The Mekong River is ranked as the 8th in terms of water discharge and as the 10th in terms of sediment load in the world. During the last 4500 years, its delta prograded more than 250 km to the south due to a tremendous amount of sediments deposited, and turned from a “tide-dominated” delta into a “wave-and-tide dominated” delta. This study aims at completing our knowledge on the fate of sediments that may be stored in estuarine or coastal systems, or dispersed over the continental shelf and slope. Sediment transport in the Mekong River Delta (MRD) coastal area was studied by numerical simulations using the Delft3D model. The model configuration was calibrated and validated from data collected in situ during 4 periods from 2012 to 2014. Then, 50 scenarios corresponding to different wave conditions (derived from the wave climate) and river discharge values typical of low flow and flood seasons enabled us to quantify the dispersal patterns of fluvial sediments close to the mouths and along the coast. Sediments mostly settled in the estuary and close to the mouths under calm conditions, and suspended sediment with higher concentrations extend further offshore with higher waves. Waves from the Southeast enhanced the concentration all along the MRD coastal zone. Waves from the South and Southwest induced coastal erosion, higher suspended sediment concentrations in front of the southern delta, and a net transport towards the Northeast of the delta. Because of episodes of Southern and Southwestern waves during the low flow season, the net alongshore suspended sediment transport is oriented Northeastward and decreases from the Southwestern part of the coastal zone (~960 × 10³ t yr⁻¹) to the Northeastern part (~650 × 10³ t yr⁻¹).

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.

Although successful in protecting planted mangrove plants, the effectiveness of emergent detached offshore structures in increasing vegetation cover has yet to be definitively determined. We selected Tien Giang Province, Vietnam as an appropriate case study to address this question. We analyzed multiyear (2000 and 2022) shoreline changes and calculated the enhanced vegetation index (EVI) together with ground truthing in pursuit of the objectives of the study. Our findings suggest that emergent detached offshore structures have yet to lead to an increase in vegetation cover or promote mangrove growth. The vegetation growth steadily increased, as did the high level of natural mangrove growth with fully grown mangrove trees, even before the structures were constructed. By 2015, all the categories increased slightly except for low vegetation cover (LVC) and medium vegetation cover (MVC). LVC decreased from 390 ha in 2010 to 291 ha in 2015, while MVC decreased from 305 ha in 2010 to 275 ha in 2015. By 2020, all the categories decreased slightly except for non-vegetation cover—Barren lands (NVC2) and MVC. NVC2 decreased slightly from 404 ha in 2015 to 368 ha in 2015. The MVC decreased slightly from 275 ha in 2015 to 212 ha in 2020. Non-vegetation cover—Intertidal mudflats (NVC1)—LVC, and high vegetation cover (HVC) increased slightly from 2015 (326 ha, 291 ha, and 249 ha, respectively) to 2020 (368 ha, 292 ha, and 298 ha, respectively). By 2022, NVC2, MVC, and HVC remained unchanged, while NVC1 and LVC increased slightly from 368 ha and 292 ha in 2015, respectively, to 380 ha and 302 ha, respectively. The increase in vegetation cover and the natural regeneration of mangrove species were partly due to the adaptation of mangrove species to the site (river mouth areas), particularly the protection provided by Ngang Island offshore, and the construction of these structures. In addition, these structures were constructed in a rather stable area (slightly eroded and estuarine area) and therefore have yet to provide any noticeable benefits for mangrove regeneration three to five years after their construction. In the future, the morpho dynamic and hydrodynamic elements of the site should be adequately considered during the design and construction of these structures to increase vegetation cover and promote natural mangrove regeneration.

This paper presents the results of applying a 3D mathematical model to study the in the Mekong riverside area’s maximum turbidity zone (MTZ). This study has established and calibrated a three-dimensional system model with a combination of hydraulics-waves and suspended sediment transport with measured data in the study area. Based on the calculated scenarios for the flood and the dry seasons, the results show the occurrence of MTZs in the riparian zone of the Mekong river with typical suspended sediment concentrations ranging from 0.04–1.0 kg/m3 dry season and 0.2–1.1 kg/m3 flood season. The location and size of the MTZs change seasonally with the interaction between freshwater and tidal oscillations. They appear more in the dry season. At high tide, seawater with high salinity intrudes into estuaries with a range of up to 50 km, causing disturbance and flocculation. While in the flood season, the distribution of the largest turbidity has moved the floodplain with a range of about 20 km.