Morphodynamics of seasonally closed coastal inlets at the central coast of Vietnam

Situated in a monsoon-prone humid tropical region, Vietnam is affected by both oceanic and continental climates causing disasters to the country like riverine flooding and storm induced damage. The coastal districts of Vietnam have a population of about 18 million habitants, account for nearly one fourth of the total population of the country and locate about 50% of the major towns and cities of Vietnam. Most of the people currently living in the coastal zone have their livelihood mainly relying on marine resources and they are also the most vulnerable to sea-related natural disasters, such as storms and floods. The natural disasters occurring in the coastal strip in the central part of Vietnam, caused by meteorological and oceanographical factors, are intensified by human interventions, like the damming of rivers for various purposes or the extensive deforestation for the creation of agricultural lands. With more than 1,000 km of coastline, the central coast of Vietnam has more than sixty inlets and river mouths discharging into the South China Sea. These systems play a vital role in social-economic activities in the region. The steep rivers with abundant natural but temporally unevenly distributed flows make the low-lying coastal plains in the region prone to inundation by flooding, while the river is almost dry during the rest of the year. Specific topographical features and hydrological characteristics of the region produce a particularly high seasonal geomorphological variation of tidal inlets and river mouths, from narrowing, shoaling or entirely closing in the dry season to widening or breaching in the flood period. Frequent disasters set back development efforts in this poorest region of Vietnam and trap people in a cycle of poverty. Stabilising inlets at the central coast of Vietnam therefore is recognised as one of the priority tasks to mitigate potential risks caused by natural disasters, especially by floods and storms on low-lying coastal plains, and to promote a safe and stable condition for social-economic development in the region. To carry out this task, Vietnam needs both substantial financial and human resources, particularly knowledge and experience in coastal engineering, which is not trivial for a developing country. Additionally, strong seasonal variation of inlets and estuaries contribute to the complexity of problems and raise a necessity to implement a strategy for inlet and river mouth stabilisation under the constraints of a shortage of resources and knowledge. This thesis focuses on tidal inlets and estuaries in a wave-dominated, micro-tidal environment under the influence of episodic river flooding in the central coast of Vietnam. Natural behaviour and morphological stability of tidal inlets, which significantly interact with channel migration, entrance shoaling or closure have been identified and analysed based on field observations, historical satellite images, topographical maps and bathymetrical data. Based on the regional natural settings and hydrodynamic-morphological features, tidal inlets along the central coast of Vietnam can be divided into two main categories, namely, (1) barrier lagoon inlets and (2) wave dominated estuary inlets. A conceptual model for channel evolution and seasonal opening/closure of tidal inlets is proposed which describes the cyclic evolution of a typical tidal inlet at the central coast of Vietnam. In the conceptual model, the inlet entrance is forced both by the alongshore current which deposits sediment in the inlet channel and by the ebb tidal and river generated currents which erode sediment from the inlet channel. The interpretation of the Escoffier diagram is extended conceptually to explain the seasonal variation of both open equilibrium and closure. The variation is regulated by the seasonal variation of river flow and littoral drift. The conceptual model indicates the two major processes which dominate in the dry and the flood season leading to a deviation from the stable and unstable equilibrium points in the Escoffier diagram. This supports our understanding of seasonal variation of coastal inlets and estuaries in a region that experiences monsoons and storms causing a large fluctuation in littoral drift and ebb flow at the central coast of Vietnam. To get deeper insight into the underlying processes and cross-sectional stability of an schematised tidal inlet, regulated by tides only and regulated by both tides and waves, the process-based morphodynamic modelling system Delft-3D has been applied. In the model the tidal period, amplitude, basin area and initial inlet dimensions were changed systematically to create different hydrodynamic environments for inlet evolution. The model successfully reproduces the evolution of the channel flow area towards equilibrium for a tidal inlet and is able to describe the main behaviour of an inlet in response to a range of tide and wave conditions and geometries. The model results are in good agreement with empirical relationships (O'Brien, 1969; Jarrett, 1976) and the analytical solution (DiLorenzo, 1988) of Escoffier's diagram. To investigate location stability of inlet channels, seven experiments were designed to cover 3 different stability ranges (poor, fair to good stability). Reliable model results increase the understanding of the processes underlying the migration and closure of a tidal inlet. It is found that tidal inlet behaviour and location stability is linked to the number of channels on the ebb delta, the curvature if there is only one channel, type of bar on the ebb delta, the migration of the updrift barrier island, the distance between the inlet throat and the outer of margin of entrance bar. The model results demonstrate that the process-based model is able to reproduce the morphological evolution of a tidal inlet fairly consistent with the Bruun et al. (1978) empirical criteria for location stability. A typical example of a tidal inlet migrating due to oblique waves which includes features such as ebb channel migration, shifting and diminishing, and the bypassing of ebb shoals from the updrift to the downdrift barriers is investigated and discussed in detail. In another case inlet closure due to prolongation of the ebb channel and infilling with littoral-drift material in the foreshore is also observed. Furthermore, the model results indicate that Escoffier's closure curve is solely applicable to the stability of the channel gorge and thus insufficient to explain the closure of a tidal inlet due to littoral sand infilling into the main ebb channel. In this study solutions are developed for the stabilisation of tidal inlets at the central coast of Vietnam. The solutions are based on the natural behaviour and evolution of two different types of tidal inlets in the region, namely 1) barrier lagoon inlets and 2) inlets formed at the mouth of wave dominated estuaries. For each type of inlet, both short-term and long-term solutions as well as structural and non-structural solutions are taken into account. The solution for the stabilisation of inlets at the central coast of Vietnam is to restrict and/or response to problems. To verify proposed solutions for the stabilisation of inlets along the central coast of Vietnam, process-based modelling is employed to simulate the evolution of a schematised tidal inlet that is stabilised by two jetties and by using river flow to flush the inlet channel. The simulation results show that the inlet after stabilisation by jetties remains open but the inlet channel is highly variable due to the accumulation and erosion of sediment in between two jetties. A sedimentation and erosion pattern is found which is related to the distance in between the two jetties and the strength of the tidal power. An optimum distance between the two jetties that takes into account the effectiveness of the jetties and the structural safety during a major flood event need further study. For inlets that are stabilised by using river flow to flush the inlet, a set of simulation scenarios in which different flushing discharges and flushing durations was designed. For a limited number of simulation scenarios, the model results show that with the same amount of flushing volume, the scenario that has a longer flushing duration and a sufficient flushing discharge is more efficient than the scenario that uses a high flushing discharge over a short duration. This means that the flushing efficiency is closely related to the flushing duration rather than the flushing discharge. Moreover, the flushing moment (at the beginning of the ebb phase or at the beginning of the flood phase) will also contribute to the efficiency of the solution but needs more study.