A conceptual model of bed deformation and bank erosion in the bifurcated reaches of the Middle Yangtze River

Abstract

The evolution of bifurcated reaches is complicated owing to flow and sediment diversion at bifurcation and subsequent differential branch deformation. Previous numerical studies have mostly focused on short-term bed deformation in bifurcated reaches, overlooking the process of bank erosion, and their practical application is often limited by high computational costs and the necessity for high-precision topographic data. Therefore, a conceptual model was proposed to simultaneously simulate bank erosion and bed deformation processes of different branches, calibrated and verified using the measurements from two typical bifurcated reaches in the Middle Yangtze River. Results show that: (i) the main branch experienced scouring, while the secondary branch showed deposition or slight scouring, with the calculated cross-sectional profiles closely matching the measurements; (ii) the mid-channel bar adjacent to the main branch exhibited more intense bank erosion due to a higher flow diversion ratio, with a bank retreat width of about 10 m, further promoting the development of the main branch; and (iii) the fluvial erosion intensity of incoming flow-sediment conditions showed a positive correlation with the deformation difference between the branches, and short branches with small bifurcation angles experienced stronger erosion compared with long branches. Besides, engineering interventions that rose the riverbed elevation at the main branch entrance by 1 m could switch the roles of the main and secondary branches, potentially promoting the transition from a two-branched system to a single-channel system.