Weather

Here’s what you need to know about high-tide flooding and the impact on Boston

High-tide flooding can cause substantial property damage and even force evacuations.

By Susannah Sudborough

August 7, 2022

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Scientists at the National Oceanic and Atmospheric Administration are predicting that high-tide flooding will happen more and more frequently over the next few decades, and this could have a huge impact on New Englanders.

According to NOAA, the northeast is particularly at risk for tidal flooding, which means more flooded shorelines, streets, and basements.

Just this past January, high tides flooded Long Wharf in downtown Boston.

High-tide flooding is when tides reach higher than normal levels and begin spilling ocean water onto streets or bubbling up from storm drains, according to NOAA. It occurs when tides reach between 1.75 and 2 feet above the daily average high tide.

This past year, Boston saw seven days of high-tide flooding — far less than the predicted 11 to 18 days, but this trend is unlikely to continue.

As sea level continues to rise, NOAA said, damaging floods that occurred only during a storm now happen more regularly. They now happen during full moons, or when there is a change in prevailing winds or currents.

NOAA predicts that by 2050, Boston could see between 50 and 70 days of high-tide flooding.

“Coastal flood warnings for significant risks to life and property will become much more commonplace as we approach mid-century,” the report says.

NOAA oceanographer William Sweet said Boston had fewer HTF days than predicted because the projections are based on what NOAA has recorded in past years, but certain circumstances can cause there to be more or less flooding each year.

This past year, from May 2021 to April 2022, he said, we were still in a La Niña period, which is characterized by unusually cold ocean temperatures in the Equatorial Pacific. During this period, winters are milder and storms tend to be further inland, which means there are fewer opportunities for coastal flooding.

Additionally, Sweet said, we are in the downturn phase of the 18.6-year nodal cycle of the moon, which causes tides to be lower. Sea levels in the northeast have also been lower, in general, the past few years, he said.

But experts predict northeast sea levels will rise again soon. After a few years of low sea levels, we are expected to leave the La Niña period early next year, Sweet said.

That’s why NOAA still predicts 11 to 18 high-tide flooding days in Boston from May 2022 to April 2023, in its annual report.

The record high number of high-tide flood days ever recorded in Boston in a year was 22.

Elsewhere in New England, high-tide flooding days are not predicted to be so numerous, but are still approaching record highs. Providence is expected to see six to 11 days, and the all-time high is 15. Newport, Rhode Island, and Nantucket are expected to see three to seven days, and the all-time high is 11.

Sweet said sea levels are rising and causing more high-tide flooding days due to some natural causes such as land sinkage, but largely due to climate change.

“Though the sea levels are still rising and falling every couple of years, the highs get higher and the lows don’t go as far back down,” Sweet said.

While minor high-tide flooding generally causes small disruptions such as road closures and stormwater backups, moderate flooding can cause damage to homes and businesses, and major flooding can necessitate evacuations, according to NOAA.

Massachusetts residents whose property is at risk from high-tide flooding should consider getting more information about the risk to their specific property and looking into ways to protect their property from flooding, Sweet said.