Gilboa Dam upgrade moving quickly
GILBOA Reconstruction work at the Gilboa Dam in Schoharie County is two years ahead of schedule, despite two interruptions caused by hurricanes or tropical storms.
Built to contain the 17.6 billion-gallon Schoharie Reservoir, one of 19 that supply drinking water to New York City, the 87-year-old concrete dam is undergoing a $400 million restoration.
The New York City Department of Environmental Protection, which oversees the city’s water supply system, provided local officials with an update on the work’s status Tuesday morning along with a close-up view of the largest public works project in the Catskills region.
DEP representatives outlined a host of features being added to the dam and facilities expected to give the structure even more durability than emergency work performed seven years ago.
The dam became a major focus after engineers in 2005 determined it didn’t meet modern design standards. They said water higher than massive flooding in 2006 had the potential to push the dam out of place, which would mean catastrophe.
For that threat, the DEP conducted an emergency project in 2006, installing 80 anchors consisting of massive steel cables drilled through the dam and mounted to the bedrock below.
The current project has seen two stoppages because of the weather. Floodwater from Tropical Storm Irene blew out a temporary bulkhead built atop the dam during construction in 2011 and damaged the work site. Work faced another stoppage as Hurricane Sandy bore down on the Northeast, deputy DEP Commissioner Paul Rush said.
“We’re currently running ahead of schedule. That’s not insignificant when dealing with a big project like this,” he said, adding contractors and workers are due credit for the project’s pace.
Rush outlined several features being added to the dam as part of the project, including dozens more anchors holding the structure to the bedrock. A new interior access being built, called an “inspection gallery,” gives engineers full access to the inside of the lower level of the dam.
Two new siphons were added to the dam to replace four smaller ones. The siphons act like straws and release water from the reservoir without affecting the spillway — an important tool since contractors are resurfacing the spillway. The spillway’s operation is tied to the amount of snow in the watershed that feeds the reservoir, and the level is maintained low enough to accommodate all that snow if were it to melt suddenly.
That scenario isn’t improbable — the level of the reservoir increased by 11 feet in one day in early March due to snow melt, officials said.
The existing two siphons, six feet in diameter, are each capable of releasing 250 million gallons of water per day. They will be replaced during one the final aspects of the reconstruction project — the construction of a new, low-level outlet to be drilled through the side of a mountain. The new release tunnel, expected to be finished in 2019, will be able to release about 1,500 million gallons of water daily — three times the capacity of the two existing siphons.
DEP Deputy Commissioner Paul Rush said the agency is discussing a call lodged by the grass roots group Dam Concerned Citizens and others who want to see a procedure put in place to govern releasing water in advance of severe weather.
The DEP released water from the Schoharie Reservoir in advance of Hurricane Sandy, one of several steps considered unprecedented up until then.
Just downstream on the Schoharie Creek, the New York Power Authority emptied water from its lower reservoir at the Blenheim-Gilboa hydroelectric project; and the state Canal Corp. opened up the movable dams along the Mohawk River to make room for potential flood water.
DEP officials gathered roughly 30 observers and shuttled them in buses and vans down to the work site on Tuesday morning.
Water thundered through one siphon as DEP regional manager Carl Davis outlined several aspects of the project. He pointed out the site of anchors visible from the spillway and the new gate system added to the top of the dam.
The gates were built into the 220-foot notch cut out of the top of the dam during earlier work. Powered by air that fills bladders which then lift steel gates, the system can be activated remotely and hold water inside the reservoir or open to release pressure on the dam.
Crews were pouring concrete as work to resurface the spillway ensued. A small concrete plant was installed to supply the material. The stone face of the original dam suffered severe erosion because of freezing and thawing, causing some of the stone to dislodge. A new design for the steps now being formed is expected to minimize the force of water on the spillway and do less damage over time.
Once complete, the dam will not only feature more than 100 anchors affixed to the bedrock, but also an additional 234 million pounds of concrete.
State Assemblyman Pete Lopez, R-Schoharie, on Tuesday said he’s seen a change for the better in the relationship between the DEP and local officials. “I’ve seen marked improvement and a change of culture within DEP, from the master controller mind set to more of a partner, facilitator,” Lopez said.
The Gilboa Dam has loomed over Schoharie Valley communities for decades and improved relations with its owners can be seen as progress, Lopez said.
Massive rainfall from Tropical Storm Irene took out sensors in 2011, leaving managers at the dam unable to monitor its status.
They called an emergency, prompting officials in Schoharie County to sound the series of sirens lining the valley, which were supposed to warn people the dam had failed. It didn’t fail, likely due to the 2006 anchoring project, and several officials and residents since then have said the sirens saved lives because people weren’t heeding warnings of potential flooding.
Lopez said people downstream of the dam are still “fragile.”
“They’re fragile emotionally, they’re fragile financially and the presence of the dam is a physical presence in our communities on a daily basis. And that’s something that we need to be able to set at ease,” Lopez said.