$($Question $5)$

Given the drainage area in figure $1$ and the indicated information,

Length of sewer from manhole $1$ to manhole $2=100m,$ and its slope $=0\mathrm{.}006\frac{m}{m}.$

Length of sewer from manhole $2$ to manhole $3=150m,$ and its slope $=0\mathrm{.}008\frac{m}{m}.$

Rainfall intensity $i=\frac{750}{t+10}m\frac{m}{h}r,$ where $t_{{?}_{?}}$ is the duration in minutes.

Assume Manning's coefficient, $n$ is $0\mathrm{.}015$

The standard dimeters available are, $250,300,375,450,525,600,750$ and $900mm.$

a$)$ Determine the size of sewer $1-2$ and sewer $2-3.$

b$)$ Determine the time of concentration that should be used for the design of sewer $2-3.$

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