An assessment of offshore wind turbine visibility in the United Kingdom

Abstract

The potential visual impact of offshore wind facilities on coastal lands has emerged as a major concern for the siting and future development of these facilities in Europe and the United States. The visibility and visual contrast of offshore wind facilities depend on complex interactions among facility size, turbine size and color, distance from shore, lighting, and weather and atmospheric conditions, as well as other factors that affect overall visibility. However, little systemic study of visibility and visual contrast of offshore wind facilities in real seascape settings has been conducted. As a result, there is uncertainty about the potential effects of offshore wind facilities on sensitive visual resource areas in coastal regions, such as national seashores, historic sites, and trail corridors. As part of a research study sponsored by the U.S. Department of the Interior's Bureau of Ocean Energy Management, an assessment of the visibility and visual contrast of 11 utility-scale offshore wind facilities was conducted in the United Kingdom in August and September of 2011. The observed facilities ranged from 25 to 140 turbines and were located within 5.5-52.0 km (3.4-32.3 mi) of 29 coastal viewpoints. Turbine power output ranged from 2.0 to 5.0 MW, with blade tip heights ranging from 113.5 to 153 m (372 to 502 ft). Study objectives included identifying the maximum distances at which the facilities could be seen from the coastal viewpoints in both daytime and night-time views and assessing the effect of distance on the degree of visual contrast associated with the facilities. Observers included a landscape architect, a geospatial visualization developer, and an archaeologist. For each facility, the observers recorded data about weather and lighting conditions, photographed the facilities at different focal lengths, and used a numeric scale to assess the facilities' degree of visibility. A total of 48 daytime observations of 11 offshore wind facilities were made, and an additional 6 observations were made at night. Weather and visibility conditions varied widely during the 10-day field study. In the course of this study, under favorable but not exceptional viewing conditions, moderate-sized offshore wind facilities were frequently found to be visible at distances exceeding 21 mi (34 km); they were visible at a maximum distance of 43 km (27 mi), as seen from an elevated viewpoint. With few exceptions, regardless of facility size or lighting conditions, on days with good visibility, offshore wind facilities were judged to be major foci of visual attention at distances of 16 km (10 mi) or less, suggesting potentially high levels of visual impact for sensitive viewers. Smaller wind facilities (25-48 turbines) were generally judged to be easily visible at distances of 23-24 km (14-15 mi). Larger offshore wind facilities (100 or more turbines) were judged likely to be seen easily by casual observers as far away as 29 km (18 mi), and were visible with extended or concentrated viewing at distances greater than 40 km (25 mi). That these distances are greater than those reported in previous studies is likely a function of the long-term trend toward larger offshore wind facilities with more and larger turbines than those assessed in previous studies. Turbine blade movement was visible in 42 of the 49 daytime observations, at distances as great as 42 km (26 mi) as seen from an elevated viewpoint, and was observed routinely at distances of 34 km (21 mi) or less. At night, aerial hazard navigation lighting was visible at distances greater than 39 km (24 mi). The study suggests that as countries begin siting offshore wind facilities with hundreds or even thousands of large wind turbines, there is potential for impacts on sensitive visual resources in coastal areas at greater distances than past studies have indicated.