HEC-RAS or HY-8 for Culverts?

[sswan60]

Modeling a system in very flat topography that. Channel has a wide flood plane once it jumps out of its ~2yr channel. We have set up a HEC-RAS and it is interesting to notice that the HW on culverts are higher using HEC-RAS than HY-8. 1.7' higher for a Q of 387 cfs.

Assuming one has accuratly inputted the data appropriately for each model, is one consider more accurate than the other? Curising the web I found one comment that indicated that HEC-RAS is considered to be more accurate than HY-8.

 

[froude]

It could be any number of things...but just a few things to consider:

1. How do the tailwater elevations compare?
2. If the roadway overtops, how does the weir-flow quantity compare?
3. Are you using ineffective flow areas in HEC-RAS to account for transition near the culvert?
4. How do the culvert losses compare?

I haven't used HY-8 much, but I would say that if you have accurate channel data downstream and upstream of the culvert, HEC-RAS may be a better modeling approach (unless the culvert has a bend or varying slope).

 

[beej67]

1.7' higher is too big of a difference to simply write off, especially for a flow that low. The models should produce reasonably similar numbers, because they're based on the same engineering principles. You should spend some time reconciling them, because one clearly has an error in assumptions or data entry. Two most likely culprits are tailwater and entrance/exit loss, IMO, but the two people I'd listen to here on eng-tips are froude (above) and francesca.

Does it overtop? If so, check to see that the length of overtopping in both models is the same.



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[francesca]

If overbank flow is involved, I'd use HEC-RAS and block out anything <3' deep AND <0.5 fps. For channel-only flow I'd use HY-8.

 

[cvg]

if tailwater backs up, say from another downstream culvert then HY-8 would not be the best method