Agglomeration characteristics under the effect of non-uniform particle sorting on a rough bed surface

Non-uniform sediment grain activity significantly influences riverbed evolution and stability. To examine the agglomeration characteristics of non-uniform particles due to water flow sorting, we conducted a flume test using various particle sizes. The motion of non-uniform particles was recorded using high-definition camera technology under different hydraulic conditions. The structural characteristics of agglomerates were analyzed using multi-target tracking technology and particle image velocimetry. The experimental results indicate that the transport velocity of large particles correlates positively with water depth, whereas that of small particles first increases, then decreases, and subsequently increases with water depth. Under the action of sorting, particles form four types of agglomerates: strip, curve, triangle, and rhombus. The head of the strip structure captures particles, leading to a pileup, while the particle crowding force in the tail weakens. Large particles move under the action of water flow, transforming the strip structure into a curve type. The tail of the curve structures are displaced by water flow interference, evolving into closed triangle and rhombus structures. The average velocity at the tail end of strip and curve structures is higher than that of triangular and rhombic structures. They are significantly affected by water flow and have relatively weak stability. Rhombic structures are the most stable, followed by triangle structures. We systematically revealed the agglomeration characteristics and evolution law of non-uniform particles under the action of water flow sorting, providing a quantitative basis for the regulation of riverbed stability.