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.

As coastal wetland ecosystems, thriving at the interface between land and water,
mangroves can benefit us in lots of ways: providing food and timber, purifying water and sequestrating carbon, protecting coastal area through wave attenuation and coastal stabilization. Regarding the protection of coastal area, mangroves damp the flow in short term and mitigate coastal erosion in long term.
In mangrove forests, the capability of mangroves to capture sediments leads to
accumulation of sediments within the mangrove forests. Hence, the elevation of the floodplain, where the mangroves grow, is usually higher than the main river bed. The slope connecting the main river channel and floodplain has been found with different angle. The topography of the mangrove area is influenced by both sediment dynamics and hydrodynamics. The physical processes behind this is still not fully understood. This study aims to study the hydrodynamics in a partially vegetated compound channel by using the 2DH RANS model in Delft3D and to verify to what extend the flow characteristics can be reproduced. The reference numerical model is set up to mimic the flume experiments so that the model can be calibrated. The influence of different physical and environmental parameters on flow characteristics is studied. Also, the limitation of Delft3D in simulating the flow in a partially vegetated compound channel has been found.

Mangroves are tidal trees commonly observed along the sheltered shorelines of most tropical (from equator to 23.5° North and South latitude) and few subtropical (23.5° to 40° North and South latitude) countries. These plants are adapted to loose wet soils, saline habitats and periodic tidal submergence. With more attention paid into the approach of building with nature, natural coastal defence strategies are gaining more importance as an asset in addressing the coastal squeeze that is prevalent not only in urban areas, but also in agriculture and industrial areas that are located along the coastline. Mangroves are receiving more attention due to their coastal protective role against wave and hydrodynamic forcings as well as their ability to adapt to sea level rise. Mangrove vegetation attenuates and damps the hydrodynamics forcings by providing obstacles to the flows and creating drag. To date and to the knowledge of the author, no study has been conducted on interaction of the wave-induced currents with mangrove vegetation. This lack of relevant studies may be due to the fact that mangrove forests and the foreshore in front of the mangroves are usually of very gently sloping bed (varying in order of 1:300 to 1:1500). This means that in order to conduct physical model experiments to study wave-induced current within a mangrove forest, a very large wave basin is required in order to conduct modelling without using a very large scale factor difference between prototype and model. This is to ensure that the relevant processes are representing prototype as closely as possible, as well as to be measureable. Numerical modelling of the interaction of wave-induced current with mangrove vegetation is yet to be conducted due to the lack of measured data for validation, both field as well as experimental measurements. An experiment by Hulsbergen (1973) was selected as validation data for current study. The main objective of the study is to understand the difference of nearshore processes for (stationary) tidal gradient-driven and oblique wave-driven current for both with and without mimic mangrove vegetation. The scope of the study involves desktop analysis of the main validation data and other relevant and similar experiments, assessment of reliability of Delft3D for the study, validation against measured data, and simulation of various hydraulic conditions for condition with mangrove forest. Among questions answered in this study are the extent of wave-induced longshore current damping within mangrove forest, the significance of wave-induced longshore current within mangrove forest, the effects of bed slope and mangrove density on wave-induced current and the extent of model’s reliability for current study. It was shown that the damping of wave-induced longshore current is more than 80% and the contribution of waveinduced current to the total velocity can be more than 70%. Of course, both of the above was specific to the bathymetry, mangrove properties and hydraulic conditions specified within current study. Furthermore, it was shown that bed slope and mangrove density affect wave-induced longshore current within the mangrove forest. It was also found that current model setup has its limitations.

Shore-normal breakwaters are constructed in coastal zones both for beach protection (erosion reduction) and port development (wave sheltering). These breakwaters have an effect on the waves, the (wave-driven) currents and hence, the sediment transport along the coast. The waves and tide force an equilibrium sediment transport along the coast in a natural unaltered coast. When breakwaters are constructed this sediment transport in the alongshore direction is (partially) blocked. This results in accretion at the updrift side of the breakwater and coastline retreat at the lee-side. Coastal erosion behind a breakwater can result in floods, destroy property or simply narrow the beach. Not only the short term effects of these structures (what happens during and short after construction) is important to know, also the long term effects need to be known; because the breakwaters are build for decades coastal influence of these breakwaters needs to be known for these time-scales as well. Therefore it is relevant to investigate the scale (in time and space) of these adverse effects beforehand.

In coastal engineering, numerical models are used to predict the impact of coastal constructions like breakwaters. High detailed models which take for all physical processes into account will result in accurate predictions but result in large computation times. Simplified models have smaller computation times and are more suitable for coastal impact predictions on larger spatial (10 - 100 km) and temporal (decades) scale. The objective of the thesis is to improve the coastline change predictions of models at decadal scale by reviewing the common practice and a new, very fast, module for lee-side wave computations and investigating different wave processes that can improve the wave modelling. This research will focus on (1) understanding of what wave processes are important for the coastline change and (2) advise on what models to use for which conditions and how to the improve model predictions.

The approach of this thesis is triple. First five different modelling approaches for wave modelling are compared, varying in computation time and usability, to a very accurate model that will be used as ground truth. The wave conditions will vary in direction and both wind waves and swell waves will be modelled. From this step de accuracy of these models for different wave conditions can be analyzed. The second step is to get an better understanding of the processes that are involved with the wave modelling. This information can be used to improve the accuracy of model approaches with high computation times. The last step is to see how this relates to sediment transport and thus the coastline change at the lee-side of a breakwater.

This can be concluded after the investigation of 3 wave model (SWASH, SWAN and the Kamphuis module)


SWAN model approach is for cases with wind waves a good model approach: the wave height, wave direction and the sediment transport are well representative. Also the setup differences are very similar to the ground truth model.
SWAN is not very accurate for cases with a small directional spreading. For these cases the diffraction is more important and there is not enough wave energy in the sheltered area nor is the wave direction well represented.
Kamphuis with Snellius (refraction) is for the case with a wide directional spreading, given the computation speed, pretty good. For the cases with a small directional spreading the wave height is not accurate.


The conclusions related to the influence of the wave processes are:


The refraction is investigated by comparing the Kamphuis module (which does not incorporate refraction) with the ground truth model. When Snell's law was applied to the kamphuis module the model results did show good results. Therefore the refraction is (especially outside the sheltered zone) very important to the wave direction.
There is a large difference for cases with small and large directional spreading. The SWAN model gives results can can be expected when no diffraction is present. Directional spreading both influences the wave height directly behind the breakwater as well as the influence lengt of the breakwater.
Diffraction does not play an important role for wind waves. The large directional spreading results in much smaller energy differences between the sheltered and the non-sheltered zone. Even SWAN without diffraction gives a good representation for wind waves. For swell waves however this is very different.Then diffraction is very important. There is a much larger difference in wave energy between the shelterd and the non-shelterd zone and for a good representation a model that can coop with diffraction is a must.
Current induced refraction of the waves has influences for waves of an incoming agle 0 to 30 degrees because this will result in rip currents along the breakwater that turn the waves up to an extra 10 to 15 degrees.



The relative sediment transport of the five modelling aproaches was also analyzed. The sediment transport proxy for the five modelling approaches showed that SWAN computes the wave height and direction well for wind waves. For swell waves however the model did not show good results. The reason is that there is no diffraction in SWAN, the diffraction computations with SWAN did not show much improvements either.

The sediment transportation proxy for the Kamphuis module with Snellius where expected to be good for wind waves, because the wave height and wave direction where well represented. However the length of the erosion pit was much smaller and also the shape of the erosion pit is very different from the other two models.

Als u Marcel Stive niet kent van de grote onderzoeksprojecten NatureCoast en NEMO of als hoogleraar kustwaterbouw die onlangs zijn driehonderdste afstudeerder afleverde, dan kent u hem vast van de Deltacommissie 2008, de Zandmotor, of wellicht van het voorwoord in Donkere Palissaden, het mooiste boek van deze eeuw over het Nederlandse kustlandschap. Waar staat Marcel Stive voor?@en