March 26, 2008

Coffer dam

This week the temporary earthen cofferdam that protects the old powerhouse area will be breached, freeing river flow at the confluence of the Clark Fork and Blackfoot Rivers for the first time in 100 years.

The event is scheduled for Friday March 28th at 11:30 a.m. To observe this historic event, the EPA and agencies are encouraging the public to use the Milltown bluff overlooking the Dam and the confluence. See the attached map (PDF) for directions to the bluff. The breach can also be seen via the Dam Cam.

Here’s the sequence of events leading up to this momentous event. First, the Clark Fork River was routed into the bypass channel along I-90 during the week of March 17th.

The temporary earthen cofferdam that has kept the powerhouse area dry will be dug out as much as possible. The steel sheet-piling around it has already been removed. Upstream of the old powerhouse area, a pilot channel is being dug that leads from the cofferdam toward the Blackfoot River.

On Friday, the cofferdam will be notched, and then the level of the reservoir will be raised by closing the radial gate on the dam. When the water rises it will start to flow down the pilot channel toward the cofferdam. As the remains of the cofferdam wash away, the flow of both rivers will spill through this channel, lowering the level of the reservoir by another 12-14 feet.

When the cofferdam breaches, there will be a surge of both water and sediment. Expect to see a very muddy river when this happens, especially near the dam. And as the reservoir drains, the water level in the river below will temporarily rise. The surge of water is expected to be about 4 feet high immediately below the dam, 3.3 feet near Pinegrove, 1.5 feet through most of Missoula, and down to six inches by the time it hits the Bitterroot. The expected surge will hit Missoula about one and a half hours after the dam is breached. (See the attached map.)

The surge won’t really look like a wave – the river will simply rise and fall with a big pulse of muddy water – but river users should be aware that this is happening. EPA is urging people to stay off the river on the 28th, because there may be debris (such as logs) that wash downstream, and they would be hard to see in the muddy water.
For more on the powerhouse removal, see CFRTAC’s Dam News 2008 (PDF) or the EPA press release (PDF).

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