Projecting of wave height and water level on reef-lined coasts due to intensified tropical cyclones and sea level rise in Palau to 2100

Abstract. Tropical cyclones (TCs) and sea level rise (SLR) 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, reducing the risks of natural disasters to coastal communities. However, projections of change in the risk to coastal reefs under conditions of intensified TCs and SLR are poorly quantified. In this study we projected the wave height and water level on Melekeok reef in the Palau Islands by 2100, based on wave simulations under intensified TCs (significant wave height at the outer ocean: SWH_o = 8.7–11.0 m; significant wave period at the outer ocean: SWP_o = 13–15 s) and SLR (0.24–0.98 m). To understand effects of upward reef growth on the reduction of the wave height and water level, the simulation was conducted for two reef condition scenarios: a degraded reef and a healthy reef. Moreover, analyses of reef growth based on a drilled core provided an assessment of the coral community and rate of reef production necessary to reduce the risk from TCs and SLR on the coastal areas. According to our calculations under intensified TCs and SLR by 2100, significant wave heights at the reef flat (SWH_r) will increase from 1.05–1.24 m at present to 2.14 m if reefs are degraded. Similarly, by 2100 the water level at the shoreline (WL_s) will increase from 0.86–2.10 m at present to 1.19–3.45 m if reefs are degraded. These predicted changes will probably cause beach erosion, saltwater intrusion into groundwater, and damage to infrastructure, because the coastal village is located at  ∼ 3 m above the present mean sea level. These findings imply that even if the SWH_r is decreased by only 0.1 m by upward reef growth, it will probably reduce the risks of costal damages. Our results showed that a healthy reef will reduce a maximum of 0.44 m of the SWH_r. According to analysis of drilled core, corymbose Acropora corals will be key to reducing the risks, and 2.6–5.8 kg CaCO₃ m⁻² yr⁻¹, equivalent to  > 8 % of coral cover, will be required to keep a healthy reef by 2100. This study highlights that the maintaining reef growth (as a function of coral cover) in the future is effective in reducing the risk of coastal damage arising from wave action. Although the present study focuses on Melekeok fringing reef, many coral reefs are in the same situation under conditions of intensified TCs and SLR, and therefore the results of this study are applicable to other reefs. These researches are critical in guiding policy development directed at disaster prevention for small island nations and for developing and developed countries.