The potential effect of utility-scale wind energy projects on the regional landscape and the possible impact of shadow flicker on nearby residents are two of the most common issues raised by decision-makers and stakeholders when a wind energy project is proposed in their community. Failure to address these concerns early in the public review process can lead to misinterpretation, diminished public confidence and project permitting delays.
Areas with the best wind resources often have high scenic value. Therefore, the possibility of dozens – or even hundreds – of wind turbines scattered across farmland, forested ridgelines and offshore locations often raises public concern.
Regulators and stakeholders routinely ask for detailed information about the locations where the project will be visible, what the project will look like, who will be affected and to what degree. Speculative claims that a wind project would have an adverse impact on scenery can put a developer on the defensive if the company is not prepared to address these issues.
Surprisingly, most local and state governments have no specific rules, regulations or standards relating to visual assessment, nor do they offer meaningful guidance concerning appropriate assessment methods. Regulations are often vague, requiring only that projects meet undefined benchmarks, such as “no undue impact” on scenic resources. This disconnect makes it difficult for developers to know how to address this critical issue.
Visual-impact assessments (VIAs), typically prepared by licensed landscape architects, evaluate the potential visibility of a proposed project and objectively determine the difference between the visual characteristics of the landscape with and without the project in place.
In preparing a VIA, visual analysts rely on a variety of geographic information system (GIS) and 3D project visualization tools for public communication and regulatory compliance. The selection of the appropriate tool set depends on the scale of the project and the expectations of the developer, review agency and general public.
There are several tools that visual analysts can use to assess the wind project’s visibility and potential impact.
Line-of-sight profiles. Line-of-sight (LOS) profiles are simple, hand-drafted or two-dimensional computer-assisted graphics used to illustrate the degree of turbine visibility above intervening landform and vegetation. Although LOS profiles may be used to illustrate whether a specific project component (e.g., nacelle) will be visible from a particular location, they are generally not appropriate for use on multiple-turbine projects, where numerous LOS profiles and cumulative impacts make interpretation difficult. GIS-based viewshed and 3D photo simulation technologies are better suited to evaluate the cumulative visibility of more complicated, multi-turbine projects.
Viewshed mapping. Also known as zone-of-visibility mapping, viewshed mapping is a GIS-based tool that maps the geographic area within which there is a relatively high probability that one or more wind turbines will be visible. Viewshed mapping is typically used early in the VIA process to identify the areas where further investigation is appropriate and to determine sensitive viewing areas and the locations of the affected viewer groups.
The two types of maps commonly used in identifying visibility are topography-only viewshed maps and vegetated viewshed maps. Topography-only viewshed maps take into account the screening effect of existing topography (e.g., bare-earth conditions). This worst-case, treeless condition analysis is used to eliminate unaffected areas from further consideration.
Vegetated viewshed maps incorporate existing forest vegetation into the topographic analysis in order to identify areas of probable screening based on existing mature forest cover. This mapping type identifies the geographic area within which one would expect to be substantially screened by intervening forest vegetation. Vegetated viewshed maps provide a more realistic representation of project visibility than the bare-earth assessment does, and they also provide a focused inventory of impacted resources.
The existing vegetation is based on the interpretation of forest areas that are clearly distinguishable using multi-spectral satellite imagery. As a result, the potential screening value of site-specific vegetative cover – such as small hedgerows, individual trees and other areas of non-forest tree cover – may not be represented.
The screening value of existing structures is not considered either. This is a particularly important consideration in populated areas, where existing structures are likely to provide significant screening of distant views. When interpreting the resulting data, it is important to recognize that viewshed maps conservatively overestimate turbine visibility in areas where a project may be substantially screened from view.
Viewshed maps are created to identify locations within the surrounding landscape in which one or more turbine high points (i.e., apex of blade rotation) might be visible. However, these maps do not indicate the magnitude of the visibility, the effect the distance has on potential project visibility or the aesthetic character of what may be seen. Although this information can be determined through custom GIS analysis, it is not commonly included as part of the initial VIA evaluation due, in part, to the limitations of publicly available data.
The accuracy of viewshed mapping may be enhanced through the use of highly detailed light detecting and ranging (LIDAR) imagery, which resolves accuracy issues presented by traditional data sources. However, LIDAR-based analysis is not common for viewshed analysis due to the high cost of acquiring site-specific data from commercial sources.
Photographic simulations. Photographic simulations effectively illustrate the degree and character of project visibility. Photo simulations are developed by superimposing a rendering of a three-dimensional visualization of the project onto a high-resolution photograph of the existing site.
Although 3D modeling and image-processing technology has been around for many years, improved hardware and software, high-resolution digital photography and publicly available geospatial data make photo simulation an increasingly accurate and cost-effective tool for project visualization. Single-frame simulations are one of the easiest ways for the public to understand a project’s visual impact and have become a routine part of a visual assessment.
Simulations are most commonly presented in a single photo frame using a 50 mm lens (film equivalent) to approximate a normal depth of field. Panoramic simulations offer a wider field of view by stitching together adjacent images. Ideally, special photographic equipment and simulation techniques are used to minimize distortion and scale issues associated with panoramic photography.
Animations. Animations are used to illustrate the changing degree and character of the project over time and/or space. As hardware, software and modeling techniques continue to evolve, project animations are becoming increasingly realistic. There are three types of animations that may be used to illustrate visibility.
Static animations are used to illustrate views of rotating blades by superimposing their movement on a static image and presenting it in a video-clip format. A similar static animation technique is becoming increasingly common to demonstrate the flash characteristics and nighttime visibility of aviation obstruction lighting required by the Federal Aviation Administration. Time-lapse photography and synchronized animations can also be used to illustrate changing visual impact at different times of day and under varying weather conditions.
Drive-through animations are most commonly used for larger projects that may require more sophisticated presentation techniques. Video animations are a valuable tool in helping the public understand the temporal nature and spatial relationship of project views within a real-world context. Drive-through animations are based on digital applications that allow the observer to view project conditions while moving through the landscape along defined project routes, such as local roadways. Current GIS technology also allows operator-controlled, real-time, virtual-reality perspectives in which the viewer can control the route and direction of the view.
Video compositing uses a combination of 3D modeling, a realistic rendering and post-production software techniques to merge, align and synchronize an animated scene within a video recording of a project’s surrounding landscape. Video compositing illustrates a detailed spatial relationship, thus allowing the public to easily understand the visual characteristics of the project within its actual context.
Animations make it easy for both reviewers and the public to understand the temporal and spatial relationships of project views. Although animations are a very useful tool for permitting and community outreach, they may be cost-prohibitive for smaller projects.
The impact of commercial- and utility-scale wind projects on scenery is regularly cited as one of the most important issues to a host community. Absent local or state regulatory guidance, it is up to the developer and visual analyst to understand stakeholder concerns and prepare a suitably detailed visual-impact assessment to assist decision-makers. State-of-the-art visualization and assessment tools should be used in the early stages of the planning process in order to reveal proposed conditions and show stakeholders the locations where the project will be visible and its effect on scenic quality.
With the increasing construction of wind turbines near residential properties, shadow flicker has also become a concern for many landowners – whether that concern is real or assumed. However, with proper planning, evaluation and documentation, many of these concerns can be prevented or mitigated.
Shadow flicker occurs when wind turbine blades rotate, casting shadows over nearby structures and the surrounding landscape. These shadows may cause a flickering effect within a structure as the shadows pass over unshaded narrow openings, such as windows. Although shadows may sweep across the landscape, flickering only occurs within a structure, and only at specific times.
When calculating potential shadow flicker, it is important to collect accurate and detailed data, including information such as appropriate turbine specifications, turbine and dwelling locations, sunshine probabilities based on historical information from the nearest major airport, and anticipated turbine operational data. The resulting information would include the potential shadow hours per year, as well as the specific days and time of day the shadow flicker might occur.
Written documentation is an important part of a shadow-flicker analysis. Emphasis on exhibits and hourly calculations tend to minimize the importance of understanding what these numbers mean and how they were derived. Although the shadow-flicker report may contain a variety of graphs and tables, there are two types of illustrations that are generally used to demonstrate potential shadow flicker.
Worst-case shadow-flicker maps illustrate the potential shadow flicker based on the assumption that the turbines are always spinning and that it is always sunny. Of course, this is not a realistic scenario, and it may be misleading to the public and regulatory agencies. However, it does provide valuable information to the developer.
Anticipated-shadow-flicker maps illustrate the potential shadow flicker based on operational data and sunshine probabilities. This is a more realistic model that can prove beneficial to the developer, the public and regulators.
Animations are an emerging tool that is proving beneficial in further explaining shadow flicker to the public and regulators. Using 3D modeling software, the number of blade revolutions per minute and the sun characteristics, the movement of the shadow from a turbine or group of turbines may be illustrated in order to mitigate concerns about the frequency of the flicker and the area of potential impact.
Due to a lack of local and state regulations or guidelines that establish a permitted degree of shadow-flicker impact, it has become standard for any structure being evaluated with more than 30 hours of shadow flicker to be considered a nuisance and require mitigation.
As such, it is important for developers to understand the potential impacts and identify appropriate mitigation measures. This information – along with a disclosure of assumptions, methodology and other relevant data – should be provided in an easy-to-read format. w
Industry At Large: Siting
Visual And Shadow Flicker Impacts Pose Double Threat
By John W. Guariglia
Shadow flicker and visual impacts are among the most common siting concerns raised in communities. So, how can they be mitigated?
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