Researchers from the University of Hawaii at Manoa are sounding an alarm that by 2050 large rain events coupled with sea level rise could cause major failure of storm drainage across 70% of Waikiki.
The study, which was published in July 2025 in Scientific Reports, was based on a model of real rain events captured by water level sensors placed throughout Waikiki’s storm drainage network. It also noted that contaminated water from the Ala Wai Canal could back up through storm drains and into the streets. The study highlighted the need for planners to act immediately to reduce future flood and health risks.
Chloe Obara, who was the lead author of the study while she was a graduate student in the Department of Earth Sciences at the University of Hawaii Manoa School of Ocean and Earth Science and Technology, said in a statement, “We’ve known that sea level rise will reduce the capacity for our drainage system to handle surface runoff, however including rainfall events, our models showed that Waikiki’s drainage infrastructure could fail sooner than we anticipated.”
Chip Fletcher, study co-author, director of the Coastal Research Collaborative, and dean of SOEST, said in a statement, “We found that by 2050, only 25 years from now, up to 70% of the Waikiki area will lose drainage during a typical rainfall event, a similar rainfall event to the one that we modeled. We need to get on top of this problem, or otherwise this problem is gonna get on top of us.”
These observations are the latest linchpin in an ongoing broader holistic effort from scientists, government officials, and leaders from nongovernmental organizations and nonprofits to increase resiliency in Waikiki, the greater Ala Wai watershed and urban coastal areas.
Obara, who now works for the City and County of Honolulu’s Office of Climate Change, Sustainability and Resiliency (CCSR), said “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 Waikiki.”
She said that 2050 may sound like the distant future, but “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 Waikiki.”
Rep. Adrian Tam (D-Waikiki) said, “I trust those that are working hard to address what is happening in this study. We at the Legislature will continue to look for other ways to mitigate the findings in the study as well.”
Holistic approach
Dolan Eversole, Waikiki Beach management coordinator for the University of Hawaii Sea Grant College Program and SOEST’s coastal processes specialist, said several complementary planning efforts and projects are part of a holistic effort to address growing concerns over beach erosion in Waikiki and storm-water management and flooding in the Ala Wai watershed, which includes Waikiki and the Ala Wai Canal.
Eversole said the Waikiki Beach Special Improvement District Association’s environmental impact statement for a beach restoration and mitigation plan is complete; although funding, expected to be in the tens of millions of dollars, cannot be secured from the state until the final design and permitting is completed.
He said the biggest expense is for the long-term restoration effort, which includes constructing T-head groins — large rock structures designed to retain sand and stabilize the shoreline, with the goal of restoring parts of Waikiki to the condition that it was in some 40 to 50 years ago.
Eversole said, “I think on the beach side we are moving the needle. There is an EIS that has been approved and there are now proposed efforts that could take effect two years. There are things starting to happen, but they are not really going to do anything for the groundwater issue, which is the backflow issue identified in the UH paper.”
Eversole added, “With respect to the storm drainage, it seems to me we are bit behind recognizing that this is going to be a major issue maybe as soon as the year 2050 if not sooner, maybe the next storm for that matter, but certainly a chronic problem as identified in the report. In the year 2050, we’ll shift from rainfall being the primary contributor to flooding to the tides coming out the storm drains being a major contributor. There certainly is a sense of urgency to do something here.”
Planning efforts abound
The city’s chief resilience officer, Ben Sullivan, said, “We are very aware of the challenge and it is one of many that we face with regards to climate change — and in particular compound flooding as an issue that multiples the concerns both of sea level rise and rain events.”
Sullivan said Fletcher has been critically important to “putting everyone on notice that we have to move faster,” and that CCSR recognized the value of Obara’s research and hired her as a flood resilience specialist.
Sullivan said the One Water Plan (808ne.ws/ One Water) is underway to produce a multiclimate hazard analysis of hot spots, areas where significant impacts from climate hazards are expected. The strategy integrates the management of stormwater, wastewater, groundwater, sea water, freshwater, graywater, and recycled water to create resource and financial efficiencies.
“It’s developing a set of capital improvement projects that allow us to move into the future with more certainty in terms of where we need to start,” he said.
Sullivan said that the city also is working on wrapping up the planning process this summer or fall for a regional plan around climate with its consultant Tetra Tech as part of City’s Adapt Waikiki 2050, which falls under the Honolulu Department of Planning and Permitting.
“The expectation is that this plan will deliver specific recommendations that would then become engineering projects and infrastructure projects to address a lot of these concerns,” he said. “One of the things identified is what are the ‘no regret solutions’ that we can take on — some of those things are going to be things like green infrastructure investments where we can get more permeable surfaces in Waikiki.”
He added that waterproofing the ground-level and migrating equipment from the basement and ground-level to higher floors is among the items under discussion, as is planning how to provide an emergency response under flood conditions.
Sullivan said that DPP will be holding a public meeting for Adapt Waikiki 2050 in the last week of August or first week of September to coincide with a public review draft . The city will post the date on the project page, 808ne.ws/40cdwpm.
Sullivan said, “I hope and aspire that over the next three or four years we actually really start moving on some of these projects, so essentially that whoever the next administration that comes in has these projects ready to go.”
Eversole said the city’s shorter-term approach complements the longer-term view, which is taking shape through the Waikiki Resilience and Sea Level Adaptation Project, which was funded from the State Office of Planning.
“It looked at the framework for adapting Waikiki. It looked at what would it look like 50 to 100 years from now. We talked about having managed retreat and canals,” he said.
Ala Wai Watershed
The Ala Wai Flood Mitigation Plan, a partnership between the city and the U.S. Army Corps of Engineers, is a key project to address the real risk of a catastrophic flood for communities surrounding the Ala Wai Canal — but it appears to have stalled again due to escalating costs.
“If there is an Army Corps project or some version of what was last proposed, that certainly would have a huge impact on stormwater and where it goes and how it gets conveyed,” Eversole said.
Eversole said most of the Waikiki storm drains — 70% — are directed to the Ala Wai and that very few drain right to the beaches, so the impacts to the beaches are more of a water quality issue, which could impact the reefs.
“If you have contaminated water, assuming the Ala Wai Canal water is contaminated, and it is now back flushing into the streets interacting with people and their properties — there’s a health issue there,” he said. “Our coral reefs are key to breaking down waves, even storm waves, and without those coral reefs — we are in grave danger. Our beaches would not exist without those coral reefs.”
Eversole said annually reefs in Hawaii provide flood protection benefits to more than 6,800 people and $836 million in averted damages to property and economic activity.
Meanwhile, a CIVic — Community Investment Vehicle to create an Ala Wai Watershed Special Improvement District is under discussion. The idea is to provide a structure to facilitate fundraising and cross-sector coordination for the district’s proposed projects. A draft bill for the Honolulu City Council says that the plan was prepared and sponsored by Hawai’i Green Growth and the Ala Wai Watershed Collaboration. The draft bill is circulating, although it has not been introduced.
Kalani L. H. Ka‘ana‘ana, CEO of Hawai‘i Green Growth, said in an email that, “The recent report and projections are a timely reminder that the impacts of climate change are no longer distant possibilities; they’re unfolding now. But the good news is that Hawaii is not starting from scratch. Across sectors and communities, partners are already working together to reimagine how we manage water, protect our infrastructure, and build resilience from the ground up.”
Ka‘ana‘ana said Hawai‘i Green Growth and ‘Aina Aloha Economic Futures and Stantec have developed the Malama Implementation Tool — a community-based resource that helps identify, assess, and advance green infrastructure projects that are grounded in local values and designed for impact.
He said Hawai‘i Green Growth’s Follow the Drop initiative, in partnership with 3Rwater, identifies and prioritizes “sites for green stormwater infrastructure from ridge to reef to help community members invest in nature-based solutions that tangibly reduce flood risk, restore ecosystems, and strengthen community resilience.”
“While the challenges ahead are real, the momentum is with us. By aligning our efforts, we can turn today’s projections into tomorrow’s progress,” Ka‘ana‘ana said. “This is the time to move forward together with urgency, with aloha, and with the confidence that collective action will shape a more resilient future for all.”
Source: The Garden Island