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Spillway gate flaw cuts water levels for Lost Creek

A design flaw discovered this summer in Lost Creek Dam's spillway gates, used only once in their 33-year history, has caused the U.S. Army Corps of Engineers to alter plans for capturing winter freshets until the flaws can be fixed.

Corps engineers say that support mechanisms used when operating the gates pose a slight risk of failure during the worst of flood events, possibly triggering unrestricted flows through the spillway, Corps officials said Friday.

So plans now are to keep the lake's surface level 12 feet lower than full so the flawed gates don't come into play, which will reduce the lake's flood-control capacity, said Jim Buck, the Corps' Rogue basin operations manager.

The result likely will mean longer and higher-volume releases into the Rogue River during winter storms, and more storms likely will cause the Rogue to swell to its banks more often than in recent years, Buck said.

The only time the Corps has relied upon those gates was during the New Year's flood of 1997, and they operated properly at that time, Buck said.

"If it were built to the current standards, it wouldn't be built that way today," Buck said. "In today's environment, it would have been designed differently."

Left closed, the gates pose no immediate danger to downstream residents, and they will not restrict the Corps' ability to fill Southern Oregon's largest reservoir each spring.

"We will be filling," Buck said. "It's just that, if we get that unusual storm, the concern is having to use them with the pressure of a full pool against them."

Fixing all three gates would cost about $7.5 million, and no money has been appropriated for repairs, Buck said.

Buck expects it to take "at least the next few years" before the work can be completed.

Similar flaws exist on spillway gates at the Corps' 1940s and '50s-era dams throughout the Willamette Valley, Buck said. Repair work has begun at the Dexter Dam near Lowell.

Lost Creek Dam was completed in 1977. Its Rogue Basin sister, Applegate Dam, which was built in 1980, has a different gate-operation system and was recently rated as safe, Buck said.

Lost Creek Reservoir is typically drawn down by Nov. 1 so its surface elevation is at 1,812 feet above sea level. That leaves 60 feet of reservoir space to capture runoff from winter storms.

Water released past the dam is funneled either through its powerhouse or through a multi-port intake tower set in the lake.

The radial gates and the spillway were designed to control water once the lake's elevation hits 1,860 feet. The gates then can be opened to control flows down the spillway, ensuring that water doesn't cascade over the top of the dam.

But the pressure of the gates' weight, plus the push of reservoir water against them, could cause enough friction that the gates wouldn't work properly when needed.

Under new "interim risk-reduction" plans completed this week, Corps hydrologists will not fill the reservoir past the 1,860-foot level from November through February, Buck said.

That way the gates will not come into play, but higher releases will be needed during and immediately after freshets to keep the elevation down.

Corps officials have discussed dropping the reservoir level below the 1,812-foot elevation to recoup some of the lost storage capacity, but no decision has been made yet, Buck said.

The agency plans a public meeting in Grants Pass some time next month to discuss these changes with the public, Buck said.

Reach reporter Mark Freeman at 541-776-4470, or e-mail at mfreeman@mailtribune.com.