New Study Finds Steep Increase in East Coast High-Tide Floods

Flooding events may triple in 15 years, increase ten-fold in 30 years for most towns analyzed, science group finds

Published Oct 8, 2014


WASHINGTON (October 8, 2014)—Flooding during high tides—something that rarely occurred in the past—is now common in some places and is projected to grow to the point that sections of coastal cities may flood so often they would become unusable in the near future, according to a report the Union of Concerned Scientists (UCS) released today, “Encroaching Tides: How Sea Level Rise and Tidal Flooding Threaten U.S. East and Gulf Coast Communities over the Next 30 Years.”

“Several decades ago, flooding at high tide was simply not a problem,” said Melanie Fitzpatrick, report co-author and climate scientist at UCS. “Today, when the tide is extra high, people find themselves splashing through downtown Miami, Norfolk and Annapolis on sunny days and dealing with flooded roads in Atlantic City, Savannah and  the coast of New Hampshire. In parts of New York City and elsewhere, homeowners are dealing with flooded basements, salt-poisoned yards and falling property values, not only because of catastrophic storms, but because tides, aided by sea level rise, now cause flooding where they live.”

The UCS study is based on an analysis of 52 National Oceanic and Atmospheric Administration (NOAA) tide gauges in communities stretching from Portland, Maine to Freeport, Texas, using moderate sea level rise projections. The analysis reveals that in the next 15 years, most of the towns analyzed could see a tripling in the number of high-tide floods each year and in 30 years a ten-fold increase compared to historic levels.

Researchers say the increases in flooding are so pervasive that Atlantic Coast communities not covered by the analysis may need to brace for similar changes.

The study found the problem will rapidly worsen as sea level rises:

  • In 15 years’ time, most of the communities analyzed are projected to experience at least 24 floods per year in exposed areas, the equivalent of flooding twice a month.
  • In 15 years’ time, 15 of the communities can expect to see at least 48 floods a year, the equivalent of flooding four times a month.
  • In 30 years’ time, most of the towns analyzed would see at least 48 floods a year, the equivalent of four floods a month.
  • In 30 years’ time, half of the towns are expected to face more than 100 floods a year, the equivalent of eight floods a month.
  • In 30 years’ time, 17 towns would face at least 180 floods a year, or 15 floods each month.
  • In 30 years’ time, nine towns could see tidal flooding 240 times or more per year.

 “This report shows that, within the timeframe of a 30-year mortgage, many East Coast communities will see dramatic changes in the number and severity of tidal floods each year, unless, of course, successful steps are taken to manage those floodwaters,” said Erika Spanger-Siegfried, report co-author and senior analyst at UCS.

Meanwhile, with only a few exceptions, most floods that now occur at high tide are classified as “nuisance” or minor floods. The UCS analysis found that in 15 years, the tides that cause today’s nuisance floods will be capable of causing “extensive” floods in seven cities—floods that are deeper, reach further inland, last longer and threaten life and property. Today, U.S. cities typically only experience extensive floods during storms and when high winds push an extreme high tide further inland. In 30 years, nearly half the towns UCS analyzed would experience extensive floods with tides alone. (See a chart on page 22 listing when nuisance floods are expected to become extensive in each town.)

The report also shows that along the East Coast, Mid-Atlantic communities will see the greatest increase in the number of floods each year because, in addition to sea level rise, the land is sinking and ocean dynamics are changing. The analysis projects that in 30 years’ time, Annapolis, Md.; Lewisetta, Va.; Washington, D.C. and Wilmington, N.C. would each see well over 300 flooding events per year.

“In the Mid-Atlantic, five locations could see areas flooded more than 10 percent of the time by 2045,” said Spanger-Siegfried. “Meanwhile, in 15 years, many of the 90 tidal floods we expect in the Atlantic City area would close roads that link west Atlantic City to the mainland, without adaptation efforts.”

“Further down the coast, Miami Beach, in 30 years, would experience more than 200 tidal floods a year,” said Spanger-Siegfried. “We expect many of these floods to be more extensive than today’s tidal floods. Some could affect much of the art deco historic district of South Beach, for example, a hub for local tourism. Without serious measures, frequent flooding will start to disrupt daily life and change the way an area functions—and could do so sooner than most of us anticipated.”

A number of cities and agencies already have had to take steps to protect their communities and property from rising seas. For example, Tybee Island and Miami Beach have started to upgrade their stormwater sewer systems to prevent seawater from backing up into pipes. Norfolk is returning some of its coastal parks into wetlands.  The U.S. Naval Academy in Annapolis is using “door dams” to protect building entrances from flooding.

“Around the world, sea level is rising in response to global warming.  As the oceans heat up, the water expands and as glaciers and polar ice sheets melt they add water to the oceans,” said Melanie Fitzpatrick, UCS climate scientist. “Only international and national actions to deeply and swiftly reduce global warming pollution can slow the pace of future sea level rise.”

The UCS report recommends that municipalities, with state and federal help, prioritize and incentivize flood-proofing of homes, neighborhoods, and key infrastructure; curtail development in areas subject to tidal flooding; consider the risks and benefits of adaptation measures such as sea walls and natural buffers; and develop long-term plans based on the best available science.

To read profiles on how tidal flooding has already changed in specific towns and see the projections for how many annual floods each town may experience in 15 and 30 years, go to: Annapolis, Md.; Charleston, S.C.; New York City; Miami, Fla.; Norfolk, Va.; Outer Banks, N.C.; Savannah and Tybee Island, Ga.; and Atlantic City, N.J.

The report also includes projections for the number of annual floods the following cities may experience in 15 and 30 years: Baltimore, Md.; Boston, Mass.; Bridgeport, Conn.; Key West, Fla.; Lewes, Del.; Ocean City, Md.; Philadelphia, Pa.; Portland, Maine; Portsmouth, N.H.; Washington, D.C.