Hydraulic Radius - An important calculation that is important for the use in finding flow rates - usually used in conjunction with the Reynolds number.
There is " open channel flow " like channels and creeks
As well as " closed flow " like pipes and closed vessels
Reynolds number refers to the turbulence or lack of turbulence of the a fluid. < 2000 laminar
> 2000 Turbulent

Fluid Dynamics is an interesting topic but is complicated and a bases of calculus knowledge and vector analysis is needed to understand this topic.
To work out the problems anyways which are not the easiest of things to do.