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Combining Local 3-D Flow Information With Unsteady Fractional Sediment Transport to Assess Pool-Riffle Self-Maintenance

Author(s): Esmaeel Bayat, Elham Vahidi, Jose F. Rodriguez, Patricia M. Saco, Gustavo A. M. De Almeida

Linked Author(s): José Rodríguez

Keywords: Fluvial hydraulics, sediment transport, size distribution, unsteady flow, stream

Abstract: Pool-riffle sequences are one of the most common geomorphological features in streams. The morphodynamics of pools and riffles has been the subject of research for over a century and has more recently attracted intense attention for their central role in providing habitat diversity conditions both in terms of flow and substrate. Initial efforts to explain the long-term stability of the pool-riffle sequences (often referred to as self-maintenance) has focused almost exclusively on cross sectional flow characteristics (either average or near bed velocity or shear stress). More recently, attention has focused on three-dimensional flow features and sediment size characteristics, but this has been done in a compartmentalised way, with studies either focusing on one or the other aspect. This paper bridges the gap between these two aspects by combining the effects of flow distribution with fractional sediment transport and sorting. Firstly, detailed 3-D flow patterns from experimental data are used on typical pool-riffle sequences to reconstruct shear stress distributions and near-bed streamlines for different flow condition. Local instantaneous bedload transport was obtained by combining these experimental results with field and simulated data from an unsteady fractional sediment transport and bed evolution model of an existing stream. Average cross sectional and �local� sediment transport (i. e. obtained by assuming sediment transport follows the streamlines) are evaluated in this paper as indications for self-maintenance. The results demonstrate that local flow can produce near-bed self-maintenance conditions even under conditions in which average flow could not, accounting for the 3-D flow field results in more self-maintenance episodes compared to traditional methods based on cross sectional variables. These results are relevant for the design of artificial pools and riffles in stream restoration projects aiming at improving habitat conditions


Year: 2017

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