Snoqualmie Falls Redevelopment

U.S. Society of Dams Project Award

Project continues and enhances engineering legacy of the falls

The Snoqualmie Falls Hydroelectric Facility is over 100 years old and played a pivotal role in the electrification of towns in northern Washington state, including Seattle and Tacoma. The first powerhouse, built in 1899, was the world’s first completely underground power plant. KCB carried out the condition assessment, design, construction services and commissioning. The project involved a combination of upgrading and/or replacing existing generating equipment and structures.

Challenges

  • Keeping and evaluating existing and aging structures in high seismic zone
  • Strict work window for heavy machinery construction in river
  • Transformer building and machine shop were unreinforced masonry buildings with poor seismic withstand capacity and prone to flooding
  • City of Snoqualmie was prone to flooding due to old diversion dam
  • Historic site with significant spiritual value adjacent to Snoqualmie Tribe lands in sensitive ecosystem

Solutions

  • KCB carried out seismic hazard assessment
  • Steel cofferdams designed to 1:100 year flood and installed within work window, minimizing impact on fish habitat
  • Structures were demolished and river bank was realigned; old timber crib retaining wall replaced with new reinforced concrete cantilever retaining wall
  • Diversion dam lowered and widened to provide flood relief to city
  • Historic architect hired during design stage; rock excavations minimized

Successes

  • Facility redevelopment met new Federal Energy Regulatory Commission regulations and license requirements
  • Power plants and weir upgraded to improve efficiency and increase installed capacity from 44 to 54 MW, powering 40,000 homes
  • Structures blended into natural background and site improved for public to enjoy
  • Project continues and enhances engineering legacy of the falls
  • Cultural, historical resources and aesthetic aspects of 110-year-old generating site were preserved

 

Paper:

Douglas, R., B. Bagnérès, E. Portfors and J. Sammet. 2012. “Redevelopment of the Snoqualmie Falls Hydroelectric Project”.