Frequency Analysis of Australian Rainfall Data as used for Flood Analysis and Design

The procedures used to obtain accurate, temporally and spatially consistent, intensity-frequency-duration (IFD) design rainfall data for Australia are discussed. These IFD design curves are used by design enigneers and scientists as input to a wide range of design flood models and other environmental studies. The basic annual maximum rainfall data for durations of 6 minutes to 72 hours are fitted using a log-Pearson Type III distribution with a small Positive regional skewness up to 0.7. Most of Australia is close to zero skewness ie. a lognormal distribution. Due to the sparsity of recording raingauges, various regression techniques were used to estimate short duration data at daily read raingauge sites. These procedures are discussed, along with the production of six master charts of rainfall intensity for various durations and average recurrence intervals (ARI) covering all Australia. From these six charts,plus a map of regionalized skewness, a full set of IFD curves can be obtained for any location using appropriate extrapolation and interpolation procedures. The IFD design curves extend from six minutes to 72 hours and ARI from one year to 100 years. Comparisons are made with USA work in this field. The paper also outlines the steps taken to automate this development work and produce CDIRS(Computerized Design IFD Rainfall System). CDIRS allows automatic determination of a full set of IFD curves (also in tabular form) for any location simply by supplying its latitude and longitude.