Waikīkī Flooding Is Becoming More Complex and Dangerous

Waikīkī is famous for its beaches, hotels and constant stream of visitors. It appears designed for taking it easy. Yet a less visible issue is intensifying beneath that image. Flooding in the area is evolving, becoming both more complicated and more hazardous.

New research indicates that the threat no longer comes only from intense rainfall or major storms. Sea-level rise, deteriorating drainage infrastructure and contaminated water now form part of the problem.

Combined, these factors are producing a different form of flood risk, with consequences for the environment and public health.

Changing flood drivers

For many years, flooding has been associated chiefly with rainstorms. In Waikīkī, however, this is beginning to shift. Flooding is increasingly caused by a combination of rainfall, ocean tides and water rising 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 the sea rises, tides are becoming a more significant factor. Flooding could eventually occur on calm days, even when there is no storm.

As a result, cities cannot depend solely on systems intended to cope with rainfall.

Drainage system failure

Waikīkī’s drainage relies on a straightforward principle: rainwater passes through drains into a canal before reaching the sea. However, the arrangement functions only when the ocean sits lower than the city.

Sea-level rise disrupts this relationship. Canal water can climb above the level of the drains, causing water to reverse direction and enter the 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 certain situations, drains may force water upwards like small fountains. At times, high tides alone can obstruct the system, without any rainfall.

These breakdowns will become increasingly common as ocean levels continue to rise.

Floodwater carries sewage

Floodwater in Waikīkī is not clean. A substantial share originates in the Ala Wai Canal, which gathers urban runoff. The water may contain sewage, bacteria and other contaminants.

During flooding, this polluted water spreads across streets and public areas. Anyone walking, driving or working in these locations could be exposed to it.

Rainfall worsens the situation. It disturbs contaminants in the canal and raises the concentration of harmful microbes in the water.

Therefore, the periods when flooding is easiest to see may also be those when it poses the greatest danger.

Groundwater adds to flooding

Flooding also has another, less obvious source: underground water.

In low-lying places such as Waikīkī, groundwater lies near the surface. This subsurface water rises along with sea levels.

Over time, it may emerge on to streets without any rain. Once the ground is saturated, it cannot take in additional water, allowing flooding to accumulate rapidly.

Groundwater can also transport pollution from ageing and leaking sewage infrastructure. Floodwater may therefore carry contaminants from both the surface and below ground.

Toxic flooding in coastal cities

The research provides a clear indication of what could happen next.

Even modest sea-level increases will cause drainage systems to fail more frequently. Greater rises could lead to widespread flooding during ordinary high tides.

Meanwhile, rainfall will become less influential. Tides and groundwater are set to become the principal flood drivers. In some circumstances, extensive areas of Waikīkī may flood 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.

Flooding could consequently move from being an infrequent event to a far more routine occurrence.

Rethinking urban resilience

The findings point to a wider challenge. Flood management is not simply about preventing water from entering an area. It also requires knowing its source and the substances it contains.

Cities will have to improve drainage networks, repair ageing sewage systems and track water quality more carefully. They will also require stronger tools for forecasting how different forms of flooding interact.

Waikīkī is not the only place facing this issue. Comparable pressures confront many coastal cities. Events there provide a warning: flooding is no longer only about water levels rising – it is also about the concealed dangers that accompany them.