roughness map of area with different tree heights

[WAsP101]

Hi Forum users,
Have been using WAsP map editor for basic digitising, ie large areas of trees (one roughness), lake (another roughness), etc. But now I wish to make a roghness map of a forested area, using the zero plane displacement height method for different coups of trees. So the first question is:

1) How do I create the tile effect (lines with different roughness values joined together) and join them together. If I had say 3 lines of different roughness values that I want to join to a common point, how do I do this please? It's a while since I did the WAsP course and the notes don't give much detail. Like I say up until now I have been just digitising single polygons.

2)For each line I can give it a roughness value and height. Assuming that I am digitising on top of a contour map (.map), if I want to give this area of digitised roughness a ZPDH of 10m and the underlying contours are 300m. Do I just set the height to 10m or 310m?

Basically I am wanting to build a map for a reasonably complex forestry situation where I know the different tree heights. So using a similar approach to Delwik etc.

Sorry for the long post, trying to be as descriptive as I can.

hope someone can help

[Mark Kelly]

Hello "WAsP101,"

1) In the Map Editor you can join 3 lines together, for example, by holding down the CTRL key, and clicking each one near the point at which they are to be joined. Then you can right click near/on the joining point and choose "anchor lines."

2) If you are wishing to use a displacement height and digitize "on top of" a terrain elevation map, then indeed you can add the ZPDH to the local elevation as you wrote. Note however that the transition from forest to non-forest area is difficult to model (most flow models cannot capture all the physics), and so there is more uncertainty around the edges of the forest, particularly when incorporating ZPDH into a terrain map. If you are only considering a few locations above forested area, instead you may consider simply subtracting the displacement height from the height AGL of these locations' hub heights (for example), i.e. instead of digitizing modified height contour lines.
I imagine if you are familiar with Dellwik et al, you are also familiar with Raupach (1994), which can be useful -- reminding that the effective/usable displacement height and roughness depend on the sparseness or denseness of the forest. Again also the transition near forests can be difficult, see e.g. Ferhat Bingöl's work from this year (and last).

--Mark Kelly, Risø / WAsP team

[WAsP101]

Hi Mark,
Thank you for the quick reply. Can you just clarify, if I was creating a roughness polygon on top of a contour map of say 300m and wanted a ZPDH of 10m. Do you have to specify (in the roughness polygon) a height of 310m? or just 10m?

thank you again

[Ole Rathmann]

Hello "WAsP101", basically the answer to having a 10m ZDPH on a 300 plateau is using a "displaced" height contour of 310m. Please notice that the situation is more complicated if the forest is on a slope - in that case you should "clip" (use the clipping option) out the forest area, save that as a separet map file, increase all elevations by 10 m, save again; then reload the original map and use "Add and replace" the forest map into it. That should give you the forest area "lifted" by the displacement height.