The 2022 revision of ASCE 7 made a long-overdue change to the criteria for considering terrain speed-up of wind. The 2016 revision in Section 26 8 set out 5 criteria for determining whether an abrupt change in topography would be considered in calculating the speed-up factor Kzt.. Effectively these criteria meant that an engineer was not required to consider a speed-up factor for non-isolated hills.
This requirement for dealing only with isolated hills was not supported by experimental measurements. In fact, the Simple Guidelines on which the ASCE 7 speed-up equations in Section 26.8 were based provides the methodology for accounting for speed-up on non-isolated hills or ridges, so called undulating or rolling terrain. Physically, wind flowing over a shadowed hill downwind of a similar sized hill will experience a speed-up, but the magnitude of the speed-up will be reduced from that experienced by the upwind hill due to the loss of energy in the flow field.
By dropping the isolated hill criterion without changing the equations for the rolling hill condition, the 2022 revision of ASCE7 will over-state the effect in rolling hill situations. This is why the 2016 revision recommended that a valid site specific procedure be used in the more complex situation.
The ICE Inc. Site Specific Procedure implemented in 2012 uses the original Simple Guideline procedures to determine topographic speed-up, thus avoiding the error of ignoring speed up on downwind hills. An Australian Field Study in the Belmont Hills of New Zealand published in 2015 compared the prediction of the speed-up prescription as specified by 7 Wind loading Standards which showed that while most were able to show speed-up on the first hill in the range of 9 hills, none of the codes, including the ASCE 7, was able to show speedup for subsequent hills. ICE presented a paper at the 2022 IASS Work Group meeting in Toronto, showing that the ICE implementation of the Simple Guidelines is able to reproduce the observed speed-up values over the entire range of hills.
This fall the ANSI/TIA 222 has announced that Revision I of the Standard has been approved. Among the many changes in this revision, the speed-up equations for topography were modified in accord with the Simple Guideline recommendations, including the rolling terrain prescription. ICE has tested the new TIA equations against the Guidelines and the ICE implementation of the Guidelines for several simple cases and finds good agreement. One of the situations dealt with by TIA as a separate category is the flat-top hill. The ICE procedures do not require this because it can be dealt with by using the escarpment prescription.
There are more complicated cases where the ASCE7 and TIA 222 are not able to provide adequate guidance. For these cases they recommend that a site specific study be carried out. For example, the special wind region designation applies to the situation where mountain ranges or other flow modification obstacles make the map wind values unrepresentative for a specific location. In this case the study includes using local wind information of sufficient duration and statistical methods to establish the return period wind or gust. After performing thousands of extreme wind derivations it has become clear that the wind map can be misleading even in areas which are not in the designated special wind regions.
The first step in evaluating the appropriate speedup profile is the characterization of the terrain roughness. The codes still retain the 3 way classification of the terrain, which sets lower bounds on the length of fetch in order to correctly categorize the terrain. In most of the cases the categorization of the terrain exposure into one of B, C, or D results in unrealistic wind profiles with height. This error is then compounded in the case where topographic speedup is experienced. For example if the tower is located on a wooded hill with the surrounding terrain being rural and fronting on a lake shore, depending on the fetches it may be impossible to assign a category which captures the wind behaviour adequately.
The problem stems from the fact that the lowest level of atmosphere is modified by interaction of wind flow with the underlying surface. When there is a large change in the surface characteristics the lowest level readjusts to the new conditions, whereas the higher levels retain the original characteristics. This means that the wind profile cannot be described by a single power law profile.
As an example, if the flow originates over a large body of water (river, lake or coastline), once it passes over the land it is modified at the lowest level, producing significant slowdown depending on the new exposure conditions, and the depth of the modified layer grows with inland distance. In the modified layer a different profile is required but this cannot be extended to the higher levels without misrepresenting the wind speeds. The net result is that a tall tower is experiencing speeds which are different from the simple power law characterization.
In the ICE site specific procedures the exposure conditions are described by the roughness length on a continuous scale, and abrupt changes in land cover or land use are explicitly taken into account to adjust the profile. This procedure is more computationally intensive and requires the use of a computer program and more detailed characterization of the surface conditions with distance from the tower site.
The codes still retain the requirement that the extreme wind be assumed to approach the topographic feature from any wind direction. Since the speed-up effect depends on obstacle type and terrain roughness which differ for different wind direction, this requirement will over-estimate the speed-up that can actually occur. ICE has developed a more detailed directional procedure which characterizes the basic wind at the airport by wind direction sector and characterizes the topographic feature and changes in exposure by direction to provide a fully documented realistic speed-up for each of the sectors to permit the selection of the overall maximum wind speed.
Hills and ridges also affect the ice accumulation on a tower in a freezing rain or in-cloud event both due to wind change with height and temperature change with height, especially for tall towers. In our site specific procedures we explicitly account for these changes in producing estimates of glaze and rime icing accumulations for specified return period as a profile with height.