I noticed today that the orifice rating curve for orifices is being calculated incorrectly in SSA. The head is being calculated to the orifice invert instead of to the orifice centerline. This probably has a minor (or no) effect if tailwater conditions are present, but it is still annoying.
Is it doing it beyond the unsubmerged orifice calculation that uses weir flow until orifice submergence? Let me know. See below:
I didn't find this to be the case running a quick test on a headwater submerged 1' diameter circular side orifice with g=32.2 ft/s^2 and PI out to 15 digits.
Their diagram in the Help is incorrect (see second image below). H for a headwater submerged orifice should be measured to the centroid of the orifice.
I hope they are using EPA SWMM engine release 5.0.019 or higher, where the Villemonte correction for downstream submergence in partially full orifices was added.
The dialog box rating curve incorrectly calculates orifice flow's H. This rating curve is not used in the hydraulic routing calculations performed by the analysis engine. I have notified the development team of the issue in the dialog box and in the help file.
The Villemonte correction changes from the EPA SWMM 5.0.19 engine are included in SSA 2012 and 2013.