the amspoort silts northern namib desert namibia formation age and palaeoclimatic evidence of river end deposits abstract detailed geomorphological and chronological investigations of the nw namibian amspoort silt formation show that the sediments are typical river end deposits this type of endoreic sediment occuring only in desert margin areas provides valuable information about the palaeo environment in the hoanib valley the fine grained deposits have buried riverine trees radiocarbon dating of the wood and luminescence dating of the sediments allow a detailed reconstruction of the aggradation processes accumulation started 10 km downstream of amspoort around the beginning of the 15th century and ended in the 19th century some kilometres upstream of amspoort this upstream shift of sedimentation during the little ice age was caused by gradually decreasing runoff resulting from aridification of the upper part of the hoanib river catchment lying east of the namib desert margin 1 200 m a s l the amspoort silt terrace is evidence of palaeo hydrological fluctuations in nw namibia at present the hoanib river erodes deeply into the silty deposits indicating that nw namibia receives more monsoonal rainfall today than during the little ice age however this contradicts the hypothesis of a continual natural aridification of nw namibia damaraland kaokoveld since the mid 19th century in the course of global climatic change rather deposition and erosion of the amspoort silts indicate that landscape degradation in nw namibia is primarily anthropogenically induced and most probably not accelerated by a decrease in precipitation

field wind erosion measurements with big spring number eight bsne and modified wilson and cook mwac samplers article i nfo the aim of this study was to compare the horizontal mass flux hmf of aeolian sediment obtained from field wind erosion measurements with the modified wilson and cook mwac and big spring number eight bsne and to analyze the effectiveness of exponential power logarithmic and rational equations to calculate the horizontal mass transport hmt for each sampler type with this purpose wind erosion was measured on fine sandy loam soil in 10 erosion events between december 4 2008 and july 1 2009 the relative efficiency of the mwac related to bsne rew b quotient between the hmf of mwac and the hmf of bsne multiplied by 100 was 247 while rew bobtained from the absolute efficiency of the bsne 85 to 95 and mwac 44 to 120 found in previous studies was between 51 and 141 the rew b increased with height as a consequence of the wind speed increase and particle size decrease which reduces the efficiency of the bsne while the efficiency of the mwac remains constant depending on the equation used the hmt of mwac was from 2 1 to 2 53 times higher than the hmt of bsne indicating that if the hmf is corrected by the rew b found in this study the hmt obtained with the mwac and bsne is similar the hmt obtained from exponential equations was 16 higher than the power equation and 62 and 11 lower than logarithmic and rational equations respectively in spite of this the hmt obtained with different equations presented a good relationship with each other pb0 05 indicating that the hmt can be corrected and compared between equations this study shows that the hmf and hmt data obtained from field measurements with the bsne and mwac are different nevertheless comparable measurements of wind erosion can be obtained with both samplers taking into account the relative efficiency and the relationship between equations found in this study

determining the burial time of single grains of quartz using optically stimulated luminescence abstract in the optical dating of sediments it is usually assumed that the optically stimulated luminescence osl signal has been completely reset by light exposure prior to burial this assumption is often not valid one approach to testing and perhaps circumventing this assumption is to examine the apparent date of last exposure to daylight of individual sediment grains this paper reports on the application for the first time of 2 new measurement protocols to the estimation of the radiation dose received during burial for individual quartz grains from an aeolian deposit of known age 10 000 year old which is considered likely to have been completely reset by sunlight at deposition additive dose laboratory doses added to the burial dose before osl measurement and regenerative dose doses added after measurement of osl from burial dose single aliquot protocols are applied to 28 and 25 individual grains respectively each grain provides an independent estimate d e of the burial dose the average d e from the additive dose protocol 21 8 1 1 gy is in good agreement with the average from the regenerative dose protocol 23 8 1 0 gy both agree well with 1 13 multiple grain regenerative dose single aliquot measurements each on 1 mg sub samples of 23 9 0 3 gy 2 9 multiple grain additive dose single aliquot measurements also on 1 mg sub samples of 22 4 0 7 gy and 3 one previously published multiple aliquot additive dose estimate of 23 5 0 6 gy using 52 sub samples each of 5 mg the resulting optical ages are in good accord with 14 c and thermoluminescence age determinations the distribution of equivalent doses in the single grains is however unexpectedly large 23 of mean d e given the very likely complete resetting of the osl signal at deposition possible reasons are discussed and it is concluded that heterogeneity in beta dosimetry is the most likely explanation the single grain optical dating protocols reported here allow a detailed examination of the dose distribution in very small samples thus they should enable accurate dates to be obtained for sediments and soils that contain poorly bleached or mixed age components as well as deposits in which quartz grains are present in extremely low abundance

topographic forcing of flow partition and flow structures at river bifurcations this paper presents the predicted flow dynamics from the application of a reynolds averaged navierstokes model to a series of bifurcation geometries with morphologies measured during previous flume experiments the topography of the bifurcations consists of either plane or bedform dominated beds which may or may not possess discordance between the two bifurcation distributaries numerical predictions are compared with experimental results to assess the ability of the numerical model to reproduce the division of flow into the bifurcation distributaries the hydrodynamic model predicts 1 diverting fluxes in the upstream channel which direct water into the distributaries 2 super elevation of the free surface induced at the bifurcation edge by pressure differences and 3 counter rotating secondary circulation cells which develop upstream of the apex of the bifurcation and move into the downstream channels with water converging at the surface and diverging at the bed when bedforms are not present weak transversal fluxes characterize the upstream channel for almost its entire length associated with clearly distinguishable secondary circulation cells although these may be under estimated by the turbulence model used in the solution in the bedform dominated case the same hydrodynamic conditions were not observed with the bifurcation influence restricted and depth scale secondary circulation cells not forming the results also demonstrate the dominant effect bed discordance has upon flow division between the two distributaries finally results indicate that in bedform dominated rivers consequently we suggest that sand bed river bifurcations are more likely to have an influence that extends much further upstream and have a greater impact upon water distribution this may contribute to observed morphological differences between sand bedded and gravel bedded braided river networks copyright c 2012 john wiley sons ltd

flow fields bed shear stresses and suspended bed sediment dynamics in bifurcations of a large river 1 channel bifurcations associated with bars and islands are important nodes in braided rivers and may control flow partitioning and thus affect downstream confluences as well as the formation and dynamics of bars however the morphodynamic processes associated with bar formation are poorly understood and previous studies have largely concerned laboratory experiments small natural streams or numerical analyses with large froude numbers high slopes and low shields stresses in these cases the morphologic changes at bifurcations are relatively rapid with predominant bed load transport and the suspended load playing a minor role in this paper the evolution of the flow structure and suspended bed sediment transport along four expansion diffluence units in the rio parana argentina are described the rio parana is a large multichannel river with a bed composed of medium and fine sands and possesses low froude numbers and high suspended bed material transport primary and secondary flow velocity components were measured with an acoustic doppler current profiler adcp along the expansion diffluence units and the backscatter signal of the adcp was calibrated to allow simultaneous measurements of suspended bed sediment concentrations the interactions between these variables show that the cores of primary flow velocity and suspended bed sediment concentration do not necessarily follow the thalweg at the bifurcation and that inertial effects on the suspended bed sediment may influence the morphodynamics of bar formation it is suggested that changes in flow stage as well as the presence of vegetation may further increase the deposition of suspended bed sediment at the bar head this study suggests that the ratio of suspended bed material to bed load is an important factor controlling the morphodynamics of bifurcations in large sand bed braided rivers

measuring flow velocity and sediment transport with an acoustic doppler current profiler an acoustic doppler current profiler adcp measures three dimensional velocity profiles within the water column using the doppler shift principle whilst the bottom tracking function and acoustic backscatter can be used to measure bed load velocity and estimate suspended sediment concentration the adcp offers many advantages over traditional single point current meters and sediment samplers including deployment from a moving launch a single instrument for both velocity and sediment transport measurements profiles of three dimensional velocity and suspended sediment and the ability to map an entire flow field limitations of adcps include a large sampling diameter close to the bed coarse measurement of vertical velocity fragmented bottom track records a poor understanding of the relation between bottom tracking and the mechanisms of sediment transport and sensitivity of the acoustic backscatter to particle size

dust raising in the dustiest place on earth the bodele in northern chad is the dustiest place on earth it is dusty because of the co location of strong near surface winds and a large exposure of low density friable diatomite data are reported from three kinds of observation on dust generation in the bodele a two automatic weather stations and pilot balloon tracking complemented with reanalysis data measuring and estimating wind velocities b sun photometers measuring atmospheric dust concentrations c a vacuum pump dust sampler and an array of mwac sediment samplers measuring in situ sediment transport with these data we develop four hypotheses about dust generation 1 the pulsing of dust output from the bodele depends primarily on the varying strength of a low level jet llj i e at this scale dust production is transport limited 2 the most prolific mechanism of dust production is the breakdown of saltating diatomite flakes as they collide with each other and with the surface 3 disturbance of flow around the large barchans particularly turbulence and convergence on their horns is a major contributor to dust production i e the dunes are dust mills 4 in consequence dust production is localised within the main exposure of diatomite to places where there are the most dunes as on the eastern and northern fringes of the exposure

effects of drying techniques on grain size analyses of fine grained sediment abstract to assess the effects of drying on subsequent grain size analyses a variety of drying approaches including drying in ambient air oven drying and freeze drying were applied to sediment collected from modern fluvial lacustrine and paludal settings containing a wide range of clay size fraction 16 61 samples were subsequently rehydrated and subjected to grain size analysis in a laser particle size analyzer and histograms compared with those from control samples never subjected to drying results indicate that drying techniques induced no effect on grain size distribution for samples containing 39 clay size material only freeze drying produced reproducible results statistically identical to the control samples

is there evidence for global scale forcing of southern hemisphere quaternary desert dune accumulation a quantitative method for testing hypotheses of dune system development luminescence dating of desert dune sediments has generated many hundreds of ages many used in reconstructions of quaternary environmental changes others in attempts to elucidate dune processes environmental and climatic interpretations of these records have proved problematic and it remains challenging to test hypotheses of the systematic response of dunefields to changes in external forcing in the past and to make predictions of the future we use a method that quantifies dune sediment accumulation to interpret dune luminescence age datasets rather than simply using the ages themselves as proxies of change the accumulation intensity method allows periods of dune sediment accumulation here over the timescale 102 105 years to be identified from compilations of dated sand sea stratigraphic sequences we apply this approach to two of the largest dune age datasets from southern africa and australia testing whether or not dunefield accumulation has co varied in the late quaternary and whether systematic relationships to external drivers at global hemispheric regional and local scales can be identified

transport equation for suspended sediment based on two fluid model of solid liquid two phase flows most theoretical and numerical studies employ diffusion theory to investigate suspended sediment distributions in fluvial rivers yet diffusion theory based descriptions are strictly valid only in the limit of small particle inertia and low concentrations this paper presents a transport equation for sediment in suspension based on a two fluid model of solid liquid two phase flows the transport equation is derived from the consideration of mass conservation for solid particles the velocity of the solid phase is obtained from an asymptotic solution of the momentum conservation equation for solid phase moreover this transport equation is a typical convection diffusion equation that includes the conventional diffusion equation as a special case when particle inertia is small and concentrations are low retaining the essential features of two fluid models the transport equation is applicable to sediment laden flows with a wide range of particle inertia and sediment concentrations comparisons of the results with available experimental observations are presented and fairly good agreement is obtained doi 10 1061 asce hy 1943 7900 0000331 2011 american society of civil engineers ce database subject headings suspended sediment two phase flow open channels solids flow author keywords suspended sediments sediment concentration two phase flow open channels

complex variations in sediment transport at three large river bifurcations during discharge waves in the river rhine river bifurcations strongly control the distribution of water and sediment over a river system a good understanding of this distribution process is crucial for river management in this paper an extensive data set from three large bifurcations in the dutch rhine is presented containing data on bed load transport suspended bed sediment transport dune development and hydrodynamics the data show complex variations in sediment transport during discharge waves the objective of this paper is to examine and explain these measured variations in sediment transport it is found that bend sorting upstream of the bifurcations leads to supply limitation particularly in the downstream branch that originates in the outer bend of the main channel tidal water level variations lead to cyclical variations in the sediment distribution over the downstream branches lags in dune development cause complex hysteresis patterns in flow parameters and sediment transport all bifurcations show evidence of sediment waves which probably are intrinsic bifurcation phenomena the complex transport processes at the three bifurcations cause distinct discontinuities in the downstream fining trend of the river differences among the studied river bifurcations are mainly due to differences in sediment mobility shields value because the variations in sediment transport are complex and poorly correlated with the flow discharge prediction of the sediment distribution with existing relationships for one dimensional models is problematic

three dimensional mathematical model of suspended sediment transport this paper presents the basic equations for a mathematical model of sediment laden flow in a nonorthogonal curvilinear coordinate system the equations were derived using a tensor analysis of two phase flow and incorporate a natural variable density turbulence model with nonequilibrium sediment transport correspondingly a free surface and the bottom sediment concentration are employed to provide the boundary conditions at the river surface and the riverbed the finite analytic method is used to solve the equations of mass and momentum conservation and also the transport equation for suspended sediment to demonstrate the method the sediment deposition for the three gorges project is considered the mathematical model specifies the boundary conditions for the inlet and outlet using data from physical model experiments the results for the mathematical model were tested against laboratory measurements from the physical model experiment good agreement and accuracy were obtained

last glacial maximum to early holocene wind strength in the mid latitudes of the southern hemisphere from aeolian dust in the tasman sea dust transported by southern hemisphere mid latitude westerly winds from australia and deposited in the tasman sea shows no evidence for stronger winds during the last glacial maximum lgm compared to the holocene features of the particle size distributions of the dust do however indicate enhanced dry deposition of dust in the lgm changing to rainfall scavenging during deglaciation and the early holocene as climate ameliorated from these results it appears that activation of desert dunefields over 40 of australia during the lgm was the result of a reduction in stabilizing vegetation and more frequent episodes of sand movement rather than of increased wind strength the lgm climate of inland australia must have been considerably more stressful for plants as a result of lower precipitation and or carbon dioxide stress to achieve the implied levels of surface destabilization enhanced atmospheric dust loads in the southern hemisphere and deposition over antarctica were most likely the result of greatly expanded source areas in the mid latitude southern continents and a weaker hydrological cycle rather than greater entrainment or more efficient transport by stronger winds during the lgm wind strength appears to have varied regionally and predominantly in high latitudes rather than uniformly for all zonal winds

the distribution of apparent dose as determined by optically stimulated luminescence in small aliquots of fluvial quartz implications for dating young sediments abstract we noted recently murray et al 1995 that quartz samples taken from in channel fluvial deposits have apparent doses ranging over several orders of magnitude we suggested that these dose distributions are best explained by a mixture of a small number of poorly bleached grains and a larger number of well bleached grains in this paper we examine the distribution of apparent dose in small aliquots each composed of 2000 grains of different particle size fractions 63 90 m 90 125 m 125 180 m 180 212 m 212 250 m diameter from two samples collected from the murrumbidgee river new south wales australia the samples are of known burial age the first being 70 years old and the second being its modern equivalent 0 2 gy the difference in the lowest apparent dose in the two samples is considered to be the result of their different burial times we suggest a method for dating recently deposited

mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system abstract a distinct series of beach ridges marking the former shorelines of large inter connected lacustrine basins in the kalahari can be clearly identified from landsat imagery and shuttle radar topography mission srtm data these basins which form the terminal sump of the okavango system in northern botswana are now almost completely dry during the quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that at its largest extent encompassed 66 000 km 2 poor chronological control has previously limited the utility of this important palaeo archive as part of a region wide lake palaeo shoreline research programme a systematic optically stimulated luminescence osl dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features twenty drill sites have generated 140 samples for dating establishing a firm chronology for multiple lake full phases in all three component basins ngami mababe and makgadikgadi of this mega lake this paper presents the final set of ages in the programme derived from four cores from the western and north eastern shorelines of makgadikgadi and uses these ages to establish a chronology of mega lake high stands during the last 300 ka providing a rare directly dated long terrestrial record of positive hydrological excursions within the southern hemisphere

optical age estimates for hyper arid fluvial deposits at homeb namibia abstract two 25 m sequences of slack water facies sediments have been dated in the kuiseb river in namibia by optical dating of sand sized quartz grains the eight samples indicate the sediments were deposited between 6 3 and 9 8 ka this revises previous age estimates for the homeb silts according to our criteria six out of eight samples displayed characteristics of partial bleaching this result was not unexpected as arid fluvial systems have high suspended sediment concentrations short flow durations and a tendency for flow peaks to occur at night

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

paleohydrology of extraordinary floods along the swakop river at the margin of the namib desert and their paleoclimate implications abstract the swakop river is a major ephemeral river drainage basin area about 30 000 km 2 crossing the western namib desert its hydrology and limited water resources depend on storms and floods therefore the hydrology and flow regime in the swakop river basin determines the availability of water for human use in this study we present a millennia long record of extreme floods using paleoflood hydrological methods and optically stimulated luminescence osl ages the record includes 44 large floods ranging in date from the late pleistocene to the present the oldest six to eight floods are dated from the late pleistocene to the mid holocene between 14 900 1700 and 5100 620 years ago with calculated peak discharges of 550 1280 m 3 s 1 eighteen floods eleven of which were between 1000 and 1600 m 3 s 1 occurred between 740 90 and 190 10 years ago additional 17 later floods occurred between about ad 1850 and 1963 with peak discharges between 100 and 1000 m 3 s 1 the historical record which partly parallels the latter period recorded 17 large floods during the period 1893 1963 flood frequency analysis ffa of the combined systematically gaged record 1963 2003 and the paleoflood record since ad 1270 740 90 years the most complete record was applied using the maximum likelihood method a two component extreme value distribution tcev estimates the 1 annual exceedance probability magnitude using only the systematic gaged record at 520 m 3 s 1 the combined paleoflood historical and gage records resulted in a much larger magnitudes for the 1 annual exceedance probability flood magnitude of 1320 1350 m 3 s 1 the 0 1 annual exceedance probability magnitude is estimated at 770 m 3 s 1 and 2100 2150 m 3 s 1 respectively the hydroclimatic interpretation of this paleoflood record is complex due to the discontinuity of the record during the late holocene our flood data show three intervals differing in their flood regime 1 1280 80 to 740 90 years bp with one extraordinary flood 200 m 3 s 1 about every 180 years 2 740 90 to 200 45 14 c years bp with one large flood per 80 years and 3 200 45 14 c years bp to ad1963 with one large flood per 7 years interval 2 represents large floods probably related to the transition from a drier climate to the colder and probably wetter climate of the little ice age lia the third period relates to the end of the lia and to a wetter period of the 19th and early 20th centuries in which more rainfall in the kalahari and namib deserts and many historical floods were documented the presence of three distinct calcretes rich in calcified rhizoliths at one of the sites two of which were dated by osl to 13 2 1 9 ka and 8 8 1 0 ka indicates at least locally higher moisture availability than in the current semiarid climate the presence of denser vegetation can be associated with increased groundwater seepage and wetter climatic conditions

how do gravel bed rivers braid sedimentary processes and bed forms leading to the onset of braiding were observed in small scale hydraulic models of gravel bed streams the laboratory streams had a variety of combinations of constant discharge and slope but identical bed material particle size distributions from initially straight channels braiding occurred by four different processes deposition and accumulation of a central bar chute cutoff of point bars conversion of single transverse unit bars to mid channel braid bars and dissection of multiple bars in these experiments the chute cutoff mechanism was the most common but the predominant braiding mechanism depends upon sediment mobility excess bed shear stress and the bed form regime at very low excess bed shear stress the central bar process dominates but at higher excess bed shear stress slip face unit bars are more common bed scour at confluences is more pronounced and propogation of alternate convergence scour and divergence deposition is more likely thus chu

sediment laden flow in open channels from two phase flow viewpoint most existing analytical and mathematical models for sediment laden flows are based on the governing equations for single phase flows and are valid only for low sediment concentration situations this paper presents an analysis on the basis of the fundamental equations for fluid solid two phase flows of the velocity and sediment concentration profiles in open channel flows a new diffusion equation is established for suspended sediment concentration from a rigorous derivation of the water sediment mixture s normal velocity in the sense of mass flux conservation the differences between this new equation and schmidt s as well as hunt s are shown to be attributable to the different approximations of sediment velocity previous formulations for velocity and sediment concentration distributions are special cases of the present model for low sediment concentration flows the developed model is extensively tested against available measurements and satisfactory or fairly good agreement is obtained

new 3 d flow interpolation method on moving adcp data 1 a simple but accurate interpolation procedure for obtaining the three dimensional distribution of three component velocity data from moving acoustic doppler current profiler adcp observation data is proposed for understanding actual flow structure within a river with complex bathymetry the three dimensional mean velocity field provides a basic picture of the flow for obtaining the three dimensional distribution of three component velocity data in this work anisotropic gridding was introduced in order to remove the random component of measured velocity data caused by the turbulence of the flow and measurement error a continuity correction based on the pressure equation was used to reduce both random and systematic errors the accuracy of the developed method was evaluated using three dimensional flow simulation data from a detached eddy simulation des by using the procedure developed the complex flow structure surrounding the spur dikes section in the uji river was successfully visualized and explored the proposed method shows superiorities in both accuracy and consistency for the interpolated velocity field as compared to the kriging and inverse distance weighted idw methods

investigations into the potential effects of pedoturbation on luminescence dating much effort has been focussed on understanding the luminescence properties of natural minerals to achieve a reliable accurate and precise dating technique however some field related aspects such as the influence or effect of post depositional disturbance on luminescence dates are as yet underexplored in the case of pedoturbation depending on its intensity the rate of sedimentation and unit thicknesses potentially the whole sedimentary record at a site can be affected this may lead to distorted osl chronologies and erroneous sediment burial ages pedoturbation can result in sediment mixing and or exhumation that affect luminescence both at the bulk and single grain level effects of these two principle processes on luminescence ages are examined using standard multigrain and single grain protocols high resolution sampling of surface gopher mounds was used to determine the efficiency of bio exhumation in resetting luminescence signal results show this is an inefficient mechanism for onsite sediment bleaching the effects on luminescence signal of bio mixing were explored by comparing a sample collected from within a krotovina infilled burrow to an adjacent undisturbed sample results show the difficulties in identifying pedoturbated samples at the single aliquot level and the possible inaccuracies in using the lowest palaeodose values to calculate osl ages where pedoturbation of samples is suspected use of probability plots of palaeodoses data is recommended from these plots it is proposed that only data falling within a normal distribution centred on the peak probability be used to calculated osl ages and to mitigate problems arising from pedoturbation

depositional environment and luminescence chronology of the hoarusib river clay castles sediments northern namib desert namibia abstract the clay castles in the hoarusib river valley are fine grained deposits in a tributary immediately upstream of a narrow gorge sedimentological analysis reveals three lithofacies associations 1 channel facies association near the mouth of the tributary 2 back flow facies association in the middle section and 3 silt and massive sand facies association in the head of the tributary overall sedimentary architecture shows that aggradation of the main hoarusib channel led to back flooding into the tributary and the deposition of massive fine grained deposits decreased discharge competency increased sediment load and the presence of narrow gorges within the main valley are suggested as reasons for the aggradation inter fingering of desert derived sediments channeled down the tributary and backflow sediments from the main river indicate an increase in both upland and desert rainfall during the course of sedimentation optically stimulated luminescence osl dating of these deposits indicates formation between 44 and 20 ka with the possibility that deposition occurred during two phases of rapid sedimentation at ca 42 and 25 ka

a sequence of fluvial and aeolian deposits desert loess and palaeosoils covering the last 60 ka in the opuwo basin kaokoland kunene region namibia based on luminescence dating abstract arid and semi arid areas are lacking in fossils for the reconstruction of the quaternary climate therefore sediments from ephemeral rivers are a welcome climatic archive in namibia restrictions concerning the palaeoclimatic evidence lie in the fact that the sediments examined usually reflect the climate of the upper reaches of the rivers rather than the local climate the upper reaches may belong to a different climatic zone or damp altitudinal zone of vegetation in order to rule out such allochthonous influences in the study of the climatic history of the escarpments between the edge of the namib desert and the savannah research is limited to sediments in basins and valley floors which are not subject to such influences research in the opuwo basin has extended knowledge of the recent quaternary climatic history of northern namibia into the period since ca 60 ka the sedimentation of the basin floor pediments began with the deposition of fluvial sands from ca 55 ka desert loess was blown in the course of aridification but this was washed away from the pediments towards the centre of the basin by ca 45 ka surface runoff was sometimes strong enough to transport gravels and sands thus geomorphological activity in this period also differed from the drier period which set in around 45 ka it was characterised by the exclusive sedimentation of desert loess even on the pediments at the basin s rim there are a total of three cambic regosol soil formation phases however as there are no indications of new clay formation or eluviation the climate can only have been slightly more humid during their formation

effects of suspended sediment on the open channel velocity distribution experiments were performed in which suspended sediment concentration in open channel bounded shear flow was varied systematically while flow depth slope and discharge were held essentially constant the effect of variation in suspended sediment concentration on velocity profile characteristics was investigated the experiments showed that the karman coefficient was independent of changes in suspended sediment concentration while the coles wake strength coefficient was very sensitive to such changes the thickness of the logarithmic part of the velocity profile decreased as sediment concentration increased the logarithmic parts of velocity profiles in sediment suspending flow were shifted downward relative to control profiles in comparable clear water flows

estimating shear stress from moving boat acoustic doppler velocity measurements in a large gravel bed river 1 moving boat acoustic doppler current profiling adcp is increasingly used to measure discharge in large rivers we investigate whether useful information about bed shear stress can be recovered from such data alternative ways to estimate local bed shear stress using the logarithmic law of the wall and spatial averaging are tested using adcp transects across lower fraser river canada repeatability is assessed by comparing estimates from outward and return boat tracks the most precise method uses the vertically averaged mean velocity and a zero velocity height based on bed grain size information the accuracy of the assumed zero velocity height can be judged by consistency between estimates using mean velocity and near bed velocity shear stress estimates from unconstrained log law fits are less repeatable and tend to overpredict and mean shear stress estimates using the depth slope product are unreliable in this river because of nonuniform flow

relationship between antarctic sea ice and southwest african climate during the late quaternary here we compare late quaternary southwest african climate records from the west coast of southern africa published winter rainfall and trade wind intensity records from a core off the coast of namibia to records of antarctic sea ice extent this comparison reveals coherent changes between antarctic sea ice extent and the southwest african winter rain region since 45 k y b p with enhanced winter rainfall and trade wind vigor during periods of increased sea ice presence we propose an oceanic and atmospheric coupling between antarctic sea ice and the winter rainfall zone of southwest africa that may lead to increased desertification in the region if global warming persists

comparison of fixed and moving vessel flow measurements with an adp in a large river a comparison of three dimensional flow velocity measurements made with an acoustic doppler profiler adp from fixed and moving vessels at cross sections of the parana river argentina was performed the purpose was to design a rapid and reliable procedure for quantifying the velocity field and related parameters such as bed shear velocity and the identification of secondary circulations in large rivers using an adp the fixed vessel measurements were performed over a period of 10 min at three vertical profiles along two of the sections these data were then compared with the results of ten moving vessel repeat transects made at each of the sections and which intersected the fixed vessel sampling locations using a number of different adp setup configurations from the velocity profiles obtained with both fixed and moving vessel measurements total bed shear velocity values were computed by applying the law of the wall the results indicate there can be significant differences between velocities obtained using the moving vessel method and fixed vessel measurements averaged over 10 min these differences in horizontal velocity can be significantly reduced by averaging five or more moving vessel transects with corresponding shear velocities calculated from five transect averages showing differences ranging between 10 and 15 dependent on the adp configuration location of the at a point vertical velocity profile in relation to large scale bed roughness may also be an influential factor and ideally the bed morphology should be quantified together with the adp derived velocities when using the adp to identify secondary flow cells it was found that although one cross section transect can provide a reasonable overall picture an average of five cross sections is necessary to resolve the finer details of flow the implications for applications that use moving vessel techniques for measurement and analysis of three dimensional flow structures including secondary flows are highlighted

depositional environment and osl chronology of the homeb silt deposits kuiseb river namibia previous studies suggest that the homeb silts of the kuiseb valley namibia i accumulated in a dune dammed lake ii are end point deposits iii represent an aggrading river bed and iv are slackwater deposits thus they have been used alternatively as evidence of past drier conditions or past wetter conditions lithostratigraphic analysis of two sediment sequences at homeb indicates sedimentation by aggradation of the kuiseb river triggered by a transition from an arid to humid climate osl ages for the sequences were obtained by the sar protocol on aliquots of 9 6 mm and 4 0 mm diameter and on single grains four millimeter aliquot minimum ages closely approximate the single grain minimum ages and are younger than 9 6 mm aliquot minimum and central ages based on these results the small aliquot 4 mm approach appears to provide ages comparable to those obtained by the more laborious and time consuming single grain method minimum ages indicate rapid deposition of the homeb silts in at least two episodes centered at 15 ka and 6 ka during climate transitions from arid to humid flash floods eroded the valley fills during slightly more arid conditions

the last millennium largest floods in the hyperarid kuiseb river basin namib desert the largest kuiseb river floods initiate at the basin s semi arid headwater downstream along the hyperarid namib desert these floodwaters are feeding shallow alluvial aquifers the only available water for human activity and for the natural ecology here we characterize the largest floods and their changing frequency and magnitudes using palaeohydrological methods along 120 km of the rivers canyon 35 palaeoflood deposit sites were identified at five of these sites we conducted stratigraphic and geochronological analyses and flood discharge estimations the upper bound of the largest flood over the late holocene is 1475 m 3 s 1 over the last 1300 years more than ten floods have exceeded 1250 m 3 s 1 an additional 33 floods exceeded 400 m 3 s 1 the last millennium was characterized by one large flood every 30 40 years during the periods 1250 1335 1355 1565 and 1715 ad to the present a slight increase in flood frequency not magnitude occurred during 1565 1715 ad one large flood in 20 years and during two short episodes 1185 1205 and 1335 1355 ad seven and four floods in 20 years respectively these episodes of increased flood frequency are associated with other proxy records of higher water availability in the namib copyright 2013 john wiley sons ltd

observations of collisions of saltating grains with a granular bed from high speed cine film high speed photography was used to record saltating sand grains colliding with a horizontal noncohesive bed of similarly sized grains impacting grain bed interaction is discussed in general the process as observed from the films is then described in terms of the apparent bed contact length abcl and various parameters of the impacting grains and any ejected grains examples are given of typical behaviour of bed grains in response to impacting grains of different sizes saltating grains that are large in comparison to the bed grains they encounter at collision can churn up the surface layers of soils and sediments so that previously buried grains become available for entrainment this process is discussed in relation to the potential release of dust particles into the airflow

three dimensional flow modelling and sediment transport in the river klaralven a three dimensional ow model that uses the rng k e turbulence model and a non equilibrium wall function was applied to the river klaralven in the southwest part of sweden the objectives were to study the nature of the ow in the river bifurcation and to investigate the short term sediment transport patterns in the river the effectiveness of three dimensional ow models depends upon 1 how well the river geometry and it surface roughness are modelled and 2 the choice of the closure model improvements were obtained by modelling the river in two parts the entire river reach and a selected part composite manning coef cients were used to account for roughness properties the method requires a calibration process that ensures the water surface pro les match the eld data the k e model under predicted both the extent of ow separation zones and the number of secondary ow regions having a spiral motion in comparison with the rng k e model the 3 d model could predict with good accuracy both the general and secondary ow elds in the river the results agreed well with the 3 d velocity measurements using an acoustic doppler current pro ler a conceptual model was developed that accounts for the development of secondary ows in a river bifurcation having two bends the main ow feature in the river cross sections was the existence of multiple counter rotating spiral motions the number of spiral motions increased as the river bends were approached the river bends also caused vorticity intensi cation and increased the vertical velocities the application of the 3 d ow model was extended by solving the sediment continuity equation the sediment transport patterns were related to the secondary ow elds in the river the sediment transport patterns at the river bifurcations are characterized by the growth of a sandbank copyright 2004 john wiley sons ltd

holocene fluvial valley fill sources of atmospheric mineral dust in the skeleton coast namibia western namibia is a significant global source of atmospheric mineral dust we investigate the relationship between dust and source sediments assessing the sustainability of dust flux remote sensing studies vickery and eckhardt 2013 have highlighted specific ephemeral fluvial systems as important contributors to dust flux including highlighting sections of valleys that are the origins of dust plumes in the period 2005 8 little is known however about the specific within valley dust sediment sources particularly whether dust is derived from modern ephemeral channel floors or older valley fill sediments many of which have been reported in the region stone and thomas 2013 as part of a region wide analysis of aeolian dust flux we investigate the sediment properties of atmospheric dust samples and valley sediments from the huab valley one of the principal regional dust sources trapped dust samples contain up to 88 very fine sand and silt when collected samples are disaggregated prior to analysis valley fill surface samples comprise 80 very fine sand and silt and the surface of the modern ephemeral channel 30 valley fill sediments were sampled at depths up to 3 6m below the present surface and reveal holocene depositional ages from 0 6 0 03ka back to 9 79 0 73ka these sediments contain 30 to 6 very fine sand and silt with levels decreasing with depth and age aeolian bedforms in the valley system nebkhas on the fill surface and climbing dunes on valley margins indicate that aeolian processes under the influence of strong seasonal easterly winds likely result in dust being winnowed out of the valley fill surfaces with sandy bedforms being constructed from the coarser component of the fill sediments the volume of valley fill sediment suggests dust sourced from holocene sediments is likely to continue into the future regardless of flow conditions in the modern channel system

environmental changes at the eastern namib desert margin before and after the last glacial maximum new evidence from fluvial deposits in the upper hoanib river catchment northwestern namibia in the upper hoanib river catchment area northwestern namibia fine grained silty deposits are widespread accumulated as river end deposits they form excellent geomorphological archives of a highly sensitive desert margin area different sedimentary complexes are separated sedimentologically and by luminescence dating the deposits give evidence of climate oscillations at the eastern margin of the namib desert during the past 50 000 years in northwestern namibia the dry conditions of the desert margin area with less than 200 300 mm mean annual precipitation prevailed at 60 40 ka and at 34 24 ka when sediment complex i accumulated during the last glacial maximum lgm fluvial activity and sedimentation were most likely interrupted due to arid conditions between 12 ka and the mid holocene higher rainfall and runoff led to the aggradation of sediment complex ii the climate was more humid than before the lgm but drier than at present after 3 ka runoff increased and the hoanib river re eroded the older deposits forming deep channels a prominent more sandy and gravely 4 m terrace in the upper and middle course of the hoanib river dates to the little ice age lia it is likely that this terrace is genetically linked with the amspoort silt formation in the lower hoanib valley d 2005 elsevier b v all rights reserved

iron and nutrient content of wind erodible sediment in the ephemeral river valleys of namibia abstract research concerning the global distribution of aeolian dust sources has principally focussed on salt clay pan and desiccated lacustrine emission areas in southern africa such sources are identified as etosha pan in northern namibia and makgadikgadi pans in northern botswana dust emitting from ephemeral river valleys however has been largely overlooked rivers are known nutrient transport pathways and the flooding regimes of ephemeral river valleys frequently replenish stores of fine sediment which on drying can become susceptible to aeolian erosion such airborne sediment may be nutrient rich and thus be significant for the fertilisation of marine waters once deposited this study investigates the dust source sediments from three ephemeral river valleys in namibia in terms of their particle size distribution and their concentrations of bioavailable n p and fe we compare the nutrient content of these sediments from the ephemeral river valleys to those collected from etosha and makgadikgadi pans and consider their relative ocean fertilising potential our results show that the ephemeral river valleys contain fine grained sediment similar in physical character to etosha and makgadikgadi pans yet they have up to 43 times greater concentrations of bioavailable iron and enriched n and p macronutrients that are each important for ocean fertilisation the known dust emitting river valleys of namibia may therefore be contributing a greater fertilisation role in the adjacent marine system than previously considered and not yet investigated given this finding a re assessment of the potential role of ephemeral river valleys in providing nutrient rich sediment into the aeolian and marine systems in other dryland areas is necessary

late quaternary palaeohydrological changes in the northern namib sand sea new chronologies using osl dating of interdigitated aeolian and water lain interdune deposits abstract ephemeral rivers today terminate in playas adjacent to the eastern part of the namib sand sea although there is evidence that these fluvial systems penetrated further west in the past there are water lain sediments exposed in interdune areas of the northern namib sand sea associated with the former course of the tsondab river which are central to palaeoenvironmental reconstruction in the region these deposits indicate that there was increased moisture availability in this presently hyper arid region and or the catchments of the rivers to the east of the sand sea in the great escarpment until now the chronology for the deposition of these sediments has been based on radiocarbon 14 c dates on carbonate materials the validity of these ages is problematic and 14 c limits the timescale to which these sediments can be dated the water lain units are bracketed by aeolian sand units which are highly suitable for optically stimulated luminescence osl dating here 24 osl ages from three sites associated with the former course of the tsondab river are presented allowing a reassessment of the timing of sediment deposits and the west to east progression of the terminus of the tsondab river during the late quaternary narabeb which is 60 km north west of the current end point of the tsondab river at tsondab vlei contains eight mud units six of which are shown to be of marine oxygen isotope stage mis 5 128 75 ka age water lain units at hartmut pan 55 km north west of tsondab vlei are dated by osl to between 16 9 0 9 ka and 12 7 0 1 ka at ancient tracks closest to the tsondab vlei 6 km west mud units are younger and placed between 12 0 0 7 and 12 8 0 8 ka 11 5 0 5 10 5 0 5 ka and after 10 5 0 5 ka this new chronology is compared with other key regional records to identify potential regional relationships and forcing factors there is greatest correspondence with the records in the north of the summer rainfall zone a revision of both old and young 14 c ages on carbonate materials calls for revision of other 14 c chronologies in the region and highlights the problems with the continued used of histograms of 14 c ages to indicate periods of increased humidity in southern africa

effect of lateral bed roughness variation on particle suspension in open channels in this paper the effect of variation of bed roughness along lateral direction on suspension concentration distribution in open channel turbulent flows was investigated starting from the mass and momentum conservation equations this study demonstrates that both the reynolds shear stress overline u v and sediment diffusivity depends on bed roughness from the theoretical analysis it is found that both the reynolds shear stress and the sediment diffusivity increase over smooth bed surfaces and decrease over rough bed surfaces at the junction of smooth and rough bed surface the effect of bed roughness on the reynolds shear stress and sediment diffusion is almost negligible including this effect suspension concentration distribution is also studied and from the hunt s diffusion equation an analytical model for predicting suspension concentration is proposed apart from this effect the effects of moveable bed roughness and stratification are also considered in the model it is observed that the rouse equation is obtained from the proposed model as a special case when the flow is considered as single phase and there is no effect of secondary current stratification and bed roughness variation on the basis of experimental data available in literature the proposed model is validated and also compared with the rouse equation to get a quantitative idea about the goodness of fit weighted relative error is calculated the comparison results and calculated errors indicate that the present model is capable of describing the suspension concentration distribution more accurately than rouse model throughout the flow depth in open channel flow

interaction between migrating bars and bifurcations in gravel bed rivers 1 in the present work we investigate the interaction between migrating alternate bars and the dynamics of river bifurcations laboratory experiments are carried out to study a y shaped bifurcation with fixed banks and erodible bed composed of well sorted sand the problem is also analyzed by developing a theoretical one dimensional model results show the occurrence of regular fluctuations in the discharge distribution at the bifurcation node which are strictly related to bar migration the effectiveness of bars in conditioning the bifurcation behavior increases with bar amplitude and decreases with bar migration speed four qualitatively different behaviors of the system are observed as the controlling parameters of the flow are varied within a range significant for gravel bed rivers the theoretical predictions are in good qualitative agreement with the experimental observations

fluid and sediment interaction over a plane bed a mathematical model is developed for the fluid and sediment interaction in an erodible channel the model is based on the conservation of mass and momentum for the fluid and sediment the interaction between the fluid and sediment as well as the interaction of sediment particles moving in the vicinity of the bed is described in the model an analysis is performed to derive the vertical distributions of the fluid velocity and sediment concentration over a plane bed in a wide open channel the analysis yields the friction factor due to the combined effects of grain roughness and sediment movement as well as the rate of sediment transport the developed model agrees with existing data for plane beds the predicted rate of transport of coarse sediment agrees with the meyer peter and mullen formula however detailed measurements of the fluid velocity and sediment concentration over a plane bed are required for a rigorous verification of the developed model

a 300 kyr record of aridity and wind strength in southwestern africa inferences from grain size distributions of sediments on walvis ridge se atlantic abstract the terrigenous fraction of sediments recovered from walvis ridge se atlantic ocean reveals a history of southwestern african climate of the last 300 kyr end member modelling of a data set of grain size distributions n 428 results in three end members the two coarsest end members are interpreted as eolian dust the third end member as hemipelagic mud the ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the se trade winds trade winds were intensified during glacials compared to interglacials changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern african aridity late quaternary southwestern african climate was relatively arid during the interglacial stages and relatively humid during the glacial stages owing to meridional shifts in the atmospheric circulation system during glacials the polar front shifted equatorward resulting in a northward displacement of the zone of westerlies causing increased rainfall in southwestern africa the equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient leading to enhanced atmospheric circulation and increased trade wind intensity

measurements in a braided river chute and lobe 1 flow pattern sediment transport and channel change this paper and its companion ashworth et al this issue discuss measurements of channel change and associated flow and sediment transport processes in a representative chute and bar reach within a proglacial gravel bed river the sunwapta river in the canadian rockies during a week in which water discharge through the reach increased then decreased a sediment wave passed into and partly along the reach at first the chute aggraded then sediment was eroded from the chute and deposited in a prograding lobe to one side of the original bar head between two distributaries measurements of velocity shear stress and gravel transport rate revealed day to day changes in the divergent pattern of flow and sediment transport the off center location of the lobe reflected an initial asymmetry in the pattern of flow and sediment transport but as the lobe grew the flow and transport gradually switched away to the other distributary width averaged bed load transport rates do not agree well with average rates inferred from bar head deposition or from volumes and spatial patterns of scour and fill the indirect estimates are considered more reliable than those based on direct sampling for necessarily brief durations by the end of the study the new lobe had almost accreted onto the original bar head supporting the idea that most braid bars are of compound origin temporal and spatial patterns in grain size distributions of the bed bed load and deposited sediment are discussed in the companion paper

characteristics and osl minimum ages of relict fluvial deposits near sossus vlei tsauchab river namibia and a regional climate record for the last 30 ka the present end point of the tsauchab river is at sossus vlei 30 km into the namib sand sea interdune deposits in three depressions west and southwest of the vlei include channel and interdune lithofacies associations but no deposits typical of river end points or of groundwater seepage into interdune areas the two lithofacies associations show that the tsauchab river extended further into the sand sea in the past it had a well developed channel and a higher flow than today that caused flooding of adjacent interdune areas osl 4 mm aliquot minimum ages indicate that the tsauchab river reached 2 3 km beyond its present end point at ca 25 ka and ca 9 7 ka and that the river was more active from 0 9 0 3 ka the eastward migration of the river end point since ca 7 ka suggests a reduction in flood magnitude accompanied by the gradual invasion of the sossus vlei area by dunes the regional data indicate an additional wet interval at ca 15 ka that is so far not recorded in the sossus area copyright 2006 john wiley sons ltd