December 14, 2007
Dam & Powerhouse
The goings-on at Milltown Dam are about to get more dramatic this winter. If everything goes as expected between now and April, we’ll witness demolition of the powerhouse, a 12 – 14 foot drop in the reservoir level, and the first time the Clark Fork River has flowed free at the confluence since 1907.
ARCO contractor Envirocon details how all of this will take place in their recent draft final design report “Stage 2A – Powerhouse and Right Abutment Removal and Stage 2 Drawdown" (PDF). This document is 90% of the way to the final plan, so while there may yet be a few changes and tweaks to incorporate – in fact we hope there are – the report gives us a good idea of how the work will proceed. Check out CFRTAC’s comments here (PDF).
In a nutshell, here’s how it would go:
• The right abutment – the part of the dam adjacent to the powerhouse on the north side – would be removed to an elevation below the existing ground surface
• The shop on the north side of the powerhouse would be demolished and removed, followed by the brick north wall and part of the roof of the powerhouse.
• The generators, weighing 19-24 tons each, would be lifted from the powerhouse by crane. The Milltown Redevelopment Working Group will save one of these for future display.
• A horse-shoe shaped earthen cofferdam would be constructed just upstream of the powerhouse, in the same location where the Montana Power Company built one in 1988 for powerhouse repairs
• A work pad and three-foot berm would be constructed just downstream of the powerhouse
• The remaining walls, floor and roof of the powerhouse would be demolished, working south, and filling the tailrace voids with rubble from the powerhouse and right abutment as the work proceeds.
• Concrete rubble, bricks and timbers from the dam and abutment would be put into an on-site repository located in the park area to the north. We are encouraging the agencies and Envirocon to look for ways to recycle the concrete and usable timbers in order to decrease the size of the repository. All steel would be recycled. The lead-based paint has already been stripped from the powerhouse walls and will be shipped to the landfill in Missoula. Asbestos in the roof will also be removed and disposed of in the Missoula landfill.
• Work pads would then be removed, and the powerhouse footprint would be armored and re-graded to create the Stage 2 channel configuration. This is a temporary channel for the river since the final river channel will eventually be constructed where the spillway currently sits.
• The Clark Fork River upstream of the dam will be redirected into the newly completed bypass channel next to I-90.
• A breach channel will be constructed from the powerhouse/abutment area toward the Blackfoot River to channel and direct river flow toward the temporary Stage 2 channel.
• The cofferdam will be removed.
• The new powerhouse channel will be breached, either by raising the level of the reservoir with the radial gate, or by allowing natural high flows to overtop the wedge of sediment separating the channel from the reservoir. At this point, the reservoir will drain by another 12-14 feet, and the combined flows of the Clark Fork and Blackfoot rivers will run into a temporary channel through the powerhouse footprint.
The channel breach and subsequent erosion as the reservoir level drops will cause the river to be a lot muddier than usual this spring. The good news is that since the Clark Fork will be in the bypass channel, all the eroded sediment will come from the Blackfoot River, which is clean. The muddy water conditions may last for several months until the Blackfoot River erodes to an elevation that is in equilibrium with the new lower level at the dam. There’s no doubt that this stage of the project will have the biggest impact to the river, but it will be short-lived.
Looking forward, the next step will be to construct a cofferdam to isolate the spillway this summer, and spillway demolition will occur in fall 2008. The new permanent channel will be created in this location, and the river will be routed into it in late winter of 2009.