Decrepit earth-fill dams ought to be improved in order to mitigate natural disasters. Since there is a recent demand for low-cost improvements, the development of a design method for optimum improvement work at a low cost is the final objective of this research. As a basic step, the risk to earth fills during heavy rains is evaluated here. The rainfall intensity and the variability of the numerous soil parameters of the earth-fills related to soil erosion are dealt with as probabilistic parameters. When the maximum overflow head on a spillway bed becomes greater than the design overflow head, overflow occurs. The peak overflow head, hp, is determined within 72 hours of the rainfall, and the various rainfall patterns are tested by the Monte Carlo method. Furthermore, the statistical values of the parameters for the soil erosion are assumed in this research, and the variability of the discharge hydrographs are derived from the values. The discharge hydrographs are applied for the flood simulation as input waves. The submerged area and the head of the flood discharge in the downstream area can be predicted. Finally, from the estimated probability of overflow, and the statistical values of the submerged area and the head of the flood discharge, the risk to the downstream area of the earth-fill dams can be evaluated.