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What a ‘channelized’ river looks like

WILMINGTON—“Channelized” is the word that Todd Menees, a river engineer for Department of Environmental Conservation, uses to describe the Deerfield River where it passes through downtown Wilmington.

The stone walls along the river, combined with the bedrock under the Route 9 bridge, have created an “all-hard, constricted river, with no dispersion of energy,” he said.

“It would have been great to pick the buildings up and move them, but that’s not going to happen,” he said.

Rivers balance their speed by dispersing their energy in flood plains or by scouring away, then redepositing, sediment.

The Deerfield in downtown Wilmington cannot release energy by digging into the hard-sided riverbank. To compensate, the Deerfield has dug down and created a 6-foot-deep “scour pool” south of the bridge.

Menees said this type of scour pool commonly forms downstream from undersized bridges and culverts.

Architect Joseph Cincotta is heading up work to repair the flood walls and bank along South Main Street and The Crafts Inn property.

A work crew has been repairing the surrounding flood walls and have filled in the scour pool, reducing its depth from 6 feet to 4 feet, Menees said.

If the town hadn’t needed to save the buildings, Menees would not have approved removing the gravel. Removing too much gravel from a river, or altering or controlling the water’s course, can disrupt flow, causing problems with flooding, erosion, or aquatic habitat.

“There’s really no choice,” he said. “You can’t just let those buildings fall into the river.”

The crew also needed to lower the water level to complete repairs. According to Menees, if raw concrete used during construction hits water, it changes the level of acidity “immediately,” harming the aquatic life.

Cincotta wants to completely fill in the scour pool, however, and has asked Menees to get second, third, and fourth opinions from a state river scientist, state fisheries biologist, and the Army Corps of Engineers, respectively.

Cincotta has suggested hard-armoring the scour pool with riprap so the river will no longer chew into the river bottom.

“I don’t take it as an insult,” said Menees. “I’ve developed a thick skin over the last 10 months.”

Cincotta said that he’s worried less about another Irene-sized flood than he is about the annual spring thaws.

“Every year, the water comes within a couple feet of the bridge,” he said, noting that the spring thaws have contributed to the collapse of the wall under the Cady & Dugan building on South Main Street.

Scour pools gradually fill in through the normal activities of a river picking up and depositing sediment, said Menees. He feels that, during high water events, the stronger waters will chew out the scour pool and then start all over again.

“Right now, none of us know what the bedrock condition is under the scour pool,” said Menees, adding that this bedrock could affect how the river releases energy.

Irene’s floodwaters also scoured under the flood walls, eventually undermining them. Menees said he pointed out cracks in the flood walls to town officials in March.

The part that failed in the wall, he said, was a concrete “box beam” across a void in the ledge that couldn’t hold up under the weight above without the gravel below.

Town Manager Scott Murphy emailed Menees in May to say the wall had failed. Menees emailed a permit back. He told Murphy to make the repairs and then send him a design for review.

“We’re not able to control the river, and we need to recognize that,” Menees said.

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Originally published in The Commons issue #160 (Wednesday, July 11, 2012).

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