Structure and self-organization of imbricated gravel bed surfaces

Imbricated gravel beds have a distinctive surface morphology, which strongly affects riverbed stability and near-bed hydrodynamic processes. High-resolution digital elevation models combined with image processing techniques were used to quantitatively characterize imbricated surface features across three representative subregions of a gravel bar in the upper Yangtze River. The results indicate that (1) the long axes of imbricated pebbles are predominantly perpendicular to the flow direction; (2) greater bed-surface coarsening corresponds to steeper slope angles, more pronounced imbrication structures, and higher surface roughness; and (3) the self-similarity of the bed surface is anisotropic, being the strongest along the flow direction, and the Hurst index exhibits a quadratic relationship with the inclination index. A field-based method for identifying imbricated clusters was proposed. The results show that where imbrication is more pronounced, clusters tend to form more linear arrangements and increasingly align with the flow direction. These findings provide a new perspective for understanding the microtopographic features of gravel bed river surfaces.