{"id":218443,"date":"2025-07-09T08:00:30","date_gmt":"2025-07-09T18:00:30","guid":{"rendered":"https:\/\/www.hawaii.edu\/news\/?p=218443"},"modified":"2025-07-09T10:37:03","modified_gmt":"2025-07-09T20:37:03","slug":"rain-events-waikiki-storm-drainage","status":"publish","type":"post","link":"https:\/\/www.hawaii.edu\/news\/2025\/07\/09\/rain-events-waikiki-storm-drainage\/","title":{"rendered":"Rain events could cause major failure of Waik\u012bk\u012b storm drainage by 2050"},"content":{"rendered":"Reading time: <\/span> 3<\/span> minutes<\/span><\/span>
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A Waik\u012bk\u012b storm drain nearly full during a king tide. (Photo credit: Â鶹´«Ã½<\/span> Sea Grant King Tides Project)<\/figcaption><\/figure>\n

Existing sea level rise models for coastal cities often overlook the impacts of rainfall on infrastructure. Researchers at the University of Â鶹´«Ã½<\/span> at M\u0101noa 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 Waik\u012bk\u012b\u2019s streets that collects rainwater and directs it into the storm drainage system) across 70% of Waik\u012bk\u012b, due to interactions with water in the Ala Wai Canal. Their study was published in July 2025 in Scientific Reports<\/em><\/a>.<\/p>\n

\"canal
Ala Wai Canal at high tide. (Photo credit: Matthew Gosner; courtesy Â鶹´«Ã½<\/span> Sea Grant King Tides Project)<\/figcaption><\/figure>\n

“We\u2019ve 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 Waik\u012bk\u012b\u2019s 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<\/a> at the UH<\/abbr> M\u0101noa School of Ocean and Earth Science and Technology<\/a> (SOEST<\/abbr>) 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.”<\/p>\n

“The many factors affecting flooding should be included in risk assessments and resiliency planning for Waik\u012bk\u012b and other coastal urban areas,” said Chip Fletcher, study co-author, director of the Coastal Research Collaborative<\/a>, and dean of SOEST<\/abbr>. “Only with accurate information can we strategically mitigate urban flood risks in Honolulu\u2019s tourism hub and other coastal areas.”<\/p>\n

A similar example of this happened in early December 2021 when a storm system brought heavy rainfall to Oʻahu<\/span>\u2019s south shore, resulting in several feet of flooding along Kal\u0101kaua 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 Waik\u012bk\u012b.<\/p>\n

Modeling Waik\u012bk\u012b\u2019s storm drainage system<\/h2>\n

SOEST<\/abbr> researchers developed a computer model of the Waik\u012bk\u012b 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.<\/p>\n

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.<\/p>\n

“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.<\/p>\n

More than 75% of the storm drainage system in Waik\u012bk\u012b 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 Waik\u012bk\u012b storm drainage system will fail by 2050, causing backflow of potentially contaminated water.<\/p>\n

“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 Waik\u012bk\u012b,” 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.”<\/p>\n

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.<\/p>\n

—By Marcie Grabowski<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

UH<\/abbr> researchers found that sea level rise and heavy rain could overwhelm Waik\u012bk\u012b\u2019s drainage system and send contaminated water into the streets by 2050.<\/p>\n","protected":false},"author":16,"featured_media":218448,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[30],"tags":[1409,1187,1467,1363,1314,158,92,942,73,9],"class_list":["post-218443","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-research","tag-disaster-emergency","tag-earth-science","tag-manoa-excellence-in-research","tag-manoa-research","tag-manoa-sustainability","tag-publication","tag-school-of-ocean-and-earth-science-and-technology","tag-sea-level","tag-sustainability","tag-uh-manoa","entry","has-media"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t