In this study, the inundation in the downstream area of the reservoir based on the assumption of the destruction of a reservoir as a representative hydraulic structure was analyzed using the DAMBRK model. With the assumption that PMF and the flood discharge resulting from a dam breach flow in from the reservoir upstream, the flood discharge flowing in from upstream was estimated to reach 1,281㎥/s for the 6 hours duration of rainfall and the maximum flood discharge resulting from the dam breach was 2,846㎥/s under breach conditions, which was 2Hd-0.4 hr. Based on these conditions, a simulation of reservoir failure was conducted in this study. In the analysis of the degree of influence of the flood on the downstream area, the time that took the first flood to reach the downstream area located 3.2 km from the reservoir was 24 min, and approximately 1 hour for that located 6.0 km from the reservoir. For the estimation of the reservoir failure outflow hydrograph, the breach flood discharge was estimated under 15 conditions, with 0.1-0.5 hr breach duration variation and within dam height scope of 1∼3Hd. As a result of this estimation, the maximum flood discharge was within the 3Hd-0.1 hr range and the minimum was within the 1Hd-0.5 hr range. The analysis of the flood in the downstream area of a reservoir revealed that from the reservoir to the downstream area, the increase in the flood discharge and level was reduced. It can thus be concluded that this result is due to the ease of geographic gratitude, the water-containing section expansion in accordance with the development of the plain and the pooling effect in midstream regions. In this research, the height of the dam, ranging from maximum to minimum, was applied. The height of the dam, however, differs at various points, and where a breach will occur in the whole dam cannot be precisely determined. Therefore, from the viewpoint of the mean height of a dam, under the aforementioned 15 conditions, it is reasonable to exclude the conditions of the mean width of the breach (Bm)-1Hd which the height of the dam breach section appears to be below one-third, and to apply the conditions of mean width of the breach (Bm)-2Hd, 3Hd which the height of the dam breach section appears to be above two-thirds.