Officials with Grant County Public Utility District in the U.S. state of Washington, working with the Federal Emergency Regulatory Commission, removed the non-failure emergency condition at Priest Rapids Dam Oct. 15.
The dam had been operating under the non-failure emergency condition since March 28, when leaking water was discovered in the dam’s spillway monoliths during an investigational drilling program. Out of caution, the reservoir behind the dam was lowered and held in what is considered the normal minimum operating range of 484.5 to 481.5 feet above sea level. The maximum reservoir elevation is 488 feet.
Grant PUD says the emergency never presented a safety risk for the public or its workers. Additionally, operations of its 955.6-MW hydroelectric plant have not been affected.
During the past six months, crews have been conducting more drilling to determine the source of the leaking and also relieve pressure in the leaking area. Workers have been installing more monitoring equipment in the spillway. Through analysis of the spillway in the full operating range of the reservoir, officials have determined the emergency designation is no longer required.
Early in the response, investigators discovered the leaking water was the result of a disbonded lift joint, or seam in the mass concrete making up the spillway’s support structures called monoliths.
The additional drain holes drilled into the spillway monoliths are now reducing the pressure in the leaking area and are allowing Grant PUD personnel to monitor the condition of the joint.
Officials are also conducting a root-cause analysis to discover why the lift joint became disbonded and are determining if and what future remedies are needed.
The 10,103-foot long Priest Rapids Dam — located on the Columbia River 24 miles south of Vantage, Wash. — was completed in July 1956. The facility is undergoing a $198 million refurbishment, with major works including the upgrade and replacement of runners, wicket gates, lubrication systems and other associated components.