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

[0208358]小尺度的簇状河床在砾石河床中普遍存在。为了更好地理解河流过程，以及对河床粗糙度特征、阶段-流量关系、河床输运和恢复计划的实施进行准确预测，需要了解它们的力学。集群微形态是砾石河床非线性系统的一个组成部分。水流结构和可携沙形成集群；一旦形成，它们又反过来影响沉积物的输运和周围的流场。簇状结构增加了河床的稳定性，延缓了沉积物的运动。自组织簇被称为“单个颗粒之间随机相遇的产物”。在自然河流中，簇状河床被认为是在大到足以造成河床沉积物选择性动员的水流消退期间形成的。一些人认为簇在准稳定平衡状态下形成，从而使流动阻力最大化，产生波长与颗粒直径之比为20。还有人认为星团是一种“运动波”或迁移，而不是稳定的结构。该项目需要广泛的实验室水槽实验，以了解集群微形态的演变。一个主要的目标是定义近床湍流结构，以提高对近床湍流和沉积物输运之间耦合的理解。将对非均匀化球体进行一系列实验，以检验团簇几何形状、团簇形成、保持稳定和解体的流量范围，以及近床水力参数与团簇床形之间的相互作用。实验室实验中变量数量的减少将使水力参数、沉积物供应和产生的颗粒簇之间的特定关系得以分离和量化。高分辨率相机和声学传感器的最新进展，结合用于图像分析和床层扫描的硬件的发展，将允许在不干扰流动的情况下，以湍流分辨频率同时测量当地沉积物运输和邻近的近床流。

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.