A new study by University of Hawai'i at Mānoa researchers revealed that Waikīkī is facing a fundamental shift in flood hazards as sea levels rise–transitioning from a flooding that is driven primarily by rainfall to events increasingly dominated by tidal processes. The team identified two key pathways that will become more significant with sea-level rise, both of which will increase public exposure to sewage-contaminated waters. They published their research recently in Scientific Reports .
"Our findings make clear that current flood management strategies for Waikīkī are incomplete," said Kayla Yamamoto, climate modeling analyst at the Coastal Research Collaborative in the UH Mānoa School of Ocean and Earth Science and Technology . "Most planning focuses on surface damage and economic loss from storms, but largely ignores the contamination dimension. Our results show that contaminated flooding will become more frequent, more extensive, and eventually a daily occurrence rather than a storm-driven one. There are currently no effective management strategies in place to address this."
Simulating future scenarios
The team used an open-source, physics-based flood model to simulate how multiple flood sources interact in Waikīkī. Unlike flood models that assess rainfall or coastal flooding in isolation, this model couples rainfall-runoff, tidal forcing, shallow subsurface behavior, and storm drain networks into a single, fully integrated framework.
"What we found is that during extreme rainfall like we've been experiencing, high tides and elevated water levels in the Ala Wai can combine to create conditions where contaminated water flows back into low-lying streets and sidewalks," said Shellie Habel, study co-author and coastal geologist with the Coastal Research Collaborative and Hawai'i Sea Grant . "As sea level rises, it will take less extreme rainfall and tides to cause similar flooding in the future."
The two key pathways they identified were: storm drain backflow, where polluted water from the Ala Wai Canal is forced into streets and public spaces in Waikīkī through drainage systems, and groundwater emergence, which brings sewage and other contaminants from aging and leaking sewage infrastructure to the surface.
The model simulations show that storm drain backflow is projected to occur even when there is no rainfall beginning when there is just one foot of sea-level rise during extreme tides, and at two feet of sea level rise under more moderate daily tidal conditions. At four feet of sea-level rise, groundwater emergence is also projected to occur without rainfall, further increasing exposure to contaminated floodwaters.
To validate their results, the researchers compared their model simulations against tide gauges, canal water level sensors, groundwater monitoring wells, and photographs of street-level flooding during three real recent storm events, including a major 50-year Kona storm in December 2021, a moderate storm in April 2023, and a five-year Kona storm in May 2024.
Implications for Waikīkī and beyond
The Ala Wai Canal is one of the most polluted waterways in Hawai'i, containing sewage, heavy metals, and pathogens such as Vibrio and MRSA. Exposure to these waters is a documented risk, with MRSA infections linked to Hawaiʻi waters already contributing to an estimated 200 deaths per year in the state. Because Waikīkī is a primary economic engine where residents and visitors are in constant contact with coastal waters, the anticipated flooding represents a growing public health and environmental crisis.
The implications of this study extend well beyond Waikīkī. Many coastal cities around the world rely on estuarine waterways to drain their stormwater, and face the same combination of aging infrastructure, rising seas, and contaminated waters.
"Our modeling framework is transferable, and we hope this study serves as a wake-up call to modernize stormwater and wastewater infrastructure, integrate contamination risk into coastal flood planning, and build early warning systems before these thresholds are crossed," Yamamoto added.
