User may inquiry why on simplified method, the ARI is always 10 years.
Based on Chapter 5 (On-site Detention) accordance to MSMA 2nd Edition, it is stated that the design storm shall be 10 year ARI accordance with the minor drainage system (Section 5.8.1, page 146).
MSMA 2nd Edition uses simplified procedure in OSD calculation, which involves the use of tables and figures, which are produced based on 10 year ARI coefficients and calculation. Unlike rational method, where users are able to assign the years of ARI. This is why it is unable to have different years of ARI for MSMA 2 OSD Simplified method design.