Critical threshold of periodic point bar scour and sediment body transport path in tidal reaches: A case study of Fujiangsha reach, Yangtze River

Abstract

The evolution of point bars in changing sections of a downstream tidal current limit is periodic. Accordingly, assessing the critical morphology and hydrodynamic characteristics of point bar scour and the sediment transport process of scour sediment bodies can support river regulation and waterway maintenance. The frequent scour of point bars in changing sections of tidal current limits within the Yangtze River directly restricts waterway stability. This study examined the Fujiangsha reach of the Yangtze River, hydrological data on sediment transport, and riverbed topography from 1950. The Jingjiang bank tail exhibited an evolutionary cycle (siltation>scour>siltation), with a primary period ranging from 3-6 years. Additionally, certain morphological and dynamic conditions were necessary for scour. The Datong station flow (Q) ranged from 20,000-40,000 m 3 •s -1 for ≥180 days•yr -1 , enabling the bank silt layers to widen. Scour occurred during flooding and was concentrated in areas 5.0-7.5 km downstream from Ebizui. When Q≥40,000 m 3 •s -1 , scouring occurred in the bank middle and lower reaches, whereas Q≥50,000 m 3 •s -1 for >50 consecutive days, scour occurred at the tail as well. Moreover, the volume of the scour shoals increased with the number of high-flow days (≥60,000 m 3 •s -1 ). Bottom sand transport mainly occurred in the low-bank zone. Before the project's second phase, the longitudinal transport of the scouring sand bodies occurred as follows: Jingjiang bank > low bank on the north side of Shuangjiansha > Fubei anabranch. During the second phase, the longitudinal transport route changed to Jingjiang bank > Fubei anabranch. The Jingjiang bank volume was also reduced; thus, its development was controlled. Owing to changes in the longitudinal transport routes, dredging should be conducted in areas where scouring sand bodies are separately transported from the tail, thereby reducing the load of dredging and maintenance for the Fubei anabranch