Modern depositional processes in a confined bedrock setting: benches of the Shoalhaven river

Abstract

The lower Shoalhaven River provides an opportunity to examine bench processes in a confined setting. Stratigraphic analysis of trenches and augur holes, ground penetrating radar, Hec-RAS modelling and geochronological techniques combine to identify that benches of multiple levels along Bull Reach are composed of coarse material and have been extensively eroded and reworked by modern events. Kermode et al. (2012) established the long-term polycyclical nature of the higher alluvial surfaces (up to 193 ka in age), and this is contrasted with the youth of the lower inset alluvial surfaces, which are shown to be less than 270 years in age. This study evaluates the relative significance of both flood regime and effects of European settlement on the geomorphic effectiveness of high magnitude events and investigates the characteristics of bench formation in this confined setting. It characterises the nature of depositional events and the relationship between facies at an event scale. Using Hec-RAS modelling, events of different recurrence intervals are compared to explore the relative impact of varying flood magnitudes. The results bring into question the theory that inundation frequencies of these surfaces are constant, or associated with formative processes. © Authors

The lower Shoalhaven River provides an opportunity to examine bench processes in a confined setting. Stratigraphic analysis of trenches and augur holes, ground penetrating radar, Hec-RAS modelling and geochronological techniques combine to identify that benches of multiple levels along Bull Reach are composed of coarse material and have been extensively eroded and reworked by modern events. Kermode et al. (2012) established the long-term polycyclical nature of the higher alluvial surfaces (up to 193 ka in age), and this is contrasted with the youth of the lower inset alluvial surfaces, which are shown to be less than 270 years in age. This study evaluates the relative significance of both flood regime and effects of European settlement on the geomorphic effectiveness of high magnitude events and investigates the characteristics of bench formation in this confined setting. It characterises the nature of depositional events and the relationship between facies at an event scale. Using Hec-RAS modelling, events of different recurrence intervals are compared to explore the relative impact of varying flood magnitudes. The results bring into question the theory that inundation frequencies of these surfaces are constant, or associated with formative processes.