The Saigon's river response to coastal dynamics

A good understanding of the hydrological cycle, and discharge in particular, in tropical tidal rivers enables reliable forecasting and decision making by researchers and policy makers.

The study proposes a low-cost technique to estimate tidally-influenced river discharge based on high frequency in-situ measurements. This technique pursues the work of Camenen et al. 2021 and estimates discharge based on a stage-fall-discharge rating curve, which requires the estimation of the river’s slope.

The application to the Saigon river (Vietnam) shows that its hydrological dynamics are ruled by a complex combination of atmospheric, continental and oceanic factors and pointed out unexpected hydrological behaviour: the 2018 rainy season was marked by heavy precipitations during four months, especially in November (typhoon USAGI). These high precipitations were associated with an increased water level; but surprisingly, the net water discharge decreased substantially during the same period (figure 9a, 9c). 

We propose an in-depth study of in-situ water level time series at several locations including in the Mekong delta, in the Saigon river and in the surrounding coastal area. The tidal signal obtained via harmonic analysis is extracted from the water level time series and the residual signal is then filtered using a low-pass filter as proposed in Trinh et al. 2020. This discriminates non-tidal fluctuations of water level and thus, permits the analysis of the impact of non-tidal signal such as storm surge or precipitation.

Our analysis supports that the coastal water level dynamics actually control the slope of the Saigon river and thus, its net discharge. Additionally, the coastal water level might be itself regulated by the seasonal flood of the Mekong delta, so that the Mekong river discharge management would indirectly influence the Saigon river dynamics, even if these two basins are hydrologically disconnected. 

The technique developed will help in better understanding the hydrological pattern of complex tropical estuarine rivers and disentangle the factors that influence their discharges as well as provide insights on tidal river interactions with coastal waters.