NUMERICAL INVESTIGATION ON EFFECTIVENESS OF PORTABLE RUBBLE MOUND BREAKWATER FOR MANGROVE RESTORATION

Abstract

 Mangrove restorations have been implemented in many countries, but the success rate is still not very high. It often fails during the initial stage of plantation due to various negative environmental impacts. In particular, high waves may cause the failure of young seedlings. A series of field surveys were conducted in a mangrove forest of Amami Oshima, Japan, in May and December 2019. Mangrove growth tests were carried out for observing the growth rate of young mangroves by planting several seeds of Kandelia obavata. In a manual wave experiment in the site, a one-month-old mangrove plant had bent and submerged due to continuous wave impacts of 10 cm high in maximum, while a six-month-old mangrove was undamaged. For investigating whether young mangroves are protected with a small breakwater, a portable reef was designed using the conventional design formula for rubble-mound breakwaters. The wave-dissipating performance of the reef was examined by the CFD model using olaFlow/OpenFOAM. Four breakwater geometries with a similar cross-sectional area were tested for local wave and tidal conditions in the mangrove forest of Amami. The numerical results showed that the mounds with an area of about 1m² can attenuate the wave heights by 48-82%. Although wave transmission varies depending on the geometry, a small rubble mound breakwater would sufficiently work to protect mangrove plants in their early growth stage.