Upper Occoquan Dam

The Fairfax County Water Authority owns and operates the Upper Occoquan Dam and Reservoir under regulation by the Federal Energy Regulatory Commission (FERC). The dam, a 70-foot-high concrete gravity dam, impounds runoff from a 595-square-mile drainage area to create a reservoir with a surface area of nearly 2.9 square miles. With approximately 8.3 billion gallons of storage, the Occoquan Reservoir is one of two major water supply sources serving nearly 1.7 million northern Virginia residents.

Even though the primary purpose of the dam and reservoir is municipal water supply, FERC has regulated this facility because of a hydroelectric plant at the site. In 2005, as part of FERC’s dam inspection review process, recommendations were made to investigate a vertical structural crack visible on an exterior face of the wall forming the concrete powerhouse intake structure. The investigations concluded that repairs to this crack were necessary to reduce seepage and, more importantly, to eliminate the potential risk of uncontrolled release of the reservoir should catastrophic failure of the cracked wall occur. Located approximately one mile upstream of the town of Occoquan, a sudden failure of the wall could potentially cause severe damage.

Fairfax Water commissioned GFT to address the dam safety-related recommendations in the FERC report, as well as design other water supply-related upgrades. The challenge was to improve the safety and stability of the powerhouse intake structure and reduce seepage without draining the reservoir. Dam safety modification alternatives selected by Fairfax Water included the installation of a PVC geomembrane liner at the upstream face of the powerhouse intake structure. Normally these liners are installed in dry conditions for new or retrofit construction. Divers, with the aid of topside personnel, moved the steel frame against the upstream face as they individually pushed the liner over more than 200 embedded anchor bolts without compromising the effectiveness of the waterproofing system.

Upgrades consisted of modifying three outlet works structures at the dam. The primary modified structure involved a large concrete intake and powerhouse building containing a 1,000-kW hydroelectric station. Removal of the hydro-station allowed for the construction of a new 70-foot-high concrete control tower and related appurtenances to increase outlet works capacity to current industry guidelines for reservoir draining. With hydro-station decommissioning at both the Upper and Lower Dams, our tasks also involved assisting the owner quality control inspection procedure compliance, surrendering its current FERC license, and transferring regulatory jurisdiction to the commonwealth of Virginia