Honolulu faces a high flooding risk from multiple sources

A new study has revealed that sea level rise will increase flood risks across Honolulu from multiple sources in the coming decades. Researchers at the University of Hawaii at Manoa School of Ocean and Earth Science and Technology found that groundwater inundation will pose the biggest flooding threat for Honolulu.

As sea levels continue to rise, the frequency of extreme high tide events will increase. Honolulu will not only be impacted from seawater washing directly onshore, but also from groundwater inundation and reverse flow through the municipal drainage system.

The researchers determined that direct marine inundation represents only three percent of the predicted flooding, while rising groundwater will be responsible for the majority of the anticipated flooding events.

Study lead author Shellie Habel is a coastal geologist and extension agent with the University of Hawaii Sea Grant College Program and UH Coastal Geology Group. 

“This is significant because many people think that sea level rise can be mitigated by seawalls,” said Habel. “But a seawall will not stop groundwater inundation. Our results highlight the need to readjust our thinking regarding the flooding that accompanies sea level rise. We want to be sure to implement flood management strategies that will be effective at mitigating flooding.”

The researchers developed a method to assess all types of flooding and to identify vulnerable infrastructure. They simulated the extent and locations of flooding from each of the three sources to produce flood maps. 

By overlapping the simulations, the team managed to pinpoint areas that will be vulnerable to combined flooding in the coming decades.

In collaboration with experts at the UH Sea Level Center, the researchers developed a statistical model that incorporated tide and sea level rise predictions. 

Using the flood simulations, the research team estimated the frequency of flooding in specific areas and identified critical infrastructure that is likely to fail and cause direct impacts, such as dangerous roadways and vulnerable storm drainage inlets. Potential impacts were found to be widespread among Honolulu’s primary urban center.

“Because each type of flooding infiltrates through unique pathways, they will require unique engineering strategies to manage,” said Habel. “The design of flood management strategies required to mitigate these impacts necessitate site-specific consideration of each mechanism to avoid being ineffective.”

The UH researchers will partner with the Honolulu Board of Water Supply, the Hawaii Sea Grant, and others to develop a coastal groundwater monitoring network. 

Data collected through monitoring, such as the effects of extreme rainfall, will improve the capabilities of the statistical model developed by the researchers. The monitoring network will also be used to inform short and long-term flood management strategies.

The study is published in the journal Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer