Flood Frequency Analysis Using L Moments: a Comparison between At-Site and Regional Approach

Abstract

Regional flood frequency analysis has been carried out for estimating peak discharge at regional level over the Kerala State, India, along with at-site flood frequency analysis. For the study, the annual peak discharges of 43 gauging stations having length of data from 14 to 47 years spread over the Kerala State were used. Using L moments and L moment ratio, best fit distribution was identified among the five distributions; Generalised Extreme Value (GEV), Generalised Pareto Distribution (GPA), Generalised Logistic (GLO), Generalised Normal (LN3) and Pearson Type III (PE3) distribution for both at-site and regional flood frequency analysis. Chi-square test, ranking method using statistical indicators and L moment ratio diagram were used for identifying the best fit distribution for at-site flood frequency analysis. It was found that GPA was the best fit distribution for 27 stations, GLO for 14, LN3 for 1 station and GEV for 2 station. After discordancy test, five different homogeneous regions were identified through heterogeneity test to carry out the regional flood frequency analysis (RFFA). After identifying best-fit distributions for each zone, flood growth curves were derived by incorporating the catchment characteristics of the basins. The distribution that was best fit in case of at-site analysis for a gauging site is found to be entirely different than the best fit distribution resulted from RFFA. In RFFA, growth curves that provide the flood magnitude for various return periods can be used to estimate the flood magnitude and frequency at ungauged sites in each region of the Kerala State in India.