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I have a section of pipe that is made of two materials. Can I just average the pipe roughness to come up with a composite roughness?
It may or may not be ok to combine the pipe roughness values to go with an average, but it certainly seems like there could be a case in which a good argument could be made for it. The best way to figure out what will work best for the network you're working with is to try one and compare the results to your field data.
Here are some ideas to try that might help out:
1) If you know where the pipe materials differ then try to add multiple pipes and use different roughnesses. For example, if you know the first 1500 ft. of the pipe is shotcrete lined followed by 1500 ft. of partially rough cut D&B followed by another 1500 ft. of shotcrete lining then you might want to try making it 3 separate pipes each with their respective roughness values. If it is not that simple consider options 2 and 3 below.
2) Use the roughness value that is the most conservative. This will assure that you design your system to handle the worst case scenario.
3) Try to model it a few different ways by taking advantage of creating child scenarios and then compare the results to see the differences, if any.