Forest height

[igarcia]

Dear Wasp team,

I am in the process of modelling a wind farm that is located in a forest, so I have some questions regarding this topic:

- First of all, what do you recommend to get the canopy height in order to calculate the displacement height? I have found this database: https://glad.earthengine.app/view/global-forest-canopy-height-2019 but I would like to know if you have recommendations on how to measure this value (besides going to the location and measuring, of course). 

- Secondly, I would like to know what is the canopy height at which we should start considering the area to be a forest in WASP. For example, for a sparse forest with 7m trees, should I calculate a displacement height or should I just input a very high roughness and forget about displacement height?

Thank you!

[Rogier]

Hi, 

We are currently working on a forest implementation where it will be possible to easily ingest displacement heights into WAsP. We have recently published a paper about this that might be useful:

https://wes.copernicus.org/preprints/wes-2021-28/

All these routines are unfortunately not yet available in a user-interface, but planned probably somewhere this year. As a rule of thumb you can use what is written there: displacement = (2/3)*tree height and roughness = 0.1*tree height. You could use these rules to create a roughness and displacement map from the layer you found. The new satellite based data layers that are presented in that paper are available here:

https://windsight.dhigroup.com/

In the paper we used a displacement from a forest that is at least 2.5 m high so in your case of 7 m trees displacement height could already become important (although still quite small).

 

 

Edited April 13, 2021 by Rogier

[igarcia]

Hi, 

Thank you very much for your fast reply, it was very useful. 

Do you recommend introducing the displacement height in the altimetry map in the map Editor, rather than substracting it to each turbine hub height in WASP? 

[Rogier]

Yes, I would do both, adding elevations to the existing map and extracting a height from your input wind climate. You then have to make sure to have the correct displacement height for all your turbines as well. Adding/subtracting the height from the turbines/climate is likely to have the biggest impact. And make sure to double check because I know from experience it is easy to make a mistake in this manual process. 

[pjindent]

Hi,

Is there an update on this issue? I am also trying to model a site in a forest.

I ran into the following issues:

1. I tried downloading the GWA landcover layers directly from the WAsP Map Editor, I can see the landcover IDs assigned to the landcover change lines, however it does not save the landcover ID information in the .map format so I saved it as a .lcv.gml file. I run into the following issues:

1a. I tried to import this file into WAsP and it is "unable to restore a map from the GML file". How do I import the GWA landcover layers in WAsP?

1b. I am able to import the elevation layer as .oro.gml file into WAsP but unable to import landcover layer in .lcv.gml format

1c. Is it possible to combine landcover and orography into a single vector file? Or should I keep them as two separate files and import them separately into WAsP?

 

2. I found an entry that indicates that WAsP is able to read displacement height directly from landcover layer from version 12.7 onwards: https://www.wasp.dk/news/nyhed?id=735ea5a0-af89-4e78-9a2b-72d232e27b74 .

2a. There is a reference to a spider grid analysis routine - "A new spider grid analysis routine is developed to process WAsP GML land cover maps". What is this and how do I use this?

2b. There is also a reference to a table - "For each cell in the zooming grid, the fraction of the total area f_i that each of a total of N land cover types in the land cover table occupies is determined and the roughness length is calculated. The displacement height is taken into account similarly to z₀ and is calculated for each cell in the zooming grid." Is the conversion of ID to displacement height and roughness automatically done by WAsP or do I need to use a specific tool/module to do this?

 

3. Re. the orographic submodule: This section describes advanced wind flow feature like computation of wind speedups due to elevation. Do I specifically need to input a .oro.gml file in WAsP for the Terrain Analysis? Do I also need to input the lcv.gml for displacement heights to be modeled with elevation?

I am at a loss to on how to do both these things (combine elevation and landcover inputs into WAsP). Any instructions would be helpful, I have gone through the online course material and I don't see any instructions on how to do this.

4. I am assuming the step to subtract displacement height to account for forest canopies is unnecessary in the current version - 12.8. Is this correct?  

 

 

[Rogier]

From what you describe it seems like what you are trying to should work. I personally would usually use the QGIS plugin for doing this. You can download the plugin here;

https://data.dtu.dk/articles/software/Using_QGIS_to_create_WAsP_maps/20495178

The video for working with displacement you can find here:

https://panopto.dtu.dk/Panopto/Pages/Viewer.aspx?id=fd8f16a9-9cee-4644-a71e-b09200ad6533

Other videos to get this point are listed here:

https://panopto.dtu.dk/Panopto/Pages/Viewer.aspx?pid=027e510f-32e6-473d-b96c-b0d800dddc2e

1a/b :

then pick "Edit > Change landcover" and assign a roughness and displacement height to a certain landcover.

Hopefully you can save this a gml.

1c: I would just keep them seperate and add them as seperate "vector map data layers":

 

2: The spider grid analysis is the default model since WAsP 12.7 so you don't have to do anything extra for this. The landcover table is not something you have to generate yourself neither, as it just made by the map editor/QGIS when you save the file as GML. It is embedded in that file.

3/4: Also this is just done automatically since WAsP 12.7. It will add the displacement heights from the landcover map to the orographic map in the program itself. In the video I linked I think there is towards the end a part where you can see it in WAsP where the displacement height shows up in the ´site effects´ tab.

 

 

 

 

Edited December 15, 2023 by Rogier