Hydraulic and Geomorphic Controls on the Evolution of Cluster Bedforms in Gravel-Bed Streams

帕帕尼科拉小规模的簇状河床在砾石河床河流中很普遍。为了更好地了解河流过程，以及发展对河床粗糙度特征、水位流量关系、推移质输运和恢复计划执行情况的准确预测，需要了解它们的机制。团簇微体是砾石床河流非线性系统的一个组成部分。水流结构和可携带泥沙形成团簇，一旦发展，它们反过来影响泥沙输运和周围的流场。团团增加了床面的稳定性和泥沙运动的延迟。自组织星团被称为“单个颗粒间随机相遇的产物”。在天然溪流中，簇状河床被认为是在水流事件退缩时形成的，足以产生选择性的河床沉积物动员。一些人认为，团簇是在准稳定平衡状态下形成的，以使流动阻力最大化，产生的波长与颗粒直径比为20。还有一些人认为，团簇是一种“运动波”或迁移性结构，而不是稳定的结构。该项目需要进行广泛的实验室水槽实验，以了解团簇微型体的演化。一个主要的目标是定义近床湍流结构，以提高对近床湍流与泥沙输移之间的耦合的理解。对于非均匀球体，将进行一系列实验，以考察团团的几何形状、团团形成、稳定和解体的流动范围，以及近床水力参数和团团床型之间的相互作用。实验室试验中变量数量的减少将使水力参数、泥沙供应和所产生的颗粒团之间的特定关系得以分离和量化。用于测量集群周围湍流的高分辨率相机和声学传感器的最新进展，再加上图像分析和河床扫描硬件的开发，将能够以湍流分辨频率同时测量当地泥沙输送和邻近的近床流动，而不会干扰流动。

0208358PapanicolaouSmall-scale cluster bedforms are prevalent in gravel-bed rivers. An understanding of their mechanics is needed for better understanding of river processes, as well as the development of accurate predictions of bed roughness characteristics, stage-discharge relations, bedload transport, and implementation of restoration plans. Cluster microforms are one component of the nonlinear system of gravel-bed rivers. Flow structure and entrainable sediment produce clusters; once developed, they in turn influence sediment transport and the surrounding flow field.Clusters add to bed stability and delay of sediment motion. Self-organized clusters have been called a "product of stochastic encounters among individual grains." In natural streams, cluster bedforms are thought to form during recession of flow events great enough to create selective mobilization of bed sediment. Some think that clusters form in quasi-stable equilibrium so as to maximize flow resistance, producing a wavelength to particle diameter ratio of 20. Still others suggest that clusters are a "kinematic wave" or migratory, rather than stable structure.The project entails extensive laboratory flume experiments to understand the evolution of cluster microforms. A major- goal is to define the near bed turbulence structure to improve the understanding of the coupling between near-bed turbulence and sediment transport when clusters are present. A series of experiments will be conducted for non-unisized spheres to examine the cluster geometry, the flow ranges under which clusters form, remain stable, and disintegrate, and the interaction between near-bed hydraulic parameters and clusters bedforms. The reduced number of variables in the laboratory experiments will allow specific relations between the hydraulic parameters, sediment supply, and resulting particle clusters to be isolated and quantified. Recent advances in high-resolution cameras and acoustic sensors for measuring turbulence around clusters, combined with the development of hardware for image analysis and bed scanning will allow simultaneous measurements of local sediment transport and adjacent near-bed flow at turbulence-resolving frequencies without disturbing the flow.