Catastrophic flood geomorphology of two bedrock gorges in the Northern Territory, Australia

Abstract

Koolpin and Jim Jim Gorges are narrow, bedrock channels developed in resistant sandstone of the Arnhem Land Plateau, Northern Territory. The tropical monsoonal climate results in rare flood events of great magnitude. Peak flow-rate estimations from rainfall of variable recurrence intervals, step-backwater flow-modelling, empirically derived boulder-transport equations and the use of fine-grained slackwater accumulations, as palaeodischarge indicators, allow quantitative estimates to be made of the magnitude of the most geomorphically -significant flows. For events with an average recurrence interval of 100, 1000 years and the maximum probable flood-discharge that the gorges experience range from approximately 2000m3Is to 6500m3Is. Channel velocities, unit stream power and bed shear-stress are also high. Additional indicators of intense flow are the presence of potholes and flutes on bedrock surfaces. The boulder fields found within these gorges provide a means of energy dissipation and their location corresponds with reductions in unit stream power within the reaches. This relationship appears to represent a self-regulation system similar to the pool-riffle sequence in alluvial channels. It is probable that the geomorphology of these gorges is deformable on the time scale of extreme events Show more