This study investigated the effect of the past drainage system and river mouth to the return flow of the 2011 Great East Japan Tsunami at the Arahama Coast and the Akaiko Coast, Sendai, Japan. At both locations, there were old river mouths, which have been closed for many years. They used to be the down stream end of the past drainage system that was confirmed from historical map. Breaching occurred at the location of these old river mouths due to the strong return flow induced by the 2011 tsunami. A detail GIS analysis was carried out based on topographic data (DEM) to obtain the flow direction and accumulation. The results revealed that the topography appears as a river basin with mild slope towards the old river mouth locations. Analysis of the flow accumulation confirms that the topographical features caused the flow to accumulate through the past drainage system towards the location of the old river mouths, which caused the breaching due to the tsunami return flow. This allowed the return flow to be effectively drained.