Hydromorphodynamics of a Large River Confluent Meander Bend Through the Seasonal Flood Wave: Pantanal Wetlands, Brazil

Rivers confluences have gained more attention in the past decades with studies related to bed morphology, hydrodynamics, flow structure and sediment transport, in both asymmetrical and symmetrical angle junctions. In contrast, meander bend functioning is not yet well understood, and few works have regarded the theme. The Paraguay and Cuiabá rivers confluence located in the Pantanal, the world's largest wetlands, is an example of a confluent meander bend in large rivers (>100 m wide). The purpose of this study is to enhance the knowledge about the hydrodynamics and bed morphology changes throughout the seasonal flood wave. Flow velocity data were collected using an acoustic Doppler current profiler (ADCP) in cross-sections before and after the confluence, while bathymetric data were collected using a single beam echo-sounder. During the low-water level, the confluence presents a tributary-dominance (Cuiabá River) regarding momentum and discharge ratios. As result, a prominent recirculation cell has been produced toward the downstream corner, which is nonexistent in the high-water level survey. Besides, secondary velocities have changed their direction from clockwise to anticlockwise during the seasonal flood. Bed morphology has also presented changes during the seasonal flood, regarding the previous hydraulic conditions. The hydraulic mole effect applied by the Cuiabá River flow in the confluent meander bend interferes in the helical motion and as well as the position of the zone of maximum flow velocity. This study provides an important framework of the hydrodynamics and morphology functioning in confluent meander bend in large rivers in different stages of the seasonal flood wave.