A rectangular channel has a bottom width of $1\mathrm{.}2$ m conveys $0\mathrm{.}72$ m$3/$s with

top width $1\mathrm{.}75$ m and uniform depth $0\mathrm{.}5$ m$.$ If a broad$-$crested weir is to be

installed at a section of the channel, analyze the minimum height of weir

that will cause critical flow above the weir.