Impacts of outliers in flood frequency analysis : a case study for Eastern Australia

At-site flood frequency analysis is a useful technique to estimate flood quantiles if reasonably long at-site flood record is available. In Australia, FLIKE software has been proposed for at-site flood frequency analysis. The advantage of FLIKE is that, for a given application, the user can compare a number of most commonly adopted probability distributions and parameter estimation methods relatively quickly using a windows interface. The new version of FLIKE has been incorporated with the multiple Grubbs and Beck test, which can identify multiple numbers of potentially influential low flows. This paper presents a case study considering ten catchments in eastern Australia, which compares two outlier identification tests (original Grubbs and Beck and multiple Grubbs and Beck tests) and two commonly applied probability distributions (Generalized Extreme Value (GEV) and Log Pearson type 3 (LP3)). The results show that the LP3 distribution with multiple Grubbs and Beck test provides more accurate flood quantile estimates than when LP3 distribution is used with the original Grubbs and Beck test. Between these two methods, the differences in flood quantile estimates have been found to be up to 61% for the ten study catchments. It has also been found that GEV distribution with L moments and LP3 distribution with the multiple Grubbs and Beck test provide quite similar results in most of the cases; however, a difference up to 38% has been noted for flood quantiles for annual exceedance probability (AEP) of 1 in 100 for one catchment. The methodology presented in this paper can be applied to other catchments/countries.