Why do my design pipes or replacement pipes have unexpected diameter, slope,
invert, etc?

It is possible that Hydra’s design may not match what you would expect by
doing your own hand calculations on a individual pipe.  Hydra’s design
algorythms consider many factors, including slope, d/D, inverts of upstream,
downstream, and adjacent pipes, diversion structures, depth considerations,

Remember that Hydra’s designs are only a starting point.  The engineer
should consider the context of adjoining pipes, cost factors, and the full range
of possible solutions, upstream diversions to remove flows, parallel pipes,
etc.  If pipe replacement is the best solution, realignment of the pipes
should also be considered.