Rain events could cause major failure of Waikīkī storm drainage by 2050

Ala Wai Canal at high tide. Credit: Matthew Gosner; Courtesy Hawai'i Sea Grant King Tides Project.

A Waikīkī storm drain nearly full during a king tide. Credit: HI Sea Grant King Tides Project

Link to soundbites (details below): https://go.hawaii.edu/r8g 

***SUGGESTED VOSOT SCRIPT BELOW***

Existing sea level rise models for coastal cities often overlook the impacts of rainfall on infrastructure. Researchers at the University of Hawaiʻi at Mānoa predicted that by 2050, large rain events combined with sea level rise could cause flooding severe enough to disrupt transportation and contaminate stormwater inlets (grate or curb opening in Waikīkī’s streets that collects rainwater and directs it into the storm drainage system) across 70% of Waikīkī, due to interactions with water in the Ala Wai Canal. Their study was published in July 2025 in Scientific Reports.

“We’ve known that sea level rise will reduce the capacity for our drainage system to handle surface runoff, however, including rainfall events in our models showed that Waikīkī’s drainage infrastructure could fail sooner than we anticipated,” said Chloe Obara, lead author of the study who was a graduate student in the Department of Earth Sciences at the UH Mānoa School of Ocean and Earth Science and Technology (SOEST) at the time of this research. “This study highlights the importance of incorporating rainfall and drainage infrastructure into coastal flood models to better understand how drivers of coastal flooding change over time.”

“The many factors affecting flooding should be included in risk assessments and resiliency planning for Waikīkī and other coastal urban areas,” said Chip Fletcher, study co-author, director of the Coastal Research Collaborative, and dean of SOEST. “Only with accurate information can we strategically mitigate urban flood risks in Honolulu’s tourism hub and other coastal areas.”

A similar example of this happened in early December 2021 when a storm system brought heavy rainfall to Oʻahu’s south shore, resulting in several feet of flooding along Kalākaua Avenue. The situation was worsened by a King Tide on the evening of December 6, which, combined with intense rainfall and onshore winds, overwhelmed the stormwater system and caused widespread drainage failure across Waikīkī.

Modeling Waikīkī’s storm drainage system

SOEST researchers developed a computer model of the Waikīkī storm drainage system. They also installed 10 sensors throughout the storm drainage system—including at street-level inlets and canal or oceanside outfalls—which recorded water depth during two rain events to calibrate and validate their model. They simulated various scenarios of sea level rise and rainfall to determine where and under what conditions the storm drainage system will experience failure.

They determined rainfall is the dominant driver of drainage backflow until sea level rises two feet. As sea levels rise further, tidal flooding becomes more influential. Once four feet of sea-level rise is reached, the dominant driver of drainage backflow was determined to be high tidal levels.

“Management practices aimed at reducing rainfall runoff will help minimize compound flooding in the short-term, but management to reduce tidal backflow, such as pumped drainage, is also urgent, as storm drains are presently impacted by high sea levels and will continue to fail as sea level rises,” said Obara.

More than 75% of the storm drainage system in Waikīkī is connected to the Ala Wai Canal, which is known to be heavily contaminated. Accounting for precipitation, the new study determined that 100% of the outfalls (end points where stormwater drains empty into the Ala Wai Canal or the ocean) of the Waikīkī storm drainage system will fail by 2050, causing backflow of potentially contaminated water.

“This research contributes to the growing body of knowledge warning of present and near future climate challenges that will affect transportation, recreation and accessibility in Waikīkī,” said Obara. “Additionally, it raises awareness of the potential health hazard posed by the presence of drainage backflow containing highly contaminated water from the Ala Wai Canal.”

With this research, the team aims to inform and prepare planners and managers so they can be better positioned to take action in Honolulu and across the state.

Link to soundbites (details below): https://go.hawaii.edu/r8g 

SOUNDBITES:

Chloe Obara, lead author and UH Mānoa Department of Earth Sciences master’s graduate

:13

“As we have higher sea levels projected into the future, compounded by larger, more frequent intense rainfall events, we can expect to see more storm drainage failure and flooding in Waikīkī.”

:19

“Though 2050 might seem like a distant future, realistically, infrastructure takes many, many years to plan, rebuild, refurbish, and so now is the time that we need to be thinking about how to retrofit or redesign the storm drainage infrastructure in Waikīkī.”

Chip Fletcher, study co-author, director of the Coastal Research Collaborative, and dean of SOEST

:16

“We found that by 2050, only 25 years from now, up to 70% of the Waikīkī area will lose drainage during a typical rainfall event, a similar rainfall event to the one that we modeled.”

Possible teaser soundbite

:05

“We need to get on top of this problem, or otherwise this problem is gonna get on top of us.”

SUGGESTED VOSOT SCRIPT

INTRO:

A new University of Hawaiʻi at Mānoa study warns that Waikīkī’s storm drainage system could fail by 2050.

VO (suggest to use file video of heavy rainfall in Waikīkī in 2021):

Researchers found that heavy rainfall, combined with rising sea levels, may flood a majority of the area.

Contaminated water from the Ala Wai Canal could also back up through storm drains and into the streets.

Researchers based their model on real rain events captured by water level sensors placed throughout the Waikīkī storm drainage network.

SOT:

Chip Fletcher, study co-author, director of the Coastal Research Collaborative, and dean of SOEST

“We found that by 2050, only 25 years from now, up to 70% of the Waikīkī area will lose drainage during a typical rainfall event, a similar rainfall event to the one that we modeled.”

Chloe Obara, lead author and UH Mānoa Department of Earth Sciences master’s graduate

“Though 2050 might seem like a distant future, realistically, infrastructure takes many, many years to plan, rebuild, refurbish, and so now is the time that we need to be thinking about how to retrofit or redesign the storm drainage infrastructure in Waikīkī.”

VO:

The study urges planners to take action now to reduce future flood and health risks.