回涡沙丘的形态特征与表面物质组成

在沙丘分类系统中,回涡沙丘是一种重要的有障沙丘形态,系运移风沙流在障碍物前遇阻堆积而成。野外测量表明,回涡沙丘具有与新月形沙丘相似的外观,其迎风坡坡度一般在2°～18°之间,背风坡坡度较陡为21°～31°。回涡沙丘高度及其与障碍物距离受障碍物高度的控制,障碍物越高则回涡沙丘也越高,沙丘离障碍物也越远,沙丘高度、沙丘与障碍物距离分别与障碍物高度线性相关。粒度特征反映了局地沙源特征,粗细颗粒均可在风力作用下形成障前堆积形态。同一地区回涡沙丘与新月形沙丘表面物质的粒度参数对比表明,由于障碍物前回旋涡流的影响,回涡沙丘表面物质经历了更为强烈的风力分选过程,物质组成更细,分选更好,更趋于正偏和窄峰态。     

新月形沙丘脊线处的风沙流结构

分析流动沙丘脊线处的风沙流结构对认识其形态-动力学过程至关重要,进而为干旱区绿洲保护和生态重建提供基础数据支撑和理论依据。利用传统的被动式集沙仪,分别对正风向与反风向情况下新月形沙丘脊线附近0~60 cm高度内的风沙运动进行观测,通过函数拟合方法分析风沙流结构。结果表明：正风向时,输沙量随高度增加符合幂函数递减的规律,气流对沙物质的输运集中在近地表30 cm高度内;反风向时,输沙量随着高度的增加先增加后减少,变化趋势近似符合双高斯函数,气流含沙量在30~50 cm高度内最多。     

Starved versus alluvial river bedforms: an experimental investigation

Laboratory experiments were conducted to investigate the formation of river bedforms under sediment supply-limited conditions, i.e. when a motionless substratum is bared by the dynamics of the mobile sediments. Three series of experiments were organized in a laboratory flume by fixing all the hydrodynamic and morphodynamic parameters but varying the thickness of the initial layer of mobile sediments which covers the rigid bottom of the flume. At the end of all the experiments, which lasted for the same amount of time, the formation of transverse sand dunes was observed. For decreasing , the rigid bottom of the flume was bared progressively earlier during the experiment and the measurements showed a clear tendency of the bedforms to lengthen, i.e. to increase their crest-to-crest distance. Moreover, under strong supply limitation, the two-dimensional transverse dunes turned into three-dimensional barchanoid forms and into isolated barchan dunes characterized by an abrupt reduction in bedform heights. A two-dimensional Fourier analysis of the bottom profile was performed, providing the amplitude of the main streamwise and spanwise harmonic components of the bottom morphology as a function of . © 2019 John Wiley & Sons, Ltd.     

柴达木盆地察尔汗盐湖北侧沙丘黏性沉积物特征

柴达木盆地察尔汗盐湖周边黏性沉积物被认为是单风向风况条件下线形沙丘和新月形沙丘两类沙丘共存的主要原因。为进一步探究察尔汗盐湖周边沙丘黏性沉积物特征,实地调查采集并测量了察尔汗盐湖北侧新月形沙丘/线形沙丘和丘间地沉积物的粒度特征、盐分含量和纵剖面水分含量变化,以期为明确黏性沉积物对沙丘地貌形成演化影响的研究提供参考。结果表明:丘间地沉积物平均粒径最大约为2.39Φ,新月形沙丘沉积物(2.41Φ)略粗于线形沙丘(2.54Φ)。沉积物以细沙为主(84%),中沙和极细沙含量都很少,丘间地中沙含量略高(18%)。区内沉积物都表现出极好的分选性,线形沙丘沉积物略优于新月形沙丘;丘间地沉积物的可溶性盐含量最高平均约为6.88%,线形沙丘(平均2.78%)明显高于新月形沙丘(平均0.80%);丘间地沉积物水分含量最高,在30cm深垂直范围内约为1.93%。新月形沙丘沉积物水分含量(平均0.12%)在1m深垂直范围内明显低于线形沙丘(平均0.55%)。除黏性沉积物(盐、粉沙和黏土)外,沉积物水分含量的空间差异也是引起沙丘形态发生变化的重要环境要素。     

Simulating study on barchan dune

Barchan dune is one of the basic forms of eolian landform, it is usually moulded by high-concentration non-saturated wind-sand flow. The formation process of barchan dune begins from “wave-grain duality” of wind-sand movement, and goes through two developmental stages of sand material accumulating (high-concertration saturated wind-sand flow) and dune form moulding (high-concentration non-saturated wind-sand flow), i.e. the processes of dissipative increase and dispersive decrease. The scattered single barchan dunes on non-sandy bed surface have obvious characteristics of mobility and unstability (not including barchan sand hill). The formation of barchan dune in wind tunnel (its dune scale is one order of magnitude larger than sand ripple scale) is helpful to know the formation mechanism of barchan dunes. One of the series achievements supported by the National Natural Science Fundation of China (No. 49371009)     

Morphological evolution of a barchan dune migrating past an offshore wind farm foundation

As the number of manmade structures installed on the seafloor is increasing rapidly, we seek to understand the impact of these immobile obstacles on marine geomorphological processes, such as the evolution of bedforms. A 5.8 m diameter monopile foundation was installed at the case study offshore windfarm approximately 30 m ahead of an approaching barchan (crescent-shaped) dune. The impact of the monopile on the dune's evolution was analysed using six multibeam bathymetry surveys spanning 20 years. To substantiate this analysis, coupled three-dimensional numerical modelling of flows and sediment was conducted in which the scour inducing bed shear stresses were calculated from the modelled turbulent kinetic energy. Following the installation of the monopile, the mid-section of the dune accelerated and stretched in the direction of the wake of the monopile. Four years after the monopile's installation the rest of the dune had caught up, flattening out the slip face within half the dune's length downstream of the monopile. Due to the modified flow field, the dune was scoured deeply at the base of the monopile to a depth of 6.8 m (supported by the model results that predicted a scour depth exceeding 2 m over a period of just a few days). The surveyed volume of material scoured amounted to 8% of the total dune volume. Whilst the process of scouring occurs at a timescale of days to weeks, the dune migrated on average by 25 m/yr. The difference in the timescale of the two processes allowed the scouring to occur through the full thickness of the dune. The scoured dune profile recovered rapidly once the dune migrated downstream of the monopile. This article demonstrates how large geomorphological features can intercept and migrate past a monopile foundation without long-lasting impacts on the integrity of the feature or the foundation. © 2020 John Wiley & Sons, Ltd.     

Migration of barchan dunes in the western Quruq Desert,northwestern China

Barchan dunes are common on Earth, Mars and Titan. Previous studies have shown that their formation, migration and evolution are influenced by the wind regime and other factors, but details vary among regions. Understanding barchan morphology and migration will both improve our understanding of dune geomorphology and provide a basis for describing the environmental conditions that affect the formation and development of these dunes on Earth and other planets. Here, we provide detailed measurements of barchan dune migration in China's Quruq Desert, in the lower reaches of the Tarim River. We monitored their migration direction and rate, and their morphological changes during migration, by comparing Google Earth images acquired in 2003 and 2014. The dunes migrated west-southwest, close to the local resultant drift direction. The migration rate averaged 8.9 to 32.1 m year⁻¹, with obvious spatial variation. In addition to the wind regime, the migration rate depended on dune morphology, density and vegetation cover; the rate was negatively related to dune height, density and vegetation cover, but positively linearly related to the length/width ratio (L_U/W) and to the decrease in this ratio from 2003 to 2014. We found correlations among the dune morphometric parameters, but the relationships were weaker than in previous research. Due to the complexity of the factors that affect the processes that underlie sand dune development and migration, the morphological changes during dune migration were also complex. Our measurements suggest that the aeolian environment played a dominant role in dune migration and its spatial variation in the Quruq Desert. These results will support efforts to control dune migration in the western Quruq Desert and improve our understanding of dune morphodynamics. © 2019 John Wiley & Sons, Ltd.     

古尔班通古特沙漠西南缘新月形沙丘内部沉积构造特征研究

沙丘内部沉积构造保存了沙丘动力演变过程的重要信息。以古尔班通古特沙漠西南缘固定、半固定新月形沙丘区为研究区域,利用探地雷达在春、秋两季对固定、半固定新月形沙丘内部构造进行探测,获取了不同规模形态的固定、半固定新月形沙丘深约8 m的内部构造图像信息。通过图像增益处理、解译和对比分析表明：（1） 古尔班通古特沙漠固定、半固定新月形沙丘共有5种沙丘内部构造雷达相,即高倾角斜层理、楔状交错层理、上凸形交错层理、低倾角-近水平层理和块状层理,其中前4种主要分布在3~5 m的浅层,而块状层理主要分布在4~5 m以下的深度。（2） 高倾角斜层理、楔状交错层理主要分布在高大新月形沙丘（链）的迎风坡上部和丘顶地带,前者为背风坡前积层埋藏而成,反映高大新月形沙丘迎风坡上部和丘顶风沙活动较频繁,沙丘“固身缩顶”后埋藏的前积纹层因风蚀而出露,后者为迎风坡风蚀坑和风蚀槽中由风沙流充填而成的构造或在丘顶风向的季节性变化形成的构造。（3） 与灌丛沙丘相关的上凸形交错层理广泛分布在新月形沙丘的迎风坡中下部,在背风坡也有局部出现,表明灌丛沙丘在沙丘表层的风沙过程中占有重要地位;而深部的块状层理可能是早期风积层受到强烈的生物扰动,原生层理消失而产生。（4） 以上沙丘内部构造的类型与组合分布特点,反映了研究区新月形沙丘总体上趋于稳定或衰退状态,这与现代沙丘“固身缩顶”的地貌变化特征相一致。例如,迎风坡中上部和丘顶常见风蚀槽,背风坡因坡度变缓、前积层发育趋缓,现代风沙活动主要集中于新月形沙丘的上部和丘顶等。由此可见,研究区固定、半固定新月形沙丘内部构造及其组合分布特征异于流动新月形沙丘,也与半个多世纪以来北疆沙漠气候变暖变湿、平均风速减弱、植被盖度增加的区域自然地理环境变化趋势有一定的吻合性。     

新月形沙丘研究进展综述

新月形沙丘是世界沙漠中分布最广泛的沙丘类型之一，在北半球沙漠中更为常见。Ｂａｇｎｏｌｄ（１９４１）全面论述了新月形沙丘的起源、运动特征和沉积构造。Ｍｃｋｅｅ（１９６６）揭示了新月形沙丘的沉积构造特征。Ｓ．Ａ．费道洛维奇（１９６０）和Ｆｒｙｂｅｒｇｅｒ等（１９７９）阐明了新月形沙丘发育的风信环境。Ｍ．Ｐ．Ｐｔｒｏｖｅ（１９６２）、朱震达等（１９８１）则深入研究了新月形沙丘的移动规律。近年来系统论模式     

察尔汗盐湖北侧沙丘沉积物颗粒微结构特征

利用扫描电镜对比分析了柴达木盆地察尔汗盐湖北侧线形沙丘和新月形沙丘沉积物颗粒的表面显微特征。结果表明：柴达木盆地察尔汗盐湖北侧线形沙丘和新月形沙丘沉积物石英颗粒表面具有明显的风成特征。颗粒以次棱角状为主,普遍发育典型的碟形撞击坑、麻坑和新月形撞击坑;察尔汗盐湖北侧沙丘沉积物以物理分解和机械搬运作用为主,化学风化作用较低,反映了当时干旱炎热的沉积环境;贝壳状断口、平行擦痕及V形坑的存在表明柴达木盆地沙丘沉积物前期曾受到冰川或流水的作用,推断出柴达木盆地周围高山的剥蚀产物或察尔汗盐湖的湖相沉积物可能是察尔汗盐湖北侧沙丘沉积物的来源之一。