Nine Mile Sediment Bypass Design-Build Improvements

LOCATION:  Spokane, Washington   |   OWNER:  Avista
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The Nine Mile Hydroelectric Development (HED), initially constructed in 1908, includes a 364-foot-long, 58-foot-high concrete gravity dam; a 225-foot-long spillway at the crest of the dam, a 120-foot-long, 5-foot-diameter sediment bypass tunnel; four intake chambers; a powerhouse integral to the dam with four horizontal Francis turbine-generator units 37.6 MW, and appurtenant transmission facilities.

McMillen Jacobs participated in a collaborative effort to advance the design, advance the schedule, and optimize construction costs. We provided final drawings and specs and self-performed the construction. We managed the dive crews, provided procurement support, and prepared the FERC packages. Design included replacing an existing access bridge, designing a platform for the trash rake storage and operations, designing a new intake structure, upgrading the existing intake deck, replacement of the fixed wheel and knife gates, coating of the SBS pipe, and removal of the existing restriction nozzle on the SBS. We also provided the analysis for hydraulics/hydrology on flow conditions through the new system and plunge pool scour and a geotechnical report with the capacity of the west abutment and support for the Scour Technical Memorandum.

The purpose of this project was to upgrade the existing inoperable sediment bypass system (SBS), install a new SBS intake structure, and to improve debris removal along the remaining powerhouse intakes. Upstream debris/sediment management improvements included primarily 3 features.  (1) The new SBS comprises debris exclusion and an expanded flow inlet, which transitions to a 60-inch diameter pipe routing the turbid water through the powerhouse, and subsequently away from the four generator intakes. The new intake “tower” included a submerged trash rack and solid bulkheads to facilitate debris removal from the SBS intake.  (2) Segmented Worthington Boom installed upstream from the SBS intake. (3) Debris-related aspect involved the setting up of the new Kunz™ trash rake, installed on rails to pull the debris out of main intakes 1 through 4 and from the upstream SBS intake and a submerged trash rack and solid bulkheads to facilitate debris removal.

Challenges included short work windows with an accelerated schedule, working within the constraints of an operating facility, and working-in-the-wet with a diving team. The project as originally designed was to take two construction seasons to complete, McMillen Jacobs team was able to complete the work within a single construction season.