By Manning's formula
$Q = A \dfrac{1}{n}R^{2/3}S^{1/2}$
Where
$A = \frac{1}{2} [ \, \frac{1}{4}\pi (1.60^2) \, ] = \frac{8}{25}\pi ~ \text{m}^2$
$P = \frac{1}{2}\pi (1.60) = 0.8\pi ~ \text{m}$
$R = \dfrac{A}{P} = \dfrac{\frac{8}{25}\pi}{0.8\pi} = 0.4 ~ \text{m}$
$S = 4/5000 = 0.0008$
Thus,
$Q = (\frac{8}{25}\pi) \dfrac{1}{0.013}(0.4^{2/3})(0.0008^{1/2})$
$Q = 1.1874 ~ \text{m}^3\text{/s}$ ← answer
