Influence of Grain Arrangement on Particle Mobility and Patterns of Variability Between Gravel Bed Rivers

Abstract The grain‐scale arrangement of gravel is important in controlling the stability of river beds, sediment supply and geomorphic adjustment of river channels. However, the influence of gravel bed texture on grain mobility is difficult to quantify because differences in flow history and sediment composition influence the development of bed structure between different channels. In this study, gravel surface observations obtained from Structure‐from‐Motion photogrammetry were compared with particle entrainment resistance forces, obtained using in situ grain force tests across 45 exposed gravel patches (2,921 grains), to investigate the influence of grain arrangement on particle mobility between rivers. Grain sorting, imbrication and surface roughness were predominantly controlled by particle size. Even when differences in grain size were accounted for, correlations between individual surface‐structure metrics and grain mobility were weak, suggesting additional complexity between sites such as grain shape variability. Each of the sampled beds was categorized into one of four groupings, based on combinations of bed surface‐structure and grain shape, using cluster analysis. Grain mobility was significantly different between bed types; for example, sites with well sorted, flat and imbricate grains were ∼25% more stable than sites with spherical, non‐imbricated grains. These findings are important for river modeling and management, because they can be used to develop better predictions for grain mobility thresholds at the‐scale. The classification of surface textures according to the four identified bed types could help to predict grain mobility with limited data, but this would require further validation.