Long-term morphological evolution of Cua Lo inlet, Central Vietnam

Sammanfattning: Cua Lo river mouth is a coastal inlet connected to an estuary at the central coast of Vietnam. The area is valuable for maritime trade, fishing industries, and tourism development. Natural disasters during the monsoon period are a problem, causing seasonal flooding. In connection to the inlet lies an elongating sand spit which causes erosion and destruction of residential areas on one side of the inlet. The objective was therefore to investigate the dynamics of the sand spit and the inlet, and to describe their long-term morphological development using a fast and simple, yet accurate model. Modelling was done using a process-based numerical approach, where both inlet migration and changes in inlet width were considered. The model base compared the longshore and inlet sediment transport, and exhibited satisfactory performance. A seasonal pattern related to the dry- and monsoon season was observed for the longshore sediment transport. Longshore transport was much larger compared to the inlet transport during the monsoon period. The sediment transport in the inlet had a clear connection to tidal variation. Two scenarios were considered for predicting inlet migration and width evolution. Migration and width evolution were modelled separately due to model limitations, over the period 2020-2049. Scenario 2 allowed for 2000 m migration, with the endpoint being a hard rock boundary. The boundary was not reached during the modelled period as the migration was only 1350 m. Scenario 1 allowed for 980 m of migration before the inlet reached the boundary limited either by longshore sediment transport changes, or a rock boundary. By 2041 the inlet was estimated to reach the boundary if the LST remained unchanged. A very narrow (4-20 m) equilibrium state took place rapidly after the width model was initiated, pointing towards seasonal or permanent closure. A sensitivity analysis on the LST and its effect on the morphological evolution suggested that the inlet could stop migrating before the 980 m boundary and that the inlet width could reach a dynamical equilibrium at that point. Actions are needed if the properties in the migration path are to be kept safe.