## A channel with two open boundaries but one forced by tide...

MOHID Water forum. Questions and discussion around MOHID Water
apalao@proes.es
MOHID User
Posts: 37
Joined: Thu Jan 01, 1970 00:00

### A channel with two open boundaries but one forced by tide...

Hi mohiders,

I´m having some troubles with the hydrodynamic model. I´m trying to simulate a channel which has an open boundary forced by tide. The other open boundary is forced by a river discharge. How should I define the tidal file and the hydrodynamic file in other to make this work? I can run the model but the results obtained are clearly wrong. In spite of the tide been defined on one of the open boundaries the tide in post-processor appears to be entering through both boundaries simultaneously. This occur even without river discharge.
Any idea?

Thanks.

Ángela

acanas.maretec@taguspark.pt
Posts: 103
Joined: Thu Jan 01, 1970 00:00
Location: Instituto Superior Técnico - Technical University of Lisbon

### Re: A channel with two open boundaries but one forced by tid

Hi Ángela,

Your wrong result seems to happen because you are considering the river boundary as an open boundary. If you signal TIDE : 1 then MOHID Water will consider tide in all domain boundaries which are open (no land cells), even if you just define a tide gauge in some specific location (e.g. in the other boundary).

What you have to do is to extend your bathymetry so that between the river boundary and the boundary of the domain is at least one land cell.

Best regards,

Ângela

apalao@proes.es
MOHID User
Posts: 37
Joined: Thu Jan 01, 1970 00:00
Hi Ângela,

I would like to confirm my suspects: If I want to simulate a channel with a forcing tide in an open boundary and a river forcing the other open boundary, I have to close the river side boundary???
In other words, there is no way to simulate a tidal wave going into through a boundary and coming out through the opposite side???

Regards,
Ángela

acanas.maretec@taguspark.pt
Posts: 103
Joined: Thu Jan 01, 1970 00:00
Location: Instituto Superior Técnico - Technical University of Lisbon
Hi Ángela,

Sorry, I do not have full knowledge about this issue but I can tell you what my experience is . Maybe this is useful for you anyway.

I usually close the river boundary and as far as I know it is a common procedure. Generally it is not a coarse approximation to do this because you usually consider a river channel in the model big enough that in the end of it the tide influence is minimum and the river flow is the determinant forcing. So it does not matter that the water will never be leaving the domain there because actually it will never happen because of river flow.

If you define TIDE : 1 MOHID Water will consider the tide in every open boundary: if you only have tide gauges in one boundary it will try to interpolate the results for the other boundary, eventually it will do it lowsily. But if you have less than three tide gauges it will not interpolate and consider the conditions of only the first gauge for the full open boundaries (the eventual second gauge will be ignored).

If you are considering the Blumberg-Khanta radiation condition you can eventually have the river as an open boundary and try to «artificially» remove the tide conditions there. I suppose you can do that by specifying a big relaxation time in this river open boundary (in the TLAG file), so that tide will not be important there. I never tried this so I do not know if this will produce good results in a model, you have to try yourself if you find it useful.

Best regards,

Ângela

apalao@proes.es
MOHID User
Posts: 37
Joined: Thu Jan 01, 1970 00:00
Dear Ângela,

Nevertheless I must insist one more time.
My simulation period is very short (is not well defined yet, but should be below 3 days). In such a time i can assume that tide is a simple wave of known constant amplitud (i.e: 3m) and period of 43200 sec (for semidiurnal tide).
Your last answer seems to point to that idea in some way, but i don't know how to simulate this case with Mohid.

I would be great if you could give some directions...

Regards and thankyou for your effort.

Ángela.

nuno@fis.ua.pt
MOHID Insider
Posts: 55
Joined: Thu Jan 01, 1970 00:00
Hi Angela,

I am a researcher at the Aveiro University and the topic of my previous work was the simulation of hydrodynamic processes in a tidal channel (with an open ocean boundary and an \"open\" river boundary).

Your simulation is very simple. You have to close the river boundary in order to perform the runs. In that way, the tide is imposed (only) at the open ocean boundary.

Using the GUI is a straightforward process: you only have to impose the bathymetry and the \"tidal file\" and then start the runs.

Hope this helps,

Cheers,
Nuno

Posts: 349
Joined: Thu Jan 01, 1970 00:00
Location: Instituto Superior Tecnico - Lisbon - Portugal
Contact:
Dear Angela,

From what I understand, you want to have a channel where in one of the boundaries you want to have a river inflow and during flood conditions you want the flow to flow out of that boundary. This is a bit hard to do and you're most likely to face some numerical problems in the boundary. You might do it if you know the water level at the river boundary and the current velocities at that boundary. Nevertheless, I advise you to extend the river boundary until a place where the tide has no influence and then set the river discharge there.

Regards,
Luis

deeptha@ktr.srmuniv.ac.in
MOHID Beginner
Posts: 4
Joined: Thu Jan 01, 1970 00:00

### No result after first column of grid - pl help

Hi, I'm trying a rectangular channel with tide at one end and closed at the other. Whatever my grid size, I'm not getting any result after the first column of the grid (where tide is imposed). What could be the reason? I'd appreciate any suggestions!

deeptha@ktr.srmuniv.ac.in
MOHID Beginner
Posts: 4
Joined: Thu Jan 01, 1970 00:00

### Result outside grid

Dear friends,
I realised that I am getting a simulation in a channel, but it is appearing outside my channel and grid! I have to figure out what is wrong here...would appreciate any help.

luis_at_mohid.com
MOHID Developer
Posts: 553
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Location: Instituto Superior Técnico - Technical University of Lisbon
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### Re: A channel with two open boundaries but one forced by tid

Hi Deeptha,

Have you solved this problem?

Can you give more details regarding your input data files?

Best regards,
Luis