The influence of grain size sorting on the roughness parametrization of gravel riverbeds

Grain size and surface roughness play crucial roles in modelling flow resistance and boundary shear stress in fluvial systems. However, the impact of grain size sorting on surface roughness, particularly for gravel-bed rivers composed of poorly-sorted sediments, has yet to be elucidated. Here we utilize a stochastic model to simulate generic riverbed surfaces, investigating the influence of sediment sorting on roughness. Through comparison with field-acquired data, we explore the relationships between grain size, sorting, presence of textural patches, and local roughness. Our analysis reveals significant spatial roughness variations on surfaces with poorer sorting conditions, driven by stochastic grain arrangements. Notably, surfaces with poorly sorted grains exhibit meterscale patches, each with distinct roughness attributes. Consequently, upon characterizing the roughness of riverbeds made up of m-scale gravel bars, the sorting of the grains needs to be considered to account for the complexity of the relationships between water flow and riverbed.