We have been simulating an overflow conduit in a wet well (well above all the conduits connecting at that node which is modeled as a storage node) as a wide rectangular conduit which has the same width as the actual weir that will be built (weirs being notably unstable in EXTRAN, we tried a different approach). The conduit has a very steep slope as we wanted to make sure that the flow would be supercritical and that the overflow discharge would not be influenced by anything downstream.

When we look at the results, we obtain very stable results but very small depths at the invert of the rectangular conduit. Normally, we had assumed that the depths would be in the same range as the depths calculated manually with a typical weir equation, which is not the case. How is the depth calculated in this case in EXTRAN? Is it simply the critical depth? Do we have a simple way around this? (increasing the Manning n or modifying the slope?).