$($b$)$ A housing area needs a channel to convey $9\mathrm{.}8\frac{m^3}{s}$ of rainfall. A trapezoidal

channel is proposed with $3$ width and located on the side slope of $3($horizontal$)$ : $4$

$($vertical$).$ If the channel is concrete lined $(n=0\mathrm{.}013)$ and bottom slope $S_0$ is $1$:$2000,$

determine the normal depth using graphical method.