Best Water/Environment: Gilboa Dam Rehabilitation

Deterioration of cut stone spillway June 2004

The Gilboa Dam, a vital element of New York City’s water system, provides the city’s residents with 14% of their water needs as well as supplying water to several upstate communities.

The New York City Dept. of Environmental Protection enlisted a joint venture of Barnard Construction Co. and D.A. Collins Construction Co. to rehabilitate the 88-year-old dam beginning in 2011. The objective of the project was to reconstruct the spillway control section to address existing deterioration, improve hydraulic flow and enhance structural stability. The work restored all features to their original functionality, upgraded the dam and provided an enhanced safety monitoring and reporting system that will include a safety report to be completed four times a year.

Modifications made to restore the dam included reshaping the spillway control section with nearly 66,000 cu yd of mass concrete; reconstructing the spillway channel slabs, plunge pool slabs and their under-drain systems with 46,000 cu yd of reinforced concrete; and building a new drainage and inspection gallery.
Modifications were also made to the existing spillway walls in order to stabilize the structure, and alterations were implemented to ensure overall safety.
In order to continue providing water to the 9 million residents served by the Gilboa dam, roughly 80% of the rehabilitation was completed in an active spillway. This created a challenging work environment, given that the spillway often rose 35 ft a day.

To circumvent these difficulties, the project team developed a sequenced diversion of water and emergency operations plan, continuously diverting between 8,100 cu ft per second (cfs) and 16,200 cfs of water around the work areas at any given time during the project.

The project was completed one year ahead of schedule, despite the added obstacle of flooding caused by Hurricane Irene. The storm caused an eight-month delay and added $19 million to the total project cost.