Research on the evolution of shoals and waterway hydrodynamic characteristics in sandy-pebble riverbed reach of the Middle Yangtze River after the operation of the TGD

The joint operation of cascade reservoirs in river basin has induced significant riverbed scouring downstream, leading to adaptive hydrological adjustments such as lower water levels under equivalent discharges, altered slopes, and increased flow velocities. These changes have resulted in a "steep slope and rapid flow" phenomenon, posing challenges to navigational safety. This study focuses on the sandy-pebble reach in the middle Yangtze River. Based on an analysis of field-measured hydrological, sediment, and topography data from 2002 to 2024, this research examines the characteristics of riverbed scouring intensity, variations in shoal and channel areas, and changes in flow conditions-specifically low-water levels, water surface gradient, and maximum flow velocity-under consistent discharge. The main findings are as follows:① In the reach from Yichang to Dabujie, both low-water and bankfull channels exhibit cumulative scouring and thalweg incision trends; The shoal area initially decreased before gradually stabilizing, while the area of deep-water channels rear progressive increased. ② In the sandy-pebble reach, the low-water level under the same flow condition shows a continuous decline; the water surface gradient remains relatively stable between Yichang and Zhicheng but increases significantly markedly between Zhicheng and Dabujie reaches low-flow periods, which are prolonged, thereby intensifying the "steep slope". ③Ongoing riverbed scouring and declining low-water levels have enhanced upstream scouring in the sandy-pebble transition zone; Additionally, the "rapid flow" phenomenon, characterized by elevated surface flow velocities in the Lujiahe reach during low-water periods, has become more pronounced. ④ In the absence of large-scale human interventions, minor riverbed scouring is expected to persist in the sandy-pebble riverbed reach; As strong erosion continues in the downstream sandy riverbed, the "steep slope and rapid flow" characteristics in the transition zone are likely intensify.