estimation of sampling efficiency of the big spring number eight bsne sampler at different heights based on sand particle size in the taklimakan desert abstract the big spring number eight bsne sampler is an important and widely used instrument in studies of aeolian sand transport its sampling efficiency is an important parameter and varies with particle size however little is known about the variations in sampling efficiency caused by the differences in particle size with height the variations in particle size with height from 10 to 200 cm were obtained from a field experiment at four sites in the taklimakan desert from may 2010 to april 2011 the variation in the sampling efficiency of the bsne sampler with particle size was estimated based on previous studies and a wind tunnel experiment it was found that the average particle size decreased with height above ground from 10 to 200 cm at three experimental sites while at the other site it decreased from 10 to 15 cm and then slightly increased the percentage distribution of particle sizes with height was similar for sizes below 63 m and between 125 and 200 m at all experimental sites but there were differences in the percentage of size classes between 63 and 125 m and above 200 m the sampling efficiencies ranged from 66 27 to 58 98 64 97 to 57 47 56 04 to 52 76 and 72 07 to 58 9 at the four sites respectively with an average of 64 84 to 57 03 at heights from 10 to 200 cm after correcting for sampling efficiency the amount of aeolian sand that passed through a section of 100 cm width 200 cm height increased by 44 5 52 1 78 8 and 26 4 at the four sites respectively with an average of 50 5 these results indicate that the sampling efficiency of the device needs to be considered when making observations across a gradient of particle sizes

provenance and transport process for interdune sands in the kumtagh sand sea northwest china abstract interdune areas are an important part of dune systems as they affect dune evolution and sand sea development however there has been little work on the provenance of interdune sediments our lack of knowledge of these sediments prevents a more complete understanding of sand sea formation and evolution in the present study we focused on the provenance of interdune sediments in the kumtagh sand sea based on their granular mineral and geochemical characteristics then further identified the spatial scope of the contribution of these sources to the interdune sediments and the associated transport mechanisms although sediment supply analysis suggested that the interdune sand may derive from the altyn tagh mountains beishan mountains and sanlongsha dune field the provenance signature based on the k rb and k ba ratios revealed that the ultimate sources were the altyn tagh mountains in the south and the beishan mountains in the north by plotting the identified provenance results on a map of the sand sea we found that the spatial scopes of contributions from these sources were separated at around 40 n however the existence of a fluvial load em1 and of reworked coarse particles em3 in the northern sand sea demonstrated that the interdune sands in this region were partly derived from the altyn tagh mountains in the south consequently by combining our results with the transport agents condition we concluded that the altyn tagh mountains supplied sediments to interdune areas by fluvial transport throughout the sand sea but that sediment in the northern sand sea was subsequently reworked and mixed with windblown sand originated from the beishan mountains

frequency and magnitude biases in the fryberger model with implications for characterizing geomorphically effective winds abstract the fryberger model uses standardized wind data to estimate regional aeolian sediment drift potential dp which allows for interpretation and classification of aeolian dune landscapes this paper identifies systematic frequency and magnitude biases in the model that result from i variations in wind direction sector range which affects frequency bin size for determining percent occurrence of transporting winds and ii use of wind speed class mid point values over more statistically representative values e g mean median or minimally classified whole knot wind speeds in the magnitude weighting factor for dp calculations the significance of these biases is tested using unclassified data from stanhope prince edward island canada frequency bias resulting from wind data aggregation into direction sectors of varying size produces statistically significant discrepancies in dp and rdp estimates for this dataset when 36 point 10 s of degree data are reduced to 16 sectors cardinal directions receive one extra direction class and the magnitude of dp and rdp values decreases by 1 4 and 2 8 respectively compared to 16 equal 22 5 sectors derived from original data this results from under representation of intermediate direction classes for this dataset the effects on rdd less than 1 and directional variability ratio rdp dp are insignificant magnitude biases result from the use of the wind speed class mid point values instead of other statistically representative measures of wind speed in dp calculations in that wind speed distributions are often positively skewed mid point values yield an over estimate of total dp by as much as 34 and rdp by up to 22 for this dataset over those derived using mean median and whole knot minimally classified values this difference is statistically significantly at both the individual wind speed direction category level and at the aggregate directionally summed level and causes rdp vectors to shift 3 5 to the south though the impacts of these frequency magnitude biases are site specific and modest the effects on rdp estimation may be more significant in complex i e multi modal wind regimes and or in environments with more frequent high magnitude winds from the cardinal directions given the increased availability of more precise unclassified wind data most of these systematic frequency magnitude biases can be avoided recommendations on reducing inaccuracies imposed by these biases include i using to the degree wind data where available and categorizing into 16 equal 22 5 direction sectors ii using either wind speed class statistical mean values or minimally classified whole knot values in dp calculations and iii recognizing that converting 36 point 10 s of degrees data to 16 direction classes may introduce a frequency bias toward the cardinal directions and will cause inaccuracies in dp and rdp estimates of an amount that depends on the wind regime further research is needed to assess the implications of such biases in different wind regimes and into the influence of localized supply and transport limiting factors on regional scale assessments of dune morphodynamics and mobility these findings are also relevant for other applications that use categorized wind speed direction data such as dust or contaminant plume dispersion modelling

wind tunnel evaluation of a wedge shaped aeolian sediment trap abstract a detailed set of tests in a wind tunnel investigated the characteristics of flow and sampling efficiency of a low cost sediment trap that can be used in wind tunnels and field applications the trap is constructed of thin sheet aluminum and has a wedge shape 32 internal inlet angle with a 20 500 mm rectangular inlet orifice that extends 40 mm beyond the wedge form relatively unimpeded airflow through the trap is achieved by the large exit area 130 500 mm which is covered with fine stainless steel mesh 62 5 m openings 60 porosity and collected all but the finer silt sediment sizes despite its angular shape the trap has good characteristics of flow and a very high sampling efficiency 90 over a wide range of wind velocities because of its near isokinetic characteristics of flow results also indicate that this trap and others that have wide side profiles are very sensitive to incident wind angle incident angles 5 significantly reduce the sampling efficiency and promote scour around the trap inlet

velocity profile of a sand cloud blowing over a gravel surface particle dynamic analyzer pda measurement technology was used to study the turbulent characteristics and the variation with height of the mean horizontal in the downwind direction and vertical in the upward direction particle velocity of a sand cloud blowing over a gravel surface the results show that the mean horizontal particle velocity of the cloud increases with height while the mean vertical velocity decreases with height the variation of the mean horizontal velocity with height is to some extent similar to the wind profile that increases logarithmically with height in the turbulent boundary layer the variation of the mean vertical velocity with height is much more complex than that of the mean horizontal velocity the increase of the resultant mean velocity with height can be expressed by a modified power function particle turbulence in the downwind direction decreases with height while that in the vertical direction is complex for fine sands 0 2 0 3 mm and 0 3 0 4 mm there is a tendency for the particle turbulence to increase with height in the very near surface layer 4 mm the movement of blown sand particles is very complex due to the rebound of particles on the bed and the interparticle collisions in the air wind starts to accelerate particle movement about 4 mm from the surface the initial rebound on the bed and the interparticle collisions in the air have a profound effect on particle movement below that height where particle concentration is very high and wind velocity is very low

frequency magnitude and characteristics of aeolian sediment transport mcmurdo dry valleys antarctica 1 due in part to the challenging environment of earth s high latitude regions available information on cold climate effects on aeolian processes in these areas remains limited data from these areas however provide insight into the physics of sediment transport by wind and the controls on erosive winds in proximity to ice caps and topographic influences this study presents a 2 year record of meteorological saltation activity horizontal saltation flux and particle size distribution data from four sites in the mcmurdo dry valleys of antarctica 2008 to 2010 saltation measurements revealed daily and seasonal patterns with spring and summer sediment transport events occurring between 09 00 and 24 00 hours due to thermally generated winds fall and winter events occur at any time of day with the strongest associated with foehn winds threshold wind speed at 4 2 m in all seasons for all locations was 10 m s 1 saltation occurred in the temperature range 40 c to 5 c westerly winds in the fall winter and easterly winds in spring summer are associated with the majority of transport events the sand in transport is mainly 250 to 500 m in diameter and poorly sorted the integrated saltation flux varies over three orders of magnitude among the sites with the lowest mean flux recorded in the taylor valley 2 9 kg m 1 day 1 and the highest in the eastern victoria valley 2271 kg m 1 day 1 for 24 hours of continuous saltation the percentage of time saltation active at these locations annually is 2 4 and 13 respectively for the victoria taylor and wright valleys

sediment transport dis continuity across a beach dune profile during an offshore wind event abstract flow dynamics and sediment transport responses over a large vegetated foredune at prince edward island canada during an offshore wind event are examined data were collected along an instrumented transect that extended from the dune crest down the lee side seaward slope of the dune across a wave cut scarp and on to the back beach when the wind direction at the dune crest was approximately crest normal less than about 15 deviation the mean near surface flow directions along the dune slope and on the back beach were generally onshore indicating reversed onshore flow relative to the regional offshore wind direction although flow patterns were consistent with a lee side recirculation eddy large excursions in flow direction were also prevalent suggesting that the eddy was unstable and alternated with highly turbulent wake flow as wind direction at the crest veered to greater than 20 from crest normal lee side winds shifted toward strongly alongshore flow with minimal directionally variability on the dune slope the wind vectors were slightly offshore whereas on the back beach they were slightly onshore wind speeds and sediment transport were greatest at the foredune crest and declined rapidly downslope due to flow expansion and deceleration in the wake zone as well as to the influence of a sparse vegetation layer mean particle counts averaged over a 15 min interval derived from laser sensors positioned at the crest were large 7 76 per second in comparison to those measured in the immediate lee of the crest 0 52 per second and farther down the dune slope these results demonstrate that sediment transport across the beach dune system was spatially discontinuous during this offshore wind event rebuilding of the dune ramp at the toe of the scarp occurred quite independently of and with a different sediment source than the broadening of the dune crest which was fed with sediment from the landward side of the foredune such process decoupling is an example of the complexity by which foredunes evolve or are maintained and as with previous research reinforces the importance of offshore and alongshore wind events to beach dune morphodynamics

laboratory studies of aeolian sediment transport processes on planetary surfaces abstract we review selected experimental saltation studies performed in laboratory wind tunnels and collision experiments performed in splash laboratory facilities that allow detailed observations between impinging particles on a stationary bed we also discuss progress in understanding aeolian transport in nonterrestrial environments saltation studies in terrestrial wind tunnels can be divided into two groups the first group comprises studies using a short test bed typically 1 4 m long and focuses on the transitional behavior near the upwind roughness discontinuity where saltation starts the other group focuses on studies using long test beds typically 6 m or more where the saturated saltation takes place under equilibrium conditions between wind flow and the underlying rough bed splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera the findings indicate that the number of ejected particles per impact scales linearly with the impact velocity of the saltating particles studies of saturated saltation in several facilities using predominantly particle tracking velocimetry or laser doppler velocimetry indicate that the velocity of the few particles having high trajectories increases with increasing friction velocity however the speed of the majority of particles that do not reach much higher than bagnold s focal point is virtually independent of shields parameter at least for low or intermediate u values in this case mass flux depends on friction velocity squared and not cubed as originally suggested by bagnold over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds measurements using horizontally segmented traps give average saltation jump lengths near 60 70 mm and appear to be only weakly dependent on friction velocity which is in agreement with some but not all older or recent wind tunnel observations similarly some measurements performed with uniform sand samples having grain diameters of the order of 0 25 0 40 mm indicate that ripple spacing depends on friction velocity in a similar way as particle jump length the observations are thus in agreement with a recent ripple model that link the typical jump length to ripple spacing a possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow quantitative understanding of saltation characteristics on mars still lacks important elements based upon image analysis and numerical predictions aeolian ripples have been thought to consist of relatively large grains diameter 0 6 mm and that saltation occurs at high wind speeds 26 m s involving trajectories that are significantly longer than those on earth by a factor of 10 100 however this is not supported by recent observations from the surface of mars which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples sullivan et al 2008 also the highest average wind speeds on mars have been measured to be electrification is seen as a dominant factor in the transport dynamics of dust on mars affecting the structure adhesive properties and detachment entrainment mechanisms specifically through the formation of aggregates merrison et al 2012 conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra terrestrial environments

using simulated emergent vegetation to alter stream flow direction within a straight experimental channel river restoration programs often use vegetation to enhance the biological functionality recreational opportunities and aesthetic beauty of degraded stream corridors yet none has used vegetation for the purpose of inducing a straight channel to meander a flume based study was designed to alter the flow pattern within a straight degraded stream corridor by using simulated emergent vegetation of varying density placed at key locations within the channel placement of vegetation zones was determined using an empirical relation for equilibrium meander wavelength based on the imposed flow rate surface flow velocities were quantified using particle image velocimetry the study showed that i flow velocity can be markedly reduced within and near the vegetation zones ii flow can be diverted toward the opposite bank and iii vegetation density controlled the magnitude of these effects

flow pattern within a permian submarine slump recorded by oblique folds and deformed fossils ulladulla south eastern australia the flow pattern within a slump in permian marine rocks of the southern sydney basin australia is recorded by folds and deformed fossils abundant brachiopod and bryzoan fossils in the slumped rocks are relatively undeformed but fossil crinoid stems have been deformed by relative rotation of individual ossicles measurement of the strain indicates that the deformation of the crinoids is consistent with flexural flow folding within the slump previous models assume that curved slump fold axes remain parallel to the enveloping bedding surface of a slump sheet detailed measurements of the orientation of slump folds in this study found fold axes to be oblique to bedding which is interpreted as a result of folds plunging downward towards the flanks of the slump or slump lobes in the present model fold axes are not generally parallel to the strike of the fold axial surface and this can explain differences between the orientations of slump fold axes and axial surfaces when these are used as directional indicators of slump movement

grainfall processes in the lee of transverse dunes silver peak nevada grainfall deposition and associated grainflows in the lee of aeolian dunes are important in that they are preserved as cross beds in the geological record and provide a key to the interpretation of the aeolian rock record despite their recognized importance there have been very few field laboratory or numerical simulation studies of leeside depositional processes on aeolian dunes as part of an ongoing study the relationships among grainfall wind speed and direction stoss sand transport rates and dune morphometry height and aspect ratio were investigated on four relatively small straightcrested transverse dunes at silver peak nevada between 55 and 95 of the total grainfall was found to be deposited within 1 m of the crest and 84 99 within 2 m depending primarily on dune size and shape grainfall decay rates on high dunes of large aspect ratio were observed to be very consistent with a weak positive dependence on wind speed for small dunes with low aspect ratios grainfall deposition was more varied and decreased rapidly within 1 m of the dune crest whereas at increased distance from the dune crest it eventually approached the smaller decay rates observed on the large dunes no dependence of grainfall on wind speed was observed for these small dunes comparison of field data with predictions from anderson s 1988 saltation model of grainfall based on the computation of saltation path lengths indicates lack of agreement in the following areas 1 deposition rate magnitude 2 variation in decay rate with wind speed and 3 the magnitude and location of the localized lee slope depositional maxima the silver peak field results demonstrate the importance of dune aspect ratio and related wake effects in determining the rate and pattern of grainfall this work confirms earlier speculation by mcdonald anderson 1995 that temporary turbulent suspension or modified saltation of relatively large grains does occur within the dune wake so that transport distances generally are larger than predicted by numerical simulations of true saltation

slipfaceless whaleback dunes in a polar desert victoria valley antarctica insights from ground penetrating radar the lower victoria valley is one of the mcmurdo dry valleys antarctica and has the largest concentration of aeolian sand dunes on the continent aeolian bedforms include elongate slipfaceless dunes which have been called whaleback dunes these dunes are composed of coarse sand and capped by granule ripples ground penetrating radar gpr profiles across one of these dunes reveal low angle inclined sigmoid tangential and convex reflections that are interpreted as strata within the dune these dipping strata record the migration of the whaleback dune and show that they are not sand mantles as previously described but are actively migrating long wavelength low amplitude bedforms resembling zibar an 800 m profile along the axis of the dune reveals low angle dips from west to east showing that the dune has accreted towards the east gpr profiles collected at 100 m intervals transverse to the dune axis reveal sets of cross stratification dipping towards the south at the eastern end of the dune and towards the north at its western end these apparent dips are resolved to show that the dune has migrated from west to east driven by westerly foehn winds and katabatic winds blowing from the polar ice cap whilst at the same time it has been building obliquely towards the south accretion towards the north at the western end of the dune is attributed to reworking of the dune sand by easterly winds blowing inland from mcmurdo sound an unconformity within the dune suggests that the wind regime has varied in the past beneath the unconformity the direction of dune migration was from west to east and eastward extension of the dune continues at the present day however this is combined with accretion towards the north and west at the western end of the dune which is not recorded in the older sediments beneath the unconformity sand wedge structures that may have taken hundreds to thousands of years to form are found within the dune and suggest that arid conditions suitable for dune construction have persisted in victoria valley for a considerable time

flow dynamics and sediment transport over a reversing barchan changli china abstract the flow and sediment transport over a reversing 6 9 m high barchan dune is examined on the changli gold coast in hebei province china wind velocity profiles are non logarithmic exhibit rapid accelerations up slope and display near surface jets at the dune crest and downwind lee slope from the windward lower slope to dune crest the wind speed at 5 cm height increased by 55 the windward slope is a slipface and therefore has a significantly greater gradient than most normal windward slopes and the magnitude of speed up is significantly greater speed up ratio of 5 25 the majority of sediment was transported in the 0 4 cm height above the bed range maximum total sediment flux occurred at the dune crest and was greater than the total flux from three other sites combined the leeward mid slope sedimentation rate is strongly influenced by the sediment plume streaming upwards and downwind from the actively reversing dune crest dunes in the process of reversing display quite different wind flow and sedimentation patterns than when the dune is in normal flow conditions

spatial and temporal variability in sedimentation rates associated with cutoff channel infill deposits ain river france floodplain development is associated with lateral accretion along stable channel geometry along shifting rivers the floodplain sedimentation is more complex because of changes in channel position but also cutoff channel presence which exhibit specific overflow patterns in this contribution the spatial and temporal variability of sedimentation rates in cutoff channel infill deposits is related to channel changes of a shifting gravel bed river ain river france the sedimentation rates estimated from dendrogeomorphic analysis are compared between and within 14 cutoff channel infills detailed analyses along a single channel infill are performed to assess changes in the sedimentation rates through time by analyzing activity profiles of the fallout radionuclides 137cs and unsupported 210pb sedimentation rates are also compared within the channel infills with rates in other plots located in the adjacent floodplain sedimentation rates range between 0 65 and 2 4 cm a1 over a period of 10 to 40 years the data provide additional information on the role of distance from the bank overbank flow frequency and channel geometry in controlling the sedimentation rate channel infills lower than adjacent floodplains exhibit higher sedimentation rates and convey overbank sediment farther away within the floodplain additionally channel degradation aggradation and bank erosion which reduce or increase the distance between the main channel and the cutoff channel aquatic zone affect local overbank flow magnitude and frequency and therefore sedimentation rates thereby creating a complex mosaic of sedimentation zones within the floodplain and along the cutoff channel infills last the dendrogeomorphic and 137cs approaches are cross validated for estimating the sedimentation rate within a channel infill

climbing zibars of the algodones abstract zibars are coarse grained low relief slipfaceless eolian bedforms that occur on sand sheets and within interdune corridors of many eolian sand seas zibar surface features consist exclusively of wind ripples and local shadow and shrub coppice dunes associated with vegetation a study of zibars located on the southwestern margin of the algodones dune field california shows that zibars and their associated interzibar areas are migrating features oriented transverse to oblique to the resultant sand transport direction zibars leave deposits formed by deposition on the zibar lee face the zibar shape modifies the near surface airflow to produce a subtle grain size change across the zibar interzibar surface flow expansion not separation leeward of the zibar crest causes the lee face deposits those preserved upon bedform migration to have the finest grain size on the zibar interzibar surface sediment coarser than that deposited on the zibar lee face is transported more slowly and less frequently and thus becomes concentrated on the zibar stoss slope and ultimately becomes interzibar deposits migration of zibars and interzibar areas can produce two characteristic depositional sequences depending upon the angle of climb for low angles of climb a sequence of stacked amalgamated interzibar deposits is preserved zibar deposits are absent amalgamated interzibar deposits consist of low angle usually less than 5 wind ripple laminae of medium to coarse sand given higher angles of climb deposits of both the entire interzibar and zibar lower lee face are preserved low angle interzibar deposits of medium to coarse sand are overlain tangentially by finer grained fine to medium sand wind ripple laminae dipping less than 15 of the zibar lee face lee face wind ripple laminae are sharply truncated and overlain by the succeeding set of interzibar wind ripple deposits this zibar interzibar sequence is distinctive and allows for the detailed interpretation of some ancient sand sheet deposits as left by migrating zibars a possible example of ancient zibar deposits is present in the entrada sandstone jurassic of southwestern colorado

sediment flux and airflow on the stoss slope of a barchan dune abstract measurements of sediment flux on the windward slope of an isolated barchan using an array of 30 sand traps provide new data that can constrain models of dune dynamics the data show that at low wind incident speeds flux increases up the dune exponentially whereas at higher wind speeds the increase with distance approaches linearity wind profile measurements conducted at the same time as the flux measurements indicate that although wind speed at a given height increases by 1 2 times from dune toe to brinkline wind shear velocity derived from the profile data decreases up the dune and is in many cases below transport threshold values this demonstrates that conventional wind profiles derived from anemometry on dunes do not measure the part of the boundary layer that is significant for sediment transport

comparison of four methods to calculate aeolian sediment transport threshold from field data implications for transport prediction and discussion of method evolution abstract aeolian sediment transport threshold is commonly defined as the minimum wind speed or shear stress required for wind driven sediment transport accurate and consistent quantification of this threshold is essential because it is an input variable in models used to predict wind erosion dune activity and dust emissions the majority of threshold quantification has been performed with analogs analytical models or wind tunnels however in the past few decades field based approaches to threshold parameterization have become more common although several methods of calculating transport threshold from field data are available their comparability is unknown to address this issue we collected high resolution sediment transport and wind measurements 1 hz on an active sand dune for 11 days and compared four different methods of calculating threshold i time fraction equivalence method tfem ii gaussian time fraction equivalence method gtfem iii instantaneous method and iv regression method time paired measurements from the two most widely used methods tfem and gtfem were strongly correlated r 0 977 however correlations between other methods varied from r 0 861 to r 0 261 to demonstrate the implications of using different threshold calculation methods we predicted mass transport which ranged from 63 6 instantaneous method to 126 6 kg per crosswind meter regression method this inconsistency suggests that the threshold calculation method could have an appreciable impact on transport predictions threshold values are similarly inconsistent when the measurement interval is modified as such we do not recommend comparing any measured threshold with another we discuss several strategies that may mitigate the impact of this issue such as clarification of semantics and method standardization we also discuss several criticisms of field based threshold measurements and re conceptualizations that could allow investigators to develop a better understanding of field based measurements overall results from this study could allow future investigators to improve threshold and transport predictions

horizontal aeolian sediment flux in the minqin area a major source of chinese dust storms abstract minqin has become one of the key dust source areas in china due to its severely degraded eco environment however little is known about how much dust is emitted and transported in this area aeolian dust samplers were mounted at 15 heights on a 50 m monitoring tower in minqin in may 2007 to monitor the horizontal aeolian sediment flux the monitoring data suggests that the total annual horizontal aeolian sediment flux over minqin is about 8700 kg m 1 of that annual total the flux in the pm63 pm20 and pm10 size classes amounted to 1730 780 and 580 kg m 1 respectively the flux in may and june accounted for 64 of the annual total with a minimum in october the horizontal aeolian sediment flux including dust flux decayed rapidly with increasing height following a modified power function in agreement with the results of several previous studies the sediment flux in minqin depends primarily on the wind s characteristics especially the maximum wind speeds which create strong aeolian transport precipitation in this arid region had limited significance for reducing aeolian sediment transport the mean diameter standard deviation skewness and kurtosis of the aeolian sediments varied with height but varied most significantly within 9 m above the surface the mean diameter of the aeolian sediment ranged from 20 m 5 6 o to 95 m 3 4 o and thus primarily represents suspended dust even though coarse particles of organic and salt aggregates are also present

the topographic signature of vegetation development along a braided river results of a combined analysis of airborne lidar color air photographs and ground measurements 1 this paper combines archived remotely sensed data airborne lidar and digital color air photographs with nonsynchronous ground observations including observations of topographic form and vegetation cover and growth to test the hypothesis that colonization of exposed river sediments by riparian trees has an impact on channel form and to quantify any impact that is identified this is achieved along a 21 km reach of the braided gravel bed tagliamento river in northeast italy where the width of the braided corridor typically exceeds 800 m lidar data are analyzed to extract a 2 m resolution digital evolution model dem and determine riparian vegetation extent height and structure within the active corridor aerial photographs are used to map the topography of the submerged parts of the corridor these data are divided into 1 km length subreaches which possess strong contrasts in vegetation height and extent joint analysis of vegetation and morphological properties of these subreaches reveals significant associations between vegetation properties and reach morphology residuals from a gamma function fitted to the topographic data for each subreach show a good fit with poorly vegetated reaches but a weakening fit with increasing vegetation cover largely as a result of the appearance of secondary peaks in the elevation frequency distribution associated with the heavily vegetated areas furthermore the overall skewness and kurtosis of the elevation frequency distribution within each of the subreaches are both significantly correlated with vegetation extent height median elevation and growth rate indicating a clear topographic signature of vegetation development along this braided river that reflects sediment accumulation within and around the vegetated patches

effects of vegetation on braided stream pattern and dynamics 1 investigations using a 16 m by 2 m recirculating experimental flume model of an ephemeral braided river indicate that the presence of large and erosion resistant plants within the channel e g trees or shrubs can have a significant impact on channel pattern and planform dynamics simulations show that these plants have two effects first they act as obstructions in some cases forcing the flow to divide this flow separation can allow the deposition of a small island immediately in the lee of the plant splitting the channel the net result is a substantial increase in the number of channels and correspondingly the braid index this is in direct contrast to previous studies where increased levels of vegetation in perennial streams have decreased the braid index second the plants stabilize braid bars and can form relatively stable islands in their lee significantly reducing the longitudinal migration of islands typically associated with braided rivers

bedform alignment in directionally varying flows many kinds of sediment bedforms are presumed to trend either normal or parallel to the direction of sediment transport for this reason the trend of bedforms observed by remote sensing or by field observations is commonly used as an indicator of the direction of sediment transport such presumptions regarding bedform trend were tested experimentally in bidirectional flows by rotating a sand covered board in steady winds transverse oblique and longitudinal bedforms were created by changing only two parameters the angle between the two winds and the proportions of sand transported in the two directions regardless of whether the experimental bedforms were transverse oblique or longitudinal as defined by the bedform trend relative to the resultant transport direction they all had trends that yielded the maximum gross transport across the bedforms the fact that many of the experimental bedforms were neither transverse nor parallel to the resultant transport direction suggests that transport directions cannot be accurately determined by presuming such alignment

deflection of sand movement on a sinuous longitudinal seif dune use of fluorescent dye as tracer abstract sand was marked by fluorescent dye in order to trace sand movement and deposition on a longitudinal seif sand dune in the sinai desert the wind regime was monitored simultaneously tracing the dyed sand was possible after light to moderate sand storms and was graphically represented on maps the dune was subjected to a seasonally bidirectional wind regime with the wind hitting the dune obliquely on either side on the windward flank the sand was transported parallel to the wind direction on the lee flank sand movement was deflected towards parallelism with the crest line sand movement was deflected if the dune had a sharp profile which favored separation of wind flow on the lee flank the deflection depended on the angle of incidence between the wind and the crest line when the angle of incidence was

fluvial islands first stage of development from nonmigrating forced bars and woody vegetation interactions abstract fluvial islands can develop from the channel bed by interactions between pioneer trees and bars although vegetation recruitment and survival is possible on all bar types it is easier for trees to survive on nonmigrating bars developed from a change in channel geometry or to the presence of a steady perturbation this field study details the first stages of development of a vegetated mid channel nonmigrating or forced bar and its evolution toward an island form over six years analysis of bed topographical changes vegetation density and roughness scour and fill depths sediment grain size and architecture and excess bed shear stress highlighted a specific signature of trees on topography and grain size segregation two depositional processes combining the formation of obstacle marks and upstream shifting deposition of sediments led to the vertical accretion of the vegetated bar during the first stage of the bar accretion bedload sediment supply coming from surrounding channels during floods was identified as a key process modulated by the presence of woody vegetation and a deflection effect induced by the preexisting topography grain size segregation between vegetated and bare areas was also highlighted and interpreted as an important process affecting the development of surrounding channels and the degree of disconnection and hence the speed of development of a growing island the heterogeneity of bedload supply can explain why sediment deposition and density of trees are not strictly related a general conceptual model detailing the first stages of evolution from a bar to an established island is proposed for relatively large lowland rivers

in channel woody vegetation controls on sedimentary processes and the sedimentary record within alluvial environments a modern example of an anabranch of the river loire france a multidisciplinary approach was used to analyse the role of pioneer trees populus nigra on bar formation in the modern fluvial system of the river loire france data were collected on seven plots located on a secondary channel chosen for their vegetation cover and their sedimentary context physical features of vegetation topographical evolution sediment grain size and flow velocity were investigated during and after floods between 2001 and 2003 woody vegetation responds morphologically to fast flowing waters and sediment deposition these adaptations influence the role played by trees on local sedimentary processes some morphological types of trees exert a strong influence during flood events by deflecting reducing or increasing the flow energy as a consequence the evolution of bedforms distribution of sediment grain size and slopes are controlled by the flood type the morphological context i e high or low energy zones the morphology of the vegetated bar and the characteristics of vegetation conceptual models are proposed to account for local processes and evolution of tree groves the first model which describes the behaviour of groves during a single flood underlines the sediment deposition downstream of the trees and the trapping of bedload sediments during the falling stage of the flood the reorientation of current streams constrained by both local bed topography and woody vegetation induces transverse sediment fluxes for these water levels the control exerted by trees on small scale sedimentary processes becomes significant the second model shows the evolution of vegetation bands over several flood events in particular it describes the influence of vegetation on the fixing and reworking of bedload sediments during the falling limb of the hydrograph the coalescence of vegetation induced bedforms and the resulting morphological changes increase the deflection power of the vegetated bar inducing the deposition of finer sediments

aeolian particle flux profiles and transport unsteadiness vertical profiles of aeolian sediment flux are commonly modeled as an exponential decay of particle mass transport with height above the surface data from field and wind tunnel studies provide empirical support for this parameterization although a large degree of variation in the precise shape of the vertical flux profile has been reported this paper explores the potential influence of wind unsteadiness and time varying intensity of transport on the geometry slope curvature of aeolian particle flux profiles field evidence from a complex foredune environment demonstrates that i the time series of wind and sediment particle flux are often extremely variable with periods of intense transport referred to herein as sediment flurries separated by periods of weak or no transport ii sediment flurries contribute the majority of transport in a minority of the time iii the structure of a flurry includes a ramp up phase lasting a few seconds a core phase lasting a few seconds to many tens of seconds and a ramp down phase lasting a few seconds during which the system relaxes to a background low intensity transport state and iv conditional averaging of flux profiles for flurry and nonflurry periods reveals differences between the geometry of the mean profiles and hence the transport states that produce them these results caution against the indiscriminate reliance on regression statistics derived from time averaged sediment flux profiles especially those with significant flurry and nonflurry periods when calibrating or assessing the validity of steady state models of aeolian saltation

spatial hydrogeomorphological influences on sediment and nutrient deposition in riparian zones observations from the garonne river france abstract this paper investigates the influence of geomorphological setting on riparian zone sedimentation within a reach of the river garonne france during three major floods the sampling design was stratified to reflect landforms constructed by fluvial processes e g floodplain lateral benches islands side channels and point bars observed sedimentation varied significantly with flood event planform context landform type and associated vegetation cover and in some cases with sample location within the landform lowest sedimentation was associated with the flood with the smallest peak discharge peak sediment concentration and sediment load sites under natural riparian vegetation experienced higher sedimentation than poplar plantations sites on concave outer banks received less sedimentation than those on convex inner banks sedimentation on floodplain sites and higher benches was lower than on low benches point bars and side channels there was considerable interdependence among these patterns reflecting the underlying geomorphological forms and processes meandering rivers tend to evolve through erosion of concave banks and deposition on convex banks point bar features tend to be built along convex banks whilst concave banks are eroded into higher floodplain and bench features as a result concave banks tend to be bordered by higher riparian margins that are less frequently flooded than convex banks where river margins are developed for agriculture the higher less frequently flooded sites are preferentially selected analyses of the quantity calibre nutrient and carbon content of the deposited sediment reveal further significant relationships which reflect the geomorphological structure of the riparian zone sediment particle size coarsens in locations with higher amounts of sedimentation the quantities of total organic carbon toc total organic nitrogen ton and total phosphorus tp all increase as the quantity of deposited sediment increases the concentration of toc and ton also increased significantly with an increase in the percentage of silt plus clay in the deposited sediments based upon the above observations a conceptual model is proposed which considers the spatial pattern in riparian zone sedimentation according to riparian morphology and flood magnitude the implications of channel incision for the functioning of the model are also discussed

long term measurements of aeolian transport directional variations over a zibar surface in the northern kumtagh sand sea abstract zibars are aeolian bedforms created by armoring of the surface by coarse grains the existence and movement of these grains strongly influence zibar morphodynamics although aeolian sediment transport has been investigated for arid surface processes with a range of grain sizes the measurement durations were short making it difficult to establish the relationship between microscopic aeolian transport processes and macroscopic geomorphological processes in the present study we conducted field observations of aeolian sediment transport above a zibar in northern china s kumtagh sand sea with a segmented sand trap during six measurement periods throughout 2014 the sampler collected 314 76 kg of sediments within 1 m above the ground the directional variation indicated that about 52 5 of the sediments came from the north northeast and northwest the southeast provided the least sediments the strongest aeolian transport occurred between august and october the weakest was in winter the aeolian sediment flux decreased with height and 54 of particles were transported within 0 1 m above the ground the transport profiles fit an exponential decay function the flux s directional variation resulted from the combined effects of sediment availability and wind regime so the calculated net transport differed from the drift potential the seasonal and directional variations of aeolian transport significantly affect the morphodynamic process of zibars the trend and the internal sedimentary structure of zibars were closely related to the aeolian transport process

populus nigra l establishment and fluvial landform construction biogeomorphic dynamics within a channelized river populations of the riparian pioneer species populus nigra l which establish on alluvial bars within river channels modulate sediment dynamics and fluvial landforms dense cohorts of p nigra have colonized gravel point bars along the channelized river garonne france during the last 20 years and have enhanced the vertical lateral and longitudinal development of the bars for this period the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to the spatial distribution and intensity of establishment and resistance of different cohorts of p nigra furthermore p nigra colonization dynamics were controlled by engineer effects of this same species this relationship is illustrated by a significant correlation between key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between 2000 and 2010 the development of wooded point bars which are discrete biogeomorphic units over the studied period appear to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment trapping and stabilization by pioneer engineer plants we propose a conceptual model of biogeomorphic unit construction for channelized lateral stable rivers we consider the resultant biogeomorphic units as functional from an ecological point of view because p nigra enhances at the cohort scale i its own inherent capacity to resist hydrogeomorphic disturbances and ii its resilience capacity as a result of successful colonization especially downstream of mature poplar stands

aeolian vertical mass flux profiles above dry and moist sandy beach surfaces abstract the vertical distribution of aeolian mass flux was investigated in a natural beach environment field experiments conducted on the beach of the eba barrier southern baltic coast poland measured the sand transport rate and the vertical mass flux distribution above dry rippled sand and a moist flat sandy surface the experiments were intended to show the changes in the vertical distribution of sand with changing wind speed all the data represent saturated flux conditions sand transport was measured using 0 5 m high vertically segmented passive sand traps while the wind speed and direction were monitored at 1 m elevation the obtained dataset comprises 65 measurements on dry surfaces and 51 measurements on moist sandy surfaces the sand transport rate above the moist surface was up to 90 higher than above the dry surface for wind speeds of 7 11 m s but higher velocities gave smaller differences between the surfaces the saltation layer was thicker above the moist surface than above the dry surface all the vertical sand flux profiles are best described by exponential decay functions analysis of the normalised flux profiles grouped by wind velocity shows that the fitted curves are less inclined for moist surfaces than dry surfaces moreover the regression coefficients depict a marked trend in which the intercept decreases and the slope increases with increasing wind speed this indicates that more sand is transported at higher elevations above the bed and less at lower elevations the proportion of total transport seems to be independent of wind speed at elevations of approximately 35 mm and 50 mm above the dry and moist surfaces respectively differences between the measured and exponential fit values of mass flux are particularly distinct close to the bed where the exponential fit either over or under predicts the measured values over predictions occur in weaker winds up to 6 7 m s whereas under predictions become more pronounced as the wind becomes stronger and when the layer in which the under prediction occurs thickens the under predictions are particularly obvious above the moist surface it is not clear whether this phenomenon is due to technique measurement error or to another source the proportion of creep is often considered to be responsible for the difference from the exponential fit which represents the saltation mode of aeolian transport this proportion is difficult to determine because the mass flux measured by the lowermost chamber of the sand trap used in the experiments represents sand transported in both saltation and creep or and reptation

morphological and sedimentary features of oblique zibars in the kumtagh desert of northwestern china abstract zibars are coarse grain armored low relief aeolian bedforms with a large spacing but without slipfaces little is known of their morphodynamic processes even though they are ubiquitous at upwind desert margins in this paper we describe the morphological and sedimentary features of oblique zibars in the northeastern kumtagh desert of china analyze the effective wind regime and discuss the significance of the coarse grained sediments for zibar morphology and sedimentary structures due to a mantle of coarse grains oblique zibars and nearby linear dunes respond to different effective wind regimes that result in the angle between their geomorphological trends topographic surveys showed that the spacing of the oblique zibars ranged between 130 5 and 357 5 m and that the height ranged between 0 6 and 3 0 m the oblique zibar surface is almost entirely covered by coarse sand and includes gravel with a particle diameter of up to 2 34 the poorly sorted positively to very positively skewed surface sediments with mean grain size ranging between 0 2 and 1 5 were characterized by a bimodal distribution the spacing between the oblique zibars is closely related to the coarsest 5th percentile of the surface sediments and can potentially be explained by a combination of the saturation length and minimum dune size theories in addition the protectionist theory combined with the surface creep and vibration of coarse grains which allows infiltration of finer materials between the grains may account for the alternating coarse and fine laminae in the oblique zibars

the origin of the transverse instability of aeolian megaripples abstract flat sand beds subjected to wind stress are unstable and the wind action results in two types of aeolian sand ripples normal ripples and megaripples the distinction between the two types is based on two characteristics i the normal ripple pattern usually has a wavelength of up to 30 cm while the megaripple wavelength is on the order of meters and ii unimodal distributions of sand grain size lead to normal ripples while bimodal distributions result in megaripples on mars the distinction between the two types is more difficult to ascertain because the length scales of normal ripples and megaripples can overlap and often there is no detailed information regarding their grain size distribution unlike normal ripples megaripples show transverse instability whose mechanism remains elusive resulting in a much larger sinuosity of the crestline than normal ripples in this study we investigate the megaripples transverse instability by using field measurements wind tunnel experiments and numerical simulations of a three dimensional ripple model we show that a coarse grains accumulate at megaripple crests with a corresponding reduction of the lateral sand transport along the crest and b the transverse instability of megaripples is generated by a positive feedback between the height of the crest and the accumulation of coarse grains with more grains accumulating on the higher portions of the crest the outcomes of this positive feedback are that the thickness of the coarse grain armoring layer along the crest is not uniform and that it correlates with the crest height in turn these height differences drive the transverse instability such that higher portions of the ripple migrate more slowly than the lower sections creating a wavy crestline an analysis of martian ripple images shows variations in the sinuosity index suggesting that this characteristic can be useful in distinguishing between normal ripples and megaripples on mars

alternate bars in a sandy gravel bed river generation migration and interactions with superimposed dunes a field study was carried out to investigate the development of alternate bars in a secondary channel of the loire river france as a function of discharge variations we combined frequent bathymetric surveys scour chains and stratigraphical analysis of deposits with measurements and modelling of flow dynamics the channel exhibited migrating bars non migrating bars and superimposed dunes possible mechanisms of bar initiation were found to be chutes associated with changes of bank direction and instability resulting from interactions between existing bars during the fall in water level after floods we propose that the reworking of bar sediments during low flows high width to depth ratio reinforced by high values of the shields mobility parameter can explain the formation or re generation of new alternate migrating bars during a subsequent flood the migration pattern of the bars was found to be cyclic and to depend mainly on i channel layout and ii the dynamics of superimposed dunes with heights and lengths depending on location and discharge value for instance the hysteresis affecting the steepness of dunes influences the flow resistance of the dunes as well as the celerity of migrating bars during flood events we compare the findings from the field with results from theoretical studies on alternate bars this gives insight in the phenomena occurring in the complex setting of real rivers but it also sheds light on the extent to which bar theories based on idealized cases can predict those phenomena copyright 2014 john wiley sons ltd

witseg sampler a segmented sand sampler for wind tunnel test the flux profile of blowing sand is the reflection of a large number of sand particles moving in different trajectories to describe the function of the flux profile requires measuring the flux of blown sand at different heights a segmented sand sampler for wind tunnel study witseg sampler has been designed and evaluated in a wind tunnel the sampler is 0 6 m high and sectioned into 60 openings that can measure the flux profile of blowing sand at 10 mm intervals special attention was paid in designing the sampler to its practicability and efficiency for the convenience of use the leading edge with inlets sand chambers and side cover are removable to maximize the sampling efficiency the leading edge is wedge shaped so that the width of the inlets is 5 mm while the width of the sand chamber is 15 mm each sand chamber has two screened vents that are connected to a vertical vent to minimize the air pressure inside the sand chamber and maximize its efficiency wind tunnel evaluation shows that the sampler is good for measuring aeolian sand flux both the measured flux profile and total flux show a reasonably good agreement with the widely accepted flux profile function and sand transport equation it has an overall efficiency of 0 91 the witseg sampler that can measure the detailed variation of blown sand flux with height proves to be a good tool for studying the flux profile of blowing sand

wind blown sand on beaches an evaluation of models five models for predicting rates of aeolian sand transport were evaluated using empirical data obtained from field experiments conducted in april 1994 at a beach on inch spit co kerry republic of ireland measurements were made of vertical wind profiles to derive shear velocity estimates beach slope and rates of sand transport sediment samples were taken to assess characteristics of grain size and surface moisture content estimates of threshold shear velocity were derived using grain size data after parsing the field data on the basis of the quality of shear velocity estimation and the occurrence of blowing sand 51 data sets describing rates of sand transport and environmental conditions were retained mean grain diameter was 0 17 mm surface slopes ranged from 0 02 on the foreshore to about 0 11 near the dune toe mean shear velocities ranged from 0 23 m s 1 just above the observed transport threshold to 0 65 m s 1 rates of transport ranged from 0 02 kg m 1 h 1 to more than 80 kg m 1 h 1 these data were used as input to the models of bagnold bagnold r a 1936 the movement of desert sand proc r soc london a157 594 620 kawamura kawamura r 1951 study of sand movement by wind translated 1965 as university of california hydraulics engineering laboratory report hel 2 8 berkeley zingg zingg a w 1953 wind tunnel studies of the movement of sedimentary material proc 5th hydraulics conf bull 34 iowa city inst of hydraulics pp 111 135 kadib kadib a a 1965 a function for sand movement by wind university of california hydraulics engineering laboratory report hel 2 8 berkeley and lettau and lettau lettau k and lettau h 1977 experimental and micrometeorological field studies of dune migration in k lettau and h lettau eds exploring the world s driest climate university of wisconsin madison ies report 101 pp 110 147 correction factors to adjust predictions of the rate of transport to account for the effects of slope and moisture content were calculated using the models of bagnold bagnold r a 1973 the nature of saltation and bed load transport in water proc r soc london ser a 332 473 504 and belly belly p y 1964 sand movement by wind u s army corps eng cerc tech mem 1 washington d c 38 pp respectively none of the models was able to produce a strong correspondence between measured and predicted rates of transport best results were obtained using the bagnold and zingg models and the kadib model was the least viable of this group the influence of sediment moisture content appeared to be the critical factor in degrading model viability overall none of the models is adequate for general applications to coastal aeolian environments where moisture content complications tend to override the predictive competence of the simple transport formulations c 1998 elsevier science b v