Lower Baker Dam Floodwall

LOCATION:  Washington   |   OWNER:  Puget Sound Energy (PSE)
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The Baker River Project consists of two hydropower developments, Upper Baker Dam and Lower Baker Dam. Lower Baker Dam, which was constructed in 1925, consists of a 285-foot-high (87-meter-high) concrete arch dam. The dam created Lake Shannon, a 7-mile-long (11.2-kilometers-long) reservoir with a surface area of about 2,200 acres (8.9 km2). Average annual inflow to Lake Shannon is about 2,700 cfs (76.5 m3/s) and can change quickly depending on power generation requirements. The reservoir level typically fluctuates up to 65 feet (20 meters) in any given year. The Lower Baker Development was constructed prior to development of federal licensing regulations for hydroelectric projects. Currently, it is applying to become a Federal Energy Regulatory Commission (FERC) licensed facility.

McMillen Jacobs designed a floodwall to raise the sides of Lower Baker Dam approximately 17 feet (5.1 meters). The purpose of the floodwall is to contain the maximum water surface elevation for an extreme flood event, and to direct potential overtopping flood flows to the center section of the dam.

The dam was analyzed using a 3D finite element model in SAP2000 to evaluate the potential impacts to the existing dam from a PMF (probably maximum flood. The model included the concrete portions of the dam, as well as representative bedrock elements to model the soil-bedrock interaction. The model was evaluated for stresses and deflections resulting from gravity, hydrostatic, seismic, and air and water temperature loading. The model was then compared to previous analyses to validate the results.

The full 12-foot (3.6 meter) thickness of the arch will be raised with parapet walls extending an additional 3.5 feet (1 meter). The section of the floodwalls adjacent to the spillway bays were designed to be shaped with typical entrance curve geometry to provide a smooth flow transition in the event of overtopping during an extreme flood. The new construction will allow for future access to the dam crest by small vehicles, and the depression upstream of the east floodwall will be filled as necessary to provide vehicle access to the floodwall. The design will also allow for future improvements to the dam crest and spill gates.

McMillen Jacobs also provided site characterization and geotechnical design services. Our team developed a multi-stage site investigation that included review of previous exploration data, a low-cost geophysical investigation to develop general soil and rock conditions, followed by a supplementary drilling program that targeted data gaps and critical foundation areas. We developed a geologic model of the site and performed geomechanical and seepage analyses to evaluate the abutment stability. Based on the results of these analyses, we developed final design recommendations for the structure foundations, seepage cutoff, excavation requirements, and slope stability.

We also performed a scour investigation that included roped rappelling off the dam crest to access exposed rock outcrops near the toe of the dam. The information we collected was used to develop rock mass characteristics, evaluate scour potential, and analyze kinematic stability.

Our teams participated in the following:

  • Owner’s engineer
  • Geologic characterization
  • Rock mechanics
  • Seepage analysis
  • Abutment stability analysis
  • Design
  • FERC/BOC coordination
  • Geologic modeling
  • Construction (upon FERC approval)