Unravelling the barotropic processes and their effect on salinity intrusion in the estuarine networks, application to the Vietnamese Mekong Delta

Abstract

This MSc thesis focuses on the hydrodynamics and salt dynamics of multi-channel estuarine delta system, with specific focus on the Vietnamese Mekong Delta (VMD). During the dry season (January - April), agricultural areas and fresh water intakes in the VMD are exposed to salinity intrusion. Interaction between river discharge and tides play a crucial role in controlling salinity intrusion in estuarine systems. However, river discharge and tidal variation in multi-channel estuarine are extremely time dependent and affect the salt intrusion. River discharge varies considerably during the wet and dry seasons and tides vary with the neap and spring cycles. Moreover, smaller river discharge and stronger tides result in further salt intrusion. This study investigates the variation in water transport distribution and tides over time and their effect on the length of salt intrusion.

The two-dimensional, depth averaged, Delft3D-Flow Flexible Mesh (DFM) model was used to explore the characteristics of tidal wave and river discharge distribution in the complex multi-channel estuarine system of VMD during the dry season. Harmonic analysis of model output shows that the barotropic DFM model simulates water level and total discharge variation in VMD reasonably well when compared with field data. It was found that secondary channels affect tidal characteristics in the VMD. Without secondary channels, tidal amplitude increases up to 20 cm in My Thuan, while in Can Tho it increases by 7.5 cm. Inequalities in net water division at junctions were observed in the VMD. The cumulative discharge from Can Tho in the lower tributaries of the Bassac river was distributed differently for Dinh An and Tran De channels at levels of 67.8 % and 26 %, respectively.

The less than 100 % sum of net water discharge distribution indicates that local water sinks affect the distribution in the Bassac river. Furthermore, in the VMD it was found that net water distribution between each channel varies considerably as a function of time because of variation in discharges, tides, and wind surges. During a period of low discharge, a strong wind surge reduces net water transport in the channels and it may even become directed toward the land as indicated by negative net water transport.

Furthermore, net water transport and tidal characteristics from the DFM were used as inputs for steady-state salt intrusion analytical models developed by Savenije (2005) and MacCready (2004). Salt intrusion reaches 30 - 50 km in both channels, with estuarine adjustment time less than ten days during normal condition without surge events.