Bed Load Sediment Transport and Morphological Evolution in a Degrading Uniform Sediment Channel Under Unsteady Flow Hydrographs. Issue 7 (5th July 2019) - Wang, Le
Abstract: Flume experiments are conducted to investigate the intrinsic links between time‐varying bed load transport properties for uniform sediments and bed surface morphology under unsteady hydrograph flows, in the absence of upstream sediment supply. These conditions are representative of regulated river reaches (e.g., downstream of a dam) that are subject to natural flood discharges or managed water releases, resulting in net degradation of the river bed. The results demonstrate that the hydrograph magnitude and unsteadiness have significant impacts on sediment transport rates and yields, as well as hysteresis patterns and yield ratios generated during the rising and falling limbs. A new hydrograph descriptor combining the influence of total water work and unsteadiness on bed load transport is shown to delineate these hysteresis patterns and yield ratios while correlating strongly with overall sediment yields. This provides an important parametric link between unsteady hydrograph flow conditions, bed load transport, and bed surface degradation under imposed zero sediment feed conditions. As such, maximum bed erosion depths and the longitudinal bed degradation profiles along the flume are strongly dependent on the magnitude of this new hydrograph descriptor. Similarly, nonequilibrium bedforms generated along the flume indicate that formative conditions for alternate bars, mixed bar/dunes, or dunes are defined reasonably well by an existing morphological model and the new hydrograph descriptor. These findings provide a new framework for improved predictive capabilities for sediment transport and morphodynamic response in regulated rivers to natural or imposed unsteady flows, while their wider application to graded sediments is also considered. Key Points: Sediment transport under flow hydrographs show differential bed load rates, hysteresis, and yield ratios during rising and falling limbs A new combined hydrograph descriptor correlates well with sediment transport and bed morphology under no sediment supply conditions Intrinsic links between unsteady flow, sediment transport, and bed morphology are considered representative of regulated river reaches