Cluster Formation and Stream-bed Armouring: A Photogrammetric Study

Abstract

Clusters are a self organising structures commonly found in natural rivers. They play an important role in river bed dynamics by providing habitat and increasing bed stability. The aim of the present study was to investigate the development of naturally formed cluster microforms from a flattened bed of graded gravel at constant flow rate. The study was laboratory based, and used photogrammetry to observe the behaviour of well graded, cohesionless sediment at flood flow conditions as clusters formed, evolved and disintegrated. Focus was primarily on the cluster formation and sediment movement, and the development of clusters was observed under varying flow rate and grain size distribution. The study of gravel dynamics using coloured particles, coupled with image analysis, has enabled in-depth observation of sediment transport and cluster development. To assist in the study of cluster dynamics, a new application of photogrammetry was developed. A digital particle tracking (DPT) program was successfully applied to recordings of sediment movement over extended experiment durations, and a cluster identification program was developed to monitor cluster evolution. Application of the DPT program to recordings of the gravel bed as it was water worked revealed large spatial and temporal variation of sediment transport rates. Image analysis was used to investigate the progression of armouring, and statistical analysis was applied to surface elevation profiles of the water-worked surface sediment to investigate the effects of armouring. Objective cluster identification was achieved by monitoring the stationary areas of the bed, and designating clusters as areas with stable groups of large particles. This tool was used in combination with DPT to obtain new insights into cluster formation. The complex interactions of clusters with the surrounding bed were studied, and the behavioural trends of cluster formation are presented in this thesis. Surface coverage of clusters on the test section increased over time, with a maximum surface coverage of around 34% observed between all experiments. Particle shape plays a role in cluster formation, where elongate stones form more stable clusters. Clustering is enhanced by the presence of a stationary object on the bed, and the presence of clusters plays a role in attenuating the sediment transport of the surrounding bed.