1D Numerical Modelling of a Complex Tidal River: Case of the River Saigon, Vietnam

Tidal rivers are complex systems at the interface between continental surfaces and the ocean. Water levels as well as river discharge are highly influenced by tidal dynamics. Therefore, the combination of a flood and tidal constraints on the water levels can be critical for flood protection. Additionally, the propagation of sediment particles or of a pollution can be difficult to apprehend due to the back-and-forth advection combined with dispersion. The River Saigon in Vietnam is a very specific example of tidal river. Indeed, its watershed is very flat and the Saigon net flow is generally negligible compared to the tidal flow [2]. The River Saigon flows through the Ho-Chi-Minh-City (HCMC) megalopolis and understanding its hydrodynamics is crucial in terms of flood risk assessment, saline intrusion, pollution and eutrophication. Nevertheless, only very scarce measurements are available to assess the flow of the River Saigon and tidal fluctuations. Local and punctual field measurements (water levels and discharges) alone do not allow to understand the complex dynamics of the Saigon river network during flood events. Numerical modelling appears essential to better identify main parameters explaining this dynamics. The purpose of this paper is thus to show the potential of 1D numerical modelling to understand the flow dynamics in a complex tidally influenced network. We used the Mage code developed at INRAE Lyon, which has been already validated on other tidal river systems such as the Adour or the Lower-Seine river systems. The calibration of friction coefficients is quite challenging for tidal rivers due to the control of the downstream water levels on the flow dynamics. The River Saigon presents additional issues such as the lack of reliable data on the upstream net discharge and the potential effects of large canals within the HCMC megalopolis on the flow dynamics. We present here the difficulties to calibrate the model of River Saigon. The upstream conditions (Saigon and Dong Nai water input) have a very low impact on the flow dynamics in the River Saigon. Also, if the Saigon flow largely influence the dynamics of HCMC canals, these canals have only a very local impact on the River Saigon dynamics. Such model would be very useful in the future for engineering purpose but also to better apprehend pollutant dynamics.