Research Progress on Coarse and Fine Sediment Exchange Near Riverbed

Sediment exchange is one of the key issues in the evolution of riverbed erosion and deposition, and its study holds significant importance for river management and engineering practices. This study systematically reviews domestic and international research progress over the past three decades, focusing on the mechanisms of coarse and fine sediment exchange near the riverbed from four aspects: theoretical models, mathematical models, experimental methods, and field data analysis. The results demonstrate that existing theoretical models (e.g., upward flux formulas based on turbulent bursting, energy balance, and diffusion theories) largely depend on empirical assumptions, lack universally applicable mechanism descriptions, and face limitations due to insufficient field data validation. Although mathematical models describing three-state transitions among suspended load, bedload, and bed material have gradually incorporated probabilistic methods such as Markov chains, they still face challenges in accurately representing sediment exchange process under non-uniform flow conditions. Experimental studies reveal that medium-diameter sediments exhibit higher upward flux due to turbulent bursting and exposure effects. Moreover, formulas for exchange layer thickness dominated by sand wave migration require modifications to account for the influence of resistance transitions in sandy rivers. This study proposes three innovative future directions: (1) integrating prototype observation data to establish unified theoretical formulas for sediment exchange; (2) developing high-precision observation technologies to overcome the bottleneck in dynamically capturing random sediment collision processes; and (3) combining machine learning with physical models to build predictive systems for sediment state transitions. This review provides systematic references for advancing the understanding of sediment exchange mechanisms and optimizing engineering practices.