Hydraulic mean depth (Dm) for a circular pipe of diameter 'D' flowing full is 0.25 D. For a circular channel, at Dm = 0.3 D, gives the condition for the maximum

Answer: Option C

Hydraulic mean depth (D_m) is defined as the cross-sectional area of flow divided by the wetted perimeter.

For a circular pipe flowing full, D_m = 0.25 D, where D is the diameter of the pipe.

For a circular channel flowing partially, when D_m = 0.3 D, it represents the condition for maximum efficiency of flow.

At this condition, both the flow rate and the mean velocity are maximized, because the hydraulic radius is at its optimum value.

Therefore, the correct answer is Option C: Both A & B