HY-8 discrepancy

[mmibrianc]

I'm using HY-8 to design a culvert, without getting into too much detail, we started our design using the predefined culvert shapes, in this case a concrete box culvert. We then had to modify the design to use a "user defined" shape to lessen the waterway opening to account for an unavoidable obstruction.

With no other changes being made to any of the other input parameters, a comparison of the results showed that the smaller waterway opening would pass more flow through the culvert than the larger opening. Intuitively, it would be the opposite.

I then ran a trial using the identical dimensions for the culvert, one using the predefined box shape, and the second by entering the same dimensions but using the user defined approach. The user defined approach showed approximately 20% more flow capacity than using the predefined box shape, even though the shapes we identical in dimension, and all other input parameters are the same.

Has anyone else had the same thing happen? Does anyone have an possible explanation why they would be different? Thank you.

 

[SMIAH]

Could you check the elevation-area table for both geometry?
Maybe it will tell where the difference is.
I don't have an explanation though.

 

[francesca]

Check your HY-8 version. One of the recent versions (not the latest) barfs on some calculations and gives what appears to be valid results.

I would also check your entry condition. If you had nice wing walls on one and not on the other, that would account for it.

 

[mmibrianc]

Thank you for the responses. The version I am using is the most recent available, version 7.2, build 4. I have reviewed the results for each geometry, the results vary somewhat considerably. The inlet type is the same for both, conventional.

The options for inlet edge condition is slightly different between the two approaches (pre-defined box culvert vs. user defined w/ a box culvert). For user defined, I am using the 'Square Edge with Headwall' condition, while under the pre-defined box, I'm using the 'Square Edge (90º) Headwall' inlet edge condition. They appear to be the same with just minor differences in terminology. Other than that small difference, the culvert models are the same.

At this time, I'm assuming that the user defined approach is not meant to model culvert shapes that have pre-defined options, such as a box culvert.