Natural flood management: lessons and opportunities from the catastrophic 2021–2022 floods in eastern Australia

Natural flood management (NFM), a nature-based solution to flood mitigation where hydrological and biophysical processes are harnessed to reduce flow velocity, erosive energy and flood risk, is an emerging global theme of water and river management. The catastrophic 2021 and 2022 floods in eastern Australia are used to assess the hydrological properties of discrete events and to start an investigation of whether widespread changes in flood hydrology are occurring. We find that most coastal rivers in New South Wales (NSW) had a noticeable decrease in flood wave celerity (increase in flood travel time) when the 2021 and 2022 floods are contrasted with equivalent floods since the 1970s and that several also exhibit increases in flood peak attenuation. For some rivers, there is a coincident trend between these changing flood properties, riparian vegetation regrowth (regreening) and geomorphic recovery over the last +30 years. These may be the first signals that passive riparian management is counteracting some of the more severe hydrological effects of floods on these rivers and that some degree of NFM is possible. Lessons from this work are that there is an immediate opportunity to implement large-scale NFM in coastal catchments of NSW, but this will only occur if we re-examine current flood mitigation and adaption strategies and recognise and prioritise nature-based solutions that include space-to-flood and corridors of river recovery, as essential parts of the modern flood mitigation toolkit.