Bed Load Sediment Transport and Morphological Evolution in a Degrading Uniform Sediment Channel Under Unsteady Flow Hydrographs

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.

Graded bed load transport in sediment supply limited channels under unsteady flow hydrographs

“…As shape parameter  was already found to have a negligible effect on overall bed load yields [Wang et al, 2015;2019] , within an adjusted general power law relationship:…”

“…Flow hydrographs are generally comprised of a quasi-steady base (or antecedent) flow of magnitude Q b , with a superimposed unsteady flow component that increases to a peak flow Q p over the rising hydrograph limb of duration T R before decreasing back towards Q b over the receding limb of duration T F . The overall magnitude, unsteadiness and shape of unsteady flow hydrographs have been described by three parameters: (i) the total water work W k ; (ii) the unsteadiness  HG ; and (iii) the hydrograph asymmetry , each which have been detailed in previous studies [Wang et al, 2015;2019]. Here, the definition of each parameter is presented briefly.…”

“…Finally, simple temporal ratio  of the rising and falling limb durations was also used [Wang et al, 2015;2019] to characterize the hydrograph shape or asymmetry, defined as follows:…”

“…One of the issues with this combined hydrograph parameter is that it proposes implicitly that the separate influences of W k and  HG on measured bed load yields and other sediment transport quantities are equivalent, which was found not to be the case for the uniform sediment experiments conducted in Wang et al [2019]. Indeed, Wang et al [2019] proposed a modified combined hydrograph parameter  m to take account of the fact that W k had a far more dominant influence on uniform bed load sediment yields than  HG , with  m taking the form:…”

“…Similarly, a bed load yield model proposed by Waters and Curran [2015] attempted to combine the influence of total water work and unsteadiness into one hydrograph descriptor parameter. As this model was only tested against their own data, its wider applicability was questioned in a recent study by Wang et al [2019], who themselves derived a modified hydrograph descriptor to better fit a wide range of both uniform and graded sediment transport yield data. As such, the current experimental study builds on the preliminary findings from Wang et al [2015Wang et al [ , 2019 and Waters and Curran [2015] to provide an improved model that better correlates sediment transport properties, overall bed load yields and bed grading characteristics with individual and combined hydrograph parameters using data obtained from a new series of flume experiments, as well as data from other recent unsteady flow sediment transport experimental studies.…”