On the effect of mid-air collisions on aeolian saltation

The effect of mid-air collisions on aeolian saltation is investigated using concentration profiles and grain velocities predicted by a numerical saltation model. The probability of a mid-air collision is found to be greater at high wind speeds. It is also found that mid-air collisions tend to reduce the number of grain/bed impacts and thus reduce the number of ejecta near the bed; this, in turn, reduces the intensity of mid-air collisions. It is suggested that this feed-back mechanism significantly influences transport rates at high wind speeds.     

Successive aeolian saltation: studies of idealized collisions

As observed by Bagnold and experimentally reconfirmed by other workers, the impact angles of saltating grains are remarkably constant over a wide range of conditions, lying between 10° and 16°. It can be shown that successive saltation contains a mechanism which very effectively confines impact angles to that range. This control mechanism is most effective at windspeeds less than about 15–30 m s^-1, depending on grain diameter and mass. The control mechanism is evaluated from model calculations of grain populations saltating over a level bed consisting of a layer of loose grains. The grains are assumed to be spherical and uniform in size and mass, also rigid and perfectly elastic. The model also describes distributions of maximum height of grain paths and of lift-off-angles. Compared to other processes involved in aeolian saltation, successive saltation is the only process with a high probability of transferring energy from horizontal into vertical grain movement. This fact, together with the calculations presented, strongly suggests that successive saltation plays a major role in saltation in air. Successive saltation of uniform grains is theoretically impossible if the ground over which saltation occurs is tilted by about 15° against wind direction. Values of tilt angles in this range are observed in nature as stoss-side angles of dunes and ripples, leading to the concept that stoss-sides are tilted up by deposition until successive saltation is subdued.     

On the modelling of two-dimensional aeolian dunes

In this study the modelling of aeolian dunes is treated from the background of the physical processes that form them. The paper concentrates on two-dimensional modelling, which can be applied to transverse and barchan dunes. The basic assumptions and equations are discussed and three models are presented: (a) a kinematic model which is based on a linear variation of the sediment transport with the topographic height; (b) an analytical model based on a boundary-layer model of the wind velocity and Bagnold's linearized sediment transport formula; (c) a generalization of this analytical model in a computer program which includes a simple routine for simulating the redistribution of sediment through avalanching. The relative importance of the simplifications is considered and indications are given for the practical applications of these three models in field studies.     

The impact process in aeolian saltation: two-dimensional simulations

ABSTRACT
A critical event in the trajectory of a sand grain saltating in air is its interaction with the surface. We examine the phenomenon of grain-bed impacts in two dimensions using a combination of dynamical computer simulations, analytical models and physical reasoning. The results indicate that the grain-bed collisions can be treated as two-body collisions with the bed particle assuming an effective mass greater than its true mass. Also, the presence of geometrical surface irregularities has a strong bearing on the interaction between saltating and surface grain populations, as well as on the formation of small-scale bedforms.     

Relationship of spatial heterogeneity for vegetation and aeolian sand soil properties on longitudinal dunes in Gurbantunggut Desert, China

On longitudinal dunes in the southern Gurbantunggut Desert, the vegetation was investigated, and the texture, water content, organic matter, total nitrogen, salt content and pH of aeolian sand soil were measured. By means of geostatistical methods, the spatial heterogeneity for vegetation and aeolian sand soil properties was analyzed. The vegetation pattern and aeolian sand soil properties were moderately to strongly spatially heterogeneous as a whole, with a spatially dependent range of 8.8–74.8 m. Because both of them had a nest structure of grading system, their spatial heterogeneity needed to be described by the fractal dimensions at different sampling scales. The autocorrelated spatial range A ₀ and fractal dimensions of the cover of vegetation and the diversity of herberious synusium were similar to these of soil water contents (SM). The coarse grain size (φ1) and sorting (σ) of soil, the diversity of herbaceous synusium and the cover of vegetation had a closer dependent range A₀, being 37.8–57.8 m. The trends of spatial variation for organic matter and total nitrogen contents were very similar to that of vegetation cover. The spatial heterogeneities of soil pH and salt content were mainly restricted by terrain and basically not related to vegetation.     

Experimental verification of aeolian saltation and lee side deposition models

We report results of experiments intended to test the validity of a model for aeolian saltation and the resulting pattern of deposition on the lee side of aeolian dunes. In steady sea-breeze conditions on a 3-m-tall dune at Point Año Nuevo, California, we measured simultaneously the near-brink wind speed and the deposition on both horizontal and lee face collector platforms. We then used the details of the deposition patterns to constrain approximate values of parameters in a numerical model of the deposition rate that incorporates the essence of the saltation process. Best fits to the data constrain a parameter that controls the probability distribution of liftoff speeds. In addition, the total vertical number flux of grains is constrained to roughly 10⁷?10⁸ grains m^?2 s^?1 at shear velocities of 0.33–0.40 m s^?1. The lee side deposition pattern, which shows the expected maximum in deposition rate at a distance of several decimetres from the brink, is also well fit by the model. In addition, simultaneous collection of horizontal and lee deposition patterns, along with the numerical simulation of these patterns, strongly implies that the windfield in the lee of this particular dune is best described as a non-recirculating wake. Grainflows on the lee face are caused by failure of grainfall depositional bumps. Our results suggest that the principal effect of increased wind speed is to increase the frequency of grainflows. rather than to increase their size, implying that very large, thick grainflows require a different mechanism.     

A new aeolian sand trap design

A vertical rod sand trap has been constructed to acquire a volumetric measure of wind-blown sand. The design is simple, construction cost is minimal, and collection efficiency is quite high. This device can be used effectively for short-term monitoring of aeolian transport.     

Toward a model for airflow on the lee side of aeolian dunes

The interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes. From 15 dunes studied during transverse flow conditions at Padre Island in Texas, White Sands in New Mexico, and the Algodones in California, distinct flow regions can generally be recognized, with the overall flow structure comparing favourably to that proposed for subaqueous bedforms. Downwind of dunes with flow separation is a back-flow eddy that extends about four dune-brink heights downwind from the brink of the dune. Beyond the separation cell, the velocity profiles can be divided into regions based upon segments separated by ‘kinks’ in the velocity profiles. The interior is an area above the dunes of relative high wind speed but low velocity gradient. Beneath the interior is the wake, which consists of two layers. The upper wake exhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow, so that the overall velocity gradient decreases downwind. The lower wake exhibits low velocity gradients and wind speeds that accelerate downwind at all heights, but primarily near the top of the layer, thereby causing the velocity gradient to increase downwind. At about eight dune heights downwind, the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent. The lowest recognizable region is the internal boundary layer. It is recognized by a relatively steep velocity gradient below the wake, and never exceeds a few tens of centimetres in height for our data set. Because of acceleration and increasing shear stress within this layer, interdune flats are at least potentially erosional. Overall, the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers. Where flow separation does not occur, simple flow expansion down the lee-face causes flow deceleration.     

Estimation of some aeolian saltation transport parameters: a re-analysis of Williams' data

A new method for analysing observed aeolian sand transport rate profiles of the kind obtained by Williams is presented. The method involves a mathematical model of aeolian saltation. Detailed information about the saltation process can be calculated from the transport rate profile by means of this model. The method is used to perform a re-analysis of Williams' trap data. Among the main findings of this analysis is that the grain borne shear stress appears to be a smaller fraction of the total shear stress than assumed by Bagnold & Owen in their theories of aeolian saltation. Other findings are that the probability distribution of the jump height of the grains does not depend much on the wind speed once the saltation is established, and that the vertical component of the mean launch velocity decreases with the grain size. It is approximately inversely proportional to the grain diameter. Our estimates of the landing angles indicate that estimates of the impact angles obtained from photographically recorded trajectories are too small due to biased sampling. The influence of grain shape on the transport characteristics is mainly due to changes in the grains' ability to jump when hitting the bed. It is found that angular grains have a lower mean jump height than spherical grains.     

The pattern of grainfall deposition in the lee of aeolian dunes

ABSTRACT
A simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation. Grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance. The deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity, the velocity being weighted by its probability density.
The resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink, beyond which deposition rates fall off roughly exponentially. An important length scale emerges that characterizes this decay with distance from the brink, the length increasing with wind velocity, and decreasing with grain diameter. It is shown that this length scale is on the order of one metre for typical grain size and wind conditions. That this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes. The position of a pivot point on the lee slope may be predicted, separating source regions from accumulation regions for grainflow avalanche deposits.
The calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits, but they provide the initial geometry for grainflow avalanches. The initial failures should coincide with the steepest gradient in grainfall deposition, slightly downslope from the grainfall maximum.     

基于微观结构的青藏高原风积沙导热系数变化机理研究

风积沙作为青藏高原一种重要的局地因素, 改变了多年冻土的赋存条件. 风积沙的导热系数特征对预报分析其对冻土赋存有利或者不利具有重要作用. 采用非稳态法对青藏高原红梁河风积沙进行了导热系数测试, 并结合电镜扫描/能谱分析, 从微观结构的角度探讨了风积沙的导热系数变化机理. 结果表明: 研究区风积沙平均粒度为242.427 μm; 标准偏差值为0.125, 分选极好; 偏度为0.359, 接近对称; 峰度值为1.086, 峰态中等; 颗粒粒径主要分布在75～500 μm之间, 沙粒均匀, 不含黏土及砾石成分, 自然堆积状态下其孔隙率为0.391. 天然状态下的风积沙颗粒呈类球形, 颗粒磨圆度高, 点与点接触, 颗粒间孔隙较大; 表面有明显撞击坑和擦痕, 这导致颗粒的比表面积增大, 连通性增强, 孔隙率增加. 干燥状态下风积沙颗粒的相互接触面积较小, 孔隙由空气填充, 导热系数较低; 而在湿润状态下, 正温时孔隙中的水间接增大了风积沙的接触面积, 导致其导热系数增大; 负温时, 孔隙内的水变成冰, 从而导致导热系数进一步增大. 天然状态下, 暖季地表风积沙含水量较低, 导热系数较低, 而冷季地表风积沙含水量较大, 导热系数较大. 此外, 风积沙为颗粒物质, 表面光滑, 颗粒之间粘性小, 孔隙未被填堵, 结构松散, 这些因素导致自然堆积状态下其渗透系数较一般细砂大, 透水性良好, 保水性差, 是防冻胀较好的换填材料.     

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.     

Internal structure of aeolian dunes in Abu Dhabi determined using ground-penetrating radar

A ground-penetrating radar survey of aeolian dunes in the Al Liwa area of Abu Dhabi reveals a variety of dipping reflectors which are interpreted as primary sedimentary structures. The interpretation of the radar profiles has been confirmed by bulldozing trenches through the study area and comparing logged sections in the trenches with the radar profiles. NNW— SSE-orientated radar profiles, approximately parallel to the prevailing wind direction, show two sets of dipping reflectors which are interpreted as sets of cross-stratification and second- and third-order bounding surfaces. Radar profiles orientated WSW—ENE across the prevailing wind direction are dominated by concave-up reflectors which are interpreted as trough-shaped scours and sets of trough cross-stratification produced by oblique progradation of barchanoid dunes. Nested troughs, with small sets of trough cross-stratification within larger troughs, may be due to reactivation following wind reversal, or the superposition of small dunes on larger dunes and the fill of large dune troughs by smaller dunes. Convex-upwards reflectors are interpreted as linear spurs on the convex portions of sinuous dunes or erosional remnants between troughs. Overall there is a tendency for the larger second-order bounding surfaces to dip downwind, which confirms Brookfield's ideas of the relative migration paths of dunes and draa.     

The soils on the calcareous sand dunes southeast of South Australia

 The properties of soils on previously dated sand dunes from Robe to Naracoorte in South Australia were examined. In these areas younger sand dunes are composed of fresh sand, but older sand dunes are composed of calcarenited sand. The soils on the sand dunes developed successionally by the age of sand dunes. The soil properties of these sand dunes differ depending on the ages of the sand dunes. The properties of sand particles in soils are as follows: (1) On the sand dunes of 4300 years B.P., A/C profile developed (Rendzina). On the sand dunes older than 125 000 years B.P. and on the plateau of Tertiary limestone, soil profiles of A₁/AB/B/C on the sand dunes of 83 000 years B.P. and A₁/A₃/B₁/B₂/C (Terra rossa) are well developed. (2) Within the sand of A/C horizons of the sand dunes with the age of 4300 year B.P., the calcite grain content is about 64%, and the quartz content is about 35%. Within the B horizons of soils on the dunes from 83 000 years B.P. to 347 000 years B.P., the calcite grain content is only 1–2%; however, the quartz grain content is about 92%. In the B₂ horizons of soils on the dune of 690 000 years B.P. and on the Tertiary plateau, there are some calcite grains but the quartz grain content is about 96%. (3) The average size of quartz grains in the soils on the sand dunes from 4300 B.P. to 347 000 years B.P. is generally smaller, but the average size of quartz on the sand dunes of 690 000 year B.P. becomes larger and the grains are well rounded. On the Tertiary limestone plateau, the average quartz size becomes again smaller, and the grains are more rounded. (4) Fe_t in B₂ horizon of the soil profiles increases clearly corresponding to the age. Iron activity expressed by Fe_o/Fe_d also shows a close relation to the chronological sequence. The B horizon of the soil profiles shows a drastic decrease of Fe_o/Fe_d according to the age. Iron crystalinity, (Fe_d-Fe_o)/Fe_t, has a tendency for a positive relation with increasing age. Received: 1 June 1995 · Accepted: 4 December 1995     

风沙堆积对下伏多年冻土影响的数值模拟

青藏高原沙漠化所产生的风沙堆积作为一种重要局地因素,其对下伏多年冻土的具体影响目前尚未清楚。通过数值方式,模拟了红梁河沙害严重地区高温退化型多年冻土在13种厚度的干湿风积沙覆盖下10年内的变化情况。结果表明：在土体浅层,沙层越厚,地温较差越小,且干沙下地温较差变化幅度明显大于湿沙;干沙越厚,最大季节融深越小,湿沙则对最大季节融深影响微弱;积沙无论干湿,沙层越厚,冷暖季年热循环量均越小,且干沙下年热循环量变化幅度明显大于湿沙。因此,风积沙对其下伏多年冻土退化有不同程度减缓,沙层越厚,减缓作用越强,且干沙减缓多年冻土退化的能力明显强于湿沙。     

Mass‐wasting of ancient aeolian dunes and sand fluidization during a period of global warming and inferred brief high precipitation: the Hopeman Sandstone (late Permian), Scotland

Large‐scale deformation structures in late Permian aeolian dune sands are associated with sand fluidization and injection. Exceptional precipitation and flooding of the desert margin are believed to have caused mass‐wasting by gravitational collapse and sliding of water‐saturated dunes, which loaded down‐dip strata, thus generating overpressure and triggering sand injection. This short‐lived but heavy precipitation seems to have been associated with a climatic change from arid Rotliegend dune deposition to widespread Zechstein marine conditions within the greater North Sea area, probably just before or coinciding with deposition of the rapidly expanding marine Kupferschiefer.     

Thresholds of aeolian sand transport: establishing suitable values

This paper assesses the practical use and applicability of the time fraction equivalence method (TFEM; Stout & Zobeck, 1996) of calculating a wind speed threshold for sand grain entrainment in field situations. A modification of the original method is used and is applied to 1 Hz measurements of wind speed and sand transport on a beach surface. Calculated grain entrainment thresholds are tested in terms of the percentage of sand transport events that they explain. It was found that the calculated thresholds offered a poor representation of the occurrence of saltation activity, explaining only about 50% of the measured transport events. Results are discussed in terms of system response time, wind speed measurement height, undetected events and sampling period. A shear velocity threshold for grain entrainment was also calculated, but this also failed to explain a high proportion of the sand transport events. The best results (67–91% of transport events explained) were found by calculating a threshold based on time‐averaged (≈ 40 s) wind velocity measurements. The applicability of a single threshold to a natural grain population is discussed. A natural surface is likely to possess a range of thresholds varying over short time scales in response to parameters such as grain rearrangement and changes in moisture conditions. The results show that calculated thresholds based on 40 s time‐averaged data consistently explain a high proportion of the recorded sand transport events. This is because such a time‐averaged approach accounts for higher frequency variability inherent in the sand transport system.     

Sedimentological characteristics and morphology of the aeolian sand dunes in the eastern part of the UAE,a case study from Ar Rub' Al Khali

The Umm Az Zimul–Al Wijan area is located in the southeastern part of the UAE and covers about 1600 km². It is bounded to the east by the Oman Mountains and to the south and west by the extensive dune area of Ar Rub' Al Khali. Field investigation revealed the common occurrence of parallel to sub-parallel sand ridges with a general east–west orientation. Also present, but less frequent, are barchan, barchanoid and star dunes. Interdune areas are classified into four types that include: dolomite, mixed ancient sabkha and ophiolitic rock fragments, mixed sand sheet and ophiolitic rock fragments, and recent sabkha. The textural characteristics of the sand forming the dunes are unimodal with a modal class in fine sand size, moderately to very well sorted, positively skewed and mesokurtic. Quartz grains are mainly subrounded to subangular. The similarity in roundness properties in different dune types in the eastern province of the UAE seems not to be influenced by dune form. Based on the heavy mineral suites and the angularity of quartz grains, it is believed that the sand sediments are mostly derived from the ultrabasic rocks forming the ophiolitic sequence of the Oman Mountains, acidic rocks of Iran and from the coastal area.     

Grain scale simulations of loose sedimentary beds: the example of grain-bed impacts in aeolian saltation

Sediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role. In order to illustrate the modelling of collections of frictional, inelastic sedimentary grains with the particle dynamics method (PDM), we use the grain impact process in aeolian saltation as a specific example. In PDM, all the forces on each particle are evaluated at a sequence of small time-steps, and the Newtonian equations of motion are integrated forward in time. Interparticle forces at grain contacts are treated as springs with prescribed stiffness (normal force) and by a Coulomb friction law (tangential force); particle inelasticity is represented by spring damping. The granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties, and the integration time-step. We find that for the range of impact speeds and impactor masses relevant to aeolian settings, grain splashes are relatively insensitive to grain stiffness, grain inelasticity and grain friction, and that the pattern of ejection from the bed is largely controlled by bed microtopography. A large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process. The splash function representing these statistics is then available for use in calculations over longer time-scales, such as the evolution of the saltation curtain. The details given here will enable the interested reader to adapt PDM modelling to other types of clastic sedimentary systems.