Channel transition
 

Hi. I am trying to understand channel transitions but I am confused about one thing. If I change width of a channel and lets say I got subcritical flow upstream naturally from conservation of momentum I can tell water level at expanded section increases.

But if I solve manning equation for expanded section its uniform depth is lower. How is uni depth from manning and transition relate to each other? Is depth obtained after expansion from momentum equation just a gradually varied flow and will catch uni depth of manning at certain distance?

Thanks to all