Study on variation and impact of low water levels in steep riverbed and rapid flowing sections of middle reaches of Changjiang River

Zhicheng—Dabujie section is a typical steep riverbed and rapid flowing section in the middle reaches of the Changjiang River.Affected by the discharge of clear water, the water level during dry seasons in this section has been continuously declining along the course, severely impacting navigation.Based on measured data, the historical hydrological and sedimentary processes and the characteristics of water level changes during dry seasons in the Zhicheng—Dabujie section were analyzed.Combining with a two-dimensional hydrodynamic numerical simulation method, we predicted the upstream influence range due to the different downstream water level drops and their impact on navigation conditions.The results indicate that: ① Since the impoundment of the Three Gorges Reservoir, the sediment transport have significantly decreased.The water level decline during dry seasons in the study section exhibits a pattern of low in the middle reach and high at both ends, and the steep riverbed and rapid flowing phenomenon in the section has worsened.② The retrogressive influence caused by the water level decline during dry seasons at Dabujie mainly affects the section below Changmenxi, with the Zhijiang—Jiangkou section being most significantly influenced in terms of flow velocity and water depth.③ The local gradients of the 200-m sections in the Lujia River channel, Zhijiang channel, and Jiangkou channel will reach above 1.2‰, 0.9‰, and 0.5‰, respectively.The navigable water depth in some shallow sections will fail to meet navigation requirements, moreover, there are several sections with navigable water depths less than 3.50 m, which will severely impact navigation.The research findings can provide references for navigation and channel planning in the middle and lower reaches of the Changjiang River.