Application of sediment studies to the management and planning of water resources in the Sydney region

Multi-decadal periods of alternating wet and dry periods have been observed in instrumental rainfall and flood records of eastern Australia. These periods are critical for the management of water resources in large cities such as Sydney. High-magnitude rainfall events are the primary source of infill events for Sydney's reservoirs. The occurrence of these rainfall events is linked with multi-decadal periods of high- and low-flood frequency, with an increase in average rainfall of 10-30% in periods of high flood frequency compared to those of low flood frequency. The instrumental record, however, is too brief to capture the full range of this variability. Sediment studies, focusing on changes in sediment deposition over time, have been used to investigate long-term hydrological variability. Discontinuous flood plains in the drowned river valley of the Hawkesbury-Nepean River act as sediment traps, preserving a record of flood events. Sediment cores collected from one of these flood plains have been analysed for particle size distribution, loss-on-ignition, magnetic susceptibility and density. These analyses, in conjunction with a chronology established through radiocarbon and optically stimulated luminescence (OSL) dating, allow a record of sedimentation over the last similar to 1000 years to he constructed. This provides a proxy flood record, which in turn has been used as a measure of hydrological variability in the region. The reconstructed flood record from the Hawkesbury-Nepean River will be used in long-term climate models, essential for the management and planning of water resources.