An Investigation of Exchange Rates in the Tam Giang-Cau Hai Lagoon System, Vietnam, through Hydrodynamic Modeling

Abstract: The Tam Giang-Cau Hai lagoon system, two inter-connected and naturally occurring coastal lagoons located in the Thua Thien-Hue province of Vietnam, comprises two inlets: Thuan An and Tu Hien. The Thuan An and Tu Hien inlets connect the lagoon system to the South China Sea and they are subjected to morphologic changes due to sediment transport effects. The magnitude of sediment transport is determined by wave breaking and long-shore currents as well as by the abundance of sediment found along the sandy coastline. Variation in the coastal sediment transport, as well as in tidal and river flows at different temporal scales significantly affect inlet geometry; a direct result being either a marked widening or narrowing of the inlets, features which are highly seasonally dependent and that also lead to inlet migration and closure. The water exchange between the lagoon and the sea is mainly influenced by the tide, however, the river flow could also affect the exchange during certain times of the year, although this has not been studied in detail in the present investigation. During the end of the winter monsoon season, inlet morphology is dominated by wave action and strong sediment deposition that can lead to a complete closure of the southern inlet of Tu Hien, subsequently creating a new inlet by means of sand spit breaching, approximately 3 km south at Loc Thuy. Inlet closure of lagoons creates unfavorable conditions for water exchange between the lagoon and the sea which can deteriorate water quality and destroy the aquatic ecosystem. This report investigates the conditions controlling the migration and morphology of the inlets with focus on the impact on the water exchange with the South China Sea. Simulations were carried out of the water circulation in the lagoon and how the lagoon exchanges its water with the sea using the ADCIRC model within the Surface Modeling System (SMS) software. This regional model for tidal circulation was used to determine water surface elevation and velocities for the lagoon system using LeProvost global tidal forcing along the ocean boundary of the model. Based on the simulated velocities, exchange rates between the lagoon and the sea were calculated for particular situations. The simulations performed in this study demonstrate that the northern inlet at Thuan An is most important for the water exchange between the lagoon system and the sea; however, the Tu Hien inlet in the south is more susceptible to morphologic changes. Although the Thuan An inlet overall dominates the water exchange between the lagoon system and the sea, the water exchange at Tu Hien inlet is crucial for the southern lagoon (Cau Hai). As Tu Hien inlet closes, the amount of water exchanged between the entire lagoon system and the sea decreases because the Thuan An inlet to the north cannot sufficiently account for the loss of water exchange through a completely closed Tu Hien inlet.