Avista crews upgrading the Nine Mile
Spillway gate system.
Reservoir to be raised and lowered throughout testing process
Avista has completed installation of new spillway gates at its Nine Mile Dam, and will be testing the new system over the next couple of weeks. Nine Mile Reservoir will be raised and lowered several times during the testing period to ensure the new spillway system is operating correctly.
The upgraded system will give Avista operators the ability to raise and lower the height of the spillway gate at any time, incrementally if needed, in order to maintain the reservoir pool at a more constant level throughout the entire year. The new system consists of metal gates supported by air-filled rubber bladders, and replaces our old wooden flashboard process, which had been in use since 1928 after Avista purchased the dam. In the past the reservoir had to be lowered each summer to accommodate installation of the flashboards.
Operators began inspecting and testing the new system this week. Tests will consist of independent cycles of each of the three spillway gate sections between the full and minimum pool elevation (about 10 feet below full pool). Beginning Sunday, Nov. 14, Nine Mile Reservoir was raised to full pool, and over the test period it will be gradually lowered and raised several times to allow for observation of the spillway gates throughout the minimum to full pool elevation range, in order to ensure the spillway gates are fully operable.
The public will see several fluctuations of the reservoir levels during this testing period, and should stay out of the section of the Spokane River above Nine Mile Dam to Plese Flats. Water levels and conditions around a dam are subject to change at any time, and river users are reminded to use caution on the water and comply with all posted notices and closures.
The reservoir level should be back to full pool by the first part of December 2010. Currently, no change in the full pool level is planned after the new spillway has been tested. To the extent possible, Avista will maintain the reservoir level at the normal full pool elevation year around; however, there will be times when the reservoir will need to be drawn down for maintenance purposes.