Determine the hydraulic radius R for a rectangular open channel with dimensions of 6 ft wide and 4 ft high (side walls) flowing at a depth of 3 ft.

Solution: R = A/P

Cross-sectional area of flow A = (6 ft) (3 ft) = 18 ft² 

Wetted perimeter P = (3 ft + 6 ft + 3 ft) = 12 ft 

Note - the open channel is "flowing partially full". There is a 1 ft "freeboard" between the water surface and the top of the side wall. 

Hydraulic radius R = A/P = 18 ft²/12 ft = 1.5 ft.