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.
Located in the estuary region of the Red - Thai Binh river system, in which the estuarine turbidity maxima occur, the deposition of navigation in waterways to Hai Phong ports is always an urgent problem that needs to be solved. At the present time, it is not easy to use dredged sediment for landfilling or other purposes. Moreover, it is also difficult to dispose of them on the land because of requiring the design and construction of dikes, requiring compaction and drainage of dumped materials. Therefore, disposing of materials at dumping sites in Hai Phong open waters is still an alternative for considering. However, the suspended sediment from the dumping sites can cause influences on the marine environment and ecosystems. Based on the characteristics of natural conditions, socio-economy and ecological environment in Hai Phong coastal areas as well as results of the modeling application (Delft3D model), this paper gives some proposals for appropriate solutions to reduce influences of sediment dumping activities in the Hai Phong open waters. They are: (1) Planning a land for reclamation of dumped materials, (2) Research on the strategies to use dredged sediments as a resource, (3) Finding a best method of dumping, (4) Considering tidal current for daily dumping, (5) Choosing disposal time during neap tide, (6) Restricting disposal in case winds come from SW and S, (7) Reducing time and the number of dumping, increasing amount of sediments in each dumping, (8) Applying technologies in disposal monitoring, (9) Regularly monitoring the environment at the dumping sites and adjacent areas.
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