Influence of Rainfall Duration on Urban Inundation Simulations

Instead of choosing a long rainfall duration in the design hyetograph, a short duration is usually adopted in the design hyetograph for storm sewer design. The reason to use a short duration is all because the size of the urban watershed is relatively small, corresponding to a very short time of concentration, like 60 to 120 min. Consequently, hydrologists believe that the design hyetograph with a short duration can already well reflect the rainfall-runoff process in urban watersheds. However, according to the record from 2018 to 2022, eight typhoons and eight rainstorms caused flooding in urban areas in Taiwan, and most of the flooding resulted from rainstorms longer than 120 min. That reveals that the adequateness of using the design hyetograph with a short duration for sewer system design should be rechecked to avoid biased design work. This study selected an urban area in southern Taiwan as a test site, where severe flooding occurred in June 2021. Different rainfall durations in the historical rainstorm record and the design hyetograph are chosen as the storm sewer model’s inputs to investigate the runoff hydrographs’ peak discharges. The results show that the largest peak discharge is not generated from the short duration rainstorm but from the rainstorm duration longer than 6 h.