Updating Australia’s Flood Guidance for Climate Change

Design flood estimation is the process of calculating either a peak flow, volume, or level with a defined probability of exceedance or average recurrence interval for the purposes of infrastructure design, planning, or decision making. The methods to be used for calculating a design flood are generally prescribed in national-level flood guidance documents. While traditionally these documents have assumed that historical data is stationary and hence representative of the future planning horizon, this assumption is no longer valid. Climate change is affecting various flood risk drivers including increasing extreme rainfalls and changing antecedence moisture conditions, resulting in altered flood exceedance probabilities. There are now mandatory requirements for corporate reporting of climate related risks. The net result is that flood guidance across the world is being updated for climate change.

While state-of-the-art regional climate modelling is invaluable for developing projections of extreme rainfall and other flood risk drivers, there are limitations associated with any single line of evidence that suggest a structured approach for evidentiary synthesis is needed. This issue is compounded in Australia, a large geographic area with relatively low population density, meaning that high-resolution regional climate modelling is only feasible in high-priority regions. Moreover, the purpose of design flood guidance is to inform flood estimation practice, and thus care is needed to ensure information is presented in a form that can be integrated into standard flood estimation practice. To this end, an approach to updating Australian flood guidance was developed to include the following elements: (1) expert elicitation (2) stakeholder engagement (3) a scientific review of literature relevant to design flood estimation, and (4) guidance preparation with stakeholder engagement to close the feedback loop. The methodology included a meta-analysis to aggregate information on extreme rainfall changes across multiple lines of evidence. The meta-analysis concluded that hourly extreme rainfalls intensify by 15% per degree of global warming while daily rainfall intensify by 8% per degree of global warming.

The updated guidance resulted in several novel outcomes. Uplift factors are recommended to be applied to design rainfalls up to and including the Probable Maximum Precipitation (PMP), and factors are provided to adjust loss rates and temporal patterns used in hydrological modelling. To estimate current flood risks it is recognised that the intensity-duration-frequency (IDF) curves based on historic data needs to be adjusted upwards to account for the embedded trend due to global warming. While not user requests could be met, for example additional guidance on the choice of temperature projection, overall, the adopted methodology ensured that the update met the user needs while being consistent with the current science.