HEC-RAS 4.1 - Bridge High Flow Analysis - Sluice Gate vs Orifice?

[aspultor]

I am modeling an existing bridge with a tailwater below the low chord and a headwater above the low chord. My understanding is that the appropriate modeling method is pressure/weir. The low tailwater condition is causing HEC-RAS to use a sluice gate equation to calculate the bridge headwater. However, if I SLIGHTLY increase (+0.3') the tailwater to innundate the low chord, HEC-RAS switches to an orifice equation, which drops the headwater by almost 2-feet. Has anyone encountered this issue with high flow bridge modeling and has advice on which equation is more appropriate?