Detecting two types of maximum turbidity in Cua Cam - Nam Trieu Estuary

The deposition zones in the estuary (also known as the maximum turbidity zone) are a natural phenomenon related to the process of land-sea interaction through hydrodynamic and bio-geochemical processes. This phenomenon usually occurs in estuaries with large tidal amplitude and river discharge. The maximum deposition zones are the places where material accumulates, including suspended sediments, dissolved nutrients, and pollutants from the continent.

Many studies around the world have shown that the formation of maximum turbidity zones is the main cause of sedimentation in some large estuaries. For example, in the Hudson River Estuary (between Jersey City and New York, USA), extreme turbidity increases the sedimentation rate for this area by up to 30cm/year. Another example is the role of the maximum turbidity zones in the northern tributary of the Yangtze Estuary to the Shanghai port sedimentation. After the completion of the Three Gorges Dam blocking the Yangtze River (in 1997), the decline of river water changed the land-sea interaction, the maximal deposition zones deeply moved to Yangtze estuary. Therefore, although the sediment flow from the continent has decreased sharply (from 419 million m3/year in the period 1950-1986 to less than 210 million m3/year in the current period (Yang et al., 2006), sedimentation due to increased maximal turbidity strongly increases the sedimentation rate of the channel into Shanghai port increased only about 30 million m3/year before 2000 and increased sharply to 42 million m3 (2006) and 57 million m3 (2008).

Being home to the largest seaport system in the North, the gateway to the sea of the northern provinces, due to the characteristics of natural conditions, the phenomenon of sedimentation in Hai Phong port channel is taking place strongly and tends to increase in recent years. In only the Hai Phong and Pha Rung port channels managed by Vietnam Maritime Administration, the dredging volume in 2019 has reached about 2.7 million m3. If other channels are included, the total annual dredging volume in this area can reach over 4 million m3/year. Dredging activities to maintain the necessary depth for the Hai Phong port system not only cause difficulties for port operations and reduce business efficiency, but also give rise to environmental problems such as dispersion of turbidity, dispersion of pollutants from bottom sediments into the aquatic environment. Therefore, many related studies with different approaches such as geology and dynamics have been conducted to find out the causes of sedimentation in Hai Phong port channel. However, no studies have been conducted to find the relationship between the formation of the maximum turbidity areas and the sedimentation of the Hai Phong port channel.

Within the framework of bilateral cooperation agreements between the Vietnam Academy of Science and Technology (VAST) and the French Institute for Research and Development (IRD), the task "Research on the relationship between the dynamics of formation of maximum deposition zones in Cua Cam - Nam Trieu Estuary with the sedimentation of the Hai Phong port channel” (code QTFR01.01/20-21) was studied by the research team led by Dr. Vu Duy Vinh of the Institute of Marine Resources and Environment in collaboration with Prof. Sylvain Ouillon and several other scientists from IRD. After two years of research, for the first time in Vietnam, the mechanism for the formation of maximum turbidity zones in the Cua Cam-Nam Trieu Estuary has been determined (the work published in the journal Marine Geology, 442, Article 106670, 2021) with the following main features:

Two types of maximum turbidity exist simultaneously on the Cua Cam-Nam Trieu Estuary with different positions at the edge of the saline wedge.

- The first maximum turbidity zone is deep inside the estuary, developed in conditions of very low salinity (from 0.1 ‰) and almost homogenous water layer (Simpson's parameter is approximately 0.65 and 7-10%). Tidal currents combined with the flocculation process of suspended sediment particles due to salinity are the main reasons for the formation of these maximal turbidity areas.

The formation mechanism and variation of the maximum turbidity zones in the Cam-Nam Trieu Estuary (Vinh & Ouillon, 2021): the maximum turbidity zones in the rainy season (figure a), and the dry season (figure b)

- The second maximum turbidity is located at the bottom (near the sea) formed in the water with very clear stratification (Simpson parameter > 10), in the salty wedge, with high salinity (about 12-15 ‰), dominated by density circulation and the presence of Transparent Exopolymeric Particles (TEP- products of phytoplankton and bacteria in the aquatic environment in the estuary).

- The first maximum turbidity areas in the dry season and the second maximum turbidity areas in the rainy season have higher turbidity than the remaining turbidity areas.

- The maximum turbidity areas in the dry season are larger in scale and intensity than those that occur in the transition season (from the dry season to the rainy season) and in the rainy season.

- The position of the maximum turbidity areas changes with the seasons: moving towards the Cam Estuary in the dry season and moving towards the Nam Trieu Estuary in the rainy season.

The results of the research team also show the very important role of the maximum deposition zones in sedimentation in Hai Phong port channel, an issue that has not been paid attention in previous studies.

Translated by Tuyet Nhung
Link to Vietnamese version

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