黄河源区达日河段辫状-分汊河道形态分析

Morphological change of braided-anabranching channel along the Dari reach in the Yellow River Source region

黄河源干流的达日河段是卵砾石河床的辫状-分汊河道,其限制性的单一河段与不稳定的多汊段交替出现,后者的洲滩与汊道变化频繁,研究其形态变化及稳定性具有较重要意义。通过分析1987-2016年Landsat和Google Earth遥感影像,选取6个区域的典型群体沙洲或江心洲,得到河段内汊道数的时空变化过程,其中1987-2004年汊道数波动较小,2004-2016年由于径流量增加使得汊道数在波动中呈上升趋势。空间上汊道数受到河宽和节点的双重约束,节点的纵向间距小于10 km时,节点间距越小,汊道数越少;节点的纵向间距大于10 km,河宽越大,汊道数较多。达日河段的水流切割沙洲和汊道淤积交替进行,使辫状-分汊河段的平面形态变化剧烈且频繁,变化周期小于10 a,但某些最稳定的江心洲可保持30 a不变。

The source region of the Yellow River near Dari County in northern Qinghai-Tibet Plateau is a gravel-bed braided-anabranching reach,in which single and multi-thread channels develop alternately along the course.The Dari braided-anabranching reach with frequent scour and deposition,is important to study the fluvial processes and channel stability.Based on Landsat and Google Earth imagery in 1987-2016,typical groups of central bars and islands in six reaches were analyzed on temporal and spatial variation patterns of braiding channel count.The channel counts fluctuated slightly during 1987-2004.The channel counts increased due to an increase in incoming runoff in 2004-2016.The channel counts along the course are constrained by channel width and the nodes(i.e.,narrow section)of mountains and hills in the valley.When the longitudinal distance of adjacent nodes is less than 10 km,the shorter the node space,the fewer channel counts.The node space is larger than 10 km,the river width is larger,with the higher channel counts.Fluvial scour bars and branch deposition channels alternatively lead to dramatically and frequently morphological change with a period less than 10 years,but the most stable islands remaining unchanged for 30 years.