Friends of the Scarborough Marsh hope to add two culverts under Black Point Road to improve Libby River water flow and reduce the threat of Phragmites in the upper river.

The Friends and the Maine Department of Transportation (DOT) are planning the culvert projects jointly, and expect to start in the fall.

“We have to get some more space under that road for water to come through,” said Charlie Hebson, a hydrologist with the Maine Department of Transportation.

Project leaders hope that allowing more water through the road will increase the salinity of the water entering the Libby River and help combat the spread of Phragmites. Phragmites is an invasive wetland grass that grows in dense stands of reeds reaching more than 10 feet tall.

“If this plant is not addressed then it will spread very quickly,” said Ryan Hodgeman, of the DOT.

There are roughly 27 acres of Phragmites in the marsh, and if nothing is done then the effected area could triple. According to Scott Lindsey, of the Maine Department of Inland Fisheries and Wildlife, if grown unchecked, Phragmites will eventually take over the habitats of native marsh plants.

The three-week $50,000 project is scheduled to begin this fall. The project may require shutting down Black Point Road for a week and then allowing one-way travel until the project is completed. If the road is not shutdown, then the project time will be doubled.

In addition to the new culverts, the project also will entail dredging on both sides of the road to help direct the water into the new culverts.

The existing six-foot culvert running under Black Point Road is too small, Hebson said. When it was installed 12 years ago the environmental impacts were not considered, Hebson said, and the major consideration was ensuring that Black Point Road would not flood.

But due to the small size of the pipe and the fact that the rest of the river is blocked by the road, twice a day water becomes backed-up at the culvert and water has scoured a pool at both ends of the road. At the highest tides the lower marsh in completely flooded while in the upper marsh the water is primarily contained in the stream.

Further reducing the salinity of the water is the amount of agricultural lands, forested uplands and developed residential property that convey fresh water runoff into the tidal areas.

Planning for the Libby River project began in January. Hebson studied a number of options for increasing the water flow and conducted tests to see what would happen with each possibility.

Some options that were considered were adding a box culvert and building a bridge. The bridge was eliminated because of the cost. While the box culvert was eliminated because it would require additional excavation, something that the DOT did not want to do because of underground utilities in the area.

Instead the project leaders focused on adding two six-by-eight culverts that would allow enough water flow to the upper river and hopefully stop the spread of Phragmites. In addition, consideration is being given to additional Phragmites control measures to eliminate persistent stands of the weed.

Although more water will be sent through, it will not increase the flood risk on property in the upper marsh, Hebson said. Studies conducted on the increased water flow using various degrees of flood levels ranging from 20-year flood up to 100-year floods indicated there was virtually no change in the high water marks.

“Basically in no situation do we make things worse,” Hebson said.

This drainage pipe on Black Point Road is too small and is causing a tidal restriction on the Libby River. Plans are now being finalized to add two more culverts to allow better flow up river.

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