Tidal Flooding Outlook and How the Moon Plays a Role

Jun 11, 2018

Credit NOAA

Massachusetts saw high tide flooding in dramatic style up and down the coastline during storms in January and March. In total, Boston saw a record-breaking 22 days of high tide flooding over the course of the past year, according to a new report from the National Oceanic and Atmospheric Administration. The lead author of the report, William Sweet, says the frequency of coastal flooding has doubled, and it’s a clear result of climate change.

“Due to sea level rise, the average trend in high tide flood frequency is now more than fifty percent higher than it was 20 years ago, and 100 percent higher than it was 30 years ago,” Sweet said.

In addition to rising sea level, New England faces a higher risk of tidal flooding due to winter storms. But another new analysis reveals that the height of our high tides is on an 18-year cycle. 

We’ve just past the peak of one of these cycles, which helps explain some of the record-breaking high tides we’ve observed recently. It also means we may have a slight reprieve from accelerating sea level rise over the next decade – giving us a window to prepare for extra high tides in the 2030s.

Stefan Talke, an Assistant Professor at Portland State University, is the lead author of a new analysis of tide gauge data from Boston Harbor that reveals the 18-year tide cycle. He says that there’s about a 6-inch sea level increase due to this 18-year change in the cycle of the moon, and at its peak, it impacts the tide quite a bit. It starts the water at a higher baseline and creates more extreme flooding.

Add to that the fact that sea level has risen about a foot over the past century, and you’ve got potential for even more flooding in the future. But again, this higher baseline that we saw in 2018 is happening due to the 18-year cycle, so the coastline will at least be in a better starting point for the next 18 years.

Still, it’s important to realize that what we saw this year is quite significant. The winter storms that flooded the coast this year were only slightly less than some historically significant ones, like the Blizzard of 1978, the Minot's Ledge Storm in April 1851, and the Christmas Storm of 1909. The Blizzard of 1978 was near the peak of the cycle, the April 1851 storm was near the peak of the cycle as well, as was the 1909 Christmas storm. So although extreme flooding is always a potential, it’s worse when we're at the peak of that moon-driven cycle.