EPHRATA — A crack in a spillway pier at the Wanapum Dam on the Columbia River is a serious situation, but not a threat of downstream flooding even in a worst-case scenario, a spokesman for the Grant County Public Utility District said.
The 2-inch-wide underwater crack extends horizontally across the upstream side of the 65-foot-wide pier called a monolith. It’s one of 12 monoliths on the spillway.
“Say this section were to fail completely,” spokesman Tom Stredwick said Monday. “The remainder of the spillway would remain intact and with the current amount of water in the river, the water through that section of the dam would still be normal for this time of year.”
The problems may arise in managing the river flow and power production from the network of Columbia River dams.
“We’re in a coordinated river system,” he said.
The utility is working with the Federal Energy Regulatory Commission and the Bonneville Power Administration to determine the long-term impacts, Stredwick said.
The first step is to assess the damage and determine what it will take to repair the Wanapum crack.
The reservoir behind the dam is being lowered 20 feet. That should be completed late Monday or early Tuesday, relieving pressure and helping with the inspection, Stredwick said.
Pressure caused a slight bowing in the dam that was first detected Feb. 24 by a staff member who noticed a curb on the road on top of the spillway was out of alignment. Engineers sent down divers who discovered the crack Thursday, 75 feet below the waterline. The crack extends all 65 feet across the monolith, which is 126 feet tall and 92 feet thick. Stredwick doesn’t think the crack extend all the way through the pier.
Continuous surveying shows no additional bowing in that section of the dam, he said.
The 51-year-old dam is a mile long, spanning the Columbia about 5 miles downstream of where I-90 crosses the Columbia River at Vantage in Central Washington. Its reservoir extends about 40 miles upriver to the Rock Island Dam, near Wenatchee. The next dam about 20 miles downriver is the Priest Rapids dam, near the Hanford nuclear reservation.
The Wanapum and Priest Rapids dams are both owned by the Grant County PUD and operated under federal licenses.
The Wanapum Dam can generate 1,092 megawatts, enough to supply 900,000 homes. The utility has 46,000 residential, business and farm customers.
