Author(s): Jinghong Deng; Yi Xiao
Linked Author(s):
Keywords: Cluster microform; Bedload particles; Flow intensity; Bed arrangement; Flume experiment
Abstract: Cluster microform is a common microtopographic structure formed under the mutual collision and deposition of individual sediment particles, plays a positive role in bed stability and provides habitat for aquatic organisms in river ecosystems. Due to the intricate interaction among flow, bedload and the distinctive bedform, research on the evolution of clusters considering the motion of individual bedload particles is fully limited. In this context, laboratory experiments were conducted in a rectangular water recirculating flume to investigate the dynamic exchanging process of cluster microforms and bedload particles under successive changing flow conditions, quantify the relationship between cluster evolution and flow intensity and examine the influence of bed arrangement on the morphology of clusters and bedload transport. Dense packed, transparent 10 mm spherical glass beads with two different array type (1) rectangular array (2) quincuncial array are considered as stationary roughness bed separately during the experiments. Black 10 mm spherical glass beads with 6 diameter spacing from each other were placed above the bed, acting as bedload particles. Experimental results indicated that the formation, stability, and disintegration of sediment clusters exhibit distinct ranges of flow intensity. With ΘR denoting the ratio of flow intensity to critical Shields number of individual particle, clusters develop gradually at 1.25 ≤ ΘR < 1.75, exist steadily at 1.75 ≤ ΘR ≤ 2.0, disintegrate slowly at 2.00 < ΘR ≤ 2.25 and significantly over 2.25. Cluster microforms are much less likely to move compared to individual sediment particles, the higher the proportion of clusters within the composition of bed materials, the stronger the bed resistance to erosion, resulting in increased stability. Furthermore, clusters and individual bedload particles demonstrate a significant dynamic exchange relationship with variations in flow conditions, exhibiting a pronounced negative correlation (P = -0.985). As flow conditions change, bed surface experiences two stable phases: one characterized by the predominance of individual bedload particles (1 ≤ ΘR ≤ 1.5) and the other dominated by clusters (1.5 < ΘR ≤ 2.25). Additionally, statistic characteristics of clusters and bedload particles vary from bed arrangement significantly. Critical entrainment flow drag force for rolling of bedload particles under the condition of a quincuncial array bed is 73.82% of that under the condition of a rectangular array bed. Meanwhile, the critical relative flow intensity ΘR for the full motion of particles on the quincuncial array bed is 0.25 lower than that on the rectangular array one.
Year: 2024