Tropical cyclones play a significant role in the formation of the hydrodynamic regime of the South China Sea, including its western part, which is bordered by the coast of Vietnam. In this region, the main hydrodynamic structure is the Vietnamese (western boundary) Current. Its structure depends on both seasonal monsoons and tropical cyclones. Therefore, the study of this dependence is an important task for Vietnamese scientists. The results of these studies are not only of scientific, but also of great practical importance, because the social and economic development of Vietnam largely depends on the activity and variability of synoptic and hydrodynamic processes in this region. This paper presents the results of joint Russian–Vietnamese studies on the dependence of the vertical structure of the Vietnamese current on tropical Pacific cyclones in the north of the South China Sea during the interseasonal winter–summer period. This study is based on numerical modeling. The calculations were performed for April–June 1999, for which the necessary initial field data are available. The simulation results indicated that the Vietnamese Current during this time period is not a uniform stream of water masses. Instead, the structure of water masses in this region is governed by a strong anticyclonic circulation and a deep cyclonic circulation, which are responsible for the complex pattern of the Vietnamese Current. The Vietnamese Current transfers coastal water masses in the summer mode (from south to north) within the anticyclonic circulation and in the winter mode (from north to south) within the cyclonic circulation.

Observation of the Hau River distributary of the Mekong River delta in Vietnam, conducted in dry and flood season (2009, 2014, and 2015), is utilized to investigate the mechanism of formation, distribution of estuarine turbidity maxima (ETM), and links with sediment transport in the system. Additionally, 3D (three-dimensional) numerical models are applied to simulate the seasonal tidal variation (flood and dry seasons) of the water and suspended sediment transport processes of the Mekong River Delta. The 3D model, with a combination of hydrodynamic-wave and suspended sediment transport, was set up and validated with measured data in the study area. The mechanism that measures ETM is the process of suspended sediment from the river when it interacts with seawater and speeds up the flocculation, combined with the asymmetry of the tidal current, which will create the region with ETM by moving in/out with the tidal current’s ups and downs. As there is surface flow velocity towards the sea, the bottom baroclinic flow has a decisive role in deposition and erosion, and it causes the suspended sediment concentration (SSC) to be maximized. During the flood season, the salt wedge near the river’s mouth, at the peak of the tide, pushes towards the sea’s direction when there are ebbing tides, with a scope of about 20 km. In the dry season, there is estuary disturbance as well; the salt wedge forms, but is relatively weak or does not exist, depending on the time of the tide. The maximum turbidity zone in the flood season moves the subaqueous delta with a scope of about 20 km and SSC of about 0.1 to 0.6 g L⁻¹, whereas in the dry season, the seawater has high salinity, and seaward SSC penetrates the estuaries to cause a disturbance and flocculation. The penetration scope is up to 50 km and creates a water mass that has high SSC, from 0.2 to 0.7 g L⁻¹, to run in/off by the tidal current’s ups and downs for several kilometers in the tidal phase.

The state of the natural environment of the marginal seas of the Northwest Pacific is largely controlled by the interaction of the atmospheric and hydrophysical processes. Tropical cyclones (typhoons), originating in the tropical zone of the Northwest Pacific and over the South China Sea basin, occupy a special place among atmospheric processes. The main destructive impact of typhoons falls on Southeast Asia. However, a significant number of are moving to the Russian Far East. The region of the South China Sea plays a significant role in the formation of tropical cyclones. This determines the importance of studying hydrometeorological processes not only in the Far East, but also in the South China Sea, and the need for cooperation between Vietnamese and Russian scientists. The main hydrodynamic structure of the western South China Sea is the Vietnamese Coastal Current (Western Boundary current), which depends not only on the seasonal monsoons but also on typhoons. The paper presents the results of joint Russian–Vietnamese studies of the dependence of the vertical structure of the Vietnamese Coastal Current on the Pacific tropical cyclones that form in the South China Sea. The study is done with numerical modeling. The period from April to June 1999 was used for modeling, provided with the necessary field data. The simulation results showed that, in general, the structure of water masses depends on the trajectories of tropical cyclones. In all cases considered, the Vietnamese Coastal Current is not a single flow, but represents a zone of eddy structures of different directions. An exception is the only situation in the condition of a tropical cyclone in the central region of the South China Sea when this current acquired the form of a single continuous flow directed from north to south only in the 200-m layer. The general patterns of changes in the dynamic structure of the Vietnamese Coastal Current for all the considered tropical cyclone trajectories include the following: areas with water transport in the northern direction prevail on the surface, while the rest of the water mass continues to flow generally in the southern direction. This transport of surface waters may be due to the influence of the emerging summer monsoon, and the rest of the water mass, which is less exposed to the still weak atmospheric processes of the monsoon type, continues to flow in the winter regime.

This paper presents the distribution features of calculated maximum significant wave height in the Southern Nha Trang bay area corresponding to maximum wind speed of 50 years return periods. Wind data from 1976 to 2006 were taken from Nha Trang Meteorological Station. Weibull distrihution function was used to estimate maximum wind speed for different return periods. Dolphin and SWAN models were used to estimate wave characteristics in the offshore and nearshore regions, respectively. The studied results show that maximum wind speeds for 50 years return period were 27, 22, 15 and 17 m/s in NE, E, SE and S directions, respectively. The areas of Southern Tre Island including Nha Trang Port, Mieu, Mot, Tam and Mun islands were strongest affected by wave action from E and S directions with significant wave height of 2 - 4 m, and lesser affected by wave action from NE and SE directions with significant wave height of 2 m. The area between Mun Island to Noi and Ngoai Islands, and the coastline from Dong Ba to Cam Ranh Headlands were strongest affected by wave action from E and NE directions with significant wave height of 4 - 7 m, and lesser affected by wave action from SE and S directions with significant wave height of 2 - 3 m. Maximum wind field blown from SW direction (V = 19 m/s) was generated a relatively strong wave field in the studv area, especialy in the area around Mun and Mot Islands with significant wave height range from 1.5 to 2.0 m. In general, for maximum wind speed of 50 years return period the southern area of Nha Trang bay was strongest affected by wave action from E and NE directions.

Hoi An is a famous tourist city in Central Vietnam with Cua Dai beach which is a long beautiful swimming beach. In recent years, tourism of the city is developing very fast. The southern part of Cua Dai beach was covered by many resorts which were located close to the beach and protected by hard structures such as seawall, revetment, groin,… At present, all properties closed to the beach and hard protection structures of resort system were destroyed by wave action. And the northern part of Cua Dai beach (public beach) was seriously eroded and has been protected by “Soft structure”. Cua Dai coast erosion problem becomes the hot topic in communication system from central to local governments and it becomes the big concern of Vietnamese and international scientists. This paper presents the initial study results of Cua Dai beach erosion features. Study results show that Cua Dai beach erosion/deposition phenomenon occurred for a long time but the rate of accretion was larger than that of erosion. From the year of 2000 to present the rate of erosion was larger than that of accretion. The main causes of Cua Dai beach erosion are the interaction between hard protection structures and wave action, dredging operation in Thu Bon river mouth... which reduced the longshore sediment transport rate from Thu Bon river mouth to eroded beach.

This paper presents the distribution features of wave characteristics based on the continued measurement wave data with interval of 1 hour from January to December 2013 in coastal waters of Ninh Thuan province. The study results show that the dominant wave directions were in East-North-East (ENE), East (E), South-South-East (SSE) and South-East (SE) with occurrence frequency of 36.7%, 20.5%, 18.2% and 11.6% respectively. During North-East (NE) monsoon the dominant wave directions were in ENE and E. During South-West (SW) monsoon the dominant wave directions were in SSE and SE. The remaining directions were rare. The maximum values of wave height (H_max) was 4.84 m (11/2013), wave period (T) was 10.9 s (11/2013). The occurrence frequency of H_s ≤ 0.5 m was 43.9%, H_s ≈ 0.5–1.0m was 21.9%, H_s ≈ 0.5–1.0 m was 21.9%, H_s ≈ 1.0–2.0 m was 27.3%, H_s > 2.0 m was 6.8% and calm wave condition (H_s ≤ 0.25 m) was 8.1%. The duration and intensity of wave action were dominant in NE monsoon period.