Acta Geographica Sinica ›› 2019, Vol. 74 ›› Issue (11): 2411-2427.

• Climate Change and Ecological Environment •

### Adjustment in the main-channel geometry of the lower Yellow River before and after the operation of the Xiaolangdi Reservoir from 1986 to 2015

WANG Yanjun1,WU Baosheng1,SHEN Guanqing2

1. 1. State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084
2. Yellow River Institute of Hydraulic Research, Zhengzhou 450003, China
• Received:2018-08-22 Revised:2019-08-01 Online:2019-11-25 Published:2019-11-01
• Supported by:
Key Program of National Natural Science Foundation of China(51639005);National Key R&D Program of China(2017YFC0405202);National Key R&D Program of China(2016YFC0402406)

Abstract:

Previous studies indicate that the lower Yellow River experienced a continuous siltation period and a continuous scouring period during the past 30 years. However, the patterns of main-channel adjustments during these two periods are not clear. Based on measured discharge, sediment load, and cross-sectional data between 1986 and 2015, the changes in the morphological parameters (width, depth, and cross-sectional geomorphic coefficient) of the main channel were analyzed. The results showed that before the operation of the Xiaolangdi Reservoir (XLDR) from 1986-1999, the main channel shrunk continuously with decreasing width and depth. Because the decrease rate in width was greater than that in depth, the geomorphic coef?cient decreased in the reach above Gaocun. In contrast, for the reach below Gaocun, the decrease rate in width was smaller than that in depth, and the geomorphic coef?cient increased. After the XLDR began operating, the main channel eroded continuously, and the width and depth increased from 2000-2015. Because the increase rate in depth was obviously larger than that in width, the geomorphic coef?cient decreased in all sub-reaches. The cross-sectional geometry of the main channel exhibited different adjustment patterns during two periods. Before the XLDR operation, the main channel mainly narrowed in the transverse direction and silted in the vertical direction in the reach above Aishan (AS); in the reach below AS, the main channel primarily vertically silted. After the XLDR operation, the main channel adjusted by widening and deepening in the reach above AS; for the reach below AS, the main channel adjusted mainly by deepening. Compared to the decrease rates in main-channel width and depth during the siltation period, the increase rate in width during the scouring period was obviously smaller, while that in depth was larger. After continuous siltation and scouring, the main-channel cross-sectional geometry changed from relatively wide and shallow to relatively narrow and deep. The pattern of main-channel adjustment was closely related to the water and sediment conditions. For the braided reach, the geomorphic coef?cient was negatively correlated with discharge and positively correlated with suspended sediment concentration (SSC) during the siltation period. In contrast, the geomorphic coefficient was positively correlated with discharge and negatively correlated with SSC during the scouring period. For the transitional and wandering reach, the geomorphic coef?cient was negatively correlated with discharge and positively correlated with SSC.