Seasonal hydro-and morphodynamics of data-limited bay and coastal inlet systems

The main objective of this study is to unravel the physical processes that control typical coastal systems in Central Vietnam while challenged by the fact that it is a data limited environment. Inlets and bays are the typical coastal system along the central coast of Vietnam. These coastal systems are strongly influenced by tropical monsoon conditions, which are characterised by variations in seasonal wave conditions and seasonal river flow. These systems are even more vulnerable to extreme weather conditions, such as floods and storms, because of the complex topography of a relatively narrow and steep mountain range which is directly connected to a dense river network in the low-lying coastal plains at the downstream end. Economic and ecological values in the coastal area are under pressure as a result of the intensification of natural disasters and human interventions. Notable examples of this are Cua Dai beach and Da Nang bay. Cua Dai beach lies adjacent to the Cua Dai inlet which is a typical seasonal varying tidal inlet connected to the catchment area of the Vu Gia-Thu Bon River. Cua Dai beach has suffered extreme erosion in the recent decade. Da Nang bay is a complex bay beach headland downstream of the Vu Gia River that discharges into this bay. Also, this system is affected by human interventions. Due to the common downstream basin of the Vu Gia-Thu Bon River system, both being typical coastal systems in Central Vietnam that experience data limitations, this study attempts to combine and understand the hydrodynamics and morphodynamics of Cua Dai inlet and the complex Da Nang embayment. In order to identify and quantify the main processes governing the evolution of Cua Dai beach to explain the morphological changes and extreme erosion in recent years, a new approach was developed. Historical shoreline positions and sediment budget changes are the two parameters of main importance in the approach to quantify the erosion processes in the Cua Dai coastal inlet. Historical shoreline changes were derived fromsatellite images and associated sediment budgets were estimated based on shoreline change rates using additional assumptions, such as defining a closure depth and a time invariant beach profile. To gain insight into the sediment transport along the Cua Dai beach, additional numerical models and empirical equations are used to investigate the variation in alongshore sediment transport induced by waves. Further analysis on how seasonal variation in both waves and river discharge impacts the morphodynamics of the ebb tidal delta and its adjacent coasts is performed based on process-based modelling.