A dam is one of the water structures that have many benefits for humans. However, flood disasters caused by dam break may have a very bad impact on human life. This study specifically analyzed the impact of flooding caused by the failure of the Karalloe dam in Bone Regency, South Sulawesi Province, Indonesia. For the first time, the selected flood discharge used in the dam break analysis was verified using the Creager graph by comparing the calculated discharge from several synthetic unit hydrograph methods (HSS) with the flood discharge measured on the automatic water level recorder (AWLR) at the monitoring point. The impact of flooding due to dam break was simulated using HEC-RAS 2D combined with ArcGIS for mapping. The calculation results of the design flood discharge based on rainfall data using the methods of HSS Nakayasu, HSS ITB I, HSS ITB II, and HSS SCS (HEC-HMS) as well as the calculation of the designed flood discharge based on the discharge data showed that the design flood discharge value which is closest to the measured discharge value and Q1000 Creager was the HSS SCS method. The flood discharge values obtained based on the HSS SCS method for Tr (return period) 2 years, Tr 5 years, Tr 10 years, Tr 20 years, Tr 25 years, Tr 50 years, Tr 100 years, and Tr 1000 years were 322.70 m3/s, 464.10 m3/s, 560.40 m3/s, 658.40 m3/s, 682.70 m3/s, 787.00 m3/s, 885.70 m3/s, and 1202.60 m3/s, respectively. The simulation results showed that 22 villages will be affected by flooding due to the failure of the Karalloe dam and the fastest standby time of the flood is 12 minutes, namely at Lookout Point 7 in Paitana Village. This result suggests that early warning system should be installed at the downstream of the dam and flood disaster mitigation should be adopted and applied to these threatened area.