Lee zone of a hill or Shadow for wind analysis

[rowingengineer]

For my code AS1170.2 you cannot apply a hill multiplier of less than one, even if you are in the shadow of the hill.

The hill multiplier is a factor that generally takes into account the increased wind speed as you travel up a hill. However I am on the flat in the lee side of a hill (ie in the shadow of the hill) and I want to know if anyone applies a reduction to wind speed for being in the "shadow" of the hill.

My thoughts are that a hill of some height would provide more shielding than a house, but to what extent.

I know for the leeward site of wind breaks and parapets that reduction in wind pressures are possible and the effect of the wind break can be up to 30H and in the code for a large parapet it is generally greater than the height of the parapet. so I am thinking that if we are within 1.5H of a significant hill that some reduction would be appropriate.

I am also thinking that a 0.8 reduction factor wouldn't be two unjustified, given that the reduction factor for a tall parapet is 0.5.

Thoughts?

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[CELinOttawa]

I don't think you should deviate from the code, personally. Also, I doubt your hill shape is going to be "sharp" like a parapet. Doesn't feel right... Maybe check a few codes; If no one allows this I'd forget about it.

 

[JAE]

What if someone someday excavates the hill away?

 

[rowingengineer]

I wouldn't be keen on using this for escarpments or similar, I was more thinking about real hills maybe even mountains, i.e. least 100m high or greater, mostly targeting hills in the 500m and greater height range.

Most of the codes in the world didn't have speed up multipliers for wind up a hill until early 70's, before this engineering judgement was used to increase the pressure. I see no reason why they reverse cannot be applied in this case. There are numerous wind damage studies that talk about reduced damage from large wind events to particular suburbs due to hills even to go as far as mentioning it in the commentary of the code, but I doubt any research specific to this has been conducted.

from the final report of cyclone Tracey

http://www.riskfrontiers.com/publications_reports.htm

is one such publication:

"In contrast, suburbs such as The Narrows and Ludmilla, which suffered relatively low levels of damage (0.3 and 0.33), were for the most part in valleys to the lee of hills so will have been
subject to reduced wind speeds."

Having been to this area, the hill they are referring to is about 10m high, so I am thinking larger hills would have better effects.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[CELinOttawa]

You could be right, but you could also be wrong. Many a problem has come from a counter-intuitive real world scenario!

 

[rowingengineer]

Having spent a bit of time on this I now understand a bit more about down-slope acceleration of winds. Hills with 400m to 500m height can have significant down-slope acceleration of wind speeds. It would be more likely that smaller hills would act like a shielding member if they has similar terrains situations where as larger mountains/hills would need to be treated as a new environment. This from what I can see has a lot to do with the the friction of the surface and slope.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[CELinOttawa]

Well now we all know... And I will be able to explain should I ever come across an informed client (or, more likely, an interested and bright intern who asks this particular question). Thanks for running this one to ground and sharing.