Riverbed evolution characteristics in the Hechangzhou braided reach under new flow-sediment conditions and waterway regulations

The Hechangzhou river reach is a typical braided reach and one of the most violently changed reaches in the lower reaches of the Yangtze River. After the impoundment of Three Gorges Dam, the sediment outflow decreased greatly, the new features of river bed evolution showed up and the navigation conditions changed accordingly in Hechangzhou braided reach. In order to restrain the scour development in the left branch and improve the navigation conditions in the right branch channel, three underwater submerged dams in the left branch were successively built. Based on a large amount of measured data, the evolution characteristics of Hechangzhou braided reach were analyzed. Results show that the riverbed in the Liuwei reach continued to cut down in 2009, accompanied by occurrences of bank line collapse and development of central bar. The control effect of three submerged dams is significant, and the measured diversion ratio of left branch in 2019 was about 64%, 12% lower than that of the highest 76% in 2002. The riverbed in the right branch changed from slow deposition to general scour, and the navigation conditions were improved. However, the left branch has experienced a significant scour in the local reach and two bank collapse events happened, which should be paid enough attention to.