Hydrodynamics of a Tropical Coastal Lagoon Influenced by Monsoon and Outer Sea Conditions: A Case Study of Ao Kung Krabaen Lagoon-Thailand
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Abstract
Ao Kung Krabaen Lagoon (AKBL) is a small and shallow coastal lagoon situated in the eastern part of Thailand. It is an important “Blue Carbon” ecosystem of this region. However, water quality of the lagoon has drastically deteriorated in recent years. To counteract the problem, a comprehensive understanding of water circulation within the lagoon and exchange with the outer sea is needed. In this study, seasonal field observations were made and then applied in three-dimensional hydrodynamic simulations using Delft3D-FLOW. Results indicate that water properties, water circulation and water exchange differ notably by season. During the southwest (SW) monsoon (July-September), circulation is at its strongest, and there are strong double-gyre circulations. This results in enhancing exchange with the sea, hence significantly reducing flushing time. The flushing time during the SW monsoon is approximately three days. During the transition period (April-May) between the northeast monsoon and the SW monsoon, sluggish circulation dominates the lagoon. The flushing time increases to around 10 days, which is the annual maximum. Water circulation within the lagoon is mostly in two-dimensional patterns owing to its shallowness, even though water exchange occurs in three dimensions. This study also demonstrates that the tropical coastal lagoons scattered around the world should be considered three-dimensionally and with attention to varying water density both inside and outside the lagoon to properly model their hydrodynamics and exchange with the outer sea.
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