金字塔沙丘形成的动力条件分析

ANALYSIS ON DYNAMICS CONDITIONS OF PYRAMID DUNES FORMATION

金字塔沙丘（或星状沙丘）形成的动力条件主要有三个方面：一是在非沙质床面的气流场辐合区，一般具有三个以上近似均布的主风向，各主风向的风力强度、出现频率及持续时间不尽相同，但年总输沙能力较为接近，而且月输沙能力具有明显的季节变化；二是不太充裕的沙源和高浓度不饱和风沙流；三是不同尺度地形条件的动力作用，和沙漠与戈壁下垫面的热力影响。在沙质床面，往往由于高浓度饱和或过饱和风沙流的作用，导致新月形沙丘或新月形沙丘链的变态，或重新组合成金字塔状的沙丘形态，但不可能发育为典型的金字塔沙丘。

The dynamics conditions of pyramid dunes (or star dunes) formation have three respects. First the flow convergent field on the non sandy bed possess: 1) over three dominant wind directions which are approximately uniform distribution, 2) their intensity and frequency and sustained time in each time are different, 3) their total sand transported quantity in a year is similar, and 4) monthly sand transported quantity has obvious seasonal changes. Second, none abundant sand source and the wind sand flow with high sand transported quantity and less accumulation. Third, the dynamics action of different scale topography, and the heating power influence on desert and Gobi under layer. On the sandy bed, the high concentration saturated wind sand flow or oversaturated wind sand flow often lead to the abnormal status of barchan dunes and barchan dune chains, or re build new pyramid like sand dunes which cannot be developed into the typical pyramid dunes.