The wind loading code is based on the flow of fluid parallel to a flat plate. The plate can have roughness, this roughness is measured by roughness length (z0), and converted to terrain category. Appendices to AS1170.2:1989 contained charts for surface roughness length and relationship to terrain category: this information for the newer codes, can now be found in the commentary, and handbooks.
The closer a layer of fluid is to the surface, the lower the wind speed, as the roughness of the surface takes energy out of the fluid and also causes turbulence which hinders forward movement. Thus the further away from the surface the higher the wind speed. In AS1170.2 the surface roughness and the influence of such roughness on wind speed is accommodated by the use of the terrain category multiplier M[z,cat]. The basic reference regional wind speed is measured at 10m above the ground in terrain category 2: this is typically at a local airport, such as Adelaide airport. The terrain category multiplier adjusts this reference wind speed for heights different than z=10m, and for surface roughness different than cat=2.
So given a reference wind speed at Adelaide airport of Vzu=45 m/s [162 km/h] at 10m above the ground, then at say 300m above the ground in the same location, have M[z,cat]=1.32, and have a wind speed of 59.4 m/s [213.84 km/h]. Then assuming that the Adelaide hills near the city, are no more than 300m high, the air mass passing over the top of the hills will be travelling at 59.4 m/s. Now this is a different issue than the layer of air at ground level, being squeezed between the upper level layers of air, and the upward sloping ground. The mass of air will increase in speed as it moves from the plains and up the slope, additionally near the peak turbulence will be caused. This phenomena is accounted for in AS1170.2 by the topographical multiplier Mt.
The topographical multiplier however does not properly account for the flow of air within the undulating terrain of the hills themselves. Within the hills the undulating terrain provides shielding and surface roughness in much the same way as houses in the suburbs create surface roughness (cat=3) and multi storey buildings in the city centre create surface roughness (cat=4). However, there is also the issue of wind leaving the plains and flowing along a valley, compared to air flow within a closed valley. The wind environment of the Adelaide hills is more complex than is accommodated by the scope of AS1170.2: 2011, and therefore using such code cannot reliably predict an extreme value for the wind speed.
Clearly if near the peak of a hill, then the wind speed is closer to that of the free stream wind which is not influenced by the ground surface: but the fluid flow is influenced by the obstruction posed by the hill. Therefore expect some disturbance in the fluid flow, this disturbance may or may not be strictly called turbulence. What ever its name, the speed of the fluid in the disturbed zone is uncertain.
More research, and guidance is required on wind environment.
- [31/01/2017] Original