SEATTLE — A massive crack in a major Columbia River dam poses enough of a risk of dam failure that Grant County authorities have activated an emergency-response plan.
Officials said there is no threat to the public from the crack in the Wanapum Dam, which is just downstream from where I-90 crosses the river.
But utility managers are lowering water levels a total of 20 feet because they fear the structure otherwise could endanger inspectors trying to get a better handle on how seriously the dam is damaged.
“At this point we already know there’s a serious problem,” said Thomas Stredwick, spokesman for the Grant County Public Utility District. “We want to make sure the spillway is stable enough that inspectors are safe when inspecting it.”
Earlier this past week, an engineer noticed a slight irregular “bowing” above the spillway gates near where cars can drive across the dam.
When divers finally took a look under water they found a 2-inch-wide crack that stretched for 65 feet along the base of one of the dam’s spillway piers.
After analyzing the data gathered by the divers and plugging it into computer models, the PUD determined late Friday afternoon that the failure risk was high enough that they needed to officially start notifying other government agencies and downstream water users.
“This is a situation that’s really changing as more information becomes available,” Stredwick said. “But there’s no immediate threat to public safety.”
Wanapum, just below The Gorge Amphitheatre and the hamlet of Vantage, is in a rural area. Failure would primarily impact fisherman, orchardists, farmers, boaters — and, of course, power generation. Wanapum currently can generate more than 1,000 megawatts of power.
PUD officials have lowered the water 6 feet behind the dam since discovering the problem, leaving many boat ramps above the dam inaccessible.
Authorities plan to let water levels drop another 14 feet by Monday.
So far the PUD has been able to continue meeting all of its power needs, but Wanapum is such a big electricity generator the utility may ultimately have to turn to buying power on the open market.
Even if the dam doesn’t fail, the significance of the damage is likely to require extensive repairs and that, too, could impact the entire Columbia River system.
“All these dams coordinate to generate energy on a regional scope,” Stedwick said. “If Wanapum is impacted, that has impacts on dams up stream as well as below.”
Officials with the Bonneville Power Administration declined to comment on the potential impact to power generation because they did not want to unduly influence energy markets.
But Kevin Wingert, a BPA spokesman, said the immediate impact would be an increase in flow from Priest Rapids Dam downstream, which would temporarily exceed the low flows needed to protect chinook salmon redds (nesting holes) through the Hanford Reach area.
He expected flows to return to normal once the drawdown was completed.
Wanapum Dam was built in 1959 and is more than a mile long. The piers supporting its 10 spillway gates are each 65 feet wide, 126 feet tall and 92 feet deep.