Cua Dai Beach located adjacent to Cua Dai Inlet is a typical, seasonally varying tidal inlet. This famous beach has suffered extreme erosion since 1995 due to an apparent irregular-periodic process, a decrease of sediment supply from the river and its estuaries and increased squeeze by coastal developments. The main objective of this study is to unravel the physical processes that control the morphological development of Cua Dai Inlet while challenged by the fact that it is a data-limited environment. In order to identify and quantify the main processes governing the evolution of Cua Dai Beach and thereby aiming to explain the morphological changes and extreme erosion in recent years, a new approach was developed. Historical shoreline positions and sediment budget changes were derived from satellite images using empirical engineering assumptions. In addition, numerical models were used to investigate in detail sediment transports and morphodynamics under the influence of seasonal waves and rivers as well as the anthropogenically-driven impacts. Results of shoreline change rates indicate that Cua Dai Beach (located on the northern side of Cua Dai inlet) experienced an average erosion of 12m/y during the period from 2000 to 2010 and erosion continued further to the north while the southern coast of the inlet accreted with a mean rate of 11m/y. The overall system showed a significant sediment loss of about 243,000–310,000 m3/y. The annual cycle of two past morphological periods has been numerically simulated to evaluate the behavior of the system without and with human interventions. The first morphological simulation without the impact of the resorts successfully reproduced an overall erosion trend at the northern coast while the formation of an ebb tidal bar was also reproduced. The second morphological simulation reproduced the impact of the resorts that have been constructed along Cua Dai Beach. Simulations indicate that the presence of the resorts has enhanced the propagation of the existing erosion further to the north. The new approach of remote sensing combined with process-based modeling has been essential to investigate the main processes that govern the morphological changes and extreme erosion at Cua Dai inlet.

Cua Dai inlet is a typical microtidal, mixed energy-wave dominated inlet in a tropical monsoon regime in central Vietnam. Both the river flow regime and coastal processes such as induced by waves and tides influence Cua Dai Inlet and its adjacent coasts. Cua Dai Beach, the northern adjacent coast of Cua Dai inlet, has experienced severe erosion since 1995 due to an apparent non-periodic cyclic process, a decrease of sediment supply from the river, estuary and squeeze by coastal developments (Do et al. in J Coast Res 34(1):6–25, 2018). The inlet channel has shifted from North to South which served as an important controlling mechanism for the creation of a new ebb shoal. However, the role of the ebb-tidal delta in relation to the channel shifting and seasonal varying hydrodynamic conditions (river discharge and wave climate) remains poorly understood. Most studies have only considered the impact of waves and tides on the development of the ebb tidal delta. No study has included the impact of a varying river discharge on ebb shoal development and inlet migration. This chapter investigates the seasonal varying hydrodynamics and sediment transport of the inlet and adjacent coasts due to the seasonal varying river discharge and wave climate. The 2DH process-based morphodynamic numerical model (Delft3D) is applied using schematized wave conditions and river discharge. Six simulations with varying dominant wave conditions for the winter and for the summer are executed in combination with varying river discharge classes that corresponding to the dry, wet and flood seasons. There exists an East North East monsoon with a flood season from September to December, an East North East monsoon with a wet season from January to March, and a dry bidirectional South East/East North East monsoon from April to August. We investigate the effect of the seasonal wave climate and seasonal river discharges at Cua Dai inlet by analyzing the effects on the resulting hydrodynamics, sediment transports and potential morphological changes through the inlet and at the adjacent coasts. Primary results indicate that the seasonal variation in the wave climate has a strong influence on the sediment transport patterns in the adjacent coasts. The variation in the river flow dominates the magnitude of sediment transport through the inlet. The results of the simulations show that the inlet generally imports sediment into the estuary except in the case of the flood season. During the flood season the estimated sediment export is significant. Interestingly, the wave direction that varies during summer also influences the magnitude of sediment import into the estuary. Waves coming from the ENE contributes to larger sediment import than waves coming from the SE.