Application of Index Flood Approach in Regional Flood Estimation Using L-Moments in Both Fixed Region and Region of Influence Framework

Abstract

The index flood method (IFM) is one of the widely used regional flood frequency estimation (RFFE) methods. Application of the L-moments-based IFM ideally requires the formation of a homogeneous region. However, homogeneity cannot generally be achieved in the case of Australian flood data. In this chapter, discordancy and heterogeneity measures are used to explore the homogeneity of 88 selected catchments in New South Wales (NSW), Australia. No homogeneous region is found based on the standard statistical tests; hence, the standard IFM is not applicable to this dataset in NSW. Therefore, an approximate IFM is developed and tested (using both fixed region and region of influence approaches) where the index flood is taken as a 10% annual exceedance probability flood (Q 10). The approximate IFM gives a median relative error in the range of 30% to 46% for design flood estimation, which is comparable to the relative error reported by the recommended RFFE technique in Australian Rainfall and Runoff, the national guide. This indicates that the IFM can be applied for design flood estimation in NSW despite the fact that an ideal homogeneous region cannot be identified. KEYWORDS: Region of Influence, Regional Flood, Flood Frequency, Homogeneity, Homogeneous Region, New South Wales, Exceedance Probability Flood, Regionalization