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| A Charleston, South Carolina resident removes debris from a drain during tidal flooding in October 2015. The city now experiences 50 days of “sunny day” flooding a year. PAUL ZOELLER/THE POST AND COURIER VIA AP |
As sea levels rise, high-tide flooding is becoming a growing problem in many parts of the globe, including cities on the U.S. East Coast. Now, new research shows that as these waters recede, they carry toxic pollutants and excess nutrients into rivers, bays, and oceans.
JIM MORRISON reports for Yale environment 360
As high-tide flooding worsened in Norfolk, Virginia in recent years, Margaret Mulholland, a biological oceanographer at Old Dominion University, started to think about the debris she saw in the waters that flowed back into the Chesapeake Bay. Tipped-over garbage cans. Tossed-away hamburgers. Oil. Dirty diapers. Pet waste.
“This water is coming up on the landscape and taking everything back into the river with it,” says Mulholland, a professor in the Department of Ocean, Earth, and Atmospheric Sciences. “I was thinking how no one is counting this stuff (as runoff pollution). It drove me nuts.”
Nuts enough that she decided to sample those waters. That’s why on a recent Saturday morning she was steering her Chevy Bolt EV toward a narrow, flooded ribbon of Norfolk’s 51st Street at high tide. Marsh grasses bordered an inlet of the Lafayette River on one side of the street. A line of houses set back from the street rose on the other. Soon she came upon an overturned trash can, its contents underwater. A few feet away was a box. She opened it, and inside was a toilet. “Oh, this is good,” she said, pulling out her phone for a photo.
It’s an apt metaphor for her pioneering research project, which she has dubbed Measure the Muck.
With global sea levels steadily rising — already up 8 inches in the past century and now increasing at an average of 1.3 inches per decade — the incidence of high-tide “sunny day” or “blue sky” flooding is on the rise, especially along the U.S. East Coast. Those flooding events now routinely wash over sections of cities, and when the waters recede they take with them an excess of nutrients and a toxic mix of pollutants that flows into rivers, bays, and oceans.
Norfolk, which experienced fewer than two days of high-tide flooding annually in the early 1960s, had 14 in 2017. In Wilmington, North Carolina, tidal flooding grew to 84 days in 2016, up from two days 50 years ago. In Lewes, Delaware at the mouth of Delaware Bay, flooding days have topped 25 in recent years, a five-fold increase over a decade ago.
An overturned trash can sits in high-tide floodwaters on 52nd Street in Norfolk, Virginia. MARGARET MULHOLLAND
According to a report from the National Oceanic and Atmospheric Administration (NOAA), high-tide flooding frequency along the southeastern coast of the United States rose 160 percent from 2000 to 2017. And with sea levels expected to rise another 3 to 6 feet by 2100 because of melting ice sheets and glaciers, scientists warn that much worse is to come. NOAA projects that as many as 85 days of high-tide flooding will occur annually along the southeastern U.S. coast by 2050.
Until Mulholland, however, few if any researchers had examined exactly how much pollution this sunny day flooding was creating. And what Mulholland found shocked her. Analysis of water samples indicates that one morning of tidal flooding along the Lafayette River in Norfolk poured nearly the entire annual U.S. Environmental Protection Agency allocation of nitrogen runoff for the river — 1,941 pounds — into Chesapeake Bay.
“That’s striking,” she says. “How do we expect to restore the bay if we’re not counting a lot of what’s going in?”
Mulholland is focused on measuring nitrogen, including ammonium, because of its effect on algae blooms, which create oxygen-depleted dead zones in bay waters. She said that other pollutants — including oil, gasoline, and trace metals — are also washing into waterways, as evidenced by the petroleum sheens visible on the water during high-tide flooding. “We can see it, and it would be great if we could measure it in the future,” she says. “But we don’t have the analytical chops to measure it so far.”
