Decreasing Culvert/Bridge Flow when Overtopping

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Decreasing Culvert/Bridge Flow when Overtopping

Daniel Denette
I am fairly new to HEC-RAS and I mostly model culverts and bridges in 1-D. The culvert that I am modeling right now has a top of roadway that is fairly low so it overtops easily, basically for all storm events. This means that I am seeing a lot of different overtopping scenarios and their respective flows through the culvert/bridge (Q bridge or Q barrel).

I notice that when I model the opening as a culvert or when I model it as a bridge and use the "Energy Only" high flow method, that the flow through the culvert decreases as the total flow and the weir flow increase (they negatively correlate). However, when I model the opening as a bridge and choose "Pressure and/or Weir" as the high flow method, the flow through the culvert increases with total flow and weir flow (they positively correlate).

For some context: For a 10 year storm flow (Q total = 360 cfs) the energy method results in culvert flows (Q barrel) of 245.5 cfs through 40 square feet of culvert while 114.5 cfs overtops the culvert (Q weir). For a 500 year storm flow (Q total = 1120 cfs) the energy method results in culvert flows (Q barrel) as little as 57.7 cfs through 40 square feet of culvert while 1062.3 cfs overtops the culvert (Q weir). I looked at other models that overtop and found the same correlations to hold true.

My intuition is that more total flow should mean deeper water and a higher energy gradient at the culvert, which should increase the flow through the culvert. It does not make sense to me that it would decrease the flow.

Can anyone explain to me why the “Energy Only” method gives the opposite correlation (and much different flows) than the “Pressure and/or Weir” method? Which one is correct? Is there a way to do “Pressure and/or Weir” method with culverts?

This has me stumped so thank you for your help!
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Re: Decreasing Culvert/Bridge Flow when Overtopping

jarvus
There are a lot of different things that could be going on, so it hard to know just from your description.  Culverts can have a lot of different flow regimes (inlet control, outlet control, open surface flow, fully submerged flow, etc.)

Is the tail water constant for all of the flows or is the tailwater higher at the higher flows?

To solve culvert/weir flow, RAS has to iterate.  Are you getting any messages about culvert/weir flow failed to converge?