| Cluster microforms are a type of bedform arising from the structural organization of the surface grains (sizes gravel and coarser) in gravel bed rivers and are broadly defined as discrete groupings of particles that sit above the average elevation of the surrounding bed surface. Clusters are important elements in gravel-bed river dynamics and have been observed to exert an influence on the bed stability, bedload transport rates, downstream particle movement, overall flow resistance, and the local flow-field characteristics. However, it is unclear as to where and when clusters form in a stream, and how clustered microtopography fits into a holistic understanding of mountain stream morphology. Additionally, there is a lack of detailed characterization data pertaining to clusters, such as their geometric properties and bedform spacing. The goal of this research was to develop a predictive relation for the presence of clusters, and to systematically characterize cluster microtopography, both geomorphically and hydraulically. This goal was carried out by conducting a laboratory study that extends prior studies and by conducting a series of field studies on three streams in the Cascade Mountains of Washington State.;The results of this thesis lead to the conclusion that clusters are bed features that form out of the structural organization of grains in cobble-size sediment within depositional mountain reaches for which the majority of' the sediment can become mobilized by yearly peak flows, e.g. riffle-pool and braided reaches. Clusters have geometric properties that are controlled by the size of the particles in which they are found and the frontal width of the cluster; cluster spacing was found to be governed by the size of the largest particle in the cluster and the local slope. If formed, clusters have the general effect on the flow field of locally shifting the elevation of the roughness layer from just above the top of the bed to the top of the cluster. Formed clusters were also found to created entrainment micro-climates and generally increase the stability of particles inside a cluster by twofold, relative to particles outside of a cluster.;This study adds information to a growing bode of data concerning the characteristics of clusters in naturally occurring clustered beds. This general information about clusters will be useful for future studies examining clusters and incorporating their presence and effects in theoretical frameworks and numerical models of natural gravel-bed river environments. |
