Multi Grain‐Size Total Sediment Load Model Based on the Disequilibrium Length

Abstract

AbstractIn natural rivers, sediment heterogeneity and flow variability control the diversity of transport modes that occur. Although these different modes contribute to the total sediment transport, a law extending from bed load to suspended load that is, relevant for a wide range of sediment mixtures and flow conditions is lacking. Besides, a transport‐limited assumption is often made in modeling of fluvial morphodynamics and thus potentially misses under‐/over‐capacity regimes associated with a particular range of grain sizes and hydraulic conditions. We present a Multi Grain‐Size Total Load model based on widely accepted concepts of sediment transport and developed within the transport length framework in combination with an erosion‐deposition formulation. The new transport length model captures the diversity of transport modes as a physical continuum. Transport capacities for single or bimodal grain sizes are reasonably predicted when compared to published data and scale with the bed shear stress through a continuously varying exponent linked to the characteristic transport height. Modeled transport lengths extend over several orders of magnitude at given flow conditions. Extremely long distances suggest that suspended transport is probably never at capacity. The model can be extended to populations of various grain sizes with a threshold of motion corrected from hiding‐exposure. However, further experimental constraints are needed to better describe entrainment and saltation in strongly heterogeneous bed load transport. The new theoretical formalism we introduce paves the way for a Multi Grain‐Size Total Load Sediment Transport model that includes the variety of transport modes in both non‐stationary and stationary regimes.