Roughness with many windbreaks

[Vinh Le Thanh]

Dear all members and admin,
I have a project, but I quite confuse to create the roughness map of project. Because the site mix between the mangroves and water; some position of site have many house focus along the river and road.
I give you the coordinate -WGS 84 -zone 48:
- Latitude: 9.183433°
- Longitude: 105.627373°
Can you give me some recomendations with this situation.
Tks a lot!

[Morten]

Dear Vinh Le Thanh,


I inserted the coordinate into Google Earth, and I agree that the roughness pattern at this site is quite complex with many small tiles and a large variation in surface roughness. From the pictures I also saw that fields are flooded in part of the year, and open water surfaces will reduce the roughness roughness. For WAsP applications you should, however, apply a roughness typical for the entire year as this is more relevant for the yearly energy production.


It would be a big job to digitize a roughness map for this site. Instead, you could try to press ‘File| Import| from Web-database| GWA Map Warehouse – roughness’ in the WAsP map editor to download a roughness map. The resolution is not ideal, so you may need to add more details for the area close to your windfarm and met mast. But you don’t have to edit the roughness far from the sites of interest, as these details should not affect the WAsP results much.


With best regards,
Morten Nielsen

[Vinh Le Thanh]

Dear Morten Nielsen,
Thank for your answer!
Actually, I try to import roughness from Wed-database, but it not conincide with the background was imported.
And I think roughess map is importance in my case because the elevation is not change too much.
I read in manual, it has some information to made me quite hesistance when decide to digitize the roughness:
- "Shouldn't digitize too large with few width meter, and the rest area digitize too small". So, in this situation with many house focus along the river and road - how to deal with this? Should I digitize detail with this site?
And I should consider the site with how much area (length & width) to decide the roughness?
Best regard,
Vinh Le Thanh

[Niels Gylling Mortensen]

Dear Vinh Le Thanh,
You are right: the roughness map is important in this case, so I will try to give you a few more hints:
1. The size (radius) of the map should be = 100 times your mast height and then some. So, for an 80 m mast, the radius should be, say, 8000 m + 50% = 12 km. The total map is then approx. 25 km by 25 km with the mast in the centre.

2. The most important roughness change is the coastline, i.e. from water (z_0 = 0 m) to land. If you download and use an SRTM elevation map in the Map Editor, this comes with the coastline already.

3. You need to generalise the land cover (roughness map) in order for the work to be practical. So, there is maybe not so many land cover classes (e.g. from south to north): the sea, marsh areas along the coast, farmland with few houses, farmland with many houses. For wind directions where a lot of the wind comes from, you can try to detail the land cover description and see if it changes the results. But you cannot specify each and every house, of course, but may specify an area of houses along a road.

4. How to digitise is described in this report http://orbit.dtu.dk/en/publications/wind-resource-assessment-using-the-wasp-software-dtu-wind-energy-e0135(259e26f3-1828-4e3f-9c37-17de375cd057).html


Hope this helps,
Niels

[Vinh Le Thanh]

Dear Niels,
Thank for your response. It's very useful with me.
Best regards,
Vinh Le Thanh