Analysis of regional composition of design flood under non-stationary conditions

To address the problem of calculating the combined design value of different regional flood under non-stationary conditions, this paper firstly constructs a varying-parameter Copula model based on the Copula function, which can consider the non-stationarity of the marginal distribution of different flood variables and the non-stationarity of their dependence structure. The non-stationary evolution characteristics of the joint distribution of floods in different regions over time are analyzed. Based on the equal reliability method and the conditional expectation combination method, a method for analyzing the regional composition of non-stationary design floods under changing environments is proposed, which can be used to calculate the design value combinations of different regional floods corresponding to a specified return period under non-stationary conditions. Based on the annual maximum 15-day flood volume (AM15FV) series at Cuntan and Yichang stations, the proposed method is used to analyze the change on design flood combinations among the Cuntan, Yichang and Cuntan-Yichang section. The results show that the AM15FV series at Cuntan and Yichang stations exhibits a decreasing trend, and their joint distribution of floods at Cuntan and Yichang stations varies with time. The design values of floods at Yichang station, Cuntan station and Cuntan-Yichang section corresponding to the specified return period tend to decrease with increasing design lifetime.