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Preparing for the Island’s future flooding

Posted on August 22, 2022

Mark Borrelli, a coastal geologist and director of the Provincetown-based Center for Coastal Studies’ seafloor mapping program, led and completed a two-year coastal resiliency and sea level rise study on Martha’s Vineyard. Borrelli presented the study earlier this month at Sailing Camp Park in Oak Bluffs. He said the event “went pretty well,” with around 30 to 40 people in attendance who had “lots of good questions” from “an engaged crowd.”

The study, titled “Mapping Storm Tide Pathways in the Six Towns of Martha’s Vineyard: Assessing Coastal Resiliency to Storms and Sea Level Rise,” found 716 storm tide pathways on the Island, which was within Borrelli’s expectations. Storm tide pathways are routes flowing water would take in low-lying coastal areas, according to the center’s website. The Martha’s Vineyard Commission was also a major partner in the study, according to Borrelli. The commission approached the center for this endeavor, and applied for the Massachusetts Office of Coastal Zone Management’s Coastal Resiliency Grant Program, which funded the project. The commission also provided additional funding for the study. “They were just wonderful partners,” Borrelli said.

Before the mapping could start, Borrelli said, the study began with a “desktop exercise” analyzing the lidar spatial data of Martha’s Vineyard, which he described as “really high-resolution, high-accuracy spatial data of the topography of the Island.” Afterward, Borrelli and his team identified the storm tide pathways on the Island using “methods we developed over the past five to six years.”

“We go to the Island and locate those using our survey-grade RTK GPS, which is just … the most accurate form of positional data you can get,” Borrelli told The Times. “We go and locate those storm tide pathways so we can find out if they’re still there, if they still exist. We [also] find new ones, because lidar data is often a year or two or three old, so we want to make sure these maps are as up-to-date as we can.”

Borrelli said there were storm tide pathways that the lidar data did not show once his team was in the field, and there were other storm tide pathways that were “ruled out” once they were “observed on the ground.” When asked about how long is necessary for storm tide pathways to be updated on the map, Borrelli said it depends, but there are two general classifications.

“Some of them are labeled as dynamic. So the ones that are labeled as dynamic are in a dune or some kind of natural feature that we expect to change over time,” he said. “Others are, you know, right across Water Street, or there’s a ferry dock, or they’re built structures that won’t change unless we change them, for the most part.”

Dynamic storm tide pathways, depending on their importance and what they flood, can be looked at every five to 10 years, according to Borrelli.

“If that road is not a primary evacuation route or a critical route in general, then you don’t have to worry too much about it,” he said. “You should be aware of it.”

The study found that “for every six-inch increase in water level, an average of 350 acres of land (between 213 – 778) acres are impacted. And a one-foot rise in sea level would permanently inundate approximately 700 acres, across a spectrum of between 464 and 1,076 acres, respectively.” The study found 47 storm tide pathways that have the potential to flood 767 acres of land that have not seen flooding in almost 100 years. For low-lying roads, depending on a storm’s severity, between 36 and 91 roads will face flooding of at least one foot of water. Additionally, around 20 miles across 58 roads will be flooded during a “storm of record,” or the highest high tide of the year.

Borrelli said storm tide pathways can help emergency managers, alongside the National Weather Service’s total water level predictions, to better prepare for potential flooding. Provincetown, which is the first place the center mapped storm tide pathways, used these two tools and moved portable flood walls, sandbags, and other “resources in place to prepare for an inundation event due to a coming storm.”

“It’s proactive, as opposed to reactive,” Borrelli told The Times.

According to Borrelli, the storm tide pathways mapping revealed some towns are more susceptible to flooding than others.

“Edgartown, unsurprisingly, has the most [storm tide pathways]. They have about 290 in Edgartown. With Chappaquiddick and the low-lying areas, that’s not too surprising,” Borrelli said. “Oak Bluffs is next, they’re the second most … Aquinnah is the least, which again is not surprising because of the high bluffs.”

Overall, Martha’s Vineyard is comparable to the other Cape and Island towns that have been mapped through similar studies, although it does have more low-lying areas. These towns include Provincetown, Nantucket, and Dennis.

“All of the features on Cape Cod and the Island are glacial, so you have the low-lying areas and the high bluffs, things like that,” Borrelli said. “While Martha’s Vineyard isn’t really more susceptible … one of the challenges is that you have 360 degrees of risk. If you’re on Cape Cod Bay, one town has a shoreline that is west-facing or east-facing, [but] you guys can get it from any direction. You’re an Island. That makes it a little trickier. Almost every storm can impact you.”

Despite the difficulties, Borrelli said the entities on Martha’s Vineyard designed to support all six towns, such as the commission, are a good way to “bring people together and share information.”


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