Evaluation of the concept of critical rainfall duration by bivariate frequency analysis of annual maximum independent rainfall event series in Seoul, Korea

This study investigated the possible problems of using the concept of critical rainfall duration (CRD) for estimating the design flood. The investigation focused on the evaluation of the difference between the annual maximum independent rainfall event series (IRES) and the annual maximum short-duration rainfall series in Seoul, Korea. Annual maximum IRES was determined by applying Freund's bivariate exponential distribution, which were also used for frequency analysis by applying the Gumbel's bivariate logistic model. Both the annual maximum IRES and the frequency analysis results were then compared with those of the conventional annual maximum fixed-duration rainfall series (FDRS). The results are summarized as follows. First, the mean rainfall intensity of the annual maximum IRES was found to be little changed regardless of their durations. Thus, most of the annual maximum short-duration rainfall series would be selected in the annual maximum IRES. Second, results of the bivariate frequency analysis of the annual maximum IRES were found to be similar to those of the annual maximum long-duration rainfall series. It means that the annual maximum long-duration rainfall series could represent the annual maximum IRES. In conclusion, the short-duration rainfall series is rather artificial and the possibility of their occurrence in reality is rare. Considering the typical shape of rainfall temporal distribution, the peak of a short-duration design rainfall could be much smaller than that of a long-duration one. The use of the concept of CRD finally could result in smaller design floods especially in small basins.