Hump-Shaped Enhancement of Coarse-Grain Transport in Sediment Mixtures Induced by Fine Grains of Different Sizes

Bedload transport of sediment mixtures is fundamental to river morphology and impacts aquatic ecology. Adding fine grains enhances coarse-grain transport, yet controlling mechanisms remain elusive. Employing discrete element simulations, we examine how fine-grain size and proportion influence coarse-grain flux. Results reveal a counterintuitive, hump-shaped enhancement of coarse-grain flux with fine-grain size. Enhancement peaks as intermediate-sized fine grains are stuck near the surface, forming a faster layer that carries coarse grains (conveyor belt), whereas much finer grains percolate deeper, only smoothing the bed to reduce friction (lubrication). With sufficient supply, percolating fine grains can fully fill the quasi-static coarse-bed voids and remain in the mobile surface layer, sustaining enhancement. We propose that the dimensionless burial depth of the fine-grain layer top is the key controlling factor and is found inversely proportional to enhancement. These results suggest a framework centered on fine-grain burial depth for future research to understand enhanced coarse-grain mobility.