East Fork Hydroelectric Project Spillway Upgrade

The East Fork Hydroelectric Project, owned by Duke Energy, is located on the East Fork Tuckasegee River in Jackson County, North Carolina. It has four high hazard potential rock fill embankments with heights ranging from 140 to 215 feet. The spillway systems at all of the dams are rock cut channel spillways in the abutments, controlled by a single Tainter gate and two earthen erodible fuse plugs. During major floods, the gates would be opened and then closed following each fuse plug activation to prevent sudden increases in flood levels in downstream areas. At Cedar Cliff, the most downstream dam in the chain, unintentional fuse plug activation could occur with storms at or less than the ten-year storm if the gate is not opened according to the operating procedures. Unintended fuse plug activation could cause substantial incremental downstream flood elevation rise. Cedar Cliff currently cannot accommodate the probable maximum flood (PMF) as required by Federal Energy Regulatory Commission (FERC) regulations. Duke conducted meetings with a team including their consulting engineers and the FERC to evaluate the spillway procedures and consequence of unintended fuse plug activation and select the best remediation option to accommodate flood flows up to the PMF and minimize downstream incremental impacts from gate inoperability or fuse plug activation. The team evaluated all potential remediation options identified during the meetings. It was concluded that the best option is to modify Cedar Cliff spillway with the construction of a labyrinth fuse gate system in an expansion of the left rock cut channel, adding an 11-foot-high parapet on the crest of the dam and upgrading the gated spillway hoist system at all of the dams with redundant wire ropes and drive trains to assure Tainter gate operability. Geologic investigations have been conducted and the remediation is now in the design phase. This paper discusses how the need for remediation was determined and the solution found from the regulatory perspective.