I have seen a value of 1 mm used for spiral wire re-inforced plastic. Can't comment on how valid that is. Also, how corrugated is corrugated? Do you know who the supplier is? Ask them
The pipe is already installed (some time ago?) so no specific info on the pipe other than: 15" dia corrugated plastic pipe.
Doing a hydrology analysis on a watershed with 3 dams and we are rehabin the most downstream dam. The upper 2 dams used cpt for their outlet works (very small dams). Since cpt is not a common product for dams and not accepted for regulated dams (in NC anyway) I don't have info on this type of pipe.
If you do not have the data then your best alternative is to run a few calcs to see what the sensitivity is. Taking some typical numbers, if you compare the flowrate with roughnesses of 2 mm and 0.1 mm in a 15" pipe you get a difference of 30%. Does this matter in your application? Only you can answer that.
Just a cautionary note. While I am not an expert in corrugated or profile-walled plastic piping I believe there have probably been many different kinds of this piping over the years, I suspect all with at least a little different effective "roughnesses". I think I have seen at least a couple basic generic categories of such piping, the first with quite deep corrugations actually encountering the flow (I think far deeper than the 1-2 mm roughness described thus far in this thread?) and another basic type that has corrugations but in effect also a kind of sort of wavy though probably from a hydraulic perspective "smoother" inner wall or liner contacting the flow than the former style (the corrugations are somehow attached or formed to the outer surface of the smoother inner wall or liner in the latter case) . You may thus want to try to find out exactly what you have.
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