Abstract Surface grain size in gravel‐bed rivers is inherently heterogeneous, with marked spatial variability that remains challenging to predict, even in the presence of regular bedforms. This study investigates surface grain size variability on self‐formed alternate bars at the scale of a single bar unit, and is divided in two parts. In Part 1, controlled laboratory experiments were conducted in a straight flume with fixed banks under steady water discharge and sediment supply conditions. High‐resolution maps of bed topography and surface grain size were created using close‐range photogrammetry and automated image analysis, while numerical simulations provided estimates of the flow field and bed shear stress distribution. Experiments show the formation of initially migrating bars, which gradually slow down, eventually leading to a steady configuration. Analysis of this equilibrium state reveals recurring periodic patterns in bed topography, shear stress, and grain size. Coarser sediment patches predominantly occur on the stoss side, upstream of the bar crest, while finer sediments concentrate near the bar front, upstream of pools. Grain size distribution is not directly correlated with bed elevation or bed shear stress but displays a systematic upstream shift with a consistent phase lag across all experiments. These patterns were unaffected by changes in discharge or channel width. Moreover, our analysis shows that the flow deflection induced by bars limits sediment transport on bar tops, which explains the lack of migration and the relatively long bar wavelength. In Part 2, a theoretical model is formulated to provide a mechanistic interpretation of the experimental results.
Redolfi et al. (2026) studied this question.