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GREENSBORO — The dam at Lake Townsend is barely 40 years old— the prime of life for many such behemoths of concrete, brick and steel.

But a nasty sort of chemical decomposition has infected the dam’s concrete, the major bulwark holding back the billions of gallons of water in Greensboro’s largest reservoir.

Now, Greensboro is in the early stages of a $34 million project to replace the existing dam with a new one directly behind it. City officials and private engineers did not take the step lightly, but considered every possible way to save the original dam before deciding it was futile, said Allan Williams, director of the city’s Department of Water Resources.

“The conclusion was that if we wanted to spend our money wisely, we really had to build a new dam,” Williams said. “We’re confident that when we get this done, we’ll have something that will be permanent for at least 50 to 80 years.”

One bright spot for city taxpayers is that with the slow economy, the project is costing Greensboro about 30 percent less than it might have in a more active building market.

City administrators estimated it would cost $50 million, based on experience with other large structures. The project includes an already complete $12 million pump station in place of one atop the old dam’s decaying concrete.

“It blew everybody away. We weren’t expecting it,” Williams said of the low bid by Charlotte-based Crowder Construction Co. “We got it at a great price with a good contractor.”

City residents will pay for the project in their water bills, which eventually will rise 5 to 7 percent to offset the cost.
The new dam should be finished in mid-2011, Williams said. Water customers will not see any interruption in service caused by the project, he said.

To prevent problems with delivery of drinking water, the project involves a delicate, two-step process taking place behind the old dam’s spillway. The spillway is a large, concrete slope that carries water discharged from the lake, both a small amount daily to refresh the creek behind the dam and any overflow from storms.

Behind the existing dam’s spillway, crews put up huge, metal walls to make a temporary “cofferdam,” which will dry out one side of the stream directly behind the old dam.

In that dry space, Crowder will build the new dam’s spillway and one wing of the new dam. Meanwhile, water discharged from the old dam will be flowing into the stream behind the temporary metal wall.

When the new spillway is done, the city can use that to carry water from the lake, allowing workers to dry out the other side of the stream and build the rest of the new dam across that area.

Except for its concrete spillway, most of the new dam will be made of packed earth reinforced by concrete block and steel bands. Modern concrete includes additives to avoid the problem that doomed the first dam, one of many structures across the nation beset by the syndrome known as alkali silica reactivity or ASR.

The Townsend dam, its spillway and nearby treatment plant were built in the 1960s to augment smaller water-supply lakes Higgins and Brandt, which flow into Townsend just east of Lake Brandt Road on the city’s northern perimeter.

Townsend’s concrete base began showing unusual wear only 20 years after it was built because of ASR, a chemical interaction not understood until several decades later. Engineers now know ASR triggers reactions inside the concrete, forming a troublesome paste that eventually destroys concrete from the inside out.

Initially stumped by the first dam’s problems, city staff figured it out five years ago after ASR became more widely known.
Lake Townsend’s marina also is being relocated to make way for the new construction, but the site has not been picked yet, Williams said.

Lake Townsend covers 1,540 acres and can supply 30 million gallons of drinking water a day. It’s a third larger than lakes Brandt and Higgins combined.

Part of the new dam’s price tag stems from federal regulations that it be built to withstand levels of rain many times greater than Greensboro has seen in recorded history.

The new dam features a spillway that eliminates the existing dam’s nine floodgates, replacing them with a maze-like structure that should save the city millions of dollars in mechanical upkeep, Williams said.

Contact Taft Wireback at 373-7350 or taft.wireback@news-record.com