The Government VN had planned to build Vam Cong Bridge (VCB) across Bassac (Hau) River at location of 7.5km downstream Long Xuyen Town and of 35.0 km upstream Can Tho City (see Fig. 1.3). It is one is largest objects of national project “Ho Chi Minh Road” lengthened along west side of Viet Nam land. There are two important facts: (i) The water discharge (including sediment and other mass...) through Hau River at this cross-section controls about 48-49% of total discharge of Mekong River entered to VN, so any it‟s change can induce large impacts on hydraulic and hydrological regime of Hau river (and Mekong river basin in general); (ii) The VCB is very big, so its concrete objects (including bridge piles, roads connected to bridge...) placed across Hau River and floodplain area of river‟s sides will generate large changes of hydraulic, flooded, erosion/deposition regimes of Hau River and closed to it areas. Among many problems induced by building VCB and related with him other infrastructures (This combined system will be note as “VCB”), most important is problem of river topographical changes. The assessments on these processes are needed to find optimal option for designing and constructing VCB. Adding to this, the predictions on impact of building VCB on river flow and flood inundation have big practical means for Vietnamese Mekong River Delta (Cuu Long river delta). 2. THE PURPOSE OF RESEARCH Assessing VCB‟s impact on Bassac river topography by numerical modeling. 3. OBJECTIVE, SCOPE AND CONTENTS OF STUDY River hydro dynamical and topographical processes in Hau River before and after building VCB for finding optimal option with minimal negative impact are main study objectives of present thesis. Hau River has pressured by heavy impacted by VCB is very small piece of very large river system in Cuu Long River Delta. So, the impact on VCB will spread for closed large area, particular in flooded season. Flow of this part of Hau River is very different for wet and dry seasons, particular for some months dry season, direction of water flow here has been changed and controlled by tidal fluctuation entered from East Sea. In general, for this part river, flow induced by upstream discharge will be 4- 5 times larger flow generated by tidal fluctuation.