I am working on a “no-rise” analysis (with a baseline FIS model). Moving upstream from the location of proposed changes my proposed conditions model shows a lower WSE profile relative to existing until an upstream bridge where the proposed profile increases about a foot relative to existing. This is due to the bridge modeling approach switching from Energy to Pressure flow for the proposed conditions model.
I changed the pressure flow criteria from EGL to water surface to see if the bridge would switch to energy flow, but it did not. Following tips from a previous RAS Solution blog post, I then added multiple interpolated sections upstream and downstream of the bridge to see if this would affect the selected modeling approach, and it did; results were as expected, with the bridge in question using the Energy high flow method.
I have a few related questions:
Why does the modeling approach switch from Energy to Pressure flow when the surface profile downstream is LOWER than existing?
Is using additional interpolated sections an appropriate “fix” for this issue? In other words, does the addition of interpolated sections increase the precision of HEC-RAS calculations such that I can place more confidence in the model results showing the bridge is not in pressure flow?
Is selecting Energy Only for this bridge justifiable? Why?