Question 12.1: The depth of a uniform steady flow of water in a 1.22 m wide...

We have to use Eq. (12.9) for the present purpose.

V=(1 / n) R_{h}^{2 / 3} S^{1 / 2}                   (12.9)

Here,                 R_{h}=\frac{1.22 \times(0.61)}{1.22+2 \times 0.61}=0.305 \mathrm{~m}=305 \mathrm{~mm} 

S(slope) =\frac{4}{10000}=0.0004 

n (from the Table 12.1) = 0.01

Therefore, from Eq. (12.9)

Or      Q =V \cdot A=\frac{1.22 \times 0.61}{0.01}(0.305)^{2 / 3}(0.0004)^{1 / 2}

Table 12.1 Values of Manning’s Roughness Coefficient n for Straight
Uniform Channels