In any flash flood evacuation operation, vehicle assignment at the inundated areas is vital to help eliminate loss of life. Vehicles of various types and capacities are used to evacuate victims to relief centers. This paper examines combinatorial optimization approach with the objective function to assign a specified number of vehicles with maximum number of evacuees to the potential inundated areas. Discrete particle swarm optimization (DPSO) and improved DPSO are proposed and experimented on. Results are presented and compared. Improved DPSO with the proposed min-max approach yields better performance for all four testing categories. Experimenting on a large number of evacuees could further improve the performance of the DPSO.