In this study, the HEC-GeoRAS model was used to estimate the changes of flood inundation depth and area of the Hyeoam-cheon basin
after the channel migration and river improvement. The target area was extremely damaged due to the heavy rainfall on 25th August,
2014 after the costruction. According to the results of the analysis, the HEC-GeoRAS model can be used to accurately estimate the
changes of flood innundation depth and area in agrecultural lands and lowlands of which the topographic changes and elevation
differences are small. The inundation depths were increased 0.99 m in heavy rainfall and 0.39 m in 100 year frequency flood. The
inundation areas in 100 year frequency flood were reduced in inundation depths under 1.5 m while they were increased in depths above
4.5 m. It was concluded that the loss of natural resevoir capacity of agricultural lands and lowlands due to the channel migration and river
improvement caused the damage of flood in 2014 and the decrease of actual effective widths of the channel added the damage.