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County to replace stormwater pipe on portion of South Wenatchee Avenue

Chelan County Public Works will replace about 650 feet of stormwater pipe in the South Wenatchee area after a second sinkhole in two months formed near Boodry Street.

The new sinkhole was discovered Wednesday by a county road crew when it was moving a piece of equipment used for paving. The sinkhole formed on the shoulder of the roadway and is marked with caution barriers and signage and is covered with a steel plate.

In late June, a large sinkhole formed in the middle of South Wenatchee Avenue – south of the new sinkhole and before the bridge between Boodry and Beuzer streets – after a period of heavy rains. That sinkhole was secured by a fence and a detour was put in place.

In response to the first sinkhole, Public Works closed the road near the bridge over Squilchuck Creek for the long term, with the goal to move up construction on a grant-funded project already planned in the area.

The Surface and Stormwater Management Utility last year received a $755,000 grant from the state Department of Ecology to replace the stormwater pipe in this area in 2021.  The project, which is under design, will not only improve stormwater conveyance along South Wenatchee Avenue where the sinkholes are but also improve water quality flowing into Squilchuck Creek. 

“We had hoped the original sinkhole was localized, but this second sinkhole exposed other impacts along the stormwater pipe,” said Eric Pierson, director/county engineer of Chelan County Public Works. “It’s become apparent we need to replace culvert before the grant dollars are available. It’s our goal to have it replaced before winter.”

The cost is expected to be in excess of $200,000.

Sinkholes occur when the surface layer of the ground collapses inward. One sign that a sinkhole may be forming is the appearance of new cracks in a roadway’s surface. They are more common after heavy rain and flooding. Roads built above drainage culverts and bridges are especially susceptible to sinkholes after rainy periods because they may be eroded by the constant flow of water and debris.

Since late June, there have been three sinkholes found on county roads, including another on Burch Mountain Road in north Wenatchee. The Chelan County road department doesn’t see many sinkholes, so this has been an abnormal summer to have three, said Brad Harn, road supervisor for Chelan County Public Works.

“When we experience flashfloods and flooding in Chelan County, we deal with more road damages – washouts and shoulder damages – than sinkholes,” Harn said. “Most of the sinkholes we see are a result of failing irrigation systems or failing stormwater culverts that were installed decades ago.”

The sinkholes on South Wenatchee Avenue and Burch Mountain Road all resulted from failing stormwater culverts that were impacted this year by abnormal amounts of high intensity rain.

The National Weather Service in Spokane reports 3.77 inches of rain has fallen since June 1 at its monitoring station at the Wenatchee water plant. This is the most rain that’s fallen during this period since 1948, when 4.47 inches fell. The Wenatchee area has seen three times the average rainfall for this time period, which typically gets 1.34 inches of rain, according to the National Weather Service.

On South Wenatchee Avenue, the sinkholes are in the vicinity of the conveyance system’s trunk line (the main stormwater culvert that runs through that area). The existing system is deteriorated beyond repair. The soil conditions over time have corroded the 37-year-old trunk line, Pierson said.

On Burch Mountain Road, the condition of the stormwater conveyance system is different than that of South Wenatchee Avenue. Cameras show the culvert is intact; however, the bottom of the culvert is scoured from the sand and velocity of the water flowing through the system, Pierson explained.

A portion of stormwater culvert was replaced last week below the Burch Mountain Road sinkhole and the road was repaired.

Last Updated: 10/17/2019 10:27 AM

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