Fish friendly designs10 September 1998
Developing an environmentally friendly hydro power facility became a reality with the McNary dam fishway hydroelectric project in the US. Zbigniew R Matus* explains how its innovative draft tube layout and power generation is a new concept for powerhouse design and operation
The US Army Corps of Engineers’ McNary dam, located on the Columbia River in the US, was virtually completed in 1953 when the first generating unit was placed into service. In addition to the dam, powerhouse and spillway structure, there is a navigation lock and two fish passing facilities, one on the north (Washington) shore and one on the south (Oregon) shore.
The original installation of the north shore fishway facility consisted of a conventional fishladder, a vertical fishlock, a holding pond and the auxiliary water supply system (AWSS). The AWSS was designed to reduce the head available at the fishladder supply ports from 18.3-21.4m, to the 0.1m needed.
The fishlock was constructed so that fish could be lifted up from the tailwater to the reservoir, if they were reluctant to use the fishladder. The fishlock was considered an experimental facility and, after several years, it was abandoned due to the complete success of the fishladder, coupled with cumbersome mechanical operation associated with use of the fishlock.
The fishladder receives a flow of approximately 5.1m/s from the reservoir through the top portion of the fishladder. This flow is augmented in the lower portion of the fishladder by the AWSS flow of approximately 49.5m/s. In the original configuration of the north shore fishway facility, this water entered the fishladder through concrete culverts and 14 supply ports spaced along the lower portion of the fishladder. The average AWSS flows of about 49.5m/s represent less than 2% of the average Columbia River flow at the McNary dam.
However, fisheries and agencies noted that approximately 90% of juvenile salmonoids (smolts) swimming downstream through the AWSS (though not the upstream migrants for whom the fishladder was designed) had been killed. This was mainly due to the severe pressure/cavitation and high water velocities created within the original AWSS where the available head was reduced from approximately 20m to less than 15cm.
In an effort to reduce the mortality rate of juvenile salmonoids which pass through the AWSS system, and to capture the untapped power, the Northern Wasco County People’s Utility District (PUD) obtained a FERC licence in 1991 to develop a hydroelectric facility.
US companies Harza Engineering of Illinois (the design engineer), and Atkinson Construction of California (the general contractor) worked on the design/build project. PUD’s request for plans called for tapping existing AWSS water supply conduits, passing water through one or two hydraulic turbines, and then discharging it into the fishladder supply pool. Due to the locations of the existing AWSS water supply conduits, non-overflow gravity dam, fishlock structure and fish ladder, the space available for the new powerhouse and water passages was very limited. Additionally, the limited size of the powerhouse made it a challenge to arrange the necessary powerhouse equipment.
The project’s stringent criteria and confined location required a creative balance between design details, construction techniques, and economic considerations. Several innovative and original arrangements were evaluated for the powerhouse and AWSS.
The existing water supply conduits pass through the non-overflow dam, exiting on the downstream side into concrete box sections from which steel pipes feed the supply ports located on the upstream side of the fishladder. In order to utilise these existing fishladder supply ports, the turbine discharge had to be directed to this area.
However, a typical vertical shaft turbine with an elbow type draft tube arrangement has the draft tube exit located approximately 180 degrees from the semi-spiral case inlet. Such an arrangement would have discharged water away from the supply ports and required expensive tunnelling and shoring work under the fishladder to redirect the turbine discharge. By rotating the draft tube from its traditional orientation to a position directly under the inlet conduit, a unique and very effective solution was found. In order to fit the draft tube in the existing trapezoidal-shaped space between the fishladder and the fishlock structure, the powerhouse was pushed as far north as possible.
When the turbine is not operating, AWSS water is diverted through an innovative turbine bypass system that incorporates the currently abandoned fishlock facility. The bypass water is conducted into the fishlock structure through two new, concrete culverts and vertical steel pipes and cascades down through energy dissipation shelves. The bypass flow then leaves through orifices cut in the lower portion of the fishlock walls and enters the same fishladder supply pool which is fed by the turbine in the operating mode.
Unlike traditional hydroelectric facility layouts, in which fish passage facilities are separate from the power generation facilities, the McNary design successfully merges the two in a cost-effective manner. Every facet of the power generation and bypass system not only addresses but also actually accentuates the traditional opposing criteria of maximising power generation while minimising fish mortality. By selecting an axial flow propeller type hydraulic turbine, rather than a customary radial-axial type, similar power generation is achieved while successful fish passage is increased.
In addition, by limiting velocities in the system and removing existing valves, right angle pipe turns and high energy dissipation structures, fish mortality reduction is successfully achieved without compromising the efficiency or cost-effectiveness of the power plant.
The 10MW installation provides the additional power needed in the region with its annual generation of approx-imately 70GWh. Equally important, the mortality rates for the smolts passing downstream through the AWSS will be reduced from the current estimation of 90% to between 6% and 14%.
McNary dam fishway hydroelectric project was a success because of its innovative and environmentally friendly design, and because it remained within the estimated budget while meeting all technical and regulatory requirements.
Throughout construction of the facilities, scheduled fishladder operation was not affected. The turbine bypass facility and the modified AWSS have met or exceeded all criteria set forth by the governing agencies.
The success of this project in part stems from the design/build and partnering approach employed by the owner. The design/build approach mandated a well thought out initial design by the engineer while enabling the use of the contractor’s construction experience and available resources to economise the design at the earliest possible stage. The design/build method also helped contain project costs by encouraging cooperation and the acceptance of responsibility between the engineer and contractor. Furthermore, this experience demonstrates how partnering can help a project run more smoothly during design and construction, and provide a higher level of co-operation, commitment, and trust between all involved parties.