As part of a plan to safely guide juvenile salmon on their journey down Washington State's Columbia river to the Pacific Ocean, a fish bypass system at the Rocky Reach dam in the US is being equipped with 29 low head, high capacity submersible pumps and auxiliary equipment
BEGINNING in the ice fields of the Canadian Rockies, the Columbia river in the US flows for over 3108km2 to the Pacific Ocean. It is the fourth largest watershed in the US, draining over 670,000km2 and receiving waters from seven states and two Canadian provinces.
The Columbia river has the second largest volume flow of any river in country and generates electric power, provides irrigation, and harbours deep-water ships that come and go across the Pacific. Millions of people depend on the river for employment in water-related industries, and for transportation.
Historically the Columbia river basin has produced some of the world’s largest runs of salmon. Estuarine habitats provide important nursery and rearing areas for young salmon, and adults use them as temporary holding areas during their return migration from the ocean to upstream spawning areas.
While a great many factors have contributed to the decline of salmon stocks in the basin, dams have clearly had a significant impact, including those through which fish passage is provided but at reduced levels from natural conditions. Overall populations of the basin’s salmon fish stocks are estimated at less than 10% of their historic size, despite major hatchery programmes.
Providing a safe passage for juvenile salmon on their run to the sea – while at the same time allowing enough water to pass through the dam’s turbines to generate electricity was a problem at the Rocky Reach dam. At 761km up the Columbia river from the Pacific Ocean, Rocky Reach was constructed in 1961 and provides the region with 1347MW of electricity.
Man-made fish ladders have long helped salmon navigate past dams during their upstream migration. The downstream migration of fish in US rivers, known as smolting, has been severely affected over the years, with a fish mortality rate of between 5-8%.
Environmental pressures have increased over the last decades and has led to the introduction of legislation which mandates that owners and operators of hydroelectric dams either set mandatory spill periods during peak migration season, which results in a major loss in power production, or install devices to aid downstream fish migration. These devices, known as fish attraction systems, are used to lure and then divert juvenile salmon, steelhead and other endangered species away from the hydroelectric turbines to a transport pipe running through the dam and then out to safety.
According to Brett Bickford, senior civil engineer of Chelan County Public Utility District (PUD) which operates the Rocky Reach, as much as 60%-70% of the average daily flow could be lost in spill periods during spring and summer, which makes it much more commercially viable to invest in a fish bypass system.’
At the Rocky Reach dam, Chelan County PUD is constructing a large fish attractor intended to allow the fish to bypass the turbines safely in a 1.2m diameter tube.
‘Scientific studies show that salmon prefer certain depths of water and velocities,’ says Stefan Abelin, director of engineering at Flygt Pump’s US operation in Trumbull, Connecticut. ‘The fish attractor is aimed at creating conditions to attract the fish toward the bypass system and away from the turbines.’
Ongoing research in this field has found that the best results are achieved by using pumps. However, no pump existed that would handle the high flow rate at an extremely low head, with the required efficiency rate. In 1998, ITT Flygt began development work on a new horizontally installed propeller pump, which would be able to meet the required duty points. After thousands of hours of CFD modelling and scale model testing, a new pump design was created. The pump utilises a planetary gear reducer to match the motor speed with the propeller rpm.
The fish attractor at Rocky Reach is powered by these new low head, high flow Flygt pumps. The pump station is being constructed to accommodate 30 horizontal flow pumps for a combined capacity of 170m3/sec. Chelan County placed an order for 29 of the new pumps and auxiliary equipment. The 90kW propeller pumps have a flow rate of 7m3/sec at a head of 0.55m, providing a combined flow rate of 175m3/sec. The auxiliary equipment includes ten racks of flap gates to prevent reverse flow, electric controls, remote supervision, control buildings, transformers, pump testing, installation, plus an extended pump and control maintenance agreement.
More fish and more electricity
The Rocky Reach bypass system is the first full-scale fish attraction project ever undertaken. Chelan County PUD hopes the new bypass will let it phase out all of its spills except for a 16% spill for 40 days each spring for Sockeye salmon which tend to travel too deep to use the bypass.
It says the slide bypass will save money because the utility will not have to spill as much water to make sure the fish can migrate past the dam. That water instead can be used to generate electricity. The public utility lost US$14.2M in power production at Rocky Reach in 2000 due to spills for all species of salmon and steelhead.
Without the system, Rocky Reach would have to spill 60-70% of its average daily flow in the spring and summer, costing an estimated US$934M in power production over the 15-year financial life of the new system.
‘We think the US$160M total cost is quite a good compared with that kind of power loss,’ Bickford says. ‘We need this to renew our license, and it’s just being a good steward of the resources to maintain multiple species of salmon.’