The coastline of Demak, Indonesia, has been eroding during the last 15 years. Coastal retreat in Demak is caused by a combination of mangrove deforestation and local subsidence due to groundwater extraction in the nearby city of Semarang. To restore the lost mangrove forest, permeable dams, consisting of bamboo poles with a brushwood filling, have been built to attenuate the waves, facilitate sedimentation at their land side, and thus create a suitable habitat for mangroves. However, existing designs required frequent brushwood maintenance. Therefore, a new type of design is proposed, consisting of only vertical bamboo poles without a filling of brushwood. Nevertheless, the hydrodynamic performance of this type of structure is not known. This study assesses the wave transformation through structures formed by bamboo poles for the physical conditions of Demak, Indonesia, with the numerical wave model SWASH. Field measurements and WaveWatch III data are analyzed to obtain the design conditions for the structures in Demak. SWASH is validated against laboratory experiments, and applied to investigate different structure designs. The model shows that for a structure consisting of two rows of bamboo poles, the transmission rate Et/Ei decreases from 75% to 55% when the row spacing in the wave direction is increased from sx = 0.42 m to sx =5.8 m. Even larger spacings do not result in less transmission, and at least three rows are needed to have a transmission rate lower than 50 % - a common wave reduction target used in restoration efforts with structures. This study thus identifies potential strategies to maximize wave attenuation by bamboo structures, which can be used to reduce wave attack along muddy coasts without the need of a brushwood filling. Hereby it provides an economically and user friendly alternative with respect to the previous brushwood structure designs, as it requires less material costs and maintenance.@en