This paper presents some results of three dimension (3D) modelling application to research characteristics of field current variation in the coastal area of Red River Delta. In the study, a 3D numerical model was set up with four vertical layers ( coordinate system). The open sea boundary conditions of hydrodynamics model have been obtained by NESTING method from the same l model for the larger marine region. Hydrodynamics model was calibrated and validated by measured data of water levels in Hon Dau National Hydrographic Station and data of currents in other sites (Ba Lat and Nam Trieu). In the studied results the temporal and spatial variation of total currents and residual currents in the coastal area of Red River Delta are obtained, in which the different roles of tidal oscillations-tide currents, fresh water from river mouths, wind stress-gradient currents and density currents were recorded (the roles of wave induced currents are ignored).

The model system for the Hai Phong coastal area was set up based on the wave climate condition between 1992-2014 and the MORFAC (the morphological acceleration factor) method in the Delft3D model. The model results of more than 50 separate simulated scenarios show the role of each wave interval and direction on characteristics of sediment transport and morphological change in the studied area. In the case of without wave, sediments from the river settle and create deposition zone in river mouths. When the wave height increases, the erosion/deposition and sediment transport in the coastal zone increase. Due to the influence of the hydrodynamics and sediment transport, the erosion/deposition rate in the dry season is higher than that in the rainy season. The morphology strongly change in the depth less than 5 - 6 m with the rate of 20 - 50 mm/year. In the water zone of over 6 m, the morphology relatively stable and reflects an accretion trend with the rate of 10 - 20 mm/year. The simulation results also show an accretion trend in the Hai Phong coastal area. It may be a consequence of the decrease in the number of big typhoons impacted on this area in the last 20 years.

Land-ocean interactions in the coastal zone are severely influenced by tidal processces. In regions of high sediment discharge like the coast of Hau river estuary, these processes are even more significant when we analyse data in flood season (September)-which belongs to Agreement of Cooperation in Science and Technology between Vietnam and US (2013-2015) and project VAST-DLT.06/15-16 (2015-2016). Our goal is to investigate the sedimentation processes. Additionally, we investigated the influence of the tidal currents in relation to the suspended sediment. Salinity (PSU-Practical salinity unit), suspended sediment concentration (NTU-Nephelometric Turbidity Units) were measured by Compac-CTD (Depth temperature conductivity chlorophyll turbidity) and OBS-3A (Turbidity and Temperature Monitoring System) instruments. The results show that the suspended sediment concentration (SSC) correlate with tidal current velocities. The tidal current velocities are up to 60 cm/s near the surface and 40 cm/s near the bottom, increasing SSC in the water column at bottom layer (24 NTU) and decreasing SSC at surface layer (8 NTU). Processes of sediment transport and deposition in flood tide in flood season are influenced by tidal currents more significantly than those in dry season. This leads to an asymmetry of the tidal ellipses and the different deposition between seasons. The analytical results imply the influence of tide and tidal currents on SSC in the coast of Hau river estuary, hence, the spread, sea water and fresh water mixing processes have difference during tidal phases and seasons.

Of the 14 inlets belonging to 12 coastal lagoons in the coastal zone of Central Vietnam, the unstable group consists of 4 inlets; the less stable group comprises 4 inlets and the relatively stable group has 6 inlets. For the feasibility and effectiveness, the constructions of stabilizing lagoonal inlets must be multi-purpose and multi-benefit, such as maintenance of ecosystems, opening to the sea for ships and boats, flood drainage and pollution limitation. They need to be combined with other development activities to reduce costs and increase benefits, for example in conjunction with seaports, fishing harbours, typhoon shelters and tourism... Solutions to stabilise the lagoonal inlets consist of 5 groups: Constructing groins for control of inlets; dredging lagoonal inlets and bottom; preventing coastal erosion outside the lagoons; stabilizing the lagoon banks and the surrounding sandy areas; regulating water supplies in the catchments into the lagoons. Depending on the natural conditions and degrees of human impact, the priority solution has been proposed for each lagoonal inlet.

In order to keep essential depths in shipping channels to and from Hai Phong ports, regular dredging activities are maintained with about 3.6 million tons of sediments per year. Due to almost all the sediments of dredged material in the channels are composed of mud and silt, they are not easy to be used for land filling or other purposes. Moreover, disposing these materials on land is facing difficulties because of requiring the design and construction of dikes, requiring compaction and drainage of dumped materials. Therefore, disposal of dredged material from shipping channels in Hai Phong sea has become urgent. Based on integrated approach, the combination of numerical modeling and multicriteria decision analysis-GIS (natural condition, socio-economic and environment-ecosystem conditions) was made and the most suitable dumping sites were proposed in the regions with water depth ranging between 18–27 m. Their total receiving capacity was estimated about 206 million tons for 40–50 years.