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Flood frequency analysis of Turkey using L-moments method

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Abstract

The index flood procedure coupled with the L-moments method is applied to the annual flood peaks data taken at all stream-gauging stations in Turkey having at least 15-year-long records. First, screening of the data is done based on the discordancy measure (Di) in terms of the L-moments. Homogeneity of the total geographical area of Turkey is tested using the L-moments based heterogeneity measure, H, computed on 500 simulations generated using the four parameter Kappa distribution. The L-moments analysis of the recorded annual flood peaks data at 543 gauged sites indicates that Turkey as a whole is hydrologically heterogeneous, and 45 of 543 gauged sites are discordant which are discarded from further analyses. The catchment areas of these 543 sites vary from 9·9 to 75121 km2 and their mean annual peak floods vary from 1·72 to 3739·5 m3 s−1. The probability distributions used in the analyses, whose parameters are computed by the L-moments method are the general extreme values (GEV), generalized logistic (GLO), generalized normal (GNO), Pearson type III (PE3), generalized Pareto (GPA), and five-parameter Wakeby (WAK). Based on the L-moment ratio diagrams and the |Zdist|-statistic criteria, the GEV distribution is identified as the robust distribution for the study area (498 gauged sites). Hence, for estimation of flood magnitudes of various return periods in Turkey, a regional flood frequency relationship is developed using the GEV distribution. Next, the quantiles computed at all of 543 gauged sites by the GEV and the Wakeby distributions are compared with the observed values of the same probability based on two criteria, mean absolute relative error and determination coefficient. Results of these comparisons indicate that both distributions of GEV and Wakeby, whose parameters are computed by the L-moments method, are adequate in predicting quantile estimates. Copyright © 2011 John Wiley & Sons, Ltd.

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