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

0208358帕帕尼科拉乌砾石河床河流中普遍存在小规模的簇状底形。 为了更好地了解河流过程，以及准确预测河床粗糙度特性、水位流量关系、推移质输运和实施恢复计划，需要了解其力学。 簇状微形态是砾石河床非线性系统的组成部分。 水流结构和可分散的泥沙会产生团簇，团簇一旦形成，又会影响泥沙输移和周围流场，增加床面稳定性，延缓泥沙运动。 自组织的团簇被称为“单个颗粒之间随机相遇的产物”。“在天然河流中，簇状底形被认为是在水流消退期间形成的，足以产生河床沉积物的选择性流动。 有些人认为团簇形成于准稳定平衡中，以使流动阻力最大化，从而产生波长与粒径之比为20。 还有一些人认为，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，以了解星团微缩形态的演变。 一个主要的目标是定义近床湍流结构，以提高近床湍流和泥沙输运之间的耦合的理解时，集群的存在。 一系列的实验将进行非unissized球体检查集群的几何形状，下集群的形成，保持稳定，并解体，和近床水力参数和集群底形之间的相互作用的流量范围。 在实验室实验中的变量的数量减少，将允许特定的水力参数之间的关系，沉积物供应，并由此产生的颗粒群被隔离和量化。 最近在高分辨率照相机和声学传感器方面取得的进展，用于测量集群周围的湍流，再加上图像分析和床扫描硬件的发展，将允许在不干扰流动的情况下，以分辨率频率同时测量局部沉积物输运和邻近的近床流动。

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.