Coastal erosion is caused by decreased sediment supply from rivers and the volume reduction in river mouth terraces, which serve as sources of fine sediment. Predicting morphological variations in coastal areas, including river mouth sandbars and terraces, requires comprehending sediment transport dynamics caused by flood flows and waves. As the first step in analyzing fine sediment transport in coastal areas, flood flow and riverbed variation analyses were conducted, including the downstream river sections with the surrounding coastal area. The initial shape of the river mouth sandbar is crucial when it comes to how it affects the morphological changes in the river mouth. However, it is difficult to give the initial topography accurately due to its considerable temporal variation and was a focal point of this analysis. Analyses of the several initial shapes of the river mouth sandbar were compared with direct observational after the flood. The processes of river mouth sandbar flushing and the terrace formation were investigated. The results show that the temporal change in water and river mouth terrace formation vary significantly depending on the initial shape of the river mouth sandbar and three-dimensional flows. This highlights the need to understand the shape of the river mouth sandbar before the flood.