HECRAS 100yr WSE lower than 25 yr

[lbgeng]

I'm assuming this must be something with my ineffective flows or critical depth or something but my 100yr WSE is nearly a foot lower than my 25yr WSE. I have 3 - three sided bottomless 20'x7' structures. Flow is 2199cfs for 25yr and 3562cfs for the 100 yr. Starting Known WSE is different for these two by 0.5'. Stream is modeled 1000' total feet and the values look pretty similar up until about 200 ft from downstream of structure.

I'm modeling this as a bridge approach option due to the client however when I did check this with the culvert option I got more reasonable results.

I'm using Energy and Pressure as requested by the client. Is there some other toggle I'm missing? Do I have something messed up with the ineffective flow regimes? Seems very strange to me.

 

[francesca]

Are you talking about the WSE through the 3-sided structure? If so then yes, you are getting supercritical flow or have very high velocity head through the 3-sided structure. Are you modeling mixed flow? Generally speaking, bridge opening design criteria consider backwater and not conditions through the bridge/culvert. Conditions inside the structure dictate scour protection. Where I am, 3,562 cfs would require at least a 100 ft span to meet the backwater criteria.