Wrightsville Dam Performed As Designed, But What’s Next?

While heavy rains pounded Vermont on Monday, July 10, Collin O’Neil was monitoring water levels at Wrightsville Detention Reservoir. 

As manager of Wrightsville Beach Recreation District, it’s something he routinely keeps track of, since high water can submerge the beach and other facilities there. Normally, the water in the reservoir sits at about 634 feet above sea level. 

But that night, the water rose behind Wrightsville dam faster than it could drain into the swollen North Branch of the Winooski River. 

When water reached just 16 feet below an emergency spillway, O’Neil contacted Montpelier officials with a warning. 

“I’ve watched the dam for 23 years and never once before tonight been concerned that it would reach that secondary spillway,” he recalled saying. 

O’Neil was far from the only vigilant one. An engineer with the Department of Environmental Conservation’s Dam Safety Program was on site that night to monitor the reservoir in person and stay in touch with state and city emergency officials.  

The next morning, Montpelierites awoke to an alert from the city government: Wrightsville had only 6 feet of storage capacity left. 

An unprecedented release down the spillway into the North Branch would “drastically add to the existing flood damage” along the North Branch and downtown, it read.

The situation developing at Wrightsville that day rattled residents already reckoning with a devastating flood. As it happened, water levels in the reservoir stopped inches short of the spillway Tuesday evening. (Hourly water-level updates are available at the U.S. Geological Survey’s webpage.) 

But if the reservoir had kept rising, what might have happened — and how should Montpelier prepare for similar events in future?

Protection, Plus a Backup

Located up Route 12 three miles north of Montpelier, Wrightsville Dam was built of earthen fill and granite in 1933–1935 by the Civilian Conservation Corps with guidance from the United States Army Corps of Engineers. Along with its reservoir, the dam was designed to provide flood control in the wake of the devastating 1927 flood. Alterations in the 1950s, ‘60s, and ‘80s created recreational areas, raised reservoir capacity, and allowed hydropower generation. Draining a 68.1-square-mile watershed, the reservoir can hold 6.6 billion gallons. 

“The dam has done its job for 90 years,” O’Neil said. “We take it for granted.” 

To understand how the dam works, imagine a faucet gushing into a porcelain sink — the reservoir — the drain of which is open.

By design, water drains through the dam to the North Branch via a primary spillway, the flow rate of which maxes out at 945 cubic feet per second, according to Ben Green, section chief and dam safety engineer with DEC’s Dam Safety Program.

(Another pathway through the dam called the penstock flows through the Washington Electric Co-op turbine; it shuts off to prevent turbine damage when the water rises higher than 660 feet above sea level.) 

If inflow exceeds outflow, then the reservoir’s surface will rise like a sink filling. There are no extra drains, so outflow can’t be increased. Floodwaters flow downstream at a controlled pace. But if they keep rising, eventually the extra water has to go somewhere.

That is where a failsafe comes in. The secondary or emergency spillway in the city’s alert message is a 155-foot-long channel made of bedrock and concrete and located 30 feet below the very top of the dam.

“That ‘freeboard’ helps ensure that overflows will always be carried by the emergency spillway channel and dam failure avoided,” explained Jeff Cueto, a retired DEC hydrologist. 

Waters reach this spillway if they rise past 685 feet above sea level (which still has never happened in the dam’s history). Like a sink overflow hole, the spillway allows extra drainage. It empties well downstream of the dam’s base to avoid scour.

With this spillage, timing matters — and it isn’t automatically deadly. Picture extra spillage from the sink to the floor, not a tsunami.

“The concern [that night] was never for dam failure,” Green wrote in an email, “simply that rising water would overflow the auxiliary spillway, [and] that could lead to additional flooding of property and related consequences.” 

To help put that possibility in perspective, Neil Kamman shared some numbers. Kamman is director of the Water Investment Division at the Vermont Department of Environmental Conservation (DEC). 

If the waters had risen to the equivalent of one foot above the 685-foot secondary spillway mark, he said, that would have sent 1,500 cubic feet per second of water rushing down it. 

Combined with flow from the primary spillway, total flow from the dam would have been one-tenth of the Winooski River’s flow at peak flood. Kamman does not believe it would have appreciably added to the flooding downtown.

However, he added, in that rare event, “it would have definitely added flooding to the North Branch and probably done a fair bit of property damage. There’s no way to sugarcoat that.”

Still, he said, Wrightsville and its sister facilities in East Barre and Waterbury did “an incredible job, because all of them took more water than we’re accustomed to seeing.” (Green, too, wrote that Wrightsville performed well during this event.)

“That’s not to diminish what happened in Barre, not to diminish the impacts to citizens in Montpelier,” Kamman added. “But I just think about, what if these weren’t there?”

The Army Corps of Engineers and a consulting firm recently calculated what would happen if water were to come over the spillway during conditions as unusual as that week’s, with all three dams in play. 

“The reservoirs filled up in a way that we had never seen before,” Kamman said. “Each facility has its own model [or calculations] in place. But nobody had ever modeled all three activating at once.” 

For that reason, he said, the models weren’t available until Thursday of flood week.

The Worst-Case Scenario

As with anything man-made, dams can fail. If a spillway erodes or if water overtops an earthen dam, that could damage or destroy the structure, potentially unleashing a catastrophic wall of water. Another hazard is if water behind the dam seeps through the fill and creates cavities.

“These larger earthen dams are fairly conservatively designed, but you have to be really careful about internal water pressures,” Cueto said. Inspecting this type of dam includes a search for evidence of seepage, he added. 

Wrightsville did not fail, and if water had come down the secondary spillway, that would not have been dam failure either. But it is classified as a high-hazard dam, meaning it is likely to cause loss of life and property damage were it to fail. 

According to the state’s Dams Inventory, inspectors with the state and the Army Corps of Engineers rated Wrightsville “Satisfactory” in an inspection last October. Since the flood, inspections have revealed no new deficiencies, according to Green. The Corps and the Dam Safety Program will soon perform more detailed visual inspections, he said.

“These [dams] are incredibly important assets,” Kamman said. “So [inspectors are] out there a lot.” 

The city included dam failure among its eight worst threat hazards in the 2021 Local Hazard Mitigation Plan. Dams of concern included Wrightsville, Graniteville’s Thurman W. Dix Reservoir dam, Marshfield No. 6 dam (Molly’s Falls), and the East Barre dam. 

The plan listed a high-priority strategy to add four inundation maps, which estimate what to expect if a dam fails, plus information about dam failure to its website during 2021–2022. Per this writer’s search, these items are not on the city’s website. 

In a January 19, 2023, annual progress report for the Local Hazard Mitigation Plan, there is a statement that the city has inundation maps for Marshfield and Wrightsville dams and that the maps “can be added to the city website.” 

According to Kamman, as more rains approached later in the first week of flooding, Dam Safety showed Montpelier emergency officials how to interpret and use the maps to inform their response.

The Bridge has asked Montpelier emergency management and planning and zoning officials when the city will add dam failure inundation maps to its website.

A Changing Future

Vermont has changed since the 1927 flood that prompted construction of the Wrightsville Dam and its sister dams. The state has regrown much of its forest, which can reduce runoff and flooding. On the other hand, the landscape now sports countless impervious parking lots, driveways, and roads. And as the planet warms, Vermont is seeing more precipitation, including heavier rainstorms.

There are many ways to prepare for floods, such as improving awareness and warning systems; floodproofing buildings; changing zoning regulations; and encouraging flood insurance. The city has laid these out in documents such as the 2022 River Hazard Area Regulations.

But should preparation also mean changing the dam? The Dam Safety Program is considering it, according to Kamman and Green.

As Green put it, they’re exploring ways to more flexibly “improve the rate of post-storm water level reduction to make the dam more resilient to back-to-back storm events.”

Now is the time to look into options and better prepare for today’s climate reality, O’Neil said. With two extra hours of rain, he added, the effect might have been much more significant.

“It’s not over yet,” O’Neil said. “Mother Nature’s like, ‘Get your act together.’”