Climate change and the morphological stability of the Tu Hien inlet, Vietnam

Historically, the coastal zone has always attracted people and across the globe the benefits of inlet systems have made communities settle there. The Tu Hien inlet and the Tam Giang–Cau Hai lagoon system in Vietnam are no exception, as approximately half a million people depend on it for their livelihood. Tidal inlets are also known for their volatile and disruptive nature, because they are dependent on the delicate balances between oceanic processes such as tidal flow, wave action and mean sea level, and riverine processes such as discharges and sediment supply. Disturbances of this balance cause changes in the behaviour of the inlet system, which will have an impact on the surrounding coastal area. The crux is that these processes can all be significantly affected by climate change. The goal of this thesis is to understand the effect of changes in climatic conditions on the morphological stability of the Tu Hien inlet. Achieving this goal was done by studying the present day behaviour of the inlet in order to use this knowledge to validate a process-based model for the present day and subsequently applying climate change scenarios to that model. Newly acquired satellite images show that in recent years (since 1999) the behaviour has changed and the inlet is not closing anymore as was the case in the time before 1999. A conceptual model has been created that captures this behaviour of the inlet and shows that in the present day the inlet is often shifting between being narrowed and open. This cycle is caused by the processes of spit growth and widening due to a certain degree of breaching. A simulation strategy has been drafted which is mainly founded on the yearly seasonal character of the forcing conditions. During a simulation of the northeast monsoon the growth of a spit should be visible; during a simulation of the southwest monsoon a status quo should be maintained. To model this behaviour an existing model (Lam, 2009) has been adapted to include sediment transport and morphology. The adapted model has been calibrated to reproduce the sediment transport up and down the Tu Hien coast and subsequently morphology simulations have been carried out to reproduce the desired morphological behaviour. The results show that after a breach the inlet mouth is narrowing and the cross-sectional area reduces until it reaches a certain stable equilibrium by the end of the northeast monsoon period. During the southwest monsoon there is little wave action and the inlet is tide-dominated, it slightly widens but a status quo is maintained. By reproducing the present day behaviour of the inlet the model has been qualitatively validated and is considered suitable for an investigation into the effects of climate change. Four climate change scenarios consisting of combinations of sea level rise, a change in fluvial run-off and a change in wave climate have been applied to the present day model. The model shows that future changes in river flows and waves are of minor importance compared to the effects of sea level rise with regard to inlet stability. The narrowing behaviour which was apparent in the present day is not observed in the climate change scenarios involving sea level rise. An increase in basin area caused by a higher mean sea level enlarges the tidal prism which results in more flushing capacity of the flows through the inlet. The increased flushing capacity ensures that a larger inlet cross-sectional area is maintained and also enhances the locational stability of the inlet. The predicted increase of the morphological stability can be considered a positive development. An increased cross-sectional area and the lack of a narrowing behaviour means that the future morphological behaviour will not have negative consequences for inlet navigability, water quality, ecology and other socio-economic factors. Even though the predictions show that climate change is likely to have a positive impact on inlet stability for Tu Hien, it is emphasised that the overall impact of climate change on low-lying coastal zones may be significant in a negative sense and deserves continuous attention.