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).     

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.     

宁夏河东沙地沙丘冻融过程的时空差异

中国的沙漠和沙地部分或全部分布在季节冻土区, 研究沙丘的冻融过程是讨论季节冻结期间沙丘风蚀和形态演变规律的条件之一。以宁夏河东沙地流动沙丘和沙障固定沙丘为研究对象, 通过野外观测和室内控制实验, 分析了沙丘的冻融过程及其控制因素。结果显示： 沙丘的冻结期在11月中旬至3月上旬, 流动沙丘各地貌部位的冻结时长和冻结层厚度均存在较大差异（背风坡面>迎风坡面>丘顶）, 背风坡脚的冻深最大。在季节冻结期内沙丘表层始终不发生冻结, 未冻层厚度的阈值约为10 cm且具有保护冻结层的作用, 流动沙丘迎风坡中在未冻层风蚀后, 地表冻结层融化再被风蚀, 如此循环过程造成其冻结层厚度远小于沙障固定沙丘的冻结层厚度。流动沙丘丘顶和背风坡面的冻结层厚度分别受短时（32 h）和较长历时（15 d）平均气温的影响。野外观测和室内控制实验均证明水分含量低于1.6%的沙丘沙不发生冻结, 冻结层硬度随含水率的增加呈幂函数递增（P<0.001）, 随温度降低呈缓慢递增。     

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.     

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.     

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.     

库布齐沙漠南缘抛物线形沙丘表面粒度特征

对库布齐沙漠南缘抛物线形沙丘特征断面上下层（0~5cm、5~10cm）沉积物采样分析结果表明,沙丘粒径与分选参数及其分布随沙丘形态、发育程度和植被生长状况发生变化。抛物线形沙丘丘体迎风坡下凹背风坡上凸,丘顶始终处于侵蚀亚环境。在顺风向断面,平均粒径从迎风坡脚到丘顶变粗,从丘顶到背风坡脚又变细,且这种变化在高大沙丘上更为明显;分选性在迎风坡为中等和较好,丘顶较差,顺风向到背风坡脚逐渐由中等变为较好;粒径频率曲线在丘顶双峰正偏,除迎风坡脚单峰正偏外,其余部位均单峰近对称。在垂直于风向的两翼断面,平均粒径在成熟沙丘由翼顶向两侧坡脚趋于变细,而在欠成熟沙丘无明显的变化趋势。翼间平地沉积物受植被等影响,平均粒径偏细但分选性差,偏度为正偏和极正偏,峰度为尖锐和非常尖锐。受不同时期风况的影响,成熟抛物线形沙丘上下层粒度参数在沙丘断面的分布较欠成熟沙丘一致。     

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.     

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.     

沙漠公路风蚀破坏规律的数值模拟研究

通过Fluent软件对沙漠公路的风蚀破坏规律进行数值模拟研究。分析公路不同横断面时风沙流扰动、增速、减速、恢复的过程,确定路基(堑)高度、边坡坡率和路面宽度等公路断面设计参数对风沙流扰动的影响,提出沙漠公路路基(堑)风蚀破坏规律。结果表明：路面宽度的改变对路基沿程风速变化影响较小,各特征点处风速变化小于5%。路基(堑)高度和边坡坡率对风沙流扰动的影响较大。随路基(堑)高度增加,风沙流流场的扰动被增强,路基(堑)风蚀破坏越显著,当边坡坡率为1:1时,路基模型高度250 mm时的迎风路肩风速为模型高度60 mm时的 1.15倍,其背风坡脚风速降低45%;路堑模型高度250 mm时的背风堑顶风速为模型高度60 mm时的1.05倍,迎风堑脚处风速降低高达80%。建议沙漠公路宜采用中低路基(堑),不宜高填深挖。当路基(堑)高度一定,边坡坡率为1:1.75或1:2时,路基(堑)沿程风速变化平缓,沙漠路肩不易被吹蚀破坏;路堑中堑脚位置处不易出现堆积。其结论与室内风蚀风洞试验结果有很好的一致性     

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.     

巴丹吉林沙漠高大沙山粒度组成及其意义

选取巴丹吉林沙漠东南部全球高差最大(约420m)的诺尔图湖东大沙山和较高的苏木巴润吉林湖西大沙山(高差约400m),对其迎风坡沉积物进行系统采样分析,讨论了高大沙山迎风坡沉积物粒度成分特点、变化规律、原因及其活动性.结果表明,全球高差最大的诺尔图东沙山和苏木巴润吉林湖西沙山迎风坡沉积物具有双层结构,表层沉积物的粒度组成...     

沙漠公路风沙土路基风蚀破坏试验研究

以沙漠公路风沙土路基为研究对象,通过室内风蚀风洞试验研究路基的风蚀破坏规律,以及路基不同断面对风沙流运动的影响。以路基高度、路基边坡坡率和路基宽度作为路基断面主要设计参数,研究不同路基断面下风沙流扰动、增速、减速、恢复的过程,以及路基周围风速流场的变化特征,分析路基病害较未病害时路基周围流场的变化。试验结果表明：路基高度和边坡坡率对风沙流运动的影响较大。随路基高度增加,路基对风沙流流场扰动增强,迎风坡坡顶处吹蚀破坏和背风坡坡底处堆蚀破坏越显著,在确定的路基边坡坡率下,路基模型高度为250 mm较模型高度为60 mm时,迎风坡坡顶风速增加1.13倍,背风坡坡底风速减小2.53倍,建议沙漠公路路基高度宜小于2.5 m。进一步,在确定的路基高度下,比较不同的边坡坡率对路基沿程风速的影响,发现当路基边坡坡率为1:1.75时,路基沿程风速变化不明显,沙漠公路风沙土路基不宜被风蚀破坏。     

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).     

A Pliocene mega-tsunami deposit and associated features in the Ranquil Formation,southern Chile

An exceptionally large tsunami affected the coastline of southern Chile during the Pliocene. Its backflow eroded coarse beach and coastal dune sediments and redistributed them over the continental shelf and slope. Sandstone dykes and sills injected from the base of the resulting hyperconcentrated flow into underlying cohesive muds, assisted in plucking up large blocks of the latter and incorporating them into the flow. Locally, the rip-up intraclasts were fragmented further by smaller-scale injections to form a distinct breccia of angular to rounded mudstone clasts within a medium to coarse sandstone matrix. Sandstone sills in places mimic normal sedimentary beds, complete with structures resembling inverse gradation, planar laminae, as well as ripple and trough cross-lamination. These were probably formed by internal sediment flow and shear stress as the semi-liquefied sand was forcefully injected into cracks. In borehole cores, such sills can easily be misinterpreted as normal sedimentary beds, which can have important implications for hydrocarbon exploration.     

Morphometric analysis of slipface processes of an aeolian dune: Implications for grain‐flow dynamics

Grain flows are an integral part of sand dune migration; they are a direct response to the local wind regime and reflect complex interactions between localized over‐steepening of a dune slipface and complex turbulent airflow on the lee slope. Grain flows are primarily responsible for delivering sediment to the base of a dune, thus driving slipface advancement; yet, there are few constraints on their morphological and spatial characteristics or the amount of sediment that is redistributed by these flows. Using a combination of high‐resolution terrestrial laser scanning and video recordings, four distinct grain‐flow types are identified based on morphology and area on a dune slipface. Grain‐flow morphologies range from small, superficial flows to larger flows that affect greater portions of the slipface, moving significant amounts of sediment. Detailed field observations are presented of the dynamics of lee slopes, including measurements of the initiation location, thickness, magnitude and frequency statistics of grain flows, as well as volume estimates of redistributed sediment for each grain flow observed. High‐resolution laser scans enable accurate quantification of bulk sediment transfer from individual grain flows and can be used to study grain flows in a variety of environments. A categorization of grain‐flow morphologies is presented that links styles of flows with wind strength and direction, turbulent airflow, sediment deposition and environment.     

风沙环境下沙漠路基风蚀破坏数值模拟研究

基于Fluent气固两相流,运用 湍流模型,对风沙环境下沙漠路基的风蚀破坏规律进行数值模拟研究。分析路基不同横断面下风沙流扰动、增速、减速、恢复的过程,以及路基障碍物的存在对风沙流扰动后造成的风速减弱区和恢复区,总结路基坡面特征点的风速变化规律,与现场实测结果作比较,二者具有很好的一致性。研究结果表明,不同路基横断面下风速减弱区和恢复区的分布对路基高度变化敏感,受边坡坡率的影响较小,不同路基高度、不同边坡坡率下路基沿程风速变化明显不同。边坡坡率一定,随路基高度的增加,路基周围流场扰动被增强,迎风坡坡顶风速增大显著,背风坡坡底风速下降愈明显;路基高度一定时,边坡坡率越小,路基沿程风速变化越平缓。当边坡坡率为1:3时,路基模型高度由1m增加到3 m时,迎风坡坡顶风速增加12%,背风坡坡底风速降低约80%。建议为避免沙漠路基的风蚀破坏,路基高度不宜太大,边坡坡率较小为宜。     

Migration and growth of aeolian bedforms

A two-dimensional analytical model is developed for the morphodynamics of aeolian dunes. The basis of the model is the sediment continuity equation, which is solved using a linearized sediment transport formula. The air flow over the topography is calculated with a steady-state boundary-layer model. This results in a series of analytical expressions for the shear stress, sediment transport, topography through time, and growth and migration of a sine-shaped dune. These expressions give quantitative relationships between bedform behavior (i.e., growth and migration) and factors such as wind velocity and surface roughness. In this way it can be seen that growth and migration rates increase for higher wind velocity, higher surface roughness and higher wave numbers (i.e., shorter wave lengths).     

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.     

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.