An evaluation of the risks to earth-fill dams due to heavy rains is conducted. Since the number of earth-fill dams is huge, the response surface method, as a simple surrogate model, is proposed to evaluate the damage costs induced by floods. A detailed flood analysis is conducted with the finite volume method to solve the shallow water equations. Then, the damage costs are evaluated with land-use data. In order to avoid time-consuming analytical efforts, the process can be simplified by the response surface method. The model is constructed by the experimental design concept. The evaluated risks obtained from the detailed and response surface methods are compared. In conclusion, it is found that the response surface method can estimate the risks and their ranking with relative accuracy.