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Projecting of Wave Height and Water Level on Reef-Lined Coasts Due to Intensified Tropical Cyclones and Sea Level Rise in Palau to 2100

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Projecting of Wave Height and Water Level on Reef-Lined Coasts Due to Intensified Tropical Cyclones and Sea Level Rise in Palau to 2100 Nat. Hazards Earth Syst. Sci., 18, 669–686, 2018 https://doi.org/10.5194/nhess-18-669-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 3.0 License. Projecting of wave height and water level on reef-lined coasts due to intensified tropical cyclones and sea level rise in Palau to 2100 Chuki Hongo1, Haruko Kurihara1, and Yimnang Golbuu2 1Department of Chemistry, Biology, and Marine Science, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan 2Palau International Coral Reef Center, 1 M-Dock Road, P.O. Box 7086, Koror 96940, Republic of Palau Correspondence: Chuki Hongo ([email protected]) Received: 4 January 2017 – Discussion started: 16 March 2017 Revised: 10 January 2018 – Accepted: 20 January 2018 – Published: 2 March 2018 Abstract. Tropical cyclones (TCs) and sea level rise (SLR) even if the SWHr is decreased by only 0.1 m by upward reef cause major problems including beach erosion, saltwater in- growth, it will probably reduce the risks of costal damages. trusion into groundwater, and damage to infrastructure in Our results showed that a healthy reef will reduce a maxi- coastal areas. The magnitude and extent of damage is pre- mum of 0.44 m of the SWHr. According to analysis of drilled dicted to increase as a consequence of future climate change core, corymbose Acropora corals will be key to reducing the −2 −1 and local factors. Upward reef growth has attracted attention risks, and 2.6–5.8 kg CaCO3 m yr , equivalent to > 8 % for its role as a natural breakwater, reducing the risks of nat- of coral cover, will be required to keep a healthy reef by ural disasters to coastal communities. However, projections 2100. This study highlights that the maintaining reef growth of change in the risk to coastal reefs under conditions of in- (as a function of coral cover) in the future is effective in re- tensified TCs and SLR are poorly quantified. In this study ducing the risk of coastal damage arising from wave action. we projected the wave height and water level on Melekeok Although the present study focuses on Melekeok fringing reef in the Palau Islands by 2100, based on wave simula- reef, many coral reefs are in the same situation under con- tions under intensified TCs (significant wave height at the ditions of intensified TCs and SLR, and therefore the results outer ocean: SWHo = 8.7–11.0 m; significant wave period at of this study are applicable to other reefs. These researches the outer ocean: SWPo = 13–15 s) and SLR (0.24–0.98 m). are critical in guiding policy development directed at disas- To understand effects of upward reef growth on the reduc- ter prevention for small island nations and for developing and tion of the wave height and water level, the simulation was developed countries. 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 commu- nity and rate of reef production necessary to reduce the risk from TCs and SLR on the coastal areas. According to our cal- 1 Introduction culations under intensified TCs and SLR by 2100, significant wave heights at the reef flat (SWHr/ will increase from 1.05– Approximately 90 tropical cyclones (TCs; also referred to as 1.24 m at present to 2.14 m if reefs are degraded. Similarly, hurricanes and typhoons) occur globally every year (Frank by 2100 the water level at the shoreline (WLs/ will increase and Young, 2007; Seneviratne et al., 2012). TCs cause large from 0.86–2.10 m at present to 1.19–3.45 m if reefs are de- waves, storm surges, and torrential rainfall and can lead to graded. These predicted changes will probably cause beach coastal erosion, salinization of coastal soils, and damage erosion, saltwater intrusion into groundwater, and damage to to infrastructure (Gourlay, 2011a). These negative impacts infrastructure, because the coastal village is located at ∼ 3 m cause major economic loss. For example, the economic cost above the present mean sea level. These findings imply that of TC Pam (category 5 on the Saffir–Simpson scale), which affected Vanuatu in March 2015, exceeded USD 449 million, Published by Copernicus Publications on behalf of the European Geosciences Union. 670 C. Hongo et al.: Projecting of wave height and water level equivalent to ∼ 64 % of the GDP of Vanuatu (GFDRR, bottom roughness, which resulted in the ability of the reef 2016). to dissipate wave energy across the reefs in Seychelles from Numerical projections indicate that climate change will in- 1994 to 2004 (Sheppard et al., 2005). Ocean acidification re- crease the mean maximum wind speed of TCs (Christensen duces the CaCO3 saturation rate and coral growth, reducing et al., 2013). Towards the end of the 21st century, the wind the calcification of corals (Anthony et al., 2008). This pro- speed and minimum central pressure of the most intense su- cess also reduces the roughness of reef flats and consequently per typhoons in the northwest Pacific Ocean are estimated higher wave energy reaches the shore. This indicates that the to attain 85–90 m s−1 and 860 hPa, respectively (Tsuboki et degraded reef may cause an increase in wave heights and wa- al., 2015). This will probably increase wave heights and wa- ter levels at shores in the future. However, there is still almost ter levels on shores at coastal areas. Additionally, sea level no quantitative study evaluating the effectiveness of reefs to rise (SLR) caused by global warming will probably increase TCs and SLR on degraded versus healthy reefs. Furthermore, the risk of coastal erosion, flooding, and saltwater intrusion if future reefs are degraded, potential supplementations (e.g., of surface water (Woodruff et al., 2013). Economic devel- by coral transplantation) will be needed. The target corals for opment and population growth are also expected to increase the transplantation are also poorly understood. along the coastal areas, increasing the potential cost of dam- This study focuses on coral reefs in Palau Islands. The ages to coastal community’s baseline damages (World Bank Palau Islands are rarely affected by TCs, but two severe TCs and UN, 2010). Therefore, the development of policies for (Typhoon Bopha in 2012 and Typhoon Haiyan in 2013) re- adaptation to TCs and SLR is essential if their projected cently impacted the islands. These TCs caused 56–83 % loss negative impacts in the near future are to be adequately ad- of coral cover on the shallow slopes of the eastern reefs dressed. (Gouezo et al., 2015), and they had a significant impact Several approaches to reduce the wave heights and wa- on coastal areas (e.g., flooding, erosion, and destruction of ter levels have been suggested, including the construction buildings). Additionally, intensity of TCs is expected to in- of sea walls and shelters, developing accurate weather fore- crease under future conditions in the northwest Pacific (Tsub- casts, and developing TC and SLR warning systems (GF- oki et al., 2015). Furthermore, there will probably be an east- DRR, 2016). However, small island nations and developing ward shift in the genesis location of TCs in the northwest Pa- countries will need to develop cost-effective strategies to ad- cific in the near future (2075–2099; Murakami et al., 2011). dress the problems associated with these phenomena, includ- This implies that Palau Islands, which currently experience ing the use of ecosystem services provided by mangroves only infrequent and lower intensity TCs, will probably be af- and coral reefs. For example, mangrove restoration has been fected by more frequent and intense TCs in the near future. demonstrated to attenuate wave height and reduce wave dam- Consequently, this study has two main aims. Firstly, we pro- age and erosion (Wong et al., 2014). More than 150 000 km vide a quantitative projection of wave heights and water lev- of the shoreline in 100 countries and territories receives some els for the healthy reefs and the degraded reefs under intensi- protection from reefs (Burke et al., 2011). Meta-analysis has fied TCs and SLR conditions by 2100, based on a numerical demonstrated that the entire reef system, from reef slope to simulation. Secondly, we estimate the potential of main reef reef flat, reduces wave height by an average of 84 % (Ferrario builder and the reef production rate necessary to reduce the et al., 2014). Coral reefs are also habitats for diverse marine coastal risk under the predicted reef degradation, based on organisms and provide various services (e.g., tourism and analysis of a drilled core that looked at past reef growth and marine products) that benefit human populations. Addition- carbonate production. ally, reefs can be considered to represent self-adapting “green infrastructure” (Benedict and McMahon, 2002) in the ocean that protects against TCs and SLR. The growth of Holocene 2 Methods reefs has kept pace with the SLR that occurred in the period 19–6 ka (Montaggioni and Braithwaite, 2009). This suggests 2.1 Study site that reef growth, as a form of green infrastructure, may be able to respond to SLR and contribute to the reduction of the The present study site is located in Melekeok state, on the wave heights and water levels at shores in the future. eastern central coast of Babeldaob, the biggest island in Palau However, approximately 75 % of the world’s coral reefs (Fig. 1a). Melekeok reef (7.501◦ N, 134.640◦ E) is located are subject to local threats such as coastal development, wa- on the eastern coast of the state.
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