A tale of two floods: Hawkesbury-Nepean valley floods of February 2020 and March 2021

Abstract. The Hawkesbury-Nepean valley is one of the largest coastal basins in NSW. It supports the local agriculture industry and is an important environmental asset. Due to its narrow sandstone gorges, which create natural choke points, floodwaters from its major tributaries can rapidly back up, rise and spill out onto the flood plain. Thus, the valley is flood-prone, with a history of disastrous events, aggravated by a constrained road network for evacuation. Two flood events occurred in the Hawkesbury-Nepean valley in 2020 and 2021, however, the impact of each of those events was different in terms of lives lost (2 fatalities compared to none) and economic losses (more than AUD 2 billion compared to less than AUD 1 billion). In this study, reasons for the variation in impacts are explored by determining an inundation likelihood map, derived using a combination of the height above nearest drainage method and streamflow forecasts, and considering antecedent hydrological and climate conditions.