FREEPORT — The town plans to replace West Street Culvert, a project meant to mitigate intense stormwater flows that have eroded the banks of Concord Gully Brook and improve water quality and wildlife habitat.

As the 82-foot-long culvert located between Woodlawn Cemetery and South Street is replaced, no through traffic will be allowed between Depot and South streets during the work, according to Town Manager Peter Joseph. The project is due to begin in September and run two or three weeks, according to Town Engineer Adam Bliss.

The town has hired J Pratt Construction of Hebron – which handled a South Street sidewalk extension in 2016 and an Elm Street-Snow Road sidewalk connector loop in 2017 – for the $261,000 project. A $95,000 Maine Department of Environmental Protection stream crossing grant, a $91,000 Environmental Protection Agency clean water grant and a $75,000 local match are funding the endeavor, according to Bliss.

The culvert replacement is one of the 2015 Concord Gully Brook Watershed Management Plan’s restoration projects of highest priority, Bliss said in a May 28 memo.

The downtown Village area is composed of mostly impervious pavement with a lot of buildings and the flow of water over those surfaces is “so powerful and erosive that stream channel degradation has created 5-foot head cuts into the native glaciomarine clay,” according to Bliss.

The culvert now in place “isn’t physically failing … but it’s a size too small,” Joseph said. It drains about half of downtown stormwater and in heavy rain events, “because … it’s not sized correctly for the flow, it therefore accelerates the erosion downstream,” he said.

The stream banks are eroded down “so there’s not a natural surface and the water that goes through there is way too quick to support the natural stream function,” Joseph said.

The culvert is 30 inches in diameter, and the replacement will be a 12-foot wide concrete arch culvert, about 11 times larger in area, Bliss said. He noted that the “new enlarged culvert shape and size decreases the velocity of the flow, thus causing less embankment erosion.”

Concord Gully is a 2.47-mile stream that starts near Stagecoach Road and flows into the Harraseeket River right below Cove Road, according to a Maine DEP watershed description. The gully watershed covers 704 acres of Freeport.

“Stormwater falling on roads, roofs and parking lots in developed areas flows quickly off impervious surfaces, carrying dirt, oils, metals and other pollutants, and sending high volumes of flow to the nearest section of the stream,” according to the DEP report. The agency included the gully on a list of Maine’s urban impaired streams.

“One of those reasons is lack of … invertebrate life in the river anymore, because of the pollution, because of erosion, because of contamination,” Joseph said, noting that the culvert replacement is one of several steps needed to make the waterway more hospitable to fish and insects.

In exchange for the town granting L.L.Bean a nearly $10 million tax break in April, the company agreed to help clean up the brook, the Times Record reported.