The new regional flood frequency estimation model for Australia : RFFE model 2015

A new regional flood frequency estimation (RFFE) model for Australia has been developed as a part of 4th edition of Australian Rainfall and Runoff, which is referred to as 'RFFE Model 2015'. To develop and test this model, flood data from 853 gauged catchments have been utilised, which includes 798 gauged catchments from the humid coastal areas and 55 catchments from the arid and semi-arid areas. The model allows the derivation of design flood estimates for annual exceedance probabilities (AEP) of 50% to 1% for small to medium catchments anywhere in Australia. In the RFFE Model 2015, the humid coastal and arid/semi-arid areas of Australia have been divided into five and two different regions, respectively. The boundaries between the arid and humid coastal regions have been drawn approximately based on the 500 mm mean annual rainfall contour line. To reduce the effects of sharp variation in flood estimates for the ungauged catchments located close to these regional boundaries, seven fringe zones have been delineated. In the humid coastal regions, a region-of-influence approach has been adopted to derive design flood estimates for ungauged catchments. In developing the prediction model for the regionalised Log Pearson Type 3 distribution, a Bayesian generalised least squares regression technique has been applied, which considers the inter-station correlation and variation in record lengths of the annual maximum flood series across different sites. For the arid/semi-arid regions, a simple index type regional method has been adopted. For easy application by the industry, an application tool has been developed, which automates the application of the RFFE Model 2015. The user is required to provide simple input data (e.g. catchment area and catchment location) to obtain design flood quantiles and associated uncertainty. This paper provides essential technical information, which will assist the user to apply the RFFE Model 2015 in practice with confidence. Further details can be found in the ARR draft chapter on regional flood methods and technical reports.