Correct option is C
Conditions for the most economical or efficient section:
From Chezy’s and Mannings equation that the velocity of flow will be maximum when hydraulic radius R will be maximum for factors like the slope of bed, and surface roughness being constant.
Hydraulic radius is maximum if the wetted perimeter is minimum.
Most economical rectangular section:
Width of the channel is twice the depth of flow (B = 2y).
Hydraulic radius is half of the depth of flow. ( R= y/ 2)
Most economical trapezoidal section:
Half of the top width must be equal to the sloping side.
The hydraulic radius must be half the depth of water.
Most economical triangular section:
The side slope has to be inclined at 45∘.
Most economical circular section:
The depth of flow(y) = 0.81D. (for maximum velocity)
The hydraulic radius(R) for maximum velocity is 0.3 D
The depth of flow(y) = 0. 95D. (for maximum discharge)
Where D = Diameter of channel
