Forecasting and Minimizing Avian Mortality Without Significant Loss of Power Generation
Publication Number: CEC-500-2005-005
Publication Date: December 2004
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Introduction
The wind turbines in the Altamont Pass Wind Resources Area (APWRA) have been killing excessive numbers of birds during its operations, which began about 20 years ago. Orloff and Flannery (1992, 1996) reported high mortality levels of raptors in the APWRA, and most recently Smallwood and Thelander (2004) reported even higher levels of mortality based on additional research following Orloff and Flannery's work. Most of the existing wind turbines were sited in the APWRA without regard to patterns of bird flights or perching, and most are mounted on short towers that position the turbine rotors at the height domains of most flights of golden eagle, red-tailed hawk, American kestrel, and the other species that are often killed by wind turbines in the APWRA.
Smallwood and Thelander collected behavior data during 2003 and 2004, and designed the study to perform spatial analysis of raptor flight heights in order to elucidate flight patterns in response to topographic features and wind conditions. The locations of flights were recorded at one-minute intervals, along with attributes of the flights, such as height above ground and specific flight behavior. One of the motivations for this new approach was the anticipated conversion of the APWRA from the existing collection of wind turbines to a new collection of modern wind turbines. This conversion has been termed Ô"repowering," and will replace the existing turbines with larger turbines mounted on taller towers at a ratio of about 8Ð10 to 1.
The repowering of the APWRA presents the opportunity to more carefully site the new turbines based on knowledge of bird flight patterns in response to topography and wind patterns. It is our objective to add to this knowledge and to support repowering efforts that substantially reduce avian mortality, especially raptor mortality. This project was made possible by the principal investigator's access to the geographic information system (GIS), additional spatial data, and GIS specialists in the Lawrence Livermore National Lab.
Subsequent to the analyses of bird flight patterns in the APWRA, we will pursue the main study goal. The study goal is to forecast avian mortality in the repowered APWRA under two scenarios (additional scenarios could be forecast later should the stakeholders show interest in doing so). Scenario One would consist of the repowered APWRA according to the 1998 environmental impact report (EIR) and any additional new planning documentation, and will be called the Power Maximizing (P-max) Scenario. We will forecast mortalities of golden eagle, red-tailed hawk, and American kestrel due to the flight behaviors recorded during the recently concluded study in the APWRA, and due to the attributes of the proposed new wind turbines and their spatial locations and arrangements in the repowered APWRA.
Scenario Two includes the same number and type of wind turbines as proposed in the 1998 repowering EIR and whatever updates the wind turbine owners would prefer to provide, but arranged within the boundary of the APWRA in a manner that a careful examination of the bird behavior data suggests would minimize avian mortality, and based on the results of the fatality associations identified by Smallwood and Thelander (2004). This scenario will be called the Bird Fatality Minimizing Scenario (F-min). F-min will include clumping of wind turbines into higher density turbine fields to the extent possible, as well as exclusion of wind turbines from saddles in ridges and ridgelines leading into major drainages. It will include minimized exposure of birds to isolated wind turbines and edge turbines, such as wind turbines on the ends of rows.
F-min will likely lead to somewhat less power generation than will P-max, because some of the sites excluded to minimize mortality are also high-energy micro-sites for power generation. Nevertheless we suspect that forecasts for F-min will indicate a much-reduced ratio of bird deaths to megawatts of power output and that forecasts for P-max would indicate a reduced ratio of mortality to power output compared to the output currently experienced in the APWRA.
Abstract
Researchers followed up on the 2004 Energy Commission final report on bird mortality in the Altamont Pass Wind Resources Area (APWRA) by geo-referencing bird behavior data collected during 2002 and 2003, and performing spatial analysis on these data to test hypotheses that could not be tested previously.
They related their 1,152 observations of raptors in flight to landscape attributes derived from a slope curvature analysis based on a digital elevation model of the landscape and ArcMap geoprocessing tools, and combined with wind directions recorded during the observation sessions.
Red-tailed hawk and American kestrel flew over convex slope structures typical of ridges and hills disproportionately more often than over concave slope structures typical of valleys, ravines and basins.
Red-tailed hawk, American kestrel, and golden eagle flew over windward aspects of ridges disproportionately more often than over leeward aspects, and these results include the shifting of flights over aspects of the ridges (and hills) as the wind directions shifted.
Locating new or existing wind turbines on the prevailing leeward aspects of ridges and hills should result in reduced encounter frequencies between flying raptors and wind turbines.
Phase II of this project is ongoing, and will compare power output and wind turbine-caused bird impacts between the wind turbine owners' preferred wind farm design after repowering and a revised design based on our knowledge of bird behaviors and mortality patterns in the APWRA.
The goal is to achieve an economically viable wind farm design that also minimizes bird mortality.
Table of Contents
PrefaceAbstract
1.0 INTRODUCTION
2.0 METHODS
3.0 PRELIMINARY RESULTS
5.0 REFERENCES
List of Figures
Figure 1. A portion of the APWRA landscape categorized into concave and convex slopes, as well as slopes intermediate between convex and concave orientations, following our use of the Jenks function to identify natural breaks in the frequency distribution of curvature values
Figure 2. A zoomed-in view of the same portion of the APWRA landscape as that in Figure 1, revealing the 10 x 10-m resolution of the raster (grid)
Figure 3. A portion of the APWRA landscape categorized into ridge versus valley features. The blue polygons indicate a tendency toward convex orientation typical of ridges and peaks; whereas, the gold matrix indicates a tendency toward concave orientation typical of valleys, saddles, ravines, and basins.
Figure 4. Distribution of flight heights above ground level among red-tailed hawks observed during behavioral observation sessions during 2003 and 2004 in the APWRA
Figure 5. Distribution of flight heights above ground level among northern harriers observed during behavioral observation sessions during 2003 and 2004 in the APWRA
Figure 6. Distribution of flight heights above ground level among American kestrels observed during behavioral observation sessions during 2003 and 2004 in the APWRA
Figure 7. Distribution of flight heights above ground level among golden eagles observed during behavioral observation sessions during 2003 and 2004 in the APWRA
Figure 8. Distribution of flight heights above ground level among turkey vultures observed during behavioral observation sessions during 2003 and 2004 in the APWRA
Figure 9. Distribution of slope aspects throughout the study area
Figure 10. Mean flight heights of red-tailed hawk over slopes of different aspects
Figure 11. Mean flight heights of turkey vulture over slopes of different aspects
Figure 12. Mean flight heights of red-tailed hawk over aspect of ridge relative to oncoming winds
List of Tables
Table 1. Flight behaviors recorded during 30-minute observation sessions in the study plots.
Table 2. Summary of chi-square tests for association between the observed number of flights and the underlying topography. Observed-divided-by-expected values express the number of times fewer or greater than the expected number of flights over ridges or valleys than would be expected by a uniform (or random) distribution of flights in the study area. Chi-square values accompanied by * indicate the test of association was significant at an α-level of 0.05, and ** indicate significance at an α-level of 0.005.
Table 3. Summary of chi-square tests for association between the observed number of flights and the underlying slope aspect. Observed-divided-by-expected values express the number of times fewer or greater than the expected number of flights over each slope aspect than would be expected by a uniform (or random) distribution of flights in the study area. Chi-square values accompanied by * indicate the test of association was significant at an α-level of 0.05, and ** indicate significance at an α-level of 0.005.
Table 4. Summary of chi-square tests for association between the observed number of flights and the aspect of the nearest ridge structure with respect to wind direction. The undefined category was omitted because it includes mostly sessions when there were no winds, so flights during these conditions were not likely to result in collisions with turbine blades. Observed-divided-by-expected values express the number of times fewer or greater than the expected number of flights over each ridge aspect than would be expected by a uniform (or random) distribution of flights in the study area. Chi-square values accompanied by t indicate the test of association tended toward significance with the P-value between 0.05 and 0.10, * indicate the test of association was significant at an α-level of 0.05, and ** indicate significance at an α-level of 0.005.