KMS XINJIANG INSTITUTE OF ECOLOGY AND GEOGRAPHY，CAS
| 风影沙丘的形态动力学研究 | |
| Alternative Title | Study on Morphodynamics of Shadow Dune |
| 赵永成 | |
| Subtype | 博士 |
| Thesis Advisor | 雷加强 |
| 2019-06-30 | |
| Degree Grantor | 中国科学院大学 |
| Place of Conferral | 北京 |
| Degree Discipline | 理学博士 |
| Keyword | 灌丛沙丘 风影沙丘 湍流结构 风况 沙源 Nebkha Shadow Dune Turbulence Structure Wind Regime Sand Availability |
| Abstract | 风影沙丘作为一种由障碍物引起的风积地貌，广泛分布于干旱、半干旱地区的荒漠与海岸地带以及火星等地外星球表面。其形成过程和发育机理因障碍物类型（如：山体、小土丘、灌丛沙丘、植被等）、形态、规模及风况条件（如： 单峰风况、 双峰风况等）而存在较大差异。 塔克拉玛干沙漠东缘台特玛湖干涸湖盆的风影沙丘主要发育在灌丛沙丘背风侧，其形态因灌丛沙丘规模、 空间分布特征等因素而差异显著，而以往关于此类风影沙丘的研究比较匮乏，其发育机理和形成过程尚不完全清楚。因此，深入了解风影沙丘的演化过程、形成环境和发育机理， 对于明确风蚀风积过程具有重要的理论意义。 本论文综合应用地貌学、沉积学和沙丘形态动力学等学科知识， 通过野外测量、遥感解译、风洞模拟及数值模拟相结合的手段，从风影沙丘空间分布格局、形态特征、 层理结构等方面，分析和探讨了风影沙丘的形成环境、发育机理以及与其他沙丘类型的关系， 得到以下结论：作为引起风影沙丘发育的障碍物（灌丛沙丘）， 台特玛湖干涸湖盆内发育的灌丛沙丘是由流水过程和风沙活动长期相互作用的产物。灌丛沙丘空间分布格局和稳定性受河流分布影响显著。 在靠近现代河道附近，灌丛沙丘上覆植被生长良好； 随着离河道距离的增加，灌丛沙丘由固定沙丘逐渐演变为半固定沙丘， 最终在干涸河漫滩和古河道附近演变为完全活化的沙丘。受灌丛沙丘形态和空间分布特征的影响，区内风影沙丘长短不一，形态多样。灌丛沙丘坡脚分离流对其周围气流分布具有重要的影响。因灌丛沙丘迎风坡坡度较陡， 气流在迎风坡坡脚处产生明显的分离现象， 导致沙丘坡脚附近风速和剪应力减小， 大量沙粒沉积， 这种沉积过程持续到坡脚处不再产生分离流为止。坡脚积沙为风影沙丘的发育提供了稳定的沙源。此外， 通过雷诺分解和象限分析法发现，坡脚分离流的存在， 将会导致对雷诺剪应力有抑制作用的湍流猝发过程（高速上抛过程（Outward interactions）和低速下洗过程（Inward interactions））的发生频率增加。瞬时雷诺应力在平均剪应力的生成中扮演着重要角色。在瞬时湍流猝发过程中，对雷诺剪应力有促进作用的低速上抛（Ejection activity）和高速下洗过程（Sweep activity） 对平均雷诺应力的贡献较大，其发生频率几乎是其他两个猝发过程（高速上抛（Outward interactions） 和低速下洗（Inward interactions））的 2倍。 当瞬时雷诺应力比平均雷诺应力高一个量级时，瞬时雷诺应力以少于 5%的时间分担率贡献了近 30%的平均雷诺应力。由此推断， 在近地表沙粒的起动过程中，瞬时雷诺应力可能扮演着非常重要的角色。 因此， 基于时间平均的风速数据预测的输沙通量因数据平均过程中的平滑作用而存在较大的不确定性。风影沙丘形态特征受风速大小、灌丛沙丘形态参数影响显著。数值模拟（CFD）和风洞模拟结果显示， 不论灌丛沙丘高度、底面直径或纵横比等形态参数如何变化，风影沙丘长度都随摩阻风速的增大以幂函数的形式减小。 CFD 模拟结果显示， 当摩阻风速小于 0.40m s-1 时，风影沙丘长度随摩阻风速的增大而急剧减小；当摩阻风速为 0.40~0.60m s-1 时，其幅度略有减小；当摩阻风速高于 0.60m s-1 时，风影沙丘长度随摩阻风速的增大基本保持不变。 此外， 灌丛沙丘底面直径的增大有助于风影沙丘的横向增宽和纵向延伸。 风洞模拟的积沙数据显示，由于灌丛沙丘高度对气流中沙粒的阻碍作用， 导致风影沙丘长度呈现随灌丛沙丘高度的增加而减小的趋势。 因此， 仅基于流场数据来量化障碍物高度对积沙范围的影响，其结果存在较大的不确定性。障碍物纵横比不能完全描述其尾流区流场结构和积沙形态。由于障碍物纵横比综合考虑了高度和宽度的变化而广泛用于风沙地貌学领域。而本项研究发现，当纵横比一定时（如：纵横比为 1:3）， 灌丛沙丘高度（如：高度为 0.30m， 0.40m，0.50m） 和宽度（如：宽度为 0.90m， 1.20m， 1.50m） 存在众多种组合情况，即在相同纵横比、不同高宽组合下的障碍物，其尾流区流场结构和积沙形态呈现明显的差异。因此，在描述障碍物周围流场结构与积沙形态时，要综合考虑障碍物纵横比和高宽绝对值本身的影响。相邻灌丛沙丘的空间排列方式对风影沙丘纵向延伸的影响显著。研究发现，发育在相邻灌丛沙丘中间的“二次沉积区” 是风影沙丘进一步发育的前提条件。该“二次沉积区” 的发育主要受灌丛沙丘横向间距的影响。当两个灌丛沙丘一前一后且沿主风向排列时，其前后间距影响着串珠状风影沙丘的形态和规模。串珠状风影沙丘形成的临界间距主要取决于上风向灌丛沙丘尾流恢复区的长度。风况和沙源供给条件共同决定风影沙丘的走向和稳定性。研究区风影沙丘发育在窄单峰风况下，其走向与合成输沙方向一致。 灌丛沙丘对气流的分离作用是单峰风况下风影沙丘发育的主要原因。 然而，单向风况并非是风影沙丘发育的必要条件。当双向风作用于不可侵蚀地表，且在地表障碍物的作用下，同样能够发育出较长的风影沙丘， 其外部形态和走向主要由风况（风向夹角和各风向的输沙率） 和沙源供给条件决定。 随风况和沙源供给条件的变化， 风影沙丘的末端也会衍生出新的沙丘类型，如：新月形沙丘、 独立于风影沙丘的线形沙丘等。 |
| Other Abstract | Shadow dune is a simple and obstacle-related aeolian dune type. They developedin some arid and semi-arid region on the Earth, such as in some desert regions orcoastal area, as well as on some outer planets, such as on the Mars. Their developmentprocesses and the formation mechanisms are varying depending on the wind regimes(e.g. unimodal or bimodal wind regimes), the type and morphology of the obstacles(e.g. hill, mound, nekbhas or vegetation). Shadow dunes in the Taitema Dry Lake areaccumulated at the lee side of nebkhas. Their morphology is influenced significantlyby the geometry and alignments of nebkhas. However, few attentions have been paidon their formation mechanisms. In view of the importance of these shadow dunes ininterpretation of the desertification processes of the Taitema Dry Lake, learning theirdevelopment processes, formation environment and mechanisms and the stability isnecessary. To learn the spatial distribution patterns, dune morphology, sedimentarystructures and the effects of wind regimes, obstacle geometry and alignment on theirformation, some field observations, remote sense measurements, wind tunnelexperiments and the numerical simulations have been performed. Some majorconclusions might be made as follows:Nebkhas in the Taitema Dry Lake were the integral productions of the fluvial andaeolian processes. The spatial distribution pattern of nebkhas is closely related to theDesert Rivers. For example, nebkhas near the modern rivers grow well and with lotsof the vegetation covered them. With the increase of the distance away from the riversbanks, nebkhas degraded increasingly. Most of them on drought floodplain andpaleochannels activated completely. The spatial patterns and morphology of shadowdunes are greatly influenced by the morphology and the alignments of these nebkhas.The flow separation at the toe region of nebkha plays a key role for the flowdynamics over them. The result shows that flow separation have been generated dueto the steep stoss slope of the nebkha. This separation flow results in the flowdeceleration, declination of the Reynolds shear stress, and therefore sand depositionnear the toe region. This flow separation process will not cease until the stoss slope isgentle enough and could not lead to the separation flow any more. Furthermore, thequadrant analysis shows that the occurrence frequency of the Outward and Inwardinteractions (these two processes play a negative role for the generation of Reynoldsshear stress) presents an increased trend due to the flow separation in the toe region.The instantaneous Reynolds shear stress is vital for the average Reynolds shearstress. In the four instantaneous turbulent bursting processes, the occurrencefrequency of the positive processes (Ejection activity and Sweeps activity) for thegeneration of the Reynolds shear stress is nearly two times higher than the negativeones (Outward interactions and Inward interactions). When the instantaneousReynolds shear stress is larger than one magnitude of the average Reynolds shearstress, the instantaneous Reynolds shear stress contributes nearly 30% to the averageReynolds shear stress within less than 5% time share rate. It might be conclude thatthe instantaneous Reynolds shear stress plays a control role for the entrainment of thestatic sand particles into movement. The sand flux predicted according to thetime-averaged wind velocity presents a big uncertainty due to the smooth effects ofthe average processes.The morphology of the shadow dune is seriously affected by the incidence windshear velocity and the obstacle geometry. No matter how does the morphologyparameters of the nebkha vary, the length of the shadow dunes decreases with thewind shear velocity as a power function. The decrement is massive when the shearvelocity is lower than 0.40 m s-1, and slightly when the shear velocity ranges from0.40 m s-1-0.60 m s-1, and nearly stable when the shear velocity is larger than 0.60 ms-1. The increase of the nebkha diameter plays a positive role for the growth andelongation of shadow dune in transverse and longitudinal direction, respectively.However, the increase of the nebkha height plays a negative role for the elongation ofthe shadow dune due to the obstruction of the windblown sand by the nekbha mass.Therefore, the structure of sand drifting flux should be integrally considered with theflow dynamics around obstacles to analyze the shadow dune formation.There is a big uncertainty when we using the aspect ratio to describe the flowdynamics and their implications on the sand deposition. The aspect ratio has beenwidely used to describe the effects of the topography or obstacles on the flowdynamics and sand deposition processes. However, results in this thesis show thateven the aspect ratio is a fix value (e.g. the aspect ratio is equal to 1:3), the height (e.g.0.30 m, 0.40 m, 0.50 m) and the width (e.g. 0.90 m, 1.20 m, 1.50 m) of thetopography could be varying in a big parameter space. The flow dynamics and thesand deposition processes over these obstacles which are in same aspect ratios, but aredifferent in height and width, present a big difference. Therefore, parameters such asaspect ratios should be using careful in description of effects of the topography on the flow dynamics and sand transporting processes.The spatial alignment of the nebkhas plays a key role for the elongation of theshadow dune. The “secondary deposition zone” between two neighboring nebkhas isvital for the development of the shadow dunes. The formation of this secondarydeposition zone depends on the transverse distance of the nebkhas. The fore-and-aftclearance of two nebkhas affects the development of the bead-like shadow dunes. Thecritical distance for the occurrence of the bead-like shadow dunes depends on therecovery distance of the upwind nebkha.The shadow dune could develop under unimodal wind regime, while theunimodal wind regime is not the unique wind regime for the shadow dune formation.The shadow dunes in Taitama Dry Lake are developed under unimodal wind regimesand orientated into the resultant sand flux direction. The flow separation anddeflection are the major reasons for their development under unimodal wind regime.However, when the bimodal winds blow over a non-erodible surface and encounterwith a topographic obstacle, some long shadow dunes could also be developed. Theirmorphology and the orientation mainly depend on the resultant wind direction and thesize of the local sand source. It mainly depends on the type, morphology of theobstacles. The shadow dunes could eject some barchans when the sand supply isinsufficient. Furthermore, some linear dunes could also be segmented from theshadow dunes. |
| Subject Area | 自然地理学 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.xjlas.org/handle/365004/15378 |
| Collection | 中国科学院新疆生态与地理研究所 研究系统 |
| Affiliation | 中国科学院新疆生态与地理研究所 |
| First Author Affilication | 中国科学院新疆生态与地理研究所 |
| Recommended Citation GB/T 7714 | 赵永成. 风影沙丘的形态动力学研究[D]. 北京. 中国科学院大学,2019. |
| Files in This Item: | There are no files associated with this item. | |||||
Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.
Edit Comment