Rising sea levels aren’t the only reason U.S. coasts are seeing low-intensity flooding more often – estuary changes driven by coastline development have also caused higher tides to spill inland.
(CN) — So-called nuisance flooding is happening more frequently in coastal cities in the continental U.S., and researchers have found that rising sea levels and coastal development have caused more high tides to result in such flooding in recent years compared to the 19th and early 20th centuries.
Nuisance flooding, also known as tidal flooding or sunny day flooding, is more destructive than its name might imply. The term refers to the minor floods that occur in low-elevation coastal cities, where high tides can swamp streets, drainage infrastructure, businesses and residences, adding to the millions of dollars in damages floods cause yearly.
Using historic water-level information obtained from 40 tide gauges that have been making observations for at least 70 years apiece, a group of climate scientists were able to compare past tidal data to contemporary changes in the tides.
“It’s the first time that the effects of tidal changes on nuisance flooding were quantified, and the approach is very robust as it is based purely on observational data and covers the entire coastline of the U.S. mainland,” said Thomas Wahl, co-author of a study published Friday in the journal Science Advances and assistant professor of civil and environmental engineering at the University of Central Florida.
Every year, the tides rise and fall. In some locations, even the highest tides fail to cross the threshold at which those high waters spill inland, resulting in nuisance flooding. But every year, the waters rise: the authors write that the global average sea level rose at an average annual rate of 3.3 millimeters, give or take 0.3 millimeters, between 1993 and 2014.
As the global sea level rises, so to do relative mean sea levels — that is, the height of the water relative to the land at a particular location, as opposed to the absolute height of the sea relative to the center of the Earth.
And as relative mean sea levels rose at each tide gauge — measuring local tides on the East, West and Gulf coasts — the absolute height of high tides rose as well, steadily closing the gap between the height of the sea level and the tipping point at which high tides cause nuisance flooding.
Almost half of the gauges recorded more nuisance flooding days caused by higher local tides, the researchers report. But not every single gauge recorded more flooding: Washington, D.C., had about 52 fewer days of nuisance flooding in 2019 due to decreased tidal range, for instance.
“While a few individual instances of these minor flooding events do not cause too many impacts, the cumulative impacts of frequent events can become very large,” Wahl said in a statement. “Hence, understanding what drives the changes in nuisance flooding is very important.”
The scientists found that the long-term rise in relative mean sea levels, or RMSL, is the primary driver for the increased incidence of nuisance flooding events, but also note that secular changes in the tides — those that are gradual, linear and nonseasonal in nature — account for more than a quarter of the nuisance flooding recorded in 2019.
“Changes are more pronounced along the U.S. East Coast, where RMSL rose faster over the last few decades compared to the West Coast, and also in many of the major estuaries, which have undergone significant changes in the tidal characteristics due to anthropogenic alterations of system geometry and river inflow,” the scientists write.
Some of those anthropogenic alterations include the deliberate redirection of rivers, dredging the sediment from the bottom of channels and harbors to affect their depth and business development near coastlines.
Estuaries, those areas where a river meets the coastline, are especially vulnerable: the scientists point to Wilmington, North Carolina, as “an extreme (but illustrative) example” of tidal shifts’ effect on nuisance flooding. (Parentheses in original.)
In 2019 alone, Wilmington saw 123 days of nuisance flooding. Before that, the city saw 1,203 flooding events caused by the rising sea level and altered tides since 1949.
“Seeing how many nuisance flooding events occurred in the past and are happening today simply because of tidal changes should be motivation for us to keep alterations to sensitive estuarine systems at a minimum as to not further exacerbate the problem, which we already face because of sea level rise,” Wahl said. “We should at least be aware of these potentially negative impacts in the planning phase of alteration projects, and it might even be possible to reverse some of the negative impacts from past decisions.”
Sida Li, a visiting student in UCF’s department of civil, environmental and construction engineering, led the study.