Projecting the risk of damage to reef-lined coasts due to intensified tropical cyclones and sea level rise in Palau to 2100

Abstract

Abstract. Tropical cyclones (TCs), sea level rise (SLR), and storm surges cause major problems including beach erosion, saltwater intrusion into groundwater, and damage to infrastructure in coastal areas. The magnitude and extent of damage is predicted to increase as a consequence of future climate change and local factors. Upward reef growth has attracted attention for its role as a natural breakwater able to reduce the risks of natural disasters to coastal communities. However, projections of change in the risk to coastal reefs under conditions of intensified TCs, SLR, and storm surges are poorly quantified. In this 15 study we assessed the current status of natural breakwaters on Melekeok reef in the Palau Islands. Based on wave simulations we predicted the potential effects on the reef by 2100 of intensified TCs (significant wave height at the outer ocean = 8.7-11.0 m; significant wave period at the outer ocean = 13-15 s), SLR (0.24-0.98 m), and storm surge. The simulation was conducted for two reef condition scenarios: a degraded reef and a healthy reef. Analyses of reef growth based on drillcores enabled an assessment of the coral community and rate of reef production that are necessary to reduce the risk 20 to the coast of TCs, SLR, and storm surges. The reef is currently highly effective in dissipating incoming waves, with the reef crest to the upper reef slope reducing wave height by 75%, and the entire reef dissipating waves by 88% of the incident wave height. However, our calculations show that under intensified TCs, SLR, and storm surges, by 2100 significant wave heights at the reef flat will increase from 1.05-1.24 m at present to 2.14 m if reefs are degraded. Similarly, by 2100 the sea level at the shoreline will increase from 0.86-2.10 m at present to 1.19-3.45 m if reefs are degraded. These predicted 25 changes will probably cause beach erosion, saltwater intrusion into groundwater, and damage to infrastructure. However, our simulation indicates that reef growth reduces the wave height by a maximum of 0.44 m at the reef flat by 2100. These findings emphasize the need for future reef formation to reduce the damaging effects of waves on the coastline. Corymbose Acropora corals will be key to reducing such effects, and 2.6-5.8 kg CaCO 3 /m 2 /y will be required to build the reef by 2100.If the RCP 8.5 scenario is realized by 2100, an increase in coral cover of >8% will be needed to reduce the impact of waves 30 on the coastline. The use of coral reef growth to reduce disaster risk will be more cost-effective than building artificial Nat. Hazards Earth Syst. Sci. Discuss., doi:10.5194/nhess-2017Discuss., doi:10.5194/nhess- -3, 2017 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 16 March 2017 c Author(s) 2017. CC-BY 3.0 License. 2 barriers. Benefits in addition to reducing disaster risk include the ecological services provided by reefs, and marine products and tourism. Research of the type described here will be required to advise policy development directed at d...