There is an old mill site having a weir, which is about 1.5 ft. high. Only about 10 ft. downstream of the weir is an 88" x 66" elliptical pipe that crosses under the road. I have figured out how much storage is behind the weir and also the storage between the culvert and the weir (which is relatively minimal).
I modeled the weir as a pond with a weir outlet and I also modelled the culvert as a pond with a weir (top of roadway) outlet.
Here are some elevations:
Top of weir = el 56.4
Bottom of weir/invert of culvert = 54.90
Top of roadway = 62.40
What I would expect to happen is that once the inflow into the culvert exceeds the culvert capacity, the weir would become submerged, and the pond elevation behind the weir would equal that of the pond behind the culvert. It's showing the pond behind the weir to be 59' +/- in a 100-yr event, and the pond behind the culvert to be 63' +/-. How can I get the two surfaces to even out?