塔克拉玛干沙漠简单线形沙丘形态动力学过程研究

采样分析、风沙观测以及计算结果表明,在不同风向作用以及沙丘形态参数改变等因素的控制下,简单线形沙丘表面起动剪切速度有明显的差异。表面的气流强度随风向的改变而有所不同。气流与沙丘正交时迎风一侧的输沙率远大于背风一侧同地貌部位的输沙率,但当气流与沙丘走向斜交时,沙丘两侧同地貌部位的输沙率差异不是太大,这是线形沙丘在动力学过程中能保持其形态的最主要原因之一。风向的多变以及沙丘形态的变化使风沙流内边界层内随高度的风速与所观测的输沙率虽也表现为幂函数关系,但相关性较低。沙丘表面被输移的沙物质的粒度分布与下垫面物质组成有一定的差异,在中低强度的起沙风范围内,被输移的沙物质的平均粒径小于下垫面沙物质的平均粒径,且随风速或输沙率的增大,粒度组成也趋于统一。总体上,地表沙物质的输移仍是一随机过程。

Studies on the Morphodynamic Processes of Simple Linear Dunes in Taklimakan Desert

Field experiments show that the airflow strength varies with wind directions on the surface of simple linear dunes. The airflow directions are similar with that Tsoar (1983) suggested, but the airflow shows some characteristics of the secondary flow in the lee side of the dune. The sand flux at the stoss slope foot is much higher than that at the lee side foot of dunes when the airflow directions are perpendicular with the dune trend. In comparison there are slight divergences of sand flux on both sides of dunes when the airflow directions are deflective with the dune trend. It is one of the main mechanisms for linear dunes maintaining their morphology in the dynamic processes. The sand flux within sand\|flowing boundary layer has not near relation with wind velocity because of the variations of wind directions and landforms. The particle sizes of sands transported by wind have some divergences with that of the underlaying materials. The mean grain size of the sands being transported is smaller than that of the underlaying materials under the controlling of the medium\|low wind activities. In generally, the sand transportation on the surfaces of dunes is a stochastic process. Under the controlling of two dominant wind directions the simple linear dunes are accumulated and elongated. The sand\|drifting winds in east direction are responsible for much of sand transportation than that in west direction do, so there are lateral migration for the simple linear dunes. The lateral migration of the simple linear dunes, time scale of the dune formation, and the other factors result in the development of the compound/complex linear dunes.