D&R Canal Dredging Reaches Midpoint

July 10, 2019

The Ashtabula, a barge on the D&R Canal in Rocky Hill, is named for the place where it was first launched — a city in Ohio located at the mouth of the Ashtabula River on Lake Erie.

Driving on the Delaware & Raritan Canal Park towpath, just north of Rocky Hill, we carefully pass a man who looks both invigorated and content as he strides along.

 

“I know him,” Garret Brown smiles. “He walks here almost every day.”

 

Brown is field engineer for Urban Dredging Consultants (UDC) of Horsham, PA, one of the contractors on a major dredging of a 10-mile stretch of the D&R Canal between Kingston and Millstone. Now in the second of its three-year phases, Brown and other workers, including staff of the New Jersey State Water Supply Authority (NJWSA), have gained a friendly familiarity with users of this popular recreational area.

 

But how familiar are citizens with the goals and strategies of the canal dredging?

 

This particular morning, through arrangement by NJWSA project engineer Sharmila Rahman, Brown is helping a journalist gain that familiarity. 

 

Most local residents know something of the Delaware & Raritan Canal, starting with its 1834 opening as a shipping shortcut between Philadelphia and New York City.

 

A canal boat on the D&R in the days it was used for commercial navigation between New York City and Philadelphia. Terminuses in Bordentown and New Brunswick connected the waterway to the Delaware and Raritan rivers, respectively. In the background is a train, which eventually made canal travel obsolete. This painting hangs in the old bridge tender's house in Griggstown, which is now a museum.

The D&R was eventually out-competed by railroads and newly-developing trucking systems. Its commercial use ended in 1932, with the state of New Jersey taking full control two years later. The Delaware & Raritan State Park, established in 1974, is now beloved by hikers, bikers, joggers and birders. 

 

But the canal’s important modern function — in terms of lives benefited — is that of a drinking water source. Under the administration of the NJWSA, established in 1981, about a million state residents are sourced drinking water from the canal. 

 

And that has led to a vital need for this dredging project.

 

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Decades of leaves falling into the canal waters, rain runoff from adjacent roads and hillsides carrying silt and small stones, and muddying material from stream overflows and Delaware River feeder canals, have all caused the main D&R to silt up. As a result, water pumping efficiency has seriously reduced.

 

The main contractor is the J.F. Brennan company of Wisconsin, with plan and design services by UDC, for which Brown works. Now in the second of three seasons, the dredging project is scheduled for conclusion in Spring 2021.

 

There are three barges in use on the canal. One standard dredger, a dredger with a booster pump, and a barge with booster pump only. Their names — “Ashtabula,” “Fox River,” and “Palm Beach” — proudly proclaimed on painted wood signs seem fanciful, but spring from a tradition of naming such vessels for the sites in which they were first launched.

 

An estimated total of 248,000 cubic yards of material will be removed during the process. Some 50,000 cubic yards was removed in the first season, which ended in November 2018.

 

Canal slurry is pumped directly into special 200-foot-long sacks called “geo bags.”  The bags are transported to the American Cyanamid superfund site in Bridgewater, where clean fill is needed to cap the area being remediated of toxic industrial wastes.

The project has been under development for eight years. Its goal is to restore the optimal water siphoning flow by returning the canal channel to a depth of seven feet (fairly close to its original 1830s design depth of 8 feet).

 

“The canal hasn’t been fully restored since its original construction,” Brown said.

 

The first task before dredging was to remove as much vegetation as possible. A state environmentalist works closely with the project managers to monitor animal species living in or around the canal, notably the wood turtle, the Indiana bat, and several fish species.

 

Because the D&R Canal is on both NJ and national rosters of places of historical significance, a cultural resource consultant is on call to evaluate any artifacts found during dredging. 

 

“We notify him if the dredging team finds anything that may be significant,” Brown says.

 

These items have included old wood track ties from the railroad branch line ran past Kingston and Rocky Hill to the Atlantic Terra Cotta Tile Company and horseshoes probably made for one of the powerful harnessed mules who plied the path along the canal, towing barges behind them (the source of the term “tow path”).

 

Early on, the canal’s banks were lined with flat reinforcing stones. Many stretches of this “riprap” can still be seen. The dredges operate at safe working distances from these and other historic infrastructure.

 

And what of the dredging process itself?

 

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It starts underwater where the rotating blades of a powerful cutter head, mounted at the end of large flexible piping, loosen material on the canal’s floor. This creates a suspension of sediment in water – the “slurry” – which is vacuum pumped into the pipe system running along the canal surface. This slurry (with an assist from the booster pumps) is then sent through culverts under Canal Road and uphill to the “dewatering pad” site.

 

There, the slurry is pumped directly into special 200-foot long sacks called “geo bags.” They’re porous enough to allow water drainage while still retaining most of the particulate matter. The drained water goes into large ponds called “sumps” where remaining fine particles settle out before the water is pumped back down to the canal. 

 

This 22-acre site includes construction office trailers. It was grasslands when purchased by the state. At project’s end, it will be restored and title transferred to Somerset County. 

 

But can you do with some 258,000 cubic yards of sludge in huge geo bags? 

 

It’s being recycled to the American Cyanamid Superfund site in Bridgewater, where huge amounts of clean fill are needed to cap the area being remediated of toxic industrial wastes. It’s a win-win for both important and environmentally sensitive initiatives.

 

 

 

 

 

 

 

 

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