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High Tide Flooding, Familiar To N.H. Coast, Is Increasing Faster Across the U.S.

A king tide appears on the streets of Hampton Beach.
Jason Moon
The flood stage for the Atlantic Ocean off Hampton Beach is 11 feet, with flooding on local streets typically possible after tides reach about 10 feet, known locally as a king tide.

A new federal report says increases in high tide flooding are accelerating along the coasts of states like New Hampshire, in a trend linked to climate change."There's a lot of heat, there's a lot of warm water," said NOAA oceanographer William Sweet on a press call Thursday. "Sea levels have been rising rather rapidly." 

The National Oceanic and Atmospheric Administration says flooding during high tides is now twice as likely as it was 20 years ago across the U.S. It could happen two or three times more often by 2030

"In addition to minor flooding events, moderate and major [high-tide flooding, between 2.75 feet and 4 feet above high tide], which trigger coastal flood warnings for significant risks to life and property, will become much more commonplace as we approach mid-century," the report said. 

Sweet said it's tied to rising sea levels as waters warm, polar ice melts and land subsides due to human impacts, including greenhouse gas emissions. Waters off the U.S. coast are now more than a foot higher on average than they were a century ago, rising millimeters or more per year. 

"We're seeing in the last several years that the number of flood days and the rate of sea level rise is higher than anticipated," he said. "It would suggest that we are under-predicting what's likely to happen in 2030." 

The annual NOAA report analyzes nearly 100 tide gauges nationwide, including Boston and Portland, Maine. It does not include stations in New Hampshire. 

It says Boston, for example, saw 11 days of high-tide flooding, which can occur when tide levels reach about 2 feet above average or a little less. NOAA predicts Boston will have up to 18 days of flooding next year, 35 days by 2030 and 95 days by 2050.

The flood stage for the Atlantic Ocean off Hampton Beach is 11 feet, with flooding on local streets typically possible after tides reach about 10 feet — known locally as a king tide. 

The beachfront town saw 53 of these tides from June of 2020 to this year, according to NOAA records – a threshold residents are accustomed to noticing, but lower than the one analyzed in the NOAA report. Hampton saw 44 of these tides in the same period 20 years ago.

A 2020 NOAA report said by mid-century, some places in the U.S. will see nearly 180 days a year of high-tide flooding, “effectively becoming the new high tide.”

This story includes reporting from WBUR’s Barbara Moran, shared through the New England News Collaborative.

Annie has covered the environment, energy, climate change and the Seacoast region for NHPR since 2017. She leads the newsroom's climate reporting project, By Degrees.

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