Modelling flood control structure in HEC-RAS

[debgallery]

I'm trying to model an existing flood control structure in HEC-RAS in order to get the downstream flow for a floodplain study. The structure is immediately downstream of a double box culvert. It consists of a concrete channel with a weir on the left side and a rectangular orifice at the downstream end of the channel

I modified FEMA's effective HEC-RAS model by adding cross-sections within the control structure, modeling the weir as a lateral structure and the orifice as an inline structure sluice gate that is always open (HEC-RAS model attached)

My first question - is using the open sluice gate an appropriate way to model a rectangular orifice? Any other suggestions are very welcome.

The second issue I'm having is that the model is showing critical or supercritical flow in some of the natural sections of creek and subcritical flow in the steeper concrete channel. This seems counter-intuitive to me. I expect there to be a hydraulic jump at the outlet of the flood control structure (between x-sec 6034 and 5932) but it appears to be the opposite.

Modelling the concrete channel at xs-6049 in Flowmaster shows supercritical flow with a Fr # of 2.43 - just what I'd expect. Hec-Ras gives it a Fr # of 0.41.

I've triple checked all my manning's n #'s, bank stations, boundary conditions etc. Do I need more x-sec's within the structure?

Thanks everyone

 

[cvg]

the orifice causes ponding above critical depth, therefore not supercritical. flow through the orifice and through the section of channel immediately upstream does not follow mannings equation