Aug 2, 2021 9:00 AM

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

Sea level rise will soon combine with a host of other environmental factors to produce dozens of floods each fall in US coastal cities.

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.

(NASA has created a web page projecting flooding changes for more than 90 coastal locations using data from the study.)

“The motivation for this idea of inflection is for planners to say: ‘This is the point at which I need to be prepared,’” Thompson says. “The reason that’s important is because we found these periods of really rapid change related to this natural cycle.”

The study arrives in the context of NOAA's annual State of High Tide Flooding report, which concluded that coastal communities experienced twice as many high-tide flooding days from May 2020 to April 2021 as they did 20 years ago. NOAA defines high-tide flooding as occurring when tides hit 1.75 to 2 feet above the daily average high tide, flooding low-lying streets and often filling stormwater pipes. As sea level rise continues, floods that arrived only with a storm now damage cars and make roads impassable during a full-moon tide or when prevailing winds drive high water over land. Galveston and Corpus Christi, Texas, for example, set records with more than 20 days of high-tide flooding, 10 times what those cities experienced two decades ago.

William Sweet, an oceanographer with NOAA, authored that annual report, but he also contributed to the Thompson study. Like Thompson, he began researching high-tide flooding after a surprise during the summer of 2009 when dozens of communities along the eastern seaboard were swamped by rising tides during clear weather. He found that a prevailing northeasterly wind and a slowing Gulf Stream—coupled with a full-moon tide, above-normal summer high tides, and decades of sea level rise—created the conditions for sunny-day flooding that summer.

NOAA has tracked high-tide flooding annually since 2014. In this year's report, Sweet notes that 80 percent of locations along the southeast Atlantic and Gulf coasts had accelerating high-tide flooding. Miami, Charleston, Savannah, Norfolk, Annapolis, parts of Boston and New York, as well as other areas on low, flat, reclaimed land, are increasingly posting flood warnings on clear days. "The inflection points, I would argue, are already occurring" in those locations, Sweet says. "The point I don't want folks to miss is that it's not a problem a decade from now. It's a problem now, and it's going to get worse."

Sweet says his work aims to prompt policymakers to act. “What we're trying to do at NOAA is keep a score and put things in a historical perspective that folks can humanize. What does it mean to my commute, to commerce at my store? These other studies pile on additional insights. They’re all pointing in the same direction,” he says. “There's going to be a flood in your future. And that future is here."

What's particularly vexing for planners and policymakers for the nearly 40 percent of Americans who live in coastal counties is that, in some places, the years leading up to an inflection point will see gradual or even imperceptible increases in high-tide flooding frequency, perhaps lulling local leaders into inaction. “That’s potentially dangerous because it can lead to complacency,” Thompson says.

Planning for the annual average number of high-tide flooding days may leave localities open to suffering from a cluster of floods during a year when the variables conspire to produce a spike. "Coastal planners, engineers―when they design something for the coastal environment, it's standard practice to look at maybe the 30-year event or the 100-year event. You're planning for some extreme," Thompson says. "When we're talking about high-tide flooding, the idea of the extreme is not the height of the event or the magnitude of it. It's actually the number of days that are extreme. You don't want to plan for the average, because then you're going to be surprised by that really bad year that happens 10, 15, 20 years before you expect it."

Mikhail Chester, an associate professor at Arizona State's School of Sustainable Engineering and the Built Environment and a leader of the Urban Resilience to Extremes Sustainability Research Network, says the study highlights the difficulty of designing long-term infrastructure like roads, storm sewers, and bridges for a rapidly-changing environment. In the past, infrastructure was designed on the assumption that the weather and climate are fairly stable. That’s no longer the case.

“You’ve got to make infrastructure agile and flexible,” he says. “You need different mental models, different ways of approaching infrastructure, because the Industrial Era–mindset that we remain rooted in is just wholly insufficient for the unpredictability and instability of the future.”

The study points to an even more unstable future with dramatically changing coastal tides. That worries Chester. "What's scary about this paper is the idea of the inflection point," he says, noting that it takes a long time to rebuild or replace infrastructure. “The question I have is, can we adapt fast enough to keep pace?”