Morphology of Cua Dai River mouth and adjacent sandy beaches in Hoi An City, Vietnam has been being eroded severely in recent years. Based on analysis of Landsat images, the long-term evolution, decadal scale, of morphology at this area since 1975 to 2015 is discussed. Morphology of river mouth in the period from 1975 to 1991 is highly similar to one in the period from 2003 to 2015, long sandspit on the left side and cuspate shoreline (cuspate foreland) on the right side, whereas the formation of double sandspit and welding between them after that was observed in the period from 1995 to 2002. From the characteristics of morphological change, there are three types of river mouth formed during the 40-year long period are classified. In addition, results of image analysis also indicate that the right bank of river shifted southward about 250 m. This is corresponding to the period with elongation of sandspit on the left after welding from offshore sandbar. The tip of cuspate shoreline on the right side was also observed to move to the south during the period that right river bank shifting southward. Besides the reduction of sediment supply to the river mouth, the shifting to south of main river channel diverting more sediment deposit on the right side of the river mouth, resulting in the erosion of the Cua Dai Beach on the left side became more serious. Countermeasure, which diverts more sediment to the left side, is highly required. Detailed effectiveness and magnitude of this structure can be evaluated based on numerical simulation.

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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.

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