How do structures in TUFLOW FV handle submergence?

[Guest Chris Nielsen]

Could you briefly explain how FV handles weir submergence? I didn’t see it in the last draft version of the user’s manual we’ve got.

[Guest Chris Nielsen]

Structures are handled in a way so that we switch from the full momentum equations to the structure equation. This takes care of submergence issues and also stability problems, especially if (for whatever reason) the structure equation is not a constriction to flow (ie an inadvertent user error).
 
The logic process for computing structure flow is as follows:
1       At the commencement of a model timestep, the structure flow (or, specific to a weir, the weir free overflow) is calculated (Q_struc) and also the standard shallow water equation flow is calculated (Q_SWE).
 
2       A check is performed between the structure flow (Q_struc) and that calculated using the Shallow Water Equation (Q_SWE), where:

Ø  IF Q_struc < Q_SWE THEN

o    Apply Q_struc to cell face

Ø  ELSE

o    Apply Q_SWE to cell face (ie ignore the structure)

 

3       If Q_struc is applied then momentum is transferred through the structure based on Q_struc and upstream velocity, which is appropriate for a weir.
 
To give you an example of how this works OK, I’ve attached a paper that Bill wrote a while back (see link) – I replicated the test that is shown in Figure 8 of this paper (see attached graph). As you can see, we get a near perfect match with theory and TUFLOW Classic results at all stages of flow regime.