The Problem

Major catastrophes are rarely the result of a single event or force. They are often the result of ‘compound events’ (extreme impacts such as floods and other natural hazards) which are caused by multiple and dependent processes. As such, they can be difficult to predict and plan for, because of their rarity and the complexity of forces that cause them.

What We Did

The research began as an academic paper, was tested through a number of consultancies and developed into a standard method used by Engineers Australia.

Tide gauges and rainfall gauges along the entire Australian coastline were paired to determine for extreme rainfall and extreme ocean levels, how likely they were to occur at the same time. This resulted in a map of dependence for the Australian coastline.

The dependence of extreme events forms an input to a method termed the ‘design variable method’ which is a bivariate flood frequency. This method uses extreme probability distributions to represent the probability of all combinations of extremes. The distribution is then matched to output from a hydraulic model to determine the water levels that get produced by pairs of events. The probability of flood exceedance is then calculated by considering all pairs of events that yield a water level above a threshold of interest.


  • For the first time, Engineers Australia is able to provide scientifically defensible guidance on estimating flood risk in estuarine catchments.
  • The implementation of the method is likely to be cost-effective for the Australian engineering community as a result of the provision of free software
  • It is likely that guidance is likely to lead to lower flood levels than traditional ‘rule-of-thumb’ methods in the majority of cases, leading to reduced costs of flood planning and defence infrastructure.

A software implementation of the approach is available online through the Australian Rainfall and Runoff website.

A number of academic papers and reports have been publish demonstrating scientific basis of the approach. One of these papers, ‘Flood risk estimation in Australia’s coastal zone‘ (2014), won the GN Alexander medal at the Hydrology and Water Resources Symposium in Perth.

Further Information

Australian Rainfall and Runoff, Book 6 Chapter 5

Australian Rainfall and Runoff Software

Australian Rainfall and Runoff Revision Reports

Zheng, F., Westra, S., & Sisson, S. A. (2013). Quantifying the dependence between extreme rainfall and storm surge in the coastal zoneJournal of Hydrology505, 172-187.

Zheng, F., Westra, S., Sisson, S., & Leonard, M. (2014). Flood risk estimation in Australia’s coastal zone: Modelling the dependence between extreme rainfall and storm surge. Hydrology and Water Resources Symposium, Perth.

Zheng, F., Westra, S., Leonard, M., & Sisson, S. A. (2014). Modeling dependence between extreme rainfall and storm surge to estimate coastal flooding riskWater Resources Research50(3), 2050-2071.

Zheng, F., Thibaud, E., Leonard, M., & Westra, S. (2015). Assessing the performance of the independence method in modeling spatial extreme rainfallWater Resources Research51(9), 7744-7758.

Zheng, F., Leonard, M., & Westra, S. (2015). Efficient joint probability analysis of flood risk. Journal of Hydroinformatics, 17(4):584-597

Zheng, F., Leonard, M., & Westra, S. (2015). Application of the design variable method to estimate coastal flood riskJournal of Flood Risk Management.

Project Details

University of Adelaide Personnel: Seth Westra, Michael Leonard, Feifei Zheng

Collaborators: Engineers Australia, WA Water Corporation, WMA Water