Waikīkī flooding is becoming more dangerous as sea levels rise

Waikīkī is widely associated with beaches, hotels and a constant stream of visitors. On the surface, it is the sort of place designed for switching off. Yet a less visible issue is intensifying. Flooding in Waikīkī is shifting, and it is becoming harder to manage - and more hazardous.

New research suggests the story is no longer simply about torrential downpours or major storms. Sea-level rise, underperforming drainage, and polluted water are now tightly linked parts of the same problem.

Combined, these pressures are producing a different kind of flood threat, one that reaches beyond property damage to affect ecosystems and public health.

Changing flood drivers

For a long time, flooding was mostly blamed on rainstorms. In Waikīkī, however, that pattern is beginning to break down. Flooding is increasingly driven by overlapping forces: rainfall, ocean tides, and even water pushing up from beneath the ground.

“Our findings make clear that current flood management strategies for Waikīkī are incomplete,” said Kayla Yamamoto, lead author of the study from the University of Hawai‘i at Mānoa.

“Most planning focuses on surface damage and economic loss from storms, but largely ignores the contamination dimension.”

Contamination is rising fast

“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,” added Yamamoto.

As sea levels continue to rise, tides take on a larger role. In time, flooding may occur even during calm weather, without any storm in sight.

That shift means urban areas cannot depend solely on infrastructure designed primarily to cope with rainfall.

Drainage system failure

Waikīkī’s drainage relies on a straightforward pathway: rainwater is meant to move through drains into a canal and then discharge into the ocean. But that only functions properly when the ocean sits lower than the city.

With sea-level rise, that relationship starts to fail. Water levels in the canal can climb above the level of the drains, causing flow to reverse and spill back into streets.

“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 study co-author Shellie Habel.

“As sea level rises, it will take less extreme rainfall and tides to cause similar flooding in the future.”

In some situations, the drains can even force water upwards like small fountains. At other times, high tides alone can be enough to stop the system from draining, even when it is not raining.

As the sea keeps rising, these kinds of failures are expected to become increasingly common.

Floodwater carries sewage

Floodwater in Waikīkī is not clean. A significant share originates from the Ala Wai Canal, which gathers urban run-off. That water can include sewage, bacteria and other pollutants.

When flooding occurs, contaminated water spreads through roads and public areas. Anyone walking, driving or working in these locations may end up coming into contact with it.

Rain can intensify the danger. It churns up pollutants in the canal and increases the concentration of harmful microbes in the water.

As a result, the moments when flooding is most noticeable can also be the moments when it poses the greatest risk.

Groundwater adds to flooding

Another contributor is harder to spot because it comes from below ground.

In low-lying districts such as Waikīkī, groundwater sits close to the surface. As sea levels rise, this underground water level rises as well.

Over time, it can seep up onto roads even without rainfall. And once the ground is already saturated, it cannot take in more water, so flooding can build very quickly.

This groundwater may also transport pollution from older, leaking sewage systems. That means floods can deliver contaminants from both the canal above and the ground below.

Toxic flooding in coastal cities

The study sets out a clear view of what may happen next.

With modest sea-level increases, drainage systems are likely to fail more often. With larger increases, flooding could spread across wider areas and occur during routine high tides.

Meanwhile, rainfall becomes less central. Tides and groundwater increasingly become the dominant flood drivers. In some scenarios, sizeable parts of Waikīkī could flood even on dry days.

“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,” noted Yamamoto.

In other words, flooding could shift from occasional events to a far more regular reality.

Rethinking urban resilience

These findings point to a wider challenge: flood management is not only about keeping water out. It also requires understanding the sources of floodwater - and what that water contains.

Cities will need to improve drainage, repair ageing sewage networks, and track water quality more closely. They will also need stronger tools to anticipate how different flood mechanisms combine and amplify one another.

Waikīkī is not unique. Many coastal cities face comparable pressures. What is unfolding here is a warning that flooding is no longer just about higher water levels - it is also about the less obvious hazards that arrive with that water.