The area o f Cua-Dai estuary and the coastal areas o f Hoi-An City have experienced complicated erosion and sedimentation in recent years. Along the coast ofHoi-An, erosion often occurs, whereas in the area o f Cua-Dai River, there is an accretion phenomenon that obstructs the waterway navigation from Cua-Dai to Cu-Lao-Cham. The occurrence o f sand dunes in the offshore location o f Cua-Dai has been recorded a number o f times in recent years. The purpose o f the study is to find out one o f the potential causes that may be related to the formation o f a new island in the estuary. Research results show that the amount o f sediment from upstream o f the Thu-Bon river has a great influence on the change o f bathymetry o f the estuary. The research results also show the trend o f coastal sediment movement in the south direction during the simulation period from January 2016 to May 2019 in the study area. This study used the numerical model Telemac which combines the hydro-morphodynamic and wave modules.