Unusual design in Concord shows how culvert is changing in face of environmental concerns - Concord Monitor

If you think today’s roads are bad, check out Portsmouth Street from a century ago.

“Digging down, about 5 feet below existing grade, we found the old wooden road. It was wooden planks placed to cross the stream, probably over 100 years ago,” said Martha Drukker, an engineer for the city of Concord who is the project leader on a very interesting culvert replacement.

Plunging down a 5-foot embankment to Mill Brook and then heading back up again on the other side must have been a chore, which is why there has been a culvert there for decades, carrying the road over the waterway that leads into a marsh on the east bank of Merrimack River, near Merrill Park in Concord.

That culvert was a standard 48-inch-diameter corrugated pipe until recently when it was replaced with a complex structure that reflects how the much-overlooked culvert has become a focus of municipal efforts to prepare for the extremes of climate change while doing less damage to wildlife.

“They’re trying to work with communities to not just go in and throw a pipe in the ground and then put a road on top of it, but to connect the needs of the environment as well as our infrastructure needs,” said Drukker.

A culvert is basically a self-contained bridge, usually built so a road can pass over a small waterway. There are tens of thousands of them in New Hampshire, although nobody really knows how many. Concord’s engineering department once estimated for me that the city by itself has 533 culverts, some are so old that records are sketchy.

The easiest and cheapest culvert is a buried pipe, but poor design often causes streambed erosion that leaves the pipe stranded well above the flow of water so it acts almost like a dam. A stream that passes through several culverts, as many do, ends up being chopping into a series of separate short segments. This creates a real obstacle to fish and amphibians and insects moving up and down waterways as needed to find better food or habitat during the year.

The goal these days is to have an “open-bottom design” allowing free organism movement, where the floor of the culvert is the actual streambed because the culvert is a three-sided concrete tunnel placed with the open side facing down, known as a box culvert.

Another problem is that culverts are usually sized to handle historical water flows. The increase in extreme storms caused by a warmer, wetter world means those flows are increasingly being exceeded, washing out culverts and blocking roads.

With those issues in mind, when the culvert on Portsmouth Street near Merrill Park began to fail, a large box culvert was the obvious replacement choice. But then another problem came up, caused by the glaciers that covered New England 15,000 years ago.

It turns out that soil in this area is glacial till, a mix of small material left behind by the retreating glaciers that wouldn’t provide a good base for a box culvert. Rather than build monstrous concrete footings to hold up the box, which would basically undo the good of an open-bottom design, engineers took an unusual step.

They turned the box culvert upside-down, so that it’s like a squared-off U, then buried it halfway, filling it with material to maintain a natural stream bottom.

“We had to set it 5 feet below the channel, and recreate the stream channel inside the box culvert,” said Drukker. That’s why they encountered the original road, which had long ago been covered.

The resulting structure is big: 40 feet long, 16 feet wide, and eight feet tall, although five of those feet are buried. It’s also not cheap: $400,000 or so, much more than pipe culvert would be. And it wasn’t easy to install, because there wasn’t much room between the road and the marsh to maneuver heavy machinery.

The culvert includes a shelf that will allow non-aquatic creatures to pass without having to go through the water or cross the road. These shelves are increasingly common even in regular box culverts to aid non-aquatic wildlife movement.

“There are some animals that do not prefer to cross in traditional upside-down U’s because they don’t like to get their feet wet,” said Linden Rayton, Merrimack River Floodplain Education Coordinator for the Society for the Protection of New Hampshire Forests. The society owns property next to the crossing.

The inverted box culvert design is unusual for New Hampshire, so unusual that the city is sharing photos and videos of the installation with the state Department Environmental Services, to help guide other communities as they seek to improve their culverts.

“Across the state, everybody is dealing with replacing culverts that haven’t been replaced for several decades. A lot of communities are trying to get up to speed and get them replaced, but it’s difficult with funding,” said Dukker.

And what about Concord itself, I wondered. Are there other culverts in the city that Dukker would like to see replaced with box culverts or inverted box culverts like this one? She chuckled at that question.

“How long a list do you want?” she said, adding: “Pretty much all of them.”

(David Brooks can be reached at 369-3313 or dbrooks@cmonitor.com or on Twitter @GraniteGeek.)

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