SRH-2D: Water appearing out of nowhere

[Hans Vahlpahl]

Hello,

I have a problem with modelling a small summer discharge of 0.013 m³/s with SRH-2D using an SMS model that has been used before to model the same amount of discharge via Hydro_AS.

Looking at the first three monitor lines there is no problem, beside the SRH-2D water arriving earlier. Since the primary objective will be a temperature modelling it is ok for the hydraulics to not align perfectly temporally.

However, after the streambed passes by a small lake, that is connected to the stream via two bed ramps that function as an inflow and outflow, additional water seems to appear out of nowhere. Despite the Inflow Boundary Condition at the start of the model area being a constant 0.013 m³/s as well as the inflow BC written in the_SIF.dat-file, a higher discharge gets recorded by the monitor lines downstream of the lake. This happens BOTH when the lake has been pre-filled via the ZONAL approach for Initial Flow Condition Method as well as when it has been left empty/DRY before modelling.

Monitor line shows some of the excess water of the pre-filled lake entering the streambed before the discharge of the Inflow Boundary Condition arrives. It is now about 0.022 m³/s instead of 0.013 m³/s.

In reality the stream passes through a culvert right after the lake. Since the discharge issues seemed to start there/after the culvert I hoped for an improvement by modelling it as an open channel in SMS. However, no change can be seen in the excel-discharge-graph.

Also, the further downstream I go the more the discharge oscillates.

 

Does anyone have an idea why or how this happens and how I could solve this?

 

Kind regards,

Hans

Edited April 21 by Hans Vahlpahl

[Hans Vahlpahl]

And sorry for the the two pictures in the end. I can't seem to get rid of them by editing.

[Donald Hendon]

What BC are you using at the exit?

[Hans Vahlpahl]

I used EXIT-ND as boundary condition.

[Donald Hendon]

Exit-ND? I am not familiar with that. Is that an SRH exit BC? The only exit conditions I see in SMS are Exit-H, Exit-Q, and Exit-EX.

[Hans Vahlpahl]

It is. It uses the total discharge at a cross-section upstream and the equilibrium channel slope at the exit to compute the water surface elevation.

I tried it with Exit-H and used the WSE, that my predecessor modelled for the same discharge with Hydro_AS at that exit BC. However that didn't really change anything.

What BC would you normally use if you only know the discharge flowing in and water depth in the middle of the modeling area taken from a pressure sensor, that is not situated close to the exit BCs?

[Donald Hendon]

So you are saying you used the Exit-H and populated with the normal depth option? If not, please share a screen shot of the Exit-ND.

Well, not really sure what you are using this for, so not sure if this method would be applicable, but since you know the inflow, you could use use Exit-Q and set the value going out the same as going in. That should eliminate your problem. 

I also notice that your channel mesh adjacent to the lake might benefit from using quads. 

[Hans Vahlpahl]

I manually typed in the WSE-value that was calculated by Hydro-AS at that Exit-BC.

Exit-ND basically only asks for the channel slope at the BC. [screenshot SRH2D-Skript]

The general problem remains that discharge is appearing in the middle of the model that has not been created by the Inlet-BC. I narrowed it down to the lake by the usage of more monitor lines. In general all the monitor lines register discharge as negative values. Only around the lake does the (-) sign not exist. (It used to be like this from the beginning. In the postings above I showed it all as positive to better compare it with the values given by Hydro-AS.)

I did two more runs with adapted models.
In the first one I raised the stream bed at the entrance to the lake so that all of the water flows into it and then back out on the other side into the actual channel, thus representing the current reality on site more accurately.

The monitor lines around the lake are distributed as shown in the picture below [edited: sorry, look at the bottom of the post]

 

A steady input of 0.013 m³/s produces the following results.

Monitor line 1 (red) shows the anticipated discharge (although negatively). At the entrance of the lake discharge starts to oscillate but kind of stays where it is supposed to be. The same observation is true for the water leaving the lake, measured at "lake exit". It appears to oscillate a bit less than at the entrance.
However on the very short distance between the lake exit and the next monitor line (right before the start of the main culvert) discharge is being registered as both positive and with a magnitude of around 0.017 m³/s, thus being higher than the Inlet-BC-discharge. The same happens inside the main culvert.
After the main culvert discharge becomes negative again but stays high at around |0.018 - 0.019| m³/s. So the problem seems to be right behind the lake exit. Possibly inside the main culvert as well. However since the water passed a smaller culvert before, which didn't cause problems, and the small part between the lake exit and the beginning of the main culvert doesn't show any abnormalities in my eyes I am a bit helpless.

I also tried to create a patch mesh at the entry and exit of the lake, which looks like this:

 

As a result the oscillating, recorded at the monitor lines "lake entrance" and "lake exit" decreased significantly. The monitor lines "before the lake", "lake entrance" and "lake exit" now show positive values instead of negative as in the first model run.

The biggest problem however remains the large increase in discharge right after the lake and inside the main culvert (Q = around 0.0207 ms).
Does anyone have an idea where this could originate from?

 

 

 

Edited May 14 by Hans Vahlpahl
movement of pictures