Displacement Height (2004-01-30)

[Old Forum Archivist]

Hello,



Here is what I read from a company doing meso maps. Is this also applicable to WAsP? We have a lot of sites entirely located in forested region.



"Displacement Height:

An additional factor to consider is that the selected heights chosen on the wind maps may not always be the height above ground. Where the vegetation is dense, the "effective ground level" is not the base of the vegetation but the middle of the vegetation canopy because the wind flow is displaced upward. The level of zero wind, called the displacement height, is typically about two-thirds the height of the top of the vegetation. In dense forests the height above ground at which the predicted wind speed actually occurs may be as much as 7-15 m (23-50 ft) higher than indicated on the maps. For example, in an area covered by forest with an average canopy height of approximately 18 m (60 ft), the Wind Map's wind speed prediction at the 65 m (213 ft) level would actually apply to a height of 77 m (253 ft) above ground [65 + 2/3(18)]."



Thanks again for any help

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Hello

see http://www.bwea.com/planning/trees.html

you'll find some really good lectures.

GH, Bonus, RES,... and Risoe's point of view about this problem.

Enjoy !

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