Severe accidents are the unexpected events that cannot be appropriately mitigated because there are no appropriate mitigation systems or strategies. Based on lessons learned from these accidents, the establishment of all possible mitigation strategies that take into account available mitigation systems is essential. Since nuclear power plant (NPP) have become larger and more complex, systematic approach to develop mitigation procedure is needed. To handle this complexity, multilevel flow modeling (MFM) is suggested and utilized to develop proper mitigation procedures for the NPP. The MFM is a well-known qualitative modeling methodology for representing complex systems at different abstraction levels of specifications. In this study, time-related information is additionally considered to reflect dynamic features to the conventional MFM model. If the time-related information is added to the MFM model, more diverse and quantitative mitigation procedures can be established. For example, in case of the water supply system with the backup tank that is described in this paper, one mitigation procedure that is the use of backup tank can be developed with the conventional MFM method. If time-to-propagate concept is applied, we can develop mitigation strategy as follow: (1) use water in the tank 1 for 3128s then use water in the tank 2 for additional 3303s then use water in the backup tank, (2) use water in the tank 2 for 2757s then use water in the backup tank. These various accident mitigation options help to mitigate accident effectively.