Introduction Acknowledgements Method ...

91129: Numerical Modeling of Tidal Dynamics and Transport in the Multi-channel Estuary of the Mekong River Paper Number: MG54B-2030

Vo Quoc Thanh12, Johan Reyns13, Herman Kernkamp3, Dano Roelvink13,Mick van der Wegen2 1 UNESCO-IHE,

Delft, Netherlands, 2Can Tho University, Vietnam, 3Deltares, Delft, Netherlands Email: [email protected]

Introduction The Mekong estuary is an area of high biodiversity in which the ecosystem has been influenced not only by natural factors but also by anthropogenic impacts (Quang et al., 2010). The former determine the water circulation such as tide propagation, river discharge and outflow plume behavior, while the latter consist of full-dyke systems in the flood zone, saline protection systems along the coast and in general much reduced mangrove forests along the river banks and coasts. Tidal propagation plays an important role in development of the delta, especially the coastal areas due to its direct impacts on saline intrusion and storm surge (Cai et al.,2014). Therefore, this research aims at investigating tidal propagation by observation analysis and numerical modelling approaches.

Figure 4. Monthly variations of tidal amplitude propagating along Bassac branch. Noticeably, all of tidal constituent amplitudes are damped completely in the flood season (from August to November) in Long Xuyen upward.

Method • Observation analysis Water level data of tidal gauging stations in the Figure 1 has been analysed by the harmonic analysis method described by Foreman (1977) and Pawlowicz et al., (2002). • Numerical modelling A hydrodynamic model, Delft3D Flexible Mesh, has been applied to simulate hydrodynamics in the research area. It is a 1D-2D-3D model developed by Deltares, running on flexible meshes, with different shape of meshes (Achete et al., 2015; Deltares, 2015). It solves the two- and threedimensional shallow equations by a Gaussian solver. Details of numerical approach and schemes are described by Kernkamp et al., (2011) and Deltares (2016). • Model description For tidal wave propagation, the mesh created covers main branches of the Mekong river from Kratie, Cambodia to the East Sea and its shelf, including the Tonle Sap lake. The net resolution varies from about 100 m in rivers to 1000 m in the shelf. Open boundaries are defined as water discharge (Kratie) and water level (the Sea). The latter was extracted from a global tidal model (TPXO 7.2, Egbert and Erofeeva, 2002). The Tonle Sap lake plays a significant role in controlling upstream discharge in the dry season. Therefore, simulated water levels at the end of the last year were used as initial conditions for next year simulation. It also reduced spin-up time of simulations (Ji, 2008).

Figure 5. Model validation of water levels of stations in the Vietnamese Mekong Delta

Figure 1. Numerical mesh and topography

Results

Figure 6. Model performance of water discharge, especially Vam Nao station because Vam Nao is the main river balancing discharge between Mekong and Bassac branches

Conclusion • Figure 2. Tidal damping of Bassac branch (left) and Mekong branch (right)

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Figure 3. Tidal phase differences of Bassac branch (left) and Mekong branch (right)

References Cai, H., Savenije, H.H.G., Toffolon, M., 2014. Linking the river to the estuary: influence of river discharge on tidal damping. Hydrol. Earth Syst. Sci. 18, 287–304. doi:10.5194/hess-18-287-2014 Deltares, 2016. D-Flow FM Hydro- and Morphodynamics: Technical Reference Manual. Foreman, M.G.., 1977. Manual for Tidal Heights Analysis and Prediction. Victoria, B.C. Ji, Z.-G., 2008. Hydrodynamics and Water Quality: Modeling Rivers, Lakes and Estuaries. JOHN WILEY & SONS, INC. Lu, S., Tong, C., Lee, D.-Y., Zheng, J., Shen, J., Zhang, W., Yan, Y., 2015. Propagation of tidal waves up in Yangtze Estuary during the dry season. J. Geophys. Res. Ocean. 120. Minh, N.N., Patrick, M., Florent, L., Sylvain, O., Gildas, C., Damien, A., Van Uu, D., 2014. Tidal characteristics of the gulf of Tonkin. Cont. Shelf Res. 91, 37–56. Nowlin, W.D.J., Pillsbury, R.D., 1982. Observations of the principal tidal currents at Drake Passage. J. Geophys. Res. 87 (C8), 5752–5770. Pugh, D., Woodworth, P., 2014. Sea-level Science: Understanding Tides, Surges, Tsunamis and Mean Sea-level. Cambridge University Press. Quang, N.X., Vanreusel, A., Smol, N., Chau, N.N., 2010. Meiobenthos assemblages in the mekong estuarine system with special focus on free-living marine nematodes. Ocean Sci. J. 45, 213–224. Zhu, X.-H., Ma, Y.-L., Guo, X., Fan, X., Long, Y., Yuan, Y., Xuan, J.-L., Huang, D., 2014. Tidal and residual currents in the Qiongzhou Strait estimated from shipboard ADCP data using a modified tidal harmonic analysis method. J. Geophys. Res. Ocean. 119, 8039–8060.

Tidal propagation is reduced significantly by increasing river discharge Numerical modelling has been applied in the Mekong Delta variously. However, most of applications are 1D hydrodynamic models for flood and salinity. This research showed good agreement between observations and simulation of 2D hydrodynamics. Hydrodynamics have been investigated and modelled. It is an important for further research on saline stratification, sediment transport and morphodynamics.

Acknowledgements This project is part of the ONR Tropical Deltas DRI and is funded under grants N00014-12-1-0433 and N00014-15-12824. The authors would like to thank the Mekong River Commission for providing most of the data . Simulations were carried out on the Dutch national e-infrastructure with the support of SURF Cooperative.