Culvert Contraction/Expansion Coeffiecients of 0.6 and 0.8?

[debgallery]

I'm working with a FEMA effective HEC-RAS model that uses contraction and expansion coefficients of 0.6 and 0.8 instead of 0.3 and 0.5 at some culverts. Chapter 5 of the manual shows that these values are used for "abrupt transitions". I have always thought that you used 0.3 and 0.5 for cross-sections adjacent to both culverts and bridges. Are culverts (as opposed to bridges) considered "abrupt transitions"? What values do you typically use for culverts?

 

[froude]

I am accustomed to using 0.3 and 0.5, as well. However, I've also run into several effective HEC-2 models which also use higher values.

My thoughts are that if the channel section is relatively uniform upstream of your structure and doesn't narrow down significantly to transition to your structure, then go with the 0.3/0.5.

I've also noticed that in many cases, changing the c/e coefficients doesn't have as large an impact on WSELs as you may suspect. I think it really just comes down to the modeler's judgement/preference.

Good luck!

 

[froude]

Sorry...I forgot to add that you may also want to compare the floodplain topwidth at the structure to the structure width. If there is a very large difference, then a 0.6/0.8 may be valid.