Currently, climate change is a serious issue all over the world, especially, the Ho Chi Minh City (HCMC) is listed as one of top ten cities which are strongly impacted by sea level rise (SLR). To protect the HCMC from the inundation problem, it is necessary to find an adaptive solution. Constructing sea dike in the Can Gio Bay located at the Saigon-Dongnai River mouth to protect the whole of the HCMC area is now one of the probable solutions proposed by the Vietnamese Government. Two sea dike plans were proposed: a sea dike connecting the Go Cong to the Vung Tau (the GCVT sea dike) and a sea dike connecting the Go Cong to the Can Gio (the GCCG sea dike). However, it would make a significant impact to the Can Gio Bay area, which is considered as "Green-Lung" of the HCMC with vast mangrove forest that is respected as one of the most beautiful mangrove forests in the Southeast Asia. Assessing the impact of the SLR and the sea dike construction in the Can Gio Bay must be vital for the sustainable development of the Can Gio mangrove forest. Especially, the sea dike construction could change the salinity environment in the Can Gio Bay. The impact of sea dike construction on the ecosystem in the Can Gio Bay must be studied to determine which sea dike design is suitable for both reducing inundation and preserving the ecosystem in the Can Gio mangrove forest.
In this study, the influence of the SLR and the sea dike construction to the hydrodynamic regime and the salinity regime in the Can Gio Bay was assessed by numerical simulations based on the scenario analyses.
In the Chapter 1, an overview of urgent issues of climate change and global warming, and the background of the HCMC were introduced, which included the causes of inundation and the solution to reduce inundation for the HCMC. The necessity and objective of the study were then presented.
Furthermore, the theoretical underpinnings for the research aims were elaborated based on the literature review.
The overview of the targeted area – the Can Gio Bay area was introduced in the Chapter 2. The physical characteristics of the Can Gio Bay area including geography location, terrain, soil, climate, hydrological system, tidal regime and
salinity were shown in detail. In addition, the Can Gio mangrove forest was introduced clearly. The social-economic aspect in the Can Gio Bay area was also shown in this chapter.
In the Chapter 3, the influence of the SLR and the sea dike construction to the hydrodynamic regime including the inundation situation in the Can Gio Bay was evaluated by the scenario analyses using the hydrodynamic model. The two-dimensional depth-averaged hydrodynamic model was applied for simulating tidal current. Because the Can Gio Bay was flat low-lying wetland area, the wetting-and-drying scheme was used to determine the tidal flat area. Totally 27 scenarios were set up based on the SLRs in the years 2050 and 2100, two types of sea dike of the GCVT and the GCCG, an thre type of operating modes for the sea dike gate and their combinations. The simulated results were validated with the observed data and it was showed clearly that the model could both provide good performance and satisfactorily simulate the hydrodynamics in the Can Gio Bay. The results of the scenarios analyses of the SLR and the sea dike construction showed that the SLR was a crucial factor that contributed to a dramatic increase of inundation in the Can Gio Bay, and the entire Can Gio Bay area would be completely submerged when the sea level increased by 1.00 m.
The sea dike construction was proved to be an efficient mechanism for controlling the water level and reducing the inundation. In the case of the GCVT sea dike construction, the Can Gio Bay becomes less navigable, the water level could be controlled and the inundated area was more effectively reduced than those in the GCCG sea dike construction. When the sea water level increased by 1.00 m, however, the effectiveness of both the CGVT and GCCG sea dikes for controlling the water level declined and the effect of inundation reduction was weakened. When the operation of sea gate was combined with sea dike construction, the effectiveness of the sea dike in reducing the inundated area became to be considerable, particularly in cases of the SLR. The closed area created by the GCVT sea dike construction, together with the operations of the gate, was proved to be effective in decreasing the inundated area, although reduction of water levels in the inner Can Gio Bay was slight.
In the Chapter 4, in order to assess the influence of river inflow discharge and tidal motion to the spatial-temporal distribution of salinity along main rivers in the Can Gio mangrove forest located in the south of Vietnam, the field observations were conducted to collect the salinity time series, spatial salinity distribution and vertical profiles of salinity during two periods in rainy season in 2017 and 2018. The period I in 2017 that was the end of the rainy season had a normal discharge from the upstream rivers and at the spring tide. The period II was the middle of the rainy season had a higher discharge from the upstream rivers and at the middle tide. All of the observed data of two periods showed a clear relationship of salinity with the interaction between tide and river inflow discharge. Upstream inflows and tidal regime drove the salinity change. All of the salinity data in the period I were higher than those in the period II, because the upstream inflow discharge in the period I was smaller than that in the Period II. The salinities in the upstream of the Long Tau and the Soai Rap Rivers were low, and they increased when the distance to the sea decreased. Moreover, the salinities along the Soai Rap River from the Phu Xuan Station, the Vam Co River to the Dong Tranh Estuary were significantly affected by the upstream flows at the Phu Xuan Station and the Vam Co River. However, this trend was not similar to that of the Thi Vai River. All of the area along the Thi Vai River had a high value of salinity, and the salinity was almost driven by the tidal regime because the upstream discharge of this river was very small as compared to others. The sudden salinity change was found along the Dong Tranh River and the Thi Vai River during the observation in the Period II. It was no evidential data collected from these sites, but sudden salinity depletion could be explained based on observations through the actual navigation. Heavy rainfall occurred during the field observation and the freshwater created by rainfall-runoff at the drylands, which water area became during ebb tide period, entered to the rivers through small creeks. In the rivers, there existed seawater bodies or bands and freshwater bodies or bands alternately without mixing due to the freshwater inflow.
However, a more detailed survey would be needed to identify the reason causing the sudden fluctuation of salinity in this area.
Finally, in the Chapter 5, the numerical simulations using nine scenarios were carried out by the hydrodynamic model coupled with the convective-dispersive model to assess the impact of the SLR and the sea dike construction on the salinity regime in the Can Gio Bay and the mangrove forest. Totally nine scenarios were set up based on the SLRs in the years 2050 and 2100, two types of sea dike of the GCVT and the GCCG, an operating mode for the sea dike gate and their combinations. The validation results showed that the model could be a worthwhile facility for simulating salinity in the shallow water areas such as the Can Gio mangrove forest. As the results of the scenario analyses, the seawater intrusion was flushed into the Phu Xuan Station via two pathways along the Soai Rap River and the Long Tau River. However, the seawater intrusion at the Phu Xuan Station could be more affected by the Long Tau River where was strongly affected by the tidal regime. At the stretch of the Vam Co intersection where was the confluence of many rivers, the tidal regime was completely overwhelmed due to the river inflow discharge. The seawater intrusion here had an unusual trend with the high seawater intrusion in the upstream zone and low salinity in the downstream zone. In general, when increasing seawater level, the seawater intrusion at most of points tended to increase almost linearly with rising sea level.
Especially, when comparing the background maximum salinity calculated by averaging the maximum salinity values at total 18 referenced points, the background maximum salinity in whole Can Gio Bay area dramatically increased by around 3.7 psu in the 0.25-m SLR and more seriously with 6.9 psu in the 0.73-m SLR. In the baseline, the horizontal salinity distribution had decreasing trend to the upstream direction in whole area of Can Gio Bay, following the indispensable principle “closer to the sea is higher salinity”. However, this principle did not comply at the stretch of the Vam Co intersection when rising sea level. There existed an unusual trend at the Vam Co intersection. The SLR made salinity in the upper area of the Vam Co intersection higher value than that in the lower part, because this region had complex features of topography and river network. Therefore, seawater was additionally received from the Ganh Rai
Estuary going to many small rivers via the Long Tau River and delivered directly to the upper part of the Vam Co intersection by many small rivers.
The solutions for the flood-prevention by the sea dike construction were proved as an efficient facility for controlling water level and reducing the inundated area in the Can Gio Bay. In contrast, the seawater intrusion strongly fluctuated. These changes also drastically differed spatially and temporally. The GCCG sea dike tended to reduce seawater intrusion in the Long Tau River, but causing a reverse flow and increasing seawater intrusion in the Soai Rap River.
On the contrary, the GCVT sea dike had stronger impact to changing in seawater intrusion, it caused significantly reduction of seawater intrusion in half of the upper region of the Can Gio Bay area. But the lower area in the exterior of the Can Gio mangrove forest, the seawater intrusion almost did not decrease and seawater was accumulated. It was be concluded that the GCVT sea dike strongly affected the fluctuation of seawater intrusion and its impact still remained in the 0.73-m SLR. However, the solutions of both GCCG and GCVT sea dike construction also affected seawater intrusion change in the Can Gio Bay area.
More specifically, if the GCVT sea dike would be operated for a long time, a half of the Can Gio Bay area would be freshened. The Can Gio Bay area is a critical mangrove forest area in the HCMC and Vietnam, which is necessary for maintaining suitable salinity concentration and reasonable inundation time for the semi-submerged area. If it would have really a strong impact on seawater intrusion, the development of the Can Gio mangrove forests would be seriously threatened.
The Chapters 1 and 2 showed the current problems in the HCMC as well as in the Can Gio Bay area involved inundation, salinity change, SLR and sea dike construction. The Chapters 3 and 5 pointed out how the SLR and the sea dike construction impact to study area as well as some suggestion contributing to the sustainable development of the HCMC. The Chapter 4 showed how river inflow discharge and tidal regime influenced on the salinity in the Can Gio Bay area. Hence, the recommendations for future research and development are:
1. Improvement of hydrological data - river discharges for the model input.
2. Enhancement of river salinity data for the model input by investing salinity observation stations in river boundary for the model.
3. Improvement of long-term field observation data to understand clearly the salinity distribution in the Can Gio Bay area based on influence of river inflow discharge and tidal regime.
4. Development of ecosystem model to understand not only salinity but also some environmental indicators in the Can Gio Bay area under the SLR and the sea dike construction.
5. The influences of the SLR and the sea dike construction on the inundation and the salinity in the Can Gio Bay area are severe. Hence it is necessary to study those effects by simulation in more extended time period.
Acknowledgments
Firstly, I would like to express my appreciation to the Japanese Government and the Ministry of Education, Culture, Sports, Science and Technology that have supported me to study at Kyushu University, Japan under providing me the MEXT scholarship.
I am thankful to Kyushu University, especially, the Faculty of Agriculture and the "International Development Research Course" of the Graduate School of Bioresource and Bioenvironmental Sciences for supporting me all time in my PhD course, doing help me to broaden my knowledge and useful for my study.
I would like to thank Thuyloi University that have support me to implement the planning study in Kyushu University. Especially, I am thankful to the Division of Science, Technology and International Affairs, South Campus of Thuyloi University for giving me a chance to go to study and helping my work.
Especially, I strongly express deep gratitude to my supervisor Professor Dr. Kazuaki Hiramatsu of Laboratory of Water Environment Engineering, who gave me an opportunity to study in Japan and gave me most orientation, motivation, guidance, support and encouraged me during time of studying in Japan.
I am very grateful to sub supervisor Assistant Professor Dr. Toshinori Tabata of Laboratory of Water Environment Engineering, who spent much time for giving me most orientation, motivation, guidance, support and encouraged me to overcome difficulties and finish the study at Kyushu University.
I would like to express special appreciation to Associate Professor Dr.
Masayoshi Harada of Laboratory of Water Environment Engineering for his support and sharing experience during time of studying Japan and thank him for valuable comments and suggestions to complete my thesis.
I am very thankful to Professor Dr. Yoshiyuki Shinogi of Laboratory of Irrigation and Water Management for valuable comments and contribution to complete my dissertation better.
I would like to express appreciation to Associate Professor Dr. Yohei Shimasaki of Laboratory of Marine Environmental Science for giving me kind support and valuable comments during my study time.
I would like to thank all members in the Laboratory of Water Environment Engineering for their help, support and sharing the nice time together.
I also thank Associate Professor Dr. Trieu Anh Ngoc - head of Division of Science, Technology and International Affairs and Division of Science, Technology and International Affairs' members, South campus of Thuyloi University who helped me to collect data related to my study area of the Can Gio Bay and gave me orientation, guidance, and support me during studying time.
I specially thank all friends in Fukuoka and Japan for sharing my trouble as well as my joys in my research process and my life.
Last but not least, a sweet word to all members of my family, especially my parent, they vitally encouraged and sympathized for my study plan.
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