Researchers quantified the effects of shear stress on fish passing through Kaplan turbines by combining data from laboratory tests, computational fluid dynamics modeling, and field studies. The results of this research are helping to reduce mortality of fish passing through turbines.
By Glenn F. Cada, Laura A. Garrison, and Richard K. Fisher, Jr.
A continuing environmental concern for hydroelectric power production is injury and mortality to fish passing through the turbines. Depending on the turbine design, fish mortality during passage can range from 5 percent to more than 30 percent.1 Improving the survival of turbine-passed fish can preserve or restore fish stocks while maintaining an important source of renewable electricity.
For more than a decade, the U.S. Department of Energy’s (DOE) Hydropower Program supported the development of “environmentally friendly” turbines — i.e., turbines that emphasize environmental attributes such as fish passage survival. Research in this field included laboratory bioassays (to measure fish responses to expected conditions inside turbines), computational fluid dynamics (CFD) modeling (to extend the results of bioassays to unstudied conditions), and field testing (to evaluate new turbine prototypes).
Integrating the results of these studies provides an opportunity to address concerns about the potential for shear stress to injure turbine-passed fish. Details are available on this approach,2 and the underlying test reports are on the DOE Hydropower Program website (http:// hydropower.inel.gov).
Mechanisms for fish injury during turbine passage
Fish passing through hydroelectric turbines can be injured or killed as a result of rapid and extreme pressure changes, cavitation, strike, grinding, turbulence, and shear stress.3,4 Until recently, little was known about the distribution and magnitude of these injury mechanisms within turbines.5 Of these, the effects of the related phenomena of shear stress and turbulence have been the most difficult to gauge.
Briefly, fluid stresses result from forces acting on an area, such as a fish body. The components of the force that are parallel to the surface area create shear stress. These fluid forces are normally a result of changing velocity within a flow field and/or turbulence. Shear stress is most obvious where two masses of water moving in different directions intersect, or where moving water slows near a solid structure.
In most natural environments, shear stresses are small and non-damaging. However, potentially lethal levels of shear stress can occur where rapidly flowing water passes near structures such as dam spillways, internal structures of hydroelectric turbines, pipelines, and canals.4 Relative scale is important. High levels of shear stress are less damaging if they occur at a scale much smaller than the size of the fish.
The effects of shear stresses on turbine-passed fish are poorly known because of the difficulties in determining their magnitudes and distributions within hydroelectric turbine systems, then recreating these scenarios in a controlled laboratory environment. Limited laboratory and field observations suggest these fluid forces can cause descaling, tearing or bruising of tissues, and even decapitation of fish.
Methods to quantify shear stress
By coupling the findings of shear stress bioassays with CFD modeling of a turbine environment, we were able to make predictions regarding the locations and extent of potentially damaging shear stresses within a common type of turbine. Comparing the resulting model predictions to estimates of injury and mortality for field tests at an operating turbine illustrates an approach for systematically evaluating and mitigating the effects of turbine-passage stresses.
Laboratory bioassays
Bioassay data for this study came from a series of experiments where juvenile fish were exposed to a high-velocity, submerged jet in a large laboratory test flume.6 Water entered the flume through a circular nozzle at velocities ranging from 0 to 23 meters per second. Contact with the boundary of the jet exposed the fish to average rates of strain (shear) up to 1,185 per second. The values of shear that caused minor injuries, major injuries, or mortalities were estimated using logistic regressions. Injuries and mortalities caused by shear stress were related to the species of the fish and also to its orientation when contacting the jet. Fish that were struck head first by the jet suffered fewer descaling and tearing injuries that fish struck from behind.
 Table 1: Comparison of Mortality of Juvenile Coho Salmon Passing Two Regions of a Kaplan Turbine at Wanapum Dam1 |
Strain rates estimated in the laboratory experiments were converted to shear stress values.7 The conditions under which minor injuries (such as descaling) to fall chinook salmon were first observed (jet velocity of 9.1 meters per second and strain rate of 517 per second) were used as a threshold value for CFD estimates of damaging fluid stresses within a turbine. The resultant shear stress value of 1,600 Pa (Pascals), the maximum shear stress corresponding to a strain rate of 517 per second, was taken as a threshold for fish injury resulting from shear stress inside the turbine. That is, areas within the turbine that had estimated shear stress values greater than 1,600 Pa were assumed to cause injury or mortality to fish.
CFD modeling
Voith Siemens Hydro Power Generation used time-averaged CFD models to estimate shear stresses within a Kaplan turbine at the 1,038-MW Wanapum project on the Columbia River in Washington. The flow passage was divided into three sections for this analysis: intake region (including semi-spiral case, stay vanes, and wicket gates); runner (hub and blades); and draft tube region. Details of the modeling effort are available.2,7
Shear stress values within the turbine were calculated for four flow rates: 255, 311, 425, and 481 cubic meters per second (cms), all at a net hydraulic head of 23 meters. These flow conditions were chosen to correspond to those used during field tests of fish passage survival at Wanapum in 1996.8
Field studies
For the field studies, a total of 1,278 fish were passed through a Kaplan turbine at the same four flow rates used for CFD modeling. In addition, fish were introduced into the intake at two depths from the turbine ceiling, 3 meters and 9.1 meters, to ascertain whether different paths through the turbine resulted in different survivals. Fish introduced closer to the ceiling were believed to pass through the runner near the hub, whereas those introduced at 9.1 meters were expected to pass in the mid-blade area.
A total of 21 coho salmon were recovered dead and 33 were not recovered and were assumed dead or preyed upon before recovery. Estimated mortalities ranged from 0 percent to 11.5 percent, depending on turbine flow rate and travel route. (See Table 1.) For a given turbine flow, mortality was significantly lower (P <0.05) at the 9.1-meter release depth (mid-blade passage) than at the 3-meter release depth (hub passage). Surprisingly, a larger predicted volume of high shear stress seemed to be associated with lower mortality. In most cases, however, the differences in percent mortality between test conditions were not statistically significant.
Because a fish passes through all three sections of the turbine (intake, runner, and draft tube), it is impossible to assign mortalities from these field tests to shear stresses experienced in, for example, the intake region. However, if it is assumed that fish followed the streamlines as they traveled through the turbine, the mortalities of fish that traveled through these areas can be compared to the predicted volumes of damaging shear stresses in these runner locations.
Significant differences in mortality between hub-passed and mid-blade-passed fish were detected only at the two highest turbine flows. (See Table 1 on page 54.) The highest mortality observed in this study, 11.5 percent at 481 cms, occurred among fish that are assumed to have passed through areas that are predicted to have relatively small volumes of high shear stresses along the hub but large volumes in the draft tube region below the hub. (See Figure 1.) The runner mid-blade was predicted to have larger flow-weighted volumes of damaging shear than the hub region under all flows. Contrary to expectations, mid-blade-passed coho salmon had lower mortalities than hub-passed fish under all flows, although these differences were statistically significant only for the two highest flows.
Results of combining the three methods
The CFD analyses predicted no large areas of damaging shear stress in the intake region upstream of the stay vanes at any of the four flows.7 Small areas of shear stress above the 1,600-Pa threshold value were found in the wakes of the wicket gates under all conditions and at the entrance edge of stay vanes at higher flows.
The Kaplan runner at Wanapum was predicted to have damaging shear stress in four areas:
— Near the surface (boundary layer) of the runner blades;
— At or near the gap between the blade tips and the discharge ring that encases the runner;
— At or near the gaps where the moveable blades are attached to the hub; and
— In the wakes downstream from the blades.
Shear stresses in the first three (runner) areas vary with flow. In the draft tube downstream from the runner, shear stresses are largest at the highest turbine flow rate. High shear stresses in the boundary layer are expected because water velocities drop from more than 10 meters per second in the main flow to zero at the blade surface over a small lineal distance. And in the boundary layer, mechanical contact (strike) and abrasion are likely to contribute to injury as well. The high shear stresses near the blade tips and hub are caused by some of the turbine flow being “squeezed” through the small gaps and creating high-energy vortices in the wake.
Finally, the spinning runner imparts a rotational motion on the bulk of the flow. CFD analyses predict that this rotation results in a vortex with high angular velocities (and thus high values of shear stress) directly downstream from the hub. Damaging shear stress also occurs where the rotating flow below the runner strikes the draft tube support piers at a sharp angle.
The sizes of the turbine regions with shear stress values greater than 1,600 Pa were quantified and normalized by the amount of water passing that point. Adjustment for localized flow rates is necessary because velocities are not uniform across a cross section of the turbine passage. Presumably, regions with higher flow rates will expose more fish than those with lower flow rates. The sizes of areas with potentially damaging shear stress were predicted to vary not only with location but also with total turbine flow rate. (See Figure 2 on page 58.) Shear stresses in excess of the threshold value comprised relatively small percentages of the flow in the regions upstream from the runner (intake and stay vane-wicket gate region). The largest predicted volumes of water exposed to damaging shear stresses in the stay vane-wicket gate region were less than 0.2 percent of the overall flow volume.
Shear stress volumes were moderate in the runner area and were predicted to decline with increasing turbine flows. This is likely due to changes in the angle at which water approaches the adjustable blades in a Kaplan turbine. At low flow rates into a turbine, the blades are flatter, i.e., tilted at a low angle so they are more perpendicular to the flow lines. At low flows, the ratio of volume of the blade wakes (with potentially damaging shear stresses) to the overall volume of the turbine is higher. At the highest flow tested, the blades are tilted at a 43-degree angle and are better aligned with the overall water-flow streamlines.
Downstream from the runner, the predicted volumes of flow containing potentially damaging shear stresses increased greatly with increasing turbine flow rates. (See Figure 1 and Figure 2 on page 58.) Even so, the volume of damaging shear stresses accounted for no more than 1.6 percent of the total turbine flow. Turbulent shear stresses in this area occur especially in the vortex immediately below the hub and near the draft tube piers. Here, the water spreads out and slows down as it is discharged to the tailwater, in some cases impacting the draft tube support piers.
It has been suggested that severe fluid forces (e.g., shear stress) are a source of injury and mortality to hydroelectric turbine-passed fish. Laboratory studies indicated that rates of strain of as low as 517 per second can injure juvenile fall chinook salmon, and even smaller values will injure sensitive fish like American shad.6 Converting that threshold to a shear stress value, our steady-state CFD model predicted that volumes of damaging shear stress were present in small percentages of the overall flow passage. Highest values were estimated to occur in the runner and draft tube areas, and even these areas did not exceed about 1.6 percent of the volume of passage, weighted by localized flow rate.
This suggests that very large portions of the water passage through large Kaplan turbines do not exhibit shear stresses that are damaging to fish. If it is assumed that mortality resulting from this injury mechanism is proportional to the flow-weighted volumes estimated by this model, then less than 0.6 percent of the fish passing through the Wanapum turbine would be killed by shear stresses under most of the flows tested.
Our findings suggest that other injury mechanisms (e.g., strike) have a greater influence on fish injury and mortality in this turbine. However, it should be remembered that fish may follow a different path through the turbine than predicted by the streamlines, and thus could have a greater or lesser exposure to threshold values of shear stress than we estimated.
Further direction for research
By integrating laboratory bioassays, CFD models, and field studies, we were able to identify areas of potentially lethal shear stresses in the regions of the stay vanes and wicket gates, runner, and draft tube. Under typical turbine operating conditions, time-averaged CFD models estimated that these dangerous areas comprise less than 2 percent of the flow path through the turbine. However, the estimated volumes of damaging shear stress in the turbine did not correlate well with observed fish mortality at Wanapum, which ranged from less than 1 percent to nearly 12 percent. Possible reasons for the poor correlation include:
— Influence of other, potentially more important injury mechanisms;
— Uncertainty about the path that fish follow through the turbine;
— Narrow range of hydraulic conditions (shear stress exposures) in the field tests;
— Uncertainty about the appropriate threshold for effects on fish; and
— Use of a steady-state, time-averaged CFD model, which may underestimate the damaging effects of instantaneous, turbulent flows.
Each of these possibilities suggests a direction for further research.
CFD models are not yet reliable for precisely predicting mortality for particular turbine systems. However, the models are useful for comparing alternative turbine designs.
Dr. Cada may be reached at Oak Ridge National Laboratory, P.O. Box 2008, Bethel Valley Road, Oak Ridge, TN 37831-6036; (1) 865-574-7320; E-mail: [email protected]. Dr. Garrison may be reached at York College of Pennsylvania, York, PA 17405-7199; (1) 717-815-6427; E-mail: [email protected]. Mr. Fisher may be reached at Voith Siemens Hydro Power Generation, P.O. Box 712, 760 East Berlin Road, York, PA 17405; (1) 717-792-7848; E-mail: richard.fisher@ vs-hydro.com.
Notes
- Cada, Glenn F., “The Development of Advanced Hydroelectric Turbines to Improve Fish Passage Survival.” Fisheries, Volume 26, No. 1. January 2001, pages 14-23.
- Cada, Glenn F., James M. Loar, Laura A. Garrison, Richard K. Fisher, and Duane A. Neitzel, “Efforts to Reduce Mortality to Hydroelectric Turbine-Passed Fish: Locating and Quantifying Damaging Shear Stresses,” Environmental Management, Volume 37, No. 6, June 2006, pages 898-906.
- Proceedings of the 1995 Turbine Passage Survival Workshop, U.S. Army Corps of Engineers Portland District, Portland, Ore., 1995.
- Cada, Glenn F., Charles C. Coutant, and Richard R. Whitney, Development of Biological Criteria for the Design of Advanced Hydropower Turbines, DOE/ID-10578, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1997.
- Cada, Glenn F., “Better Science Supports Fish-Friendly Turbine Designs,” Hydro Review, Volume 17, No. 6, November 1998, pages 52-61.
- Neitzel, Duane A., et al., “Survival Estimates for Juvenile Fish Subjected to a Laboratory-Generated Shear Environment,” Transactions of the American Fisheries Society, Volume 133, No. 2, March 2004, pages 447-454.
- Garrison , Laura A., Richard K. Fisher, Jr., Michael J. Sale, and Glenn F. Cada, “Application of Biological Design Criteria and Computational Fluid Dynamics to Investigate Fish Survival in Kaplan Turbines,” HydroVision 2002 Technical Papers CD-Rom, HCI Publications, Kansas City, Mo., 2002.
- Fish Survival Investigation Relative to Turbine Rehabilitation at Wanapum Dam, Columbia River, Washington, Report prepared by Normandeau Associates Inc., John R. Skalski, and Mid Columbia Consulting Inc. for Grant County Public Utility District No. 2, Ephrata, Wash., 1996.
Acknowledgments
The authors thank Jim Loar, Mike Sale, and Fotis Sotiropolous for their ideas and suggestions during the conduct of this effort. Mark Bevelhimer of the Environmental Sciences Division, Oak Ridge National Laboratory, reviewed the manuscript. This work was supported by the Wind and Hydropower Technologies Program, Office of Energy Efficiency and Renewable Energy, U. S. Department of Energy.
Glenn Cada, PhD, research staff member in the Environmental Sciences Division of Oak Ridge National Laboratory, helped design and interpret the laboratory fish tests. Laura Garrison, PhD, is assistant professor of mathematics with York College of Pennsylvania. Richard Fisher, Jr., P.E., is senior vice president with Voith Siemens Hydro Power Generation. Garrison and Fisher carried out the computational modeling.
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