Wind loads - topography effects

[Cruezr]

I have a situation where two houses are located on opposite sides of a gully.
The gully is 20m wide at the bottom, is approx 500m wide at the "top" and has a rise of about 25m (so a big flattish V-shape).

Most winds codes (I presume) consider topographic changes starting from the bottom of the hill/escarpment and going up, assuming unimpeded wind load to the bottom of the hill. But what happens in the case of a gully that slopes into the base of the hill then back up the other side ?
Is there any consideration given to the wind having to get down into the gully first then back up the other side, or does the wind skip across the gully and hence reduce the topographical effect ?

As a comparison, what happens when the gully is more like a ravine say only 100m wide and quite deep...common sense suggests there is little wind in the bottom of the ravine and the wind skips across the top, thus effectively nullifying the topographical effect?

Interested to hear members thoughts.

 

[JAE]

Not sure what code you are under.

In ASCE 7-10, section 26.8 requires the use of topo effects ONLY when ALL of three conditions are met:
To paraphrase:
1. The hill, ridge or escarpment is "isolated" and unobstructed for a distance of 100 times the topo height.
2. The hill, ridge or escarpment protrudes above the height of other topo features over a 2 mile radius by a factor of 2.
3. Your structure is on the upper 1/2 of the hill, ridge or escarpment.

For your case it wouldn't apply since you aren't technically on an isolated hill, ridge, etc.



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[msquared48]

I would worry more about wind being channeled up the valley, parallel to the ridges, a funneling effect.

Happens all the time in foothill and mountain valleys...

Mike McCann, PE, SE (WA)

 

[EngineeringEric]

Don't forget the elevation factor.... it can help a lot and maybe squash any concerns you have if they apply to your case and are not utilized when calculating the force.