Regionalisation of design rainfalls in Victoria Australia for design flood estimation by joint probability approach

The currently adopted rainfall-based design flood estimation technique in Australia known as the Design Event Approach is widely criticised and currently being challenged by a more holistic approach known as the Joint Probability Approach to design flood estimation. In this new approach, the input variables to the runoff routing model are specified by probability distributions. For example, the distribution of rainfall duration is specified by an exponential distribution and the conditional distribution of rainfall intensity is expressed in the form of intensity-frequency-duration (IFD) curves. These distributions are identified by analysing historical pluviograph data on or near the catchment of interest. For practical application, it is desirable to regionalise the IFD data so that this can be derived at various locations in a region by simplified technique. This paper presents how the distribution of rainfall intensity can be regionalised in the state of Victoria in Australia. This uses data from 23 pluviograph stations from South-east Victoria. In the study, regional relationships were developed between joint-probability IFD data and currently available Design Event IFD data. These regional relationship along with the existing extreme design rainfall and probable maximum precipitation data were used to derive joint probability IFD data, which were then used to estimate design floods using the Joint Probability Approach. Further research is required to validate this regional relationship before it can be applied in practice.