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.

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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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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. @en

Shoreline-change data are of primary importance for understanding coastal erosion and deposition as well as for studying coastal morphodynamics. Shoreline extraction from satellite images has been used as a low-cost alternative and as an addition to traditional methods. In this work, satellite-derived shorelines and corresponding shoreline-change rates and changes in volumes of coastal sediments have been estimated and evaluated for the case of the data-rich North-Holland coast. This coast is globally unique for its long in situ monitoring record and provides a perfect case to evaluate the potential of shoreline mapping techniques. A total of 13 Landsat images and 233 observed cross-shore profiles (from the JAaRlijkse KUStmeting [JARKUS] database) between 1985 and 2010 have been used in this study. Satellite-derived shorelines are found to be biased in seaward direction relative to the JARKUS-derived shorelines, with an average ranging 8 m to 9 m over 25 years. Shoreline-change rates have been estimated using time series of satellite-derived shorelines and applying linear regression. The satellite-derived shoreline-change rates show a high correlation coefficient (R2> 0.78) when compared with the JARKUS-derived shoreline-change rates over a period of 20 and 25 years. Volume changes were calculated from the satellite-derived shoreline-change rates using assumptions defining a closure depth. Satellite-derived volume changes also show a good agreement with JARKUS-based values. Satellite-derived shorelines compare better with in situ data on beaches that have intertidal zone widths ranging from one- to two-pixel sizes (30 m-60 m). The results show that the use of Landsat images for deriving shorelines, shoreline-change rates, and volume changes have accuracies comparable to observed JARKUS-based values when considering decadal scales of measurements. This shows the potential of applying Landsat images to monitor shoreline change and coastal volume change over decades.@en

Cua Dai Inlet is a typical, seasonally varying tidal inlet in central Vietnam. Since 1995 the northern adjacent coast, known as Cua Dai Beach, has experienced serious erosion. The decadal scale behavior of this inlet appears to reflect a nonperiodic cyclic process. Inlet channel shifting from north to south has welded the abandoned ebb-tidal delta with Cua Dai Beach, leading to accretion but subsequently triggering erosion. Although erosion of Cua Dai Beach was exacerbated by decrease of sediment supply from the estuary and ebb-tidal delta and by coastal developments, the channel shifting to the south, and the ebb shoal development were important primary controlling mechanisms. This study aims to quantify the main erosional processes in and near the Cua Dai coastal inlet and adjacent beaches since 1995. First, satellite data were used to detect shoreline change trends and to estimate volume changes. Second, alongshore, wave-driven sediment transports were estimated using numerical models. Observed shoreline changes indicate that, during the period from 2000 to 2010, erosion rates at the northern side of the inlet were on average 12 m/y. Close to the inlet, erosion rates were larger, up to 19 m/y. At the same time, the southern coast of the inlet was found to accrete with a mean rate of 11 m/y. Calculated alongshore sediment transport rates explain the observed erosion and accretion patterns. The overall system lost a significant sediment volume, which is estimated to amount to 243,000–310,000 m3/y. A logical conclusion is that the effects of the shifting of the inlet channel to the south caused erosion of the northern adjacent coast, whereas human interventions in the river catchment, the estuary, and along the coast contributed importantly to the overall sediment deficit of the inlet system and its beaches and to the shifting erosion pattern toward the north.@en