Three Decadal Morphodynamic Evolution Of A Large Channel Bar In The Middle Yangtze River: Influence Of Natural And Anthropogenic Interferences

The study focused on the question of how the natural and human forcing has affected the morphodynamic evolution of bars in the middle Yangtze River. A large channel bar was investigated using satellite remote sensing data during 1986-2017. A novel algorithm was proposed to select images acquired at the similar river stage. The bar was delineated from selected images and surface area, volume and centmid were quantified to examine bar morphodynamics. The results indicated a four-stage evolution process, including a steady extension (1986-1998), a quick shrinkage (1998-2002), a fluctuated shrinkage (2002-2012), and a fluctuated extension (2012-2017). The first stage was characterized by the steady growth at a rate of 0.087 km(2)/a, and a centroid translation towards the inner bend by 400 m. The second stage was the most dynamic episode, with the bar degradation at a rate of 0.71 km(2)/a, and a downstream migration by 850 m. In the third stage, the bar was eroded at a rate of 0.05 km(2)/a, and migrated towards the inner bend by about 80 m. In the fourth stage, the bar was deposited at a rate of 0.12 km(2)/a, and traveled upstream by around 400 m. The bar volume exhibited the similar trend. The analysis showed that the four stages were separated by the consecutive floods, the closure of Three Gorges Dam and the installation of two river engineering works. The results suggested that the bar evolved much more dynamic under the natural flow (1985-2003) than that of the regulated (2003-2012). The findings revealed that the evolution was largely governed by the flow and sediment supply while the human forcing can affect the process by altering these regimes. This study also demonstrated the great usefulness of combining multi-temporal imagery and river stage records in the assessment of bar morphodynamics.