Numerical investigation of downstream variations in bed‐surface characteristic grain size

Abstract The understanding of downstream fining in the characteristic grain sizes of river beds is well established, whereas downstream coarsening remains poorly understood and is often considered counterintuitive. This study has employed a one‐dimensional (1D) coupled mathematical model of nonuniform sediment transport to elucidate the mechanisms controlling downstream fining/coarsening, with emphasis on the role of sediment supply. To ensure physical realism, the model was first benchmarked against the classic field dataset from the East Fork River, Wyoming. The simulation reasonably reproduced general trends in channel degradation and surface coarsening (armoring) observed during the 1975 snowmelt flood, confirming the model's ability to capture sorting processes under sediment‐deficit conditions. Building on this validation, numerical experiments were performed for two grain‐size classes (sand and gravel) under both clear‐water flow and overloaded sediment‐supply conditions. The results show that sediment supply and its composition relative to that of bed materials play an important role in controlling downstream variations in characteristic grain sizes. Downstream coarsening and bed aggradation were observed at a bed slope of , far below the slope range (greater than 0.02–0.03) associated with the mobility reversal hypothesis proposed by Solari and Parker (2000). The simulated downstream coarsening can be attributed to the deposition of substantial amounts of fine sand, which causes pronounced surface fining upstream; as this fining effect weakens downstream, the characteristic bed‐surface grain size increases downstream. By integrating field validation with theoretical exploration, this work demonstrates that downstream coarsening can be a predictable outcome of specific imbalances between sediment supply and transport capacity, providing a physically based explanation that warrants further verification through laboratory experiments and field observations.