Nine Mile Dam Sediment Bypass Design-Build

Conceptual design, Engineering services, Final design, Environmental/permitting, Self-performed construction, Engineering during construction, Testing & commissioning
Alternative delivery (D-B, progressive D-B, CM/GC, CMAR, EPC), Professional services
Spokane, WA
Avista Corporation
why it matters

Nine Mile Dam's Sediment Bypass System (SBS) was rendered inoperable in 2010 due to debris accumulation caused by the installation of Obermeyer gates. Despite temporary clearance, the SBS became blocked again in 2011 and ceased to operate. Avista Corporation investigated the issue and proposed solutions involving increased flow rates and a robust debris-clearing system. Improving the SBS and safeguarding the turbine was essential for the efficient and safe operation of the dam, reducing the risk of damage, downtime, and costly repairs.

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As the design-build lead, McMillen played a key role in this collaborative project to design and construct this SBS system. Our involvement encompassed various aspects, including providing final drawings and specifications, managing dive crews, offering procurement support, and preparing Federal Energy Regulatory Commission (FERC) packages. The project focused on upgrading the existing SBS, installing a new SBS intake structure, and improving debris removal for the powerhouse intakes.

The modifications to the SBS were designed to redirect the silty and sandy bedload of the Spokane River, minimizing potential turbine damage caused by sediment. The new SBS featured debris exclusion measures and an expanded flow inlet, which directed turbid water through a 60”-diameter pipe, diverting it away from the generator intakes. The new intake structure incorporated a submerged trash rack and solid bulkheads to facilitate efficient debris removal. We also managed the coating of the SBS pipe.

A segmented Worthington® boom was installed upstream from the SBS intake to further enhance debris management. This cable-mounted boom was strategically positioned to redirect surface debris away from the dam's west wing wall and the natural eddy. The downstream boom cable was connected to a sliding mounting arm attached to the new SBS intake tower, ensuring continuous exclusion of surface debris and directing it downstream, away from the SBS intake.

Additionally, we implemented a new Kuenz™ trash rake system, installed on rails, to extract debris from the main intakes and the upstream SBS intake. A submerged trash rack and solid bulkheads were also incorporated to facilitate efficient debris removal. The collected debris was then disposed of in nearby trucks for proper removal.

Additional work included:

  • Replacement of the pre-cast access bridge with a steel bridge
  • Installation of a 60” motor-actuated knife-gate valve inside the powerhouse
  • Removal and retrofit of the existing restriction nozzle on the SBS outlet
  • Confined space entry into the sediment bypass pipeline (60” pipe) to perform sandblasting removal of the existing pipeline lining system and application of the new industrial pipeline lining system 
  • Development of an extensive safety plan that addressed monitoring access, full ventilation system, air quality monitoring, etc.
  • Design, installation, and integration of controls on the sediment bypass knife gate valve, wheel gate, and water level transducers
  • Hydraulic power unit manufacturing, installation, and component connection for a new emergency wheel gate
  • Updates to emergency lighting
  • Management of divers and underwater welders
  • Demolition of the existing concrete intake deck, pre-cast access bridge, and steel building
  • Environmental protection measures including conversion to bio-friendly construction fluids, redundant containment measures for concrete work, and detailed pre-work plans

Through our comprehensive contributions to this project, we successfully upgraded the SBS, improved debris management, and enhanced the overall efficiency and reliability of the powerhouse intakes. Our expertise in design, construction, and project management allowed us to deliver a solution that ensures the longevity and optimal performance of the facility.


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