Both drone pictures are courtesy of S.A.J. Their volumetric porosity ranges between n ≈ 0.5 and 0.9. The structures are formed by arrays of bamboo poles with a diameter of d ≈ 0.15 m, distributed over a width of approximately 1.5 m in the streamwise direction. ( a) Pictures of bamboo structures built by the Indonesian Ministry of Marine Affairs and Fisheries in Demak, Indonesia. The aim of this study is thus to develop a design tool t o calculate this resistance, which could be implemented in large-scale flow models to optimize the performance of future designs. Predicting the impact of the bamboo structures on spatial flow patterns requires quantifying the resistance forces exerted by the structures in currents. However, this aspect has received less attention in existing designs 10. Although the structures are designed for wave attenuation, they can also affect local currents, which in turns influences sediment transport and mangrove habitat creation. Since waves lose energy as they pass through the structures, the calmer hydrodynamic conditions behind the poles enhance sediment deposition, and favour mangrove expansion 5. The width of the structures varies between 0.7 and 1.5 m in the flow direction, and their volumetric porosity ranges between n ≈ 0.5 and 0.9, where n is defined as the ratio of the fluid volume to the total volume. Some of the configurations constructed in a pilot project in Indonesia, consisting of groups of cylindrical bamboo poles driven into the soil, are presented in Fig. Bamboo and brushwood structures have consequently been built to counteract erosion at degraded mangrove sites in South East Asia and South America 3, 5– 9. Mangrove deforestation can increase the exposure of the remaining forest to wave action, causing coastline retreat, and hindering the natural recovery of the forest 5. Regardless of their economic and societal value, 30 % of the mangrove forests have disappeared around the world over the last 50 years 4. Mangrove forests effectively function as extensive wood fences that protect coastal communities from storms 1, 2 by attenuating waves and currents, and by preventing erosion 3. The application of the model can guide the design of future mangrove restoration efforts. The model also showed that anisotropic structures with small spanwise spacing and large streamwise separation maximize the bulk drag coefficient, and the energy dissipation per pole. The model could reproduce measured bulk drag coefficients from the literature within a deviation of 20%. We developed a model that predicts the bulk drag coefficient of dense arrays of emergent cylinders in a current, including blockage, sheltering and a balance between turbulence production and dissipation. Currents cause drag forces on the poles, which depend on flow acceleration through the elements (blockage), and the distance from wakes of upstream cylinders (sheltering). Improving future designs requires a better understanding of how the bamboo poles affect waves and currents. The lack of design rules for the structures has led to mixed success rates in their implementation. Bamboo structures, formed by poles driven into the soil, are being used to create a sheltered environment for mangrove restoration. In spite of their economic and societal value, mangrove forests have experienced a worldwide decline due to human activities. Mangrove vegetation provides natural protection against coastal hazards like flooding and erosion.
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