Retrofitting a Deep Water Plant Intake to Improve Fish Passage

Design and installation of a one-of-a-kind modification to an existing hydropower intake system was completed by a diverse team of engineers and constructors. This modification, called the Round Butte Dam selective water withdrawal (SWW) project, is already helping restore salmon and steelhead runs to their ancestral spawning grounds. The facility consists of five large steel structures constructed in an inland reservoir.

The SWW system consists of a selective withdrawal bottom (SWB) structure that is hydraulically connected to the power intake; a 40-foot-diameter, 135-foot-long vertical flow conduit; a floating top structure that contains the surface withdrawal and fish collection components; a 230-foot-long access bridge; and a floating fish transfer facility.

Located in Lake Billy Chinook, the reservoir impounded by Round Butte Dam, the SWW project was needed to reestablish anadromous fish runs above the dam. The dam was originally constructed in the early 1960s with a fish passage system for upstream and downstream migration, but the downstream migration was not effective. Instead, in 1966 that fish passage system was abandoned and a hatchery was built at the base of the dam.

Round Butte Dam is part of the 465-MW Pelton Round Butte Hydroelectric Project on the Lower Deschutes River in Oregon, which is co-owned by Portland General Electric and the Confederated Tribes of the Warm Springs Reservation of Oregon. CH2M HILL provided design and construction oversight for the project.

This work presented many unique challenges, not the least of which involved connecting the structure to the powerhouse intake, 270 feet under water.

Designing the structure

The three-unit powerhouse at Round Butte Dam has a peak flow of 14,000 cfs through a deep-water intake. The intake is a concrete structure with three trashracks that create a face area 54 feet wide by 70 feet high. The SWB needed to enclose this entire intake in such a way that no gaps larger than ¼ inch were created. The exact dimensions of the intake were not known.

ASI Group performed a preconstruction three-dimensional (3D) sonar survey during the summer of 2006 to map the intake structure, trashracks and bottom contours. Because of the logistics of the tower and intake structure, a truss and winch system was designed and installed on the tower deck so that the sonar equipment used to survey the intake could be hung from the tower. The sonar system was deployed and operated to map the four pier noses and bottom contour for the excavation work would be needed to install the SWW. Images featuring thousands of cloud points were plotted to complete the 3D mosaic.

The structure also had to be designed to be assembled in the dry at a site where no land was available. The only shoreline access available was a small pad on the dam’s right abutment, just enough space to operate one crane and unload materials. From this location, all five large structures were assembled, using multiple techniques. Once the 1.4-million-pound structure was erected, it had to be placed at the bottom of the lake and sealed against the intake while keeping powerhouse outages to a minimum.

PGE and CH2M HILL conducted several workshops in 2005 with industry experts to identify viable alternatives. After the workshops, the team decided the construction methods used to build and place the SWB structure would be critical to the design. As a result, the general contractor, Barnard Construction Co. of Bozeman, Mt., and several specialty subcontractors — including Dix Corp. of Spokane, Wash., Associated Underwater Services (AUS) of Spokane, Wash., and Thompson Metal Fabricators (TMF) of Vancouver, Wash. — were hired at the 25 percent design phase to outline the means and methods to be used in construction and to collaborate on the final design. A series of investigations was then undertaken to define the design variables and understand the design and construction constraints.

The geometry of the structure was complex as a result of the large flow areas needed to pass water through the control gates and from the 40-foot-diameter vertical flow conduit that conveyed water from the top to the bottom of the structure. This complex geometry also meant designing unconventional framing and bracing to support the high gravity and seismic loads. The team designed the structure to be hung from the top of four columns during construction and placement, supported off four jack locations temporarily when first set on the bottom, and then anchored with 11 steel pipes in the final operating condition.

Building the structure

Because of the configuration of the dam, there is virtually no work space available on the dam crest. To overcome site limitations, designers decided to use a pontoon barge with a central moon pool to assemble the SWB structure in the water, 50 feet offshore at the right abutment. Materials were delivered to the barge using a land-based ringer crane and a barge-mounted crane.

Because dive work at depths of 200 to 270 feet is time-consuming, expensive and a significant safety concern, the structure was designed so that remotely operated vehicles could perform nearly all the underwater work. Performing this excavation, drilling and grouting without making a single dive was a major challenge. All equipment was controlled from the barge and observed using ROVs while divers were kept on standby for emergency situations.

The SWB was erected first, in two stages. During the first stage, the outer framing was built on the pontoon barge, supported by a temporary floor. Once the framing work was completed, the SWB was lifted by the rod jacks and suspended by the truss system to allow removal of the temporary floor. As work on the SWB was completed, it was partially lowered into the water to provide better access to the top of its 70-foot frame.

Other structures then built were the top structure, access bridge, fish transfer facility and vertical flow conduit. Total construction time was 26 months.

Installing the structure

Organizing the work approach, given the project’s limited onshore workspace, was one of the most challenging aspects of the Round Butte SWW project. The completed structure was to be massive — more than 300 feet high and weighing roughly 5 million pounds. Yet the only shoreline access available was a small pad on the dam’s right abutment.