The applicability of the mega nourishment concept for the purpose of combating erosion in Cua Dai beach

Abstract

Nourishments are considered to be a popular soft measure against coastal erosion. In the Netherlands, the concept of a localized mega nourishment has been introduced as an innovative approach, aiming to counteract the anticipated enhanced coastal erosion. The positive results of the Sand Engine implementation lead the way for applying the mega nourishment concept as an innovative solution in many retreating coastlines around the world. In central Vietnam, Cua Dai beach experiences ongoing erosion that propagates to the north over the years which significantly undermines the existence of the coast. The complexity of the prevailing climate as well as the various human interventions that have taken place, complicate the understanding of the system and the determination of a long term solution to the problem of erosion.
The present research investigates the concept of mega nourishment as a measure against the problem of erosion in the Cua Dai beach. A consistent approach has been followed for this purpose, addressed through several research steps. Initially, an overview of the recorded morphological evolution and of the relevant processes has been created as well as an assessment of the stakeholder relations in the area, through available literature. Then, the prevailing dynamics of the overall region that includes Cua Dai beach have been analyzed with the use of representative conditions of the influencing forcing mechanisms, based on characteristic past morphological periods. Extreme flood and storm events have been also investigated. For this purpose numerical modeling simulations with Delft3D have been performed. A design approach has been selected and the primary and secondary design parameters have been determined, formulating the final design solutions. Assessment of the design solutions has been realized, in terms of a qualitative analysis but also an assessment of their seasonal behaviour. For the latter part, numerical modelling simulations have been also made.
An erosive trend has been identified in the north coastline where Cua Dai beach is located that has been present before any interventions in the coast took place. In addition, the trend of formation of an ebb tidal bar has been highlighted as well as its interaction with the north coast. The presence of resorts along Cua Dai beach was found to enhance the erosive pattern. The placement of sandbags across the public Cua Dai beach strengthened further the erosion. The extreme flood and storm events do not alter the erosive trend of the north coast that is present under regular seasonal conditions, but can explain the enhancement of the inspected morphological trends.
The selected design approach that concerns the inclusion of the influencing stakeholders in the financing of the nourishment design solutions, led to the formation of three design alternatives that follow the mega nourishment concept. The required volume, the design lifetime, the location and the shape of the designs formed the main design parameters. The seasonal assessment of the behaviour of the designs has been performed separately for each design alternative, based on the level of diffusion and the trends formed in the alongshore sediment transport gradients. All design solutions have shown a degree of spreading around them, which is directed to the expected locations. Thesis findings suggest that sand nourishment applications that follow the mega nourishment concept can be an option for the problem of erosion in Cua Dai beach.