Author(s): Katherine Heays; Heide Friedrich; Bruce W. Melville
Keywords: Cluster formation; Armouring; Sediment transport; Photogrammetry; Image processing
Abstract: The interaction between a gravel bed in a river and the localised flow is complicated, and knowledge of this is important for modelling the processes and characteristics of a stream channel. Stream bed armouring is a common phenomenon in gravel-bed rivers. Armouring occurs when selective entrainment of smaller, more unstable particles leaves a bed with a larger average surface gravel size than that of the underlying bed (Best 1993; Chin 1985; Nino and Musalem 2000). The armour layer of the gravel bed generally consists of cluster formations, where small and medium sized particles congregate around larger particles to form more stable structures (Brayshaw et al. 1983). In order to further investigate the mechanics of cluster formation, an experiment using well graded sediment was conducted in the Hydraulics Laboratory at The University of Auckland. Formation of the armour layer over a test section in the flume was observed at three different flow rates. Flow data were collected with an Acoustic Doppler Velocimeter probe and gravel-bed properties were identified using photogrammetry. In order to enable identification of patterns which form due to variation in gravel size, different sized gravel particles in the test area were painted in different colours. At the beginning of each experiment, a 15 minute video was taken from aerial and side views, and then photographs and short videos were taken at regular intervals for the remainder of each experiment. To obtain quantitative results regarding the characteristics of the armour layer, image analysis was conducted using the programs Adobe Photoshop, Image J and Matlab. This paper details the methods developed to enable the retrieval of quantitative information from such experiments.