For a circular channel (with $r_0$ as the radius of the channel) to be efficient,

the half subtended angle at the centre with respect to the water level must be ${151}^{\circ }10\text{'}$

depth of flow must be $1.88 r_0$

depth of maximum velocity must be $1.62 r_0$

the half subtended angle at the centre with respect to the water level must be ${90}^0$