The Tacoma Second Supply Project provides for the water supply and wildlife conservation needs of the Puget Sound region in an economical and environmentally sensitive manner through the efficient and cost-effective use of available water resources and multiple wetland improvements. The project includes improvements to Tacoma Water’s headworks and fish facilities and the construction of 34 miles of pipeline from the headworks through South King County to Tacoma.

Installing prefabricated steel bridge section for pipeline support system.	Installing bridge abutments and placing remaining bridge pipeline section.

Parsons constructed 28.3 miles of this pipeline consisting of 48- to 90-in. steel pipe, including 30-ft-deep trenching and 60-ft-deep microtunnel shafts in an urban and suburban setting. The microtunnels crossed several major highways, the Puyallup River, and the Burlington–Northern and Union Pacific Railways.

Parsons also constructed a state-of-the-art chemical feed facility and upgraded the Tacoma Water headworks and fish bypass facility:

• The new chemical feed building handles chlorination, fluoridation, and pH adjustment for corrosion control. Facility testing and startup were completed in August 2005, and the building is now treating water.

• The existing headworks site was originally constructed in 1911. The modifications increased the water supply system’s capacity from 72 to 160 mgd, provided fish screens to the intake water, and constructed a fish ladder, fish trap, and sorting facility. Major structural modifications involved raising the existing 17-ft-high by 152-ft-long gravity dam by 6.5 ft, constructing a concrete scour apron downstream of the dam, raising and extending the existing intake structure, and raising and modifying the existing settling basin.

Work restrictions from the fish windows and from the requirements to maintain a high-quality water flow to Tacoma’s largest water supply system allowed for in-river work to be performed only during the specified time frame from June 1 to October 15. These restrictions required that construction sequencing be performed over three in-water construction seasons.

Constructing intake and power screening structures with 96-in. steel pipe outlet.	Bypass flume for intake structure.

During the first in-water year, Parsons placed 40- and 100-kip anchors through the existing dam. Parsons also installed both the concrete forms (to raise the dam by 6.5 ft) and the Parsons-designed frames and lagging (for water diversion) on the upstream face of the existing dam. Parsons designed and installed a system to close the existing intake structure, allowing for modifications to be performed in the second in-water season. The third season involved raising the northern half of the dam crest by 6.5 ft, raising the existing settling basin walls to accommodate the increased flow from the higher dam, installing a new 168-in., dual-leaf gate and a sluice gate (for flow distribution between the water supply system and the new fish-handling facility)—as well as constructing the new fish-handling facility, built downstream of the new settling basin, that contains new fish screens, fish bypass, fish ladder structures, and a new fish trap and sorting structure.

• Upstream water quality barrier that separated the dredging work and the initial construction of the new intake structure from the main river channel.

• Water diversion structure attached to the upstream face of the dam, consisting of structural steel frames and stop logs to divert the river around the new dam crest construction.

• Water supply division pipes that provided water to the city’s water system, bypassing the new intake structure construction and the in-river cofferdam system used to divert the river from the rock excavation within the river bed.

• Placement of the new concrete apron downstream of the dam.

Environmentally friendly super sack cofferdam allows dewatering for in-river work.

The in-river cofferdam water diversion barrier, conceived and designed by Parsons’ project management team, consisted of a series of “super sacks” and heavy tarps. The super sack cofferdam was constructed of poly bags (filled with salmon-spawning gravel) with lifting straps that were stacked in the water on top of the heavy tarps, which were secured to the inside of the cofferdam. This system worked well because the cofferdam could be built to any configuration, and its height could be varied to accommodate the variable river depth and extremely irregular river bed. An additional advantage of this design was its ability to dispose of the spawning gravels (within the super sacks) directly into the river. This method was not only approved by the regulatory agencies, but it was also appreciated by them because the gravel created a huge spawning bed downstream of the new dam. Salmon and steelhead have already been witnessed stopping to spawn in the gravel.

Parsons crews worked with Tacoma Water personnel and the U.S. Army Corp of Engineers to complete the challenging headworks project in an efficient and timely manner—while maintaining an excellent safety record. On October 18, 2005, Tacoma Water’s Second Supply Project pipeline, headworks dam modifications, and treatment facility began delivering water through its entire 34-mile length. For more information, visit the Tacoma Water website.

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