$($c$)$ Estimate the normal depth in a trapezoidal channel with a bottom width of $4\mathrm{.}0$ m $,$ side slopes of $3(H)$:$1(V)$ and a bed slope of $0\mathrm{.}008.$ The Manning's $n$ value is $0,025$ and the discharge is $40\frac{m^3}{s}.($Use trial and error method$).$

$(10$ marks$)$