Mechanism of adjustment of scouring and silting of Chenglingji—Jiujiang reach in the middle reaches of the Yangtze River after impoundment of the Three Gorges Dam

The construction of a large-scale water conservancy project has caused the adjustment of the scouring and silting of the downstream river channel, which has a significant impact on the ecological environment, flood control and channel transportation. Since the trial experimental impoundment of the Three Gorges Dam in 2008, the downstream erosion of Chenglingji has intensified. To explore the adjust characteristics of scouring and silting and the driving mechanism of the downstream erosion of a river channel during a water control project, the spatial evolution characteristics and transverse morphological variation of the reach from Chenglingji to Jiujiang reach were analyzed. The results show that the scouring of the low flow channel in the Chenglingji—Jiujiang reach has intensified, and the morphological changes since 2008 have mainly been concentrated in the medium flow channel, of which the proportion of the evolution volume accounts for more than 90%;the transvers changes shows scouring and incising, while a sectional view displays narrowing and deepening. On the basis of the established empirical relationship between the reach-scale medium channel dimensions and the accumulated sediment transport capacity in each discharge interval during 175 m experimental water storage period from 2008 to 2016, this paper indicates that the discharge of 29 000—31 000 m³/s at Luoshan Station and that of 31 000—33 000 m³/s at Hankou Station can be regarded as characteristic discharges corresponding to the average top levels of braided channel bars, which are of morphological significance in constraining the passing flow in the medium flow channel and guaranteeing that the water transport capacity is the strongest. The geomorphic impact of flow below the characteristic discharge was more prominent during 2008—2016;this impact resulted in the adjustment of scouring and silting being mainly concentrated in the medium flow channel. Moreover, river channel erosion has intensified because, in addition to the construction of waterway regulation projects, the volume of transported sediment with d>0.125 mm has largely diminished.