Optimising the design of bamboo structures for mangrove restoration

Abstract:

Mangrove ecosystems protect coastal areas from flooding and erosion. Despite their economic and societal importance, 30% of the mangrove forests worldwide have disappeared over the last 50 years due to human activities. Mangroves can recolonize degraded areas, but they require locations with low wave action where the coastline is either stable or accreting. Bamboo structures are consequently built at eroding mangrove coastlines in order to dissipate waves, prevent erosion, and provide a protected environment for mangrove establishment. However, existing structures have not always succeeded in mitigating erosion due to the lack of design guidelines. Optimising future designs relies on understanding how the structures influence waves and currents, and how they affect the local sediment dynamics. We consequently conducted flume experiments to measure wave transformation, hydrodynamic forces and flow velocities inside cylinder arrays, which imitated the bamboo poles of a structure, with varying cylinder distributions, orientations and densities. The laboratory measurements provide relationships of wave transmission, wave reflection and drag coefficients for cylinder arrays with porosities between n=0.64-0.9 and Keulegan-Carpenter numbers between KC = 10-21. The results also give insights on which designs could maximise wave energy dissipation per element, and thus reduce the material costs of a structure. For instance placing the elements in rows with small lateral spacings (increasing flow acceleration between the cylinders) and big downstream spacings (decreasing sheltering) increases the forces and dissipation per element. Future work will investigate how the presence of the structures, and their distance from the coastline, influences the morphodynamic conditions required for mangrove establishment. The present work thus contributes to the development of design guidelines for future mangrove restoration efforts.