Abstract: Rainfall events in the Three Gorges Region (TGR) of the Yangtze River tend to be concentrated on flood season. The coincidence of such rainfalls with flood events occurring in the upper reaches of the river may result in adverse impacts on the flood control of the Three Gorges reservoir. To investigate the characteristics of this coincidence, this paper calculates the joint distribution of the annual maximum rainfall in TGR and the annual maximum flood in the upper Yangtze River based on the Archimedean Copula function, and examines the risk of their coincidence from aspects of both time and magnitude (including peak value and hydrograph). The results indicate that temporal coincidence manifests a bimodal distribution with two peaks, and the annual maximum daily areal rainfall and 7-day areal rainfall in TGR generally occur earlier than the annual maximum flow and 7-day flood volume in the upper Yangtze River. The probabilities of both the flood peak coincidence and hydrograph coincidence increase with the increasing design frequency, and the coincidence that the TGR rainfall occurs earlier than the upper Yangtze River flood has a higher probability in both cases. For a given design frequency, the probability of hydrograph coincidence is greater than that of flood peak coincidence. This study would help the flood control scheduling of the Three Gorges reservoir.

Key words: rainfall and flood, Three Gorges region, upper Yangtze River, Archimedean Copula function, coincidence risk