what would be the change in water speed going from an opening of 10ft height by 25ft width into a tube 5ft height by 18ft width if the water was traveling at 1.2mph?
Use continuity considerations, that the quantity of water moved per second is the same in both cross-sections, and ignore hydraulic considerations for simplicity.
10*25 * 1.2 mph = 5*18 * x mph
x = 10*25*1.2/(5*18) = 3.33 mph approx.