Large meandering bends with high width-to-depth ratios: insights from hydro-sedimentological processes

Abstract

Morphodynamics and hydro-sedimentological processes in meandering rivers are one of the most complex phenomena observed in alluvial channels. Because of the complex nature of the interaction between flow structure, morphology, sediment transport, and bank roughness, a better understanding of these morphological units is needed, particularly for low-gradient rivers characterized by large meandering bends with high width-to-depth ratios. The present research provides an accurate description of the interaction between suspended bed-sediment transport, flow structure, and bed morphology on three consecutive bends characterized by width-to-depth ratios higher than 50. The study focuses on a selected reach of the Colastiné River, which is a secondary channel of the Paraná River, Argentina. Acoustic measurement techniques with high spatial-time resolution were employed during two different events - a bankfull and a medium-flow stage event - to capture the three-dimensionality of the flow velocity, suspended bed-sediment transport, and variations in bed morphology. Although the core of maximum velocity shifts from bank to bank at the bend entrance, following the thalweg shifting, the suspended bed-sediment concentration remains in the center of the channel because of the strong influence of the secondary currents and the bed morphology. Two types of secondary flows are well-defined in the cross section: a unidirectional flow toward the outer bank generated by the topographic steering effect along the point bar, and the classical helical motion confined to the thalweg zone. Close to the outer bank, the core of maximum velocity shifts toward the center of the channel because of the presence of macro-roughness induced by banklines and downed trees, which in turn generate high turbulence patterns along the outer bank. The bed morphology shows an extended point bar occupying almost half of the channel width (in the cross section apex) and spreading downstream to the entrance of the following bend. The thalweg shows an abrupt change from bank to bank, producing high curvature at each bend entrance. The findings reported herein show a lack of correlation between the cores of maximum velocity and suspended bed sediment, exhibiting different behaviors than those observed in smaller alluvial channels with lower width-to-depth ratios.