the sedimentary structure of linear sand dunes linear sand dunes dunes that extend parallel to each other rather than in star like or crescentic forms are the most abundant type of desert sand dune1 but because their development and their internal structure are poorly understood they are rarely recognized in the rock record2 models of linear dune development2 3 4 5 6 have not been able to take into account the sub surface structure of existing dunes but have relied instead either on the extrapolation of short term measurements of winds and sediment transport or on observations of near surface internal sedimentary structures from such studies it has not been clear if linear dunes can migrate laterally2 7 8 here we present images produced by ground penetrating radar showing the three dimensional sedimentary structure of a linear dune in the namib sand sea where some of the world s largest linear dunes are situated these profiles show clear evidence for lateral migration in a linear dune moreover the migration of a sinuous crest line along the dune produces divergent sets of cross stratification which can become stacked as the dune height increases and large linear dunes can support superimposed dunes that produce stacked sets of trough cross stratification these clear structural signatures of linear dunes should facilitate their recognition in geological records

coastal foredune topography as a control on secondary airflow regimes under offshore winds aeolian sediment transport under offshore winds is usually discounted from sediment budget considerations of coastal dunes results presented here indicate however that depending on wind orientation and foredune morphology positive contributions to the sediment budget can arise from offshore winds lee side airflow patterns seaward of the foredune crest under offshore winds were studied at three sites of varying foredune geometry along magilligan strand northern ireland ultrasonic anemometers recorded 1 hz wind velocity data at 1 intervals descriptive statistics and wind rose diagrams were used in the analysis of these data under perpendicularly offshore winds each site exhibited a different type of lee side airflow pattern a tall 11 4 m sharp crested foredune produced flow separation with a recirculation cell a lower 6 6 m more rounded foredune crest exhibited lee side flow that remained attached but was deflected parallel to the crest while flow over a low incipient foredune crest resulted in attached flow with no deflection in the lee the morphology of the lee slopes best explained these airflow adjustments more obliquely offshore winds were also found to affect the secondary airflow patterns with a smooth transition from flow separation to flow attachment as the incident angle decreased at higher wind velocities this smooth transition was replaced with a more abrupt switch between the secondary airflow types this threshold was located at similar approach angles to those recorded for desert dunes 90 20 these findings have significant implications for the post storm recovery and long term evolution of beach dune systems where the predominant winds are offshore and offer an explanation for the presence of many beach dune systems in lee coast locations copyright 2010 john wiley sons ltd

complex spatial feedbacks of tephra redistribution ice melt and surface roughness modulate ablation on tephra covered glaciers tephra fallout from the 2011 grimsvotn eruption onto svinafellsjokull iceland created an ice ash landscape of a type that is rarely studied but is nevertheless common in glaciovolcanic regions we used terrestrial laser scanning tls to measure ice surface topography and absorption at high spatial resolution confirming ablation rates either reduce or increase under thick insulating and thin reduced albedo ash deposits respectively fourier transform analysis of the tls data identified that a three fold increase in aerodynamic roughness was attributable to an increase in larger 0 2m surface features moreover tls measurements revealed the importance of ash redistribution by meltwater in generating differential melting which modifies roughness and ash patchiness such that the net effect of these spatial ash ice feedbacks was to reduce ablation rates by up to 59 the modulating effects of these previously undocumented feedbacks on ablation rates are therefore significant and must be correctly parameterized if ashcovered glacier mass balances are to be predicted correctly

dynamic processes acting on a longitudinal seif sand dune abstract field measurements were made on a longitudinal dune in the sinai desert in order to understand its morphology and dynamics the field measurements contradicted the wind structure indicated by the helicoidal flow theory rather it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank the essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line the occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line when this angle is 40 the velocity of the deflected wind drops and deposition takes place on the lee flank the angle of incidence in each wind storm is changed intermittently between 30 and 100 along the dune because the dune meanders and because of the sinuous outline of the crest line in this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line as a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month peaks and saddles along the crest line advance at an average rate of 0 7 m per month the lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition this can explain the formation of peaks along the crest line of the dune

classification of debris covered glaciers and rock glaciers in the andes of central chile in the dry andes of chile 17 to 35 s debris covered glaciers and rock glaciers are differentiated from true glaciers based on the percentage of surface debris cover thickness of surface debris and ice content internal ice is preserved by an insulating cover of thick debris which acts as a storage reservoir to release water during the summer and early fall these landforms are more numerous than glaciers in the central andes however the existing legislation only recognizes uncovered or semicovered glaciers as a water resource glaciers debris covered glaciers and rock glaciers are being altered or removed by mining operations to extract valuable minerals from the mountains in addition agricultural expansion and population growth in this region have placed additional demands on water resources in a warmer climate as glaciers recede and seasonal water availability becomes condensed over the course of a snowmelt season rock glaciers and debris covered glaciers contribute a larger component of base flow to rivers and streams as a result identifying and locating these features to implement sustainable regional planning for water resources is important the objective of this study is to develop a classification system to identify debris covered glaciers and rock glaciers based on the interpretation of satellite imagery and aerial photographs the classification system is linked to field observations and measurements of ice content debris covered glaciers have three subclasses surface coverage of semi class 1 and fully covered class 2 glaciers differentiates the first two forms whereas debris thickness is critical for class 3 when glaciers become buried with more than 3 m of surface debris based on field observations the amount of ice decreases from more than 85 to 65 85 to 45 65 for semi fully and buried debris covered glaciers respectively rock glaciers are characterized by three stages class 4 rock glaciers have pronounced transverse ridges and furrows that arch across the surface which indicates flow produced via ice class 5 rock glaciers have ridges and furrows that appear linear in the direction of flow indicating reduced flow from limited internal ice and class 6 rock glaciers have subdued surface topography because the movement of the rock glacier has ceased ice content decreases from 25 45 to 10 25 to 10 from class 4 to 6 respectively examples from digital imagery aerial photographs and field photographs are provided for each class the classification scheme can be used to identify and map debris covered glaciers and rock glaciers to create an inventory this will help improve recognition of these landforms as an important water resource in the dry andes of chile which will aid in sustainable planning and development in basins that hold the majority of the population and support a large share of the economic activity in chile

relations between dune morphology air flow and sediment flux on reversing dunes silver peak nevada relations between wind speed sediment flux and dune morphology were measured for two reversing dunes situated in the south western part of the silver peak dunefield in clayton valley west central nevada the larger dune was 120 m in length with a height of 12ae5 m and the smaller dune 80 m long and 6 m high both dunes were sharp crested aligned approximately e w perpendicular to the dominant wind direction and had slightly concave stoss profiles twenty seven rotating cup anemometers were placed 0ae3 m elevation along n s transects on each of the dunes at each anemometer site a passive wedge shaped sediment trap was used to measure sediment flux amplification of wind speed was observed towards the crest on the stoss side of both dunes with speed up factors ucrest ubase ranging from 1ae50 3ae19 with a corresponding increase in sediment flux by 1 2 orders of magnitude in general the ratio of crest flux to base flux qc qb increased with increasing incident basal wind speed on both dunes direct measurements of the stoss slope variation in sediment flux relative to the dune crest are in good agreement with owen s transport model friction speed u was approximated from near surface 0ae3 m point wind speed although not all assumptions of the owen model are upheld the modified model performance is sufficiently robust to predict short term variation in stoss sediment flux on the study dunes improved models that adequately account for variation in sediment flux under changing air flow and transport conditions are necessary for the prediction of longterm evolution of dunes in this regard further progress in model development will require increased understanding of the spatial and temporal variability of airflow and the short term response of sediment flux to these flow conditions

parabolic dune development modes according to shape at the southern fringes of the hobq desert inner mongolia china since the 1970s parabolic dunes at the southern fringe of the hobq desert inner mongolia china have exhibited many different shapes v shaped u shaped and palmate each with a unique mode of development in the study area parabolic dunes are mainly distributed in regions a b and c with an intermittent river running from the south to the north we used high resolution remote sensing images from 1970 to 2014 and rtk gps measurements to study the development modes of different dune shapes the modes are characterized by the relationship between the intermittent river and dunes formation of the incipient dune patterns the predominant source supply of dunes and the primary formation of different shapes v shaped u shaped and palmate most parabolic dunes in region a are v shaped and closer to the bank of the river the original barchans in this region exhibit disconnected arms behavior with the sand blown out of the riverbed through gullies the nebkhas on the disconnected arms acquire the external sand source through the fertile island effect thereby developing into triangular sand patches and further developing into v shaped parabolic dunes most parabolic dunes in regions b and c are palmate the residual dunes cut by the re channelization of river from transverse dune fields on the west bank are the main sand source of region b the parabolic dunes in region c are the original barchans having then been transformed the stoss slopes of v shaped parabolic dunes along the riverbank are gradual and the dunes are flat in shape the dune crest of v shaped parabolic dune is the deposition area which forms the arc shaped sand ridge their two arms are non parallel the lateral airflow of the arms jointly transport sand to the middle part of dunes resulting in a narrower triangle that gradually becomes v shaped palmate parabolic dunes have a steeper stoss slope and height the dune crest of the palmate parabolic dune is the erosion area which forms a long and narrow trough between nebkhas by the funnelling effect this process forces sand towards lee slopes which transform from concave original barchans into convex ultimately resulting in the formation of palmate parabolic dunes

estimating aerodynamic roughness over complex surface terrain 1 surface roughness plays a key role in determining aerodynamic roughness length zo and shear velocity both of which are fundamental for determining wind erosion threshold and potential while zo can be quantified from wind measurements large proportions of wind erosion prone surfaces remain too remote for this to be a viable approach alternative approaches therefore seek to relate zo to morphological roughness metrics however dust emitting landscapes typically consist of complex small scale surface roughness patterns and few metrics exist for these surfaces which can be used to predictzofor modeling wind erosion potential in this study terrestrial laser scanning was used to characterize the roughness of typical dust emitting surfaces playa and sandar where element protrusion heights ranged from 1 to 199mm over which vertical wind velocity profiles were collected to enable estimation of zo our data suggest that although a reasonable relationship r 2 0 79 is apparent between 3 d roughness density and zo the spacing of morphological elements is far less powerful in explaining variations in zo than metrics based on surface roughness height r 2 0 92 this finding is in juxtaposition to wind erosion models that assume the spacing of larger scale isolated roughness elements is most important in determining zo rather our data show that any metric based on element protrusion height has a higher likelihood of successfully predicting zo this finding has important implications for the development of wind erosion and dust emission models that seek to predict the efficiency of aeolian processes in remote terrestrial and planetary environments

a decade of surface change on a namib linear dune survey data from return visits to a linear dune in the namib desert provide information about change in dune form over a decade the data demonstrate that although change on these large features is slow there is none the less considerable movement over this time scale the dunes are therefore confirmed to be currently active and not pleistocene relics

surface roughness and bulk heat transfer on a glacier comparison with eddy correlation more than 100 simultaneously measured profiles of wind speed and temperature were analyzed to determine the aerodynamic and thermal roughness lengths of glacier ice the results proved to be extremely sensitive to measurement height which is difficult to define precisely on the hummocky terrain of a glacier therefore aerodynamic roughness lengths were estimated from the microtopography of the surface yielding 2 46 mm for ice and 5 5 mm for snow despite the indeterminate nature of the profile analysis it did support the view that the thermal roughness length must decrease as the roughness reynolds number increases a relationship which has been modelled in the literature good agreement between bulk transfer estimates and eddy correlation measurements of fhe sensible heat flux density on melting ice and snow was obtained by combining the aerodynamic roughness length with a suitable model for thermal roughness length

the effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer the effect of turbulent flow structures on saltation sand transport was studied during two convective storms in niger west africa continuous synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 hz the shear stress production was determined from the vertical and streamwise velocity fluctuations the greatest stress bearing events were classified as turbulent structures with sweep ejection inward interaction and outward interaction described according to the quadrant technique the classified turbulent structures accounted for 63 5 per cent of the average shear stress during the first storm and 56 0 per cent during the second storm the percentage of active time was only 20 6 per cent and 15 8 per cent respectively high saltation fluxes were associated with sweeps and outward interactions these two structures contribute positively sweeps and negatively outward interactions to the shear stress but have in common that the streamwise velocity component is higher than average therefore the horizontal drag force seems primarily responsible for saltation sand transport and not the shear stress this was also reflected by the low correlation coefficients r between shear stress and saltation flux 0 12 and 0 14 respectively while the correlation coefficients between the streamwise velocity component and saltation flux were much higher 0 65 and 0 57 respectively

measurement and parameterization of aerodynamic roughness length variations at haut glacier d arolla switzerland spatial and temporal variations in aerodynamic roughness length z0 on haut glacier d arolla switzerland during the 1993 and 1994 ablation seasons are described based on measure ments of surface microtopography the validity of the microtopographic z0 measurements is established through comparison with independent vertical wind profile z0 measurements over melting snow slush and ice the z0 variations are explained through correlation and regression analyses using independent measurements of meteorological and surface variables and parameterizations are developed to calculate z0 variations for use in surface energy balance melt models several independent variables successfully explain snow z0 variation through their correlation with increasing surface roughness caused by ablation hollow formation during snow melt non linear parameterizations based on either accumulated melt or accumulated daily maximum temperatures since the most recent snowfall explain over 80 of snow z0 variation the z0 following a fresh snowfall on an ice surface is parameterized based on relationships with the underlying ice z0 snow depth and accumulated daily maximum temperatures none of the independent variables were able to successfully explain ice z0 variation although further comparative studies are needed the results lend strong support to the microtopo graphic technique of measuring z0 over melting glacier surfaces

geomorphology of sand dunes in the northeast taklimakan desert three types of sand dunes exist in the taklimakan desert namely compound complex crescent dunes and crescent chains compound dome dunes and compound complex linear dunes besides these three compound complex types single simple dunes are also distributed throughout the sand sea the compound complex linear dunes are developed under acute bimodal wind regimes though the ratios of the resultant drift potential rdp and the drift potential dp are the same as that near the border and adjacent area of the sand sea the compound complex crescent and dome dunes are developed respectively because of divergence of the sand available the stress of the sand moving winds and the time scales of dune formation the sand supply for the dunes is not from lopo nor in the east as previous studies suggested but mainly from local alluvial or lacustrine deposits the grain size component does not correlate evidently to the morphology parameters of the sand dunes analyses of the dp and drift direction suggest that the northeast taklimakan is an area of low wind energy and the resultant drift direction rdd coincides well with the distribution morphology and scales of the dunes

the effect of wind speed and bed slope on sand transport this paper reports on a wind tunnel study of the effects of bed slope and wind speed on aeolian mass transport the use of a sloping wind tunnel has enabled estimation of the friction angle to be about 40 for saltating particles in the range 170 540 m a formula relating dimensionless mass transport to friction speed and bed slope is proposed and mass transport data for five uniform sand samples and one non uniform sand sample are shown to fit the equation well in particular the relationship reveals an overshoot in mass transport slightly above threshold collisions a feature also evident when previous experimental data is re examined as the number of mid air collisions between the saltating particles increases greatly with wind speed the overshoot may occur as a result of increasing energy losses resulting from the collisions finally it is demonstrated that data for saltating snow shows a similar overshoot in the dimensionless transport rate

why deposits of longitudinal dunes are rarely recognized in the geologic record dunes that are morphologically of linear type many of which are probably of longitudinal type in a morphodynamic sense are common in modern deserts but their deposits are rarely identified in aeolian sandstones one reason for non recognition of such dunes is that they can migrate laterally when they are not exactly parallel to the long term sand transport direction thereby depositing cross strata that have unimodal cross bed dip directions and consequently resemble deposits of transverse dunes dune parallel components of sand transport can be recognized in ancient aeolian sands by examining compound crossbedding formed by small dunes that migrated across the lee slopes of large dunes and documenting that the small dunes migrated with a component in a preferred along crest direction over the large dunes

alpine turf exfoliation pans in lesotho southern africa climate process morphological linkages abstract although alpine pans have been reported from a variety of mountain regions these have received limited research attention and are thus amongst the least known of closed basins this study investigates macro and fine scale morphological attributes and the process dynamics of alpine turf exfoliation pans in the high drakensberg of lesotho southern africa climate data temperature precipitation and wind are used to better ascertain climate surface process linkages and how these may be associated with the observed morphological phenomena thirty pans were assessed for both macro and fine scale aspect controlled morphological attributes during 2005 tinytag temperature loggers recorded ground temperatures on various pan riser aspects to establish the potential for cryogenic activity similarly wind speed and direction data from 2001 were used to identify the potential role of wind as an erosion and transportation mechanism the number and total weight of detached turf clumps accumulated at the base of various pan riser aspects were determined in july 1999 september 2001 and september 2004 it is suggested that the pans originated through initial turf disruption by animal trampling and turf burning followed by the cryogenic up heave of sediments the pans typically elongate towards the southeast as they enlarge demonstrate most active erosion on eastern and southeastern riser aspects and host seasonal micro echo and micro climbing dunes along such riser aspects it is concluded that an annual cyclic seasonal pattern of contemporary climate driven geomorphic processes dominated by strong northwesterly winds control the fine scale morphological evolution of alpine turf exfoliation pans in the lesotho highlands

measuring glacier surface roughness using plot scale close range digital photogrammetry glacier roughness at sub metre scales is an important control on the ice surface energy balance and has implications for scattering energy measured by remote sensing instruments ice surface roughness is dynamic as a consequence of spatial and temporal variation in ablation to date studies relying on singular and or spatially discrete two dimensional profiles to describe ice surface roughness have failed to resolve common patterns or causes of variation in glacier surface morphology here we demonstrate the potential of close range digital photogrammetry as a rapid and cost effective method to retrieve three dimensional data detailing plot scale supraglacial topography the photogrammetric approach here employed a calibrated consumer grade 5 mpix digital camera repeatedly imaging a plot scale 25 m2 ice surface area on midtre lovenbreen svalbard from stereo pair images digital surface models dsms with sub centimetre horizontal resolution and 3 mm vertical precision were achieved at plot scales 4 m2 extraction of roughness metrics including estimates of aerodynamic roughness length z 0 was readily achievable and temporal variations in the glacier surface topography were captured close range photogrammetry with appropriate camera calibration and image acquisition geometry is shown to be a robust method to record sub centimetre variations in ablating ice topography while the dsm plot area may be limited through use of stereo pair images and issues of obliquity emerging photogrammetric packages are likely to overcome such limitations

highly variable aerodynamic roughness length z0 for a hummocky debris covered glacier the aerodynamic roughness length z0 is an essential parameter in surface energy balance studies but few literature values exist for debris covered glaciers we use microtopographic and aerodynamic methods to assess the spatial variability of z0 for lirung glacier nepal we apply structure from motion to produce digital elevation models for three nested domains five 1 m2 plots a 21 300 m2 surface depression and lower 550 000 m2 of the debris mantled tongue wind and temperature sensor towers were installed in the vicinity of the plots within the surface depression in october 2014 we calculate z0 according to a variety of transect based microtopographic parameterisations for each plot then develop a grid version of the algorithms by aggregating data from all transects this grid approach is applied to the surface depression dem to characterize z0 spatial variability the algorithms reproduce the same variability among transects and plots but z0 estimates vary by an order of magnitude between algorithms across the study depression results from different algorithms are strongly correlated using monin obukov similarity theory we derive z0 values from the meteorological data using different stability criteria we derive median values of z0 between 0 03 m and 0 05 m but with considerable uncertainty due to the glacier s complex topography considering estimates from these algorithms results suggest that z0 varies across lirung glacier between 0 005 m gravels to 0 5 m boulders future efforts should assess the importance of such variable z0 values in a distributed energy balance model

monitoring surface change on a namib linear dune in tackling the apparently intractable problem of linear dune initiation and maintenance there has been a move away from large scale deductive models to smaller scale field studies of individual dunes this paper reports a study of surface change on a large complex linear dune in the namib desert southern africa the dune surface responds to a markedly seasonal wind regime in summer westerly winds erode sand from the west flank of the dune and deposit it on the easterly lee side of the dune crest in winter this pattern is reversed easterly winds erode sand from the east slope and deposit it on the west slope the crest therefore moves back and forth some 15 m each year returning at the end of a year s cycle to its position at the beginning the position of the base of the dune appears to remain fixed even though sand is moving throughout the dune system the dune does extend northward along some resultant of the westerly and easterly winds despite relatively high levels of activity especially at the dune crest there is no evidence of the breakdown of the linear dune form the conclusion must therefore be that linear dunes can be maintained in bimodal wind regimes and are not necessarily related to unidirectional parallel regimes as others have suggested

a physically based method for estimating supraglacial debris thickness from thermal band remote sensing data in order to account for the effects of debris cover in model scenarios of the response of glaciers to climate change and water resource planning it is important to know the distribution and thickness of supraglacial debris and to monitor its change over time previous attempts to map surface debris thickness using thermal band remote sensing have relied upon time specific empirical relationships between surface temperature and thickness limiting their general applicability in this paper we develop a physically based model that utilizes advanced spaceborne thermal emission and reflection radiometer aster thermal band remotely sensed imagery and is based on a solution of the energy balance at the debris surface the model is used to estimate debris thickness on miage glacier italy and is validated using field debris thickness measurements and a previously published debris thickness map the temporal transferability of the model is demonstrated through successful application to a separate aster image from a different year using reanalysis meteorological input data this model has the potential to be used for regional scale supraglacial debris thickness mapping and monitoring for debris up to at least 0 50 m thickness but improved understanding of the spatial patterns of air temperature aerodynamic roughness length and thermal properties across debriscovered glaciers is needed

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

high resolution mapping of supra glacial drainage pathways reveals link between micro channel drainage density surface roughness and surface reflectance this paper reports on the use of a small unmanned aerial vehicle suav carrying a standard compact camera to construct a high resolution orthomosaic om and digital elevation model dem over the lower reaches of the glacier midtre lovenbreen svalbard structure from motion sfm techniques were used to build the om and dem and together these reveal insights into the nature of supra glacial drainage major meandering supra glacial drainage pathways show clear dynamism via meander cutoffs and abandoned channels in addition the imagery reveals a very extensive network of smaller channels that may well carry substantial amounts of water this network of channels is in part controlled by the structure of the glacier but in turn these channels have a significant impact on the ice surface roughness of the ice surface is higher where channels are most extensive in addition we find a relationship between channel density and surface reflectance such that greater channel density is associated with lower reflectance values given the role of surface reflectance and roughness in the energy balance of glaciers it is therefore apparent that extensive networks of small supra glacial channels across such glaciers have the potential to have an important impact on energy exchanges between the atmosphere and the ice surface copyright 2015 john wiley sons ltd

secondary airflow and sediment transport in the lee of a reversing dune lee side windspeed and sediment transport were measured over a small 1 2 m transverse ridge in the silver peak dunefield west central nevada usa using an intensive array of 25 cup anemometers and seven total flux traps during crest transverse and transporting flow conditions u0 3crest 8 4 m s 1 windspeed near the surface of the lee slope averaged half 48 per cent that of crest speeds dimensionless speeds in the separation zone ranged from 0 2 to 0 8 that of the outer flow u12 along the boundary of the separation cell windspeed increased by 10 per cent of the crest speed before separation equilibrium of upper and lower wake regions was not observed by the documented eight dune heights suggesting that wake recovery may not occur over closely spaced dunes sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs this suggests that a significant amount c 70 95 per cent of sediment transported over the crest moved as fallout for this data set flux was approximately proportional to the cube of the near surface windspeed u0 3 and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection beyond the interdune transport direction progressed from oblique to crest transverse this indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest transverse flows regarding the grain size and sorting properties of transported sediment there was no significant difference in mean grain size over the dune although in general particles were finer and more poorly sorted in the lee copyright 1999 john wiley sons ltd

wind direction and complex sediment transport response across a beach dune system evidence from a field study on wind flow and sediment transport across a beach dune system under onshore and offshore conditions including oblique approach angles indicates that sediment transport response on the back beach and stoss slope of the foredune can be exceedingly complex the upper air flow measured by a sonic anemometer at the top of a 3 5 m tower located on the dune crest is similar to regional wind records obtained from a nearby meteorological station but quite different from the near surface flow field measured locally across the beach dune profile by sonic anemometers positioned 20 cm above the sand surface flow form interaction at macro and micro scales leads to strong modulation of the near surface wind vectors including wind speed reductions due to surface roughness drag and adverse pressure effects induced by the dune and wind speed increases due to flow compression toward the top of the dune as well as pronounced topographic steering during oblique wind approach angles a conceptual model is proposed building on the ideas of sweet and kocurek sedimentology 37 1023 1038 1990 walker and nickling earth surface processes and landforms 28 111 1124 2002 and lynch et al earth surface processes and landforms 33 991 1005 2008 geomorphology 105 139 146 2010 which shows how near surface wind vectors are altered for four regional wind conditions a onshore detached b onshore oblique attached and deflected c offshore detached and d offshore oblique attached and deflected high frequency measurements of sediment transport intensity during these different events demonstrate that predictions of sediment flux using standard equations driven by regional wind statistics would by unreliable and misleading it is recommended that field studies routinely implement experimental designs that treat the near surface wind field as comprising true vector quantities with speed and direction in order that a more robust linkage between the regional upper air wind field and the sediment transport response across the beach dune profile be established copyright 2012 john wiley sons ltd

the surface energy balance of an active ice covered volcano villarrica volcano southern chile the energy balance of bare snow and tephra covered ice near the glacier equilibrium line elevation on villarrica volcano southern chile was investigated during 2004 and 2005 combining meteorological surface temperature and ablation measurements with energy balance modelling a tephra thermal conductivity of 0 35 w m 1 k 1 and a critical tephra thickness of 0 1 m tephra windblown tephra impurities lowered snow albedo but increased snowmelt by only an estimated 0 28 m w e over the same period the net mass balance impact of supraglacial tephra at villarrica volcano is therefore positive as thick ash and lapilli mantle most of the glacier ablation zones probably reducing annual ablation by several metres w e in the accumulation seasons frequent melting events were recorded with modelled daily snowmelt rates of up to 50 mm w e

including debris cover effects in a distributed model of glacier ablation 1 distributed glacier melt models generally assume that the glacier surface consists of bare exposed ice and snow in reality many glaciers are wholly or partially covered in layers of debris that tend to suppress ablation rates in this paper an existing physically based point model for the ablation of debris covered ice is incorporated in a distributed melt model and applied to haut glacier d arolla switzerland which has three large patches of debris cover on its surface the model is based on a 10 m resolution digital elevation model dem of the area each glacier pixel in the dem is defined as either bare or debris covered ice and may be covered in snow that must be melted off before ice ablation is assumed to occur each debris covered pixel is assigned a debris thickness value using probability distributions based on over 1000 manual thickness measurements locally observed meteorological data are used to run energy balance calculations in every pixel using an approach suitable for snow bare ice or debris covered ice as appropriate the use of the debris model significantly reduces the total ablation in the debris covered areas however the precise reduction is sensitive to the temperature extrapolation used in the model distribution because air near the debris surface tends to be slightly warmer than over bare ice overall results suggest that the debris patches which cover 10 of the glacierized area reduce total runoff from the glacierized part of the basin by up to 7

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

large scale controls on the development of sand seas in northern china abstract although sand seas are located in arid regions given that the proportions of dune areas vary greatly from region to region they are clearly influenced by other factors as well as aridity this paper discusses the distribution pattern and major features of sand seas and large fields of stabilized dunes in northern china and demonstrates the main influences on their development it appears that the availability of onsite loose sediments not wind strength is critical to the occurrence of the sand seas on a regional scale tectonic endorheic basins or areas of alluvial fans or forelands of mountain ranges are advantageous to the accumulation of fluvial sediments and thus are ideal locations of large sand seas in an arid climate the large dune fields currently stabilized by vegetation were active sand seas during the drier periods of the late pleistocene and early holocene the distribution patterns of chinese sand seas suggest that the height or size of the dunes in the sand seas are determined more by the availability of sands than any other potential factors as the sand seas in china account for a large portion of deserts in the middle latitudes of the northern hemisphere taklamakan is the second largest sand sea on earth knowledge of these sand seas past changes is important for understanding climatic changes in the terrestrial regions of mid latitudes

the effect of surface slope on saltation threshold a new wind tunnel has been constructed to study the mass transport properties of wind blown sand this report on the first set of experiments in the new wind tunnel concerns the effect of slope on threshold friction speed the results of this series of static threshold experiments and one dynamic threshold experiment for a range of particle diameters and bed slope angles show that provided the effects of reynolds number variation and interparticle cohesive force are accounted for the static friction angle is independent of slope and close in value to the measured static angle of response

geomorphology of desert sand dunes a review of recent progress through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes as developments with equipment allowed field measurements became more sophisticated however by the mid 1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported most importantly the attempt to measure the stresses imposed by the wind on the sand surface proved impossible and the use of shear or friction velocity as a surrogate for shear stress also failed to deliver at the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux in a development paralleled in fluvial geomorphology aeolian geomorphologists have attempted to measure and model turbulent structures over dunes progress has recently been made through the use of more complex numerical models based on computational fluid dynamics cfd some of the modelling work has also suggested that notions of dune equilibrium form may not be particularly helpful this range of recent developments has not meant that field studies are now redundant for linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground penetrating radar images of the internal structure of the dunes the paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form at least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies more recently attention has shifted although not completely to modelling approaches and very considerable progress has been made in developing models of dune development it is clear however that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology

synchrony of southern hemisphere late pleistocene arid episodes a review of luminescence chronologies from arid aeolian landscapes south of the equator abstract luminescence dating chronologies that have been reported in various studies of late pleistocene arid land aeolian sequences from australia southern africa and south america are reviewed with the objective of determining whether any pan hemispheric synchroneity existed in the timing of landscape responses to late quaternary climate changes south of the equator the results broadly show that contemporaneous regional arid land aeolian activity occurred in australia and southern africa during the periods ca 65 41 and 36 9 ka the relatively limited luminescence data from the south american subcontinent also point to recurrent arid land aeolian deposition between 63 and 8 ka with increasing frequency in the period ca 32 8 ka records from all three continents suggest a period of reduced aeolian activity between 41 and 36 ka overall the data are indicative of extensive landscape instability of the low to mid latitudes in the southern hemisphere during the last glacial period particularly at the last glacial maximum which accords with mainstream opinions aeolian activity during the holocene has generally been localised the rigour with which comparisons can be made between the different studies however is constrained by variations in practices between laboratories and changes that have occurred to luminescence dating procedures over the years

aerodynamic roughness of gravel surfaces abstract the interactions between surface winds and gravel surfaces that can be characterized by aerodynamic roughness length have important implications for sediment mobilization transport sedimentation and the development of desert pavements gobi deserts wind tunnel results of systematic tests carried out to determine the aerodynamic roughness length of gravel surfaces of different size and coverage at a range of free stream wind velocities are reported the aerodynamic roughness of gravel surfaces is a function of gravel size gravel coverage and free stream wind velocity the 1 30 law proposed by bagnold based on nikuradse s results of water pipe flow or taking the aerodynamic roughness length as a fixed percentage of roughness element height is not supported for gravel surfaces the aerodynamic roughness of gravel surfaces decreases with free stream wind velocity the variation of aerodynamic roughness with gravel coverage can be described by quadratic curves maximum aerodynamic roughness length occurs at gravel coverage range above 40 75 increasing with gravel size multivariate models based on free stream wind velocity and gravel coverage have been developed for predicting the aerodynamic roughness of different gravel sized surfaces the maturity of the roughness effects of gravel surfaces increases with gravel coverage in general the roughness effects are fully developed when the gravel coverage is over 15

temporal trends in grain size measures on a linear sand dune within aeolian dune systems spatial patterns of grain size variation have been recognized but little has been said about temporal changes increasingly it is becoming clear that linear dunes are associated with bi directional wind regimes which are often seasonal in the namib sand sea where linear dunes are aligned roughly north south winds blow from the west in summer and from the east in winter in response to this regime sand is eroded from the west slopes and deposited on the east slopes in summer and eroded from the east slopes and deposited on the west slopes in winter preliminary evidence from a study of a single namib linear dune reported here confirms that this seasonal aeolian regime induces seasonal responses in some grain size measurements due to the dynamics of sand transport on the dune the characteristics of the sand source immediately upwind of the sample point and the nature of the deposit thus time of sampling is crucial to the results obtained

structure from motion photogrammetry a low cost effective tool for geoscience applications abstract high resolution topographic surveying is traditionally associated with high capital and logistical costs so that data acquisition is often passed on to specialist third party organisations the high costs of data collection are for many applications in the earth sciences exacerbated by the remoteness and inaccessibility of many field sites rendering cheaper more portable surveying platforms i e terrestrial laser scanning or gps impractical this paper outlines a revolutionary low cost user friendly photogrammetric technique for obtaining high resolution datasets at a range of scales termed structure from motion sfm traditional softcopy photogrammetric methods require the 3 d location and pose of the camera s or the 3 d location of ground control points to be known to facilitate scene triangulation and reconstruction in contrast the sfm method solves the camera pose and scene geometry simultaneously and automatically using a highly redundant bundle adjustment based on matching features in multiple overlapping offset images a comprehensive introduction to the technique is presented followed by an outline of the methods used to create high resolution digital elevation models dems from extensive photosets obtained using a consumer grade digital camera as an initial appraisal of the technique an sfm derived dem is compared directly with a similar model obtained using terrestrial laser scanning this intercomparison reveals that decimetre scale vertical accuracy can be achieved using sfm even for sites with complex topography and a range of land covers example applications of sfm are presented for three contrasting landforms across a range of scales including an exposed rocky coastal cliff a breached moraine dam complex and a glacially sculpted bedrock ridge the sfm technique represents a major advancement in the field of photogrammetry for geoscience applications our results and experiences indicate sfm is an inexpensive effective and flexible approach to capturing complex topography

particle size and sorting characteristics of sand in transport on the stoss slope of a small reversing dune studies of the particle size and sorting characteristics of sand on the stoss slope of a 6 m high reversing dune show that the sand in transport is generally finer and better sorted than surface sand at the same position on the slope the sand in transport becomes coarser and more poorly sorted as wind speed and rates of mass transport increase toward the dune crest these patterns reflect changes in the competence of the wind which is capable of transporting larger grains and a wider range of grain sizes as its speed increases in space and time our field observations suggest that the particle size and sorting characteristics of surface sand are highly dependent on antecedent wind conditions and are not an invariant property of the dune as is widely assumed the wide range of particle sizes on the surface as well as its change through time also has important implications for modeling sediment transport on dunes transport thresholds may vary by as much as 30 on the stoss slope of the study dune

relations between morphology air flow sand flux and particle size on transverse dunes taklimakan sand sea china field experiments on two isolated transverse dunes with different scales in the taklimakan sand sea show that particle size and slope control the threshold shear velocity and sand flows on the stoss of the dunes the decreased threshold shear velocity and the unsaturated sand flows are the main reasons why sands are not deposited at the toe of the dunes the particle size controls sand flux through its influence on the aerodynamic roughness elements and threshold shear velocities the sand flux is different for dune surfaces with different particle size distributions under the control of the medium to low stress of the drifting sand winds the results of the field experiments and particle size analysis showed that sand transport had little influence on the particle size distributions the mean particle size being transported on the stoss is lower than that on the dune surface it is difficult to determine whether or not the wind is fully saturated with sands when sand flux calculation is mainly dependent on the shear velocity and the mean particle size the sands within some range of the particle size are transported and developed into simple transverse dunes and then the interdune surface is coarsened and alternated with the draas in the taklimakan sand sea copyright 2002 john wiley sons ltd

the effect of unsteady winds on sediment transport on the stoss slope of a transverse dune silver peak nv usa rapid 10 s measurements of sediment transport and wind speed on the stoss slope of a transverse dune indicate that the majority of sand transported is associated with fluctuations in wind speed with a periodicity of 5 20 min duration increases in the sediment transport rate towards the dune crest are associated with a small degree of flow acceleration the increase in wind speed is sufficient however to greatly increase values of the intermittency index so that the duration of saltation is extended in crestal regions of the dune the pattern of sediment transport on the stoss slope and therefore the locus of areas of erosion and deposition change with the regional wind speed erosion of the crest occurs during wind speed events just above transport threshold whereas periods of higher magnitude winds result in deposition of sand upwind of the crest thereby increasing dune height although short term temporal and spatial relations between sand transport and wind speed on the stoss slope are well understood it is not clear how these relations affect dune morphology over longer periods of time

dynamic processes of a simple linear dune a study in the taklimakan sand sea china samples of dune sands surveys of the morphology and field measurements of wind velocity and direction of a simple linear dune in taklimakan sand sea show that the airflow and sand flux vary with the change of wind direction on the dune surface decrease of the airflow stress on the lee flank does not result in much decrease of the sand flux because of the low threshold shear velocities and the airflow conditions there are no significant relations between the sand flux on the lee flank and the angle of incidence of the airflow the low threshold shear velocities and the maintenance of the sand flux at the lee flank are the main mechanisms keeping the linear shape of the dunes measurements of the sand flux shows that it reaches a maximum on the crest of the dune the grain size of the transported sands has some differences compared to that of the dune surface the sands transported are finer than that on the dune surface but better sorted under the influence of the medium to low wind activity the field experiment results exhibit that it is possible for the dunes to be shaped as linear dunes during the processes of accumulation and elongation

scale dependent roughness of a glacier surface implications for radar backscatter and aerodynamic roughness modelling the roughness of the snow free surface of the glacier midre lovenbreen svalbard has been investigated on scales between 1 mm and 300 m it is shown that the roughness is reasonably well described by scale free fractal models for scales longer than a few metres and shorter than about 100 mm however there is a break in the behaviour between these scales which can be characterized by a definite scale length of 70 500 mm and a root mean square height variation between around 6 and 70 mm the aerodynamic roughness length contributed by these features is estimated to be 0 3 1 5 mm features on this scale are consistent with the observed microwave backscattering properties of the glacier

a distributed energy balance melt model of an alpine debris covered glacier distributed energy balance melt models have rarely been applied to glaciers with extensive supraglacial debris cover this paper describes the development of a distributed melt model and its application to the debris covered miage glacier western italian alps over two summer seasons sub debris melt rates are calculated using an existing debris energy balance model deb model and melt rates for clean ice snow and partially debris covered ice are calculated using standard energy balance equations simulated sub debris melt rates compare well to ablation stake observations melt rates are highest and most sensitive to air temperature on areas of dirty crevassed ice on the middle glacier here melt rates are highly spatially variable because the debris thickness and surface type varies markedly melt rates are lowest and least sensitive to air temperature beneath the thickest debris on the lower glacier debris delays and attenuates the melt signal compared to clean ice with peak melt occurring later in the day with increasing debris thickness the continuously debris covered zone consistently provides 30 of total melt throughout the ablation season with the proportion increasing during cold weather sensitivity experiments show that an increase in debris thickness of 0 035 m would offset 1 c of atmospheric warming

calculating ice melt beneath a debris layer using meteorological data generalized numerical models of sub debris ice ablation are preferable to empirical approaches for predicting runoff and glacier response to climate change as empirical methods are site specific and strongly dependent upon the conditions prevailing during the measurement period we present a modified surface energy balance model to calculate melt beneath a surface debris layer from daily mean meteorological variables despite numerous simplifications the model performs well and modelled melt rates give a good match to observed melt rates suggesting that this model can produce reliable estimates of ablation rate beneath debris layers several decimetres thick this is a useful improvement on previous models which are inappropriate for thick debris cover

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