In the wide open channel described, what is the hydraulic radius R?

• A. 0.5 m
• B. 1.0 m
• C. 1.5 m
• D. 2.0 m

Answer: (B)

The hydraulic radius is the cross-sectional area of flow divided by the wetted perimeter (R = A/P). In a wide-open channel, the width is much larger than the depth, so the wetted perimeter is essentially the width and the flow area is width times depth. That makes R approximate the depth of flow, since A ≈ b·y and P ≈ b, giving R ≈ (b·y)/b = y. If the channel described has a depth of 1.0 m, then the hydraulic radius is about 1.0 m.