The 2011 Great Tohoku Earthquake and Tsunami in March had caused a lot of damages to people and infrastructure in Japan. With a giant tsunami height plus the subsidence due to earthquakes have destroyed most of the nearshore constructions and coastal zone areas. According to measured data from the survey group (Mori et al. 2011); maximum tsunami wave height measured at the Nanakita area is about 14m. This Tsunami made the topography of this region changed drastically. Many breaching places occurred along the coast and river mouth. This study will focus on investigating the mechanisms of sediment transport and hydrodynamics leading to the river mouth was closed in about five months after the event and then breached in a different location during the river flood.