Catastrophic stripping: response of the Shoalhaven River to high-magnitude, low-frequency flood events, NSW, Australia
The lower Shoalhaven River gorge has seen episodes of catastrophic stripping both in the modern and palaeorecord. Ortho-rectified aerial photographs, one-dimensional hydraulic modelling (HEC-RAS), stratigraphic analysis and OSL dating combine to identify that benches along Bulls Reach have been extensively eroded and reworked by modern events. Kermode et al (in prep) 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 150 years in age. HEC-RAS modelling of floods from 20-100 years ARI shows that velocities and shear stress in the reach surpass that predicted for widespread entrainment and transport of sediments and re-working of vegetated bars and benches. There is contention regarding the relative roles of flood regimes, the impact of European settlement and the role of vegetation (clearance and/or regrowth) on catastrophic stripping episodes. Early European settlement cleared banks along portions of the Shoalhaven River gorge; an alteration influencing the response of the river to high magnitude flood events. Further, the building of Tallowa Dam has altered the flow regime, and although this has little impact on the discharge of less frequent, higher magnitude floods, the retention of bedload sediment has substantially altered the substrate composition of downstream landforms. 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.