modeling width adjustment in meandering channels widening in sinuous channels occurs when the retreat of the outer bank exceeds the advance of the opposite bank an analytical model is presented to simulate width adjustment in meandering channels of non cohesive bank material resulting from bank erosion of two interactive processes basal erosion and bank collapse bank collapse refers to the avalanche of non cohesive material in the upper part of bank above water surface resulting from over steepening of the bank surface due to basal erosion the rate of basal erosion including lateral erosion and bed degradation is calculated as a function of the longitudinal gradient of sediment transport rate and strength of secondary flow the transverse bed slope is treated as a variable that increases as channel sinuosity increases until the transverse bed slope reaches its maximum value by simplifying the bank collapse process for non cohesive materials the present study shows that the rate of bank line retreat is determined by lateral erosion rate near bank bed degradation rate sediment grain size and difference between flow depth and bank height the time dependent widening processes of two meandering channels in the laboratory are selected to test applicability of the model the result shows that the simulated bank lines at individual time intervals closely match the experimental measurements whether the sinuosity of a meandering channel will increase or decrease is primarily determined by didtribution of the lngitudinal gradient of sediment transport rate along the channel

riverbank change sixmile island ohio river usa abstract soil exploration collection of hydrologic and meteorologic data and sequential comparative photography were used to investigate erosion deposition on alluvial banks of an island in the ohio river from april 1980 through april 1982 major erosion episodes coincided with flood events and were caused primarily by exit of bank recharge waters from pervious bank soil layers after flood recession waves created by wind and or vessel passage were of little significance in causing bank failure floodwater tractive forces runout of precipitation derived waters and wave action were effective only in removing soft sediments or loose debris from upper bank failures sediments deposited by spring floods if allowed to dry for two to three months strongly resisted subsequent wave attack

interpreting geoproxies of late quaternary climate change in african drylands implications for understanding environmental change and early human behaviour abstract climate and environmental change in the late quaternary are widely regarded as key drivers of early human development and dispersal in africa robust records of change are required to assess the nature of potential impacts today s late quaternary palaeoenvironmental palaeoclimate reconstructions from the interior southern african basin kalahari are more spatially detailed have a stronger chronometric underpinning and span a longer time range than those of 10 20 years ago but are no less controversial reconstructions for the last 150 ka are usually interpreted in terms of changes in hydrological balance from conditions more humid or more arid than today and expressed in climatological terms as the changing latitudinal interplay between monsoon easterlies and tropical and mid latitude westerlies with more data than ever before why then is the record not yet delivering a more coherent picture of regional climate change this paper deliberately addresses current challenges and controversies including issues often ignored or neglected with the view that it is essential to do so if more robust interpretations of the past are to be generated in the future sources of palaeo data from the interior are predominantly geoproxies reconstructions not based on highly resolved organic or isotope records but on interpretation of spatially extensive geomorphological proxies of past environments despite being increasingly datable using osl their inclusion in syntheses has proved complex not least when interpretations appear to conflict with records from other sources consequently they are sometimes removed from regional syntheses of climate change these omissions are often replaced by spatially exaggerated interpretations of higher quality local records allowing more spatially coherent timeslice interpretations to be presented yet this is counter intuitive to the spatial complexity of conditions present in the palaeoenvironmental proxies or in climatic and environmental systems today in addition the quest to link regional records to master records from ice and ocean cores can force interpretation within a temporal scale that may be inappropriate for their analysis underplaying the reality of complexity within the environmental system there are not necessarily good or bad records of terrestrial palaeoenvironments in interior southern africa rather there are records that contain different suites of information about past land surface hydrological and climate conditions preserved at temporal scales which can sometimes be incomparable consideration of these factors may resolve apparent conflicts between datasets resulting in a better understanding of past environmental dynamics and revealing the nature of environmental variability which may be a key facet to which human development has responded

effects of riparian vegetation on stream bank subaerial processes in southwestern virginia usa riparian vegetation is frequently used for stream bank stabilization but the effects of vegetation on subaerial processes have not been quantified subaerial processes such as soil desiccation and freeze thaw cycling are climate related phenomena that deliver soil directly to the stream and make the banks more vulnerable to fluvial erosion by reducing soil strength this study compares the impact of woody and herbaceous vegetation on subaerial processes by examining soil temperature and moisture regimes in vegetated stream banks soil temperature and water tension were measured at six paired field sites in southwestern virginia usa for one year results showed that stream banks with herbaceous vegetation had higher soil temperatures and a greater diurnal temperature range during the summer compared to forested stream banks daily average summer soil water tension was 13 to 57 per cent higher under herbaceous vegetation than under woody vegetation probably due to evapotranspiration from the shallow herbaceous root system on the bank in contrast to summer conditions the deciduous forest buffers provided little protection for stream banks during the winter the forested stream banks experienced diurnal temperature ranges two to three times greater than stream banks under dense herbaceous cover and underwent as many as eight times the number of freeze thaw cycles during the winter the stream banks under the deciduous forests were exposed to solar heating and night time cooling which increased the diurnal soil temperature range and the occurrence of freeze thaw cycling study results also indicated that freeze thaw cycling and soil desiccation were greater on the upper stream bank due to thermal and moisture regulation of the lower bank by the stream therefore subaerial erosion and soil weakening may be greater on the upper stream banks additional research is needed on the influence of subaerial processes on both subaerial and fluvial erosion copyright 2006 john wiley sons ltd

insights into river bank erosion processes derived from analysis of negative erosion pin recordings observations from three recent uk studies recent studies of river bank erosion in three catchments in the uk have been characterized by the persistent occurrence of negative erosion pin results the cause of these negative recordings is considered with reference to field data from the afon trannon nant tanllwyth and river arrow and to a laboratory study of freeze thaw and desiccation processes it seems that there is potential for and in some cases evidence of a number of different circumstances that generate negative results but none of these alone is sufficient to explain all incidents factors considered include deposition of sediment during high flows soil fall from the upper parts of the bank on to lower erosion pins loosening of the soil surface and expansion contraction of the soil mass with fluctuations in temperature and moisture content movement of the erosion pin within the bank and human interference each has its own implications for the use of erosion pins further issues arise when including negative data in subsequent data analysis and it is demonstrated that attempts to correlate erosion rates with hydro meteorological data in order to ascertain causes of erosion will be influenced by the way in which negative data are handled it is thus suggested that any study of river bank erosion using erosion pins should state whether or not negative data were obtained and if so how they were included in data analysis failure to include this information could lead to comparison of mean erosion rates that reflect bank processes very differently the studies presented here offer a clear example of the value of anomalous field data results which do not appear to fit expected patterns can reveal as much about the processes in operation as those that do

present and past microbial life in continental pan sediments and its response to climate variability in the southern kalahari abstract terrestrial climate archives are rare in the arid southwestern african region which makes paleoclimate and paleoenvironmental studies difficult since there are only ephemeral lacustrine systems in the area in this study a continental pan playa is evaluated as a climate archive climate has a strong impact on the pan ecosystem causing adaptation of indigenous microorganisms to varying temperature precipitation and salinity conditions here a combined approach of inorganic and organic geochemical investigations including lipid biomarker analyses was carried out to examine the response of indigenous microbial communities to environmental changes and to characterize the nature abundance and depth distribution of recent phospholipids and past glycerol dialkyl glycerol tetraethers gdgts microbial life within the sediments of witpan located in the southern kalahari lipid biomarkers contain information about changes in biogeochemical processes and climate variation therefore we tested here whether they can be used to reconstruct paleoclimatic changes such as past precipitation periods in arid terrestrial ecosystems despite the extreme environmental conditions with rather low toc values restricted availability of water and substrates in the pan system bacterial life was observed along the depth profile of witpan bacterial membrane phospholipid life markers showed their highest abundance in the surface layers indicating that microbial life in witpan is strongly influenced by near surface processes a series of saturated branched and unsaturated phospholipid fatty acids plfas were detected and several phyla of bacteria such as gram positive and gram negative bacteria were present some plfas and intact archaeal membrane lipids point to the presence of a halophilic microbial community in the surface layers biomarkers for past microbial life archaeol branched and isoprenoid gdgts were absent or had very low concentrations during the dry holocene sequence below the surface sediments however during the last glacial maximum lgm considered to represent a period with increased precipitation an increased abundance of these biomarkers was observed thus these results demonstrate the potential of microbial biomarkers in pan systems to preserve climate signals over geologic timescales the data indicates that microbial biomarkers can be used to trace paleo precipitation periods in semi arid to arid environments and that pan structures can form suitable geo archives for biomolecules in areas where other terrestrial archives are missing

kalahari valley calcretes their nature origins and environmental significance abstract calcretes that form in non pedogenic settings have been widely reported in the geomorphological and geological literature yet they are still poorly understood in comparison with pedogenic varieties this may be because there are assorted types of non pedogenic calcretes often loosely referred to as groundwater calcretes but encompassing groundwater phreatic open valley or confined channel calcrete types forming within vadose and phreatic environments in different geomorphological settings relatively few studies have described the detailed petrological characteristics of such calcretes leading wright and tucker calcretes international association of sedimentologists reprint series 1991 vol 2 blackwell scientific publications oxford p 10 to suggest that much more work is needed to define the groundwater calcrete facies and to devise criteria for its recognition so that the various non pedogenic calcretes can be identified within the quaternary as well as further back in the geological record most descriptions of groundwater calcretes are highly generalistic and do not take into account the variability that may occur as a result of calcretes forming in different positions within a landscape and affected by separate controls thus the range of groundwater calcretes that exists now needs to be studied individually to identify patterns within the distinct types as a step towards expanding our knowledge of the variety of non pedogenic calcretes this paper describes the petrology micromorphology and mode of development of one of these types of calcrete namely that which forms within valley locations but not in definite fluvial channels late quaternary to holocene valley calcretes are described from various dry valleys mekgacha across the kalahari region of central botswana with the majority of samples collected from trunk and tributary valleys of the okwa samples have been analysed in thin section and under scanning electron microscope in order to determine the carbonate matrix type and calcrete micromorphology in general most samples consist of grains of quartzose kalahari sand cemented by fine crystalline often glaebular grain coating and pore filling micrite cement types are fairly consistent and not as micromorphologically variable as has been noted for other kalahari calcretes biological inputs are prevalent in many samples and include networks of calcified rootlets these characteristics reflect the fact that the calcretes formed in a relatively near surface environment with relatively high rates of evaporation

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

a novel proxy for terrestrial organic matter in sediments based on branched and isoprenoid tetraether lipids we propose a novel tracer for terrestrial organic carbon in sediments based on the analysis of tetraether lipids using high performance liquid chromatography mass spectrometry hplc ms analysis of terrestrial soil and peats shows that branched tetraether lipids are predominant in terrestrial environments in contrast to crenarchaeol the characteristic membrane lipid of non thermophilic crenarchaeota which is especially abundant in the marine and lacustrine environment based on these findings an index was developed the so called branched and isoprenoid tetraether bit index based on the relative abundance of terrestrially derived tetraether lipids versus crenarchaeol this bit index was applied to surface sediments from the angola basin where it was shown to trace the outflow of the congo river furthermore analyses of particulate organic matter from the north sea showed relatively higher bit indices in water column particulate organic matter near large river inputs a survey of globally distributed marine and lacustrine surface sediments shows that the bit index in these environments correlates with the relative fluvial input of terrestrial organic material making this index generally applicable the new proxy allows the rapid assessment of the fluvial input of terrestrial organic material in immature sediments up to 100 ma old

last glacial maximum dune activity in the kalahari desert of southern africa observations and simulations abstract it has long been understood that as ephemeral landscape features sand dunes are highly sensitive to environmental change and thus their distribution and the timing of their development may provide clues to past climate dynamics the relationship between climate and dune activity however is neither simple nor straightforward with a range of controls affecting the balance between erodibility the availability of sediment for deflation and erosivity the potential for sediment transport to explore such complex systems over large spatial and temporal scales a number of dune activity indices dai have been created that incorporate wind speed and moisture balances to calculate the potential for and degree of dune mobilisation using modern weather station data these indices have generally been shown to provide reasonable indications of dune activity potential until recently however the detailed quantitative data required to inform these equations has not been available for past climate scenarios and attempts to determine the relative importance of the various controls of dune activity have relied on rough estimations of climatic parameters this paper combines data from monthly general circulation model gcm outputs from the coupled ocean atmosphere gcms for 21 ka with the most detailed dai equation presently available to calculate the potential for dune reactivation in southern africa during the last glacial maximum lgm 18 24 ka based on these data and calculations it is indicated that there was significantly less potential for dune activity across southern africa at 21 ka when compared to the aeolian sediment records from the region this study poses serious and fundamental questions about 1 the reliability of the model outputs 2 the degree to which dais are able to account for the complexity and dynamics of aeolian systems and or 3 the interpretation of dune records as palaeoclimatic proxies at millennial time scales

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

numerical analysis of river channel processes with bank erosion this paper presents a numerical analysis of river channel processes with bank erosion the model can be used for investigating both bed deformation and bankline shifting in 2d plan form the basic equations are used in a moving boundary fitted coordinate system and a new formulation of nonequilibrium sediment transport is introduced to reproduce the channel processes the model was applied to examine the morphological behavior of experimental channels temporal changes in the plan form in a meandering channel can be classified into two patterns meander developing and meander straightening comparison of the observed and calculated results indicates that the model is applicable to both channel changes under various hydraulic conditions on the basis of the numerical findings the paper clarifies the influence of hydraulic variables on the location of bank erosion and bed scouring the model also was used to investigate the effect of alternate bars on bank erosion and to investigate the development of channel meandering from an initially straight channel

morphological modelling of rivers with erodible banks a bank erosion mechanism and provisions to account for the associated planform changes and input of bank erosion products are added to a two dimensional depth averaged model of river morphology the model is applied to a reach of the meandering gravel bed river oh e eger in the former state of czechoslovakia the agreement with observations is poor but this can be ascribed to shortcomings in the flow and bed topography submodels rather than to shortcomings in the bank erosion submodel better results are expected when a three dimensional flow model equations for sediment mixtures and a bank accretion mechanism are included this inclusion will have to be preceded by fundamental research on bank accretion mechanisms and on hiding and exposure effects in the relationship for the influence of gravity pull on sediment transport direction 1998 john wiley sons ltd

effects of uncertain topographic input data on two dimensional flow modeling in a gravel bed river 1 many applications in river research and management rely upon two dimensional 2d numerical models to characterize flow fields assess habitat conditions and evaluate channel stability predictions from such models are potentially highly uncertain due to the uncertainty associated with the topographic data provided as input this study used a spatial stochastic simulation strategy to examine the effects of topographic uncertainty on flow modeling many equally likely bed elevation realizations for a simple meander bend were generated and propagated through a typical 2d model to produce distributions of water surface elevation depth velocity and boundary shear stress at each node of the model s computational grid ensemble summary statistics were used to characterize the uncertainty associated with these predictions and to examine the spatial structure of this uncertainty in relation to channel morphology simulations conditioned to different data configurations indicated that model predictions became increasingly uncertain as the spacing between surveyed cross sections increased model sensitivity to topographic uncertainty was greater for base flow conditions than for a higher subbankfull flow 75 of bankfull discharge the degree of sensitivity also varied spatially throughout the bend with the greatest uncertainty occurring over the point bar where the flow field was influenced by topographic steering effects uncertain topography can therefore introduce significant uncertainty to analyses of habitat suitability and bed mobility based on flow model output in the presence of such uncertainty the results of these studies are most appropriately represented in probabilistic terms using distributions of model predictions derived from a series of topographic realizations

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

effect of sediment transport boundary conditions on the numerical modeling of bed morphodynamics abstractexperimental sediment transport and river morphologic studies in laboratory flumes can use two sediment supply methods an imposed feed at the upstream end or a recirculation of sediment f

meander migration and the lateral tilting of floodplains the response of a meandering river to lateral tilting of the floodplain that supports the river is studied theoretically here the lateral direction is perpendicular to the overall direction of the river on the floodplain it is found that for most natural meandering rivers the river will migrate down the tilt direction to a lower elevation however the theory also shows that under some conditions especially when the froude number of the flow in the channel is very small the river migration is biased in the opposite direction two major contributions to the response of the river to lateral tilting identified in the theory are 1 perturbation of the cross section averaged flow velocities and 2 modification of the near bank flow velocity variation along the channel the theory is one of the first attempts to develop a quantitative understanding of the effects of floodplain tilting on meandering rivers and it provides a framework for future studies

branched and isoprenoid tetraether bit index traces water content along two marsh soil transects surrounding lake qinghai implications for paleo humidity variation abstract the bit index represents the relative abundances of branched glycerol dialkyl glycerol tetraethers bgdgts and the isoprenoid gdgt crenarchaeol while bgdgts are produced mainly by soil anaerobic bacteria crenarchaeol is known to be a biomarker for aerobic ammonia oxidation by chemolithoautotrophic thaumarchaeota particularly in the open ocean or lakes thus the index in marine and lacustrine settings has been widely used as a proxy for soil input here we have investigated gdgt distribution along two transects extending from the lake shore marsh to upland soils on the ne qinghai tibetan plateau the results show that soil water content swc correlates positively with concentration of bgdgts and negatively with concentration of crenarchaeol consequently there is a significant positive correlation between swc and the bit index our study highlights a new potential application of bit as a humidity proxy in loess soil and peat deposits

modelling basin effects on earthquake ground motion in the santiago de chile basin by a spectral element code summary simulations of strong ground motion within the santiago de chile metropolitan area were carried out by means of 3 d deterministic wave propagation tool based on the spectral element method the simulated events take into account the pronounced interface between the low velocity sedimentary basin and the bedrock as well as topography of the area to verify our model we simulated a regional earthquake recorded by a dense network installed in the city of santiago for recording aftershock activity after the 2010 february 27 maule main shock the results proof the alluvial basin amplification effects and show a strong dependence of spectral amplification in the basin on the local site conditions moreover we studied the seismic response due to a hypothetical mw 6 0 event occurring along the active san ramon fault which is crossing the eastern edge of the city the scenario earthquakes exhibit that an unfavourable interaction between fault rupture radiation mechanism and complex geological and topographic conditions in the near field region may give rise to large values of peak ground velocity in the basin finally 3 d numerical predictions of ground motion are compared with the one computed according to ground motion prediction equations selected among the next generation attenuation relationships in terms of ground motion peak values and spectral acceleration the comparison underlines that the 3 d scenario simulations predict a significantly higher level of ground motion in the santiago basin especially over deep alluvial deposits moreover also the location of the rupture nucleation largely influences the observed shaking pattern

1d finite volume model of unsteady flow over mobile bed summary a one dimensional 1d finite volume method fvm model was developed for simulating unsteady flow such as dam break flow and flood routing over mobile alluvium the governing equation is the modified 1d shallow water equation and the exner equation that take both bed load and suspended load transport into account the non equilibrium sediment transport algorithm was adopted in the model and the van rijn method was employed to calculate the bed load transport rate and the concentration of suspended sediment at the reference level flux terms in the governing equations were discretised using the upwind flux scheme harten et al 1983 hll and hllc schemes roe s scheme and the weighted average flux waf schemes with the double minmod and minmod flux limiters the model was tested under a fixed bed condition to evaluate the performance of several different numerical schemes and then applied to an experimental case of dam break flow over a mobile bed and a flood event in the rillito river tucson arizona for dam break flow over movable bed all tested schemes were proved to be capable of reasonably simulating water surface profiles but failed to accurately capture the hydraulic jump the waf schemes produced slight spurious oscillations at the water surface and bed profiles and over estimated the scour depth when applying the model to the rillito river the simulated results generally agreed well with the field measurements of flow discharges and bed elevation changes modeling results of bed elevation changes were sensitive to the suspended load recovery coefficient and the bed load adaptation length which require further theoretical and experimental investigations

a framework for the analysis of noncohesive bank erosion algorithms in morphodynamic modeling in this paper we analyze the performance of several non cohesive bank erosion algorithms to be embedded into two dimensional models for river hydro morphodynamics on non moving meshes to avoid the complexity of analyzing two dimensional model results arising from the non linear interaction between flow fluvial transport and bank erosion we develop a simplified framework in detail we reduce the two dimensional morphodynamic model to a cross sectional model under the assumption of longitudinal morphodynamic equilibrium and apply bank erosion algorithms therein to build candidate bank erosion models we break down bank erosion algorithms into three modeling steps identification of the bank computation of sediment fluxes due to bank erosion and bank updating different potential models are created by choosing different options for each step we assess model performance against surveyed bank erosion over a flood event in the transitional selwyn river new zealand this study is preliminary to implementation of bank erosion in a fully two dimensional setting to model braided planform dynamics

two dimensional depth averaged model simulation of suspended sediment concentration distribution in a groyne field summary river training structures such as spur dikes are effective engineered methods used to protect banks and improve aquatic habitat this paper reports the development and application of a two dimensional depth averaged hydrodynamic model to simulate suspended sediment concentration distribution in a groyne field the governing equations of flow hydrodynamic model are depth averaged two dimensional reynold s averaged momentum equations and continuity equation in which the density of sediment laden flow varies with the concentration of suspended sediment the depth averaged two dimensional convection and diffusion equation was solved to obtain the depth averaged suspended sediment concentration the source term is the difference between suspended sediment entrainment and deposition from bed surface one laboratory experiment was chosen to verify the simulated flow field around a groyne and the other to verify the suspended sediment concentration distribution in a meandering channel then the model utility was demonstrated in a field case study focusing on the confluence of the kankakee and iroquois rivers in illinois united states to simulate the distribution of suspended sediment concentration around spur dikes results demonstrated that the depth averaged two dimensional model can approximately simulate the flow hydrodynamic field and concentration of suspended sediment spur dikes can be used to effectively relocate suspended sediment in alluvial channels

channel adjustments to changing discharges powder river montana sixteen years of annual surveys reveal how powder river responds to varying discharges during 1978 the second largest recorded daily mean discharge occurred cutbank migration bed degradation net bank erosion and overbank deposition all contributed to increase the channel area at 12 cross sections by an average of 62 during the ensuing years the channel area decreased as sediment was stored in low lying benches adjacent to the active bed of the channel the survey data indicate that the balance between bank erosion and deposition varies with discharge in years when the annual maximum daily mean discharge is 3 s a flow of 60 m 3 s has a recurrence interval of 1 1 yr bank erosion and deposition are approximately equal in years when the annual maximum daily mean discharge is between 60 and 150 m 3 s a discharge of 150 m 3 s has a recurrence interval of 2 7 yr bank deposition exceeds bank erosion and the channel contracts often by developing benches in years with higher discharges the channel expands through net bank erosion these results demonstrate that the channel of powder river is influenced by a wide variety of formative discharges powder river9s recent history of expansion and contraction and the development of prominent benches cannot be explained by equilibrium models based on a single channel forming discharge

development and testing of riverbank stability analysis the ability to predict the stability of eroding riverbanks is a prerequisite for modeling alluvial channel width adjustments and a requirement for predicting bank erosion rates and sediment yield associated with bank erosion however there are a number of limitations of existing bank stability analyses that limit their physical basis and predictive ability some of these limitations are addressed through the development of a new bank stability analysis the new approach is applicable to steep cohesive nonlayered riverbanks that fail along planar failure surfaces pore water and hydrostatic confining pressure terms are included in the analysis the failure plane is not constrained to pass through the toe of the bank the predictive abilities of four bank stability analyses lohnes and handy 1968 huang 1983 osman and thorne 1988 and the present analysis were assessed using field data the new analysis is the most successful of the tested analyses in terms of predicting the stability of riverbanks wit

bank and near bank processes in an incised channel abstract gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion the interaction of these forces control streambank mechanics hydraulic forces exerted by flowing water on in situ bank toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower bank profiles seepage forces exerted on in situ bank material by groundwater downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength data from a study site on goodwin creek ms usa clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore water pressures negative pore water pressures matric suction have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear a stable bank can be transformed into an unstable bank during periods of prolonged rainfall through 1 increase in soil bulk unit specific weight 2 decrease or complete loss of matric suction and therefore apparent cohesion 3 generation of positive pore water pressures and therefore reduction or loss of frictional strength 4 entrainment of in situ and failed material at the bank toe and 5 loss of confining pressure during recession of stormflow hydrographs relatively small frequent flows during the winter have the ability to erode failed bank materials maintain oversteepened unstable bank surfaces and promote prolonged periods of bank retreat channel migration and high yields of fine grained sediment confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe which steepens the bank for example more than 2 m of bank retreat occurred during the study period at the research site on goodwin creek northern mississippi the loss of matric suction negative pore pressures due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability apparent cohesion friction angle soil bulk unit weight and moisture content were measured in situ matric suction was measured continuously in situ with a series of five pressure transducer tensiometers a bank failure algorithm which combines the mohr coulomb approach for saturated conditions and the fredlund modification for unsaturated conditions was developed for layered cohesive streambanks the resulting equation has been used successfully to investigate the role of matric suction positive pore water pressures and confining pressure for layered streambanks composed of cohesive materials

assessing the paleoenvironmental potential of pliocene to holocene tufa deposits along the ghaap plateau escarpment south africa using stable isotopes the tufa deposits of the ghaap plateau escarpment provide a rich yet minimally explored geological archive of climate and environmental history coincident with hominin evolution in south africa this study examines the sedimentary and geochemical records of ancient and modern tufas from buxton norlim limeworks groot kloof and gorrokop to assess the potential of these sediments for providing reliable chronologies of high resolution paleoenvironmental information chronometric dating demonstrates that tufa formation has occurred from at least the terminal pliocene through to the modern day the stable isotope records show a trend toward higher more variable 18o and 13c values with decreasing age from the end of the pliocene onwards the long term increase in 18o values corresponds to increasingly arid conditions while increasing 13c values reflect the changing proportion of c3 c4 vegetation in the local environment analysis of the thabaseek tufa in particular provides valuable evidence for reconstructing the depositional and chronological context of the enigmatic taung child australopithecus africanus collectively the results of the present study demonstrate the potential of these deposits for developing high precision records of climate change and ultimately for understanding the causal processes relating climate and hominin evolution

case study two dimensional model simulation of channel migration processes in west jordan river utah the paper presents the application of a two dimensional depth averaged numerical model to simulate the lateral migration processes of a meandering reach in the west jordan river in the state of utah a new bank erosion model was developed and then integrated with a depth averaged two dimensional hydrodynamic model the rate of bank erosion is determined by bed degradation lateral erosion and bank failure because bank material in the west jordan river is stratified with layers of cohesive and noncohesive materials a specific bank erosion model was developed to consider stratified layers in the bank surface this bank erosion model distinguishes itself from other models by relating bank erosion rate with not only flow but also sediment transport near the bank additionally bank height slope and thickness of two layers in the bank surface were considered when calculating the rate of bank erosion the developed model was then applied to simulate the processes of meandering migration in the study reach

present day lunette sediment cycling at witpan in the arid southwestern kalahari desert abstract witpan is a pan depression in the southwestern kalahari desert the pan fringing lunette dune has a number of notable characteristics a highly active crest the lower lunette pediment is cemented and the lower lunette supports a highly developed and active gully system geomorphological and sedimentological considerations are used to evaluate the interaction of wind and water in shaping contemporary lunette processes it is considered that like other pans in the area groundwater activity influences pan floor sediment availability and transport that the present highly sea sonal climatic regime permits the interaction of aeolian transport of sediment to the dune and its return to the pan floor by water during storm events creating a cycling process favoured by sedimentary properties that promote surface runoff

piping sapping erosion ii identification diagnosis although erosion of streambanks and other shorelines by emergent seepage is widespread this erosion mechanism termed piping or sapping has not been recognized as important to the overall erosion process the mechanism is complex and interactions with other bank and shore processes tend to mask the effects of piping sapping direct evidence water emerging from a soil face and carrying away soil particles is rarely encountered several types of indirect evidence are presented and illustrated in this paper including cavities formed by piping deposits of dislocated particles below piping zones blind gullies staining produced by persistent seepage outflow and particular types of localized failures slab toppling block shearing and tensile falls caused by undercutting due to piping sapping the interactions of this erosion mechanism with other erosion deposition processes are described the purpose of this paper is to facilitate identification and evaluation of piping sapping erosion particularly

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 i theory in this paper a slope stability analysis for steep banks is used in conjunction with a method to calculate lateral erosion distance to predict bank stability response to lateral erosion or bed degradation the failure plane angle failure block width and volume of failed material per unit channel length may be calculated for the critical case these parameters define the bank geometry following failure and form the starting point for subsequent analyses the calculation procedure is illustrated by a worked example following mass failure slump debris accumulates at the bank toe the debris is removed by lateral erosion prior to further oversteepening or degradation generating further mass failures any process based model for channel width adjustment must account for the combined effects of lateral erosion and mass instability in producing bank instability the approach adopted here represents a marked improvement over earlier work which does not account for changes in bank geometry due to lateral er