High-tide coastal flooding in Hawaii and other U.S. coastal regions to accelerate

Hawaii and other U.S. coastal regions will experience an accelerating number of high-tide flooding days starting in the mid-2030s, according to a newly published study.

Led by University of Hawaii at Manoa and appearing this week in the journal Nature Climate Change, the study found that the twin effects of sea level rise and natural fluctuations in the tides will lead to the growing frequency of coastal flooding over a nearly 10-year period.

Not only will many more coastal areas experience flooding at high tide, there will be times when the flooding comes in clusters that will make for severe weeks or possibly months, according to the study.

“The change over that decade will be drastic,” said lead author Phil Thompson, director of the UH Sea Level Center and assistant professor of oceanography.

The research, largely funded by the NASA Sea Level Change Team, is an extension of a scientific paper that Thompson wrote in 2019 featuring a novel statistical model to project future flooding days for Honolulu while figuring in tidal range and sea-level projections.

This time, Thompson joined researchers in different parts of the U.S. to analyze tide gauge data from 89 coastal locations across the country, including 10 locations in Hawaii and U.S.-affiliated Pacific islands.

The projections come from the same model using National Oceanic and Atmospheric Administration sea level rise scenarios for the 21st century. According to NOAA, sea level is rising at a rate of about one-eighth of an inch per year, but the rate is expected to accelerate as the oceans warm and expand as glaciers and ice sheets melt.

Thompson said the most rapid increases in flooding at high tide in the United States are expected along the Gulf of Mexico and Pacific coastlines, including Hawaii.

With many cities on the Atlantic Coast already plagued by so-called sunny-day flooding, the mid- to late-’30s will be the tipping point from which a regional issue transitions into a national issue because it will then affect a majority of the nation’s coastlines, he said.

One of the major influencing factors in the projected dramatic rise in sea level between the mid-’30s to the mid-‘40s is the 18.6-year nodal cycle, which is tied to the angle of the moon’s orbit in relation to the equator. The cycle causes sea levels to drop or to rise at an increased rate depending on which half of the cycle is in effect.

Until the mid-’30s, the nodal cycle will have a suppressing impact on sea level rise, Thompson said. But after then, ocean levels will rise at an increasing rate until reaching a peak in the mid-’40s.

Additionally, the research team found that annual cycles in tides and sea level are likely to combine with large-scale anomalies, such as ocean eddies, to stack ocean layers and create clusters of high-tide flooding events and extreme weeks and months.

Nuisance flooding often causes minor erosion, road closures, temporarily unusable parking lots and inaccessible homes and businesses. Cesspools in Hawaii also have been shown to be vulnerable. The low-lying industrial area of Mapunapuna is notorious for high-tide flooding.

In Honolulu, there were 15 nuisance flooding events in 2017, the most on record to that point, with the highest event reaching more than 18 inches above normal, although it happened on a December night when few were aware of it.

By Thompson’s calculations, Honolulu will experience between 56 and 150 of those “record” nuisance days per year by the mid-’40s.

While a handful of annual flooding episodes is a nuisance, a future of increasing events and coastal flooding seasons is likely to create hardships and financial burden. Coastal communities had better prepare, Thompson said.

“The accumulated impact can add up a lot,” he said. “If a business has to shut down for many days, that’s a problem. How do you make rent that month?”

The city’s Oahu’s Resilience Strategy document includes a climate adaption strategy that calls for, among other things, a coordinated effort to protect transportation infrastructure, water systems and housing stock from sea level rise and other future climate change-related phenomena.