One of the potential hazards that can cause major problems in coastal areas is erosion. In order to solve this problem, the notion of sustainable coastal development has garnered growing attention around the world. Coastal zone management mechanisms aim to ensure the sustainability of resources and the environment. For example, natural coastal protection using vegetation such as mangrove trees is currently preferred in many places in the world. However, there are challenges in the development of this natural form of coastal protection, e.g. mangrove seedling trees can be damaged by the waves or the current before they grow strong enough, and thus require appropriate protection until at least two years after planting. To solve this problem, a natural coastal protection system that combines a main natural protection and a temporary man-made structure is proposed. After a process of weighing temporary man-made-structure alternatives, the geobag dyke was selected. This study aimed to quantify the effects of various geobag dyke configurations and geobag unit weights on wave height reduction. Laboratory experiments were conducted on a narrow wave flume using a mangrove model as main natural protection and geotextilegeobag models as temporary man-made structures. Various wave conditions were generated during the laboratory tests. This paper focuses on the experimental results of wave transmission through the protection system in order to determine the most effective geobag dyke configuration to reduce the wave height. Based on experimental modelling, the most effective geobag dyke configuration uses relatively heavy geobag units with a moderate dyke slope (1:1.5).

This is a study on the effect of hull roughness on ship resistance components (especially full viscous resistance), which is tested using wind-tunnel experiments and Computational Fluid Dynamics (CFD) simulations. With the wind-tunnel experiment, a full viscous resistance analysis can be carried out to further explore changes in the friction and pressure resistance only without the wave resistance. In the experiments, the roughness model used sandpaper with an average roughness height (k a ) = 162 µm, that then it was predicted equal with k s = 1475 µm. In the CFD simulations, the roughness parameter was represented by an equivalent sand grain roughness height (k s ), and this was varied by several levels. The results indicated that there was a significant increase in ∆Ct (up to 73.7%) and ∆Cf (up to 106.96%), but only a slight increase in ∆Cp (up to 10.57%). The trend of the increase in resistance due to k s and Reynolds numbers were also discussed. The parameter k s were very influential on ∆Cf, but had only a slight effect on ∆Cp. With the significant results about the increase in ship resistance due to the roughness, both the friction and the pressure resistance component will lead to an increase in fuel consumption on a ship then it will increase levels of carbon emissions in the air.