Demak is a regency within the province of Central Java, Indonesia, with a mud-mangrove coast bordering the Java Sea. The region is facing a rapid retreat of the coastline, threatening the livelihood of a large part of the population. The main cause of the erosion is the deforestation of the green belt of mangroves. This has disturbed the delicate sediment balance in the area drastically. This MSc thesis was carried out within BioManCO. This is a project of Delft University of Technology and Universitas Diponegoro and aims to develop a bio-morphodynamic model for mangrove-mud coasts. This will eventually be used to identify the conditions under which autonomous reforestation of a sustainable mangrove green belt will take place, restoring the natural coastal protection. Semi-permeable dams are already being implemented to restore the sediment balance in the area. In this approach, however, the existence of a fluid mud layer is neglected. The observation of relatively steep slopes of the interface between mud and water indicates potential mud transport within the mud layer. Such a transport would contribute to the shoreward flux of sediment and thus to the restoration of the coastal profile. If a hybrid dam is implemented, it will block the flow of sediment and might therefore defy its own purpose; attenuating flow and waves in order to capture sediment and restore the eroded coastal profile. The objective of this thesis is to assess wave damping as a driving mechanism for set-up of the fluid mud layer at the coast of Demak and to identify under what conditions such a set-up can exist. Significant attenuation of waves can be achieved by viscous dissipation of wave energy in the mud layer. The set-up of the fluid mud interface is hypothesised to be balancing the wave force resulting from the reduction of wave energy in shoreward direction. To gain insight in the damping of waves and, more generally, in the dynamics of the coastal system of Demak, a field campaign has been carried out. Based on these measurements a SWAN-Mud model has been set up and has been coupled to an idealised model that calculates the equilibrium slope based on modelled wave-damping. The field observations show that the interface level is indeed sloping upwards towards the coast. This slope, however, does not seem to change significantly during the field campaign, indicating that the occurring waves are not able to move the layer. A strong daily variation in wave height and period, dependent on the prevailing wind system, is observed. SWAN-Mud is able to reproduce these measurements convincingly, even with the simple schematisation used in this thesis. The damping of the waves is influenced by the water depth and the wave period, and to a lesser extent by the wave height. It is also strongly dependent on the thickness and viscosity of the mud layer. The use of a fluid mud module to model the dissipation of waves at the coast of Demak is proven to be necessary. The developed conceptual model assumes a balance between the wave force in the mud layer and a pressure gradient due to a set-up of the fluid mud interface. This model shows that waves are able to force positive slopes in shoreward direction. However, for the range of mud parameters, water depths and wave characteristics as measured in Demak, these calculated slopes are too mild in comparison with the observed slopes. The adopted approach neglects the yield stress in the fluid mud layer. This internal strength might be able to balance and thus maintain the observed slopes after first being forced by the waves. Over time, this could lead to a build-up of sediment against the coast which could potentially be colonised and fixated by mangrove species. This build-up has a possible implication for the management of the coastal area of Demak. The hybrid dams might indeed be blocking a restoration mechanism of the mud coast which defies the original purpose of building these dams.

Across the world, the presence of humans in coastal regions is always increasing. Hydraulic forcing from extreme events is a large risk around the global coastlines, the risk being complicated by the increased human presence along the coasts.
The knowledge that vegetation can be used as a coastal protection measure is not something new. The benefits of vegetation have been seen throughout history and are being researched on till date. If we know to a certain confidence what the wave heights are at the shoreline, coastal defence structures, used as a hybrid protection measure, can be designed accordingly. Not much research has been done on the wave behaviour on shallow foreshores.
Localised studies have been previously done on tropical vegetated coasts, but there is a lack of efficient and accurate analysis on a large scale. As we bring more clarity on how waves transform and attenuate in a typically vegetated coast, some questions get answered, and some more questions arise, which also happened during the research done for this thesis.
Observed data, be it laboratory or field, is very crucial in validating numerical models. A laboratory experiment was done in the TU Delft Laboratory of Fluid Mechanics flume, for a complete vegetation-free profile, where the surface elevations were observed for different wavemaker input conditions.
A lot of numerical models have been developed that predict wave transformation and dissipation through vegetated foreshores. However, these models lack validation from observed data. This thesis first focuses on understanding the wave transformation for two unique (and mainly theoretical) wave conditions: a regular sinusoidal wave and a bichromatic wave. It was checked if the transformation is reflected in the models – SWAN and SWASH, which they did.
The research proceeded on to validating the models by comparing the wave heights observed in the laboratory experiment versus when the models were inputted with the same conditions, including inputting the observed data into the models. When the laboratory conditions were replicated, the SWASH results obtained correlated quite well with what was observed in the laboratory. The same was not true in the case of SWAN.
When a spectral analysis was done for the observed data, a presence of very low frequencies (VLF) as well as some minor higher frequencies was noticed. To check its effect, if any, on the model results, they were filtered out. Both the original and filtered data was inputted into the models. The difference in the foreshore region was more distinct in the filtered case, i.e., making a bichromatic elevation input purer resulted in more pronounced undulations in the wave heights than what was predicted in the unfiltered data. This result does not fit well with the existing knowledge on wave dissipation processes. It is widely known that the presence of VLFs and higher frequencies are the driving mechanisms that result in the undulations in the foreshore region, but the predicted results were exactly opposite to this knowledge.
What can be thought of from the anomaly is that the presence of various frequencies (that is, waves with different periods) counteract each other’s effects and make the undulating wave heights milder, but when the signal is made purely bichromatic, it leads to more distinct undulations. This proposition is also backed by the similar SWASH results for the laboratory condition-replicating theoretical inputs. This anomaly needs further investigation.
Another interesting observation was that the changes happening in the offshore region did not affect the results in the foreshore region, for varying parameters in SWAN.
SWASH can be concluded as a better model for predicting wave heights, especially in the foreshore region. SWAN could not predict the fluctuations in the wave heights. Obtaining the wave heights at the shoreline with SWAN, and designing a dike with those results, for example, will lead to disastrous consequences, as SWAN underestimates the wave heights.
The study is limited by the consideration of hydrodynamics only, and by the many simplifications made to simulate the conditions. One of the recommendations formulated is to obtain field data and to make a similar comparison with the models to corroborate (or correct) the observations made.
This study tried to see the correlation between the models and observed data in the laboratory, for simple (and somewhat purely theoretical) cases. It is, nonetheless, a starting point for more complicated cases, the basis for which can be laid on this study.

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.