Semimountainous areas in Yamaguchi and Shimane Prefectures suffered severe damage brought by floods on July 2013. The damages in these areas were characterized by the vulnerability of their backlands that consists of agricultural lands. In order to investigate the mechanisms of land erosion on these areas where existing high-resolution data was not prepared, post-disaster RTK-GPS surveys and numerical simulations were conducted for Abu River in Yamaguchi. The surveys were aimed at providing the high-resolution topographic datasets of these regions in order to evaluate the influences of micro-topography on channel incisions by the flood flow. The results of the numerical calculations of movable bed-evolution well reproduced the process of the channel incisions on these areas, and it implies that the micro-topography is one of the most influential factors on erosion and channel formation processes on such semimountanious regions.