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Spokane River

Nine Mile Dam Spillway Improvement Project starts this summer   

Tags: Avista Utilities, Electricity, Infrastructure upgrades, Washington, Hydro power, Renewable Energy, Spokane River

This summer and fall, Avista will replace the wooden flashboards at Nine Mile Hydroelectric Development with operable spillgates. The upgraded system will allow Avista to pre-program desired heights and give operators the ability to raise and lower the height of the spillway at any time, incrementally if needed, so that we can maintain the reservoir pool at a more constant level throughout the entire year.

Click to enlarge graphic
Click above to enlarge graphic.
Click to enlarge graphic
Click above to enlarge graphic.
What will the new spillway consist of?
The new spillway will consist of metal gates supported by air-filled rubber bladders. Compressors will inflate the bladders to approximately 25 psi to raise the gates. When stream flows are high enough, the bladders will deflate to lower the gates so water can flow over the top. The new, automated technology will be much more flexible and precise when managing the reservoir elevation.

What is the construction plan and timeline?
While the timeline is subject to change, the construction period is anticipated to be from July through October 2010. The new spillgates have been ordered and we expect delivery in July.

Once the new spillgates are delivered, we will remove some concrete from the top of the existing dam and replace it with new concrete to provide a good foundation to erect the new gates. A trough will be excavated in the dam crest for the compressed air lines that will be used to inflate the bladders.

The reservoir level must be lowered during construction to erect the new spillgates. During this period, the pool level will be 2 to 4 feet below the spillway crest, which may bring the level lower than what it has been in the past when the flashboards were erected each year in mid-summer. Construction should be complete and the reservoir level back to full pool by November of 2010.

How will the new spillway change Nine Mile operations?
No change in the full pool level is planned after the new spillway is installed. To the extent possible, we will maintain the reservoir level at the normal full pool elevation year around.

In the past the reservoir had to be lowered each summer to accommodate installation of the flashboards. When the flashboards were pulled in early winter or spring of each year, the material was lost and had to be replaced. Additionally, erecting the wooden flashboards each year is a labor intensive operation. The new spillway will result in less regular maintenance for Avista and enable the generation of additional power.
For project information, contact Steve Schultz, Project Manager, Senior Engineer.
Posted by  System Account  on  6/17/2010
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