Engineer: Dam integrity not in doubt during Irene (with photos)

Even with an evacuation alarm sounding and record levels of water crashing over Gilboa Dam spillway

Even with an evacuation alarm sounding and record levels of water crashing over Gilboa Dam spillway during Hurricane Irene, engineers remained confident that the structure would hold.

New York City Department of Environmental Protection engineer Tom DeJohn acknowledged that some extra precautions were taken during the unprecedented flood that accompanied the storm. But he said the dam’s integrity was never in serious question, even after the agency lost all communication with its monitoring devices.

“There was no signs of the dam doing anything other than what it was intended and designed to do,” said DeJohn, who observed the flow from the dam’s western parapet during the peak flow on Sunday, Aug. 28.

DeJohn helped author a 20-page report issued after Gilboa’s emergency action plan was activated shortly after noon on the day Irene arrived. He said activating the plan was more a matter of precaution than actual concern for the safety of the dam.

“It was really an abundance of caution that led us to do what we did in terms of getting eyes on the dam,” he said.

Still, the report completed five days after the storm highlights the tense moments leading up to record-setting water levels at the dam, which holds back the 19.5-billion-gallon Schoharie Reservoir that serves New York City. The report also shows how city officials scrambled to get to the dam amid flooding roads and high winds.

Initial forecasts from the National Weather Service predicted the level of the Gilboa Dam would peak at 1,131.4 feet above sea level, or just slightly below moderate flood stage. But once the storm hit this area, this prediction proved to be vastly inaccurate.

Shortly before 11 a.m. on the day of the storm, visual monitors at the dam reported that a 200-foot-long temporary bulkhead built to facilitate a $350 million rehabilitation project had failed as department officials expected. By that time, the main access to the dam through the west gate had been cut off because of flooding over the state Route 990V bridge.

Then, shortly after noon, all electronic communication with the dam’s monitoring instruments was lost as the storm knocked out power to the area. Agency officials alerted Schoharie County’s Board of Supervisors and Emergency Operation Center, which sounded the dam’s siren system and ordered a full evacuation.

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DEP officials ordered design teams with the rehabilitation project to do an emergency inspection via helicopter. But conditions from the storm made an aerial observation of the dam impossible, according to the report.

Gilboa’s water level ultimately reached 1,137.7 feet. Inspections from various areas of the dam — including both parapets and the eastern embankment — showed that it seemed to be holding fine, even if parts of it were hidden by the extraordinary amount of water flowing over the dam.

“No leakages were noted on the earthen section or obvious stability issues with the control section and the spill line across the crest appeared true and level,” the report states. “However, the extraordinary spill elevation obscured most of the more detailed features of the control section.”

Using massive spotlights set up at the base of the dam, department monitors continued to watch the water as it rapidly receded during the early morning hours of Aug. 29. Inspections conducted afterward showed that both the stone and earthen parts of the dam had weathered the flood mostly unscathed.

Damage was done to the dam’s steel siphons, though the amount is still questionable. DeJohn said the spillway and a construction area below the dam were damaged by the high water, but the damage is consistent with the type that is expected during sustained high water levels.

A failure at the nearly century-old dam could spell catastrophe downstream. The 150-foot-deep Schoharie Reservoir and all its billions of gallons of water could come crashing down through the Schoharie Valley in the event of a serious collapse at Gilboa.

The outlook after a dam breach is bleak, according to projections calculated by emergency planners: More than 150 bridges across the region would be closed indefinitely, with the Thruway shut down from Saugerties to Syracuse. Five major power lines crossing the creek and countless substations would be turned off by National Grid, effectively blacking out large swaths of the Capital Region.

Downstream safety concerns prompted the department to conduct an emergency $24 million project to install 80 massive cables through the dam to anchor it to the bedrock below. Completed in December 2006, the project also cut a 220-foot-long notch on the dam’s western edge and installed four gigantic siphons to help release water from the reservoir.

All our photos

From the farms of Schoharie County to the streets of the Stockade, our photographers captured the flooding in dozens of photos you can see by clicking HERE

The emergency repairs helped alleviate some downstream concerns as Irene approached. But Bob Mann Jr., supervisor of the downstream town of Blenheim, said nothing would have changed the unprecedented nature of the storm.

“This really was a very unique event that shouldn’t be repeated any time in the near future,” he said.

That doesn’t mean there’s no room for improvement. Mann was dismayed by a lack of communication after the alert was sounded and was confused over why a more accurate prediction of the water level wasn’t made far in advance of the storm.

“Certainly, the model needs to be reworked,” he said.

Schoharie County spokeswoman Karen Miller was largely pleased with the NYC Department of Environmental Protection’s response during Irene. Though she hadn’t read the report on Gilboa, she said the department seemed to act consistently with the advanced planning for an epic flood event.

“The storm we had was totally unusual,” she said. “That is not something I anticipate that we’ll see again hopefully for very long time.”

Categories: Schenectady County

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