The Gooseberry Causeway has long been at least partially blamed for many of coastal Westport’s environmental troubles, from nitrogen pollution in the Westport River estuary to erosion at East Beach. But the results of a three-year, $1.325 million Buzzards Bay Coalition study point elsewhere — to the island itself, sea level rise and overall climate change.

The study

The causeway issue “has always been one of the big outstanding questions on Buzzards Bay," Mark Rasmussen, the president of the Buzzards Bay Coalition, said at the study’s outset. "It's always been too scientifically complicated to settle and it's very frustrating to never have an answer."

But the coalition’s three-year “Gooseberry Causeway Impact Study” was launched to provide those answers. Funded mostly by a $1 million private grant, the $1.325 million study brought together scientists and partners from Boston University, the Woods Hole Group and the Virginia Institute of Marine Science who sought to understand the causeway and its impact on Buzzards Bay, from the Westport River east to the Slocum River in Dartmouth.

Other results focusing on Dartmouth, the Slocum River and Allens Pond, are expected later this year.

The causeway

The causeway was built by the United States Army Corps of Engineers in 1943, replacing a smaller causeway built in 1922 but destroyed in the Hurricane of 1938.

The report notes that “the timing of the Gooseberry Causeway construction coincides with the beginning of dramatic changes to this stretch of coastline, leading government officials, coastal scientists and residents to question whether this manmade structure may be contributing to some or all of these changes.”

Scientists studied the area’s hydrology, used computer models and spent many months to answer that question. The takeaway? Other issues are at play.

Takeaway 1: Tidal and current action, and Gooseberry Island itself, not the causeway, affect the flow of water into the Westport River, and thus shapes its health.

Scientists used computer modeling and other techniques to simulate what would happen if the causeway was removed or modified, and found that the counter-clockwise circulation of water on the west side of the island would be the same with or without the causeway.

Though the island creates that counter-clockwise rotation, “the flow of water near the inlet is controlled by the tides moving water into and out of the Westport River. The causeway causes no impact to the tidal movement of water.”

While northwest water movement along the shoreline during storm events does deposit sediment at the mouth of the river and adds to the spit at the west end of Horseneck Beach, “the sand deposits do not overwhelm the tidal power that flushes water in and out of the inlet.”

Takeaway 2: The causeway is not causing or accelerating the erosion of East Beach — “erosion would be occurring with or without the causeway.”

Scientists found that the circulation pattern on either side of the island — counter-clockwise to the west and clockwise to the east — “drives the loss of sand primarily to deeper waters offshore.”

Further, scientists found that if the causeway were not there, “intensified longshore currents could further accelerate erosion on the western end of East Beach, exacerbating sediment loss beyond current levels.”

So why does East Beach continue to erode? In large part erosion is exacerbated by sea level rise, the study concludes.

“The historic source of sand for East Beach (from underwater) has been depleted over time,” the authors note. “Deepening water due to sea level rise is exacerbating the problem because it prevents sand in deeper offshore areas from moving onshore.”

Takeaway 3: Sea level rise and climate change, not the causeway, will continue to erode fragile areas throughout lower Buzzards Bay, including East Beach.

“Under climate change scenarios, the causeway does not strongly influence sand and water movement in the region,” the report’s authors note.

While the island, its sandbar and the shape of the shoreline control the movement of water and sediment, the causeway does not.

These findings were partially borne out by sediment samples in marsh areas, which showed that “Westport salt marshes are not building sediment height as fast as sea levels are rising, so they are susceptible to drowning as sea level rises.”

“More storms mean more frequent events when the waves are big enough to move large amounts of sand.”

For more on the study, visit the coalition’s report page.