[H-CG23] Adaptation to climate change and its social implementation

[HCG23-09] Effects of coral growth and reef formation under increased wave heights due to intensified tropical cyclones and sea level rise by 2100

*Chuki Hongo^1,2,3, Akira Iguchi⁴, Masashi Kiguchi⁵ (1.Nanki Kumano Geopark Center, Wakayama Prefectural Government, 2.Wakayama University, 3.Nanki Kumano Promotion Council, 4.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 5.Institute for Future Initiatives, The University of Tokyo)

Keywords:Green infrastructure, Coral/reef-based breakwater, Sea level rise, Tropical cyclone, Reef crest

Numerical projections indicate that by 2100 global warming will increase the mean tropical cyclone maximum wind speed. Intensified tropical cyclones and rising sea level will increase wave heights and increase the risks of coastal damage by the year 2100. The construction of gray infrastructure such as seawalls will be required to reduce coastal damage. However, the cost of construction will increase. Coral reefs act as green infrastructure by forming coral and reef-based natural breakwaters. Healthy coral reefs can reduce this damage and are low cost and maintenance free. Numerous studies have investigated the effectiveness of reef crests associated with upward coral growth in dissipating waves on reefs currently and by 2100. However, estimates of wave dissipation by fully emergent reef crests at tectonically active sites and by non-reef structures are poorly understood. We use a wave simulation model to estimate changes that will occur by 2100 in wave height on coral reefs with emergent reef crests and non-reef structures under variable tropical cyclone wave conditions and sea level rise in the Ryukyu Islands and the main island of Japan. Our findings indicate that the more emergent the reef crest, the greater the effectiveness of wave dissipation. Moreover, our data show that the effectiveness of wave dissipation is diminished by reefs with less emergent crests and by non-reef structures. For example, the emergent reef crest of the Ohara reef of Kume Island is exposed even at high tide. The future significant wave height was found to increase to a maximum of 2.46 m (84.6% wave dissipation) for degraded corals/reefs and to a maximum of 2.45 m (86.4% wave dissipation) for the healthy corals/reefs on the Ohara reef by 2100. If reef ecosystems are degraded by 2100, the effectiveness of wave dissipation by coral/reef growth will be reduced. To maintain wave reduction by corals/reefs, reef conservation and restoration efforts, including direct coral transplantation, will be required. Our method is applicable to other areas in the Indo-Pacific and Caribbean regions. Inexpensive but effective natural breakwaters will probably be needed for small island nations. To accumulate knowledge about coral/reef-based natural breakwaters, further researches in other areas and on the effects of reduced coral growth rates in future climate change on green infrastructure will be needed.