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Design flood estimation is often required in hydrological design, floodplain management and soil conservation studies. Australian Rainfall and Runoff (ARR), the national guide for flow estimation in Australia, currently recommends the Design Event Approach (DEA), which is a rainfall-based flood estimation method. The method considers the probabilistic nature of rainfall intensity in combination with other model input parameters, such as rainfall temporal patterns and initial loss. The key assumption involved in the DEA is that the representative design input and parameters in modelling can be defined in such a way that they are “annual exceedance probability (AEP) neutral”, i.e. it produces a flood peak output that has the same AEP as the rainfall input. However, this assumption is generally not satisfied and the arbitrary treatment of various flood producing variables can lead to inconsistencies and significant bias in flood estimates for a given AEP. In recent years, there has been considerable research on the development and application of the Joint Probability Approach/Monte Carlo Simulation Technique (MCST) to design flood estimation. The previous applications of this method with Victoria and Queensland data have shown that MCST can overcome some of the limitations associated with the DEA, and this can produce more accurate design flood estimates. The wider application of the MCST needs regionalisation of various input variables to the rainfall-runoff model. These input variables include various rainfall parameters, such as duration, intensity, temporal pattern and losses. Of these, rainfall intensity, i.e. Intensity-Frequency-Duration (IFD) data, is the most important one. This paper focuses on the regionalisation of the IFD data for New South Wales (NSW) so that this can be used with the MCST to obtain design flood estimates. This study uses 83 pluviograph stations across NSW to regionalise the IFD data. In the application, complete storm rainfall events are selected from each of the 83 pluviograph stations in NSW. An exponential distribution is fitted to the selected partial duration series data for each station to obtain rainfall quantile estimates. A polynomial is then fitted to obtain rainfall quantile estimates for any duration and Average Recurrence Interval (ARI). These are then used to construct an IFD table for each station. The IFD tables from the selected 83 pluviograph stations form the basis of regionalisation of the IFD data in NSW as discussed in this paper. An ‘Inverse Distance Weighting (IDW) method’ is adopted to obtain IFD estimates for a location of interest (where regional IFD estimation is needed) from the nearby pluviograph stations. The IDW method assigns weight to pluviograph stations as a function of distance from the location of interest in such a way that the weight is inversely proportional to distance. It has been found that three to five nearby stations (depending on the spatial density of the available pluviograph stations) can be used to obtain reasonably accurate regional IFD estimates for NSW. The new IFD data is applied with the Design Event Approach to two NSW catchments for estimating design floods. The application of the MCST to these study catchments is yet to be undertaken which will require derivation of the stochastic rainfall duration, temporal patterns and losses, which is currently being undertaken.

Regionalisation of intensity-frequency-duration data: A case study for New South Wales