Fox Hill dam’s future is under study
A flood control dam on the Norwalk River — the grass-covered earthen dam near Fox Hill condominiums — has been upgraded to a “high hazard” classification by state and federal authorities, triggering studies of its future.
“Although the dam is not in imminent danger of failure and is generally functioning as originally planned, there is the possibility of a storm occurring that would overtop the dam and cause failure,” says a preliminary draft of a study on the dam. “If the dam fails, there are serious consequences.”
A failure of what is known as “dam number two” of the Norwalk River Flood Control Project could be very serious, even life threatening.
“Total estimated damages from a catastrophic breach of Dam No. 2 would approach $700,000 and the potential for loss of human life would be significant,” the draft study says.
This is not a constant looming threat, however. Most of the time there isn’t that much water behind the dam — a flood control dam lets normal stream flow pass through, and a large flood pool only builds up behind the structure in times of very wet weather, heavy long-lasting rains, possibly made worse by snow melt.
And the potential threat is something state and federal officials keep an eye on, according to Ben Smith, a hydrologist with the Natural Resource Conservation Service (NRCS), a federal agency that oversees many dam projects.
“No, it’s not really something to be worried about. There’s an emergency action plan in place,” Smith said. “...If there’s a major storm, they go out and look at the dam ... Will it overtop? Is there going to be any sort of problem? And then (they) work with local emergency management people to get anyone out of harm’s way...
“It’s not like something sitting unattended,” Smith said. “They send people out to look at this in major rain events.”
Still, the conditions that lead to a dam breach do occur — remember hurricane season? — and there could be real problems.
A dam project is being studied by state and federal agencies with a goal described as “reducing the potential for flood damages and loss of life from a catastrophic breach.”
The dam — and what the state proposes to do about it — is expected to be the subject of a public hearing in the coming months, though no date has been set.
“There’ll be a draft plan,” First Selectman Rudy Marconi told the selectmen earlier this fall. “...The cost of improving the dam is $6 million. The cost of removal is $3 million.”
No hearing date has been set, as the studies are on-going, according to Smith, the NRCS hydrologist.
“We’re still working on it. Nothing is set yet,” Smith said in a Nov. 25 email response.
“The project is moving slowly, Smith said in mid-October. “...We are hoping to schedule a public participation meeting toward the end of the year to present that draft plan and preferred alternative, and to hear comments and concerns then.”
The state, too, is anticipating further study of the project, according to Charles Lee of the Connecticut Department of Energy and Environmental Protection (DEEP).
“I am in the process of requesting an extension of the federal assistance to allow more time to study, make recommendations, and provide for public outreach and comments,” Lee told The Press.
The “preferred alternative” Smith referred to would be a recommendation as to what should be done, from a list of options under study — which range from repairing the dam to tearing it down.
Marconi said the town is keeping an eye on developments, and has a real interest in what happens with the dam.
“The major concerns at this point are impact on wetlands and changing the wetland levels, when the water comes up and recedes. Changing that dam will have an effect on Great Swamp,” Marconi said. “...And that’s what we need to drill down into.”
The town is waiting for the state study to be finished, and the planned public hearing on the dam’s future to be scheduled.
Floods in ’50s
The “dam number two” that is under study is officially part of the Norwalk River Flood Control Project, dating back to the mid-1950s.
Following flooding from back-to-back hurricanes in mid-August, the devastating flood of October 1955 sent the Norwalk River on a rampage through its valley, tearing through Branchville, Georgetown and Wilton on its way to Norwalk after 12 to 14 inches of rain fell between Oct. 14 to Oct. 17, wiping out bridges, stranding a train in Cannondale.
Water was said to reach eight to 12 feet deep in parts of Georgetown after the dam at the Gilbert & Bennett wire factory gave way.
The Norwalk River Flood Control Project was started, but never completed. Only two dams were built out of five in the original plan — the one near the Fox Hill condos and another completed in Wilton.
Although the dam is a flood control structure that allows normal water flow to pass through it, a sizable pool of water does back up behind it only in times of heavy rain — when it is apparent to drivers on the western end of Farmingville Road, near Ligi’s Way.
And a breach in the dam during a big storm — which is when such a thing would be likely to occur — could make flooding worse.
“The hazard from a breach, depending on the construction, might be worse than the hazard from the original flood it was designed to protect (against), because they happen so fast,” said Smith of the NRCS.
There are a variety of options for what should be done with a potentially problematic dam.
“The discussion is going to be what the options are,” said Town Wetlands Agent Beth Peyser. “You could get rid of a dam, you could repair a dam, you could replace a dam, you could move a dam…”
Smith of the NRCS suggested one more: Doing nothing.
“When we’re looking at these things, we’re required to look at several options,” he said. “One of those options is the decommissioning option. Another option is the rehabilitation option — in most cases structural measures. And then there’s another option we have to look at which is the ‘no federal action’ option.”
The “no federal action option” looks at what “the sponsor” — the state DEEP, in this case — “could do to relieve the hazard of the dam, without federal funding.”
The study being done is in the hands of consultants for state DEEP and federal NRCS.
The studying of options appears to have been triggered by a reclassification of the dam.
“The project is needed because the DEEP and NRCS both concur that Dam No. 2 is now a high hazard (class “c”) dam based on current criteria and that properties and significant infrastructure downstream are at risk if a catastrophic breach were to occur,” says the draft study done by consultants.
“They did a risk assessment,” First Selectman Marconi told The Press. “Unbeknownst to the town the state had hired a consulting company to assess that particular location of the dam, and as a result of the review the ‘risk evaluation’ was changed, making a higher risk of breaching.”
He added: “They felt it needed to be modified and upgraded if we’re going to keep it in place...
“Then what they did is look at the floodplain downstream,” Marconi said. “And all of their calculations resulted in a determination that if the dam were removed but the spillway were still in place — so normal flows will still go through the spillway, in the event of the storm — it will overflow, but given the downstream floodplain availability, they feel there would be minimal damage.”
The study by the consultants says dam number two “provides 1,162 acre-feet of flood storage to the top of dam, and 87 acres of wetland wildlife habitat in the upstream storage area.”
The wetland wildlife habitat would be in the Great Swamp that runs from Farmingville Road to parts of Ivy Hill Road.
The purposes of the Norwalk River Dam No. 2 rehabilitation project, according to the state and federal authorities overseeing the studies, include the following:
• “Comply with applicable design, performance and safety standards by reducing the potential for flood damages and loss of life from a catastrophic breach;
• “Continue to provide flood damage reduction as designed for the 100-year, 24-hour recurrence interval flood event downstream of the dam;
• “Maintain existing wetland wildlife habitat and recreational values of the impounded forested wetland and riparian systems.”
The draft study does suggest that “catastrophic” events are possible, in the event the dam backed up a lot of water in a storm, and then failed.
“There is a need to comply with current state and federal safety and performance standards for a high hazard dam and to continue to provide the current levels of flood protection and wildlife habitat,” the consultants’ study says.
“There is a potential for loss of life from a catastrophic dam failure…
“The sponsor (state DEEP) wants to continue to provide 100-year flood protection and wildlife habitat in a manner that reduces risk of loss of human life and is both cost effective and environmentally acceptable.”
The draft study lists three properties as “potentially impacted structures” in the event of dam removal: houses at 4 Brookside Road (Drive), at 91 Great Hill Road and the Ace Tire building on Route 7 — 861 Ethan Allen Highway.
“Elevation certificates (EC) prepared by a licensed land surveyor are needed for each structure,” the draft study says. “...With the EC, each structure can be evaluated to determine if raising the BFE (base flood elevation) from the dam removal will increase flooding at each structure or newly put the structure in the 100-year floodplain.”
The state consultants’ draft study considers the area’s traffic, as well as residences in areas downstream from the dam, in calculating that there is “population at risk” of 30 people.
The study is based on 2013 average daily traffic count from the Connecticut Department of Transportation, approximately 15,000 - 19,000 vehicles travel on the following roads daily in Ridgefield: Route 35 (Danbury Road) just below the dam; Shields Lane; Cross Brook Lane; Limestone Road; Great Hill Road, and Route 35 (Danbury Road) further downstream of the dam…”
While the potential for flooding comes only in times of very wet weather, the problems from a dam breach could be particularly serious, not in the broad relatively flat area around the dam itself, but downstream in areas where the valley narrows.
“There’s a lot of storage behind this dam, even though it’s not very tall, it has a lot of area. The area gives it the volume,” said Smith of the NRCS.
“If you have a narrow valley downstream, things can reconverge … if there was to be a clog downstream, you could get a wall situation … you may have a big wide area for a while, things spread out, but then they reconverge.”
According to Smith, another concern is that weather is becoming more extreme — and has been since the flood control project was designed in the 1960s and the dam was built in 1979.
“Between the time that this was designed and the time it’s being evaluated, now, there is a different extreme rainfall study published by NOAA (National Oceanic and Atmospheric administration),” he said. “The difference in those two studies for 100 year, 24-hour storm — it has gone up significantly — and that is what we typically design to protect from.”
Marconi said the town wants to work with the state and federal agencies studying the dam.
“We will continue to meet,” he said, “and to work toward a resolution to be sure that, at the end of the day the people of Ridgefield will all have an opportunity at a public hearing to review what the final recommendation may be.”