Priest Rapids Dam spillway leak cause identified
Grant PUD/courtesy photo The Grant County PUD has determined the cause of the leaking at Priest Rapids Dam.
EPHRATA — The leaks in spillway supports at Priest Rapids Dam are due to separation between concrete blocks poured at different times during construction. Grant County PUD officials said at least four of the 22 spillway supports are affected.
The investigation was prompted by the results of test drilling done at the dam in March. Utility district officials did some investigation at Priest Rapids as a precautionary measure; a crack at Wanapum Dam discovered in 2014 required repairs that cost about $86 million.
Water leaked into the test holes, prompting PUD officials to lower the reservoir behind the dam. The spillway supports, also known as monoliths, are stable but PUD officials said the problem does merit further investigation, according to a press release from the utility. There is no threat to property or humans.
The problem is a “disbonded lift joint,” the press release said, meaning the pieces don’t fit together as tightly as they did. That has resulted in leaking, about 3 to 4 gallons per minute, at the four joints, which are near the base of the spillway pillars.
“To date inspection drilling has occurred through about half of the spillway and will continue through the remaining monoliths,” the press release said. Drilling should continue about four weeks, said Rich Wallen of the PUD’s power production division at Tuesday’s commission meeting.
“The spillway structure is stable and no movement has been detected," the press release said. Once the investigation and analysis are completed, PUD officials will decide what repairs are needed beyond the drilling, if any are needed at all. "The inspection drilling is reducing pressure from the water inflow through the disbonded area of the joint lift.”
The reservoir has been lowered about 3 feet, still within the normal operating range at Priest Rapids. The reservoir will be maintained between 481.5 to 484.5 feet above sea level (which is how water elevation in a dam reservoir is measured). The reservoir will stay at the lowered level at least until the inspection work is completed. The maximum elevation is 488 feet above sea level.