Gilboa Dam Stabilization Project
Commissioner Emily Lloyd was in Gilboa on December 14 to mark the end of the Gilboa Dam stabilization project. In the span of one year, the DEP completed a $24 million project to make the dam safe by installing 80 anchoring cables that pin the dam down to the bedrock below.
The stabilization project consisted of four main stages:
- A debris boom across the Schoharie Reservoir to keep debris off the Dam. (Completed December 2005)
- The removal of a large notch from the top of the Dam in order to lower water levels, decrease pressure on the Dam and reduce the likelihood that water levels could rise to the point where the stability of the Dam would be threatened. (Completed February 2006)
- The installation of four large siphons over the Dam to increase the amount of water that can be drained from the reservoir. (Completed March 2006)
- The installation of 80 anchoring cables through the top and the front of the Dam. The cables were placed through holes drilled in the Dam and down into solid bedrock beneath. They were then anchored in place and tightened, creating an anchoring system that will help to hold the Dam in place. (Completed December 2006)
A $300 million full-scale reconstruction of the dam will begin in 2008. That project will add up to 12 feet of mass to the dam, reconstruct the spillway and dam face, install a new diversion tunnel around the dam from the Schoharie Reservoir into Schoharie Creek, and construct a floodgate at the top of the dam that will allow for quick releases of water in order to help control flooding downstream. The reconstruction will also bring the Dam up to the stringent State standards for new dam construction.
The Gilboa Dam Stabilization and reconstruction projects are part of DEP's long standing commitment to upgrade its dams to modern design criteria. As part of a comprehensive program to assess the condition of dams at reservoirs in the Catskill/Delaware watershed, an engineering evaluation in 2005 of the Gilboa Dam at the Schoharie Reservoir showed that the dam did not meet State criteria and could potentially become unstable in an extremely large flood, even larger than the record flood in the region of January 1996. Assumptions that were made in the original design and construction of the dam were found to be outmoded and did not apply to dams as built today.
The following excerpts from the Executive Summary of the Emergency Action Plan presents the general responsibilities of DEP's West-of-Hudson Division during an emergency situation at Gilboa Dam / Schoharie Reservoir:
Emergency Action Plan Inundation Maps
50 Year Storm Event Flooding Scenario Inundation Maps
In an ongoing effort to keep residents updated on the status of the Gilboa Dam and Schoharie Creek, the following are versions of the presentations the agency has been showing at public sessions:
- On December 14, Commissioner Emily Lloyd was in Gilboa to mark the end of the Gilboa Dam stabilization project. In the span of one year, the DEP completed a $24 million project to make the dam safe by installing 80 anchoring cables that pin the dam down to the bedrock below.
- On September 18, DEP Commissioner Emily Lloyd and Schoharie County Board of Supervisors Chairman Earl VanWormer announced progress on a number of issues. DEP committed to paying $370,000 for Schoharie County to install an early warning siren system that would alert downstream residents of potential danger at the Dam. DEP will also reimburse the County for most of its expenses related to emergency planning for the Dam.
- Commissioner Lloyd also announced that New York City will install floodgates in the top of the Dam as part of the full-scale reconstruction that will begin in 2008. The floodgates will be in addition to release works in the base of the Dam that will exceed existing State guidelines for capacity to release water downstream.
- For more information, read the press release here - Schoharie County Board of Supervisors Chairman Earl Vanwormer and DEP Commissioner Emily Lloyd Announce Joint Progress on Several Gilboa Dam Issues
- New York City Deputy Mayor Dan Doctoroff announced on March 13 that the City will implement a comprehensive spill control program at the Gilboa Dam after the full-scale reconstruction project is complete at the dam in 2011. The spill control program will be modeled after programs previously in place at the City's Pepacton and Neversink reservoirs. At those reservoirs, the DEP has maintained voids through the winter equal to one-half of the melted snow pack surrounding each reservoir. If the snow pack is light, then voids have been equal to the amount of water that would runoff into each reservoir from a one-inch rainstorm occurring over a six-hour period. This will be the first time the City has had a spill control program in place at Gilboa. Deputy Mayor Doctoroff made his announcement at a meeting in Schoharie with county and local officials. Doctoroff, DEP Commissioner Emily Lloyd and the county and local officials then traveled to the Gilboa Dam, where they met the media and toured the ongoing stabilization project. View a photo essay of the Deputy Mayor's visit. (Link will appear in a new browser window.)
- A debris boom to keep the work site safe and clear of floating debris was installed on December 22. View a photo essay of the installation. (Link will appear in a new browser window.)
- The contract for the installation of siphons was awarded on January 6 to D. A. Collins Construction. Field work began the next day. Four siphons are now in place with a total capacity of around 500 MGD. View a photo essay of the siphon and small notch installation. (Link will appear in a new browser window.)
- New equipment was crafted in January 2006 that allowed the intakes to the Shandaken Tunnel to be opened wider. The original maximum flow through the Tunnel of 540 million gallon per day (MGD) has been increased to 590 MGD. As water levels in the reservoir drop the flow through the Tunnel decreases steadily.
- A small interim notch was installed at the Dam on February 11 to help keep water levels down. View a photo essay of the siphon and small notch installation. (Link will appear in a new browser window.)
- A much larger notch was completed on February 18. The large notch is 220-feet wide by 5.5-feet high.
- The number of anchoring cables has been increased as the design process has progressed, going from 47 up to 79. The contract for installation of the anchors was awarded in March, and actual anchoring work began April 26. The project was halted for 18 days in June and July following region-wide flooding that affected the Schoharie Valley. When the anchors are complete the Gilboa Dam will meet State dam stability guidelines for existing dams. As of October 12th, 62 of the 79 cables had been installed.
- Surveillance lighting has been installed for continuous nighttime monitoring. A trailer at the site facilitates 24-hour electronic monitoring of the Dam since December 2005. Video monitoring was added in February 2006.
- A joint DEP/Schoharie County OEM project has distributed weather alert radios to residents of the downstream areas.
- Automated monitoring of the snow pack in the Schoharie Reservoir watershed. (February 2006)
- Flow monitoring at the Ashokan Waste Channel in Ulster County. (February 2006)
- Remote computer monitoring of the Dam and of the Gilboa and Prattsville stream gauges. (March 2006)
- New, redundant elevation gauges were installed at the intake chamber where water enters the Shandaken Tunnel from the Schoharie Reservoir. (June 2006)