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Hi, I am having problems to model wind driven currents in a very shallow lagoon (maximum depth is 0.75 m).

There is a variable called THRESHOLD DEPTH FOR WIND which I had to modify as the default value is 1 m which will not produce any currents. When I modified this to 0.05 m it produced unrealistic values such as negative depths, below bottom levels free surfaces and very high speeds. The User manual p.44 has indicates that the THRESHOLD DEPTH FOR WIND is to avoid unpysical velocities in tidal flats or dry zones.


Does anybody know how to fix this? Is there a physically adequate value for the COEFFICIENT OF WIND INFLUENCE that can be used when THRESHOLD DEPTH FOR WIND is lower than 1 m?

Thank you in advance!

Hello

if you use a very low value for the depth threshold you will generate extreme forcing values for wind which is not physically accepted but also challenging numerically.
Moreover, if you use a constant value in space, you will apply the same wind force for any water depth in the domain. I suggest to use a space varying value (that depend on the water depth, for instance), an expression like max(0.1,H%R(I)/2.d0) could be used to define the threshold for any node I.
In all case, you have to use advection schemes that satisfy tidal flats and use raisonable time steps.

wit my best regards

Riadh

Hi
For this kind of problem, I would say that 2D is not the best solution to study wind effects.
You probably should move to 3D...

Kind regards

Hello Christophe

for a lake of maximum water depth of 75cm, not sure that 3D will perform better than 2D.

Riadh

Hi Riadh and C.coulet, I decreased the COEFFICIENT OF WIND INFLUENCE in 2 orders of magnitude (1.2615E-5) from the default value and I am getting now reasonable results for the THRESHOLD DEPTH FOR WIND = 0.05.

I was wondering if this is something physically adequate to do that makes physical sense? Would this be a more realistic (or unrealistic) solution than creating an spatially variable wind field that depends on water depth, which does not happen in reality and it requires more pre-processing work?

The problem is very much 2D given the shallowness of the lake, that is why I didn't set-up the model in 3D in the first place.

Thanks for your interest in this topic!

Hello

To answer your questions, the best way is to compare with measurements. If not satisfied, you need to model wind with space-time variation.

kind regards

Riadh