water level) Flood risk model: f + 1 = , , x: model variables (e.g., levee height, settlement size) Observation Simulated observation Operator True world OSSE is the idealized numerical experiment to check if the proposed algorithm properly works. Model parameter: (e.g., social memory decay rate) External forcing (High water level) Flood risk model: f + 1 = , , x: model variables (e.g., levee height, settlement size) Simulated observation Operator Model parameter: (e.g., social memory decay rate)