$($Please do step by step$)$

You are designing a sewer line that carries a flow rate of $0\mathrm{.}08\frac{m^3}{sec}$ and you do not want the flow to exceed $\frac{2}{3}$ of the pipe depth $($diameter$).$ If the slope was $0\mathrm{.}02$ and the manning coefficient for the concrete pipe was $0\mathrm{.}013.$ What is the most economic $($smallest$)$ pipe diameter you would choose.

Hint: use an iterative process where you assume the diameter first.