Guidance on using an updated collision risk model to assess bird collision risk at onshore wind farms
NatureScot published a guidance document on collision risk modelling. The document aims to provide a standardised approach to assessing collision risk at onshore wind farms. A standardised approach would allow for impact assessments to be more readily comparable and help facilitate the cumulative impacts of multiple wind farms regarding collision risk.
The approach outlined in this guidance builds on previous methodologies outlined for collision risk modelling (Band et al., 2007), with the model’s output being the predicted number of birds per year which will collide with wind turbines at the wind farm. The model requires several inputs including bird survey data (flight density, heights, distribution), bird behaviour, turbine details (rotor diameter, blade size, pitch, rotor speed) and bird details (body length, wingspan, flight speed). The model outputs can then be used to establish the significance of the mortality at the wind farm concerning the local or national population of that species. The model comprises five stages.
- Stage 1 estimates the density of bird flights in the vicinity of the turbines.
- Stage 2 provides an estimate of the number of bird passages through rotors based on Stage 1.
- Stage 3 calculates the probability of a collision when a bird passes through the rotors.
- Stage 4 calculates the average probability of collision based on the outputs of Stage 2 and Stage 3, as well as the proportion of time that the turbines are not operational (e.g. for maintenance).
- Stage 5 accounts for the proportion of birds that are likely to avoid the turbines because they have been displaced from the site or because they take evasive action to avoid the rotors.
The guidance document also reiterates the need to outline the uncertainties in the model for collision risk within an impact assessment. These uncertainties could include the variability in flight activity data, such as limitations in the estimates of flight heights during surveys and the lack of knowledge of night-time behaviour. It could also include limitations within the model itself. Such limitations are variability in bird dimensions and flight speed, the simplification of the shape of birds and turbine blades within the model, and the uncertainty arising from the turbine options available at the time of the assessment, including the size and speed of rotors.
References:
Band, W., Madders, M., & Whitfield, D. P. (2007). Developing field and analytical methods to assess avian collision risk at wind farms. Birds and wind farms: risk assessment and mitigation, 259-275.
Naturescot (2024) Guidance on using an updated collision risk model to assess bird collision risk at onshore wind farms