The proper assessment of design flood is a major concern for many hydrological applications in small urban watersheds. A number of approaches can be used including statistical approach and the continuous simulation and design storm methods. However, each method has its own limitations and assumptions being applied to the real world. The design storm method has been widely used for a long time because of the simplicity of the method, but three critical assumptions are made such as the equality of the return periods between the rainfall and corresponding flood quantiles and the selections of the rainfall hyetograph and antecedent soil moisture conditions. Continuous simulation cannot be applied to small urban catchments with quick responses of runoff to rainfall. In this paper, a new flood frequency analysis for the simulated annual peak flows (FASAP) is proposed. This method employs the candidate rainfall events selected by considering a time step order of five minutes and a sliding duration without any assumptions about the conventional design storm method in an urban watershed. In addition, the proposed methodology was verified by comparing the results with the conventional method in a real urban watershed.

Recently, many studies on wind speed frequency analysis reported that mixture distribution is appropriate for the distribution of wind speed data. Since the annual maximum wind speed data are a subsample of wind speed data, the characteristics of the annual maximum wind speed data are highly likely related to those of wind speed data. Hence, in this study, the fit of mixture distribution for annual maximum wind speed data measured in Korea was assessed. A two-component Gumbel mixture distribution was employed, and an expectation-maximization algorithm was used as fitting method. The fit of the mixture distribution was compared with those of the Gumbel, Weibull, Generalized Extreme Value, and Kappa distributions, based on several goodness-of-fit measurements. The results show that the fit of the mixture distribution for the annual maximum wind speed in Korea was superior to the fits of other distributions. Additionally, the mixture distribution led to good fits also at those stations where it was not selected as the most appropriate distribution.