Powering up for fish passage16 December 2008
A new fish collection facility is currently under construction at the Pelton Round Butte project in the US. Scot Lawrence and Chad Croft give more details
For the first time in over 40 years, Chinook salmon, steelhead and sockeye salmon will have the opportunity to complete their life cycles in the Deschutes river basin above the Pelton Round Butte hydro project in Oregon, US. Construction is currently underway on a selective water withdrawal (SWW) structure at Round Butte dam and is scheduled for completion in spring of 2009. This one-of-a-kind structure is designed to pass downstream-migrating juvenile fish and improve water quality in the lower Deschutes River while maintaining current hydroelectric generating capacity.
Pelton Round Butte project
The Pelton Round Butte hydroelectric project is operated by Portland General Electric Company (PGE), and is licensed under the Federal Power Act to PGE and the Confederated Tribes of the Warm Springs Reservation of Oregon as joint licensees. It is the largest hydro project contained entirely within the state of Oregon.
The project consists of three dams located in sequence on the Deschutes river. Water released from Round Butte dam flows directly into Pelton dam and then into the reregulating dam. Each of these includes a dam, powerhouse and reservoir. The three reservoirs associated with the project include Lake Billy Chinook at Round Butte dam, Lake Simtustus at Pelton dam, and the reregulating reservoir at the most downstream reregulating dam. Overall, the project operates as a modified run-of-river system, where average daily discharge from the reregulating dam is approximately equal to the average daily inflow to the Round Butte dam. Round Butte and Pelton dams are ‘store and release’ facilities that operate in the peaking mode.
The Pelton Round Butte project spans a 32km reach of the incised canyon of the Deschutes river, with the downstream-most development located 161km upstream of the confluence of the Deschutes river with the Columbia river. The project is located near Madras in Jefferson County, Oregon, in the Deschutes river basin.
Selective water withdrawal
Fish passage facilities to capture downstream-migrating wild salmon and steelhead smolts were provided at the Round Butte dam when it was originally constructed. However, stratification of currents in Lake Billy Chinook caused by differences in the water temperature of the three rivers, coupled with the physical location of the collection facilities, resulted in low numbers of migrating smolts arriving for downstream transport. As a result, the facilities were phased out of service between 1968 and 1973. A hatchery located at the Round Butte dam tailrace was constructed as mitigation for the loss of steelhead and spring Chinook salmon production upstream of the dam in 1973.
The Federal Energy Regulatory Commission (FERC) issued a new 50-year operating licence for the Pelton Round Butte project on 21 June 2005. A major component in the issuance of the new licence was a requirement to reintroduce anadromous fish above the project. This included the need to construct facilities for passing juvenile fish downstream to mature in the Pacific Ocean and passing returning adults back upstream of the project to naturally spawn. In addition to the Chinook and sockeye salmon and steelhead, it is anticipated that bull trout will also be passed to re-establish genetic connectivity between the populations above and below the project.
Another major component of the new operating licence is the need to meet State and Tribal water quality standards. Currently, temperatures in the lower Deschutes river downstream of the project are about 2°C cooler in the early spring and about 2°C warmer in the fall and early winter months compared to historic records.
Downstream migrating juvenile fish typically migrate in the top 12m of the reservoir. The combination of an existing deep water exclusive outlet from the reservoir (power tunnel intake) and conflicting temperature characteristics of the three rivers created confusing surface currents for fish. This condition caused the original downstream fish migrant collection facility to fail.
The 82m tall selective water withdrawal (SWW) structure at the Round Butte forebay is being constructed to accomplish the goals of effective fish passage, compliance with water quality standards and maintenance of generation.
When operational, the SWW will modify the surface currents in Lake Billy Chinook to mimic the downstream flow of the three rivers in order to aid migration. The SWW will literally be passing the flow of the entire Deschutes river through the top structure during the majority of the year, thus inducing the surface currents.
By selecting either top or bottom water withdrawal (or a combination of the two) from the reservoir, the SWW structure will re-establish historic lower Deschutes river temperatures to pre-dam conditions. Computer modelling was used to model the lower Deschutes river temperatures as a result of typical project operation throughout the year. Multiple scenarios, or blends, of surface withdrawal mixed with bottom withdrawal were run in the model until the appropriate temperature profile was found to meet the temperature criteria.
The major objectives of the SWW project are to:
• Reorient the surface currents toward the dam to allow downstream migrating fish to find their way to the SWW.
• Safely capture downstream-migrating salmonids attempting to leave the reservoir using the selective water top (SWT) facility while excluding fish from the turbine intakes. One hundred percent of power generation flows are screened for fish exclusion.
• Provide for the safe and efficient sorting, enumeration, tag detection, marking and loading of downstream migrating fish for transport below the dam.
• Manage the downstream water quality during late summer and fall by mixing surface and deep waters to control the discharge temperature and dissolved oxygen (DO).
• Meet seismic standards, as well as wind, wave and hydraulic loading that an underwater structure is subject to.
• Achieve all of the above while maintaining hydroelectric generating capacity.
Efforts to develop a design to meet the above objectives began as early as the mid-1990s. This effort involved collaboration between the project owners, over 22 local, state and federal agencies, environmental groups and engineering/consulting firms. Research included utilizing Doppler radar to measure streamflows of the three rivers in various parts of Lake Billy Chinook, and the development of 3-D hydrodynamic and temperature computer models to evaluate reservoir conditions based on varying flows through the structure. A number of potential structures were evaluated, some of which are shown as models in Figure 2.
There were numerous factors that rendered the concepts infeasible, including cost and constructability. The final design was the result of value engineering analysis and computer modelling, and finally, construction of a 1:20 scale physical model at ENSR/AECOM in Seattle Washington. PGE’s engineering department and the engineering firms of CH2M Hill, EES Consulting, and ENSR/AECOM were instrumental in arriving at the final design.
The final design is shown as a 3-D rendering in Figure 3. The SWW consists of three major components: a selective water bottom (SWB); vertical flow conduit (VFC); and selective water top (SWT).
The SWW will be placed directly in front of the existing powerhouse intake tower and will include a floating fish transfer facility (FTF) which will be attached to the shore via an access bridge. The access bridge holds the SWT and FTF in place laterally and provides for personnel and equipment access.
The SWB directs water from the SWT to the power intake and also provides for direct water withdrawal from the bottom of the reservoir into the existing power tunnel. Surface water and fish are drawn into the fish collection entrances (FCE), which are the openings of both V-screens on the SWT. Flow for power generation passes through more than 836m2 of stainless steel wedge wire screening with gaps to keep fish out. By the time the fish reach the apex of each V-screen, the flow has been reduced from 85m3/sec to 0.85m3/sec.
Once the fish have reached the apex, or capture point, they cannot swim out of the structure and are routed into U-shaped channels where the flow from each V-screen is combined. The combined flow of 1.7m3/sec is further screened and reduced to 0.34m3/sec. At this point, the fish approach a separator where large fish (greater than 38cm long) are separated from smaller fish. The large fish (typically Kokanee and Bull Trout) are held on the structure in a holding tank and then sorted by species and either returned to the reservoir or trucked downstream of the project dams. The small fish are then pumped off of the SWT to the FTF where they are further automatically sorted by size and then manually sorted by species. The fish will then be tagged and the downstream migrating smolts will be trucked and released below the project dams to continue their journey to the ocean.
The SWT is made of 2.3M kg of concrete and steel which provides a floating surface collector. The SWT is 27m wide x 46m long x 18m tall. The SWT is capable of passing up to 198m3/sec of surface water at fish criteria. The SWB is an 18m wide x 18m long x 21m tall. This 590,000kg steel structure has the capacity to pass a total of 255m3/sec (99m3/sec at fish criteria). The VFC (12m diameter x 41m long steel pipe) connects the SWT to the SWB. The VFC is not rigidly connected to the SWT or SWB. The joints are similar to a ball and socket type which allow for the movement of the SWT and slight rotation of the VFC as needed for reservoir fluctuations and wind, wave and seismic loads.
The SWT floats on top of the VFC and is designed to operate over a 6m reservoir fluctuation. If the reservoir needs to be drawn down more than 6m, the VFC is lifted off of the SWB by jacks inside the SWT. The SWT and VFC can then be moored in a deeper portion of the reservoir. The VFC has buoyancy tanks attached to its perimeter to reduce the load of its own weight on the SWB.
The SWW is a unique structure; it is the only known floating surface fish collection facility that is coupled with power generation. As previously mentioned, water drawn through the SWT for attracting/collecting downstream migrating fish is piped to the power tunnel for power generation. The concept has been contemplated elsewhere but has not been implemented. The V-screen technology for fish collection, as well as a floating surface fish collector is not unique, and is in operation elsewhere. However, other floating fish collection facilities use pumps to create the attraction flow for fish, not hydro power turbine flow. According to CH2M Hill, as far as it knows, the SWW is the only structure of its kind in the world.
Barnard Construction Company of Bozeman, Montana, and Dix Corporation of Spokane, Washington, are partners in the construction project. Thompson Metal Fab of Vancouver, Washington, is the primary steel fabricator. On-site work was initiated in October 2007.
A tremendous amount of shoreline work was completed during the early stages of construction, which included an anchor block for the access bridge, roadway realignment, retaining walls, and kilometres of conduit for power and control cables.
Another important effort early on was the need to make room for the SWB at the bottom of the lake. The contractor excavated rock directly in front of the existing power tunnel intake at 85m deep. Underwater excavation was accomplished by use of a crane barge that dropped a 16,000kg chisel to pulverise the basalt rock. A clam shell device and air lift system was used to remove the loose material from the area. Remote operated vehicles (ROV) equipped with cameras, sonar, and positioning systems were used to give the contractor a set of eyes at depth to observe progress. Multiple sonar surveys were conducted and a template of the SWB bottom was placed to make sure the SWB would fit.
Due to the size of the structures to be built and extremely limited construction space on the dam, the majority of the SWW is being constructed on the water on construction barges, next to the dam. The closest large staging area is about 1.6km away from the dam via a narrow, steep grade road, which is also the main access to and from the dam.
Figure 4 shows the SWB being erected on a construction barge. During the week of 13 October 2008 the SWB barge was floated into place in front of the existing intake tower. The SWB was suspended from the barge by a series of stainless steel threaded rods and lowered 82m deep in front of the power tunnel intake structure. The SWB was then anchored by drilling, setting, and grouting 11 piles through the SWB and 12m into the bedrock at the bottom of the reservoir. The Round Butte powerhouse was shut down during critical phases of setting and anchoring the SWB. However, powerhouse shutdowns have been minimised as much as possible. The contractor has utilised normal nightly shutdowns due to peak load operations to conduct the majority of the deep in-water work.
Figure 5 shows how the SWT concrete floats in the water. Note the large 15m x 15m square hole that the 12m diameter VFC slides into. There is an array of concrete floats that make up the base of the SWT. These floats provide buoyancy for the structure and were convenient to use as a construction platform/barge for the steel erection.
Figure 6 shows the SWT with approximately 75% of the structural steel framing erected. The black steel in the picture is the floor and lower sides of the V-screens. Figure 6 also shows the SWB completed and partially submerged and ready to be moved out in the forebay and in front of the existing power tunnel intake structure.
The SWT, VFC and access bridge are also being constructed on the water. The FTF is being built off-site and then assembled on the water.
The SWW is scheduled to be operational in the Spring of 2009, ready for the first set of Chinook and Steelhead juveniles that were planted in the upstream tributaries in 2007 and 2008.
Scot Lawrence, hydro licensing project manager, and Chad Croft, project manager for Pelton Round Butte, at Portland General Electric. www.PortlandGeneral.com
You can track the progress of the construction project at http://DeschutesPassage.com/.
|Project fact file|
Round Butte dam