沙质地表与砾质戈壁风沙运动对比研究——以敦煌莫高窟窟顶风沙运动为例

Contrastive Study on Sand Drift Movement over Sandl and and Gravel Gobi Desert in Extremely Arid Regions——A Case Study on Sand Drift Movement over Mogao Grottoes in Dunhuang

通过野外观测资料分析,阐明在沙质地表下,风速相当时不同风向风沙流的结构不同,即含沙量影响风沙流结构。一般随着含沙量的增大,各层的绝对输沙量相应增加,相对输沙量在下层增加,上层减少。但也有不一致的情况,本次野外观测就出现了下层减少上层增加的情况。戈壁风沙流结构与沙质地表显著不同,其风沙流结构呈独特的"象鼻"效应,即在一定高度有一个最大输沙量,且这个高度随风速的增大而增加。西北风作用下,自鸣沙山至窟顶输沙量逐渐减少。戈壁具有一定的阻沙效应,风速为7.7 m/s时,每100 m阻沙率达10%左右。沙山前沿固沙带对输沙量起着重要作用,有防护措施与无防护设施相比较,西北风的输沙量可减少95%左右。偏东北风在缺少沙源的情况下,主要以输送为主,偏南风实际输沙量明显小于偏西北风,输沙量由沙源地到窟区逐渐减少。

After analyzing the field-observed data, in this paper it is revealed that sand drift structures over sandy lands are different under different wind directions when wind speed is similar, that is sediment concentration affects sand drift structure. Generally, the absolute sand transport at all heights of windstream is increased with the increase of sediment concentration, whereas the relative sand transport is increased at lower height but decreased at upper height. However, there is also an inconsistent situation, in this field observation, for example, the sand transport was decreased at lower height but increased at upper height. Sand drift structure over Gobi desert is significantly different from that over mobile sandland. The roughness of Gobi surface increases with the increase of wind speed. With the increase of windstream height, the sand transport rate over Gobi desert has a unique ＂elephant nose＂ effect, that is the sand transport comes to a maximum value at certain height, and this height is increased with the increase of wind speed. Fidd observation reveals that sand transport is gradually decreased from Mingsha Mountain to Mogao Grottoes under northwest wind. Gobi desert with rough surface can block sand to a certain extent, and the sand blocking rate per hundred meters can be as high as 10 % when wind speed is 7.7 m·s^- 1. Sand transport under northwest wind can be reduced by 95 % after taking some measures for controlling sand drift. Northeast wind plays an important role in sand transport, sand transport under south wind is obviously less than that under northwest wind due to insufficient sand sources, and sand transport is gradually decreased from the sand-resource areas to Mogao Grottoes.