Question $5.($contd$.)$

b$)$ The diagram below shows two small watersheds $($I and II$)$ feeding into a reinforced concrete storm sewer. Details of each watershed are given in the table on the next page. Water from each watershed flows into a catcla$-$basin al the upstream end of the sewer, as shown. Rainfall intensity vs duration is given in the graph. First, calculate the flow in each pipe for the critical storm$($s$).$ Then determine the diameters of the two sewers and the flow velocities, given that the slope of each sewer is $0\mathrm{.}0017,$ the length is $100$ mn $,$ and Manning's $"$ n $"$ is $0\mathrm{.}013.$ Furthennore, the pipes should ilow at approximately $\backslash (2/3\backslash )$ full $(\backslash (\backslash$mathrm$\{$d$\}/\backslash$mathrm$\{$D$\}\backslash ))$ or less at the calculated flows $($constant $"\backslash ($ n $\backslash )").$ Use only commercially$-$available pipe sizes $\backslash ((\backslash$mathrm$\{$mm$\})$: $375,450,525,600,675,750.(15\backslash )$ marks$)$