Experimental study on flow structures in arc-shaped bank erosion zones

Arc-shaped bank erosion is a common type of bank erosion in the middle and lower reaches of the Yangtze River.It has the characteristics of complex three-dimensional (3-D) flow structures, rapid expansion, and strong destructive capacity.The general and pocket types of arc-shaped bank erosion models were designed on the basis of actual bank erosion events.3-D flow fields in general and pocket-type bank erosion zones were obtained from generalized tests by using particle image velocimetry technology.Flow structure and turbulence characteristics were analyzed.Results showed the following: ① The arc-shaped bank erosion zone can be divided into the main flow, mixing, and backflow zones in accordance with the flow structures and distributions of turbulent intensity.② In the mixing zone, the velocity gradient and turbulence intensity were large, and the width of the mixing zone gradually increased along the main flow direction.The maximum values of the flow velocity at the backflow zone of the general and pocket types were approximately 0.19 and 0.27 times the average velocity of the mainstream, respectively.③ The vertical average flow velocity at the backflow region of the general bank erosion zone was 48%—76% larger than that in the pocket-type bank erosion zone.The backflow center of the general bank erosion type was closer to the outer side of the bank erosion zone than that of the pocket-type bank erosion zone, thus facilitating the transport of sediment from the bank erosion zone to the main flow area.④ Under the condition of high mainstream flow velocity, the exchange rate between the water in the general-type bank erosion zone and the main flow was higher than that in the pocket-type bank erosion zone, and the opposite was true under low mainstream flow velocity.