The analysis of floods meeting with tides and the rational selection of the combination of floods and tides are vitally important in planning design for the tide affected rivers. The model for the risk probability of the combination of floods and tides was established in this paper. Firstly

the different copula functions were used to build the bivariate joint distribution of annual maximum flood discharge and its corresponding tidal level and the annual maximum tide level and its corresponding flood discharge

respectively. Then

based on both mentioned above the risk probability analysis models for the combination of floods and tides were put forward. The case study of the combined risk probability of floods and tides for the estuary of the Moyang River was conducted. The results indicated: (1) The combinatorial risk probability of the design flood discharge with the fiftyyear return period and the average value of annual maximum tide level was 6。89％; (2) The combinatorial risk probability of the design tide level with the fifty-year return period and the average value of annual maximum flood discharge was 4。77％. The models for combined probability of the floods and the tides can be used to measure the risk probability of the combination of floods and tides

which can provide scientific basis for the rational selections for the combined probability of floods and tides.