Normally I would put a drop at the manhole to reduce as much velocity as possible. From the manhole the velocity will be assumed to be zero. The down stream pipe will be considered as a pipe with a submerged inlet.
If you don’t put a drop use the principal of conservation of momentum to...
Your explanation of the question is not clear. If 30000cfs flows at approx 9 fps through a rip-rap 25', the water level will be about 20' high, using mannings formula??? Something is not right with you question.
You did not mention something about the slope of the rip-rap. The chart you...
