Many US coastal communities already struggle with flooding at high tides, a problem that will become "chronic" in the coming 15 years due to global warming, scientists said on Wednesday.

Even as shorelines grow more populated, sea levels are swelling due to melting glaciers and polar ice sheets, putting more populations at risk, said a report by the Union of Concerned Scientists.

Particularly dire consequences are expected along the US East Coast and the Gulf Coast, the report warned.

"Our analysis shows that increases in tidal flooding will be substantial and nearly universal," said the report, based on tidal gauges in 52 coastal communities from the northeastern state of Maine down to Florida and along the Gulf Coast of Texas.

"That means the steady creep of sea level rise will force many communities largely unfamiliar with tidal floods today to grapple with chronic flooding in the next 15 to 30 years."

Scientists said the floods, while not catastrophic, could harm key infrastructure such as bridges and roads and cause property damage.

From 1880 to 2009, global sea levels rose about 20 cm. Today, oceans are rising at an even faster rate.

"No longer an intangible global trend, sea level rise has arrived on the doorstep of communities scattered up and down the East Coast, delivered by the tides," it added.

"In the next 15 years alone, two-thirds of these communities could see a tripling or more in the number of high-tide floods each year."

The biggest changes are expected in the mid-Atlantic, said the report.

Washington and Annapolis, Maryland, can expect more than 150 tidal floods per year by 2030.

In southeastern towns like Savannah, Georgia, and Lewisetta, Virginia, "extensive flooding is expected to occur with tides alone on a regular basis within one or two decades", said the report.

"By 2045, even more places can expect to see extensive flooding, including Ocean City, Maryland, and Myrtle Beach, South Carolina."

In some places, the changes may be dramatic. For instance, the northeastern town of New London, Connecticut, currently experiences tidal floods about twice per year, but that could rise to 35 in 30 years.

Spots along Texas' Gulf Coast that rarely see tidal floods "could face 35 to 70 tidal floods per year by 2045", the report found.

Some places, like Norfolk, Virginia, have put up tide gates to keep floods out of downtown business districts, but that's not an option everywhere.

Miami, the world's seventh-richest city, is built on porous limestone and is already losing water wells to incoming saltwater. The city of 417,000 faces more frequent flooding of business and residential areas.

(China Daily 10/09/2014 page10)