Grant County Public Utility District workers are drawing down water in the reservoir behind Wanapum Dam after a 65-foot-long, 2-inch-wide crack was discovered last week on a spillway section along the bottom of the dam.
Chuck Allen, public affairs officer for the PUD, said the cause of the crack has not yet been determined. A team of engineers is working with the Federal Energy Regulatory Commission to evaluate the crack and determine short- and long-term repair needs.
The reservoir, known as Lake Wanapum or the Wanapum forebay, a technical term for the body of water behind a dam, is being lowered 20 feet below the normal level, from 580 feet to 560 feet, to reduce pressure.
Most irrigation water in the Columbia Basin comes from water behind the Grand Coulee Dam. “A small number” of irrigators withdraw water from the Wanapum reservoir, Allen said. Most of them draw from Rock Island Dam.
“It’s not irrigation season right now, so currently there is no impact to irrigation,” Allen said.
It’s too early to tell whether irrigation will be affected, Allen said.
“We’ve got to find out what’s going to happen, how repairs will be facilitated and what river levels will be when we begin making repairs,” he said.
The crack was discovered Feb. 24, when a dam worker noticed a section appeared to be out of alignment by a few inches, Allen said. Engineers realized that section had moved a couple inches. A diver was sent down to examine the structure beneath the water and discovered the crack. The crack is currently under water, Allen said.
“Safety is our number one priority,” he said. “The amount of monitoring and examination we’re doing of the dam is very extensive and exhaustive. We’re going to make sure everything we do going forward is going to be done in the safest manner possible.”