【作者】 薄天利；

【导师】 郑晓静；

【作者基本信息】 兰州大学 ， 工程力学， 2010， 博士

【摘要】 沙漠化或者荒漠化问题是我国,特别是西北地区,的最大环境问题之一。定量预测沙漠或荒漠边缘的扩展速度、准确评估相关防治措施的有效性,有着十分重要的现实意义。风成沙丘场的演化过程是一类典型的时空多尺度问题,目前采用不同手段所进行野外观测,包括对其它星球,如火星,都还难以获得其演化的全过程,也尚未见对其成功模拟的例子。因此,对风成沙丘场的演化过程建立可靠、有效的定量模拟的方法,不仅是非常必要的,而且也是从一个侧面对多尺度科学问题的探索,有着十分重要的科学意义。本学位论文对风成沙丘场提出了一种耦合尺度普适模型,其中不仅包含了沙粒碰撞与跃移、风沙流形成与持续、沙丘场演化这三种时空尺度且跨越8-9个数量级及其相互的耦合,而且可适用于不同来流风场和地表等情况。该模型的主要创新点为1)建立了一种计算沙丘场局地风场的新模式,使得对由沙丘场地表形态变化引起的风速变化的处理更加合理并接近实际;2)在引入一个宏观上远小于沙丘场尺度而微观上又包含了足够多的沙粒运动微观信息的细观尺度上的“沙体元”后,首次提出了“沙体元”的“覆盖因子”和“传输因子”这2个风沙运动研究中的新的统计量,由此搭建起“沙体元”的厚度和在给定时间段内整体移动的距离这些宏观量与风沙流中的沙粒在沙床表面的侵蚀与沉积、反弹或溅起、平均跃移长度和时间、沙粒冲击床面的平均速度等这些微观量联系的“桥梁”。本学位论文以这一新的模型为基础,采用空间并行方案,即将沙丘场划分为多个子区域,每个处理器计算一个子区域内沙丘场的形成发展过程,使得对面积在数百平方公里的风成沙丘场长达百年演化过程的定量模拟得以实现。定量再现出的在单一和多个主风向作用下风成沙丘场的演化过程和不同的沙丘形态与野外观测结果定性一致,沙丘移动速度随高度的变化规律和沙丘高度随沙源厚度变化的规律与野外观测结果定量吻合。由此得到了野外观测难以获得的来流风速和主风向数、沙源厚度和粒径对沙丘场演化过·程、沙丘形态和移动速度以及相互碰撞影响等的定量规律,并给出了1)沙丘场形态发生变化的临界沙源厚度；2)沙丘高度和移动速度分别随沙粒粒径和来流风速等因素变化的拟合公式；3)沙丘尺度的概率密度分布函数等。在将该模型推广到不同地表情形的沙丘场演化和扩展过程后,1)给出了火星新月形沙丘在演化过程、形态和移动速度等方面与地球新月形沙丘的定量上差异,以及火星在目前条件下仍可形成沙丘并在火星沙尘暴的作用下发生移动的理论分析依据；2)沙漠和荒漠边缘的扩展速度以及随风速、粒径以及植被盖度和草方格铺设方式及其退化率变化的定量规律；3)草场沙化强度随单位面积过牧量和放牧面积变化的拟合公式。本文的研究为沙漠或荒漠边缘扩展速度的预测、相关防治措施的设计及其有效性的准确评估提供了可靠的定量分析途径和理论依据。更多还原

【Abstract】 Desertification or sand desertification has been becoming one of the severe environmental problems in China, particularly in Northwestern China. It is of a very important and practical meaning to quantitative predicting the spreading rate of desert boundary and evaluating the effectiveness of related prevention strategies. The evolution process of Aeolian dune field is a typical issue involving multiple temporal and spatial scales which is still out of accurate description through current field observation on earth and other planets like Mars or successful simulation with existing numerical models. Therefore, to establish an available method aiming for the evolution process of aeolian dune field, as an exploratory probe into multi-scale science, is of an urgent necessary and significant scientific value.This dissertation put forward a universal scale-coupled model concerning aeolian dune fields. This model incorporates underlying physical processes including the collision and saltation of sand particles, the formation and development of wind-blown sand flow, and the evolution of sand dune fields as well as the coupling among them which spans three specific tempo-spatial scales ranging 8-9 orders of magnitude. It therefore is suitable for various incoming wind and surface conditions. The main highlights of innovation in this dissertation are:1) to establish a new mode for calculating local wind velocity fields around sand dunes so as to obtain a more reasonable and realistic description to the variation of wind velocity raised by the change of surface morphology in a dune field and 2) to introduce a meso-scale "sand body element" which is much smaller than the scale of dune fields in macro but containing micro information on the movement of sufficient number of sand particles, for the first time put forward two new statistic quantities in the study of wind-blown sand movements, i.e. the coverage factor and transportation factor of "sand body element", and therefore build a "bridge" between macro quantities such as the thickness and transportation length within a given period of "sand body element" and micro quantities describing the erosion and deposition of sand surface, rebound and ejection of sand particles including the average velocity particles impacting sand bed, saltation length and time etc. The novel model proposed in this dissertation made a quantitative simulation of the whole evolution process of an aeolian dune field with an area of hundreds of square kilometers over one century.Based on the novel model, this dissertation successfully reproduced the whole formation and evolution processes of aeolian dune fields under the action of single and multiple prevailing wind directions. The simulated achieved a qualitative agreement in dune pattern and a quantitative correspondence in changing law of dunes’ migration speed with dune height and that of dune height with thickness of sand supply comparing with field observations, and further revealed quantitative laws on the influence of incoming wind velocity, kinds of prevailing wind directions, thickness of sand supply and sand diameter to the evolution of dune fields, dune patterns, and collision behaviors of sand dunes. Results presented 1) the threshold thickness of sand supply determining dune patterns; 2) fitting formula governing the dune height and migration speed varying with sand diameters and incoming wind velocities; and 3) a probability distribution function of dune size etc. By extending this model to describe the evolution and spreading of dune field under various surface conditions, it can realize 1) the quantitative differences between barchans dunes on earth and Mars in evolution processes, dune patterns and migration speeds etc. and theoretical evidence for the possibility of forming dunes under the action of sand storms taking place on Mars under the contemporary atmospheric conditions; 2) boundary of desert and the quantitative laws of its spreading rates changing with wind velocities, sand diameters, vegetation coverage, laying manners of straw checkerboard barriers and grassland degradation rates etc.; 3) a fitting formula describing the grassland degradation intensity varying with overgrazing sheep units per area and grazing area. The results given out in this dissertation provided reliable quantitative analysis methods and theoretical evidences for predicting the spreading rate of the boundary of deserts, designing related prevention strategies and evaluating their efficiency.更多还原