Copula-Based Analysis of Flood Peak Level and Duration: Two Case Studies in Taihu Basin, China

Recently, several bivariate and multivariate methods have been used to analyze the relationship of flood peak discharges with corresponding hydrograph volumes and durations in many basins. In some flat tidal areas, such as the Taihu Basin of China, the flood peak level and the corresponding flood duration have received more attention than the flood peak discharge and other flood characteristics in flood control and flood management. In this study, avoiding the complex problem of positive-negative flows, a pragmatic approach for bivariate flood frequency analyses based on flood peak level data is proposed. Commonly used bivariate copulas and techniques for fitting such copulas are applied to analyze the correlation between flood peak levels and durations based on peak-over-threshold flood series in Pingwang and Wuzhen. The goodness-of-fit test shows that the extreme-value copulas are not suitable for these two cases. Along with the tail dependence analysis, the Student copula (ν=5) is chosen as the best-fitting model. A comparison between the joint return period (years) associated with the event (H>hT or D>dT) obtained through the Student copula (ν=5) and other bivariate copulas demonstrates that ignoring the tail dependence results in a significant overestimate of flood risk. The joint return periods of arbitrary combinations of flood peak levels and flood durations are obtained through the final fitted copula. Furthermore, a comparison between the cumulative probability curves of flood peak levels obtained through different univariate and bivariate methods confirms the correctness of this work.