Research into the effects of sudden pressure changes inside a turbine on fish is providing valuable insight to guide the design of fish-friendly turbines. Installation of a new unit at 603-MW Ice Harbor is intended to provide field results to evaluate the success of this work.
By Richard S. Brown, Martin L. Ahmann, Bradly A. Trumbo, and Jason Foust
Hydropower is a tremendous renewable energy resource in the Pacific Northwest that is managed with considerable cost and consideration for the safe migration of salmon. Recent research conducted in this region has provided results that could lessen the impacts of hydropower production and make the technology more fish-friendly. This research is now being applied during a period when great emphasis is being placed on developing clean, renewable energy sources.
The configuration and operation of hydropower facilities on the mainstem Columbia and Lower Snake rivers is greatly influenced by the need to recover and sustain salmon and steelhead that are protected under the Endangered Species Act. The U.S. Army Corps of Engineers (USACE) maintains and operates eight hydropower projects on the Lower Snake and Columbia rivers that form part of the Federal Columbia River Power System.
As a component of a much larger effort to improve salmon passage through these hydropower projects, USACE implemented the Turbine Survival Program (TSP) to evaluate the turbine passage environment and to optimize the design and operation of large propeller-style turbines for safe fish passage. In 2004, the USACE TSP team, with support from other regional engineers and fish biologists, began developing criteria and guidance toward the design of new turbines with potential to reduce direct and indirect sources of mortality to seaward-migrating juvenile salmon. A primary focus of the design and operational changes is minimizing the differential turbine pressures and resulting risk of barotraumas, which are injuries caused by a rapid decrease in pressure.
How pressure affects fish
Fish passing through hydroturbines are exposed to various forces that may cause injury (e.g., shear forces, blade strike and pressure changes).1,2,3,4,5 Although blade strike is one of the most apparent sources of injury, the probability of strike from a large propeller-style turbine runner blade is relatively low, especially for small fish.6 However, all fish passing through turbines are exposed to various levels of pressure change. As fish pass between the runner blades, they are exposed to a sudden drop in pressure. Fish then return to near surface pressure as they exit the draft tubes and enter the tailrace. The severity of this pressure change depends on many variables, including the design and operation of the turbine and the specific route a fish follows as it passes through the turbine.
The exposure of fish to differential turbine pressures can lead to barotrauma, which may include a ruptured swim bladder; eyes popped outward (exopthalmia); rupture of blood vessels (hemorrhaging); and gas bubbles (emboli) in the vasculature, organs, gills and fins.4,5 Research conducted by Pacific Northwest National Laboratory (PNNL), a Department of Energy Laboratory operated by Battelle, in cooperation with and funded by USACE, has defined the relationship between survival and exposure to differential pressures that lead to barotrauma in juvenile chinook salmon. In what is the most comprehensive research on barotrauma to date, PNNL scientists have examined injuries from barotrauma among a broad range of fish sizes and conditions, pressure changes and rates of pressure change, as well as a range of total dissolved gas levels.
Results from the extensive biological tests conducted by PNNL indicated the primary source of injury results from exposure to a sudden drop in pressure and the associated expansion and rupture of the fish's swim bladder.7 The swim bladder is like a balloon inside the body cavity of the fish that aids in buoyancy regulation. When the swim bladder expands as a result of a rapid decrease in the surrounding pressures, it can crush the blood vessels and internal organs of the fish. In addition, rupturing of the swim bladder can cause severe injury or death as escaping gas can pop the eyes outward on the fish and tear through the organs, muscles and blood vessels.7
However, if the differential in pressure change is minimized during hydroturbine passage, then mortality from barotrauma would be reduced or eliminated because the potential for excessive expansion and rupture of the swim bladder is reduced.
|Fish exposed to rapid decompression can suffer from exopthalmia (eyes popped outward).|
Pressure criteria for design of new turbines
USACE is using information provided from the PNNL barotrauma studies to establish new pressure limits for replacement turbines. Applying a minimum pressure criterion will reduce the differential turbine pressures juvenile fish are exposed to and will minimize the risk of barotraumas. Selecting an acceptable limiting pressure criterion requires consideration of the acclimation depth of fish prior to turbine passage and the maximum change in pressure the fish can be exposed to without risk of injury.
For application to design of replacement turbines within the Columbia River system, the likelihood of barotrauma injury and mortality to seaward migrating juvenile chinook salmon was evaluated. PNNL researchers have found that the average maximum depth at which a juvenile chinook can attain neutral buoyancy (acclimation) is about 23 feet (6.9 meters).8 Based on the PNNL research, the expected mortal injury (mortality or injury highly associated with mortality) resulting from barotraumas for juvenile chinook acclimated to 23 feet with exposure to a minimum turbine pressure of 7.5 psia (50.5 kPa or ½ atmospheric pressure) is about 29%. However, if the minimum pressure exposure for these same fish were increased to twice this level, to 14.7 psia (surface pressure, 101 kPa or 1 atmospheric pressure), the expected mortal injury resulting from barotrauma would be reduced to about 2%.
Although applying a minimum pressure requirement that eliminates the risk of barotraumas may be desirable, it must be weighed against other potentially off-setting factors, such as risk of decreased direct injury and reduced turbine efficiencies.
USACE recently contracted with Voith Hydro in a collaborative effort to design and supply two new turbines for installation at the 603-MW Ice Harbor Lock and Dam on the Lower Snake River in southeast Washington State. The contract is for the design and supply of a fixed-blade runner and one optional adjustable-blade runner.
A primary goal of this effort is to develop design guidelines and criteria to significantly reduce the risk of injury to migrating juvenile salmonids. As such, the USACE and Voith Hydro design team selected 14.7 psia (1 atmospheric pressure) as a minimum design pressure target. During the design phase, modifications to the existing stay vanes, runner, discharge ring, and draft tube geometries also are being developed and evaluated using computational fluid dynamics (CFD) models and physical hydraulic models. These tools allow designers to target safe pressure limits for juvenile fish, reduce potential for blade strike and minimize exposure to mechanical and hydraulic shear forces. As the design progresses, turbine performance is evaluated and trade-offs are assessed to assure reasonable levels of power generation and turbine efficiency are maintained.
The new turbine runners, which are about 290 inches in diameter, are being designed for a head range of 84 to 103 feet, typical of many Lower Snake and Columbia river hydropower projects. Installation of the new turbines at Ice Harbor Lock and Dam will begin in 2015. Once they are installed, USACE will conduct field tests to determine estimates of survival of juvenile fish passing through the new runners. The implementation of an acoustic telemetry study will provide appropriate passage survival estimates for comparison with existing runners. If successful, the minimum pressure design criterion and other design concepts resulting in improvements to the turbine water passageway may provide a more biologically, economically and mechanically sustainable alternative for future turbine replacements.
|PNNL researcher John Stephenson observing juvenile chinook salmon inside of a hyper/hypobaric chamber used to expose fish to simulated pressure changes associated with turbine passage.|
The need for barotrauma research on other species
Most of the research on barotrauma related to dam passage has been limited to juvenile salmon, specifically chinook. Continued research is needed to determine the thresholds of pressure change to which other species and age groups of fish can be safely exposed. Some species may be at a disadvantage compared to salmon because salmon have a connection between their swim bladder and the back of their throat called the pneumatic duct. This allows them to expel gas from their swim bladder when decompressed, likely reducing injury during hydroturbine passage.7 Many other species do not have this opening and are likely more susceptible to injury when exposed to the rapid pressure changes of passage through a hydro turbine.
In addition, unlike juvenile chinook salmon, many other fish species add gas to their swim bladder using a bed of vasculature, which allows them to be neutrally buoyant at much greater depths.9 Because these fish may enter the turbine after residing in deeper water than do downstream migrating salmon, there is a higher likelihood that these fish may experience barotrauma during turbine passage.
DOE has a goal of adding more turbines to existing hydro infrastructure, with emphasis on environmentally sound and sustainable solutions. However, the lack of information regarding the effects turbine passage may have on non-salmonid fish species has slowed this process. As such, additional research is necessary to identify pressure tolerances of other fish species so further advances in fish-friendly turbines continue to be developed for riverine environments throughout North America and the rest of the world. Partnerships among scientists and engineers as we have described above could hasten the realization of DOE's goal.
The authors thank the U. S. Army Corps of Engineers' Portland and Walla Walla Districts for funding and critical input from staff, as well as staff of Pacific Northwest National Laboratory, Columbia Basin Research, School of Aquatic Resources, University of Washington, Corps Turbine Survival Technical Team, Symbiotics LLC and University of British Columbia Department of Zoology.
1Deng, Z., et al, "Evaluation of Fish-Injury Mechanisms during Exposure to Turbulent Shear Flow," Canadian Journal of Fisheries and Aquatic Sciences, Volume 62, No. 7, July 2005, pages 1513-1522.
2Deng, Z., et al, "Evaluation of Blade-Strike Models for Estimating the Biological Performance of Kaplan Turbines," Ecological Modeling, Volume 208, No. 2-4, Nov. 10, 2007, pages 165-176, doi:10.1016/j.ecolmodel.2007.05.019.
3Cada, G., et al, " Efforts to Reduce Mortality to Hydroelectric Turbine-Passed Fish: Locating and Quantifying Damaging Shear Stresses," Environmental Management, Volume 37, No. 6, 2006, pages 898-906, doi:10.1007/s00267-005-0061-1.
4Brown, R.S., et al, "Assessment of Barotrauma from Rapid Decompression of Depth-Acclimated Juvenile Chinook Sal-mon Bearing Radiotelemetry Transmitters," Transactions of the American Fisheries Society, Volume 138, No. 6, 2009, pages 1285-1301.
5Brown, R.S., et al, "Quantifying Mortal Injury of Juvenile Chinook Salmon Exposed to Simulated Hydro-Turbine Passage," Transactions of the American Fisheries Society, Volume 141, No. 1, 2012, pages 147-157.
6"Fish Entrainment and Mortality Study; Niagara Power Project," FERC N. 2216, report prepared for New York Power Authority by Acres International Corporation, Canada, 2005.
7Brown, R.S., et al, "Pathways of Barotrauma in Juvenile Salmonids Exposed to Simulated Hydroturbines Passage: Boyles Law vs. Henry's Law," Fisheries Research, Volume 121-122, 2012, pages 43-50.
8Pflugrath, B.D., et al, "Maximum Acclimation Depth of Juvenile Chinook Salmon: Implications for Survival during Hydroturbine Passage," Transactions of the American Fisheries Society, Volume 141, No. 2, 2012, pages 520-525.
9Fange, R., "Gas Exchange in Fish," Reviews in Physiology, Biochemistry and Pharmacology 97, 1983, pages 111-158, doi: 10.1007/BFb0035347.
Stephenson, J.R., et al, "Assessing Barotrauma in Neutrally and Negatively Buoyant Juvenile Salmonids Exposed to Simulated Hydro-turbine Passage Using a Mobile Aquatic Barotrauma Laboratory," Fisheries Research, Volume 106, 2010, pages 271-278.
This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.
Richard Brown, PhD, is senior research scientist with Pacific Northwest National Laboratory. Martin Ahmann, P.E., is senior hydraulic engineer and Bradly Trumbo is a fishery biologist with the U.S. Army Corps of Engineers. Jason Foust, PhD, is hydraulic engineer with Voith Hydro.