Gilboa: Questions and answers on the integrity of the dam

PHOTOGRAPHER: Edward Munger Jr.

The day after Tropical Storm Irene’s floodwaters had already receded, a rumor spread that sent people scrambling for high ground.

Somebody erroneously said the dam broke, referring to the massive Gilboa Dam that looms upstream of the Schoharie Valley.

The dam holds back 17.6 billion gallons of water that’s used to quench the thirst in New York City — a volume so great that a failure in Gilboa would mean catastrophe for communities from the Schoharie Valley all the way to Amsterdam and then to Schenectady.

Managed by the New York City Department of Environmental Protection, the dam became the focus of concern when engineers in 2005 determined the 182-foot-high concrete and stone structure didn’t meet current design standards.

A massive flood, in other words, could push the entire dam out of the way, releasing a sudden and crushing blow of water crashing down the valley.

Flooding from Tropical Storm Irene surpassed flooding considered “the big one” that hit the valley in 1996 — but the Gilboa Dam stood firm.

Many believe that’s because the DEP in 2006 completed a $24 million emergency project — drilling 80 massive steel cables through the dam and anchoring them to the bedrock. The DEP also installed siphons which take some of the pressure off of the dam itself by releasing water to the spillway.

Planning that followed led to a roughly $350 million, four-year construction project begun in 2009 and stunted, temporarily, by the floods of tropical storms Irene and Lee.

As construction work continues, the New York City Department of Environmental Protection in late August fielded 10 questions from The Sunday Gazette on how the work is progressing. Here are the questions and answers:

Q: What is the status of the dam’s integrity following last year’s record-breaking flood?

A: DEP conducted a thorough assessment of the Dam’s integrity following Hurricane Irene and concluded — and maintains — that the dam is safe and secure. This was confirmed by the Independent Technical Review Team, whose membership was approved by NYS DEC Dam Safety.

Q: What work is being undertaken this summer?

A: Major tasks under construction now include reconstruction of the stepped concrete spillway on the areas outside the temporary diversion structure. The contractor is working from the toe towards the top in these areas. This includes construction of a new full length inspection gallery within the spillway, which will connect the east access stairway to the west adit and gate house. Within the gallery there will be a drain system to relieve potential uplift pressure. Other major items include: reconstruction of the side channel floor and plunge pool slabs with rock anchors to protect from extreme spill events; installation of anchors in the west training wall to improve stability; repointing the masonry on the north and east retaining walls and upstream dam face; placement of stone fill on the west upstream earthen embankment to improve slope stability; excavation of the east side access stairwell leading to the new inspection gallery; and various electrical duct banks for security and data.

Q: A low-level outlet is expected to be part of this project and should enable the DEP to release water from the reservoir. What is the status of this part of the project ?

A: The new low level outlet (LLO) will be constructed under a separate contract. A nine-foot-diameter tunnel will connect the new LLO facility downstream of the dam through a gate shaft located on the eastern abutment on the eastern side to an intake crib on the bottom of the reservoir. The design is under revision due to impacts resulting from Hurricane Irene. Completion of the LLO is projected for 2018. DEP is required to conduct a study of potential releases to the Schoharie Creek below Gilboa Dam as part of its permit with DEC by December 31, 2014.

Q: Officials in Schoharie County have said they wanted to improve communication with the DEP as it relates to the construction activity. Have there been any discussions or progress to that end?

A: Communication has significantly improved with Schoharie County through the duration of this project. Recent improvements since Hurricane Irene include the inclusion of the County’s retained engineer in monthly project update meetings, and transmittal of monthly progress reports to the County’s retained engineer. DEP continues to make formal presentations to the county on a semi-annual basis. DEP provided a tour to county officials this past spring.

Q: During the hurricane, a “bulkhead” built on top of the dam was destroyed. Is another bulkhead in place now?

A: There was a temporary wooden bulkhead constructed on the eastern end of the spillway crest to protect the construction workers below during the initial spillway crest demolition activities. This bulkhead was destroyed by the floodwaters. This has since been replaced with robust steel sheeting driven into the reservoir sediment upstream and backed up by stone fill. The contractor uses this area to stage the loading of barges with stone fill for the upstream west earthen embankment work.

Q: What are some of the project’s elements that are expected to be undertaken during upcoming months?

A: Elements of work to be undertaken during the upcoming months are as follows:

a. Continue to place concrete for the stepped concrete dam face and plunge pool slabs,

b. Continue spillway channel slab demolition and concrete placement,

c. Continue work on the east side gallery stairwell,

d. Installation of the necessary electrical and instrumentation throughout the project,

e. Complete required security improvements,

f. Continue installation of post tension anchors in the west training wall,

g. Installation of two new siphons to provide snow pack mitigation for the residents of the downstream communities and provide reservoir elevation control as necessary,

h. Commence with west gate house staged demolition and reconstruction,

i. Monitor and maintain the existing dam monitoring instrumentation,

j. Maintain storm water controls,

k. Complete stage one of the stone fill on the western upstream earthen embankment.

Q: Gauges and indicators were destroyed throughout the valley during the storm – some on or near the Gilboa Dam were put out of commission, what were those and have they been replaced?

A: The flood waters cut communications lines to the 6 extensometers used to indicate movement of a portion of the spillway control section concrete monoliths. The communications lines were repaired and temporarily armored, and the extensometers placed back into service last fall. New extensometers and concrete buried communications lines are being constructed.

Q: Some are convinced the DEP’s project to drill 80 cables through the dam and anchor them to the bedrock likely kept the dam in place during the record-breaking flood of 2011 – how is the pressure those cables exert on the dam described in terms of weight and how much more weight will be added to the dam as part of the stabilization project?

A: The rock anchors installed in 2006 added approximately 65,000 tons, and the concrete buttress currently under construction will add nearly 100,000 tons.

Q: What is the cost of the current construction under way?

A: The original bid price for the Construction Contract was $121 million and there has been approximately $23 million in change orders, bringing the total construction cost to approximately $144 million.

Q: How many personnel, from engineers to laborers, are involved in upgrading this dam?

A: Well over 100 engineers and personnel are involved in the upgrade work.