Morphological modelling of Lai Giang Inlet, Vietnam.

The Lai Giang inlet located in Binh Dinh province incorporates various features of the estuaries in the Central part of Vietnam. Belonging to the micro-tidal and wave-dominant coast and influenced by the monsoon regime, the inlet has a seasonal character. During the dry season, as the river flow diminishes, the wave action causes high level of sedimentation and closes up the inlet eventually. In flood season, as the river discharge is high, the channel is scoured and the inlet begins to migrate. The high sedimentation level and migration of the Lai Giang inlet has been a serious problem of Binh Dinh province for a long time, because it is the only exit for the floodway. It is an anchorage and also the connection between the sea and the aquaculture area of Hoai Nhon district. The high level of sedimentation at the entrance of the inlet prevents river flood from flowing smoothly, thus leading to overflow in lowlands and navigation issues. In recent decades, the exploitation and protection of Lai Giang area have been studied in various forms of scientific researches and projects by different scientists and local professional agencies. However, the studies have only focused on hydraulics, hydrology and on adjusting the flow of Lai Giang river. There are only general and basic studies on the entrance of the inlet. The main objective of this research is to understand the morphological behaviour of Lai Giang inlet. The specific interest is focused on the main factors which are the tidal characteristic, the wave climate and the river flow during the flood season, and the interaction between all these factors that influence the morphological changes. The study starts with the collecting and analysing all the documentations to come up with a conceptual model of the Lai Giang inlet to explain how the sedimentation and the migration processes happen. Then, the Delft3D modelling software, which can model (tidal) flow, waves and sediment transport, has been applied to confirm the hypothesis and gain further knowledge. According to the data analysis, the conceptual model as well as the descriptive and quantitative result of the model, we can make the following main conclusions: 1. The wave climate in this area has seasonal characteristic and is dominated by two main directions; Northeast and Southeast in winter and summer monsoon, respectively. 2. During the summer monsoon, the longshore sediment transport moves towards the north, bypasses the entrance of the inlet and gradually builds up on the down-drift spit due to the Southeast Wave. 3. During winter monsoon, the Northeast wave intensifies the southward longshore sediment transport leading to the large amount of sedimentation in front of the inlet. At the same time, the significant river flow flushes away the sediment deposits at the main ebb channel located nearer to the up-drift spit. Thus the sedimentation could not take place at the up-drift spit. The sediment displacement at the up-drift and down-drift spit made the inlet migrate to the north gradually. Finally, the possibility to stabilize the inlet is discussed to give the optimum solution for this area.