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1.
Textural variations within different representative types of dune sediments in Kuwait 总被引：2，自引：2，他引：0
Samples were collected from the surfaces of four types of typical dunes in order to identify variations in textural characteristics
over their bodies. These dunes are barchan, climbing dune, falling dune, and nabkha. Statistical parameters vary from position
to another and show that each dune has its own characteristics. It is well recognized that all the sediments of the studied
dunes tend to be finer from borders toward the mid dune. Histograms and bivariate diagrams successfully differentiate between
different localities within all studied dunes. The climbing dune shows high uniformity where medium sand represents the mean
grain size of 91% of collected samples. Samples from barchan and falling dune show lowest variability in statistical parameter
values compared to other dunes. On the other hand, nabkha sediments are more variable and show higher values of average statistical
parameters. All studied dunes are coarser than surrounding dunes in regional areas and other comparable dunes. But particularly,
the barchan sediments in Kuwait are characterized by larger grain size, better sorting than other comparable dunes in the
upwind (Iraq) and downwind (Saudi Arabia) and other parts of the world. 相似文献
2.
EDWIN D. McKEE 《Sedimentology》1966,7(1):3-69
The type, scale, and relative abundance of sedimentary structures in four kinds of dunes at White Sands National Monument, New Mexico, were determined by examination of vertical sections on walls of trenches cut through the dunes both in a windward direction and at right angles to this direction. Analysis of cross-stratification in all dunes examined indicated certain common features: sets of cross-strata mostly are medium- to large-scale; nearly all laminae dip downwind at high angles (not uncommonly at 30°-34°); most bounding surfaces between sets of cross-strata are nearly horizontal on the upwind side, but have progressively steeper dips to lee, downwind; and individual sets of cross-strata tend to be thinner and the laminae flatter near the top than at the bottom of a dune in vertical section. Sparse but distinctive structural features that are characteristic of the four types of dunes are varieties of contorted bedding, rare ripple laminae, and either local scour-and-fill bedding, or festoon bedding. Other structures, apparently limited to either one or two types of dunes, are the concave-downward foresets in some parabolic dunes; the low-angle reverse dips of upwind strata on high transverse dunes; and the almost horizontal laminae which represent apparent dip in sections normal to wind direction in dome-shaped and transverse dunes. Describing cross-stratification in terms of three dimensions, dune structure at White Sands consists dominantly of the tabular planar sets, with units thickest near the dune base, thinner above. To a lesser extent the sets are of simple (non-erosion) tabular form and relatively uncommonly, of the trough type. Wedge planar forms are scarce. The planar forms characteristically are of two classes in nearly equal proportions: those in which bounding surfaces are virtually horizontal and those in which they dip at moderate to high degree. A brief comparison is made between the structures of dunes that are characteristic of one effective wind direction, as at White Sands, and certain others formed by winds of two or more directions. Seif dunes of Libya, reversing dunes of the San Luis Valley, Colorado, and star dunes in Saudi Arabia are discussed as examples of complex dunes formed by multi-directional winds. 相似文献
3.
Variations in wind velocity and sand transport on the windward flanks of desert sand dunes 总被引：6，自引：0，他引：6
N. LANCASTER 《Sedimentology》1985,32(4):581-593
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. 相似文献
4.
The pattern of grainfall deposition in the lee of aeolian dunes 总被引：1，自引：0，他引：1
ROBERT S. ANDERSON 《Sedimentology》1988,35(2):175-188
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. 相似文献
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. 相似文献
5.
沙丘背风侧气流及其沉积类型与意义 总被引：6，自引：2，他引：6
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. 相似文献
6.
Recent Monte Carlo computer simulations have modelled the local, geometric mechanism by which mixed populations of differently sized particles order themselves when shaken, such that larger particles rise at the expense of the smaller. This process, called size segregation, is distinct from the mechanism of sifting where smaller particles fall between the interstices of larger particles. Size segregation is applied here to the sand grain populations found on the surfaces of dunes, which are involved in a sorting process driven by the wind. We present data from four barchan dunes located in the Eastern Province of Saudi Arabia. A simple model is used to demonstrate that size segregation, driven by saltation impacts, is energetically plausible at the wind velocities typically encountered on dune surfaces. Laboratory tests which support this model are also discussed. 相似文献
7.
N. LANCASTER 《Sedimentology》1989,36(2):273-289
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. 相似文献
8.
Abbas Al-Harthi 《Environmental Geology》2002,42(4):360-369
Serious hazards have taken place in urban areas and road construction in Saudi Arabia because of the presence of accumulations of drifting sand dunes. Several researches, which carried out investigative work to solve this problem, were reviewed. Three locations of dune fields along the area between Jeddah and Al-Lith were investigated. The dune forms was identified. Detailed field investigations showed that barchan dunes are dominant in the area. The sands from the studied locations were found to be similar in grain size and shape parameters. Mineralogically, the sand reflects the composition of the surrounding igneous and metamorphic rocks. Regression analyses were performed and empirical relationships between dune height, width, windward length, slip-face length, and rate of movements were developed. Relatively strong relations exist between these parameters. The most important geometric parameter controlling dune movement with wind speed and direction is determined to be the dune height. A reasonable similarity occurs between the barchan dunes in the study area and those existing in Al Nufud, Al Jafurah sand seas, and Khulays area. The studied dune fields pose some natural hazards on the roads, and the surrounding buildings and constructions in the villages along the area between Jeddah and Al-Lith, especially during wind storms. 相似文献
9.
Zhibao Dong Zhengcai Zhang Guangqiang Qian Wanyin Luo Ping Lv Junfeng Lu 《Environmental Earth Sciences》2013,69(1):267-277
Star dunes have received less study than other major dune types, though they are widely recognized to represent a major dune type that develops under a multi-directional wind regime. Several types that include simple, compound, and complex star dunes are identified in the south of China’s Kumtagh Desert. It is suggested that the formation and development of these star dunes is controlled by wind regime, the underlying and surrounding topography, and sediment availability. A complex wind regime and rich sediment availability are generally required for the development of star dunes. Especially, wind regime appears to be the most important control factor. The wind regime under which star dunes arise is characterized by the drift potential, amount of variability in drift direction, and the direction distribution mode of the drift potential. It is strongly suggested that a rectangular bimodal wind direction distribution mode has unique significance in star dune formation. Under this mode, star dunes can develop in areas with a directional variability index typical of linear dunes or even barchan dunes. A development model is proposed for star dunes based on the following evolution: barchan dunes → transverse ridges → dune networks → simple star dunes → compound star dunes → star dunes atop complex linear dunes. 相似文献
10.
Large barchan-shaped sand deposits have been observed in the north west of Torres Strait. These deposits share characteristics of both subaerial barchan dunes and subaqueous sand banks. A study of satellite imagery indicate that the deposits migrate in the direction indicated by their horns (10-15 m west per year), and that sediment is shed from their horns, features that are characteristic of barchan dunes. However the orientations of sand dunes superimposed upon the sand banks indicate the presence of mutually-evasive channels and circulation of sediment around the sand bank, a characteristic of subaqueous sand banks. The presence of mutually-evasive channels is the criteria used to categorise the deposits as sand banks.Barchan forms are known to exist in regions with limited sediment supply and unidirectional current or wind regimes. In the Torres Strait both these criteria are met. Previous work has demonstrated the presence of a net westward current through the Torres Strait that is driven by the southeast trade winds. The relatively high displacement of the wind-driven currents during the trade wind season relative to the monsoon appears to provide the necessary ‘unidirectional’ regime to form barchans. The low, and typically eastwards, displacement of the residual monsoon season current appears to have a negligible affect on the barchan form. While seasonal wind-driven currents appear to maintain the barchan shape of the sand banks, tidal currents actively maintain mutually-evasive channels observed by variations in dune orientation on the sand banks. A sediment starved environment combined with bedload transport attributed to both wind driven and tidal currents is concluded to create a unique hydrodynamic environment where sand banks can attain a barchan form. 相似文献
11.
JOHN H. HOYT 《Sedimentology》1966,7(2):137-143
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). 相似文献
12.
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. 相似文献
13.
Prof. Dr. Michael Sarnthein Prof. Dr. Eckart Walger 《International Journal of Earth Sciences》1974,63(3):1065-1087
Fairly constant winds from N to NNE (Fig. 2) prevail at present at the Western Sahara coast. Accordingly, a relatively narrow field of barchan dunes of only 80 km width reaches the coast SE of Cape Blanc (Fig. 1). Very uniform pebble plains form their ground of advance in the study area 60 km wide and 18 km long. Height H, volume V, and distance D from the southern border of the study area were determined for 963 dunes from aerial photographs (Figs. 5 and 6). Data on the dune advance rate were estimated for the particular region byCoursin (1964). Consequently it was possible to calculate a dune sand discharge amounting to 93 000 m3/yr/80 km crossing the southern border of the study area at the time the aerial photographs were taken. Based on the areal distribution pattern of the dunes this sand flow probably might increase threefold within the next 800 years (Fig. 7). Corresponding to the dune sand-discharge QT a saltation sand-discharge (Q and q), 50–100 times larger, of 5,0 and 7–13 Mio m3/yr/80 km, respectively, reaches the Atlantic from the Sahara. The estimates were derived from two independant calculations: the dune advance rate and the wind data. If one compares the wind transported load from the Sahara with that of the mouths of large rivers (e. g. Niger River: 40 Mio. m3/yr) it seems only of minor importance. Because of the relatively coarse grain sizes (Md≈220μm) the wind sand supply is deposited mainly along the strand line. Consequently, remarkably wide sebkha plains are built forward and the shelf becomes unusually narrow. Several independent criteria (e. g. Fig. 7) suggest a fairly young age, close to 500 years of the recent barchan field. A different wind direction, from the NE, and a lowered sea-level might have resulted during the ice-ages in as much as 5 times larger wind load (? 25 Mio m3/yr) arriving at the shelf edge and from there flowing down to the deep sea as turbidity currents. The present wind load has a content of iron oxides of roughly 1.2 per thousand. This value increased to 3.2 per thousand in Pleistocene dune sands. 相似文献
14.
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. 相似文献
15.
The effect of unsteady winds on sediment transport on the stoss slope of a transverse dune, Silver Peak, NV, USA 总被引：5，自引：0，他引：5
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. 相似文献
16.
Temporal trends in grain-size measures on a linear sand dune 总被引：4，自引：0，他引：4
IAN LIVINGSTONE 《Sedimentology》1989,36(6):1017-1022
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. 相似文献
17.
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. 相似文献
18.
Toward a model for airflow on the lee side of aeolian dunes 总被引：8，自引：0，他引：8
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. 相似文献
19.
YanYan Yang LianYou Liu XiaoYan Li PeiJun Shi GuoMing Zhang YiYing Xiong YanLi Lyu LanLan Guo Bo Liang MengDi Zhao JiaDong Dai XiYang Zuo XuJiao Han 《Sedimentology》2019,66(2):590-603
Aeolian sand entrainment, saltation and deposition are important and closely related near surface processes. Determining how grains are sorted by wind requires a detailed understanding of how aerodynamic sand transport processes vary within the saltating layer with height above the bed. Grain‐size distribution of sand throughout the saltation layer and, in particular, how the associated flux of different grain size changes with variation in wind velocity, remain unclear. In the present study, a blowdown wind tunnel with a 50 cm thick boundary layer was used to investigate saltating sand grains by analyzing the weight percentage and transport flux of different grain‐size fractions and the mean grain size at different wind velocities. It was found that mean grain size decreases with height above the sand bed before undergoing a reversal. The height of the reversal point ranges from 4 to 40 cm, and increases with wind velocity following a non‐linear relationship. The content of the finer fractions (very fine and fine sand) initially increases above the sand bed and then decreases slightly with height, whereas that of the coarser fractions (medium and coarse sand) exhibits the opposite trend. The content of coarser grains and the mean grain size of sand in the saltation layer increase with wind velocity, indicating erosional selectivity with respect to grains in multi‐sized sand beds; but this size selectivity decreases with increasing wind velocity. The vertical mass flux structure of fine sand and very fine sand does not obey a general exponential decay pattern under strong wind conditions; and the coarser the sand grain, the greater the decrease rate of their transport mass with height. The results of these experiments suggest that the grain‐size distribution of a saltating sand cloud is governed by both wind velocity and height within the near‐surface boundary layer. 相似文献
20.
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. 相似文献