Design flow in SSA

[derryck1234]

I have a question regarding design flow. In the analysis of my model. My peak flow is less than my design flow, however, the pipe is said to be surcharged? This does not make sense to me? I can imagine that what it must mean is that the design flow is the flow for that particular pipe only, but when it is connected into a system, the other pipes may cause the water to get backed up within the system, so that even although the pipe has not yet reached its design flow, it is still surcharged.

The other thing which bothers me, is that I read up on the exact definition of the design flow to find out that it is the flow in the pipe under gravity, but flowing full. This is actually not the most efficient depth of flow. We all know from undergraduate hydraulics that the pipe in fact gives the most flow at approxiamely 81%?

Thanks

Derryck