Flooding represents 44% of all disasters worldwide, and it is one of the most predominant natural disasters in the Asia–Pacific region, with the Philippines being among the vulnerable countries due to the unique characteristics of the region that favor the formation of typhoons. Most notably, Typhoon Ondoy (Ketsana) in 2009, which affected Marikina City, left it flooded due to the river overflowing. This research employs FastFlood, an open-source, rapid flood simulation tool, to model flood dynamics in Marikina and assess the vulnerability of key infrastructures under different rainfall intensities over an extended period. Rainfall intensities of 18 mm/hr, 56 mm/hr, and 90 mm/hr, reflective of Typhoon Ondoy, were used to simulate flood events that lasted over 1, 3, 6, and 12 hours. Outputs from the FFS-tool were then exported to QGIS for visualization and flood risk analysis. The results indicate that moderate rainfall can result in flooding in low-lying areas, with flood levels up to 7.43 meters under extreme scenarios. Infrastructure vulnerability increases with rainfall intensity, especially in areas along the Marikina River. Despite being in early development, FastFlood proves to be valuable for its rapid modeling capabilities and interactive flood hazard mapping features. However, current limitations, like the lack of calibration and reliance on default data, highlight the requirement for further validation using real-world flood events. (Initial thesis abstract before final thesis defense).