Corps to blast 38-year-old concrete plug

From the February 6, 2002 edition of McKenzie River Reflections

BLUE RIVER: The U.S. Army Corps of Engineers will remove the last of a 35.5-foot-long concrete plug during the week of February 4 from a diversion tunnel through Cougar Dam. The work is part of the multiple-phase temperature control construction at the dam, which is on the South Fork of the McKenzie River.

When Cougar Dam was being built in the 1960s, the diversion tunnel was used to divert river water away from the construction area, giving crews a dry area in which to work. Upon the dam’s completion in 1963, however, the tunnel was plugged with about 550 cubic yards of concrete and has not been used since.

The plug removal - also called a lake tap - is designed as a well-choreographed exercise in controlled blasting techniques. More than two-thirds of the plug has already been removed in previous blasts. The final blast, however, is expected to be the most critical since it will be the one that actually clears the tunnel.         The work is being performed under a contract with ASI RCC, Inc., of Buena Vista, Colorado. In the summer of 2001, ASI began work on a $9.8 million contract to install regulating gates inside the diversion tunnel, reline it to maintain its structural integrity, and then remove the plug.

When the blast removes the plug, water currently being stored behind the dam will again be diverted through the tunnel for about 45 minutes to flush sediment from the tunnel. The re-opened tunnel will be used to manage flows on the South Fork of the McKenzie River during the three-year construction of the tower, according to Corps public affairs specialist Heidi Helwig.     “On about March 1, the Corps will continuously divert water through the tunnel until reservoir levels are about 157 feet below normal winter operating levels,” Helwig said. “This will allow workers to begin construction on the water intake tower, which will include attaching a new weir system to the face of the existing tower. As a result of the drawdown, all boat ramps on Cougar Reservoir will be closed during the three years of construction.”

The new weir system is designed to allow dam operators to draw water from differing depths in the reservoir, while mixing it to the optimum water temperature for release below the dam. The changes will improve river conditions for salmon and resident fish in the McKenzie River, matching natural conditions needed by the fish.         

To minimize impacts to fish during construction, the Corps, in coordination with the U.S. Fish and Wildlife Service and the Oregon Department of Fish and Wildlife (ODFW), will maintain a residual pool above the dam. It also will collect adult bull trout-listed as a threatened species under the Endangered Species Act below the dam and transport them to release sites above the reservoir. In addition, the Corps has been working closely with ODFW, the U.S. Forest Service and the U.S. Geological Survey to monitor water quality and fish health throughout construction.

Following completion of the Cougar Dam upgrades, the Corps is scheduled to add another water temperature control structures to Blue River Dam. The work at both locations was authorized as part of the Corps’ Willamette Temperature Control Project.

Image above: A 50-ton steel bulkhead was removed from Cougar Dam’s diversion tunnel last year. Since that time, crews excavated and re-lined the old tunnel while installing two new sets of gates to control water flowing out of the reservoir. This week, the last seven feet of a 35 foot long concrete plug was to be blasted out by explosives.