三峡水库干流典型消落带泥沙沉积过程

Sedimentation Processes in a Typical Riparian Zone Along the Yangtze Mainstream of the Three Gorges Reservoir

选择三峡水库中游干流典型消落带断面，现场调查泥沙沉积速率随高程分布，探讨三峡水库特定水位调节模式和长江上游泥沙输移的季节性特征对消落带泥沙沉积的影响。结果表明，消落带泥沙净沉积主要发生在145-168 m高程，2010年累积净沉积厚度为1.1-39.9 cm，随高程增加逐渐减小，145-155 m平均泥沙沉积厚度为14.9 cm，155-168 m平均泥沙沉积厚度为2.6 cm；2013年累积净沉积厚度为3-80 cm，表明三峡水库干流消落带泥沙沉积过程迅速。泥沙粒径随高程增加逐渐变粗，体现在砂粒体积分数和中值粒径逐渐增大。消落带下部沉积泥沙颗粒组成与干流悬移质泥沙接近，而消落带上部沉积泥沙明显粗于干流悬浮泥沙。消落带泥沙沉积速率与水库水位调节密切相关，雨季水库低水位运行，长江悬移质输沙量的增加导致了消落带下部大量泥沙沉积；旱季水库高水位运行，长江悬移质输沙量减少导致消落带上部泥沙沉积速率降低，消落带河岸侵蚀对沉积泥沙贡献逐渐增大。消落带淹水时间随高程增加而缩短，长时间淹没有利于消落带下部泥沙沉积。综合推断，三峡水库干流消落带下部泥沙主要来源于雨季河流悬移质泥沙的沉积，而消落带上部沉积泥沙主要来自旱季消落带河岸侵蚀产沙。

A typical transect of the riparian zone along the Yangtze mainstream of the middle Three Gorges Reservoir is selected to determine the lateral distribution of sedimentation rates against elevation, and to explore the effect of the reservoir water level regulation and the seasonal variation of suspended sediment concentrations of the Upper Yangtze River. It is found that the net sedimentation mainly takes place within the portions between 145-168 m. The cumulative net sediment depth ranges from 1.1-39.9 cm during the 2010 survey and it decreases with the increase of the elevation. The average sediment depth is 14.9 cm between the＆amp;nbsp;elevations of 145-155 m, and 2.6 cm in the portions above. The sediment depth ranges from 3-80cm during the 2013 field survey, indicating the severity of the sediment accretion during the initial period of dam operation. The riparian sediments become coarser with the increase of the elevation, which indicates that the sandy fraction of the bulk sediment increases at a higher elevation. The sediment grain-size composition at lower elevations is close to that of the suspended sediment, while that at a higher elevation is coarser than the suspended sediment. The sedimentation in the riparian zone is closely related to the water level regulation induced by the regular dam operation. During a wet season when the reservoir water level is low, a high fluvial suspended sediment yield leads to a significant sedimentation in the lower portions of the riparian zone. During the dry season when the fluvial suspended sediment supply is limited, much fewer sedimentation occurs at the upper portions. Relatively longer inundation duration also contributes to a higher sediment depth in the lower portions of the riparian zone. It can thus be concluded that the sediment in the lower portions of the riparian zone is due to the fluvial suspended sediment yield during the wet season, while the sediment in the upper portions of the riparian zone originates from the bank erosion.