Hydro Review

Snoqualmie Falls Plant No. 2: Restoring a Renewable Resource

The second powerhouse at the 44.4-MW Snoqualmie Falls project began operating in late 1910. Redevelopment work currently under way at the site will add 10 MW of capacity by 2013. Snoqualmie Falls No. 2 is the most recent inductee into the Hydro Hall of Fame.

By Elizabeth A. Dubreuil and Roque Bamba

Puget Sound Energy's (PSE) 44.4-MW Snoqualmie Falls Hydroelectric Project, about 30 miles east of Seattle in the foothills of the Cascade Range, features two powerhouses. Plant 1 began operating in 1898 and was inducted into the Hydro Hall of Fame in 1998.1 Plant 2, about a quarter-mile downstream from Plant 1, began operating in 1910. It is one of this year's Hydro Hall of Fame inductees.

Both Snoqualmie Falls plants are undergoing major upgrades to their aged infrastructure. When completed in 2013, this work will add 10 MW of capacity to the project. The redevelopment ensures that the words of Charles Baker, who first developed power generation at Snoqualmie Falls, will ring true:

"... Snoqualmie ... will stand for centuries, and for centuries to come it will still have the same duty to perform and perform equally faithfully. The same rock chamber will be there and will contain generation after generation of new water wheels and generators, each better and more extensive than their predecessors, with the same never ceasing din of industry converting the waste energy of Nature to the uses of mankind."

Developing the Snoqualmie Falls site

In the 1890s, a Cornell-educated civil engineer named Charles Baker contemplated harnessing the energy at Snoqualmie Falls for electricity production. Initially, Baker thought power from the 270-foot waterfall would serve only nearby uses. But as transmission technology advanced, he refined his plan to include markets in the growing Puget Sound cities of Seattle and Tacoma.

Baker convinced his father, prominent Chicago businessman William T. Baker, to bankroll the venture. The son's plan, never before accomplished, was to build the plant completely underground so as to protect its equipment from the wet winters of the Pacific Northwest and the near-constant spray of the falls. Under Charles Baker's supervision, construction of Plant 1 at Snoqualmie Falls began in April 1898. This plant contained four Pelton turbines and first generated electricity in late 1898. Commercial power production began on July 31, 1899. The plant's capacity was 6 MW, transmitted at 32,000 volts to Seattle, Tacoma, Everett, and small towns in between.

Baker always anticipated adding a second plant at the site. However, owing to his father's death and a series of business missteps, Baker lost ownership of the Snoqualmie Falls facility in 1903.

The chief rival of Baker's Snoqualmie Falls Power Co. was Seattle Electric Co., formed in 1900 by Boston-based engineering consultants Charles A. Stone and Edwin S. Webster. Seattle Electric consolidated most of the lighting, traction, and related subsidiary businesses in Seattle. Before long, Stone and Webster would own the Snoqualmie Falls project.

But first, in 1905, the newly formed Seattle-Tacoma Power Co. (which evolved out of Baker's holdings after his departure) added a fifth Plant 1 unit, with a Francis turbine. This addition, which filled the remaining space in the powerhouse, increased capacity of the plant by 5 MW.

A few years later, the company asked consulting engineer W.A. Brackenridge to recommend the most practical and efficient way to further increase power output at Snoqualmie Falls. Brackenridge developed four possibilities, three involving modifications to Plant 1. The fourth, Brackendridge's preferred option, called for a new plant down river from the falls. He believed a new plant provided the following advantages:

  • Ability to use the total available head;
  • No risk of damaging existing machinery (from expanding the subterranean powerhouse);
  • No construction delay waiting for completion of a new penstock shaft; and
  • Ability to accommodate future enlargement.

To convey water to a new powerhouse below the falls, Brackenridge recommended building a tunnel, running from just upstream of the falls to a forebay a quarter-mile downstream, perched high above the powerhouse. By constructing penstocks from the ridge-top forebay to a powerhouse on the river's edge, he calculated, the new plant could achieve nearly the same head as Plant 1's vertical, 260-foot-long penstock.

 

Brackenridge also suggested that the tunnel be constructed large enough to accommodate future expansion of Plant 2. Seattle-Tacoma Power, now under Stone and Webster's ownership, proceeded with construction of Plant 2 and related structures. Based on drawings by Stone and Webster, Plant 2 was built in 1910.

Plant 2 boosted Snoqualmie Falls' capacity by 9 MW. The new plant would take advantage of the fixed-crest diversion dam Baker had built a dozen years earlier, just upstream from the falls, to serve Plant 1. But a second intake had to be constructed above the falls, on the opposite side of the river from Plant 1's intake, to divert water into Plant 2's conveyance tunnel. The tunnel emptied into an open forebay, which in turn tied into a 7-foot-diameter penstock below the gatehouse. Through the penstock, running down a steep hillside, water reached the new aboveground powerhouse.

Construction of Plant 2 required impressive engineering ingenuity. To remove debris and rock and deliver equipment and tools, engineers constructed a narrow-gauge railway. The railway branched off the mainline that served Plant 1 during its construction, then crossed the river (via a ferry) upriver of Plant 1's machine shop and warehouse.

To allow for the possibility of future expansion, Plant 2 was designed and built with one end wall of temporary construction, finished with galvanized iron. And Plant 2's intake, tunnel, and forebay were sized for installation of three 10,000-horsepower units, even though the powerhouse initially housed just one turbine. Finally, although a single Plant 2 penstock was installed in 1910, the ridge-top gatehouse was designed with space for adding two more penstocks in the future.

The first unit in Plant 2 reportedly began operating at the end of 1910.

Expanding Plant 2

In 1956 and 1957, the Plant 2 powerhouse was expanded by PSE-predecessor Puget Sound Power and Light Co., which over the previous 40-plus years had acquired dozens of small utilities and numerous power-generating plants across western Washington State. Puget Sound Power and Light added five more bays to Plant 2's original four-bay structure and installed a new vertical turbine with more than double the output of the two horizontal turbines earlier planned for installation by Stone and Webster. This expansion also included installation of a second penstock with a larger capacity than the original 1910 penstock.

With its added turbine, the expanded Plant 2 had a capacity that doubled that of the five older Plant 1 units. At this point, the combined total capacity of both plants was 44.4 MW.

Snoqualmie Falls Plant 2 today

Today, Snoqualmie Falls Plant 2 consists of a concrete intake structure just upriver from the crest of the falls. This intake structure, with a capacity of 1,800 cubic feet per second, features two 10-foot head gates, a bar rack, and mechanically cleaned trashracks. Water flows 1,215 feet through a 12-foot-diameter underground tunnel lined with unreinforced concrete. The tunnel conveys diverted water to a 100-foot-long forebay between the tunnel exit and gatehouse.

The gatehouse, which controls water flow into the three penstocks, is a concrete building with a bar rack, mechanically cleaned trashrack, and three 8-foot head gates. The penstocks are surface-installed on piers between the gatehouse and Plant 2 powerhouse. The penstock that supplies Unit 1 is 7 feet in diameter and 600 feet long, of riveted construction. The penstock that supplies Unit 2 is 7 feet in diameter and extends 75 feet from the gatehouse. At that point, both penstocks combine into one 10-foot-diameter, 515-foot-long penstock running to the powerhouse.

The powerhouse is a flat-roofed, rectangular concrete building 46 feet wide by 120 feet long by 40 feet high. It contains Unit 1, a horizontal Francis unit with a horsepower of 10,000 that was installed in 1910, and Unit 2, a vertical Francis unit with a horsepower of 28,000 that began operating in 1957.

In 1991, the owner replaced the runner in Unit 2 because it was near the end of its useful life. The generator was rewound in 1995 because of damage caused by a fault on the transmission system. And in 1997, Puget Sound Power and Light and Washington Energy (Washington Natural Gas) merged to form PSE.

The Unit 2 runner was replaced again in 2004 because of damage resulting from the quantity and type of sediment passing through the units.

The current Snoqualmie Falls No. 2 powerhouse (at left) contains a single turbine-generator unit with a capacity of 9 MW. The proposed redevelopment (at right) will house two turbine-generator units.

Expanding for the future

While Snoqualmie Falls' power-generating infrastructure has performed reliably for more than a century, most of the facility's key equipment and structures are showing their age. But that is soon to change.

Under a 40-year operating license issued by the Federal Energy Regulatory Commission in 2004 (and amended in 2009), PSE suspended generation at Snoqualmie Falls in the spring of 2010 to launch a major redevelopment. The construction work, scheduled for completion in 2013, involves upgrades to aging energy infrastructure at Plant 1 and Plant 2. When complete, this work will add 10 MW of capacity, bringing the project to a total of 54.4 MW. The improvements are expected to cost about $240 million.

Barnard Construction of Bozeman, Mont., is general contractor for the work. Klohn Crippen Berger of Vancouver, British Columbia, is the design engineer. Other companies involved include GeoEngineers (environmental compliance) and Kleinfelder (quality assurance).

Work to be performed includes:

  • Intakes. New intakes will convey water to both powerhouses. The structures will include a trashrack, coarse- and fine-debris trashrakes, gates, gate guides and hoists, gate controls, personnel access, air vents, covered gate slots, and handrails.
  • Tunnel and forebay to Plant 2. The tunnel's concrete lining will be removed, and temporary stabilization support will be provided. The rock surface will be geologically mapped, the tunnel floor lowered, and a new concrete lining installed. Shotcrete lining is planned to help increase capacity. The open-cut rock forebay will be widened at the downstream end to improve approach flow conditions to the gatehouse. Rock cuts will be close to vertical to match existing conditions, and the forebay's narrowest point will be widened by about 15 feet.
  • Gatehouse to Plant 2. The gatehouse is at the downstream end of the forebay and at the crest of a precipitous slope. The new Plant 2 gatehouse will lie on a substructure extension built upstream of the existing substructure. The new substructure will contain slots for bulkhead and intake gates. A new gatehouse structure about 46 feet wide and 19 feet deep will be constructed. A coarse trashrack with 12-by-12-inch openings will be set upstream of the new intake structure.
  • Penstocks. The two vertical penstocks at Plant 1 will be replaced with a single 8-foot-diameter penstock with a complex bifurcation to supply water to all five units. A new 7-foot-diameter welded penstock will be built for conveying water to a new Unit 1 in Plant 2. Both penstocks to Plant 2 will be modified to incorporate a bifurcation necessary to divert water to new flow bypass valves.
  • Plant 2 bypass facility. The bypass-valve systems and pipe work will be located upstream of Units 1 and 2 and be enclosed by the new powerhouse structure. The valves will operate in conjunction with the turbines so as to maintain a constant discharge into the river should one or both of the units trip off-line.
  • Plant 2 powerhouse structure. The existing superstructure, together with a large part of the substructure, is being removed to allow for the replacement of Unit 1 and installation of the three flow bypass valves. A steel structure with a concrete face will be built to provide the required functionality. The structure will utilize exterior architectural treatments to keep the essential character-defining elements of the existing design.
  • Turbines and generators. In Plant 1, the 1905-vintage Unit 5 will be replaced with a more efficient 6-MW horizontal Francis unit. In Plant 2, the 25-MW Unit 2 will remain, but a new 12.5-MW vertical-shaft unit will replace the 1910-vintage, horizontal-shaft Francis turbine, Unit 1.
  • Electrical and controls. Electrical design will include grounding, communication, station service power, batteries, switchgear, transformers, and alarms. Control systems will be upgraded and integrated into a distributed system that will contain the control logic to bring units from both plants into operation and maintain the required river flows.
  • Tailraces. Minor changes will be made to the floor of the tailrace for Plant 1 to improve flow and drainage when the units are not operating. The tailrace of Plant 2 for the new Unit 1 will be deepened to match that of Unit 2.

Note

  1. Freeland, C., "Hydro Hall of Fame: Honoring Three 100-Year-Old Facilities: Snoqualmie Falls No. 1," Hydro Review, Volume 17, No. 6, November 1998, pages 40-42.

 

Elizabeth Dubreuil is a senior cultural resource scientist and Roque Bamba is an energy resources implementation manager with Puget Sound Energy.

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