Modeling arch bridges

[Bridget]

Why HEC-RAS disregards the momentum answer, even though neither EG line or water surface are above the maximum low chord? This happens when dealing with arch bridges. I tried also to model the bridge with a straight low chord and piles with increasing width, to simulate the bridge arches, but the answer is the same.
Thanks if someone can help me.

[jarvus lunt]

The bridge method that RAS defaulted to may be lower than the maximum low chord, but that doesn't mean that the momentum method didn't hit the low chord. There are also other reasons that RAS may disregard the momentum method.

Go to the bridge specific output table and look at the bottom. Are there any warnings, such as, "for the final momentum answer, the upstream energy was lower than the downstream energy...."

[Bridget]

I get this warning:"Momentum answer is not valid if the water surface is above the low chord or or if there is weir flow. The momentum answer has been disregarded"
So the method used is "Energy only".
Thanks for you reply