The Environment Canada Wind Profile equation used in their site-specific recommendation calculates the wind pressure profile as a single equation containing 6 variables, including the wind speed at 10 m elevation from a map, the roughness length for the region as well as at the crest of the feature, and derived parameters which are related to the feature characteristics. A sample of the summary is shown below for Version 2.1, 2016 and Version 2.2, 2019.
Version 2.1 2016 Applied in March 2016
Version 2.2 2019 Applied in February, 2023
The version 2.1 equation was introduced in 2016 after extensive discussions between EC and industry which pointed out that the 0.1 power law for wind speed does not correctly represent actual winds for towers. The range of exponents adopted by EC to make the correction is from the ASCE 7 guideline, but the range is appropriate for 3 second gust winds, whereas the S37 requires an hourly average wind. Although ASCE 7 also supplies exponents appropriate for hourly averaged winds, the EC justification for their choice is that it is meant to provide gust information; however the gust effect factor in the code is meant to account for the gust component, so the recommendation should supply the hourly average wind.
In the example shown above for V 2.1 there is a change in exponent required because there is a change in roughness length on the fetch to the tower (going from 0.4 to 0.05 m). This necessitates two different equations to be used in the calculation. However, it can be seen that both equations in the example have the same exponent. This is the case for other profile calculations which I have seen, and is necessitated by the fact that if the different exponents were used for the two equations the profiles would not match up at the transition height.
In the latest reports based on V2.2 as shown in the second example there is a single equation used, meaning there is no change in roughness at the tower from the regional average. In many of the cases there actually is a change in roughness as there is for the V2.2 report shown above.
The tower in the second example is located on an escarpment, and is assumed to be surrounded by fully developed forest with roughness length of 0.7 m. In actuality it is located on an escarpment within 500 m from a Bay, so the roughness goes from 0.002 m over the water fetch of 3 km to 0.45 m averaged over the escarpment fetch. The correct description leads to a factor of 2 increase in the predicted wind pressure over the tower height, but the user of the report is not aware of the error and can make a totally erroneous design without being aware of it.
This example shows the need for the equation to deal with a change in roughness so as not to be constrained to an erroneous approximation. This can be easily done by keeping the original logarithmic profile for the wind in the equation which would avoid the mismatch between the assumed power law profile and the logarithmic term inside the brackets in the equation.
I have detected such issues in other cases as well, for example a tower located on a wooded 200 m high ridge is calculated as if there is no ridge at all.
The main problem with the EC recommendations report is the lack of detail in describing the situation to the user. Although it is possible to decipher the various parameters in most cases, as discussed in a previous blog on this topic, in this case the user would also have to go to Google Earth to be able to see the actual situation. In most cases, if the user were given the necessary description he could catch such errors.
The Site Specific report has to permit the user to see the assumptions underlying the calculation, with a location map to demonstrate that the correct situation is used in the report. Industry should press this issue with Environment Canada through the S37 Committee.