The tide is high and rising


Live near the the ocean is a dream for many people. But in recent years, the effects of climate change have made it a risky proposition. Hurricanes have become more intense, leading to larger storm surges and heavier rains that pound the coasts, erode beaches and destroy homes. This storm threat has been compounded by so-called harmful floods which occurs during high tides, even in calm and clear weather. New study shows untimely flooding is being exacerbated by dredging and the construction of piers and jetties that were meant to make coastal life easier but actually redirect the flow of incoming ocean waters and make high tides higher than ever before.

Published Friday in the newspaper Scientific progress, the study compared high and low tides measured over the past 70 years with an older database of historical tide measurements taken in the mid-1800s and recently found in piles of old boxes stored in an annex of the National Archives in College Park, Maryland. The researchers found that of 40 coastal tide gauges operated by the National Oceanic and Atmospheric Administration (NOAA), nearly half had measured more days of harmful flooding due to higher local tides. Cities built along coastal estuaries showed the greatest tidal changes.

These daytime flooding hit coastal residents in places like Miami; Norfolk, Virginia; and Charleston, South Carolina. “Even if they don’t kill people, these harmful floods cost us a lot of money, destroy property and prevent people from getting to work,” says the study’s author. Thomas’s choice, assistant professor of civil, environmental and construction engineering at the University of Central Florida.

The study is the first to link harmful flooding to the combination of sea level rise caused by climate change and deliberately designed changes in coastal geography that facilitate the flow of water inland. lands. Wahl says he and his colleagues have documented the effects of rising seas eight to nine inches around the world since the 1880s, with most of this increase occurring over the past 20 years. “We know that sea level rise is the main reason for harmful flooding and why we will see it a lot more in the future,” says Wahl. “But then we thought there might be other things that are making this problem worse. This is what kicked off this idea of ​​how changes in daily tides, rises and falls, could also have played a role in worsening the problem of harmful flooding.

Wahl and graduate student Sida Li collected data for tide stations along the Atlantic, Gulf and Pacific coasts. The eastern and gulf coasts are more susceptible to harmful flooding than the Pacific or Alaskan coasts due to their higher rates of sea level rise, gently sloping submarine topography, and features geological, such as the porous limestone found in Florida, the study concluded.

Wilmington, North Carolina, was the worst case the team had found. The city experienced 123 days of harmful flooding in 2019 alone, and an additional 1,203 flooding since 1949 due to changes in historical tidal conditions and rising seas. The Wilmington tide gauge is located upstream in the Cape Fear River, 29 miles from the mouth of the estuary. The tidal amplitude, which is the difference between up and down, has increased by about 1¼ feet since 1936 due to dredging and channel deepening which facilitated the flow of tidal water upstream from the ocean.

Dredging since the early 1900s has doubled the depth of the Delaware River and increased the tidal range in Philadelphia as well as in the estuary at the mouth of the Hudson River in New York City. In recent years, airline pilots have raised concerns about damaging flooding around Jamaica Bay, which surrounds JFK Airport. In fact, federal scientists have listed airports in San Francisco, Oakland, New Orleans, Miami, Fort Lauderdale, Newark, and Washington, DC, as the most vulnerable to sea level rise and harmful flooding.

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