Water pressure on the face of the 50-year-old Wanapum Dam is the likely cause of a large crack, the Grant County Public Utilities District said Wednesday.
Engineers and analysts have determined the 65-foot-long crack was not caused by seismic activity, problems with the dam's foundation, Army activities at the nearby Yakima Training Center or the operation of the dam's spillway gates, according to a Grant PUD press release.
The Columbia River dam is about an hour-and-a-half northwest of the Tri-Cities off Highway 243.
The fracture was detected by divers Feb. 27, three days after a worker noticed a shifting of a spillway pier.
Chuck Allen, public affairs officer for Grant PUD, said divers periodically inspect parts of the dam, but in this case divers were specifically inspecting for the anomaly.
Allen said officials are trying to determine how deep into the structure the fracture extends. It had opened up about 2 inches at its widest point, he said.
Officials continue to conduct investigative drilling to determine where the fracture extends into the dam monolith. Six holes already have been drilled and more are scheduled.
The idea, Allen said, is that each drill hole gets to the point of the fracture. Once they reach the fracture, drilling will continue until reaching a point where the fracture is not located.
"Essentially, the fracture is like a pocket that has developed in the structure," he said.
Reservoir levels have been lowered from 571 feet to between 545 and 541 feet, a 4-foot range allowing for normal dam operations under lowered reservoir levels, Allen said.
An intermediate level of 562 feet is being discussed, he said. That level is the lowest possible for allowing the dam to operate fish ladders normally and to allow reservoir boat launches, currently dry, to be used.
All recreation areas on the reservoir are closed, and Allen said their re-opening likely would be no earlier than June, when evaluation and analysis of what caused the fracture are completed.
At that point, officials will move to a repair and stabilization phase, ensuring the spillway is stable before reservoir levels are raised to the intermediate level.
Allen said officials are uncertain how long repairs will take.
The dam is operating at 50 to 60 percent of capacity, generating between 400 and 500 megawatts instead of the 700 megawatts it would generate if the reservoir were at normal levels.
Allen said operating capacity depends on a number of factors, including the amount of reservoir water and number of generating units operating.
Seven of the dam's 10 generating units are working, with the other three down for routine maintenance.
Water is too low to have water naturally flowing through them, and pumps are being installed to get water to the top of the ladders from the lower reservoir. Water will then flow down the ladders to the downriver side of the dam, allowing fish to be able to navigate the ladders.
Until fish ladders are modified, Allen said area stakeholders, such as tribes and regulatory agencies, would like the dam to conduct a trap-and-haul process, hauling Chinook salmon by truck from Priest Rapids to the Wanapum reservoir.
The ladder modifications will be evaluated and tested, Allen said. If favorable, the trap-and-haul process will not be needed in the future, he said.
The reservoir drawdown also has affected 11 irrigators on the Wanapum reservoir with surface water withdrawal rights, Allen said.
Grant PUD is working with each of them individually on permits and irrigation adjustments, he said.
-- Tri-City Herald intern Matt Benoit is a Washington State University student: 509-947-9277, firstname.lastname@example.org; Twitter: @Matt_Benoit