The changes in the design storms and temporal patterns between the first and second editions of MSMA

This paper investigates the changes in the design storms and temporal patterns between the first and second editions of MSMA or the Manual Saliran Mesra Alam (the Urban Stormwater Management Manual for Malaysia). MSMA is published by the Department of Irrigation and Drainage Malaysia (D.I.D). MSMAware is a drainage design software for MSMA. The software computes: design storm, temporal pattern, Rational Method, Time-Area Method, On-Site Detention (OSD), detention basin and culvert design. Software is developed by Ir. Dr. Quek Keng Hong. The design storms are computed using the Intensity-duration-frequency (IDF) formulas from the first and second editions of MSMA for a study catchment in Kuala Lumpur and the results compared. The objective is to determine the changes in design rainfall intensities between the first and second editions of MSMA. For durations of between 15 to 700 min, the IDF estimates using the first edition of MSMA (MSMA1) were mostly higher than those estimated using the second edition (MSMA2). In the study, out of 14 stations, 10 of them (or 71%) were higher than the first edition curve, while the remaining 4 stations (or 29%) were lower than the first edition estimates. It is concluded that the design storms estimated based on the second edition for Kuala Lumpur can be up to about 26% higher than the first edition for duration below 700 minutes, for 71% of the stations. Overall, the quality of the storm data in the second edition of MSMA is better as the new data is more up-to-date. The IDF data in the second edition covers longer storm durations from 5 minutes to 72 hours, and the lower range ARI of 0.5 to 12 months. There are now 135 stations in the second edition compared to only 35 previously. No IDF data is provided for East Malaysian states of Sabah and Sarawak. The changes in the IDF data is expected to change the magnitudes of design storm. In the second edition, the formula for computing the IDF data has changed from a polynomial formula to an empirical equation. The storm intensities have changed due to the changes in the formula used. In the first edition, the data used were up to about 1983 or 1990. For instance, the data used for the Federal Territory was only up to 1983 in MSMA1. However, in the second edition, the data used were more up-to-date. The temporal patterns for 15, 30, 60, 180 and 360 minute durations for the second edition of MSMA are generally lower than those for the first edition for the first one-third duration but rise above the latter for the remainder. Areal reduction factor (ARF) is given in the first edition of MSMA but not in the second edition. It is recommended that the procedure given in the first edition of MSMA should be applied for the second edition to account for the catchment averaging effect of point precipitation.