Sunny-Day Flooding Is About to Become More Than a Nuisance

During the summer of 2017, the tide rose to historic heights again and again in Honolulu, higher than at any time in the 112 years that records had been kept. Philip Thompson, director of the Sea Level Center at the University of Hawaii, wanted to know why. “Where did this come from?” he asked. “How often is this going to happen? Is this our window into the future?”

What Thompson and a group of researchers discovered is that the future has arrived. The summer of ’17 was a glimpse of the watery reality coming to Honolulu and other coastal communities. The study, published this June in Nature Climate Change, found that higher and more frequent tides will reach an inflection point in the 2030s, particularly along the West Coast and at islands like those in Hawaii, making what’s been labeled as “nuisance flooding” common.

“Many areas along the East Coast are already experiencing recurrent impacts,” Thompson says. “In the mid-2030s, these other areas are going to catch up rapidly. So then it’s a transition from being a regional East Coast issue to a national issue, where a majority of the nation’s coastlines are being affected by high-tide flooding on a regular basis.”

How regular? The study, which included researchers from NASA and the National Oceanic and Atmospheric Administration, shows that sunny-day floods will cluster in the fall, creating a nightmare for cities and businesses. Streets will be impassable, cars will be damaged in parking lots, and stormwater systems will be strained. In addition, tidal flooding also fouls local waterways with pollutants including oil, gasoline, trace metals, and nitrogen, spawning algae blooms that create oxygen-depleted dead zones. 

Thompson notes that high-tide flooding is subtle, damaging a community with a thousand cuts―or, in this case, dozens of days a year when arriving at work or shopping for groceries becomes a hassle or even impossible. “If it’s happening 10 or 15 times in a month, it becomes an issue,” he adds. “A business can’t keep operating with its parking lot under water. People lose their jobs because they can’t get to work. Those impacts can really accumulate quickly.”

The study adds to growing research on the variables driving increasingly high tides. Like sea level rise, high-tide flooding varies from place to place. Among the factors increasing sunny-day flooding are local land subsidence, the effects of El Niño, the slowing of the Gulf Stream along the Atlantic coast, water temperature, and ocean eddies.

While the role of the moon’s so-called “wobble” in nuisance flooding made headlines, it’s nothing new, and the label is misleading. The moon is not wobbling; its angle relative to Earth’s equator changes ever so slightly as it orbits, something first reported in 1728. The cycle takes 18.6 years. Half of that time it suppresses tides, and during the other half it amplifies them. The effect is especially strong in places that have a single high tide or a dominant high tide during a single day, like much of the West Coast.

While the moon’s angle is now amplifying tides, sea level rise has not been significant enough in some places to top flood thresholds. That will change during the next cycle in the 2030s, the study concludes. Those higher sea levels coupled with another lunar cycle will drive a national leap in high-tide flooding, starting with what Thompson and researchers call “a year of inflection.”

Those years will differ from place to place because of local variables. That means La Jolla likely will have 15 days of high tide flooding in 2023, 16 days in 2033, and 65 days in 2043. In Honolulu, they project two days of flooding in 2033 and 65 days in 2043. In St. Petersburg, Florida, the jump is from seven days in 2023 to 13 days in 2033 and then to 80 days in 2043.