The dynamics of star dunes: an example from the Gran Desierto, Mexico

Observations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the Gran Desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type. Such interactions lead to deposition of sand in the central parts of the dune, giving rise to its pyramidal shape, as well as to some extension of the linear arms. The major arms of the dune studied are oriented NE-SW, or transverse to summer SSE and winter NNW winds. An avalanche face up to 10 m high develops during the course of each season. Flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune, where it is deposited as wind ripples migrate into zones of locally reduced flow velocity. Reattachment of the separated flow occurs on the lower part of the N or S arms, parallel to the flow. Spring westerly winds move sand obliquely up the S and N arms of the dune and outwards on the E arm. Large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms. Formation of star dunes in the Gran Desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge. Convergent leeside secondary flows are developed, which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune. Examples of star dunes at different stages of their development can be documented.     

Variations in wind velocity and sand transport on the windward flanks of desert sand dunes

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

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 is higher than on the crest line or the windward flank and longitudinal sand transport occurs. When the angle is less acute (> 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.     

沙丘背风侧气流及其沉积类型与意义

在腾格里沙漠东南缘对现代沙丘表面气流、沉积过程的野外观测结果表明,由于区域气流、沙丘形态及其相互作用等的不同使沙丘背风坡气流发生变化,在此发现三种背风坡次生气流 :分离流、附体未偏向流和附体偏向流。前者以弱的反向流为特征多发生在横向气流条件下坡度较陡的背风坡;后二者具有相对高的风速,其中附体流多发生在坡度缓和的背风坡,其方向在横向气流条件下保持原来的方向,而在斜向气流作用下发生偏转且其强度为原始风入射角的余弦函数。根据背风坡气流方向及强度,作者阐述了不同区域气流环境中沙丘背风坡沉积过程、层理类型及特征,探讨了交错层产状与区域气流方向之间的关系.     

Wind regimes and aeolian geomorphology in the western and southwestern Tengger Desert,NW China

Wind is the primary control on the formation of aeolian geomorphology. In this study, we combined wind regime data from automated weather stations in the western and southwestern Tengger Desert of the Inner Mongolia region in China with remote‐sensing data to analyse the relationship between the wind energy environment and aeolian geomorphology. Tengger Desert is one of the main dust storm sources in northwestern China. Therefore, efforts aimed at controlling desertification and dust storm require a deeper understanding of the processes that govern the formation and subsequent evolution of dunes in this area. Wind speed was largest in the northwest (3.3 m/s in the Xiqu station) and smallest in the southeast (1.2 m/s in the Haizitan station). Potential sand transport was also largest in the northwest (195 in the Jiahe station) and smallest in the southeast (33 in the Tumen station). The sand‐driving wind (5.92 m/s) directions were from the NW and SE quadrant across the study area, at >76% of all sand‐driving wind, reaching 99% in the Tumen station. The sand‐driving wind in the NW quadrant reached >48%, and in the SE quadrant, >12% of all sand‐driving wind in all stations. In the study area, sand dunes included crescent, dune networks, transverse, and coppice dunes. Dune crest directions had similar trends from upwind to downwind, at 133° in the middle region, and 124° in the southwestern region. Mean dune spacing changed with dune patterns; the maximum spacing for crescent dunes was 147 m, for dune networks 118 m, and for transverse dunes it was 77 m. The mean crest length was 124 m (maximum) for crescent dunes in the northwest, 121 m for transverse dunes, and 84 m for dune networks. However, because of gullies in the southern region, the mean crest length was only 58 m (least) for the crescent dunes in that area. The defect density ranged from 0.007 to 0.014. The spatial differences in dune patterns reflected the evolution of the dune field, where older dunes had been formed upwind and younger downwind. Copyright © 2014 John Wiley & Sons, Ltd.     

Internal sedimentary structure of linear dunes modelled with a cellular automaton

Linear dunes are the most common type of dune found on Earth and exist on several extra-terrestrial bodies, but despite this abundance their internal stratigraphy has not been commonly agreed. A cellular automaton is deployed to simulate the development of linear dunes, starting from a flat bed, under bi-modal oblique wind regimes of varying degrees of asymmetry. The internal stratigraphy of the linear dunes is monitored by keeping track of (buried) erosion surfaces, avalanche deposits and vertical accumulation, as well as the age of last subaerial exposure of the sediments. The simulations show the initial pattern-coarsening of a network of small dunes into fewer larger longitudinal ridges via bedform interactions and Y-junction dynamics. Three newly recognized types of bedform interaction are identified that relate to initial Y-junction dynamics: longitudinal crest-splitting, which creates free dune tips that can interact with adjacent dunes, and laterally oscillating interactions that lead to ephemeral Y-junctions (normal or reverse). The results show that these three bedform interactions leave no persistent signatures in the stratigraphic record. However, a further three bedform interactions involving the superposition of one dune onto another – merging, cannibalizing and repulsion (known from transverse dune field dynamics) – do leave specific evidence in the internal stratigraphy of the remaining dune, a buried interaction surface at a specific inclination. The preservation potential of this interaction surface varies between the three types. After the initial pattern-coarsening phase, the linear dunes become larger and more independent and their crest orientation follows the net resultant transport direction. The stratigraphies of mature dunes under wind regimes of differing asymmetry show that under (nearly) symmetrical winds the dune accumulates mainly vertically, with strata dipping parallel to the flanks, while under more asymmetrical wind regimes the internal stratigraphy resembles that of transverse dunes.     

The structure and development of foredunes on a locally prograding coast: insights from ground-penetrating radar surveys, Norfolk, UK

The internal structure of coastal foredunes from three sites along the north Norfolk coast has been investigated using ground‐penetrating radar (GPR), which provides a unique insight into the internal structure of these dunes that cannot be achieved by any other non‐destructive or geophysical technique. Combining geomorphological and geophysical investigations into the structure and morphology of these coastal foredunes has enabled a more accurate determination of their development and evolution. The radar profiles show the internal structures, which include foreslope accretion, trough cut and fill, roll‐over and beach deposits. Foredune ridges contain large sets of low‐angle cross‐stratification from dune foreslope accretion with trough‐shaped structures from cut and fill on the crest and rearslope. Foreslope accretion indicates sand supply from the beach to the foreslope, while troughs on the dune crest and rearslope are attributed to reworking by offshore winds. Bounding surfaces between dunes are clearly resolved and reveal the relative chronology of dune emplacement. Radar sequence boundaries within dunes have been traced below the water‐table passing into beach erosion surfaces. These are believed to result from storm activity, which erodes the upper beach and dunes. In one example, at Brancaster, a dune scarp and erosion surface may be correlated with erosion in the 1950s, possibly the 1953 storm. Results suggest that dune ridge development is intimately linked to changes in the shoreline, with dune development associated with coastal progradation while dunes are eroded during storms and, where beaches are eroding, a stable coast provides more time for dune development, resulting in higher foredune ridges. A model for coastal dune evolution is presented, which illustrates stages of dune development in response to beach evolution and sand supply. In contrast to many other coastal dune fields where the prevailing wind is onshore, on the north Norfolk coast, the prevailing wind is directed along the coast and offshore, which reduces the landward migration of sand dunes.     

An empirical model of aeolian dune lee-face airflow

Airflow data, gathered over dunes ranging from 60-m tall complex-crescentic dunes to 2-m tall simplecrescentic dunes, were used to develop an empirical model of dune lee-face airflow for straight-crested dunes. The nature of lee-face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee-face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low-speed (0–3O% of crestal speed) back-eddy flow, typically occur leeward of steep-sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment. As each of the three types of lee-face flow is defined by a range of wind speeds and directions, the nature of lee-face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross-strata.     

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 ( γ ), 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.     

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, straight‐crested 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 1 ) 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 7 ) 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.     

The origin of bounding surfaces in ancient aeolian sandstones

Three orders of aeolian bounding surface are arranged in a hierarchy based on their extent and regularity. First order surfaces are the most extensive. They are flat-lying bedding planes cutting across all other aeolian structures and are attributed to the passage of the largest aeolian bedforms—draas—across an area. First order surfaces cut across second order surfaces, which are gentle to moderately dipping surfaces bounding sets of cross-strata. Second order surfaces are attributed to the passage of dunes across draas, or to longitudinal dunes migrating across the lower ice slopes of draas. Third order surfaces bound bundles of laminae within coscts of cross laminae and are due either to local fluctuations in wind direction and velocity or to changes in airflow patterns caused by configurational changes in dune patterns. All these bounding surfaces could be explained by wind variations and dune migration, but the rates of dune migration relative to probable sediment deposition rates are incompatible with this general explanation of the form and spacing of the bounding surfaces. The concept of climbing bedforms of different hierarchical order together with subsidence provides a better explanation. Analogous bounding surfaces in aqueous bedforms have already been attributed to climbing bedforms of differing hierarchical order.     

Proterozoic aeolian quartz arenites from the Hornby Bay Group, Northwest Territories, Canada: Implications for Precambrian aeolian processes

The Hornby Bay Group is a Middle Proterozoic 2.5 km-thick succession of terrestrial siliciclastics overlain by marine siliciclastics and carbonates. A sequence of conglomeratic and arenaceous rocks at the base of the group contains more than 500 m of mature hematitic quartz arenite interpreted to have been deposited by migrating aeolian bedforms. Bedforms and facies patterns of modern aeolian deposits provided a basis for recognizing two sequences of aeolian arenite. Both sequences interfinger with alluvial—wadi fan conglomerates and arenites deposited by braided streams. Depositional processes, facies patterns and paleotopographic position of the arenites are consistent with modern sand sea dynamics.Distal aeolian facies in both sequences are composed of trough crossbed megasets deposited by climbing, sinuous-crested, transverse dunes. Megasets comprise a gradational assemblage of tabular to wedge-planar cosets formed by deflation/reactivation of dune lee slopes and migration of smaller superposed aeolian bedforms (small dunes and wind ripples). Megasets in the proximal facies are thinner, display composite internal stratification and have a tabular-planar geometry which suggests that they were formed by smaller, straight-crested transverse dunes. Most stratification within the crossbeds is inferred to have formed by the downwind climbing of aeolian ripples across the lee slopes of dunes.Remarkably few Precambrian aeolian deposits have been reported previously. This seems anomalous, because most Precambrian fluvial sediments appear to have been deposited by low sinuosity (braided) streams, the emergent parts of which are prime areas for aeolian deflation. Frequent floods and rapid lateral migration of Precambrian humid climate fluvial systems probably restricted aeolianite deposition to arid paleoclimates. Thus the apparent anomaly may reflect non-recognition and/or non-preservation of aeolianites and/or variations in some aspect of sand sea formation and migration unique to the Precambrian. Reconstruction of the Hornby Bay Group aeolianites using recently developed criteria for their recognition suggests that the latter reason did not exert a strong influence.     

Sand movement patterns in the Western Desert of Egypt: an environmental concern

Wind action is the most dominant agent for erosion and deposition in the vast Western Desert of Egypt. Analysis of wind data from seven meteorological stations distributed along the Western Desert reveals that this desert is characterized by high-energy wind environments along the northern and southern edges and low-energy wind environments throughout the rest of the desert. Accordingly, sand drift potential follows the pattern of wind energy. Maximum sand drift potential was observed at the southern edge (571 vector units, which equals 40 m³/m width/year). Sand drift direction was observed towards the southeast except at the southern part of the desert where the trend of sand movement was towards southwest. The major dune type recognized on satellite images was the simple linear type. Linear dunes are generally associated with bimodal wind regime. Rates of sand drift potential and sand dune migration were greatest at East of Owinate region at the extreme southern part of the desert. Measurements of crescentic sand dune advance from two satellite images reveal a maximum advance rate of about 9 m/year at the southern part of the desert. Dune movement creates potential hazard to the infrastructures in this open desert.     

AIR AND SAND MOVEMENTS TO THE LEE OF DUNES1

The large and extensive transverse and barchane dunes of coastal South West Africa are strongly oriented under the influence of predominantly southerly winds. During periods of strong winds (40–50 miles/h) deposition occurs on the lee slope in three ways: (1) sand is blown over the crest of the dune and falls on the lee slope; (2) rapid deposition near the dune crest results in periodic slumps and slides down the lee slope; (3) eddy currents developed to the lee of the dune pick up sand from the surface downwind from the dune and transport it to the lee slope. The size and strength of the lee eddy is surprising. With winds in the 40–50 miles/h range frequent gusts lift fine sand from the downwind surface to a height of several feet. Less frequently sand is picked up from a low position on the lee slope and redeposited higher on the slope. The addition of material to the lee slope by the eddy is much less volumetrically than the contribution directly over the dune crest from the windward direction; however, with strong winds the removal and transportation of sand from the area downwind of the lee slope back to the lee slope appears to be important in the deflation of this surface. The width of the area influenced by the lee eddy during strong winds is about equal to the height of the dune. Observations in low dunes from 1 to 20 ft. high at Sapelo Island, Ga., U.S.A., confirm the presence of a well developed eddy to the lee of these dunes during strong and moderate winds (20–50 miles/h).     

Erg deposits in the Lower Jurassic Wingate Sandstone, northeastern Arizona: oblique dune sedimentation

The Lower Jurassic erg (aeolian sand sea) deposits of the Wingate Sandstone on the Colorado Plateau are beautifully exposed near Many Farms, Arizona. These 3-D outcrops allow a detailed study of structures and sequenses in the erg body. The erg sequence comprises chiefly oblique dune deposits. The dune facies are in most cases characterized by a well-developed tripartite upbuilding. Each dune coset contains unusually thick and intricate bottomsets, medial low-angle dipping toesets, and upper steeply dipping foresets. The foresets reveal significant across-crest transport of sand and dip within a narrow range of directions towards the ESE. The bottomset beds are composed of compound cross-bedding that documents strong along-crest transport towards the SSW, whereas the toeset beds reveal upslope, downslope, and along-crest transport of sand. The ancient dunes apparently formed in a directionally varying wind flow with prevailing winds (early summer) from the NW and periodic strong winds (late summer) from the SW. The dunes were oblique not only to seasonal transport directions, but also to the resultant annual transport direction and dune migration direction. This was caused by the interaction of the dune system with the primary winds which resulted in secondary airflow and significant along-crest transport of sand. The erg deposits at Many Farms are separated by a number of super bounding surfaces suggesting several episodes of erg formation and destruction. The initial erg system was dominated by transverse dunes, but overlying ergs only contained oblique dunes. All erg systems were bounded to the SW by wide regions of erg margin environments in which aeolian sand sheet, fluvial, and lacustrine facies were deposited. Even though fluvial deposits are absent from the main part of the sequence at the study area, the effects of this system are reflected within the erg deposits at Many Farms.     

Determination of sand dune characteristics through geomorphometry and wind data analysis in central Iran (Kashan Erg)

The combination of wind measurements and remotely sensed geomorphometry indices provides a valuable resource in the study of desert landforms, because arduous desert environments are difficult to access. In this research, we couple wind data and geomorphometry to separate and classify different sand dunes in Kashan Erg in central Iran. Additionally, the effect of sand-fixing projects on sand dune morphology was assessed using geomorphometry indices (roughness, curvature, surface area, dune spacing and dune height). Results showed that a Digital Elevation Model of the National Cartographic Center of Iran (NCC DEM) with 10-m resolution and accuracy of 54% could discriminate geomorphometry parameters better than the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data with 30-m resolution and Shuttle Radar Topography Mission (SRTM) data with 90-m resolution and 45.2 and 1.6% accuracy, respectively. Low classification of SRTM DEM was associated with too many non-value points found in the DEM. Accuracy assessment of comparison ground control points revealed that ASTER DEM (RMSE = 4.25) has higher accuracy than SRTM and NCC DEMs in this region. Study of curvature showed that transverse and linear sand dunes were formed in concave topography rather than convex. Reduced slopes in fixed sand dunes were established due to wind erosion control projects. Measurements of dune height and spacing show that there is significant correlation in compound dunes (R ² = 0.546), linear dunes (R ² = 0.228) and fixed dunes (R ² = 0.129). In general, the height of dunes in Kashan Erg increases from the margin of the field to the center of the field with a maximum height of 120 m in star dunes. Analysis of wind data showed that sand drift potential is in low-medium class in Kashan Erg. Linear sand dunes in Kashan Erg show that they are following a global trend in forming of these. Finally, established of geomorphometry method in dune classification will help researchers to identify priority of land management and performance assessment of sand dunes fixing projects in arid arduous environment.     

Deflection of sand movement on a sinuous longitudinal (seif) dune: use of fluorescent dye as tracer

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 < 40°, sand on the lee flank was transported parallel to the crest line; when the angle of incidence was nearly perpendicular to the crest, movement along the lee flank abated and deposition occurred. Where the dune was low, flat and blunt, as in a zibar dune, there was no boundary-layer separation and no deflection of sand movement on the lee flank. The deflected movement along the lee flank resulted in elongation of the longitudinal (seif) dune.     

Morphology, internal structure and mechanics of small longitudinal (seif) dunes in an aeolian horizon of the Proterozoic Dhandraul Quartzite, India

The excellently preserved metre-scale, linear bedforms in an aeolian horizon of the Proterozoic Dhandraul Quartzite, India, show oppositely dipping strata arranged in a zigzag pattern. The strata are dominantly of translatent type, deposited by along-crest migrating ripples preserved on the flanks of dunes. The bedforms thus may be interpreted in a morphodynamic sense as longitudinal (seif) dunes. In order to determine the regional palaeoflow pattern, the migration directions of ripples preserved at the top of sheet sandstones that are associated with the dune cross-strata and internally show subhorizontal translatent strata were measured. A directionally varying flow with a mean direction nearly parallel to the mean axial trend of the dunes is indicated. The kinematics of the dunes were thus largely the result of alternate operation of two oblique flow components, each of which was deflected at a dune crest into an along-crest flow on the downwind flank of the dune. The deflected flow formed along-crest migrating ripples, which in turn deposited climbing ripple strata. Alternate deposition on the two opposite flanks resulted in near vertical accretion of the dunes, as is indicated by the zigzag pattern of stratal arrangement.     

Sand transport model of barchan dune equilibrium

Erosion and deposition over a barchan dune near the Salton Sea, California, is modelled by book-keeping the quantity of sand in saltation following streamlines of transport. Field observations of near-surface wind velocity and direction plus supplemental measurements of the velocity distribution over a scale model of the dune are combined as input to Bagnold-type sand-transport formulae corrected for slope effects. A unidirectional wind is assumed. The resulting patterns of erosion and deposition compare closely with those observed in the field and those predicted by the assumption of equilibrium (downwind translation of the dune without change in size or geometry). Discrepancies between the simulated results and the observed or predicted erosional patterns appear to be largely due to natural fluctuation in the wind direction. Although the model includes a provision for a lag in response of the transport rate to downwind changes in applied shear stress, the best results are obtained when no delay is assumed. The shape of barchan dunes is a function of grain size, velocity, degree of saturation of the oncoming flow, and the variability in the direction of the oncoming wind. Smaller grain size or higher wind speed produce a steeper and more blunt stoss-side. Low saturation of the inter-dune sandflow produces open crescent-moon-shaped dunes, whereas high saturation produces a whaleback form with a small slip face. Dunes subject to winds of variable direction are blunter than those under unidirectional winds. The size of barchans could be proportional to natural atmospheric scales, to the age of the dune, or to the upwind roughness. The upwind roughness can be controlled by fixed elements or by the sand is saltation. In the latter case, dune scale may be proportional to wind velocity and inversely proportional to grain size. However, because the effective velocity for transport increases with grain size, dune scale may increase with grain size as observed by Wilson (1972).     

Interaction between aeolian, fluvial and playa environments in the Permian Upper Rotliegend Group, UK southern North Sea

The Permian Upper Rotliegend Group in offshore UK Quadrants 42, 43, 47 and 48 comprises a sequence of mixed aeolian/fluvial/playa deposits. These deposits are up to 300 m thick and contain a record of the interaction between desert fluvial systems and adjacent aeolian and playa environments. The relative dominance of water vs. wind transport and deposition in this stratigraphic package was a function of fluctuations in the discharge of ephemeral fluvial systems and changes in water table/playa level driven by a combination of climatic change and syndepositional tectonics. The Rotliegend sedimentary record is punctuated by numerous surfaces recording erosion by wind and water. The origin of these surfaces is mostly climatic, with periods of increased runoff resulting in fluvial incision, especially near active faults. During periods of reduced runoff, wind erosion of fluvial deposits occurred, with fluvially derived sand being reworked into expanding aeolian dune fields. Wind erosion also occurred as a rising water table isolated dunes from their sediment supply, resulting in deflation of dunes down to the water table. These surfaces formed in a basin that was subsiding. Thus, even in a background of overall increasing accommodation space, climatically driven falls in the water table allowed for periods of erosion. The occurrence of significant erosion, especially near syndepositional fault zones, resulted in a sedimentary record that shows pronounced lateral as well as vertical facies variations.