大型海底、海岸和沙漠沙丘的形态和迁移特征

摘要

The migration of submarine, coastal and desert dunes is influenced by their morphological parameters. Analysis of the wave height and length of some representative large-scaled dunes indicates that these parameters are highly different among the submarine, coastal and desert dunes, with the height being the smallest for the submarine dunes and the largest for the desert dunes for the same range of lengths. Such phenomena are mainly due to the difference in the thickness of the fluid over the bedforms and secondly due to the sediment supply. In shallow marine environments, the water depth represents a limiting factor for upward growth of the dunes, whilst on land the thick atmospheric layer provides the space for dune growth; in deserts, relatively abundant sand supply, as compared with the coastal environment, results in full growth of the dunes. There tends to be deviations of the height-length data points from statistical H-L curves; numerical experiments demonstrate that such deviations may be associated with the bedload transport rate. Along the transport pathways, if the transport rate increases or decreases, then the resultant H-L data may systematically deviate from the standard curve; likewise, rapid decrease in the transport rate can reduce significantly the dune migration rate, which causes upward accretion of sand and may form extremely high dunes. This behavior can be used to control the rate of dune migration, on the basis of a distance-duration curve and appropriate design of dune height. An example of deriving the distance-duration curve is provided in the present study. The designed height may be realized by artificial measures such as a fencing system, low walls and trenches in front of the dunes, and the deployment of armoring layers on the bed.%海底、海岸和沙漠沙丘的迁移与其波高、波长等形态参数有关.对一些代表性大型沙丘的分析表明,海底、海岸和沙漠沙丘的形态特征存在着较大的差异,在相同波长条件下,海底沙丘波高最小,海岸沙丘波高次之,沙漠沙丘波高最大.其原因主要是流体厚度的差异,其次是物质供给条件的不同:浅海地区的水深限制了沙丘向上生长,而陆地气流厚度较大,给沙丘的向上生长提供了较大的空间;沙漠环境的沉积物供给远大于海岸环境,因而导致沙丘高度的差异.沙丘形态参数经常偏离统计的波长-波高曲线,数值实验结果显示,这与推移质输运率有关.推移质输运率的沿程变化可以使形态参数系统性地偏离波长-波高曲线;推移质输运率的沿程突变可使沙丘迁移受阻,造成沉积物的垂向堆积,形成超高的沙丘.由于大型沙丘的迁移特征受控于推移质输运率和沙丘高度,因此可以通过波高设计来计算迁移距离-历时曲线,进而控制沙丘迁移动态.文中给出了一个获取迁移距离-历时曲线的算例.设计波高可通过人工篱笆、隔挡墙、沟渠、表面护层等措施而实现.