Sea-level rise and coastal hazards push O‘ahu’s infrastructure toward functional collapse: a multi-hazard exposure assessment

Abstract

Understanding the consequences of sea-level rise (SLR) requires accounting for multiple coastal hazards and their impacts across infrastructure and communities. We assess island-wide exposure of buildings, roads, wastewater systems, parks, and essential services on O‘ahu due to passive flooding, wave-driven flooding, and coastal erosion under intermediate and intermediate-high SLR scenarios. Results show that urban Honolulu experiences the highest absolute impacts to residents and buildings, while rural regions including Koʻolaupoko and the North Shore face disproportionate impacts to wastewater and roads; in some windward communities, half of residents live below 3 m elevation and along non-redundant access routes. A critical threshold emerges between 0.9 and 1.2 m of SLR (2073–2102), marking a transition from localized disruptions to widespread infrastructure failure. By mid-century, 1,200 buildings and over 6,000 residents may be affected. By end-of-century, 58,000 residents, 200 km of roads, seven schools, and wastewater infrastructure are affected. 32% of the coastal highway along northeastern O‘ahu becomes impassable, and nuisance flooding increasingly affects low-lying roads in Honolulu. Along oceanfront homes, flooding and eroded cesspools pose public health risks and limit safe ocean access. We find that passive flooding assessments alone omit 62–73% of the impacted area at SLR below 1.2 m compared with analyses that include erosion and wave-driven flooding, leading to substantial underestimation of coastal infrastructure risk. These results are conservative, as storm-driven flooding, groundwater rise, and drainage backflow are not considered. The framework applied here can inform similar assessments in other coastal regions by integrating high-resolution hazard modeling with community-level infrastructure, enabling risk characterization and supporting proactive, place-based adaptation decisions.