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Thank you for the suggestion and thanks to those who commented and voted. This is a great idea so I will progress the status of this idea to "Gathering Support". As a workaround, please consider the comment below:
I agree that the sluice gate can be used so the gate can be set to a fixed opening or adjusted during the analysis. They can still be used in a static way but when water upstream is less than the opening height, ICM will use a thin plate rectangular weir to determine the flow through the sluice and when the gate is submerged, ICM will use an orifice equation to determine the flow. So if you're looking at an extreme event and you're trying to calcualte the max flows, then probably this should be deemed as an acceptable approach as it will solve an orifice equation for the rectangluar cross-section. However, below is my recommendation as a better workaround:
Some municipalities where ICM is mandated (e.g., Dubai Municipality) would expect modellers to represent the non-circular openings using a circle of equivalent area to the non-circular opening. I agree with this approach and my guess is that this would better match the performance of a rectangular orifice (a factor that might impact the difference here is the width/height ratio) so if the width is more than double the height, I would recommend some stress testing to ensure that the results won't be hugely impacted.
Taking this a step further, I'd also like to have the option of modelling irregular shaped orifices. Occasionally you do come across things like this, e.g. in the attached image. Often when in reality two different metal plates have been bolted together. Sometimes it is a rectangle over a circle, so you end up with a truncated circle. And I've also seen a circular orifice with a partially closed circular door, so you end up with a 'crescent moon' shaped opening. At present, there is no good work-around for these, as I don't believe there are any zero-length link objects to which you can give a custom shape. So you normally have to calculate the area of the irregular shape and model the opening as an equivalent area circular orifice or rectangular sluice. But this will never perfectly match the pass-forward flow rate at different upstream water depths, which can cause issues in the model if the orifice is part of a CSO structure, as predicted spill performance can be effected.