Developing the IsoProbability Curves of Combination of Rainfall and Water Level for Urban Drainage Infrastructure Planning—A Case Study in Ho Chi Minh City

To ensure efficiency and cost-effectiveness, drainage systems often need the capability to handle drainage such that the overload phenomenon only occurs with a return period greater than a certain value specific to the system. For urban areas situated in low-lying regions and influenced by tides, the flow in the drainage system depends on two natural factors: rainstorm rainfall in the area (R), and water level at the outlet (H) in the time of rainstorm. Consequently, the return period of the overload phenomenon in the drainage system will be determined by the combination of R, H causing the overload. Before conducting hydraulic calculations, it is essential to construct probability curves for the R, H combinations. This article aims to introduce a method for constructing iso-value curves representing the return period of R, H combinations. The method is applied to the case of Ho Chi Minh City as an example.