Cohesion experiment videos

While cohesion is thought to be an important control on sediment transport, few studies have systematically examined the role of cohesion in river morphodynamics. In this study we use simplified, small-scale laboratory experiments to investigate how increasing sediment cohesion affects the morphometrics of fluvial channels. Experiments were conducted in a laminar flume with a mixture of angular, sand-sized plastic particles and different amounts of xanthan gum—a proxy for cohesive biofilms—to tune cohesion among grains. With increasing cohesion, we observe a transition from highly mobile, braided to single thread meandering channels, then to straight, gully-like channels with headcuts that exhibit decreasing distance of retreat with increasing cohesion. Particle image velocimetry (PIV) shows that cohesion decreases sediment transport rate even when discharge is increased, suggesting an increase in critical shear stress. Bank width measurements show that this leads to narrowing of the channel with increasing cohesion. However, strong qualitative differences in observed channel morphodynamics suggest that beyond changing critical shear stress, cohesion alters the fundamental processes that govern channel erosion, including bank strength and the formation of aggregates. Our work suggests a novel approach of using sediment cohesion to explore the transition between transport limited and detachment limited channels.尽管黏聚力被认为是控制泥沙输移的重要因子，但目前鲜有研究系统探讨其在河流地貌动力学中的作用。本研究通过简化的小型室内实验，探究泥沙黏聚力升高对河流河道地貌形态参数的影响。实验在层流水槽中开展，以棱角状砂粒级塑料颗粒与不同剂量的黄原胶——作为黏性生物膜的替代介质——混合，以此调控颗粒间的黏结强度。随着黏聚力升高，我们观测到河道形态发生连续转变：从高活动性的辫状河道，过渡为单一曲流河道，最终演变为笔直的类冲沟河道，其溯源侵蚀陡坎的后退距离随黏聚力升高而逐渐缩短。粒子图像测速法（Particle Image Velocimetry, PIV）结果表明，即便增加流量，黏聚力仍会降低泥沙输移速率，这表明临界剪切应力有所升高。岸宽测量数据显示，该效应会导致河道随黏聚力升高而逐渐缩窄。然而，观测到的河道地貌动力学过程存在显著的定性差异，这表明除了改变临界剪切应力之外，黏聚力还会改变调控河道侵蚀的核心过程，包括岸坡强度与颗粒团聚体的形成。本研究提出了一种全新的研究路径，即通过调控泥沙黏聚力，探究输移限制型河道与侵蚀限制型河道之间的形态转换机制。