Flooding in the Vietnamese Mekong Delta: Perspectives in the Context of Projected Climate Change

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1 Post-graduate Research Programme on Adaptation to Climate Change (PRoACC) in the Mekong River Basin UNESCO-IHE and Partners in the Mekong region POLICY BRIEF Flooding in the Vietnamese Mekong Delta: Perspectives in the Context of Projected Climate Change Introduction The Vietnamese Mekong Delta (VMD) (Figure 1) is located at the very end of the Mekong River (MR); therefore, it is influenced not only by local but also regional hydrological conditions (e.g. precipitation from the upstream segments). The VMD accounts for the local precipitation and sea level in the East and West Sea while the MR is contributing to the delta with the upstream discharge, often considered from Kratie, Cambodia. Due to the pressure of the global climate change, the hydrological changes in the Mekong Basin have been recently analysed; most of the works were done by the Mekong River Commission (MRC) and its research partners (e.g. Hoanh et al., 2010). It was projected that the flood hydrograph may change its pattern (e.g. from a two-peaks to a one-peak flood) and a sharp-hydrograph may appear more frequently in the future. Hence, it is important to prepare the local residents and authorities for the possibility of unexpected events to happen, from which they might be able to figure out appropriate strategies to adapt to different future climate changes scenarios. The four main implications of the projected climate change identified in the sub-project are presented below: 1. The projected flood hydrograph might be significantly different if compared to the past events. 2. Flood dynamics in the VMD would be an actual concern in the future with the modification of splitting discharge between the Bassac and Mekong River and therefore a flood warning system is required. 3. The coastal area along the East Sea would be affected greatly by the future sea level rise, which prompts for the adequate adaptive measures (e.g. mangrove forest). 4. The Ca Mau Peninsula would be highly subjected to the sea level rise and the modification of discharge proportion between the Mekong and Bassac. 1

2 Figure 1: The Mekong river network after the Kratie gauging station and its section in the Vietnamese Mekong Delta. 1. The projected flood hydrograph might be significantly different if compared to the past events With the impact of climate change, the projected flood hydrograph ( Figure 2) (at the Kratie gauging station leading to correspondent patterns in the VMD) might change its pattern significantly (e.g. from a two-peak to a one-peak flood). Such a modification might give significant impacts on the available agri- aquaculture systems in the VMD whose seasonal calendars which were well-adapted to the past flood hydrographs. The lesson was learned from the serious flood in

3 Daily discharge (m 3 s 1 ) 100,000 90,000 80,000 70,000 60,000 50,000 40,000 30,000 20,000 10,000 0 J J A S O N D Recorded events (1991, 1996, 1997, 2000) Projected events (2019, 2021, 2046, 2047) Figure 2: Recoded and projected daily discharge during the flood period in different high flood years in Kratie. 2. Flood dynamics in the VMD would be an actual concern in the future with the modification of splitting discharge between the Bassac and Mekong River and therefore a flood warning system is required The local residents of the VMD receive benefits from the annual normal flood. However, the behaviour of the future fluvial floods were projected to be different from the past events (e.g. acute rising and falling flood; Figure 2), and hence, a flood warning system is required to strengthen the capacity of local residents in the VMD for the better adaptation strategies. Changes of the temporal discharge distribution during the flood period might lead to significant modifications of the river morphology, leading to serious erosion and/or unexpected sedimentation. In addition, the proportion of discharge entering the VMD is projected to be different from the past, resulting in more water routed along the Mekong than in the Bassac. However, the Bassac currently keeps an important role to sustain a vast agricultural area within the VMD and with the depletion of discharge along the Bassac, stresses of water requirement along the Bassac might increase significantly. Figure 3 presents the projected future water levels at Tan Chau and Chau Doc with the projected sea level increase of 30 cm in comparison to that of the year

4 Figure 3: Range of simulated water levels at Tan Chau (TC) and Chau Doc (CD) with the past and future events. 3. The coastal area along the East Sea would be affected greatly by the future sea level rise, which prompts for the adequate adaptive measures (e.g. mangrove forest) With the greater magnitude of differences between the spring and neap tides in the East Sea in comparison to the West Sea (Figure 4) and because of the fact that upstream discharge was mainly routed along the Bassac and Mekong to the East Sea ( Figure 1), the rise of the sea level (especially during the spring tide) in the East Sea would results in greater impacts on the coastal area. The lessons were learned from the annual flood in 2011 (e.g. in Soc Trang, Tra Vinh and Can Tho). Therefore, it is important to design the proper adaptive strategies to sustain and improve the livelihood of local residents in different scenarios of sea level rise. Figure 4: Measured sea levels in the East (A) and West (B) Sea on the selected dates in The Ca Mau Peninsula would be highly subjected to the sea level rise and the modification of discharge proportion between the Mekong and Bassac The Ca Mau Peninsula experiences the complex hydraulic regimes as it is strongly affected by both the East and West Sea. In this study, the levels of detail in the models used were not enough to fully describe the future hydrological changes in the area. However, the general trend resulted from this study is that fluvial floods would be stronger during the flood period (Figure 5), while from the earlier studies one (Vastila et al., 2010) may conclude that the saline intrusion would be more serious in the dry season. With the increase of extreme events, it is very likely that the farming systems have to be adapted to the new hydrological environment. 4

5 Figure 5: The highest (A) and the most likely (B) simulated water level in the VMD in the flood period (Q Kratie : ) (Scenario 1) and probability of the occurrence; the nonflooded area along the coasts went beyond the scope of the applied model. Conclusion It can be concluded that if the IPCC climate change scenarios are assumed, the flooding patterns in the Vietnamese Mekong Delta are to be undergo considerable change. This presents considerable challenges to adapting the whole socio-economic system of the Vietnamese Mekong Delta, which however are not immediate. It should also be noted that the changes in the Mekong hydraulic regime due to developments upstream, e.g. due to the hydropower development, were not considered (as this was not the purpose of this study), and their influence may have even greater influence on the whole Mekong Delta. References Hoanh, C.T., et al., Impacts of climate change and development on Mekong fow regime. First assessment MRC Technical Paper No. 29. Mekong River Commission, 2010: Vientiane, Lao PDR. Tateishi, M., V. L. Nguyen, et al. (2007). "Salt water intrusion in the Mekong River estuary, Vietnam: Observation at low flow season in May 2005." Sci. Rep., Niigata Univ. (Geology) 22: Västilä, K., M. Kummu, C. Sangmanee, and S. Chinvanno Modelling climate change impacts on the flood pulse in the Lower Mekong floodplains. Journal of Water and Climate Change 01:

6 Acknowledgements This policy brief is based on research conducted by Van Pham Dang Tri (a,b), Dimitri Solomatine (b), Ioana Popescu (b), Ann van Griensven (b), and Nguyen Hieu Trung (a). This project would not have been successful without the support of the local knowledge institutions, which supported the post-doctoral researchers: Asian Institute of Technology (AIT), Thailand Institute of Geographic Sciences and Natural Resources Research (IGSNRR), Chinese Academy of Sciences (CAS), China Changjiang River Scientific Research Institute (CRSRI), China Can Tho University, Vietnam Southern Institute of Water Resources Research (SIWRR), Vietnam Vietnam Institute of Meteorology, Hydrology and Environment, Vietnam Water Resources University (WRU), Vietnam Furthermore, the project would not have been possible without the support from the Mekong River Commission. Box. The PRoACC approach. PRoACC is organised around post-graduate research projects which focus on different aspects of climate change adaptation (physical, social, economic, institutional) and are integrated. The projects are developed in close collaboration with key stakeholder groups from the Mekong River Basin. Besides carrying out the research, the scientific findings will be translated for the development and implementation of better policies in the field of climate change adaptation. In addition, the capacity building aspect of this post-doctoral programme is significant. All post-doctoral fellows come from the region and are hosted by local/regional knowledge institutes. This will strengthen regional cooperation further and the objective is that the post-doctoral fellows will continue to work on related topics in the region after the programme is finished. PRoACC is funded by the Netherlands Ministry of Development Cooperation (DGIS), through the UNESCO-IHE Partnership Research Fund. It has not been subjected to peer and/or policy review by DGIS or UNESCO-IHE, and therefore does not necessarily reflects the views of these institutions. For additional information about PRoACC please contact Dr. Wim Douven or Prof. Stefan Uhlenbrook at UNESCO-IHE / or the PRoACC website at 6