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Flooding surges in U.S. coastal cities, thanks to relentlessly rising sea levels

The incidence of high-tide flooding — also called “sunny day flooding,” “nuisance flooding” or “saltwater flooding” — has increased by 50 percent in just the last eight years, a new report says.

In the “flood year” running from May 2017 through last April, about one-fourth of coastal cities tied or broke their previous record for tidal flooding; leading that list were Boston, above, and Atlantic City.
REUTERS/Brian Snyder

Last year was a record-breaker for flooding in cities along the U.S. coasts, according to a new federal assessment  — and the horrific trio of hurricanes Harvey, Maria and Irma wasn’t really the problem.

The analysis, issued by the National Oceanic and Atmospheric Administration last week, looked closely at flooding in nearly 100 locations around the country, as measured by tide gauges that have been in place for a century or so.

And there was plenty of flooding outside the hurricane zones, much of it chronic and attributable to rising sea levels. Some was driven by lesser storms, like the nor’easters of last February and March. But in nearly half the cases, weather was considered a minor contributor or no factor at all.

The incidence of this high-tide flooding  — also called “sunny day flooding,” “nuisance flooding” or “saltwater flooding” — has increased by 50 percent in just the last eight years, and 100 percent in the last 30, according to NOAA’s obviously solid data.

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In the “flood year” running from May 2017 through last April, cities whose gauges went over the historical high-tide mark by at least 20 inches saw this happen, on average, six times  — an all-time record.

And that average includes a number of notable extremes. About one-fourth of these cities tied or broke their previous record for tidal flooding; leading that list were Boston and Atlantic City, with 22 days each of seawater-inundated infrastructure; San Diego, somewhat anomalously, posted 13.

Obviously this type of flooding is not as sudden or dramatic as, say, the misery visited on the Texas coast by Hurricane Harvey. But neither is it trivial.

A peer-reviewed analysis by the Union of Concerned Scientists, published last summer, looked at the question of how much nuisance flooding is enough to make people abandon their coastal homes for higher ground. Based on expert opinion from hydrologists, planners, realtors and others in a position to offer the best guesses, the UCC researchers arrived at this benchmark:

Coastal flooding that occurs 26 times per year, and covers at least 10 percent of land area, has reached a point of “effective inundation” where “current use is no longer feasible.”

In historical terms, too, last year’s experience is notable. The report’s principal author, Gregory Dusek, told the Insurance Journal about a tide gauge at Charleston, South Carolina, whose records go to back to 1921, and thus include the effect of the 1928 Okeechobee hurricane, which set a record high-tide mark of .75 meters above the long-term mean.

Last year, Dusek said, “we exceeded that four times in Charleston, and we’re expected to exceed it at least four times this year.”

Because sea-level rise is driven by ocean temperatures as well as glacial meltwater, NOAA considers it likely that mild El Niño conditions predicted for the rest of this year and next may bring unusually frequent tidal flooding  — even by the standard of recent years  — to about half the cities considered in the assessment, the report says:

As a whole, high tide flood frequencies during 2018 are predicted to be about 60% higher across U.S. coastlines as compared to trend values at the start of this century (i.e., year 2000). Along the Northeast Atlantic and Northwest Pacific Coasts (where steeper topography often buffers the extent of impacts), 5‐6 days are predicted or a 100% and 10% increase since 2000, respectively. Along the flatter and more‐vulnerable Southeast Atlantic and Gulf Coasts, about 2‐3 flood days are predicted in 2018 or a 160% and 50% increase since 2000, respectively. Along the Southwest Pacific Coast about 2 days of high tide flooding are predicted (70% increase). Little or no tidal flooding is expected (as in all cases, not considering wave or rainfall effects) within the Caribbean, Hawaii or U.S. Pacific Islands (Kwajalein Island being an exception). …

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Breaking of annual flood records is to be expected next year and for decades to come as sea levels rise, and likely at an accelerated rate. Already, high tide flooding that occurs from a combination of high astronomical tides, typical winter storms and episodic tropical storms has entered a sustained period of rapidly increasing trends within about two-thirds of the coastal U.S. locations.

Though year‐to‐year and regional variability exist, the underlying trend is quite clear: Due to sea level rise, the national average frequency of high tide flooding is double what it was 30 years ago.

Since the main driver of sea-level rise is global warming, these trends can be expected to continue indefinitely  — an observation you will not find in the report, which makes no mention of a warming planet or its changing climate.

This is the Trump administration’s NOAA, after all, which serves a White House where such linkages are inconvenient and therefore not for official citation. Also, a White House whose occupant happily trashed, early on, a bipartisan policy of making sure that future federal investments in infrastructure weren’t likely to be in the path of saltwater inundation. Developers and realtors didn’t care for that at all.

But the risk to infrastructure is already significant and growing worse. Andrea Dutton, a University of Florida geologist, told the UK Guardian that new patterns of flooding in her state’s southern portions have made fish swimming over roadways a common sight, and have prompted TV and radio stations to add tide forecasts to their reports.

The situation is aggravated by porous limestone bedrock, which routes saltwater flow to the surface, and a sort of second coastline formed by tidal portions of the Everglades.

They used to get just one day a year of tidal flooding, now it’s two months of it in the fall. Engineering can help delay things but ultimately the oceans will win.

Extreme events like hurricanes may be the breaking point but this sort of frequent flooding is the taste of what is coming in the future on a permanent basis. We need to rethink our relationship with the coastline because it’s going to be retreating for the foreseeable future.

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NOAA’s main page on its tidal flooding assessment, with links to the full report and other resources, is here.

Correction: A previous version of this article said that roadkill fish had become a common sight in south Florida. Andrea Dutton was describing live fish on the roads.