Floods are the most common and costliest natural disaster in Australia. However, the Flood Risk Assessments (FRAs) employed to manage them are hazard-focused and tend to overlook exposure and vulnerability. This leaves potential for Australian FRAs to make better use of a technique which holistically incorporates all three flood risk components. In this study, flood exposure assessment and mapping for the Hawkesbury-Nepean Catchment (HNC), a flood-prone region in Australia, was conducted. Three flood exposure indicators—population density, land use type, and critical infrastructure density—were selected to derive the flood exposure index (FEI). Results demonstrated that Statistical Areas Level 2 (SA2s) on or near the floodplain, located near the eastern border of the HNC, are severely or extremely flood-exposed due to the significant presence of flood-exposed assets such as hospitals or police stations. The Wahroonga (West)—Waitara SA2 was the most exposed SA2 in the catchment (extreme exposure). This was followed by the Acacia Gardens, Glendenning—Dean Park, and Cambridge Park SA2s (all severely exposed). The Goulburn SA2 was also identified as severely flood-exposed even though it remains outside of the floodplain. This is due to its many exposed assets as Australia’s first inland town. All selected indicators were found to either strongly or moderately positively correlate with the FEI. Ultimately, this novel FEI can assist in the reduction of flood risk in the HNC, as well as foster community resilience strategies. Additionally, the developed scalable and replicable methodology can be applied to other flood-prone regions of Australia.

Floods are the most common and costliest natural disaster in Australia. Australian flood risk assessments (FRAs) are mostly conducted on relatively small scales using modelling outputs. The aim of this study was to develop a novel approach of index-based analysis using a multi-criteria decision-making (MCDM) method for FRA on a large spatial domain. The selected case study area was the Hawkesbury-Nepean Catchment (HNC) in New South Wales, which is historically one of the most flood-prone regions of Australia. The HNC’s high flood risk was made distinctly clear during recent significant flood events in 2021 and 2022. Using a MCDM method, an overall Flood Risk Index (FRI) for the HNC was calculated based on flood hazard, flood exposure, and flood vulnerability indices. Inputs for the indices were selected to ensure that they are scalable and replicable, allowing them to be applied elsewhere for future flood management plans. The results of this study demonstrate that the HNC displays high flood risk, especially on its urbanised floodplain. For the examined March 2021 flood event, the HNC was found to have over 73% (or over 15,900 km2) of its area at ‘Severe’ or ‘Extreme’ flood risk. Validating the developed FRI for correspondence to actual flooding observations during the March 2021 flood event using the Receiver Operating Characteristic (ROC) statistical test, a value of 0.803 was obtained (i.e., very good). The developed proof-of-concept methodology for flood risk assessment on a large spatial scale has the potential to be used as a framework for further index-based FRA approaches.

Floods are the most common and costliest natural hazard in Australia. However, the Flood Resilience Assessments (FReAs) employed to manage them lack a focus on adaptive capacity and tend not to be incorporated into established flood risk frameworks. This leaves potential for Australian FReAs to make better use of a methodology which holistically incorporates more accurate flood resilience characterisations into flood risk frameworks. In this study, a FReA and mapping for the Hawkesbury-Nepean Catchment (HNC), a flood-prone region in Australia, were conducted. Nine flood resilience indicators were selected to derive the Flood Resilience Index (FReI). Results demonstrated that Statistical Areas Level 2 (SA2s) on or near the floodplain, located near the eastern border of the HNC, present moderate to very high levels of resilience due to increased socio-economic development and urbanisation in the region. Ultimately, this novel FReI can contribute to knowledge bolstering flood resilience in the HNC, as well as assist in flood risk reduction. Additionally, the developed scalable and replicable methodology can be applied to other flood-prone regions of Australia.