Abstract
Rivers inherently show heterogeneous sediment sizes and can also show a strong sediment supply variability in time because of natural episodic events or as a consequence of human activities, which alter the characteristics and dynamics of alluvial bars at the macro-scale. The impact of the combination between sediment size heterogeneity and sediment supply variation, or even with other forcings (i.e. hydrology, channel geometry) remains poorly documented. In this work, a physics-based numerical model is applied on a trained reach of a sandy-gravel bed river to investigate the combination of these parameters on bar morphodynamics. The morphodynamic computations are performed with a two-dimensional depth-averaged hydrodynamic solver, internally coupled to a sediment transport and bed evolution module, which estimate the transport of graded sediment and model bed stratigraphy, respectively. A 1 km long reach of the Loire River at Bréhémont (France) is selected to conduct the numerical investigations. The interaction between several forcing mechanisms induces highly complex bar morphodynamic processes in this area.A comprehensive set of high-definition data is available, which allows to study the river morphodynamics for a succession of three flooding events and a period of low flows. Based on this model, a variety of scenarios is presented with the aim of exploring the implications of sediment gradation, geometrical and boundary forcing effects on in situ bars morphodynamics.
Highlights
Rivers often present a wavy bed topography due to the presence of periodic bars [1], corresponding to large sediment deposits alternating with deeper areas
Our actual understanding and the modelling of the impact of sediment grading on bar morphodynamics remains limited [20], recent research works [5, 7] have shown the relationship between graded sediment and bars properties, under simple configurations in terms of geometry, hydrology and sediment supply conditions
The present study is meant to better understand the phenomena induced by the combination of hydrology, sediment supply, channel curvature and graded sediment on bar morphodynamics in natural or trained alluvial riverbeds composed of low mode bars, i.e. alternate and central bars
Summary
Rivers often present a wavy bed topography due to the presence of periodic bars [1], corresponding to large sediment deposits alternating with deeper areas (pools). The present study is meant to better understand the phenomena induced by the combination of hydrology, sediment supply, channel curvature and graded sediment on bar morphodynamics in natural or trained alluvial riverbeds composed of low mode bars, i.e. alternate and central bars. To reach this goal, a two-dimensional fully-nonlinear model is used to simulate the morphodynamics of a 1 km long reach of the Loire river at Bréhémont wherein free, hybrid and forced bars coexist [3]. A variety of scenarios are proposed to study the bar morphodynamics in response to the various forcing mechanisms
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