The role of barrier reef protection to storm surge during tropical cyclones in a narrow lagoon, New Caledonia, south west Pacific

Pacific islands are exposed to coastal hazards, particularly storm surges resulting from low pressure, extreme winds, and waves. While coral reefs naturally protect shorelines from incident waves, the diversity of reef geomorphologies induce various coastal responses, complicating hazard prediction based on offshore conditions. Understanding lagoon storm surge dynamics is therefore crucial for developing adaptation strategies to address global warming and sea-level rise. This study investigates the storm surge response of the narrow and shallow lagoon of Poe in New Caledonia using the SCHISM-SWAN coupled circulation-wave numerical model under tropical cyclone conditions. A retrospective of the 2019 cyclonic season shows that maximum storm surge is concentrated in the narrowest part of the lagoon, where human infrastructures are located, increasing exposure to extreme water levels. Modifying the reef structure to a more irregular form reveals that macro-scale reef features can mitigate wave setup intensity. Additionally, water depth variations over the reef crest significantly affect wave setup, with up to 30 % difference between high and low tides. A broader generalization based on 258 synthetic cyclone tracks highlights that within the lagoon, wave setup accounts for approximately 70 % of the storm surge, rising to 90 % during the most extreme event. These results showing that the wave contribution is essential for accurate storm surge prediction. The findings provide critical insights for improving coastal hazard assessment, land use planning, and resilience strategies in reef island environments facing climate change.