HY-8 Velocity Limitation

[0000jz]

I am using HY-8 for the analysis of existing cross-pipes, trying to determine headwater elevations. The drainage manual i am using lists conflicting limitations for the HY-8 software. In one instance, it is noted that HY-8 should not be used where entrance velocity exceeds 5 ft/s. In another instance, it is noted that HY-8 should not be used where stream velocity exceeds 5 ft/s. I cannot find any reasoning behind these limitations to determine whether to use stream velocity or culvert entrance velocity as the determining factor.

Can anyone provide clarification on which velocity should be used as the determining factor?

 

[cvg]

neither is a limitation with HY8, but you will need to clarify with the author of the "drainage manual" to find out what they were thinking

 

[0000jz]

Interesting use of quotes... as if you could tell that the drainage manual was not actually a legitimate drainage manual. Since this regulatory manual has had multiple authors over many years, it would be quite impossible to find out what the "author" was thinking.

Neither is expressly listed as a limitation of HY-8. But I am wondering if it has something to do with the fact that HY-8 ignores the approach velocity head.

 

[LincolnPE]

The entrance loss in HY-8 is computed as a function of barrel velocity, and is therefore typically conservative estimating headwater elevation. Typically HY-8 use is restricted by culvert/bridge size (NBIS), when gradually-varied flow computations are preferred on the larger structures.

 

[beej67]

HY-8 is no more or less restricted than FHWA HDS-5, which is how you'd do a culvert by hand if you didn't want to use HY-8. The only real step "up" from HY-8 is HEC-RAS, which does gradually varied flow as Lincoln points out.



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