The coastline of Demak, Indonesia has been eroded during the last 15 years. To restore the natural coastal protection which existed out of mangroves forest, permeable dams, consisting of bamboo poles with a brushwood filling, have been built to attenuate the waves and facilitate sedimentation behind the dams and thus creating a habitat for mangroves. However these designs required a lot of maintenance, so a new type of design is proposed without a filling of brushwood, containing only vertical bamboo poles. Next to this the possibility to include aquaculture in the design is proposed. Therefore this study assesses the wave transformation by the new designs in Demak, Indonesia with the numerical wave model SWASH. In order to do that first the hydrodynamic conditions are analyzed to obtain the design conditions for the structures, and then SWASH is validated against laboratory experiments to find the right drag coefficient values. The design wave conditions are based on the local water depth, offshore wave heights and periods and local bathymetry. The offshore waves are based on a dataset from WaveWatch III, which compares well with measurements of a local storm event. The dataset of WaveWatch III is filtered and extrapolated to determine the wave conditions for return periods(R) of 1 and 5 years. This resulted in offshore waves with Hm0 = 2.08 and 2.39m, and Tp = 6.9 and 7.5 s respectively. The water level is mainly influenced by the tidal elevation and the surge levels. The surge levels are obtained from a risk assessment of the coast of North Java (Willemsen et al., 2019), which vary between 0.63 to 0.68mfor R = 1 and 5 years respectively. The tidal levels are determined using the water depth measurements of two transects from Van Domburg et al. (2018) and analysed with Matlab tool Utide by Codiga (2011), resulting in a spring tidal range of 85 cm and a neap tidal range of 50 cm.
Four different configurations from the experiments of Jansen (2019) are used to validate SWASH: single row, longitudinal(the spacing in flow direction is longer than the lateral spacing), open uniform and dense uniform configuration. These configurations are modelled in the numerical wave model SWASH by use of the vegetation module. The most influential factors are: the drag coefficient, the way to describe mass conservation and the number of stems(cylinders) per m2. For the drag coefficient the bulk drag coefficient of Gijon Mancheno et al. (2021) is used, which contains factors for sheltering, blockage and the KC state of the flow. For the densely packed configurations the bulk drag coefficient proved to have a better agreement than the drag coefficient of a single cylinder. The sensitivity to the number of stems per m2 is small when implementing the longitudinal configuration as an average amount of stems perm2 or by specifying the individual rows of the configuration and so locally increasing the number of stems. Three methods of describing the mass conservation in SWASH are evaluated: by means of a cross sectional approach that is expressed by the blockage factor in the bulk drag coefficient Gijon Mancheno et al. (2021), by a volumetric approach due to activating the porosity in SWASH which means that the blockage factor cannot be included in the drag coefficient and by a combination of these two. For the longitudinal configuration the best agreement is found the cross sectional approach and for the single row configuration the best agreement is found by the volumetric approach.
Once SWASH is validated, the designs of several structures are investigated. Firstly, a design consisting out of two rows of bamboo poles is considered where the spacing between the rows is varied to find an optimum distance. The transmission rate Et /Ei decreased from 75% to 55% with a spacing sx = 0.42 m to 5.8 m, larger spacings did not result in less transmission. If one wants to be conservative at least three rows are needed to have a lower transmission rate of 50 %. When mussels are considered, the structures have to be placed in deeper water as mussels can only grow between MLWS and 40 cm above the bed. To provide enough space for mussel growth, the poles have to be placed more sparsely. The effect of a larger water depth in combination with a limited pole length and sparse structures is larger than the extra drag and frontal area provided by mussels, especially since they did not cover the whole pole length, and resulted in high transmission rates. It is thus recommended to, or place a high number of rows of mussel poles or place a few rows without mussels followed by poles for mussels. This decision however also depends on benefits that mussel poles may bring and the cost of the materials, therefore a cost/benefit analysis is required.
This thesis found an efficient design that can be used in reducing wave attenuation along muddy coasts without the need of a brushwood filling. Hereby it provides an economically and user friendly alternative with respect to the current design, as it requires less material and maintenance.