Up-scaling Dune Morphodynamic Models for the Study of Bedload Transport in Large Rivers

This paper presents preliminary results from a first attempt to give a channel-scale perspective to models of sediment transport over dunes, thereafter integrating effects at all bedform scales. Recent morphodynamic models have demonstrated the relationships between composition and rate of sediment transport, and the probability distribution of bed elevation relative to mean bed level (Ps) over a train of dunes. These models are yet essentially theoretical and there exists only limited data to implement them. Lately, the analysis of Ps curves for new data from the Mississippi River revealed the potential interest of this approach for assessing, although mostly qualitatively for the moment, the sediment transport stage and the erosion-deposition pattern. Just as bed elevations vary over dune backs, crests, and troughs, bed elevations over a meander planform vary in the alongstream direction from bend scours to point-bar tops. The range and shape of channel-scale Ps curves, and their time variation, thus can provide information on bedload-transport conditions, especially in large rivers where it is difficult to obtain. The survey line in this study is located just upstream of Baton Rouge in Louisiana and is nearly 50-km long, hence comprising several meander bends and low-sinuosity reaches of the Mississippi River. Two surveys were conducted during different high-stage events. The bathymetry was measured with a high-precision echo sounder and a 1MHz transducer. Data on flow conditions are those reported by the USACE. The Ps curves compare well with those from experimental data sets at smaller scale. This study shows that depth-continuous models for sediment continuity are promising for scale-integrated analysis of sediment transport in large rivers.