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 During the past four decades, the number of days a year that tidal
waters reached or exceeded National Oceanic and Atmospheric
Administration flood thresholds more than tripled in many places,
the analysis found. At flood threshold, water can begin to pool on
streets. As it rises farther, it can close roads, damage property
and overwhelm drainage systems.
Since 2001, water has reached flood levels an average of 20 days or
more a year in Annapolis, Maryland; Wilmington, North Carolina;
Washington, D.C.; Atlantic City, New Jersey; Sandy Hook, New Jersey;
and Charleston, South Carolina. Before 1971, none of those locations
averaged more than five days a year. Annapolis had the highest
average number of days a year above flood thresholds since 2001, at
34.
The analysis was undertaken as part of a broader examination of
rising sea levels Reuters plans to publish later this year.
As many Americans question the causes and even the reality of
climate change, increased flooding is already posing a major
challenge for local governments in much of the United States.
“Chronic flooding is a problem our coastal managers are dealing with
every day,” said Mary Munson, executive director of the Coastal
States Organization, a Washington nonprofit representing 35 states
and territories. “Flooding causes the quality of life in these
communities to decrease along with the property values, while the
flood insurance rates go up.”
In Charleston, for example, a six-lane thoroughfare regularly
becomes impassable when high tides block rainwater from emptying
into the Atlantic Ocean, restricting access for half of the city to
three hospitals, four schools and police headquarters. The city,
which has more than 120,000 residents, has $200 million in
flood-control projects underway.
Laura Cabiness, director of public service for Charleston, said
street flooding has always been a problem in the low-lying city. But
more recently, she said, “it’s deeper than usual and higher than
usual, and the tide has remained higher longer.”
For its analysis, Reuters collected more than 25 million hourly
tide-gauge readings from nearly 70 sites on the Atlantic, Gulf and
Pacific coasts and compared them to NOAA flood thresholds.
Reuters then narrowed the analysis to include only the 25 gauges
with data spanning at least 50 years. Nineteen gauges were on the
Eastern Seaboard, three on the West Coast, and three on the Gulf
Coast. Comparing the years prior to 1971 to the years since 2001,
the average number of days a year that readings exceeded flood
thresholds had increased at all gauges except two: those in St.
Petersburg, Florida, and Woods Hole, Massachusetts.
The trend roughly tracks the global rise in sea levels. The oceans
have risen an average of 8 inches in the past century, according to
the 2014 National Climate Assessment. Levels have increased as much
as twice that in areas of the Atlantic and Gulf coasts where the
ground is sinking because of subsidence – a process whereby natural
geological forces or the extraction of underground water, oil or gas
cause the ground to sink.
The most dramatic increases in annual flood-level days occurred at
10 gauges from New York City to the Georgia-South Carolina border, a
stretch of coast where subsidence accounts for as much as half the
rise in sea level in some locations, according to U.S. Geological
Survey studies.
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Charles Chesnutt, a coastal engineer with the Institute for Water
Resources, a policy and planning arm of the U.S. Army Corps of
Engineers, said the evidence “is very compelling and suggests we
ought to be looking more seriously at the problems that are coming
at us now.” The Corps of Engineers is the lead federal agency on
coastal flood control projects.
The Reuters findings are supported by a pair of soon-to-be-published
studies from scientists at NOAA and Old Dominion University in
Norfolk, Virginia. Reuters adapted its methodologies from those
scientists and sought their input.
Old Dominion University researchers Tal Ezer and Larry Atkinson
found in their study that the U.S. East Coast is “a hotspot of
accelerated flooding,” and that flooding outside of storm events has
increased in frequency and duration.
They found that changes in the Gulf Stream may be contributing to
increased flooding from rising sea levels. The current off the
Atlantic Coast pulls water away from the shore as it flows
northeastward from Cape Hatteras. The researchers said that as the
climate has warmed, the current has weakened, so it’s not pulling as
much water away.
The NOAA study examines flooding at 45 tide stations around the
United States. It is expected to be released this summer.
Flood thresholds are indicators, not confirmation, of flooding, but
scientists say the tide gauge readings are a reliable measure of
increased flooding.
When seas hit the flood threshold in Annapolis, the 306-year-old
city that is home to the U.S. Naval Academy, forecasters expect
water to start ponding in the historic city dock area. A few inches
more, and water begins reaching backyards and the tops of storm
drains in some areas.
During high tides on April 30 and May 1, and again on May 16, more
than six inches of water swamped restaurants and shops in historic
buildings along the city dock. Makeshift flood walls of boards and
garbage cans blocked doorways. People removed their shoes and rolled
up their pants to wade to work.
(Edited by John Blanton)
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