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

Washington State University, Pullman WA

Investigators

Abstract

0208358 Papanicolaou Small-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.