The design rainfall analysis was conducted for the design of the urban drainage system, using rainfall data with various intervals including annual and short-interval (minutes and hourly) data from 4 representative stations in Bangkok Metropolis. Five methods of analysis comprise: the Composite Hyetograph, the Yen and Chow, the Pilgrim and Cordery, the Huff, and the Kiefer and Chu. Rainstorms with 30-, 60-, 120-, 180- and 240-minutes duration at 2- and 5- years return period were studied. The structure, hypothesis and constaints of each method were analysed and compared in order to understand the differences, drawbacks and advantages of each particular method. Conclusions drawn from the analysis were that the design rainfall produced by various methods differed according to different concepts and hypothesis of each method employed. Basically the analysis was carried out by using the relationship among rainfall depth, intensity, duration and frequency. One finding was that the IDF Curve had changed somewhat, from the one normally referred to in several reports/papers. For certain duration, the rainfall intensity for the 6- to 24-hours duration increased. These changes were associated and conformed with changes in the urban's environment which, in turn, effect rainfall's pattern as explained in several references. It was therefore suggested that the IDF Curve be updated and improved using the most current data. The design rainfall generated by each method has a specific character. The Composite Hyetograph Method produced peak intensity at the initial interval and tapered off later on. These intensities were equal for all duration and at 5-year return period, the corresponding intensity was 170 mm/hr. The Yen and Chow gave a triangular shape pattern with the internsities vary inversely with the duration. At 5-year return period and 30 minutes duration, the intensity was 220 mm/hr. The Pilgrim and Cordery Method imitated the real situation, it gave the intensity similar to the actual rainfall. At 5-year return period and 30 minutes duration, the maximum intensity was 150 mm/hr and the differences from the actual rainfall were found to be between 0-30%. For the Huff Method, the 50% probability level and the duration of the maximum value of 1/4 and 2/4 of the total duration were selected for designing. The differences from the actual rainfall were between 0-30%. The Kiefer and Chu method which needs the analysis of the IDF Curve produced the r-values (the ratio of the time before peak to the total rainfall duration) between 0.1 to 0.4. At 5-year return period the intensity was 210 mm/hr. In conclusion the pattern of design storms for the Bangkok area were highest in between 20 to 50% of the total storm duration. And when comparing the generated design value with the actual one, each method had both significant similarity and difference patterns for different duration. Thus, the selection of design storm analysis method was hinged upon many factors such as: the availability of data, the conditions of the watershed, the criteria for design storm and etc. To improve the reliability of the results, besides using the most up-to-date data, the combination of methods which take into account all the positive points of each one, such as the Composite Hyetograph and the Kiefer and Chu methods, should be employed.
