A trunk sewer, $1200$ mm in diameter $($n$=0\mathrm{.}013),$ is placed on a slope of $0\mathrm{.}003$ between two manholes $120$ m apart. The average design flow in the line is $667$ L$/$s and the minimum and peak flow are $0\mathrm{.}5$ and $2\mathrm{.}5$ of the average flow, respectively. What would the depth of flow in the pipe at average flow?