When the impossible happens: how February 2022 event redefines extreme floods in Australia

Floods are recurring natural disasters worldwide, and they are causing increasing loss of life and property damage due to climate change and the occupation of flood-prone areas. In this context, flood modelling is an important tool to assess flood hazard and guide mitigation strategies. However, in situ hydro-climate observation networks required to provide reliable flood data are declining due to station closure, which jeopardizes the building of accurate flood models, particularly in low-latitude regions. This paper analyses an extreme flood event that occurred in eastern Australia in February 2022 which provides a point of reference for future floods in coastal catchments and humid sub-tropical climates, especially around the Pacific. This event combined some of the most extreme rainfall ever seen in Australia with initial soil water conditions close to saturation. It resulted in significantly lower peak time differences along river systems than in past historical events and led to the largest total insurance claim for a single flood event in Australia. The event is compared to a large dataset of observed floods across Australia from 1954 onwards to evaluate its “surprise” effect, or more precisely, how the inclusion of a single flood event can influence subsequent design flood calculations. The 2022 flood was found to be the event with the largest impact on the 1-in-100 years values (1% annual exceedance probability or AEP) for five- and ten-day total runoff. Including this single event increases these values by 13% averaged across 62 sites, respectively, with a maximum increase of up to 30% for certain sites. This impact is particularly significant considering the amount of hard infrastructure, such as roads and levees, that is built in reference to the 1% AEP level. Overall, the 2022 flood and the associated data collected for this paper presents an opportunity to use this event as reference for extreme flood scenarios in lower latitudes under combined riverine and coastal influence.