Debris flow is a natural phenomenon that can cause some damage and fatalities. Debris flow from Mount Merapi frequently happens through the Putih River, so it is necessary to do research. Anticipating the occurrence of debris flows can be done with predictions to reduce the number of casualties and material losses. This research used a simulation method based on Ashida, Takashi, and Mizuyama equation that applied the Nakayasu synthetic unit hydrograph through modeling on SIMLAR V2.1 application by modifying the hyetograph pattern and the rain intensity value. The test result contained flood area, velocity, volume, flow height, and the value of degradation and aggradation that happened in each simulated pattern. The velocity value in simulation with rain intensity of 56.8 mm and one peak hyetograph is 1.36 m/s, and in simulation with rain intensity of 56.8 mm and two peak hyetograph is 1.42 m/s. It means the parameter increases by 4.4%. Based on the results, it can be concluded that the hyetograph pattern is very influential on the impact of lava floods. The increase in the hydrograph value is also directly proportional to the increase in flow velocity. The value of rain intensity and rain pattern also affects the number of areas affected by the lava flood. The increasing value of the intensity and pattern of rain is also directly proportional to the increase in the area affected by flooding.