Formation condition for concave-bank deposition in the meanders of the Middle Yangtze River and its application

In what kind of meanders concave-bank deposition (CBD) can occur remains an open question. This study investigated the characteristics and major influencing factors of CBD in the meanders of the Middle Yangtze River (MYR), and proposed a formation condition for CBD. Concave-bank deposition occurred in 10 of 23 meanders in the MYR, including two types: counter point bar (CPB) and concave-bank bench (CBB). CPB occurred in both mildly-curved and sharp bends, while CBB only occurred in sharp bends. Hydrodynamic conditions and sediment supply are two major factors controlling the formation of CBD. The incoming sediment load remained in an amount of 0.68 × 108 t/y at Jianli and 0.77 × 108 t/y at Luoshan after Three Gorges Project operation, therefore, CBD development was not limited by a lack of sediment and instead is mainly dependent on the hydrodynamic conditions in the MYR. Flow separation zones were observed in the typical CBD-type bends of the MYR, which indicated that CBD was the result of flow separation or recirculation. An adverse longitudinal water surface gradient is the necessary condition for the formation of concave-bank flow separation. Therefore, a formation condition for CBD was proposed based on this assumption, which considered the combined effects of channel resistance, riverbed slope, channel widening, and curvature. CBD happens when the channel widening and curvature terms are greater than the channel resistance and longitudinal channel slope terms. The proposed condition was verified based on the data of the 23 bends in the MYR and 26 bends in 12 other rivers. The results indicated that more than 85% of the bends were correctly identified. Therefore, the proposed formation condition provided considerably high accuracy.