FLOW STRUCTURE, BED MORPHOLOGY AND SEDIMENT TRANSPORT IN A LARGE MEANDERING RIVER

LUCAS DOMINGUEZ RUBEN; RICARDO SZUPIANY; JORGE ABAD; CARLOS RAMONELL; HECTOR DANIEL FARIAS

Conferencia; 2nd International Conference on the Status and Future of the World?s Large Rivers; 2014

Meandering rivers appears as one of the most common configuration in natural channels. Flow within a channel bend is characterized by helical flow (central region cell), which redistributes the streamwise momentum within the cross section, producing erosion and sedimentation conditions in the outer and inner bank respectively. At the bed, radial pressure gradient produce flow towards the inner bank. In addition to this classical central region cell that cover a large portion of the cross section, a cell with smaller secondary flow intensity and opposite in rotation, is often observed near the outer bank which is believed to play an important role in bank erosion processes. While there is a significant amount of studies that analyze the flow structure in bends, the majority of them correspond to numerical modeling, laboratories channels and, lesser extent, natural channels; thus, they mostly cover low width/depth ratios (typically from 5 to 20). Moreover, as a consequent of numerical and laboratory complexity, the understanding of sediment transport still presents a limited knowledge. The present study shows results of the flow structure, bed morphology and suspended sediment transport of a succession of three bends of a large meandering branch of the Paraná River, whose width/depth ratios range between 50 and 60. The data were obtained with acoustic Doppler current profilers (aDcp) and a specific calibration curve that relate the returned acoustic signal intensity and suspended bed sediment concentration. Differences respecting smaller width/depth ratios rivers were observed, such as: i) absence of secondary cell near outer bank, ii) small width of the central secondary cell relative to cross section, iii) non-correlation between cores of flow and suspended sediment and iv) important turbulent generation at the outer bank produced by river banks irregularities, such as erosion notches and steps and fallen trees.