On Monday, Sept. 14 during its annual meeting awards ceremony held this year in New Orleans, La., the Association of State Dam Safety Officials presented its 2015 National Rehabilitation Project of the Year award to the New York City Department of Environmental Protection’s Gilboa Dam.
Key data:
- Gilboa Dam is 2,024 feet long, 155 feet high, and more than 150 feet wide at its base; Impounds 19.6 billion gallons of water in Shoharie Reservoir;
- Utilizes 314 square miles of drainage area at the northern-most tip of Catskill Mountains watershed;
- Supplies water to Shandaken Tunnel, which is part of the largest gravity-fed, unfiltered water supply system in the world; and
- Supplies about 15% of drinking water to New York City on a daily basis.
Graphics/NYCDEP
Gilboa Dam rehabilitation
Gilboa Dam was built from 1919 to 1927 at a cost of US$7.8 million. According to NYCDEP, the organization began a thorough investigation of Gilboa Dam’s integrity after the flood of 1996 overtopped its spillway by 6.7 ft, a record at the time.
An investigation completed in 2003 found that Gilboa Dam would require a comprehensive rehabilitation and upgrade because it likely did not meet modern U.S. dam safety standards. In 2004, NYCDEP began a program to renovate the dam located in the Catskill Mountains, approximately 125 miles north of Manhattan.
Gilboa Dam impounds Schoharie Reservoir, which collects water from a 314-square-mile section of the Catskill Mountains watershed. It is the northernmost reservoir in New York City’s water supply system and its capacity is about 19.6 billion gallons of water.
It diverts that water via gravity through the 18-mile-long Shandaken Tunnel, which discharges into the Esopus Creek where it travels another 5 miles before entering Ashokan Reservoir. From Ashokan Reservoir, the water flows south through the Catskill Aqueduct to New York City. The result is delivery of about 1.2 billion gallons of water, about 15% of the City’s drinking water, per day to the City. This water supply is the largest gravity-fed, unfiltered water supply system in the world.
Continued safety
Additional engineering work in 2005 found that Gilboa Dam had a marginal safety factor for flood conditions — conditions similar to 1996 — and that the dam could potentially fail under the pressure of a larger flood.
Following that report, NYCDEP said, “[We] moved immediately to make emergency repairs to the dam and protect the 8,000 residents who lived downstream.”
In 2006, NYCDEP reported that it cut a 220-foot-long by 5.5-foot-deep notch from the top of the westernmost portion of the dam to control water spilling from Schoharie Reservoir and allow for the installation of 80 anchoring cables into the top and outer face of the dam.
“These post-tensioned anchors significantly improved the safety of the dam by pulling it tighter to the bedrock below. Temporary siphons were also installed to remove water from Schoharie Reservoir, over the dam’s spillway and into the creek below, providing NYCDEP with more control over the level of water storage in the reservoir,” said the agency.
Continuing improvements
Work on Gilboa Dam’s rehabilitation began in 2011 and was scheduled for completion in 2016, but it was completed in October 2014, two years ahead of schedule. However, additional construction at the site will continue until approximately 2020.
The Gilboa Dam rehabilitation project is part of a $400 million program to build and improve other facilities near the dam. Construction work began via six separate construction contracts, some of which are still being executed.
The contracts include:
- Installation of spillway crest gates and operating equipment;
- Site preparation;
- Dam reconstruction;
- Low-level outlet construction;
- Shandaken Tunnel intake chamber improvements; and
- Environmental site restoration.
This includes a permanent release tunnel that would give NYCDEP the ability to release water from Schoharie Reservoir around the dam and below it into Schoharie Creek.
NYCDEP said, several new features were added to the dam during its rehabilitation, including an inspection gallery inside the dam that runs its entire length. The gallery — which also includes instruments to constantly measure stress on the dam — will allow engineers to visually inspect the inside and outside of the dam on a regular basis. The dam was also designed with 3-, 6- and 12-foot steps that dissipate the energy of water as it spills from the reservoir. The east and west abutment walls that support Gilboa Dam were also strengthened through the installation of 40 post-tensioned anchors, or steel cables that pull them tight to the bedrock.
NYCDEP manages New York City’s water supply, providing more than 1 billion gallons of high quality water each day to more than 9 million New Yorkers. This includes more than 70 upstate communities and institutions in Ulster, Orange, Putnam and Westchester counties who consume an average of 110 million total gallons of drinking water daily from New York City’s water supply system.
