also been identified in sedimentary and geochemical signatures in deep sea sediment cores from the north atlantic confirming the link between millennial scale climate variability and ocean thermohaline circulation it is shown here that two of the most prominent north atlantic events the rapid warming that marks the end of the last glacial period and the bolling allerod younger dryas oscillation are also recorded in an ice core from taylor dome in the western ross sea sector of antarctica this result contrasts with evidence from ice cores in other regions of antarctica which show an asynchronous response between the northern and southern hemispheres

dating the siple dome antarctica ice core by manual and computer interpretation of annual layering the holocene portion of the siple dome antarctica ice core was dated by interpreting the electrical visual and chemical properties of the core the data were interpreted manuallyandwith acomputeralgorithm the algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the gisp2 greenland ice sheet project 2 ice core 10 be stratigraphic ties to the dendrochronology 14 c recordandthedatedvolcanic stratigraphy thealgorithm interpretation ismorecon sistent andbetter quantifiedthanthe tedious and subjective manual interpretation

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

evolution of salt marsh cliffs in muddy and sandy systems a qualitative comparison of british west coast estuaries salt marsh cliffs in the muddy severn estuary are mostly strong and tall they are retreating in response to the erosive attack of wave and tidal currents chiefly through toppling failures and rotational slips in the sandy solway firth and morecambe bay systems marsh cliffs are strong only in their upper parts where a dense root mat of marsh grasses binds the sediments here cantilever and toppling failures are the main response of the cliffs to tidal and wave erosion the differences between the three estuarine systems in the mechanisms of marsh cliff erosion are partly reflected in the mode of preservation of the cliffs on the restoration of the conditions leading to renewal of marsh growth

high resolution holocene n2o ice core record and its relationship with ch4 and co2 1 nitrous oxide n 2 o concentration records exist for the last 1000 years and for time periods of rapid climatic changes like the transition from the last glacial to today s interglacial and for one of the fast climate variations during the last ice age little is known however about possible n 2 o variations during the more stable climate of the present interglacial holocene spanning the last 11 thousand years here we fill this gap with a high resolution n 2 o record measured along the european project for ice coring in antarctica epica dome c antarctic ice core on the same ice we obtained high resolution methane and carbon dioxide records this provides the unique opportunity to compare variations of the three most important greenhouse gases after water vapor without any uncertainty in their relative timing the co 2 and ch 4 records are in good agreement with previous measurements on other ice cores the n 2 o concentration started to decrease in the early holocene and reached minimum values around 8 ka 260 ppbv before a slow increase to its preindustrial concentration of 265 ppbv

kinetics of conversion of air bubbles to air hydrate crystals in antarctic ice the depth dependence of bubble concentration at pressures above the transition to the air hydrate phase and the optical scattering length due to bubbles in deep ice at the south pole are modeled with diffusion growth data from the laboratory taking into account the dependence of age and temperature on depth in the ice the model fits the available data on bubbles in cores from vostok and byrd and on scattering length in deep ice at the south pole it explains why bubbles and air hydrate crystals coexist in deep ice over a range of depths as great as 800 meters and predicts that at depths below 1400 meters the amanda neutrino observatory at the south pole will operate unimpaired by light scattering from bubbles

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

a channel evolution model for subtropical macrochannel systems a channel evolution model cem represents stages of channel development in response to specific types of disturbance in recent years classic incised disturbed cems have provided process based understanding of channel adjustment and formed the cornerstone for river restoration and rehabilitation while broadly applicable to alluvial systems in temperate and semi arid regions these models cannot be assumed to be universally applicable lockyer creek in south east queensland australia has notable macrochannel morphology and is subject to high hydrological variability typical of many subtropical climates the aim of this paper is to present a case study of channel adjustment and evolution in lower lockyer creek to determine if existing cems adequately describe processes of channel adjustment and the associated trajectories of change typical of river systems in subtropical settings lockyer creek has recently been subjected to a spate of flooding resulting in significant channel erosion this offers an ideal opportunity to investigate the nature and rate of channel adjustment processes and place them in context of longer term geomorphic adjustments in these systems specifically we address two questions firstly do the classic incised disturbed cems adequately represent the observed macrochannel adjustment secondly if current cems are inadequate what is the channel evolution model for these systems of which lower lockyer creek is an example results show that these are non incising systems where wet flow bank mass failures wbmfs are the dominant process of channel adjustment they occur within the channel bank top boundary resulting in no change to overall bank top width furthermore subsequent floods deposit sediment in the failure scars and failure headwalls generally do not retreat beyond channel bank top channel adjustment has not followed the evolutionary stages for incised disturbed channels and a new four stage macrochannel cem is outlined for these subtropical systems the proposed cem illustrates a cyclical pattern of erosion by channel bank wbmf followed by re aggradation through deposition and oblique processes contributing to bank rebuilding this cem provides sufficient information to determine the stage of macrochannel adjustment enabling decisions to be made over whether intervention is required or will be successful

regime equations for natural meandering cobble and gravel bed rivers data obtained from 48 stable reaches of upland rivers in the uk were stratified by stream type to develop regime equations specifically for natural meandering cobble and gravel bed rivers c3 and c4 stream types according to the rosgen classification multiple regression models were applied to derive equations for reach averaged values of bankfull width mean depth slope meander arc length and sinuosity in bankfull discharge and associated bed material load the caliber of the bed material bank vegetation density and valley slope the equations show that their cross sectional dimensions are primarily determined by the bankfull discharge bank vegetation and bed material size whereas their profile and plan form are very strongly influenced by the valley gradient although bankfull bed material load only appears to have a minor influence on channel morphology its effect is implicit in the value of bankfull discharge because this corresponds to the flow that transports most of the bed material load

changes in atmospheric carbonyl sulfide over the last 54 000 years inferred from measurements in antarctic ice cores we measured carbonyl sulfide cos in air extracted from ice core samples from the west antarctic ice sheet wais divide antarctica with the deepest sample dated to 54 300 years before present these are the first ice core cos measurements spanning the last glacial maximum lgm the last glacial interglacial transition and the early holocene the wais divide measurements from the lgm and the last transition are the first cos measurements in air extracted from full clathrate bubble free ice this study also includes new cos measurements from taylor dome antarctica including some in bubbly glacial ice that are concurrent with the wais divide data from clathrate glacial ice cos hydrolyzes in ice core air bubbles and the recovery of an atmospheric record requires correcting for this loss the data presented here suggest that the in situ hydrolysis of cos is significantly slower in clathrate ice than in bubbly ice the clathrate ice measurements are corrected for the hydrolysis loss during the time spent as bubbly ice only the corrected wais divide record indicates that atmospheric cos was 250 300 parts per trillion ppt during the lgm and declined by 80 100 ppt during the last glacial interglacial transition to a minimum of 160 210 ppt at the beginning of the holocene this decline was likely caused by an increase in the gross primary productivity of terrestrial plants with a possible contribution from a reduction in ocean sources cos levels were above 300 ppt in the late holocene indicating that large changes in the cos biogeochemical cycle occurred during the holocene

spatio temporal changes in river bank mass failures in the lockyer valley queensland australia wet flow river bank failure processes are poorly understood relative to the more commonly studied processes of fluvial entrainment and gravity induced mass failures using high resolution topographic data lidar and near coincident aerial photography this study documents the downstream distribution of river bank mass failures which occurred as a result of a catastrophic flood in the lockyer valley in january 2011 in addition this distribution is compared with wet flow mass failure features from previous large floods the downstream analysis of these two temporal data sets indicated that they occur across a range of river lengths catchment areas bank heights and angles and do not appear to be scale dependent or spatially restricted to certain downstream zones the downstream trends of each bank failure distribution show limited spatial overlap with only 17 of wet flows common to both distributions the modification of these features during the catastrophic flood of january 2011 also indicated that such features tend to form at some optimum shape and show limited evidence of subsequent enlargement even when flow and energy conditions within the banks and channel were high elevation changes indicate that such features show evidence for infilling during subsequent floods the preservation of these features in the landscape for a period of at least 150 years suggests that the seepage processes dominant in their initial formation appear to have limited role in their continuing enlargement over time no evidence of gully extension or headwall retreat is evident it is estimated that at least 12 inundation events would be required to fill these failures based on the average net elevation change recorded for the 2011 event existing conceptual models of downstream bank erosion process zones may need to consider a wider array of mass failure processes to accommodate for wet flow failures

abrupt climate change around 22 ka on the siple coast of antarctica a new ice core from siple dome antarctica suggests the surface temperature increased by 6 c in just several decades at approximately 22 ka bp this abrupt change did not occur 500 km away in the byrd ice core or in climate proxy records in the siple dome core indicative of the mid latitude pacific this demonstrates there was significant spatial heterogeneity in the response of the antarctic climate during the last deglaciation and draws attention to unexplained mechanisms of abrupt climate change in antarctica

monitoring and numerical modelling of riverbank erosion processes a case study along the cecina river central italy riverbank retreat along a bend of the cecina river tuscany central italy was monitored across a near annual cycle autumn 2003 to summer 2004 with the aim of better understanding the factors influencing bank changes and processes at a seasonal scale seven flow events occurred during the period of investigation with the largest having an estimated return period of about 1 5 years bank simulations were performed by linking hydrodynamic fluvial erosion groundwater flow and bank stability models for the seven flow events which are representative of the typical range of hydrographs that normally occur during an annual cycle the simulations allowed identification of i the time of onset and cessation of mass failure and fluvial erosion episodes ii the contributions to total bank retreat made by specific fluvial erosion and mass wasting processes and iii the causes of retreat the results show that the occurrence of bank erosion processes fluvial erosion slide failure cantilever failure and their relative dominance differ significantly for each event depending on seasonal hydrological conditions and initial bank geometry due to the specific planimetric configuration of the study bend which steers the core of high velocity fluid away from the bank at higher flow discharges fluvial erosion tends to occur during particular phases of the hydrograph as a result fluvial erosion is ineffective at higher peak discharges and depends more on the duration of more moderate discharges slide failures appear to be closely related to the magnitude of peak river stages typically occurring in close proximity to the peak phase preferentially during the falling limb but in some cases even before the peak while cantilever failures more typically occur in the late phase of the flow hydrograph when they may be induced by the cumulative effects of any fluvial erosion copyright 2008 john wiley sons ltd

slump blocks intraformational conglomerates and associated erosional structures in pennsylvanian fluvial strata of eastern canada pennsylvanian fluvial channel sandstones in new brunswick and nova scotia contain numerous examples of eroded mudstone surfaces including in situ mudstone beds boulders and slumped blocks the eroded surfaces bear a variety of structures including linear scours flutes longitudinal furrows and rill marks a block of interchannel mudstone up to 40 m in extent displays a basal slip plane slump related deformation and evidence of intense corrasion on a channel floor mudstone clasts from small pebbles to boulders over 4 m long are common immediately above channel base erosion surfaces and represent a lag clasts over 20 cm diameter are commonly fluted occasionally on all sides suggesting clast rotation rill marks occur on large mudclasts and in situ mudstone surfaces and indicate emergence and erosion by surging water or surface runoff preservation of the delicate erosional structures depended on a highly cohesive mud substrate and subsequent rapid burial a previous interpretation of the mud blocks and their surficial features as the result of mud intrusion is inconsistent with the field evidence

bed coarsening riffle shortening and channel enlargement in urbanizing watersheds northern kentucky usa abstract stream systems naturally respond to watershed land use dynamics particularly in urban developments with unmanaged impervious areas such urban provoked alterations to channel morphology cause water quality impairments have adverse effects on aquatic biota and pose risks to adjacent public infrastructure over the past four years we have collected detailed hydrogeomorphic data at 40 unique stream locations throughout northern kentucky with at least two rounds of annually repeated surveys at 70 of the sites and three rounds of surveys at 50 of the sites analysis of this time series data encompassed measured rates of instability across three distinct dimensions including 1 channel cross sections 2 longitudinal profiles and 3 bed material particle composition regression analyses between geomorphic change and 2011 watershed imperviousness indicated stream cross sections in urban suburban watersheds tend to be getting larger their overall shape is both deepening and widening additionally stream riffle lengths are shrinking and their pools are becoming both longer and deeper and finally their bed material composition is coarsening particularly in streams in the early stages of watershed development by documenting fluvial geomorphologic dynamics in such detail this study highlights the process by which unmitigated urbanization homogenizes stream habitat and degrades aquatic ecosystems this improved process based understanding of the urban induced channel response sequence has clear implications to both stormwater management and stream ecosystem restoration particularly in stream systems where headcut migration is a primary driver of channel instability

needle ice processes and sediment mobilization on river banks the river ilston west glamorgan uk abstract this paper examines the role of needle ice processes in the bank erosion system of a site on the river ilston west glamorgan south wales uk bank retreat rates were monitored with erosion pins over a 2 25 year period concurrent detailed data were also obtained on river flow levels and ambient air and stream temperatures unique data on river bank thermal regimes are presented along with rare information on the morphological properties and sediment yield characteristics of needle ice growths needle ice occurred even with mild freezing temperatures if sufficient bank moisture was present with average growth rates of approximately 1 mm h 1 bank material incorporation and mobilization within needle ice was significant sediment yields of up to 4 02 kg m 2 of bank face were observed and represented between 32 and 43 of total bank erosion recorded most sediment was incorporated within needle ice when growth became stressed because of overrapid cooling and or limited moisture supplies which supports the models of outcalt and fukuda four mechanisms of direct needle ice sediment transport on river banks were identified direct particle fall sediment laden rivulets sliding failures and toppling failures however the indirect preparatory role of needle ice and frost disturbance of bank surfaces is also emphasized and the enhanced fluvial erosion which takes place when bank materials have first been conditioned by frost related processes is noted both indirect and direct effects are expressed in a simple conceptual model and a number of the important system linkages examined combinations of needle ice action and subsequent fluid entrainment of weakened bank material appear to be the dominant mechanism for bank erosion here pronounced and persistent erosional notches in the lower parts of the bank profile were observed showing that in winter even single low magnitude flow events can effect significant channel width adjustment here

cyclic sedimentation in tertiary lower rhine basin germany the liegendrucken of the brown coal open cast fortuna mine abstract in the lower rhine basin at the northwestern headlands of the schiefergebirge in western germany the oligocene miocene koln formation once was splendidly exposed in the former fortuna mine one of the open cast mines of the regional brown coal mining industry there a complex 60 m thick clastic marine shoreline succession was studied coastal marine sediments were formed along a high energy coast in the basal sediment fill of the basin due to repeated ingressions of the tertiary north sea a number of subtidal to supratidal coastal onlap cycles developed this study provides first knowledge about cyclic sedimentation in the tertiary of the lower rhine basin

hydro physical characterization of contrasting soils in a semiarid zone of the ebro river valley ne spain summary the arid agricultural lands of the monegros desert in the central ebro basin ne spain show contrasting white patches wps and dark patches dps evidencing their different soil composition the white patches have shallow soils with low or nil crop development in most years soil at the dark patches uses to be deeper and winter cereal attains better development this paper analyzes soil hydro physical features in order to identify differences in soil physical properties between white and dark soils for this purpose 20 pairs of wp dp soils were studied on several agricultural plots each sampled pair was under the same soil condition three soil treatment st were studied freshly tilled soil mb cropped c and fallowed soil f the gypsum and calcium carbonate content organic matter om content and particle size distribution for the 0 10 cm depth soil were determined field measurements for the 1 10 cm depth soil layer were soil bulk density b soil penetration resistance spr saturated hydraulic conductivity k 1 10 and sorptivity s 1 10 the water retention curve wrc was determined in the lab on undisturbed soil cores for the 1 10 cm depth soil layer additional field measurements of soil surface crust sorptivity s crust and hydraulic conductivity k crust at saturation were also performed the results show that wp soils have significantly p b k and s values in both the surface crust and the 0 10 cm depth soil layer on average k 1 10 in dp was 1 4 times higher than that measured in wp dp showed a better defined microstructure and retained more water retention at near saturation conditions than wp soil treatments have a significant effect on b spr s 1 10 k 1 10 and on the wrc parameters overall b and spr was the lowest under mb with maximal s 1 10 and k 1 10 values under c compared to c and f mb presented the highest volume of structural porosity and the lowest water retention at near saturation conditions

multidecadal variability of atmospheric methane 1000 1800 c e 1 we present a new high precision high resolution record of atmospheric methane from the west antarctic ice sheet wais divide ice core covering 1000 1800 c e a time period known as the late preindustrial holocene lpih the results are consistent with previous measurements from the law dome ice core the only other high resolution record of methane for this time period and confirm most of the observed variability multidecadal variability in methane concentrations throughout the lpih is weakly correlated or uncorrelated with reconstructions of temperature and precipitation from a variety of geographic regions correlations with temperature are dominated by changes in northern hemisphere high latitude temperatures between 1400 and 1600 c e during the onset of the little ice age times of war and plague when large population losses could have reduced anthropogenic emissions are coincident with short periods of decreasing global methane concentrations

stochastic erosion of composite banks in alluvial river bends the erosion of composite river banks is a complex process involving a number of factors including fluvial erosion seepage erosion and cantilever mass failure to predict the rate of bank erosion with these complexities a stochastic bank erosion model is suitable to define the probability distribution of the controlling variables in this study a bank erosion model in a river bend is developed by coupling several bank erosion processes with an existing hydrodynamic and morphological model the soil erodibility of cohesive bank layers was measured using a submerged jet test apparatus seasonal bank erosion rates for four consecutive years at a bend in the brahmaputra river india were measured by repeated bankline surveys the ability of the model to predict erosion was evaluated in the river bend that displayed active bank erosion in this study different monsoon conditions and the distribution functions of two variables were considered in estimating the stochastic bank erosion rate the probability of the soil erodibility and stochastic stage hydrographs for the nth return period river stage additionally the influences of the deflection angle of the streamflow longitudinal slope of river channel and bed material size on bank erosion rate were also investigated the obtained stochastic erosion predictions were compared with the observed distribution of the annual average bank erosion rate of 45 river bends in the brahmaputra river the developed model appropriately predicted the short term morphological dynamics of sand bed river bends with composite banks copyright 2014 john wiley sons ltd

causes of planform stability of a low energy meandering gravel bed river cher river france abstract for at least two centuries the lateral mobility of the meandering reaches of the cher river france has been very low this article aims to identify the main causes of this behavior two not mutually exclusive explanatory hypotheses are proposed under the first hypothesis the natural mechanisms of loop migration would have been inhibited or blocked by the presence of bank protections under the second hypothesis a decrease in the frequency and or intensity of morphogenic hydrological events since the nineteenth century would have reduced the frequency of bedload mobilization and or reduced the capacity of the river to erode its banks to test these hypotheses the diachronic evolution of the planform was reconstituted at different time scales using a gis and field surveys morphological transformations were characterized and quantified eroded and vegetated areas length of eroded banks rates of bank retreat and the critical discharges of bedload mobilization and of lateral erosion were estimated engineering works in the riverbed were identified and when possible dated the results show that meander morphodynamics have been highly constrained and disrupted by engineering works probably for over a century however the meanders still have noticeable potential for bedload mobility and lateral erosion and hence for self restoration

methyl chloride variability in the taylor dome ice core during the holocene methyl chloride ch3cl is a naturally occurring ozone depleting trace gas and one of the most abundant chlorinated compounds in the atmosphere ch3cl was measured in air from the taylor dome ice core in east antarctica to reconstruct an atmospheric record for the holocene 11 0 kyr b p and part of the last glacial period 50 30 kyr b p ch3cl variability throughout the holocene is strikingly similar to that of atmospheric methane ch4 with higher levels in the early and late holocene and a well defined minimum during mid holocene the sources and sinks of atmospheric ch3cl and ch4 are located primarily in the tropics and variations in their atmospheric levels likely reflect changes in tropical conditions ch3cl also appears to correlate with atmospheric ch4 during the last glacial period 50 30 kyr b p although the temporal resolution of sampling is limited the taylor dome data provide information about the range of natural variability of atmospheric ch3cl and place a new constraint on the causes of past ch4variability

evidence for substantial accumulation rate variability in antarctica during the holocene through synchronization of co 2 in the taylor dome dome c and dml ice cores high resolution records of atmospheric co2 concentration during the holocene are obtained from the dome concordia and dronning maud land antarctica ice cores these records confirm that the co2 concentration varied between 260 and 280 ppmv in the holocene as measured in the taylor dome ice core however there are differences in the co2 records most likely caused by mismatches in timescales matching the taylor dome timescale to the dome c timescale by synchronization of co2 indicates that the accumulation rate at taylor dome increased through the holocene by a factor two and bears little resemblance to the stable isotope record used as a proxy for temperature this result shows that different locations experienced substantially different accumulation changes and casts doubt on the often used assumption that accumulation rate scales with the saturation vapor pressure as a function of temperature at least for coastal locations d 2004 elsevier b v all rights reserved

numerical simulation of hydrodynamics and bank erosion in a river bend we present an integrated analysis of bank erosion in a high curvature bend of the gravel bed cecina river central italy our analysis combines a model of fluvial bank erosion with groundwater flow and bank stability analyses to account for the influence of hydraulic erosion on mass failure processes the key novel aspect being that the fluvial erosion model is parameterized using outputs from detailed hydrodynamic simulations the results identify two mechanisms that explain how most bank retreat usually occurs after rather than during flood peaks first in the high curvature bend investigated here the maximum flow velocity core migrates away from the outer bank as flow discharge increases reducing sidewall boundary shear stress and fluvial erosion at peak flow stages second bank failure episodes are triggered by combinations of pore water and hydrostatic confining pressures induced in the period between the drawdown and rising phases of multipeaked flow events

computer program for stability analysis of steep cohesive riverbanks the ability to predict the stability of eroding riverbanks is a prerequisite for modelling alluvial channel width adjustment and a requirement for predicting bank erosion rates and sediment yield associated with bank erosion mass wasting of bank materials under gravity occurs through a variety of specific mechanisms with a separate analysis required for each type of failure this paper presents a computer program for the analysis of the stability of steep cohesive riverbanks with respect to planar type failures planar type failures are common along stream channels destabilized by severe bed degradation existing stability analyses for planar type failures have a number of limitations that affect their physical basis and predictive ability the computer program presented here is based on an analysis developed by darby and thorne the software takes account of the geotechnical characteristics of the bank materials the shape of the bank profile and the relative elevations of the groundwater and surface water to estimate stability with respect to mass failure along a planar type failure surface results can be displayed either in terms of a factor of safety ratio of resisting to driving forces or probability of failure the computer analysis is able to determine the relative amounts of bed degradation and bank toe erosion required to destabilize an initially stable bank data for the analysis are supplied in the form of either hec 2 hydrographic survey data files or user supplied bank profile data in conjunction with user supplied geotechnical parameter values some examples using data from the upper missouri river in montana are used to demonstrate potential applications of the software copyright 2000 john wiley sons ltd

numerical simulation of widening and bed deformation of straight sand bed rivers ii model evaluation in this paper the numerical model presented in the companion paper is tested and applied assessment of model accuracy was based on two approaches first predictions of evolution of a 13 5 km reach of the south fork of the forked deer river in west tennessee were compared to observations over a 24 yr period results suggest that although the model was able to qualitatively predict trends of widening and deepening quantitative predictions were not reliable simulated widths and depths were within 15 of the corresponding observed values but observed change in these parameters at the study sites were also close to these values simulated rates of depth adjustment were within 15 of observed rates but observed rates of channel widening at the study sites were approximately three times those simulated by the model in the second approach the model was used to generate relationships between stable channel width and bank full discharge the model was able to successfully replicate the form of empirically

anatomy of a channel bank collapse structure in tertiary fluvio lacustrine sediments of the lower rhine basin germany an exceptionally well preserved synsedimentary deformation feature is described from uppermost miocene fluvio lacustrine coastal plain sediments exposed in lignite mines of the lower rhine basin germany the feature is interpreted as a complex multi stage channel bank collapse structure rotational slumping of bank material into the channel along two failure surfaces extending below the channel toe generated a series ofminor compressional thrust backthrust structures within the muddy channel fill sediments a reconstruction of the failure event is proposed which is linked toa lowering of river stage and subsequent channel abandonment the accurate description of this unusually well preserved channel bank collapse structure may have general application in the interpretation of similar but less well exposed preserved phenomena elsewhere

european impacts on downstream sediment transfer and bank erosion in cobargo catchment new south wales australia abstract upland valley fills in cobargo catchment on the south coast of new south wales represent substantial sediment source zones contemporary channels within these fills are up to 8 m deep and 50 m wide for catchment areas 2 virtually all banks are eroding downstream of this sediment source zone sediment transfer zones are characterised by sinuous channels that are partially choked by sands released from upland valley fills coarse sands stored in point bars deflect flow to outer banks where colluvial footslopes are eroded approximately 50 of banks are eroding in this section of the catchment other than the 10 km river reach upstream of the river mouth the remainder of the catchment is a bedrock confined sediment throughput zone characterised by fluctuations in channel bed elevation the downstream 10 km of the catchment is a sediment accumulation zone in which erosion is restricted to occasional concave banks prior to european settlement of cobargo catchment upland valley fills were largely unincised and middle transfer reaches of the catchment comprised swamps vegetation clearance along with disturbance and drainage of swamps transformed discontinuous water courses into continuous channels working from a base point of intact upland valley fills almost 50 of available material has been removed from the upper catchment sediment source zone contributing 2 9 10 6 m 3 of material to the lower catchment of this roughly 65 has been stored along the channel bed and floodplain this gives a total sediment contribution to the delta of 1 0 10 6 m 3 sufficient materials are stored in transient storage units along transfer zones to maintain current rates of sediment throughput to the delta for several decades cobargo catchment experienced dramatic acceleration of channel incision and bank erosion processes within a few decades following european settlement of the catchment i e around 1830 transition in landscape form and associated sediment delivery were rapid with a negligible lag interval i e within a few decades of disturbance however given the extensive volume of sediment released from upland valley fills landscape recovery i e refilling of upland valleys will likely take thousands of years

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

pore water pressure and streambank stability results from a monitoring site on the sieve river italy to investigate the role of pore water pressures in the stability of a streambank a series of tensiometers and piezometers was installed in a bank of the sieve river tuscany italy fluvial entrainment at the bank toe was monitored by repeated cross profiling erosion pins and marked pebbles fluctuations in matric suction measured at the tensiometers reflected the overall response of pore water pressures to rainfall evapotranspiration rising and drawdown of the river stage and variations in water table an expression was derived for the safety factor with respect to mass movement of the upper bank incorporating the failure criterion for unsaturated soils and the normal mohr coulomb criterion for saturated conditions variations in matric suction have important effects on the stability of the streambank during low flow periods the shear strength term due to the matric suction allows the bank to remain stable at a steep angle however during rainfall and flow events reduction in matric suction and increase in unit weight of the material from vertical and lateral infiltration may be sufficient to trigger a mass failure without development of significant positive pore water pressures during the rising limb of high flow events the factor of safety increases as a consequence of the stabilizing confining pressure of the water in the river despite a reduction in matric suction during drawdown in the river when the suction values are still low and the confining pressure in the river decreases to zero the factor of safety falls to lower values than those experienced prior to the runoff event measurements of fluvial entrainment reveal that although the processes mechanisms and the frequency of retreat of basal and upper bank zones differ significantly the amount of retreat at the bank toe due to fluvial erosion is comparable to that of the upper portion of the bank due to mass failure copyright 1999 john wiley sons ltd

late quaternary fluvial dynamics of the jarama river in central spain the jarama river and its catchment provide a valuable natural archive to study and reconstruct past environmental conditions in central spain this region is highly prone to changes in the hydrological cycle under predicted climate warming in particular with respect to aridity periods rainfall variability and the occurrence of extreme flood events this paper presents 15 exposures covering a time span of the last 44 ka which were documented in the field and of which seven exposures were soil chemical analyzed regarding soil texture organic matter as well as carbonate and iron content ages are based on radiocarbon dating on 32 samples latest holocene sediments were found within the channel belt where permanent remobilization of sediments takes place due to the migration of the meandering river course in more distal floodplain positions sediment sequences show a complex fluvial architecture referring to periods of varying character and intensity of alluviation as well as periods of geomorphic stability indicated by soil formation furthermore sedimentation patterns vary along different river sections aggradations of coarse gravels took place between 40 and 18 ka cal bp over the entire valley floor alluviation of fine material was documented between 17 and 16 at 7 5 between 5 1 and 3 3 at 2 8 between 2 1 and 1 5 at 1 0 and around 0 4 ka cal bp until recent times astonishingly in late holocene times between 4 2 and 3 1 ka cal bp aggradations of coarse gravels even in distal floodplain areas overlap with sedimentation of fine material in adjacent river sections pointing to a complex constellation of parameters involved periods of soil formation were detected around 43 at 31 between 16 and 12 6 after 7 5 until 5 1 between 2 8 and 2 1 and at times after 1 5 ka cal bp phases of geomorphic stagnation were found at 38 ka cal bp and for a duration of 200 years at 3 0 ka cal bp and refer to absent geomorphic or pedogenic processes all these patterns are an expression of the interaction between climatic variations tectonic impulses and human influences a comparison with other terrestrial archives enabled us to reconstruct late pleistocene and holocene palaeoenvironmental conditions on a regional scale and furthermore to link specific stages of floodplain development to prevalent influencing variables thus with emphasis on the fluvial system response a model of a cause and effect relationship is presented that concentrates on rapid climate changes long lasting climatic deteriorations and the role of human interventions

a 350 year atmospheric history for carbonyl sulfide inferred from antarctic firn air and air trapped in ice during 1650 1850 a d and increases throughout most of the twentieth century measurements of cos in modern air and in the upper layers of the firn at south pole indicate ambient annual mean mixing ratios between 480 and 490 ppt with substantial seasonal variations peak mixing ratios are observed during austral summer in ambient air at south pole and cape grim tasmania 40 41 s provided cos is not produced or destroyed in firn these results also suggest that atmospheric cos mixing ratios have decreased 60 90 ppt 10 16 since the 1980s in high latitudes of the southern hemisphere the history derived for atmospheric mixing ratios of cos in the southern hemisphere since 1850 is closely related to historical anthropogenic sulfur emissions the fraction of anthropogenic sulfur emissions released as cos directly or indirectly needed to explain the secular changes in atmospheric cos over this period is 0 3 0 6 index terms 0325 atmospheric composition and structure evolution of the atmosphere 0365 atmospheric composition and structure troposphere composition and chemistry keywords atmosphere composition carbonyl sulfide stratosphere sulfate aerosol