Slide could block river, flood Yakima Valley farmland

A widening crack in Rattlesnake Ridge south of Union Gap, Wash., threatens to precipitate a massive landslide that could block Interstate 82 and the Yakima River. Authorities have urged about 50 people to evacuate.

UNION GAP, Wash. — A potential 4 million-cubic-yard landslide on Rattlesnake Ridge near Union Gap has prompted U.S. Rep. Dan Newhouse to ask Gov. Jay Inslee to proclaim a state of emergency for Yakima County.

Such a proclamation would be vital for any federal response when the slide occurs.

State and local officials expect about 20 acres of hillside could slide anytime between now and early March and block Interstate 82, the Yakima River and jeopardize irrigation canals serving the thousands of acres of farmland in the Lower Yakima Valley.

However, they believe chances of that are remote, and that it’s far more likely any slide will end up in a large rock quarry that is suspected of being the cause of the slide.

A crack in the surface of the ridge was detected in October and for some time the ground as been sliding at a rate of 1.6 feet per week. It’s occurring in the gap, a natural break in the ridge that separates the upper from the lower valley. The city of Union Gap is immediately to the north.

“As you may know, the valley is home to over 200,000 residents as well as the thousands of commuters to and from the area,” Newhouse wrote to the governor. Newhouse said the river provides invaluable resources to the region from recreation to irrigation.

Disruption to transportation and water infrastructure “poses devastating effects for the state and the region’s overall economy,” wrote Newhouse.

Several residents below the slope were evacuated Jan. 11 when a rainstorm accelerated concern.

Scott Revell, manager of Roza Irrigation District in Sunnyside, said in a worse case scenario, a slide could affect the Roza, Sunnyside Valley and Union Gap irrigation districts. They service more than 166,000 acres of farmland.

A slide could potentially reach the Sunnyside Valley Irrigation District dam just downstream from Parker, he said.

The Roza canal goes through a tunnel and is five miles from the slide area but officials want to divert water from the river into the canal at Roza dam upstream of Selah to reduce flooding if the river is blocked at the gap, Revell said.

The Roza could take only 1,000 cubic feet of water per second and the river is running at 2,500 cfs, he said. Furthermore, the canal’s concrete lining could be damaged if water freezes, he said.

A dozen concrete expansion joints are open right now for improvements and running water would damage those areas, he said, adding he’s trying to speed up that work in case the canal has to be used.

“We view it as a last resort to save human life,” he said.

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