2D connection inside one vs connecting two flow-areas

[sondrfo]

Hi,

Are there any difference between how flow is computed for connections between two 2D flow areas, and for connections inside one 2D flow area? I am doing a dam break analysis (full 2D), and are considering two options:
(1) Calculate the whole strech as one 2D-area with a internal connection simulating the dam. Doing this I have to fill my reservoir with water with “pre-simulation” (no initial condition available in the reservoir.)

or

(2) Make two separate 2D areas, one for my reservoir and one for the strech downstream of the dam in order to be able to set a initial condition for my reservoir (2D area). Connection between the two as a 2D/SA Connection.

If I use the same cell-size, etc. in the two cases, will I get different results based on the method I use?

[Jarvus]

If you select the 1D weir equation*, I think technically the flow is computed in the same manner, however, you can get differences.

I’ve commented on this before.

Short answer: I generally recommend trying to do everything in a single 2D area because it tends to be more stable.

Longer answer: If the dam break is inside of the 2D area, the flow through the dam break gets updated for each 2D iteration. If the the dam is between two different 2D areas, the weir flow computation is either lagged (if 1D/2D iterations is off) or the iterations are on the time step (if 1D/2D iterations are on). But in either case, it tends to have more stability problems.

Yes, if the dam is inside of the 2D area you have to fill it (RAS needs to let you specify a starting WSE for interior dams). You can add a hydrograph boundary condition line inside of the 2D area immediately upstream of the dam and give it an extremely large flow just to fill the reservoir up quickly at the start of the run. Writing out a restart file, after the reservoir is filled can also save some time for future runs.

* If the dam is inside of the 2D area, you also have the option of using the 2D domain instead of the 1D weir equation which is not an option if using two, 2D areas. However, this equation assumes open channel flow, so it is not generally a good choice at the start of the dambreak where the weir flow assumptions are more valid. Using the 2D domain equation when the flow is actually weir flow seems, in my experience, to produce too high of velocities.

[sondrfo]

Thanks a lot for an informative answer.

Do you have a good answer for my next question?

When viewing the results from the simulations dam-break simulations, the hydrograph created in RasMapper (RM) (Drawing a Profile line close to the dam downstream) and the hydrograph in the “Stage Flow Hydrograph” (SFH) are different. Most of the time the results in SFH are not as smooth as the ones from RM. It also seems that the results showed in SFH are much more sensitive to changing parameters (such as timestep, max iterations), than the results in RM. I find this both using one single 2D area and using 2D/SA connections.

Where does the RM and SFH get the results from, and are any of them “more” accurate than the other?

[cameron]

2D Connection lines are more accurate than profile lines drawn in Mapper. The 2D connection lines are grabbing results along the cell faces where as most of the time profile lines are not along cell faces and have to interpolate from the results.

[Luis Partida]

Yes^^ I recommend using dummy 2D inline structures if you are looking for the best results. Just do a weir with a height of 0.01-ft above the terrain. Make sure you either use the 2D equation or drop the weir coefficient to 0.2 or lower if you expect complete submergence

[Author]

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