open TELEMAC-MASCARET The mathematically superior suite of solvers

Dear Forum,

Regarding my MSc dissertation I'm trying to build a nested model of the Solent area on the south coast of England (Southampton). The difficulty is a fairly individual tide of the area. Because of reflection of the french coast, amphidromic points and the Isle of Wight Southampton and harbours around the Solent show a double high tide manner. The tide somewhat rotates in an anti clockwise manner around the island.

I tried to model this in a smaller domain until I realised that this pattern is caused by wider scale hydrodynamics so I modelled the whole English Channel.

What I'm trying now is to extract the pattern from the English Channel model to again force the smaller domain and by this reproducing this special tidal pattern.

What I did so far is in BlueKenue I used the open boundary of the nested model as line and extract along a line the FreeSurface and scalar of the velocity vector in U and V. From This i created a lbf file. In the lbf file the west-east asymmetry is apparent but if I force the nested model with it, it seems like the tidal signal advects almost simultaneous from west to east. By far not producing reasonable tidal curves, even though the tidal curves in the English Channel model were ok.

What I forced the model with was SL VU and VV until now. I'm trying now just to use the SL. As option for liquide boundary conditions I'm using the Thompson option and in terms of velocity profiles I had used option 2, using the U and V values from the liquid boundary file.

I have the feeling I'm missing something! For instance, does the line of extraction has to lie on nodes in the larger model? Or anything?
If someone has any Ideas or experience they would be really appreciated.

Attached I'll put a picture of the model domain to as well my lbf and .cas file. Maybe someone can provide some help.

Additionally I provide a picture of the actual area of Interesta nd my lbf file but I think the format should be alright.

Further I finished the run where I used only the FreeSurface elevation as forcing, and this comes way closer to what it should look like than using the velocity as well. But I got a strange late third high tide during one cycle. To show you what I mean I added a figure of a tidal curve.

Hi Tim

Did you ever resolve this problem? I am having a similar issue trying to run a tidal simulation in the English Channel and not recreating the double lows.

Oliver

Hi Oliver,

after different attempts in terms of resolution of my nested model I was able to resolve the double high, but the difference in amplitude between the first and second peak was about 3m. I presume this false representation has to do with a distortion of the signal due to downscaling, as my nested model didn't had the same cell size as my English Channel Model.

However, I build two different English Channel models afterwards. One using a depth criterion determining the cell size, which was of rel. high resolution. This worked fine but was of high computational expenses. Another one was of an universal 5 km resolution only increasing towards my study area (Lymington). This showed the dupable high tide as well but also showed a massive phase shift in terms of tidal elevation.

If you have powerful computational resources I would recommend to build a relatively high resolved English Channel model. It literally depends on the size of your study area you are aiming for and your resources...

hope that helps,

Tim

Hi Tim, thanks for your response I haven't had time to reply until now. I did wonder if it was a scaling issue and unfortunately building a large high res model is not feasible for me.

Thanks, Oliver

Hi Oliver,

I think it also depends on how you want to force it. I used the TPXO model for domains equal or larger than the Solent region and figured out that this wasn't sufficient to reproduce the correct tidal signal. The wider geometric setup was important as reflections on the french coast, amphidromic points and to some degree reflection on the southwards travelling tide from the North Sea is important for the tides in the English Channel. I think it depends on the size of your study area and if you have measurements you can force your model with it might work.

Cheers,
Tim

Ok, yes this is a similar conclusion I came to. I was trying to extract boundary conditions for a relatively small domain from a larger English channel domain. However, unfortunately that larger channel model was not detailed enough to provide good boundary conditions. I will remake this channel model at some point.

Thanks for your help

Oliver

Hi Olivier

I have a coarse grid and want to extract boundary condition for a finer grid.

May i ask how did you do the nesting? Do i need to modify any subroutine?

Thanks in advance
Homayoon