Flooding during high tides, which used to be rare, is now common in some places and could worsen to the point that sections of coastal cities may flood so often they would become unusable in the near future, according to a report by the Union of Concerned Scientists (UCS).

The Cambridge, Mass. group’s study, “Encroaching Tides: How Sea Level Rise and Tidal Flooding Threaten U.S. East and Gulf Coast Communities over the Next 30 Years,” looks at how often 52 cities flood have flooded in the past and are expected to flood in the future.

The group analyzed 52 National Oceanic and Atmospheric Administration (NOAA) tide gauges in communities from Portland, Maine to Freeport, Texas, using moderate sea level rise projections. 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, according to the analysis.

“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.

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 report says. The analysis projects that in 30 years’ time, Annapolis, Md.; Lewisetta, Va.; Washington, D.C. and Wilmington, N.C. would each see more than 300 flooding events per year.

Five Mid-Atlantic cities could see areas flooded more than 10 percent of the time by 2045, while in 15 years, many of the 90 tidal floods in the Atlantic City area would close roads that link west Atlantic City to the mainland, unless mitigation efforts are taken, the analysis found.

“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 researchers said that East Coast communities not covered by the analysis may need to brace for similar changes.

According to the UCS study:

• 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.

While most floods that now occur at high tide are minor or so-called nuisance 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.

A recent report by the National Oceanic and Atmospheric Administration (NOAA) found that eight of the top 10 U.S. cities that have seen an increase in nuisance flooding are on the East Coast, with Annapolis and Baltimore, Maryland, leading the list with an increase in number of flood days of more than 920 percent since 1960.

Nuisance flooding has increased on all three U.S. coasts between 300 and 925 percent since the 1960s, the NOAA report said.

The UCS scientists report that 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.

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.

The study contains profiles on how tidal flooding has already changed in specific towns and projections for how many annual floods each town may experience in 15 and 30 years: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.

*This story previously appeared in our sister publication Insurance Journal