Mechanism of Morphology Adjustment of The Chenglingji-Jiujiang Reach in the Middle and Lower Reaches of the Yangtze River after Impoundment of the Three Gorges Dam

The impoundment of Three Gorges Dam(TGD) has greatly altered the water and sediment conditions in the middle and lower reaches of the Yangtze River, resulting in the channel erosion continuously developing downstream: during the period of 2002-2008, the scouring mainly occurred in Yichang to Chenglingji. After 2008, the erosion from Chenglingji to Jiujiang has intensified. However, the current research in the Chenglingji to Jiujiang reach is based on the difference in the amount of scouring and silting in longitude channel dimensions. Little work has been undertaken to reveal the reason and mechanism. Therefore, it is extremely important to carry out the research on the mechanism of the adjustment of scouring and silting of section. In this study, a total of 232 measured fixed sections of the Chenglingji to Jiujiang reach, 473 km in length, after the impoundment of the Three Gorges Reservoir from 2001 to 2016 were used to assess the channel adjustment characteristics of the reach, including the change of the river patterns scoured and the variation of the reach-scale parameters of the medium flow channel of the reach, and the driving mechanism of the adjustment is further studied.

The results show that since the experimental impoundment in 2008, the scouring of the Chenglingji-Jiujiang reach mainly occurred in medium flow channel, which is more obvious than that in the initial storage of the Three Gorges Reservoir, and the part between medium water level and bankfull water level was slightly silted; it is believed that the channel adjustment changes are directly driven by the alterations of flow duration and sediment reduction.In addition, the increase of the implementation of the waterway regulation engineering in the middle and lower reaches of the Yangtze River since 2008 has also aggravated the erosion of the medium flow channel, but it is not decisive. The main driving factor remains the water and sand conditions.This paper derived a critical threshold by analyzing the empirical relationships between the reach-scale medium flow channel dimensions and the fluvial erosion intensity of each flow interval, which is of morphological significance in shaping the medium flow channel. This result can provide reference for the study of the impact of the impoundment of the Three Gorges Dam on the evolution of the riverbed downstream the dam.