Flood control dam: Problems and plans
Could a flood control dam end up causing a flood? Well, if so much rain fell that it overtopped the dam, causing a breach, all the water the dam had held back would flow downstream.
With a flood storage capacity of 1,162 acre-feet of water, and an increasing amount of development downstream, the Great Swamp flood control dam near the Fox Hill condominiums has been reclassified as a “high hazard potential” structure — meaning “failure will cause probable loss of human life or serious economic damage.”
It’s a concern.
The state counts 180 residence and 100 commercial structures, as well as roads and bridges, in downstream areas that could be affected by a dam breach.
“Anytime a dam overtops, you could have a dam failure,” said Kristen Walker of the U.S. Department of Agriculture’s Natural Resources Conservation Service.
She’s among a group of state and federal officials studying the dam, which generally goes little noticed but in heavy rains sometimes backs up a pool of water across Farmingville Road.
“A breach would cause a very quick release of all the water impounded behind the dam,” says a release from the officials monitoring the dam. “Little or no warning would be available to people downstream.”
The Great Swamp dam was built in 1979 and will be 40 years old next year. It’s 440 feet long and 10 feet high and stands at the outflow of an 87-acre wetland and wildlife habitat — Ridgefield Brook, which becomes the Norwalk River.
A dozen or so Ridgefielders — some from the Fox Hill condominiums, near the dam, others from Quail Ridge, on the other side of Great Swamp — listened Thursday night, April 19, in the Ridgefield Recreation Center as officials and engineers discussed making the dam safer.
Collaborating on the project are the state Department of Energy and Environmental Protection (DEEP), the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS), and consultants from Schnabel Engineering.
Whatever project is eventually decided on, the U.S. Natural Resources Conservation Service is expected to carry 100% of the design cost, and 65% of the construction cost, with the state DEEP picking up the other 35% of the construction cost.
The DEEP owns and maintains the Great Swamp dam among some 250 state dams — and keeps a bureaucratic eye on many more private dams.
“There are about 5,000 dams in Connecticut,” Peter Spangenberg of DEEP’s dam safety division said Thursday night. “We can’t keep track of all of them.”
With its “high hazard potential” classification, the Great Swamp dam is due for inspection every two years.
“A well-maintained dam, even if it has a high hazard classification, isn’t that much of a risk to the community,” Spangenberg said. “A poorly maintained one is.”
“We did the inspection this year — it is in good condition,” said Walker
“It has been kept up beautifully,” said Spangenberg.
The dam was designed to last 100 years, and should have another 60 of years of useful life. But things change.
Ray Frigon, supervising analyst with the DEEP, told The Press before Thursday’s meeting that current re-examination of the dam was prompted by what’s gone on downstream.
“After several decades of development in the area, the dam has a different hazard classification at this point in time,” he said. “If the dam were to fail, it has the potential to cause, certainly, destruction of property.”
Frigon said there is also “potential for loss of life, just given the acreage that drains to the Great Swamp area, which is where the dam is located.”
Frigon added, “Loss of life and loss of property, it’s under the worst imaginable scenario that you could think of.”
Flood of ’55
A “worst imaginable scenario” that happened is what led to the dam’s construction. Though built in 1979, the dam was conceived as part of the Norwalk River Flood Control Project, prompted by the flood of 1955.
More than a foot of rain fell on southwestern Connecticut over two days in mid-October 1955, sending rivers rampaging, wiping out roads, bridges, and homes, and leaving millions of dollars in property damage and four people dead.
Kristin Walker of the NRCS told Thursday’s meeting she’d seen pictures of flood of ’55.
“Absolutely terrifying,” she said, “… homes just moving down the river, people on the roof.”
Five dams were originally planned as part of the Norwalk River Flood Control Project. Two eventually got built — the Great Swamp dam in Ridgefield and one on Spectacle Brook in Wilton. Three never did, their designs tied up with plans for the controversial and now abandoned Super 7 expressway project.
The plan’s key flood control structure, the Miller’s Pond dam, was to have been on Route 7 near the intersection of Florida Hill Road — right on the Norwalk River. It was never built.
The two other dams that weren’t built? One was to have been at Candee’s Pond on Cooper’s Brook near Branchville, another on Comstock Brook in Wilton.
Since its purpose is flood control, the Great Swamp dam has a spillway that allows the stream to flow through it — it backs up water only when there’s too much of it. With very heavy rain, more water reaches the dam than can go through the spillway, and substantial amounts of water pool behind the dam.
After a big storm, the dam continues a slow release of the water held back.
“It meters the water out slowly through a spillway,” said the DEEP’s Frigon.
A question with the Great Swamp dam is whether — in a “probable maximum precipitation” event — the spillway would allow enough water to pass through to avoid having the pool build up to the point that it overtops the dam.
“The spillway sizing may not be big enough,” Spangenberg, director of the Connecticut Dam Safety Agency, said April 19.
Among the options being studied to correct the problem are “overtopping protection,” a kind of hardening of the dam so it can better survive overtopping; making the top of the dam higher; increasing the capacity of the spillway so more water goes through and less backs up; and decommissioning and removal of the dam.
A resident in the audience Thursday night asked about the possible removal of the dam.
“It’s an option for the owners to consider,” said Wade Biddex of Schnabel Engineering, the consultants on the project.
“I’ve seen it out West, where it was built to protect agricultural land. Maybe there’s no houses downstream,” he said.
“In a case where you have a lot of downstream property, I can’t see that happening,” he added.
“These dams were built for a reason.”