FLOODFLOW FREQUENCY MODEL SELECTION IN AUSTRALIA

Uniform flood frequency guidelines in Australia and the United States recommend the use of the log Pearson type 3 (LP3) distribution in floo d frequency investigations. Many investigators have suggested alternat e models such as the Generalized Extreme Value (GEV) distribution as a n improvement over the LP3 distribution. Using floodflow data at 61 si tes across Australia, we explore the suitability of various flood freq uency models using L-moment diagrams. We also repeat the experiment pe rformed in the original US Water Resource Council report (Bulletin 17B ) which led to the LP3 mandate in the United States. Our evaluations r eveal that among the models tested, the GEV and Wakeby distributions p rovide the best approximation to floodflow data in the regions of Aust ralia that are dominated by rainfall during the winter months, such as southwest Western Australia and Tasmania. For the remainder of the co ntinent, the Generalized Pareto (GPA) and Wakeby distributions provide the best approximation to floodflow data. The two- and three-paramete r log-normal models and the LP3 distribution performed satisfactorily, yet not as well as either the GEV or GPA distributions. Other models such as the Gumbel, log-normal, normal, Pearson, exponential, and unif orm distributions are shown to perform poorly. Recent research indicat es that regional index-flood type procedures should be more accurate a nd more robust than the type of at-site procedures evaluated here. Nev ertheless, this study reveals that index-flood procedures should not b e restricted to a single distribution such as the GEV distribution bec ause other distributions such as the GPA distribution perform signific antly better in the most densely populated regions of Australia.