Storage unit with throttled outlet causes mass balance error when using EXTRAN surcharge method

[nstrahl]

When modeling closed (pressurised) storages with a throttled outlet the storage head gets "stuck" at the same value. This only happens with the EXTRAN surcharge method, which is the method I am most interested in using.

I have attached a very simple example of a storage with an outlet that throttles the flow to a max. of 0.4 m³/s. The storage inflow is defined as a timeseries and is larger than what can be released through the outlet. I would expect the storage to pressurize and slowly release the pressure through the outlet drawing the water level back down and ultimately emptying the storage. This is what happens when using the SLOT method, which yields a mass balance error of 0%. However, the storage head remains constant with the EXTRAN method and results in a mass balance error of over 200%.

Version: Release 5.2.4.

swmm-extran.zip

[dickinsonre]

I will say that I often see this and find the slot better but the slot often needs a smaller time step.

[dickinsonre]

I tried a time step of 0.01 seconds and it did not help for Extran surfachage. I could use a time step of 1 second for surcharge and it was the same answwer., In general you are using a min step oif 0.1 seconds which does not help but in your case it does not help to set it to 1 second.

[cbuahin]

@nstrahl, let me take a look as well and get back to you.

[NandanaPerera]

I think I have observed this behavior myself too. Under the EXTRAN surcharge method, once a storage node is surcharged, the head does not decrease. This results in an overestimation of flows and final storage, leading to a high continuity error. This issue does not seem to be related to the routing time step.
As a workaround, what I tried was to modify the storage curve to assign a zero area above the current rim elevation (e.g. after 2.6001m). Then adjust the rim elevation to a significantly higher value.
Addressing this issue would be beneficial to ensure both methods provide consistent results.