Frequency analysis is a useful tool for flood risk assessment, but the definition and calculation of return periods and design values in a multivariate framework are difficult tasks. In this paper, by constructing the joint distribution of flood duration, peak discharge and volume, we introduced the definitions of multivariate "OR", "AND" and secondary return periods and discuss their differences in identifying the safe and dangerous regions in a critical level. The rationality and reliability of information provided from them in flood risk management and engineering design were then analyzed, respectively. The traditional "OR-AND" approach in multivariate return periods definition is limited in the identification of safe and dangerous regions, while the secondary return period based on the Kendall measure would be more rational. Consequently, mistakes in the recognition of safe and dangerous event can be avoided by the use of second return period, which, obviously, is better for the flood risk management. For a given second return period, the combination of flood duration, peak discharge and volume design values calculated from the most-likely approach, may have an advantage to meet the requirement that with low cost to bear greater risks in engineering design. Furthermore, the information provided from multivariate design values would be more considerate and reliable than the univariate ones.