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

Existing sea level rise models for coastal cities often overlook the impacts of rainfall on infrastructure. Researchers at the University of Hawai‘i at Mānoa discovered that by 2050, large rain events combined with sea level rise could cause flooding severe enough to disrupt transportation and contaminate stormwater inlets across 70% of Waikīkī on O‘ahu, Hawai‘i, due to interactions with water in the Ala Wai Canal. Their study was published recently 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. Only with accurate information can we strategically mitigate urban flood risks in Honolulu’s tourism hub and other coastal areas,” said Chip Fletcher, study co-author, director of the Coastal Research Collaborative, and Dean of SOEST.

SOEST researchers developed a computer model of the Waikīkī storm drainage system. They also installed ten 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 currently and until sea level rises two feet. 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.

Over 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 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 to allow Honolulu to continue serving the people of Hawai‘i.