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).

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.

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he system of coastal bodies of water in Vietnam consists of three main types as estuarine waters, lagoons and bays distinguished by the features of origin, evolution, dynamics factors, morphology, structure and ecosystem. In some cases, there exist of transitional waters in morphology between types, or this type is a secondary component of others. The closed degree, circulation, eroded – accumulated correlation, water salinity are the most important features of coastal bodies of water. The estuarine waters are divided into two kinds as delta and estuary. The deltas consists of fluvial- dominated (for examples, Red River Delta), wave – dominated (Thu Bon Deltas), and tide – dominated ones (Mekong Delta). The estuaries of which Bach Dang and Dong Nai are most typical locate on the coastal areas where the tide dynamics and eroded process are being dominated. Having 12 unites and total area of 458km2, the lagoons concentrate in coastal zone of the Vietnam Centre where generally there exists of strong wave, tide not large in range, and abundantly sandy sediments. By the closed degree, they can be divided into three kinds including nearby closed, very closed, and fully closed lagoons. The combination between enclosed degree of lagoons and local hydro-climate factors has formed three saline groups as brackish, brackish – haline, and haline lagoons. The coastal bays consists of 48 unites with the total of 4000 km2 and distinguished into bights/ shelters (called as Vũng by Vietnamese) when the area of waters is below 50 km2, and bays (called as Vịnh by Vietnamese) when the area of waters is from 50 km2 and larger. By the closed feature of waters, they divided into five degrees as very open, open, semi – closed, nearby closed, and very closed bays. From them, the most popular degrees are open 46%), then semi – closed bays (29%). Comparing with estuarine waters and lagoons, the coastal bays are rather stable in morphology and dynamical processes. They are different in geographical features by four coastal regions as North Vietnam, North of Centre Vietnam, South of Centre Vietnam, and South Islands Group.