investigation of the sand sea with the tallest dunes on earth china s badain jaran sand sea abstract china s badain jaran sand sea features the tallest dunes on earth and a unique mega dune lake landscape it had been explored little until the 1990s though early scientific explorations surrounding the sand sea had begun by the early 20th century heated debates now focus on the desert environment and particularly how the mega dunes and desert lakes develop and evolve this paper reviews the status of these debates and summarizes the supporting evidences the environmental research mainly concerns formation and evolution of the sand sea and its relationship with climate change the proposed formation time ranges from the early pleistocene to the holocene opinions vary about climate change on different time scales the reconstructed climate change history is shorter than the sand sea s history with the longest record extending to the late pleistocene the mega dune research focuses on sediments dune morphology and formation processes it remains unclear whether the mega dunes result primarily from wind action control by the underlying topography or groundwater maintenance the sources of lake water are also debated but there are four main hypotheses atmospheric precipitation groundwater from nearby areas precipitation and snowmelt in remote areas such as the qilian mountains and the qinghai tibetan plateau or paleowater that formed during past periods of wet climate we believe that the sand sea deserves further study in terms of its dune geomorphology evolution and hydrology and their responses to climate change meteorological and hydrological observations and monitoring in the sand sea are particularly necessary

the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

grain size distribution patterns of a point bar system in the usri river india grain size distribution patterns in a point bar system of the usri river india were critically analysed in the light of log normal log hyperbolic and log skew laplace distribution models sand samples were collected from the cross bedding foreset of different sizes of bedform the objectives were to i study whether bedform heights have any role in grain size distribution patterns ii offer a best fit statistical model iii study the downstream variation of size sorting in a point bar system and iv study the mechanism of grain sorting the results indicate that the bedform heights have no role in grain size distribution patterns quantitatively when the errors in three distribution models were analysed it was observed that the log normal distribution is the best fit statistical model and the next one is the log skew laplace however in the upper reaches of the river log normal distribution is the best fit model in the case of large bedforms whereas in the lower reaches the log normal model is the best fit one in the case of small bed forms it is also observed that within a point bar for large and small bedforms there is a tendency for mean grain size to decrease downstream between point bars for large bedforms there is no consistency in decreasing grain size downstream whereas for small bed forms the decrease of grain size downstream is observed except near the confluence at palkia with distance of transport the coarser and finer fractions of sediments are gradually chopped off the coarser fractions are buried below the advancing bedforms on the lee sides and the finer ones are transported further downstream thus the finer admixture giving rise to the fining upward sequence overlies a carpet of coarser materials this mechanism provides a clue to the process of grain sorting in the fluvial environment an interpretation has been offered for the log normality of the grain size distribution pattern during prolonged transportation in a fluvial environment the larger grain size fractions are gradually chopped off and buried below the advancing bedforms on their lee sides on the other hand the finer fractions are transported further downstream in suspension thus the narrow intermediate size fraction takes active part in the distribution patterns leading to the generation of unimodality and a symmetric distribution pattern downstream which are the main criteria for log normality similarly increase of bedform size is the effect of increase of stream power and froude number leading to the selective segregation of bed materials thus the intermediate size fractions take a more active part than the coarser and the finer size fractions in developing log normality besides the hydrodynamic parameters of the usri coarsening of grain size downstream has been attributed to i the aggrading nature of the usri downstream and ii the contribution of coarser materials to the usri by its tributaries and bank erosion copyright 2006 john wiley sons ltd

on the nature of transverse variability in a coastal plain estuary even though estuarine processes are inherently three dimensional most of the previous studies have focused primarily on the axial longitudinal and vertical variabilities recent surveys in the lower delaware bay indicate the presence of significant transverse variability in the salinity distribution and residual circulation pattern conceptual models are developed to examine the mechanisms which might explain the observed transverse structure results indicate that both the gravitational effect and wind forcing are capable of generating substantial transverse structure in drowned river valley estuaries with triangular cross sections

depth and age in the north pacific the concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer in past studies problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor in this study we evaluate different tectonic histories for the quiet zones and construct a revised isochron chart for the north pacific we check and modify the current sediment thickness charts using new deep sea drilling data and revised ideas concerning reflectors in the southwestern north pacific averaging the synbaps digital data base into half by half degree elements and correcting for sediment load we plot basement depth versus age for the entire north pacific because of the skewness in the data associated with seamounts we present the data as contours about the mode and find that two thirds of the observations lie within 300 m of this value on crust between 0 and 80 ma the ocean floor subsides with the square root of age on ocean floor older than 100 ma the depths in general flatten with age with areally correlateable swells and valleys the hawaiian and line island swells correlate positively and the mariana basin valley negatively with geoid height the flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor further the correlation of the swells and valleys with geoid height presents evidence for organized convection in the upper mantle beneath older lithosphere in the north pacific

modeling influence of stratification on lateral circulation in a stratified estuary abstract the dynamics of lateral circulation in the passaic river estuary is examined in this modeling study the pattern of lateral circulation varies significantly over a tidal cycle as a result of the temporal variation of stratification induced by tidal straining during highly stratified ebb tides the lateral circulation exhibits a vertical two cell structure strong stratification suppresses vertical mixing in the deep channel whereas the shoal above the halocline remains relatively well mixed as a result in the upper layer the lateral asymmetry of vertical mixing produces denser water on the shoal and fresher water over the thalweg this density gradient drives a circulation with surface currents directed toward the shoal and the currents at the base of the pycnocline are directed toward the thalweg in the lower layer the lateral circulation tends to reduce the tilting of isopycnals and gradually diminishes at the end of the ebb tide a lateral baroclinic pressure gradient is a dominant dri

particle size variation of aeolian dune deposits in the lower reaches of the heihe river basin china abstract the grain size characteristics of aeolian dune and gobi sands in the lower reaches of the heihe river desert basin ejina of northwestern china in central asia were investigated aimed at a sedimentological study of the dune sediments and their response to aeolian processes and source area grain size parameters mean standard deviation skewness and kurtosis were determined for sand samples from dune crests interdunes and gobi areas the aeolian dunes are mainly composed of medium and fine grained sand sorting improves with decreasing grain size a clear relationship occurs between skewness and sorting in that sands become progressively zero skewed sk1 0 as sorting improves suggesting that improved sorting is accomplished by loss of both relatively coarser and finer grains in the desert in comparison with many other desert areas the dunes in the ejina are composed of relatively coarse sands are poorly sorted and relatively platykurtic in log probability grain size distributions most dune sands in the ejina are composed of two distinctive saltation populations truncated at about 1 8 and 2 5 this represents two differing aeolian transport conditions and presumably produces two separate saltation populations in opposite air flow directions sand dunes exhibit grain size and sorting patterns in which sands become finer better sorted and more finely skewed from interdune to crest which is a similar pattern to that in the namib desert this pattern can be best explained by differential movement of sand in creep and saltation populations at a regional scale dune sands tend to become finer from southwest to northeast in the desert basin which coincides with the basin hydrological setting but not with the compound direction of the basin wind system it appears that local scale factors such as hydrological processes and source materials rather than regional aeolian processes are responsible for the grain size variations of dune sands in the ejina desert

secondary circulation in a highly stratified estuary observations from the hudson river estuary are presented to discuss the interaction between secondary flows and stratification in the vicinity of a headland during highly stratified conditions analysis of the cross stream momentum balance suggests that centrifugal accelerations are in near balance with the cross stream baroclincity results emphasize that the interaction between stratification and secondary flows can set up a cross stream internal seiche this results in a reduction and subsequent reversal of the secondary flows downstream of a headland simple scaling arguments suggest that the spin down time for tidally driven eddies is related to stratification

comparison of particle size characteristics of the tertiary red clay and pleistocene loess in the chinese loess plateau implications for origin and sources of the red clay the origin of the tertiary red clay underlying the pleistocene loess in the chinese loess plateau remains controversial although several lines of evidence have suggested a wind blown origin this study examines the particle size parameters of the late miocene and pliocene red clay by comparing it with those of the late pleistocene loess the particle size distribution of a total of 15 339 loess and 6394 red clay samples taken from 12 loess sections along a north south transect and two red clay sequences at lingtai and jingchuan was systematically analysed the median grain size skewness and kurtosis of the late pleistocene loess all show a systematic southward change and are principally influenced by distance from source region the spatial and temporal differentiation of dust deposits is expressed in a skewness kurtosis median grain size ternary diagram from which the distance to the source region can be inferred the particle size characteristics of the tertiary red clay sediments are very similar to those of the palaeosols within the late pleistocene loess deposits suggesting an aeolian origin for the red clay based on the comparison of red clay and loess in the ternary diagrams it is inferred that the source sink distance was greater in the neogene than in the last and penultimate interglacials and that the dust source region in north western china underwent a progressive expansion during the period from at least 7ae0 ma to the present

grain size characteristics of surface sediments of nebkhas at the southern margin of the mu us dune field china abstract the grain size characteristics of nebkhas are considered to differ from those of mobile dunes in relation to differences in vegetation influences however minimal research has been conducted in this respect therefore this study investigates the grain size characteristics of surface sediments of a nebkha field with 50 80 vegetation cover at the southern margin of the mu us dune field the grain size composition and parameters were determined at different positions on nebkhas and in interdune areas the results indicate that the nebkha sediments were significantly finer than those of mobile dunes had weaker sorting and were more positively skewed and more leptokurtic in relation to the interception of fine particles by vegetation associated prevention of movement and pedogenesis we infer that the influence of pedogenesis resulted in minimal differences between grain size compositions and parameters in samples obtained from different positions on nebkhas between nebkhas and in interdune areas the creep populations of interdune sediments can be transported to the windward toe but further transportation along the windward slope of a nebkha to the crest or leeward slope is limited because of vegetation interception

grain size characteristics of sediment transported during dust storms abstract the grain size characteristics of sediment transported by wind in creep saltation and suspension during 15 dust storms in the slims river valley yukon territory were investigated grain size analysis of the transported sediment indicated that the suspended sediment is significantly finer and better sorted than the sediment transported in either surface creep or saltation the data also show that during dust storms the mean particle size of the suspended sediment decreases as a power function of height the grain size distributions of the sediment transported in suspension and surface creep are characterized by strong positive skewness which results from the selective removal of surface particles small enough to be transported by the wind at a given velocity in contrast the grai size distribution of the saltation samples are negatively skewed which is thought to result from the selective removal of the finer particles into suspension from those particles initially lifted from the surface into the air stream thus the proportion of sediment initially entrained into the air stream which returns to the surface in saltation will lack the fine grains removed in suspension and will therefore tend to be less positively or negatively skewed

the use of grain size distribution patterns to elucidate aeolian processes on a transverse dune of thar desert india the grain size distribution of aeolian dune sands in the thar desert india was analyzed and compared with three model distributions log normal log hyberbolic and log skew laplace to determine the best fit statistical model in total 51 samples were collected along a single transect over a transverse dune of which 15 were from the stoss side 12 from the crest and 24 from the lee side samples were collected during a calm period in the afternoon of a winter s day it was observed that of these 51 samples 33 fit best to a log hyperbolic distribution 14 fit best to a normal distribution and only four fit best to a laplace distribution however it was further observed that of 24 samples from the lee side 13 fit best to a normal distribution eight fit best to a hyperbolic distribution and three fit best to a laplace distribution of 12 samples from the crest of the dune 11 fit best to the log hyperbolic distribution only one to the laplace distribution but none to a normal distribution of 15 samples from the stoss side of the dune only one sample best fits a normal distribution 14 fit best to a log hyperbolic distribution and none best fit to a laplace distribution during sample collection a calm period prevailed and there was no dusty wind it was therefore assumed that in the initial stage a mixture of coarse medium and fine sands was laid down on the stoss side of the dune as wind speeds increased and saltation started the coarser fractions were segregated and lagged behind on the stoss slope in the final stage when the remaining intermediate and finer fractions reached the dune crest the finer fractions were winnowed away to suspension from the crest of the dune as a result a narrow range of intermediate sized sediments was deposited by rolling down the lee side to explain the development of log normality in such a situation both the coarser and finer fractions to which the skewed distributions can be attributed are separated from the initial mixture of coarse intermediate and fine fractions hence the main criteria for the development of a normal distribution is the lack of skewed fractions and the concentration of the narrow intermediate size fractions in the final grain size distribution this is also corroborated with the index of symmetry which is a measure of the difference between the angle of two slopes of the hyperbolic distribution as represented by the coarser and finer fractions copyright 2010 john wiley sons ltd

provenance of loess material and formation of loess deposits on the chinese loess plateau abstract results of a multiple isotopic chemical and mineralogical analysis of loess from the three northwestern inland basins the junggar basin the tarim basin and the qaidam basin and the loess plateau region of china are summarized they suggest a qualification of the conventional views that the three northwestern basins were the important source areas of the loess plateau and that the sand deserts were the primary single source of the chinese loess it is argued that the gobi stony desert in southern mongolia and the adjoining gobi and sand deserts the badain jaran desert tengger desert ulan buh desert hobq desert and mu us desert in china rather than the three inland basins are the dominant source areas of the loess plateau however although these gobi and sand deserts are regarded as the main source regions they serve as dust and silt holding areas rather than dominant producers the mountain processes including glacial grinding frost weathering salt weathering tectonic processes and some fluvial comminution in the gobi altay mts hangayn mts and the qilian mts have played an important role in producing the vast amounts of loess sized material for forming the loess plateau

grain size distributions of bed load inferences from flume experiments using heterogeneous sediment beds abstract the grain size fractions in the bedload transported over the five heterogeneous sediment beds of different values of bed roughness were computed from the flume experiments the existence of an entrapment factor associated with the sorting observed from the bed to active layer was modeled based on the modified critical shear stress to estimate the grain size fractions in the transport layer under given hydraulic conditions the efficiency of these models was tested with the observed data subsequently the patterns of observed grain size distributions in the transport layer were tested to identify the distributions developed in the active layer due to sorting using three probability density functions pdf such as log normal log hyperbolic and log skew laplace tests indicated that a log skew laplace distribution fitted best for 49 a log hyperbolic for 31 and a log normal for 20 out of forty five bedload samples collected under unidirectional flow with changes in flow discharge and bed roughness the results of this study would be useful to specify the grain size distributions in the bedload formed under different hydrodynamic conditions in various sedimentary environments

variations in wind velocity and sand transport on the windward flanks of desert sand dunes the magnitudes of increases in wind velocity or speed up factors have been measured on the windward flanks of transverse and linear dunes of varying height on transverse dunes velocity speed up varied with dune shape and height for linear dunes speed up factors varied principally with wind direction relative to the dune with dune shape and dune height the main effect of velocity speed up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas this is greatest for large dunes with winds of moderate velocity blowing at a large angle to the dune changing ratios of base to crest sand transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase on linear dunes the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi directional wind regime

wind blown origin of the pliocene red clay formation in the central loess plateau china abstract in order to determine the depositional processes of the pliocene red clay formation deposited directly beneath the plio pleistocene loess in the chinese loess plateau four red clay sections spanning over 400 km are studied grain size analysis of closely spaced samples in the sections shows that the particles of the red clay are very fine with the sand fraction 63 m being negligible and that all the curves indicating changes in different grain size parameters have a similar pattern the grain size records in the upper part of the four sections are almost identical with the median grain size centered at 4 8 m the ree patterns of 28 samples from the jiaxian red clay section are all characterized by lree enrichments relatively flat hree and slight negative eu anomaly being similar to those of eolian loess and the average upper continental crust these lines of evidence point to a wind blown origin of the red clay deposits accumulation of the loess soil sequences in the chinese loess plateau during the past 2 6 ma can be therefore regarded as the continuation of the pliocene atmospheric dust deposition observations of spatial grain size changes in the loess plateau suggest that the eolian red clay might be transported mainly by the westerlies differing significantly from the overlying loess that was transported essentially by the east asia winter monsoonal winds

grain size characteristics of dune sands in the central taklimakan sand sea abstract compared with the sand grain size of the other sand seas central taklimakan sand sea has some of the finest sands seen globally the dunes are composed of fine and very fine sands with a diameter between 2 00 and 4 00 0 25 0 063 mm there are differences in the grain size distributions for different dune types mean grain size of the compound complex crescent dune sands is 3 08 the compound dome dunes 3 21 the compound complex linear dune sands between 2 63 and 3 41 and the star dunes 2 81 from the northern edge of the sand sea towards southern edge the components of fine and very fine sands increase closely related to the wind regimes time scale for dune development and underlying sediments compared with the foregoing there are some differences for the compound complex linear dunes in the centre of taklimakan though the pattern of coarser crests does exist the sorting parameters indicate that the sands on the west flank of compound complex linear dunes are better sorted than that on the east flank and crest sampling on the superimposed dunes developed on the surface of the complex linear dunes suggests that the patterns of finer crest coarser crest all existed in central taklimakan under the conditions of low energy wind regime when fine sand is available the frequency of the pattern for finer crest and better sorting will increase sampling on the simple crescent dunes developed on the interdunes suggests that the simple dunes are developed originally with nearly the same grain size distributions and parameters with the development of the dunes and evolution of the dune morphology the grain size distributions of the dune sands varied under the actions of the wind regimes time scale and underlying sediments

lateral circulation generates flood tide stratification and estuarine exchange flow in a curved tidal inlet abstractcross channel transect measurements of microstructure and velocity in a well mixed and curved tidal inlet in the german wadden sea show the occurrence of significant late flood stratification this stratification is found to be a result of lateral straining this study observes a strong single cell lateral circulation which is strongly pronounced at late flood and absent during most of ebb this tidal asymmetry is caused by a systematic interplay between centrifugal forcing and the lateral baroclinic pressure gradient during flood a positive feedback between the terms generates strong lateral circulation whereas during ebb a negative feedback leads to a suppression of the cross channel exchange a theoretical framework based on vorticity is developed which allows lateral and longitudinal circulation to be studied in a consistent way with this framework it is possible to show that the tidal asymmetry of the lateral flow is a major driver of residual longitudinal estuarine circulation here ide

variation in particle size distribution over a small dune the variation in particle size distribution for a sequence of sand samples collected from the surface of a small barchanoid dune along a sampling line parallel to the windflow is studied by means of the hyperbolic distribution the modal or typical value of the log size increases linearly with distance from the windward foot to the crest of the dune and falls off steeply on the lee side the spread of the log size distribution locally near the modal value is roughly constant on the windward side but decreases significantly down the slip face two other parameters of the four parameter hyperbolic distribution can be considered largely constant a comparison of the log size distributions for heavy and light mineral grains based on a selected set of the samples showed that heavy and light minerals follow different hyperbolic patterns however the difference in modal size of the distributions for the two types of minerals was virtually constant among the samples

secondary currents in a curved stratified estuarine channel this paper presents a study of secondary circulation in a curved stratified channel in northern san francisco bay over a 12 5 hour tidal cycle secondary currents were strong at times varying by up to 35 cm s from top to bottom but relatively transient as the balance between centrifugal and lateral baroclinic forcing changed over time the short travel time around the bend did not allow a steady state balance to develop between centrifugal and lateral baroclinic forcing during the flood tide the confluence of two streams with different velocities produced a strong lateral gradient in streamwise velocity as a result lateral advection was a significant term in the streamwise momentum balance having the same order of magnitude as the barotropic and baroclinic pressure gradients and the frictional terms during the first part of the ebb secondary currents were induced by lateral baroclinic forcing the direction of the secondary circulation reversed later in the ebb as the baroclinic forcing became weaker than the centrifugal acceleration the gradient richardson number showed that stratification was stable over most of the tidal cycle decreasing the importance of friction and allowing secondary currents to persist

formation of the highest sand dunes on earth characterization of dune morphology has historically been based on relationships between dune forms and wind regimes with dune height shown to be sensitive to atmospheric boundary layer depth sand availability and sediment properties while these parameters have been used in numerical simulations to model the occurrences of some types of dunes they cannot alone explain the great diversity in form and size seen on earth and on other solar system bodies here we present results from our studies of dune formation in the badain jaran desert in western china where earth s tallest dunes occur we measured the variability of the dune morphology in this desert on the basis of landsat etm data and we detected the bedrock landforms beneath the aeolian sands by applying gravity methods wind records from stations at the periphery of the desert and srtm topographical data were examined also to augment the interpretation our studies demonstrate that in addition to average wind parameters dune height is highly sensitive to local geology subsurface characteristics and topography and interactions between changing climate conditions and aeolian and fluvial processes these additional factors need to be considered in the interpretation and simulation of dunes on earth we anticipate that analysis of anomalous dune heights like those seen in the badain jaran may also provide critical information on subsurface characteristics and environmental conditions on earth and on other planetary bodies

morphologic and sedimentologic patterns of active aeolian dune fields on the east coast of maranhao northeast brazil the eastern portion of the maranhao coast is characterized by large active dune fields located in a tropical setting this article combines the analysis of dune field morphologic patterns with the classical analysis of grain size and heavy minerals to study the sedimentary dynamics of the active aeolian system in this region based on the heavy mineral analysis the main sedimentary suppliers feeding the system are the westward alongshore drift bringing sand from the coast east of the parnaiba river mouth and the river itself the absence of well defined variation patterns of the two morphometric parameters studied dune spacing and crest length reflects complex sedimentary dynamics and transport pathways inside the aeolian system despite the unidirectional wind during the wet season the interdune plains are flooded and the system is intensely reworked by intermittent drainages during the dry season the deposits formed by the drainages and interdune lakes become temporary internal sedimentary sources for the system due to this combined aeolian fluvial transport the sediment source area has a planar geometry with contributions from the beach and interdune plains and not linear as expected in a typical case of source only in the beach the areal limits of the dune fields is the main boundary condition controlling the dune field patterns as dune spacing and crest length by changing the sedimentary inflow outflow balance and interactions between migration dunes copyright 2015 john wiley sons ltd

geomorphoclimatic characteristics and landform information in the ejina basin northwest china natural environment of mid latitude deserts of central asia is undergoing rapid changes one of its elements is the relief what needs to be particularly emphasized is its morphogenesis and landform evolution in this study a geomorphological research in the ejina basin inner mongolia is carried out with regard to identifying the main landforms and the contemporary geomorphic processes we consider ancient and modern landforms as a unit and provide a general explanation for their evolution related environmental indications of existing landforms are discussed on the basis of climate geomorphology results show that the current basin is under the conditions favorable for the development and dominance of landforms originated from physical processes such as desiccation deflation and aeolian the landform system is characterized by its variable structure which is conditioned by specific hydrodynamics processes that existed during glaciations it is suggested that landform patterns in ejina are under the strong influence of rapid and or intensive geomorphic processes these signatures reflect the regional dynamics of landform evolution as characterized by a very effective integration of tectonic and climate driven morphogenetic processes based on the present day climate data we infer an obvious linkage of desert gorges and pediments in the basin to ancient humid phases which provided high energy runoff for the formation of these landforms and were associated with erosional features under more humid and colder climate during the last glacial period desert plains and aeolian dunes in the central basin are the related products of contemporary climate clear evidences proving the landform inconsistency and landform instability in the ejina are presented indicating that the main geomorphic unit is experiencing transformation from alluvial plain to desert plain and then to aeolian dunes however the geomorphodiversity in the ejina should be a compound result of complex surficial processes other than glaciation frost weathering fluvial alluvial action desiccation deflation and aeolian processes as well as climatic changes have played important roles

numerical modelling of the sydney harbour estuary new south wales lateral circulation and asymmetric vertical mixing abstract a fully calibrated three dimensional hydrodynamic model of the sydney harbour estuary was used to determine the dominant forcing of regulating estuarine circulation in a sinuous channel under conditions of low river discharge during dry weather the system characterized by stratified flood and mixed ebb during spring tides and stratified flood and ebb during neap tides was analysed using model results of current profiles density field eddy viscosity and gradient richardson number at a mooring station and cross channel transect the magnitude and direction of lateral circulation was modulated in the flood ebb and spring neap cycles due to changes in the interactions between the tides and the complex geometry during spring tides a differential advection induced lateral baroclinic pressure gradient tends to drive the lateral circulation and stratifies the water column when the along estuary circulation is strong enough to break the stratification induced by the lateral baroclinic pressure gradient centrifugal force works in concert with nonlinear advection establishing a classical two layer helical flow across the channel channel bends also induce vertical mixing as a result of the overturning of the density fields the lateral circulation redistributes the differential advection induced lateral shear of the along estuary momentum and is transferred to either reinforce or cancel the along estuary circulation the lateral bathymetry variability in a channel is therefore the key factor causing the intratidal asymmetries in the along estuary circulations stratification suppresses lateral circulation during neap tides and generates a three layer lateral circulation structure in the water column during stratified ebb

mixing and circulation at the confluence of two rivers entering a meandering reservoir 1 a field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows ebro and segre with distinct physical chemical characteristics as they join at the upstream end of ribarroja reservoir in northern spain given the short average residence time of water in the reservoir the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam in winter inflows have similar densities 0 o 10 5 and their spatial distribution is largely determined by inertial forces and in particular by the discharge ratio downstream of the confluence both rivers flow side by side and largely unmixed over long distances in summer with 0 of o 10 3 the flow fields at the confluence are largely controlled by buoyancy forces atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms leading to localized high mixing rates mixing in general though is weak at this time of the year in fall and early winter density differences are largely controlled by conductivity differences between the incoming flows the warmer ebro water with larger thermal inertia flows beneath the colder segre water the spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong likely as a result of mixed water being denser than either of the incoming flows

provenance of aeolian sediment in the taklamakan desert of western china inferred from ree and major elemental data abstract to identify the provenance of aeolian deposits in the extensive dune field in the taklamakan desert is of great importance for understanding the formation of this the largest sand sea in china the opinions from earlier studies are quite dipolar and are as follows a local origin of the dune sands in different parts of the desert on the basis of various heavy mineral assemblages and b strong homogenization of the sands in the entire taklamakan on the basis of geochemical data from whole rock samples by separately examining the ree characteristics and major elemental composition in coarse and fine fractions of the samples from aeolian deposits this paper provides new data for interpreting sources of aeolian deposits in the taklamakan the sampling sites are distributed in four different fluvial systems i e the areas of the keriya river niya river and cele river in the southern part and the northern margin near the tarim river fig 1 our results show that there are some significant differences in concentrations of trace elements and in ree features between the coarse and the fine fractions of the aeolian sediment the major elemental and ree data suggest that the coarser sand is different from area to area in the desert rather than a homogenization of the entire basin the fine fractions mainly silts are more homogenized the regional difference of coarse fractions in the study areas is consistent with the fluvial and wind systems in the basin this confirms that the sands are often mixed between the northern and southern parts but there is much less mixing along the east west direction it should be emphasized that not only glacial and aeolian processes but also fluvial and lacustrine processes have jointly contributed to the formation of the huge sand sea in addition little variation is found between old and young sands in two sediment sequences in the central part of the desert indicating consistency of sand sources in a given site during the last 40 ka

the development of large aeolian bedforms abstract large aeolian bedforms complex and compound varieties of dunes or draas dominate over wide areas of many sand seas and are well represented in the rock record analyses of allometric relationships between dune height and spacing in several sand seas suggest that draas develop by vertical growth of existing dunes in the namib and other sand seas draa tend to occur in areas of low total and net sand transport rates the distribution of such bedforms in a sand sea reflects long term patterns of deposition which are related to regional scale changes in wind regimes and sand transport rates superimposed bedforms are almost ubiquitous on draa and develop only after a minimum size of draa has been reached formation of superimposed dunes reflects spatial and temporal variations in secondary flow patterns developed on the draa measurements of winds and depositional patterns on complex linear dunes in the namib sand sea suggest that an important mechanism for their growth is the migration of superimposed dunes from areas of high to low wind velocity on draa flanks leading to the deposition of sediment by bedform climbing

grain size variations on a longitudinal dune and a barchan dune abstract the grain size characteristics of the sand upon two dunes a 40 m high longitudinal dune in the central namib desert and a 6 0 m high barchan in the jafurah sand sea of saudi arabia vary with position on the dunes on the longitudinal dune median grain size decreases sorting improves and the grain size distributions are less skewed and more normalized toward the crest though sand at the windward toe is distinct elsewhere on the dune the changes in grain size characteristics are gradual an abrupt change in grain size and sorting near the crest as described by bagnold 1941 pp 226 229 is not well represented on this dune coarse grains remain as a lag on concave slope units and small particles are winnowed from the sand on the steepest windward slopes near the crest avalanching down slipfaces at the crest acts only as a supplementary grading mechanism on the barchan dune median grain size also decreases near the crest but sorting becomes poorer though the grain size distributions are more symmetric and more normalized the dune profile is a gaussian curve with a broad convex zone at the apex upon which topset beds had accreted prior to sampling grain size increases and sorting improves down the dune s slipface however this grading mechanism does not influence sand on the whole dune because variations in wind regime bring about different modes of dune accretion on both dunes height and morphology appear to influence significantly the grain size characteristics

turbulent stresses and secondary currents in a tidal forced channel with significant curvature and asymmetric bed forms acoustic doppler current profilers are deployed to measure both the mean flow and turbulent properties in a channel with significant curvature direct measurements of the reynolds stress show a significant asymmetry over the tidal cycle where stresses are enhanced during the flood tide and less prominent over the ebb tide this asymmetry is corroborated by logarithmic fits using 10 min averaged velocity data a smaller yet similar tendency asymmetry in drag coefficient is inferred by fitting the velocity and estimated large scale pressure gradient to a one dimensional along channel momentum balance this smaller asymmetry is consistent with recent modeling work simulating regional flows in the vicinity of the study site the asymmetry in drag suggests the importance of previously reported bed forms for this channel and demonstrates spatial and temporarily variations in bed stress secondary circulation patterns observed in a relatively straight section of channel appear driven by local curvature rather th

three dimensional tidal flow around headlands field measurements of tidal flow around a headland indicate secondary circulation induced by flow curvature the secondary flow defined to be the flow in the plane normal to the direction of the vertically averaged current is directed toward the headland near the bottom and seaward near the surface consistent with theoretical predictions the strength of the secondary flow varies from 5 to 15 of the streamwise flow it is strongest when the water column is stratified because of both the enhanced shear of the streamwise flow and the reduced frictional damping of the secondary flow the transverse exchange accomplished by the secondary flow significantly influences the structure of the streamwise flow causing a broadening of the transverse shear and changing the horizontal and vertical distributions of momentum it may also have a significant influence on horizontal dispersion in the vicinity of headlands

the importance of aspiration and channel curvature in producing strong vertical mixing over a sill on the basis of observations of the time dependent tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast channel curvature induces a cross channel circulation that can rapidly overturn a stratified flow eighteen along channel sections of density velocity and dissipation rate of turbulent kinetic energy were collected in and around the tacoma narrows of puget sound a site suspected of driving a strong vertical circulation in adjoining main basin rapid inflow to the narrows on flood from one channel of a triple junction reduces dynamic pressure allowing dense water from below sill depth to be uplifted or aspirated into the narrows we estimate water from below 150 m 3 times the sill depth is drawn into the narrows on a 3 m flood tide once in the narrows the flow remains stratified until it passes a 50 bend where a strong secondary circulation overturns the 50 m deep water column and generates intense turbulent mixing cross channel velocities of up to 0 4 m s 1 are observed and maximum values of exceed 10 3 w kg 1 upon leaving the sill stratification is reestablished and turbulence decays a similar set of sequences occurs on ebb except that the outflow bypasses the flood inflow channel and instead discharges into colvos passage the third branch of the triple junction colvos passage ultimately discharges the ebb effluent back into main basin enhancing the impact of mixing at the narrows by discharging the mixed product far from the source scaling of the cross channel momentum equation suggests that below a threshold value of along channel velocity stratification should suppress secondary circulation for a given vertical shear radius of curvature and channel width above the threshold velocity the magnitude of the cross channel velocity is roughly consistent with predictions for unstratified flow we estimate the maximum effective eddy diffusivity that aspiration and mixing in the narrows can produce in main basin to be 10 3 m2 s 1

an interpretation of trends in grain size measures abstract the mean grain size sorting and skewness of a sedimentary deposit are dependent on the sediment grain size distribution of its source and the sedimentary processes of i winnowing erosion ii selective deposition of the grain size distribution in transport and iii total deposition of the sediment in transport if a source sediment undergoes erosion and the resultant sediment in transport is deposited completely the deposit must be finer better sorted and more negatively skewed than the source this trend is referred to as case i the lag remaining after erosion on the other hand must be coarser better sorted and more positively skewed case ii if sediment in transport undergoes selective deposition the resultant deposit can either be finer case iiia or coarser cas iiib than the source but the sorting will be better and the skew more positive although exceptions to these trends may occur they suggest that comparison of one sediment must be made with another for the proper identification of the sedimentary process and therefore it is not possible for a single grain size distribution to identify the depositional environment the trends also suggest that the skewness of a grain size distribution has been widely misinterpreted and implies neither the truncation of one of the tails nor the mixing of more than one mode rather a skewed sediment is the natural result of the sedimentary process in a system of related environments these trends can be used to identify both the probable source and the probable deposit and by inference the net sediment transport paths among sedimentary deposits such an analysis provides a rapid understanding of the sedimentary processes identifies patterns of erosion and accretion and may suggest transport processes

geochemical evidence of the sources of aeolian sands and their transport pathways in the minqin oasis northwestern china identification of aeolian sand sources occurring in oases of desert environments is of great importance for understanding desertification processes and for developing strategies for sustainable development in arid regions combined with wind data and hierarchical cluster analysis we analyzed the spatial characteristics of major and trace elements of sands sampled at the margins of the minqin oasis northwestern china and its adjacent deserts the badain jaran desert and the tengger desert with the purpose to identify the aeolian sand sources and their transport pathways in the region the spatial distribution revealed by bivariate plots of cr ni cr v y ni al v zr hf zr hf and ternary plots of major and trace elements showed that sands between the west b badain jaran desert bm the dune belt between badain jaran and the minqin oasis and tne dune field located in the northeast margin of the minqin oasis and southeast tsw dune field located in the southeast margin of the minqin oasis sides of the oasis have different provenances while the composition of sands in the minqin oasis m and in the dune field located in the south margin tm is associated with both the variations in abundance of k rb ba and sr were used as indicators of aeolian transport processes our results show that while aeolian sands from the badain jaran desert can be transported over mountains and over long distances by northwest winds to the west sides of the minqin oasis they cannot directly reach neither bypass the oasis to the east side our interpretation is that the oasis can act as an effective barrier to stop the migration of dune fields both in the badain jaran desert and the tengger desert however the extensive occurrence of aeolian sands in the minqin oasis indicates that its role in preventing desert encroachment should not be overestimated c 2014 elsevier ltd and inqua all rights reserved

coeval changes in grain size and sedimentation rate of eolian loess the chinese loess plateau to determine the temporal spatial relationship between strength of the winds transporting the chinese loess and aridity conditions of the loess source area six loess sections located respectively in the northern middle and southern loess plateau were studied grain size distributions were analyzed on loess paleosol samples that are closely spaced in the sections time scales were established by tuning each of the grain size records to theoretical changes in the earth s orbital parameters results obtained show that during the last two glacial cycles particle size of the loess co varies with apparent sedimentation rates suggesting that variations in winter monsoon intensity and the aridity of the loess source area are tightly coupled this proximal relationship is in striking contrast with the observation of distal eolian clays in north pacific sediments we propose that this phenomenon may result from the proximity of the loess plateau to the source area and from the significant control of the winter monsoon on the expansion retreat changes of the deserts in the northern and northwestern china

patterns of grain size distribution in some point bars of the usri river india abstract the patterns of grain size distribution of point bar sediments on the usri river india were compared to log normal log hyperbolic and log skew laplace distribution models sediment samples were collected from a specific size of bedform ranging in foreset thickness from 23 to 40 cm from the source to the mouth of the river over a distance of about 90 km the hydrodynamic parameters of the river were also measured at four point bars the results indicate that a the log normal distribution is the best fitted distribution model among the three distribution models b after a certain distance of transport the log normal distribution becomes log hyperbolic and c though there is a tendency for grain size to decrease downstream within each point bar together with improvement of sorting this pattern is not consistent along the entire course of the usri river from one equivalent bar position to the next or from the source to its mouth

observations of turbulence in a partially stratified estuary the authors present a field study of estuarine turbulence in which profiles of reynolds stresses were directly measured using an adcp throughout a 25 h tidal day the dataset that is discussed quantifies turbulent mixing for a water column in northern san francisco bay that experiences a sequence of states that includes a weak ebb and flood that are stratified followed by a strong and eventually unstratified ebb and flood these measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification examination of individual reynolds stress profiles along with profiles of richardson number and turbulent froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects using the measured turbulence production rate p the dissipation rate e is estimated the observed turbulence had values of e nn 2 20 all of the time and e nn 2 200 most of the time suggesting that the observed motions were buoyancy affected turbulence rather than internal waves however at times turbulent froude numbers in much of the upper water column were less than one indicating important stratification effects taken as a whole the data show that stratification affects the turbulent velocity variance q2 most severely that is observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes finally the dataset is compared to predictions made using the popular mellor yamada level 2 5 closure these comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions the length scale does not appear to compensate for these errors and as a result similar errors are seen in the eddy viscosity predictions it is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self transport from the near bed region to the overlying stratification

grain size variation on dunes in the southwest kalahari southern africa abstract grain size parameters mean standard deviation skewness and kurtosis were determined for 83 sediment samples from six dune cross profiles and one 28 km long profile on dunes in the southwest kalahari desert in southern africa although some studies elsewhere and previous studies in the kalahari have reported significant grain size variation over dune cross profiles data presented here show no systematic variation of grain size parameters except that sediments from the crest of a dune are almost invariably better sorted indicated by the standard deviation of the sample than samples from the base or plinth of the dune the proportion of fines 63 m in plinth sediments is also greater than in crest samples there is no significant trend in grain size parameters along the length of a linear dune this lack of significant pattern corresponds with similar findings for central australian dunes

size distributions in suspension over sand pebble mixture an experimental approach experiments were conducted in a laboratory flume to estimate the grain size distributions in suspension at different heights over five heterogeneous sediment beds of different values of bed roughness the analysis had been performed to study the influences of velocity bed roughness and ripple height on the size sorting which developed the specific grain size distribution in suspension the observed size distributions in suspension were tested using three statistical distributions such as log normal log hyperbolic and log skew laplace tests indicated that log hyperbolic distribution does not fit in suspension even the original beds grain size distributions were log hyperbolic it is observed from the experimental findings that the flow velocity influences the overall grain size distribution leading from log normal to log skew laplace distribution with the increase of velocity but the bed roughness does not have any role in changing the size distribution from log normality the results of this study would be valuable to hydraulic engineers and sedimentologists to predict the nature of bed material from the size distributions of suspended materials

the role of streamline curvature in sand dune dynamics evidence from field and wind tunnel measurements abstract field measurements on an unvegetated 10 m high barchan dune in oman are compared with measurements over a 1 200 scale fixed model in a wind tunnel both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune confirming significant flow deceleration upwind of and at the toe of the dune acceleration of flow up the windward slope and deceleration between the crest and brink this pattern including the widely reported upwind reduction in shear velocity reflects observations of previous studies such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition this is not observed in the field wind tunnel modelling using a near surface pulse wire probe suggests that the field method of shear velocity derivation is inadequate the wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune evidence provided by reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave unstable streamline curvature these additional surface stresses are not recorded by the techniques used in the field measurements using the occurrence of streamline curvature as a starting point a new 2 d model of dune dynamics is deduced this model relies on the establishment of an equilibrium between windward slope morphology surface stresses induced by streamline curvature and streamwise acceleration adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress whereas convex streamline curvature and deceleration have the opposite effect the relationships between form and process are investigated in each of three morphologically distinct zones the upwind interdune and concave toe region of the dune the convex portion of the windward slope and the crest brink region the applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field

a model to study the grain size components of the sediment deposited in aeolian fluvial interplay erosion watershed abstract aeolian fluvial interplay erosion areas with complex dynamics and physical sources are the main suppliers of coarse sediment in the yellow river understanding the composition distribution and sources of deposited sediments in such areas is of great importance for the control of sediment transport in rivers in this paper a typical aeolian fluvial interplay erosion watershed the dongliu gully was studied and the frequency distribution curves of sediments deposited in the stream channel were fitted using the weibull function sources of deposited sediment in the stream channel were analyzed based on the law of the conservation of matter results showed that the hilly zone accounted for 78 of deposited sediments which were dominated by material with a median grain size d 50 of 0 093 mm and the desert zone accounted for 22 of deposited sediments which were dominated by material with a d 50 of 0 01 mm wind erosion dynamics accounted for 72 of deposited sediments while water erosion dynamics accounted for only 28 this research provides a theoretical basis for the control and management of rivers with high sediment content

sediment transport patterns determined from grain size parameters overview and state of the art abstract grain size trends have been applied in many diverse sedimentary environments to determine sediment transport paths generally coinciding with information from tracer studies current measurements and the orientation of sedimentary structures the different methods proposed to date are critically analysed and compared with reference to recent field studies it is concluded that the two dimensional methods produce comparable results and may in fact complement each other in spite of the advances several problems still exist which include the sampling method and density the choice of trend types the relative weight of grain size parameters and the interpretation of results these are discussed together with possible solutions

grain size characteristics of linear dunes in the southwestern kalahari abstract linear dunes in the southwestern kalahari sand sea exhibit characteristic grain size and sorting patterns in which sands become coarser better sorted and more finely skewed from interdune to crest in addition at a regional scale sands tend to become finer and better sorted in the direction of transport these patterns can best be explained in terms of the differential movement of sand in creep and saltation populations whereby the saltating population moves more rapidly and is concentrated downwind and in dunes slower moving creep populations remain close to source areas and in interdunes the fines will tend to be concentrated in interdunes because they can penetrate between coarse grains where they are protected from further movement this type of model probably explains most grain size and sorting relationships in desert sand seas

late pleistocene aeolian activity in the cold desert of ladakh a record from sand ramps abstract ladakh northwest india lies in the rain shadow zone of the indian summer monsoon ism where sand ramps provide a good record of past aeolian activity here we present an archive from five sand ramps that is based on their sedimentological structure and optically stimulated luminescence osl chronology sedimentary architecture indicates that the sand ramps are composite records of aeolian hill slope erosion and fluvio lacustrine activity the osl chronology suggests that aridity and aeolian activity was dominant during 25 17 ka and

grain size characteristics of namib desert linear dunes the results of a systematic sampling of linear dunes in the northern namib sand sea show that they are composed of two distinct groups of sands a fine well sorted near symmetrical crest and slip face population and a coarse moderately sorted positively skewed plinth population these may be explained in terms of the aeolian sorting processes operative on the dunes which involve differential rates of movement of creep and saltation load on windward flanks and avalanching on leeward slopes leading to the concentration of coarse sands towards the dune base

size frequency distribution of longitudinal dune rippled flank sands compared to that of slipface sands of various dune types rippled flanks of longitudinal dunes and the slip faces of various dune types can be distinguished granulometrically by comparing the top to bottom trend of changes of sorting in the sand population of each individual slope flank sands which are prone to sorting processes through migration of ripples alongslope are always better sorted upslope whereas slipface sands which are controlled by avalanche and grainfall of sands become better sorted downslope considering the absolute values of the bottom samples the sands of the slip faces are both better sorted and the size distribution more symmetrical than those of the rippled flanks applying this approach in investigating ancient sediments could improve palaeogeographical interpretations