by Paulina Ćwik
In a nutshell:
- Overwash is a newly-intensifying phenomenon impacting the Pacific Islands.
- Produced by a combination of storm swell and wave dynamics with coral reefs, overwash waves flood low-lying islands, compromising freshwater supplies.
- Research is underway to improve forecasting and bolster resilience to these events as they become more frequent due to climate change.
Climate change is already reshaping the world in profound ways, and the Pacific Islands are on the frontlines of this transformation. There are over 30,000 Pacific Islands, many of which are atolls —narrow, ring-shaped coral reefs with central lagoons, rising only a few feet above sea level. Despite their small individual landmasses, these islands collectively cover more than 300,000 square miles (approximately 15% of the Earth’s surface) and are home to over 2.3 million people, with 1.4 million residing in U.S.-affiliated islands. These communities, along with their surrounding ecosystems, have coexisted on the islands for hundreds of years. However, the current pace of sea level rise, unprecedented in the last 5,000 years, now threatens these low-lying island systems with flooding, coastal erosion, and storm surges.
Recent events and studies highlight a critical and escalating issue: the increasing frequency and severity of wave-driven flooding, or overwash, which poses a significant threat to vulnerable Pacific islands [1]. The primary source of freshwater for populated atoll islands is rainwater that infiltrates the ground and forms freshwater ‘lenses’ floating above the denser saltwater. The renewal of these freshwater lenses depends entirely on groundwater recharge from rainfall. Many native plants or food crops, such as taro or coconut trees, have adapted to directly access this shallow groundwater to meet their moisture needs. Consequently, any degradation of these fragile freshwater resources can have severe consequences for both the atoll population and the ecosystem.
An illustrative case occurred on January 20, 2024, when Roi-Namur, an island on Kwajalein Atoll (the second largest atoll in the Pacific Ocean, with a mean elevation of 1.8 meters) in the Republic of the Marshall Islands (Fig.1), was struck by a series of powerful waves. Unlike typical rogue waves, this overwash event was the result of a complex interaction between storm swell and wave dynamics over coral reefs, producing a massive surge of water that overwhelmed the island’s natural defenses. The incident caused significant flooding and infrastructure damage. Overwash events (Fig.2) lead to saltwater intrusion into freshwater aquifers essential for drinking water, and cause damage to crops and ecosystems vital to local communities. Additionally, the loss of biodiversity and cultural heritage sites is a profound concern.
Collaboration between scientists, policymakers, and local communities is essential to address these challenges effectively.
Unfortunately, overwash episodes are not uncommon. Similar events occurred for example in 2020 on the island of Ofu in the Manuʻa Island group of American Samoa [2], in 2015 in Vanuatu [3], in 2014 in Roi-Namur [4], or in 2005 on the island of Pukapuka in the Cook Islands [5]. The combination of higher sea levels and storm-driven waves is expected to exacerbate these events, causing more frequent and severe flooding. This increase in frequency will limit recovery time between events [6] and may push the coping capacity of island communities to a level that seriously jeopardizes their ability to maintain permanent habitation [7].
Researchers are actively studying these dynamics to develop a better understanding of the events, evaluate coastal flooding hazards, and devise mitigation strategies [8-12]. The goal is to enhance the resilience of island communities through improved forecasting, robust infrastructure, and sustainable water management practices. Collaboration between scientists, policymakers, and local communities is essential to address these challenges effectively.
The flooding at Roi-Namur in early 2024 demonstrated the immediate and long-term challenges posed by sea level rise and overwash events, emphasizing the pressing need for effective adaptation strategies not only in the Marshall Islands but across all Pacific Islands. Despite contributing only a fraction of global CO2 emissions, these islands suffer disproportionately from climate impacts. The cultural richness of the Pacific Islands is immense, with more than 1,750 unique languages spoken across the region. This linguistic diversity underscores the deep cultural heritage and identity tied to these lands. Many islanders face the imminent prospect of relocation, risking the loss of their history and identity. The cultural and emotional toll of displacement is profound, as their connection to their land is deeply rooted in their sense of self and community, now endangered by the looming threats of climate change.
Protecting these communities requires a multifaceted approach that integrates scientific research, traditional knowledge, and innovative strategies. Supporting ongoing research and policy initiatives is crucial for mitigating the impacts of climate change on these vulnerable regions. Through collaboration and dedicated efforts, the sustainability and resilience of the Pacific Islands can be ensured, preserving their unique cultural and ecological heritage for future generations.
References:
- Cheriton, O.M., Storlazzi, C.D. and Rosenberger, K.J., 2016. Observations of wave transformation over a fringing coral reef and the importance of low‐frequency waves and offshore water levels to runup, overwash, and coastal flooding. Journal of Geophysical Research: Oceans, 121(5), pp.3121-3140
- Cheriton, O.M., Storlazzi, C.D., Oberle, F.K., Rosenberger, K.J. and Brown, E.K., 2024. Extreme low-frequency waves on the Ofu, American Samoa, reef flat. Coral Reefs, pp.1-7
- Hong, I., Pilarczyk, J.E., Horton, B.P., Fritz, H.M., Kosciuch, T.J., Wallace, D.J., Dike, C., Rarai, A., Harrison, M.J. and Jockley, F.R., 2018. Sedimentological characteristics of the 2015 tropical cyclone pam overwash sediments from Vanuatu, South Pacific. Marine Geology, 396, pp.205-214.
- Quataert, E., Storlazzi, C., Van Rooijen, A., Cheriton, O. and Van Dongeren, A., 2015. The influence of coral reefs and climate change on wave‐driven flooding of tropical coastlines. Geophysical Research Letters, 42(15), pp.6407-6415.
- Terry, J.P. and Falkland, A.C., 2010. Responses of atoll freshwater lenses to storm-surge overwash in the Northern Cook Islands. Hydrogeology journal, 18(3), p.749
- Bailey, R.T., 2015. Quantifying transient post‐overwash aquifer recovery for atoll islands in the Western Pacific. Hydrological Processes, 29(20), pp.4470-4482
- Storlazzi, C.D., Elias, E.P. and Berkowitz, P., 2015. Many atolls may be uninhabitable within decades due to climate change. Scientific reports, 5(1), p.14546
- Vetter, O., Becker, J.M., Merrifield, M.A., Pequignet, A.C., Aucan, J., Boc, S.J. and Pollock, C.E., 2010. Wave setup over a Pacific Island fringing reef. Journal of Geophysical Research: Oceans, 115(C12)
- Quataert, E., Storlazzi, C., Van Rooijen, A., Cheriton, O. and Van Dongeren, A., 2015. The influence of coral reefs and climate change on wave‐driven flooding of tropical coastlines. Geophysical Research Letters, 42(15), pp.6407-6415
- Van Dongeren, A.P., Lowe, R., Pomeroy, A., Trang, D.M., Roelvink, D., Symonds, G. and Ranasinghe, R., 2013. Numerical modeling of low-frequency wave dynamics over a fringing coral reef. Coastal Engineering, 73, pp.178-190
- Gawehn, M., van Dongeren, A., van Rooijen, A., Storlazzi, C.D., Cheriton, O.M. and Reniers, A., 2016. Identification and classification of very low frequency waves on a coral reef flat. Journal of Geophysical Research: Oceans, 121(10), pp.7560-7574
- Beetham, E. and Kench, P.S., 2018. Predicting wave overtopping thresholds on coral reef-island shorelines with future sea-level rise. Nature communications, 9(1), p.3997.
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