A Union Gap Irrigation District pipeline running through the site of a slow-moving landslide on the western slope of Rattlesnake Ridge is damaged and will not be usable for next year’s irrigation season, officials say.
The concrete pipe, which is buried in a hillside roughly 50 feet above Thorp Road, irrigates 3,222 acres of land between the gap and Cheyne Road, north of Zillah. It serves about 170 customers, said district manager Patrick Andreas. District officials believe the pipe was damaged sometime within the last year as a result of the estimated 8 million tons of rock and soil inching down the hillside just east of Union Gap.
“Instead of being round, the pipe is now egg-shaped, and there’s huge cracks,” Andreas said.
Photos of the pipe show what appear to be at least three long cracks along the pipe’s walls, as well as chunks of cement and grout on its floor. Ray Wolverton, a facilitator for the district and a former member of its board, was among those who discovered the damage. He said the cracks are about 150 feet long and deep enough to stick a hand into.
“I was surprised it was that bad,” he said.
District employees first suspected the pipe was damaged in July after two mudslides in the area, which indicated the pipe might be leaking. The cracks were discovered in late October.
Dave Norman, state geologist with the Department of Natural Resources, said his department doesn’t have enough information to determine if the landslide caused the damage to the pipe.
The damage has serious implications for growers who use its water to irrigate their fields, Wolverton said. If the district doesn’t implement a plan to resupply water to those affected by March — when the growing season typically starts — those farmers could experience significant crop losses, he said. In a report Wolverton authored about the cracks, he estimated the annual crop value of the area at $40 million, and estimated that the land, orchard and vineyard value is between $40 million and $60 million.
The Union Gap Irrigation District canal diverts from the Yakima River through the Selah gap, then runs through Moxee and around Rattlesnake Ridge before ending just north of Zillah. Andreas said the plan to resupply water to those between the ridge and Zillah is two-fold: First, they’ll divert water from the Sunnyside Irrigation District canal through a new pipeline and intricate system of pumps, which will return water to the lower portion of those affected. Then they’ll divert water from the Roza Irrigation District canal through a newly built pipeline to return water to the upper portion.
Wolverton said he isn’t sure if the plan will be fully implemented by March.
“If we had another four months, I would say I don’t have any doubt it will be done,” he said.
He said there were delays in getting the project off the ground, and the district is still working on a plan to supply electricity to the new pump systems. Officials would not provide an estimated cost for the plan, saying not every detail had been finalized. They speculated it could be in the millions.
The line was in use this past irrigation season.
The landslide was detected in October 2017.
As geologists studied the slide and tried to predict its impact, officials with the Yakima Valley Office of Emergency Management rushed to prepare for what — at the time — was thought to be an impending landslide.
About 60 people were evacuated from homes at the base of the ridge, but after a Seattle-based consulting firm hired by the state determined the slide didn’t pose a danger, they were allowed to return. That firm, Wyllie & Norrish Rock Engineers, confirmed what other geologists studying the slide at the time were reporting: That the slide was unlikely to become a fast-moving, catastrophically damaging landslide.
Geologists say the 20-acre, 200-foot-deep mass could continue sliding for years, if not decades, and will likely fall bit by bit into the quarry pit at the slide’s base.
Dozens of GPS units and seismometers remain on the hillside as geologists continue to monitor the slide. In a report released Monday, geologists with the state Department of Natural Resources say the slide continues to move, but slowly. Different parts of the slide move at different speeds, and speeds range from 0.02 feet per week to 0.94 feet per week.